Dissertationen: „Helium Superfluid“

1

Matthias, John Robert. „Quantum evaporation from superfluid helium“. Thesis, University of Exeter, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.390167.

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2

Rorai, Cecilia. „Vortex reconnection in superfluid helium“. Doctoral thesis, Università degli studi di Trieste, 2012. http://hdl.handle.net/10077/7868.

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2010/2011
La presente tesi concerne la modellizzazione e simulazione numerica, attraverso l'equazione di Gross-Pitaevskii (chiamata anche equazione di Schroedinger non lineare), della dinamica dei vortici quantistici nell'elio superfluido e in particolare del fenomeno della riconnessione. La riconnessione si verifica qualora due vortici approssimativamente antiparalleli, si intersecano e si scambiano le estremità. Questo fenomeno è stato osservato sperimentalmente e risulta essere una caratteristica essenziale della turbolenza quantistica.
XXIII Ciclo
1983

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3

Boatwright, Adrian Lional. „Molecular behaviour in superfluid helium clusters“. Thesis, University of Nottingham, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.478961.

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4

Blancafort, Jorquera Miquel. „Theoretical reaction and relaxation dynamics in superfluid helium nanodroplets“. Doctoral thesis, Universitat de Barcelona, 2019. http://hdl.handle.net/10803/668116.

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The study of superfluid helium has been carried out mainly by physicists. In recent years, taking advantage of the potentialities presented by superfluid helium nanodroplets (HeNDs) as inert matrices at very low temperatures (0.37 K), the chemical community became involved in its application to high-resolution spectroscopy. More recently (early 2000s), this community began to be involved in research using HeNDs to investigate chemical reactivity in this quantum solvent. As for the theoretical studies on the dynamics of physicochemical processes in HeNDs, they have been possible about five years ago and the number of theoretical dynamics studies, despite their interest, is very scarce. The main objective of this thesis is to contribute to the development of the research in this area. To introduce the reader into the topic, Chapter 1 is divided into four sections: the first one describes the properties of helium, the second one considers the history of the discovery and research carried out on the superfluidity phenomenon, the third one outlines the properties of superfluid helium nanodroplets, and the last one gives an overview of the applications and fields of study implying HeNDs. The theoretical and numerical methods used to describe superfluid liquid helium are detailed in Chapter 2. In the first section attention has been paid on the density functional theory (DFT) and its time dependent extension for real-time simulations (TDDFT). The second section describes the main density functionals used and the third section is aimed to present the numerical methods employed to perform the TDDFT calculations. The following four chapters contain the original studies carried out in this thesis. The investigation of the capture process of a Ne atom by a HeND can be found in Chapter 3. Here, the atom is treated using classical mechanics and the influence of energy and angular momentum is examined for a wide set of initial conditions. The microscopic mechanism, energy and angular momentum exchanges and vortex formation have been extensively analysed. The present contribution corresponds to the first systematic analysis of the influence of angular momentum in the capture process and vortex formation. Chapter 4 represents a natural evolution from Chapter 3 and describes the formation of a neon dimer or neon adduct inside a superfluid helium nanodroplet, treating both atoms classically. Analogously as in the previous chapter, angular momentum has also been taken into consideration and the mechanism, energy an angular momentum exchanges and vortex formation are analysed. These two chapters complement and extend two previous investigations of our group where the Ne atoms were treated using standard quantum mechanics at zero angular momentum. The contents of Chapter 4 correspond to the second theoretical investigation on bimolecular reaction dynamics in HeNDs. The following two chapters use a full quantum hybrid approach to explore rotational and vibrational energy relaxation dynamics. Chapter 5 corresponds to the first theoretical study reported so far on the rotational energy relaxation dynamics of molecules in HeNDs. This process has been studied using several isotopes of the H2 molecule (fast rotors) and considering a set of initial excitations and nanodroplet sizes. The last investigation (Chapter 6) is centred on the study of the vibrational energy relaxation in HeNDs. Thus, the influence of the energy gap between the vibrational levels, molecule-helium interaction energy and nanodroplet size on the vibrational relaxation dynamics has been analysed, taking as a reference the I2@(4He)100 doped nanodroplet which was recently studied in our group. To the best of our knowledge it is the first time that the influence of these key factors has been examined. Finally, in Chapters 7 and 8 the main conclusions and a summary in Catalan are presented.
Les nanogotes d’heli superfluid (HeNDs) són matrius inerts i nanoreactors ideals a baixa temperatura (0.37 K). Això ha atret l’atenció de químics doncs permeten realitzar espectroscopia d’altra resolució, estudiar la reactivitat i sintetitzar en condicions especials. L’estudi teòric de la dinàmica de processos en HeND ha estat possible tan sols fa cinc anys i, tot i el seu interès, n’hi ha molt pocs estudis. L’objectiu d’aquesta tesi és contribuir a la recerca en aquesta àrea. El Capítol 1 descriu les propietats de l’heli, la història de la superfluïdesa i les propietats i aplicacions de les HeNDs. La teoria del funcional de la densitat (DFT) i l’extensió de la mateixa depenent del temps (TDDFT), els principals funcionals per HeNDs i els mètodes numèrics es presenten al Capítol 2. Els següents capítols contenen els estudis originals d’aquesta tesi. En el Capítol 3 s’investiga la captura de Ne en una HeND on l’àtom es tracta clàssicament. El mecanisme, els intercanvis d’energia i moment angular i la formació de vòrtexs s’han analitzat àmpliament. Aquest és el primer anàlisi rigorós de la influència del moment angular en la captura i formació de vòrtexs. El Capítol 4 descriu la formació de Ne2/Ne-Ne en HeND tractant ambdós àtoms clàssicament. El mecanisme, bescanvis d’energia i moment angular i formació de vòrtexs també s’han estudiat. És el segon estudi sobre reaccions bimoleculars en HeNDs. Els Capítols 3 i 4 complementen i amplien dues investigacions del nostre grup on els àtoms es van tractar quànticament amb moment angular zero. En els propers dos capítols es consideren les relaxacions rotacional i vibracional utilitzant enfocs quàntics híbrids. El Capítol 5 correspon al primer estudi teòric de la relaxació rotacional de molècules en HeNDs, i s’han considerat varis isòtops de H2 i excitacions inicials i mides de nanogota. El Capítol 6 detalla la influència de la separació energètica vibracional, interacció molècula-heli i mida de nanogota en la relaxació vibracional en HeNDs, agafant com a referència el sistema I2@(4He)100. És el primer cop que s’examina l’efecte d’aquestes propietats clau. Els Capítols 7 i 8 presenten les principals conclusions i un resum en català, respectivament.
Las nanogotas de helio superfluido (HeNDs) son matrices inertes y nanoreactores ideales a baja temperatura (0.37 K). Esto ha atraído a los químicos pues posibilitan realizar espectroscopia de alta resolución, así como estudiar de la reactividad y síntesis en condiciones especiales. La dinámica teórica de procesos en HeND ha sido posible tan sólo hace cinco años y, a pesar de su interés, todavía hay muy pocos estudios. Esta tesis pretende contribuir a la investigación en esta área. El Capítulo 1 describe las propiedades del helio, la superfluidez y las propiedades y aplicaciones de las HeNDs. La teoría del funcional de la densidad (DFT) y su extensión dependiente del tiempo (TDDFT), los principales funcionales para HeNDs y los métodos numéricos se presentan en el Capítulo 2. Los siguientes capítulos contienen los estudios originales de esta tesis. En el Capítulo 3 se investiga la captura de Ne en una HeND donde el átomo se trata clásicamente. El mecanismo microscópico, intercambios de energía y momento angular y formación de vórtices se han analizado ampliamente. Este es el primer análisis detallado de la influencia del momento angular en la captura y la formación de vórtices. El Capítulo 4 describe la formación de Ne2/Ne-Ne en HeND tratando ambos átomos clásicamente. El mecanismo, intercambios de energía y momento angular y formación de vórtices también se han estudiado. Los Capítulos 3 y 4 complementan y amplían dos investigaciones de nuestro grupo donde los átomos se trataron cuánticamente con momento angular cero. En los dos capítulos siguientes se estudian las relajaciones rotacional y vibracional utilizando enfoques cuánticos híbridos. El Capítulo 5 corresponde al primer estudio teórico de la relajación rotacional de moléculas en HeNDs, y se han considerando varios isótopos de H2, excitaciones iniciales y tamaños de nanogota. El Capítulo 6 detalla la influencia de la separación energética, interacción molécula-helio y tamaño de nanogota en la relajación vibracional en HeND, habiéndose tomando como referencia el sistema I2@(4He)100. Es la primera vez que se examina el efecto de estas propiedades clave en la dinámica. Los Capítulos 7 y 8 presentan las principales conclusiones y un resumen en catalán, respectivamente.

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5

Bryan, Matthew S. „Inelastic Neutron Scattering of Nanoconfined Superfluid Helium“. Thesis, Indiana University, 2018. http://pqdtopen.proquest.com/#viewpdf?dispub=10842052.

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The dynamics of liquid ⁴He confined in a mesoporous powder FSM-16 are reported in this dissertation, including the roton linewidth, excitation spectrum, and Compton profile. With an ordered triangular lattice structure, FSM-16 is a high surface area porous glass with hexagonal pores a few nanometers in diameter. Neutron backscattering results examined the roton linewidth as a function of temperature. Observed linewidths in confinement are consistent with the theoretical and experimental results of the bulk liquid. The temperature-filling phase diagram was explored at intermediate fillings and low temperatures. The maxon and roton excitations are used as indicators of density for a thin-film that transitions into a three dimensional confined fluid. The resulting excitation spectrum at low fillings does not correspond to the bulk liquid at any pressure. The deep inelastic neutron scattering results found an enhanced single particle kinetic energy, with full pore and thin film liquid deviating from the bulk momentum distribution in shape.

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6

Lorenson, Claude Pierre. „Dynamical properties of superfluid turbulence /“. The Ohio State University, 1985. http://rave.ohiolink.edu/etdc/view?acc_num=osu148726339902566.

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7

Cregan, Robert Fraser. „Defects on the free surface of superfluid helium“. Thesis, University of Exeter, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.261949.

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8

Feng, Cheng. „Superfluid helium droplets : from spectroscopy to magnetic nanoparticles“. Thesis, University of Leicester, 2015. http://hdl.handle.net/2381/32439.

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This project applies superfluid helium droplets in two big areas: spectroscopy and nanoparticles formation. Helium droplets, consisted of more than 103 helium atoms, provide a non-friction and very cold (~0.37 K) environment for clusters/nanoparticles growth, which offers unique advantages as a medium in applications of both spectroscopy and nanostructures formation. By sequentially adding NaCl and water molecules, the dissolution process of NaCl in water can be experimentally studied for the first time. This is achieved by recording the infra-red spectra of NaCl(H2O)n (1 ≤ n ≤ 7) complexes in O-H stretching region in combination with ab initio calculations. In particular, two bands have been assigned to dissolution of NaCl in water. The existence of quantum vortices in helium droplets has been firmly evident from spherical Ag nanoparticles formed in a chain. Quantum vortices provide a new route for the formation of one-dimensional nanostructures, which has been exploited in the fabrication of novel nanowires. Finally the pioneering work on the fabrication of both antiferromagnetic and ferromagnetic nanomaterials is reported for the first time, taking Cr and Ni as examples respectively. It is found that we can force ferromagnetic ordering in magnetic materials in superfluid helium. In particular, antiferromagnetic chromium has been discovered to convert to ferromagnetic, and the magnetism of ferromagnetic materials can have hugely enhanced magnetic moments that are close to the theoretical limit. This will have potential for applications in biomedical science, data storage and energy efficient technologies in the longer term.

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9

Ohya, Masayoshi. „Stability of superconducting magnets cooled by superfluid helium“. Kyoto University, 2006. http://hdl.handle.net/2433/143795.

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Kyoto University (京都大学)
0048
新制・課程博士
博士(エネルギー科学)
甲第12421号
エネ博第128号
新制||エネ||32(附属図書館)
24257
UT51-2006-J412
京都大学大学院エネルギー科学研究科エネルギー応用科学専攻
(主査)教授塩津正博, 教授宅田裕彦, 助教授白井康之
学位規則第4条第1項該当

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10

Coppens, François M. G. J. „Ultrafast quantum dynamics of doped superfluid helium nanodroplets“. Thesis, Toulouse 3, 2018. http://www.theses.fr/2018TOU30145/document.

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Dans cette thèse, nous étudions deux aspects de la dynamique d'impuretés atomiques interagissant avec des nanogouttes d'hélium superfluide (He) : la photo-excitation d'alcalins sur une nanogoutte et le dopage de nanogouttes contenant des tourbillons (vortex) quantiques avec des atomes de gaz rares. Nous utilisons la théorie de la fonctionnelle de la densité d'hélium ainsi que sa version dépendante du temps pour en faire la description théorique. Le premier aspect a été effectué dans le cadre d'une collaboration avec des expérimentateurs sur la photo-excitation du rubidium (Rb). Les alcalins sont une sonde très intéressante des gouttelettes d'hélium car ils résident dans leur zone de surface, où il a été prédit qu'un taux de condensation de Bose-Einstein de 100% était possible en raison d'une densité inférieure à celle de l'hélium superfluide. Nos simulations montrent que les états excités 5p et 6p désorbent à des échelles de temps très différentes, séparées par 2 ordres de grandeur (~100 ps et ~1 ps pour 5p et 6p respectivement). Ces résultats sont en accord avec ceux de l'expérience pompe-sonde à l'échelle femtoseconde qui a étudié la photodesorption d'atomes de Rb. Cependant, dans nos simulations, l'excitation vers 5pPi_{3/2} aboutit à un exciplexe RbHe lié à la surface, contrairement à l'expérience où RbHe est éjecté. L'introduction de la relaxation de spin de Pi_{3/2} à Pi_{1/2} nous a permis de résoudre ce désaccord, l'exciplexe RbHe ayant alors assez d'énergie pour désorber. Le deuxième aspect concerne une investigation purement théorique inspirée par les travaux récents de Gomez et Vilesov et al., où les tourbillons quantiques étaient visualisés en dopant les nanogouttes d'hélium avec des atomes d'argent, puis en les faisant atterrir en douceur (soft landing) sur un écran de carbone. Les images au microscope électronique montrent de longs filaments d'agrégats d'atomes d'argent qui s'étaient accumulés le long des coeurs des vortex. La formation de réseaux de tourbillons quantiques à l'intérieur de nanogoutelettes dopées par du xénon est également mise en évidence par diffraction de rayons X qui montrent des pics de Bragg caractéristiques d'agrégats de xénon piégés dans les coeurs des vortex. Nous avons d'abord étudié des collisions frontales entre un atome de xénon, héliophile, et une nanogoutte de 1000 héliums, et comparé les résultats à ceux d'une étude précédente sur le même processus avec le césium (Cs), qui est héliophobe. Dans le cas de Xe une «boule de neige» se forme autour de lui quand il entre dans la nanogoutte, et il lui faut beaucoup plus d'énergie qu'au Cs pour qu'il puisse en ressortir. Quand il le fait, il emporte des héliums avec lui, contrairement au Cs. Nous avons ensuite simulé des collisions entre Ar/Xe et des nanogouttes d'hélium superfluides pour différentes vitesses initiales et paramètres d'impact afin de déterminer leur section efficace de capture. Ces simulations ont ensuite été répétées pour des gouttelettes hébergeant un vortex quantique. On observe que l'impact des impuretés induit de grandes déformations de flexion et de torsion de la ligne de vortex, allant jusqu'à la génération d'ondes de Kelvin hélicoïdales qui se propagent le long du coeur du vortex. Ar/Xe est bien finalement capturé par le vortex, mais pas dans son coeur. Nous avons également découvert que l'existence d'un réseau de 6 lignes de vortex dont les noyaux sont remplis d'atomes d'Ar donne une rigidité accrue à la nanogoutte qui permet de stabiliser le système nano-goutte + vortex même à de faibles vitesses angulaires. Nos simulations impliquant des nanogouttes d'hélium comportant des tourbillons quantiques ouvrent la voie à d'autres investigations sur des nanogouttes hébergeant un ensemble de vortex, en collision avec de multiples impuretés
In this thesis we investigate two aspects of the dynamics of atomic impurities interacting with superfluid helium (He) nanodroplets, namely the photo-excitation of alkalis on a nanodroplet and the doping process of nanodroplets hosting quantised vortices with noble gas atoms. For the theoretical investigations we use He density functional theory and its time-dependent version. The first aspect involves a joint experimental and theoretical collaboration that focusses on the photo-excitation of the alkali rubidium (Rb). Alkalis are a very interesting probe of He droplets since they reside in their surface region, where it has been argued that almost 100% Bose-Einstein condensation could be achieved due to a density that is lower than in bulk superfluid He. In our simulations we find that states excited to the 5p and 6p manifold desorb at very different timescales, separated by 2 orders of magnitude (~100 ps and ~1 ps for 5p and 6p respectively). This is in good agreement with experimental results where the desorption behaviour of photo-excited Rb atoms is determined using a femtosecond pump-probe scheme. However, in our simulations excitation to the 5pPi_{3/2}-state results in a surface-bound RbHe exciplex, contrary to the experimental case where the RbHe exciplex desorbs from the droplets surface. Introducing spin-relaxation from Pi_{3/2} to Pi_{1/2} into the simulations, the RbHe exciplex is able to desorb from the droplet's surface, which resolves this contradiction. The second aspect concerns a purely theoretical investigation that is inspired by recent work of Gomez and Vilesov et al., where quantised vortices were visualised by doping He nanodroplets with silver atoms, subsequently "soft landing" them on a carbon screen. Electron-microscope images show long filaments of silver atom clusters that accumulated along the vortex cores. Also the formation of quantum-vortex lattices inside nanodroplets is evidenced by using X-ray diffractive imaging to visualise the characteristic Bragg patterns from xenon (Xe) clusters trapped inside the vortex cores. First, head-on collisions between heliophilic Xe and a He nanodroplet made of 1000 He atoms are studied. The results are then compared with the results of a previous study of an equivalent kinematic case with cesium (Cs), which is heliophobic. Xe acquires a "snowball" of He around itself when it traverses the droplet and much more kinetic energy is required before Xe is able to pierce the droplet completely. When it does, it takes away some He with it, contrary to the Cs case. Next, collisions between argon (Ar)/Xe and pristine superfluid He nanodroplets are performed for various initial velocities and impact parameters to determine the effective cross-section for capture. Finally, the simulations are then repeated for droplets hosting a single quantised vortex line. It is observed that the impact of the impurities induces large bending and twisting excitations of the vortex line, including the generation of helical Kelvin waves propagating along the vortex core. We conclude that Ar/Xe is captured by the quantised vortex line, although not in its core. Also we find that a He droplet, hosting a 6-vortex line array whose cores are filled with Ar atoms, results in added rigidity to the system which stabilises the droplets at low angular velocities. Our simulations involving droplets hosting quantum vortices open the way to further investigations on droplets hosting an array of vortices, involving multiple impurities

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11

Gould, S. G. „Observation of Freedericksz transition in superfluid '3HeA“. Thesis, University of Manchester, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.377649.

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12

Forbes, Andrew Christopher. „Roton-roton scattering in liquid helium HE II“. Thesis, University of Exeter, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.280645.

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13

Singh, S., L. A. De Lorenzo, I. Pikovski und K. C. Schwab. „Detecting continuous gravitational waves with superfluid 4He“. IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/625336.

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Direct detection of gravitational waves is opening a new window onto our universe. Here, we study the sensitivity to continuous-wave strain fields of a kg-scale optomechanical system formed by the acoustic motion of superfluid helium-4 parametrically coupled to a superconducting microwave cavity. This narrowband detection scheme can operate at very highQ-factors, while the resonant frequency is tunable through pressurization of the helium in the 0.1-1.5 kHz range. The detector can therefore be tuned to a variety of astrophysical sources and can remain sensitive to a particular source over a long period of time. For thermal noise limited sensitivity, we find that strain fields on the order of h similar to 10(-23)/root Hz are detectable. Measuring such strains is possible by implementing state of the art microwave transducer technology. Weshow that the proposed system can compete with interferometric detectors and potentially surpass the gravitational strain limits set by them for certain pulsar sources within a few months of integration time.

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14

Karimäki, J. (Janne). „Structure and dynamics of vortices in superfluid helium-3“. Doctoral thesis, University of Oulu, 2012. http://urn.fi/urn:isbn:9789514297397.

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Abstract In this thesis, the structure and the dynamics of vortices are studied from the standpoint of the hydrodynamical theory of superfluids. In the hydrodynamical theory a superfluid is described by a continuous order parameter field. In the case of superfluid helium-4 this field is a complex-valued function of position and time. However, in superfluid helium-3 the order parameter is a complex-valued 3 × 3 matrix. The first part of this work consists of studies on structures that appear in the order parameter field, when a vessel filled with superfluid helium-3 in the A phase (3He-A) is rotated in an external magnetic field. Among the most common of these structures are the so-called continuous vortices. They exist in several different forms. In addition to vortices, other possible structures include the vortex sheet of 3He-A that was discovered at the Low Temperature Laboratory of Helsinki University of Technology (currently Aalto University) in late 1993. In this thesis, these structures were studied by finding stationary vortex configurations that minimize the free energy of the superfluid. An algorithm for minimizing the free energy was implemented by writing a computer program. This program was then used to study the structure of a few vortex types, inferred to be the most probable ones. In addition, regular lattices formed by these vortices, including the vortex sheet, were studied. A phase diagram for vortex lattices was constructed by comparing the free energy of various lattice structures as a function of rotational velocity and external magnetic field. The study of vortex structures also lead to a discovery of a new type of vortex in 3He-A, later named the LV3 vortex. In the second part of the work, the dynamics of vortices was studied using a filament model of vortex motion, which also has its theoretical justification in the hydrodynamical model of superfluids, but where the detailed structure of the vortex core is not relevant. The specific problem under consideration here was the motion of a quantized vortex in a rotating elongated cylinder filled with superfluid, and how the motion of the vortex depends on temperature and the rotational velocity of the vessel. The study of vortex motion was simplified using scaling laws. A new type of scaling law was discovered, which both simplified the specific problem under study, and made the results more general. In summary, the research in this thesis touched upon two somewhat complementary areas, i.e. the structure of continuous vortices in 3He-A and the dynamics of thin vortex lines, which is more applicable to superfluid 4He or to the B phase of superfluid helium-3 (3He-B). However, these areas complement each other in advancing the general scientific understanding about the properties of superfluids.

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15

Bierau, Frauke [Verfasser]. „Trapping biomolecular ions in superfluid helium droplets / Frauke Bierau“. Berlin : Freie Universität Berlin, 2011. http://d-nb.info/1026265991/34.

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16

Hendry, Philip Charles. „The nucleation of quantised vortex rings in superfluid helium“. Thesis, Lancaster University, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.303023.

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17

Ahlstrom, Sean L. „Quasiparticle beam profile measurements in superfluid helium 3-B“. Thesis, Lancaster University, 2016. http://eprints.lancs.ac.uk/83024/.

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This thesis details experiments in the ballistic regime of superfluid 3He-B at temperatures around 100 ➭K. In this thesis, the quasiparticle beam profile from a Black-Body radiator and a vibrating wire resonator is measured using a quasiparticle camera which consists of 25 tuning forks. The vibrating wire resonator is found to have a very thinly spread beam at velocities slightly higher than the pair-breaking critical velocity which spreads out at higher velocities. At 21 mm/s the vibrating resonator is found to have a more focused beam than the Black-Body radiator where both beam profiles are found to be focused around the centre of the quasiparticle camera. A theoretical model is created in order to model the spread and intensity of the experimental beam profile from the Black-Body radiator. In this thesis, the response of some of the probes to thermal damping is measured in the form of the geometrical constants γ and λ. The geometrical constant λ is related to the velocity dependence of the probe to thermal damping and is found for two wires of varying diameter, a tuning fork and its overtone mode in which all the values of λ were close to unity. The geometrical constant γ is related to the sensitivity of a probe to thermal damping. This is found for a Black-Body radiator and then used to derive the sensitivity for other wires in the cell in thermal equilibrium with it. Two resonant modes of a small tuning fork are used to do a Black-Body radiator calibration on the area around the tuning fork which is used to find the sensitivity of 25 tuning forks (pixels) in the quasiparticle camera.

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18

Courts, Samuel Scott. „Superfluid turbulence in two-fluid flow of helium II /“. The Ohio State University, 1988. http://rave.ohiolink.edu/etdc/view?acc_num=osu148759165817599.

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19

Qu, An. „Experimental study of metastable solid and superfluid helium-4“. Thesis, Paris 6, 2017. http://www.theses.fr/2017PA066052/document.

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L'hélium solide métastable est un candidat possible pour la supersolidité. Notre équipe a démontré en 2011 qu'on pouvait obtenir de l'hélium solide métastable à des pressions inférieures à la pression de fusion à l'aide d'une onde acoustique focalisée. Cependant, une instabilité inattendue apparaît lorsque la pression locale du crystal atteint 21 bar c'est à dire 4 bar sous la pression de fusion. J'ai donc commencé ma thèse en étudiant le temps d'apparition de l'instabilité, et j'ai confirmé qu'elle apparaît toujours dans des phases de décompression de l'onde sonore, c'est à dire à une pression inférieure à la pression de fusion. Ensuite, j'ai étudié la limite de cavitation de l'hélium superfluide à pression négative. En utilisant une méthode interférométrique développée par mon prédécesseur Fabien Souris, j'ai mesuré directement la densité de cavitation de l'hélium superfluide métastable. J'ai trouvé que, à 1 K, l'hélium superfluide cavite lorsque sa densité locale a diminué de 8.4%. En utilisant une équation d'état bien établie théoriquement, on peut convertir ce résultat en pression de cavitation pour le comparer avec ceux obtenus par d'autres groupes. À ma grande surprise, mon résultat n'est pas compatible avec ces derniers. Cette incompatibilité soulève des questions intéressantes quant à la possibilité de nucléation de la bulle sur des vortex quantifiés. Enfin, j'ai étudié la dynamique de la bulle d'hélium déclenchée par la cavitation. En analysant l'équation du mouvement de la bulle et le transfert de chaleur correspondant, j'ai expliqué avec succès pourquoi la durée de vie de la bulle a une transition dramatique quand l'hélium passe de liquide normal à superfluide
Metastable solid helium is a possible candidate for supersolidity. In 2011, our group has demonstrated that we could obtain the metastable solid helium at pressures below the melting pressure using a focused acoustic wave. However, an unexpected instability occurs when the local pressure of the crystal reaches 21 bar which is 4 bar below the melting pressure. So I started my thesis by studying the appearance time of the instability, and I confirmed that it always appears at the low pressure swing of the acoustic wave. Then, I studied the cavitation limit of superfluid helium at negative pressure. Using an interferometric method developed by my predecessor Fabien Souris, I directly measured the cavitation density of metastable superfluid helium. I found that at 1 K, superfluid helium cavitates when its local density is lowered by 8.4%. Using a theoretically well-established equation of state, this result can be converted to a cavitation pressure in order to compare our results with those obtained by others groups. To my surprise, my result is not consistent with the others'. This incompatibility raises interesting questions about the possibility of nucleation of the bubble on quantified vortices. Finally, I studied the dynamics of the helium bubble triggered by cavitation. By analyzing the equation of motion of bubble and the corresponding heat transfer, I have successfully explained why the bubble's lifetime has a dramatic transition as the helium passes from normal liquid to superfluid

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20

Mulheran, Paul Alexander. „A microscopic theory for quantum evaporation from helium-II“. Thesis, University of Exeter, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.280662.

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21

Patel, Hetal. „Experimental study of two-dimensional helium mixture films absorbed on graphite“. Thesis, Royal Holloway, University of London, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.252098.

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3He-4He mixture films adsorbed on graphite were studied down to temperatures below 1 mK using heat capacity and nuclear magnetic resonance (NMR) to probe the system. This work represents the first measurements in this temperature range of the various systems studied. The work focused on two types of mixtures, 3He °He mixtures within the second layer, and 'He floating on three layers of 4He. For mixtures within the layer, a homogeneous mixed phase was observed, followed by a phase interpreted as a Fermi liquid of 'He quasiparticle excitations tunneling within a solid 'He lattice. For the second type of mixture, the expected occupancy of the first excited surface normal state was observed. This is accompanied by anomalous behaviour at low temperatures (T < 10 mK) which arises due to a low concentration of 3He atoms being pushed into the third superfluid 4He layer. An NMR spectrometer utilising a cooled differential preamplifier operating at 4K was developed and used to measure the magnetic properties of helium mixtures. However, its signal to noise ratio was too low to provide the required accuracy in the measurements. An automated computer controlled data acquisition system was also developed to measure heat capacity and magnetic properties of the mixtures. The accompanying software required to process the data collected from both types of measurements was also developed and used successfully to obtain the data presented in this thesis

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González, Flórez Ana Isabel [Verfasser]. „Biomolecular Ions in Superfluid Helium Nanodroplets / Ana Isabel González Flórez“. Berlin : Freie Universität Berlin, 2016. http://d-nb.info/1105472221/34.

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23

Choi, Myong Yong Baer Tomas. „Infrared laser spectroscopy of isolated biomolecules in superfluid helium nanodroplets“. Chapel Hill, N.C. : University of North Carolina at Chapel Hill, 2006. http://dc.lib.unc.edu/u?/etd,353.

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Thesis (Ph. D.)--University of North Carolina at Chapel Hill, 2006.
Title from electronic title page (viewed Oct. 10, 2007). "... in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemistry." Discipline: Chemistry; Department/School: Chemistry.

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Levitin, Lev. „Experimental studies of the superfluid phases of confined helium-3“. Thesis, Royal Holloway, University of London, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.530789.

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25

Godfrey, Simon Paul. „Vortices and stability in boundary layer flow of superfluid helium“. Thesis, University of Newcastle Upon Tyne, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.327247.

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26

Silva, Filipe Ribeiro Ferreira da. „Electron driven reactions in complexes embedded in superfluid helium droplets“. Doctoral thesis, Faculdade de Ciências e Tecnologia, 2009. http://hdl.handle.net/10362/5340.

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A thesis submitted to the University of Innsbruck for the doctor degree in Natural Sciences, Physics and New University of Lisbon for the doctor degree in Physics, Atomic and Molecular Physics
The research work performed under the course of this thesis at the Nano-Bio Physics Group of the Institute of Ion Physics and Applied Physics, University of Innsbruck, deals exclusively with electron driven reactions in complexes embedded in helium nanodroplets. Helium nanodroplets provide a special and exotical environment that is not reachable with other techniques. The cold environment of the helium nanodroplets (0.38K), is a perfect tool to study complex systems in their ro-vibrational ground state. Dopants are added to the helium nanodroplets in a pick up cell allowing to control accurately the growing of clusters‘ size in helium droplets. The research activities described in this thesis cover the interaction of low and intermediate energies (0 – 100 eV) electrons with a wide range of simple and complex molecules in a very cold environment. Electron impact ionisation and free electron attachment to different systems were studied. Different halogenated molecules were used to study the size of solvated cations and anions. Clusters of the rare gas argon were also investigated and compared with argon cluster ions formed upon electron impact of pure neutral argon clusters. Several biomolecules and molecules with biological interest have been studied, these including: some amino acids as Glycine, L-alanine and L-serine embedded in helium nanodroplets. Several features were assigned as helium solvation and fragmentation. In the case of L-serine, a magic octamer S8H+ cluster was observed and identified. Free electron attachment experiments to L-serine shows very rich chemistry observed here for the first time in amino acids embedded in helium nanodroplets. Positively and negatively charged ions from He nanodroplets doped with acetic acid were also investigated. Chemistry triggered by low energy electrons was discuss and compared with previous studies especially with single, gas phase molecules. Preliminary studies on L-valine show strong indication for peptide bond formation at cold temperatures and triggered by low electron energy, close to 0 eV.

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Griswold, Donald L. „Dynamics of the vortex line density in superfluid turbulence /“. The Ohio State University, 1986. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487267024995376.

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28

Khan, K. B. „The nonlocal-nonlinear-Schroedinger-equation model of superfluid '4He“. Thesis, University of Exeter, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.267224.

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29

Idowu, Olusola Clement. „Hydrodynamics of coupled normal-fluid and superfluid motion in helium II“. Thesis, University of Newcastle Upon Tyne, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.323480.

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30

Spence, Daniel James. „A new route for the fabrication of nanoparticles in superfluid helium“. Thesis, University of Leicester, 2015. http://hdl.handle.net/2381/31403.

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This project intends to establish a new route for the fabrication of nanoparticles using superfluid helium droplets as the growth medium, which offers a high degree of control over the growth of nanoparticles, allowing tailoring to specific applications. Starting from the characterization of the UHV superfluid helium droplet source, we progressively add more and more dopants to the droplets, leading to the formation of small clusters composed of a few molecules and/or metal atoms, and large nanoparticles composed of >104 atoms. The helium droplet source and small clusters are characterized using mass spectrometry while nanoparticles are investigated by transmission electron microscope imaging. By sequential doping we have obtained binary clusters, unlocking the possibility of producing heterogeneous clusters of any two or more materials. In addition, we have fabricated core-shell nanoparticles, and have provided microscopy images that clearly show core-shell structure for nanoparticles fabricated by this route for the first time. Finally, in this project we have obtained the first direct evidence for the presence of quantized vortices in nanoscale superfluid helium, which are then utilized as templates for the growth of nanowires. This opens up new a landscape of helium nanodroplet technology and new possibilities for nanoscience and nanotechnology.

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Sherwin-Robson, Lucy Kathleen. „Numerical experimentation and analysis of quantum turbulence in superfluid helium II“. Thesis, University of Newcastle upon Tyne, 2016. http://hdl.handle.net/10443/3496.

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The study of turbulence in superfluid Helium II suggests that at least in part the rules of classical turbulence are obeyed. The question posed is, whether the tangles of quantised vorticity that represent turbulence in a superfluid are directly analogous to the swirls and eddies found in turbulent classical fluids. A cornerstone of classical turbulence has been the evidence of the Kolmogorov scaling and this has been observed in some experimental studies of superfluid turbulence. Here we contrast quantum turbulence in various scenarios to further our understanding and confidence in such modelling as well as to search for evidence of any adherence to Kolmogorov. In all numerical simulations presented here turbulence in the superfluid is driven by motions of the normal fluid. My work approaches the superfluid turbulence through three distinct normal fluid models. In most physical experiments with superfluid helium, turbulence is generated in two ways. Firstly, thermally (by applying a heat flux, as in thermal counterflow) and we model this by using a uniform normal fluid. Secondly, mechanically (by stirring the liquid) and we model this in one of two ways; either a synthetic turbulence using a kinematic simulations (KS) flow or with a frozen snapshot from a direct numerical simulation (DNS). We determine the difference between thermally and mechanically driven quantum turbulence. Using the kinematic simulations model we find that in the latter the energy is concentrated at the large scales, the spectrum obeys Kolmogorov scaling, vortex lines have small curvature, and the presence of coherent vortex structures induces vortex reconnections at small angles. In contrast, when we employ our uniform normal fluid we find the energy is concentrated at the mesoscales, the curvature is larger, the vorticity field is featureless and reconnections occur at larger angles. Our results suggest a method to experimentally detect the presence of superfluid vortex bundles. We show that vortex tangles with the same vortex line density have different energy spectra, depending on the driving normal fluid, and identify the spectral signature of two forms of superfluid turbulence: Kolmogorov tangles and Vinen tangles. By decomposing the superfluid velocity field into local and nonlocal contributions, we find that in Vinen tangles the motion of vortex lines depends mainly on the local curvature, whereas in Kolmogorov tangles the long-range vortex interaction is dominant and leads to the formation of clustering of lines, in analogy to the ‘worms’ of classical turbulence. Finally, we compute the frequency spectrum of superfluid vortex density fluctuations for tangles of the same vortex line density, but which are driven by two different normal fluid models. Taking our measurements in a sufficiently small cube to eliminate any filtering effect, we observe the f−5/3 that has been experimentally observed within the Kolmogorov tangles whereas for the Vinen tangles we find a flat and featureless spectrum.

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Stiles, Paul L. Miller Roger E. „Infrared spectroscopy of metal cluster-adsorbate systems solvated in superfluid helium nanodroplets“. Chapel Hill, N.C. : University of North Carolina at Chapel Hill, 2006. http://dc.lib.unc.edu/u?/etd,204.

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Thesis (Ph. D.)--University of North Carolina at Chapel Hill, 2006.
Title from electronic title page (viewed Oct. 10, 2007). "... in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Computer Science." Discipline: Computer Science; Department/School: Computer Science.

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Reneuve, Jason. „Modélisation de la structure fine de la turbulence quantique et classique“. Thesis, Lyon, 2019. http://www.theses.fr/2019LYSEN036/document.

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Cette thèse est constituée de deux parties, dont l'axe commun est la modélisation de phénomènes de petites échelles pour des écoulements turbulents.Dans une première partie on s'intéresse à l'influence des rotons sur la dynamique d'un modèle d'hélium superfluide. On commence par une calibration d'un modèle non-local d'interaction dans le but de reproduire la relation de dispersion expérimentale de l'hélium, mesurée par diffraction de neutrons. On utilise ensuite ce modèle calibré pour réaliser des simulations numériques directes (DNS) de l'équation de Gross-Pitaevskii, afin de sonder le phénomène de reconnexion des tourbillons quantiques. Ce phénomène est étudié en détail via une analyse géométrique et énergétique des résultats des DNS. On compare alors systématiquement ces résultats à ceux du modèle local afin d'étudier l'influence des rotons sur l'écoulement aux échelles de l'ordre de l'Angstrom.Dans un second temps on cherche à décrire la structure spatio-temporelle de la turbulence homogène et isotrope. Pour cela on commence par une analyse des propriétés statistiques du champ eulérien de vitesse, basée sur l'évaluation de ses incréments spatio-temporels. On utilise les données issues d'une DNS des équations de Navier-Stokes mises à disposition par l'Université Johns Hopkins. On propose ensuite un champ aléatoire spatio-temporel pour la vitesse eulérienne, en caractérisant d'abord la structure de ses corrélations par une approximation gaussienne. On modifie ensuite cette approximation par une mesure multi-fractale afin de reproduire les aspects non-gaussiens observés dans la DNS, tels que les hauts niveaux des coefficients d'asymétrie et d'aplatissement
This thesis consists of two parts that share a common theme : the modeling of small-scale phenomena in turbulent flows.In a first part we focus on the influence of rotons on the dynamics of a model of superfluid helium. We begin by a calibration of a nonlocal model of the interaction, aiming at reproducing the experimental dispersion relation of helium, as measured by neutron scattering methods. This model is then used to perform Direct Numerical Simulations (DNS) of the Gross-Pitaevskii equation, in order to probe the reconnection of quantum vortices. This phenomenon is studied quantitatively through a geometrical and energetical analysis of the results of the DNS. We then systematically compare these results with those of the local model, so as to study the influence of rotons on flow scales of the order of the Angtstrom.The goal of the second part is to describe the spatio-temporal structure of homogeneous and isotropic turbulence. To achieve it we start by a standard analysis of the statistical properties of the eulerian velocity field, by computing its spatio-temporal increments. We use the data from a DNS of the Navier-Stokes equations, hosted and made available by the Johns Hopkins University. We then propose a random, spatio-temporal eulerian velocity field, by first characterizing the structure of its correlations through a gaussian approximation. This approximation is then modified by a multifractal measure in order to reproduce the non-gaussian features, as they are demanded by the observed high level of skewness and flatness of increments

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Graves, Max. „Path Integral Quantum Monte Carlo Study of Coupling and Proximity Effects in Superfluid Helium-4“. ScholarWorks @ UVM, 2014. http://scholarworks.uvm.edu/graddis/299.

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When bulk helium-4 is cooled below T = 2.18 K, it undergoes a phase transition to a superfluid, characterized by a complex wave function with a macroscopic phase and exhibits inviscid, quantized flow. The macroscopic phase coherence can be probed in a container filled with helium-4, by reducing one or more of its dimensions until they are smaller than the coherence length, the spatial distance over which order propagates. As this dimensional reduction occurs, enhanced thermal and quantum fluctuations push the transition to the superfluid state to lower temperatures. However, this trend can be countered via the proximity effect, where a bulk 3-dimensional (3d) superfluid is coupled to a low (2d) dimensional superfluid via a weak link producing superfluid correlations in the film at temperatures above the Kosterlitz-Thouless temperature. Recent experiments probing the coupling between 3d and 2d superfluid helium-4 have uncovered an anomalously large proximity effect, leading to an enhanced superfluid density that cannot be explained using the correlation length alone. In this work, we have determined the origin of this enhanced proximity effect via large scale quantum Monte Carlo simulations of helium-4 in a topologically non-trivial geometry that incorporates the important aspects of the experiments. We find that due to the bosonic symmetry of helium-4, identical particle permutations lead to correlations between contiguous spatial regions at a length scale greater than the coherence length. We show that quantum exchange plays a large role in explaining the anomalous experimental results while simultaneously showing how classical arguments fall short of this task.

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Pakpour, Fatemeh. „Investigation of quantum turbulence in superfluid ⁴He using injected ions and He₂* molecules in the zero temperature limit“. Thesis, University of Manchester, 2014. https://www.research.manchester.ac.uk/portal/en/theses/investigation-of-quantum-turbulence-in-superfluid-4heusing-injected-ions-and-he2-molecules-in-the-zerotemperature-limit(d0650559-8afa-47a2-af63-021bb8115ee2).html.

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The decay of quantum turbulence in superfluid (_^4)He in the zero temperature limit wasinvestigated. Quantum turbulence was created by impulsive spin-down from a specific angular velocity to rest. A measurement of the vortex line density was performed using the scattering of charged vortex rings. The vortex line density decayed as L ∝ t^(-3/2) which is the characteristic of quasiclassical turbulence forced at large length scales. The interaction of metastable spin-triplet 〖He〗_2^* molecules with quantized vortex lines in superfluid (_^4)He at temperatures below 200 mK was studied too. The molecules were generated during an injection of electrons from a sharp metal tip at high voltage. They were detected as a current into a metal collector after ionization upon colliding with the collector surface. The detected current was suppressed by even a small rotation indicating the trapping of the molecules on quantized vortices. The presence of (_^3)He impurities at the level of 0.3 ppb strongly suppressed the detected signal. The temperature dependence of the detected signal showed a sharp peak, most probably associated with the condensation of (_^3)He atoms on vortex cores. The time of flight of the molecules as a function of temperature was measured. It was observed that there are three regimes for 〖He〗_2^* molecules transportation in superfluid (_^4)He : ballistic regime for T < 100 mK, diffusive for T > 200 mK, and an intermediate regime between them. The vortex lines were created by either rotation or ion injection. The trapping diameter of the molecules on quantized vortices was found to be 96 ± 6 nm at pressure of 0.1 bar and 27 ± 5 nm at 5.0 bar. It was also demonstrated that a tangle of vortices moving in superfluid (_^4)He are capable of conveying the 〖He〗_2^* molecules through the drift region.

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Enrico, Michael Paul. „Ballistic excitation scattering processes in superfluid '3He-B and spin-lattice relaxation in copper down to 7#mu#K“. Thesis, Lancaster University, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.261014.

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37

Vilà, Casanovas Arnau. „Quantum dynamics of physicochemical processes in superfluid (4)He nanodroplets“. Doctoral thesis, Universitat de Barcelona, 2015. http://hdl.handle.net/10803/313040.

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The present thesis presents several theoretical studies on the dynamical processes of physicochemical interest involving superfluid helium (4He) nanodroplets. It has been developed a theoretical approach and computationally implemented. This consists in the first stage on the theoretical study of chemical reaction dynamics in superfluid helium nanodroplets. The description of the system is fully quantum and consists in a hybrid method in which different theoretical approaches are combined to describe the different components of the system. The liquid helium (superfluid) has been described through the Density Functional Theory (DFT), using a phenomenological energy functional. Atomic and molecular impurities have been described by means of standard quantum dynamics. The methodology developed in the present thesis has been applied to the study of various physicochemical processes of interest. These studies con be grouped as: - Study of the structure, energy and electronic spectroscopy of helium nanodroplets doped with atomic oxygen in different electronic states. In this work, the influence of the electronic state of oxygen and energy structure of the drop has been determined to be the induction of anisotropy on the helium density (for the open-shell cases), as well as the high quality of helium nanodroplets acting as a matrix for high-resolution spectroscopy confirmed. - Investigation on the dynamics of the photodissociation of homonuclear diatomic molecules in superfluid helium nanodroplets. In this context, the method developed has been initially applied to the case of molecular chlorine. Some interesting results related with the energy exchange phenomenon and generation of quantum (confined) resonances due tot the interaction of the molecules with the helium nanodroplet have been reported. Moreover, the relaxation process of the nanodroplet after the photodissociation has been also studied. This methodology developed has been also applied to the case of two halogen homonuclear diatomic molecules more, Br2 and I2. Different phenomenology and mechanism have been found for these cases, even producing full recombination for the biggest droplet considered. Furthermore, in order to determine the role played by the mass of the atoms of the diatomic homonuclear molecule on this process, we have studied the photodissociation dynamics of prototypic “isomers” of Cl2. - We have modeled the process of the capture of a neon atom by superfluid helium nanodroplets. We have restricted to the case of zero angular momentum as a first approach to the problem at the quantum level. We obtained the time scale of this process. Also, it was found that the energy exchange does not take place at any time, but until the velocity is low enough (Landau’s critical velocity) there is no mechanism for the exchange energy. This leads to a constant (mean) velocity of the quantum particle (Ne atom), since the friction force is zero. The resulting wave packet in momentum representation has a Gaussian type profile. - The synthesis reaction of a dimer of van der Waals (Ne2) in helium nanodroplets has been investigated. This process starts with a drop initially doped with Ne atom. Then, a second Ne atom is captured, which initiates the process of the synthesis. We have restricted to the zero angular momentum case for computational reasons. A great variety of phenomenology (reaction mechanisms) depending on the collision velocity and the droplet size has been found.
La present Tesi doctoral comprèn diferents estudis teòrics sobre processos fisicoquímics relacionats amb nanogotes d'heli (4He) superfluid. Per a la seva realització s'ha desenvolupat la metodologia teòrica i implementat a nivell computacional. Es tracta d'un primer estudi teòric en qual s'estudia la dinàmica de reaccions químiques en nanogotes d'heli superfluid. La descripció del sistema és completament quàntica, combinant diferents aproximacions teòriques per descriure els components del sistema. L'heli líquid s'ha descrit a través de la teoria del funcional de la densitat (DFT), utilitzant un funcional fenomenològic. Les impureses atòmiques i moleculars s'han descrit mitjançant dinàmica quàntica. La metodologia desenvolupada ha estat aplicada a l'estudi de diversos processos d'interès quimicofísic. Aquests són: - Estudi de l'estructura, energètica i espectroscòpia electrònica de nanogotes d'heli dopades amb oxigen atòmic en diferents estats electrònics. - Estudi de la dinàmica de la fotodissociació de molècules diatòmiques homonuclears en nanogotes d'heli superfluid. El mètode desenvolupat s'ha aplicat inicialment al cas del clor molecular. Posteriorment s'ha realitzat els casos del brom i el iode, així com d'"isòtops" artificials del clor amb l'objecte de determinar la influència de la massa sobre aquest tipus de processos. S’ha pogut determinar la formació de ressonàncies quàntiques, les qual s’han estudiat amb detall posteriorment. - Modelització del procés de captura d'un àtom de neó per part de nanogotes d'heli superfluid. S'ha estudiat el cas de moment angular nul com una primera aproximació a la problemàtica a nivell quàntic. - Reacció de síntesi d'un dímer de van der Waals (Ne2) en nanogotes d'heli. Partint d'una gota inicialment dopada amb un àtom de Ne, la captura d'un segon àtom de Ne inicia el procés de la síntesi. S'ha determinat una gran varietat de fenomenologia (mecanismes de reacció) segons la velocitat de col·lisió i la mida de la gota.

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Sherrill, David Semmes. „Measurements of the longitudinal nuclear magnetic resonance in superfluid helium-3 as a function of magnetic field /“. The Ohio State University, 1987. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487329662147149.

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39

Carcy, Cécile. „Investigation of the Mott transition with metastable Helium atoms“. Thesis, Université Paris-Saclay (ComUE), 2019. https://pastel.archives-ouvertes.fr/tel-02890916.

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Dans cette thèse, nous transférons un condensat de Bose-Einstein d'Hélium métastable dans un potentiel cubique 3D (réseau optique). Ce système constitue une réalisation expérimentale de l'Hamiltonien de Bose-Hubbard où interviennent deux échelles d'énergie: la force d'interaction entre les atomes U et l'énergie cinétique J. Suivant le rapport U/J, l'état fondamental de ce système est soit un superfluide (SF), soit un isolant de Mott (MI). Ces deux phases sont séparées par une transition appelée transition de Mott dont nous avons étudié quelques propriétés au cours de cette thèse. Grâce à l'utilisation d'atomes d'Hélium dans un état métastable, il est possible de détecter ces atomes de manière individuelle et en 3D après un long temps de vol. Ainsi, on a accès à la distribution en impulsion du gaz piégé résolue à l'atome unique, ce qui permet de mesurer les fonctions de corrélation en impulsion à n'importe quel ordre. Nous avons démontré que les corrélations d'ordre 2 et 3 en impulsion sont celles d'un système décrit par une matrice densité gaussienne pour un isolant de Mott loin de la transition. Dans la phase MI, nous avons par ailleurs étudié l'augmentation de la cohérence à une particule en se rapprochant de la transition de Mott. Pour finir, nous avons comparé les distributions en impulsion mesurées expérimentalement à des distributions numériques exactes calculées à partir de l'algorithme Monte Carlo et de nous paramètres expérimentaux pour un panel de températures. Ces comparaisons nous ont permise de mesurer la température des nuages étudiés nécessaire à l'étude de la transition de phase entre un état SF et l'état thermique (NF) qui apparait lorsque le gaz est chauffé au delà d'une certaine température. En particulier, nous avons mesuré l'évolution de la fraction condensée le long des transitions de phases SF-MI et SF-NF. Nous avons montré que si l'évolution de la fraction condensée en fonction de la température permet de mesurer la position de la transition de phase SF-NF, l'inhomogénéité des nuages étudiés rend la mesure de la position de la transition de Mott plus compliquée. Dans ce cas, nous avons donc étudié d'autres observables
In this work, we transfer a Bose-Einstein condensate of metastable Helium atoms to a 3D cubic lattice potential (optical lattice). This system is an experimental realization of the Bose-Hubbard Hamiltonian that depends on two parameters that are the interaction strength U between the atoms and the kinetic energy J. Depending on the value of the ratio U/J, the ground state of the system is either a superfluid (SF) or a Mott insulator (MI). In the experiment, we investigated the Mott transition that separates the SF and the MI phases. Thanks to the use of He$^*$ atoms, one can detect the atoms individually and in 3D after a long time-of-flight. As a result, we access the in-trap momentum distribution of the lattice gases probed with a single atom sensitivity and one can compute the momentum correlations at any order. We demonstrate that the 2 and 3-particles correlations of a Mott insulator deep in the MI phase are the ones of a system described by a gaussian density operator. In the MI phase, we investigate the restoration of the first-order coherence on approaching the Mott transition. Finally, by comparing the momentum distributions measured in the experiment with Quantum Monte Carlo numerical simulations performed with the experimental parameters and calculated for a wide range of temperatures, we calculated the temperature of the lattice gases probed, allowing us to investigate the transition between a SF and a thermal gas (NF) that occurs when increasing the temperature of the system. We notably have measured the condensate fraction across the SF-NF and the SF-MI transitions. We demonstrated that when probing trapped systems, if the condensate fraction is a good observable to locate the position of the phase transition, it is not the case for the SF-MI transition. We thus probed different observables

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Sy, Ndeye Fatimata Issaga. „Turbulence de grille oscillante à basses températures“. Thesis, Université Grenoble Alpes (ComUE), 2016. http://www.theses.fr/2016GREAI062/document.

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Cette thèse a pour objectif l’étude comparée de la turbulence isotherme en hélium I (HeI, fluide classique) et en hélium II (HeII, dont une partie du fluide est inviscide) par une analyse lagrangienne. Les analogies et/ou différences du comportement dynamique en écoulement classique ou superfluide devraient ainsi nous renseigner sur les caractéristiques multi-échelles intrinsèques de la turbulence. Ce type de recherche, qui constituait une première mondiale en début de thèse, semble promis à un bel avenir (à titre d'exemple, des mesures similaires de suivi lagrangien en HeI/HeII, en aval d’un barreau oscillant, sont actuellement en cours à Charles Université, Prague).Nous avons opté pour l’écoulement canonique de grille oscillante car il est isotrope, sans vitesse moyenne (autorisant ainsi un temps d’observation long) et homogène par plan. Nous basons nos mesures sur la visualisation de microsphères creuses de verre, dont la densité est voisine de celle de l’hélium liquide. Cette expérience étant nouvelle, il a fallu procéder à sa conception et à son dimensionnement, à sa mise en place ainsi qu'à sa calibration. Le cryostat que l’on a conçu est entièrement en verre, permettant ainsi une observation multi-angles de l’écoulement. Le système de visualisation utilisé est basé sur l'imagerie haute cadence en éclairage "backlight" (diffusion avant). A partir de la reconstruction des trajectoires des particules, nous avons pu réaliser des analyses lagrangiennes à une particule (à un temps et à deux temps), mais aussi étudier la dispersion relative de paires de particules.La résolution spatiale et temporelle de nos mesures nous a permis de pleinement caractériser les échelles inertielles de la turbulence, tandis que les échelles sont plus marginalement résolues. Dans ces conditions, nous trouvons que les caractéristiques de la turbulence en HeI sont en accord avec les mesures de la littérature dans des écoulements similaires en fluide classique. Ce même comportement est également observé en HeII
This thesis aims at comparing the isothermal turbulence in Helium I (HeI, classical fluid) and in Helium II (HeII, which has one inviscid component) through a Lagrangian analysis. Analogies and/or differences between the classical and superfluid behaviors are expected to shed new light on the intrinsic multi-scale properties of turbulence. This is a pioneering study, as no similar cryogenic experiment had been performed prior to this thesis, which opens a full range of possible future studies (as an example, similar measurements of Lagrangian tracking in the wake of an oscillating cylinder, are currently in progress at Charles University in Prague). We opted for an oscillating grid turbulence generation, which produces isotropic and homogeneous (per plane) turbulence, with no mean velocity (hence allowing longer observation times). As tracers we use hollow glass microspheres, which are almost neutrally buoyant in liquid Helium. This being a new facility, significant effort was dedicated to the design and the tailoring of the experiment, its implementation and the calibration of the flow. The cryostat hosting the experiment is fully transparent (made of glass), allowing visualization at multiple angles. Measurements were performed using high speed imaging in backlight illumination. Particle trajectories were reconstructed using Lagrangian tracking, from which we performed analysis of single particle statistics (single time and two times) as well as relative dispersion of pairs of particles. The spatial and temporal resolution of our measurements give us access to the dynamics of the flow at inertial scales, while dissipative scales are marginally resolved. In these conditions, we find that turbulence in HeI behaves in agreement with previous results obtained in similar flows using classical fluids. Interestingly, the same turbulence properties stands also for superfluid conditions

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Anderson, Charlotte Rain. „The effects of introducing static and dynamic disorder on the low-energy excitations of superfluid â´He“. Thesis, University of Liverpool, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.367070.

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Ödlund, Erika. „Virtual instrumentation: Introduction of virtual“. Thesis, Linköping University, Department of Electrical Engineering, 2007. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-8316.

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The Large Hadron Collider (LHC) is the next large particle accelerator developed at CERN, constructed to enable studies of particles. The acceleration of the particles is carried out using magnets operating at about 1.9 K, a temperature achieved by regulating flow of superfluid helium. For economical reasons, control of the helium flow is based on feedback of virtual flow meter (VFT) estimates instead of real instrumentation.

The main purpose of this work is to develop a virtual flow meter with the possibility to estimate the flow by means of two different flow estimation methods; the Samson method that has previously been tested for the LHC, and the Sereg- Schlumberger method that has never before been implemented in this environment.

The virtual flow meters are implemented on PLCs using temperature and pressure measurements as input data, and a tool for generating the virtual flow meters and connect them to the appropriate physical instrumentation has also been developed.

The flow through a valve depends, among others, on some pressure and temperature dependent physical properties that are to be estimated with high accuracy. In this project, this is done by bilinear interpolation in twodimensional tables containing physical data, an approach that turned out to be more accurate than the previously used method with polynomial interpolation.

The flow measurement methods have been compared. Since they both derive from empirical studies rather than physical relations it is quite futile to find theoretical correspondencies, but the simulations of the mass flows can be compared. For low pressures, the results are fairly equal but they differ more for higher pressures. The methods have not been validated against true flow rates since there were no real measurements available before the end of this project.

Le Grand Collisionneur de Hadrons (Large Hadron Collider, LHC) est le prochain grand accélérateur de particules du CERN, construit pour permettre l’étude des particules. L’accélération des particules sera réalisée en utilisant des aimants supraconducteurs qui fonctionneront à 1.9 K et la température sera régulée en contrôlant le débit d’hélium superfluide. Pour des raisons économiques, la régulation du débit d’hélium sera basée sur les réponses des estimations des débitmètres

virtuels (Virtual flow meters, VFT) au lieu d’instrumentation réelle.

Le but principal de ce projet est de développer un débitmètre virtuel qui estimera le débit avec deux méthodes différentes ; la méthode Samson qui a déjà été mise en oeuvre pour le LHC, et la méthode Sereg-Schlumberger qui n’a pas encore été implémentée dans cet environnement.

Les débitmètres virtuels seront implémentés sur des PLCs avec des mesures de température et de pression comme données d’entrée. De plus, un outil pour générer les débitmètres et les relier avec l’instrumentation physique adéquat a été développé.

Le débit à travers d’une vanne dépend entre autres des propriétés physiques qui dépendent à leur tour de la température et de la pression. Ces propriétés devront être estimées avec une grande précision. Dans ce projet, cela est fait en appliquant une interpolation bilinéaire dans des tableaux de deux dimensions. Cette méthode s’est montrée plus précise qu’avec une méthode d’interpolation polynomiale.

Les deux méthodes de mesures de débit ont été comparées. Elles dérivent toutes les deux des études empiriques et non physiques, alors les similarités théoriques sont donc peu pertinentes, mais les résultats des simulations des débits peuvent être comparés. Pour des pressions basses, les méthodes sont quasiment équivalentes, mais les différences sont plus importantes pour les pressions plus hautes. Étant donné qu’il n’y avait pas de mesures disponibles avant la fin de ce projet, les méthodes n’ont pas été validées avec des débits réels.

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Schmidt-Wellenburg, Philipp Anton [Verfasser], Oliver [Akademischer Betreuer] Zimmer und Klaus [Akademischer Betreuer] Schreckenbach. „Production of ultracold neutrons in superfluid helium under pressure / Philipp Anton Schmidt-Wellenburg. Gutachter: Klaus Schreckenbach. Betreuer: Oliver Zimmer“. München : Universitätsbibliothek der TU München, 2009. http://d-nb.info/1054434026/34.

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Hilcker, Michaela [Verfasser], Norbert [Akademischer Betreuer] Pietralla und Joachim [Akademischer Betreuer] Enders. „A superfluid liquid helium target for electron scattering experiments at the S-DALINAC / Michaela Hilcker ; Norbert Pietralla, Joachim Enders“. Darmstadt : Universitäts- und Landesbibliothek Darmstadt, 2020. http://d-nb.info/1215838816/34.

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Premke, Tobias [Verfasser], und Alkwin [Akademischer Betreuer] Slenczka. „Limitations of Superfluid Helium Droplets as Host System Revealed by Electronic Spectroscopy of Embedded Molecules / Tobias Premke. Betreuer: Alkwin Slenczka“. Regensburg : Universitätsbibliothek Regensburg, 2016. http://d-nb.info/1098531019/34.

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Deepak, Deepak. „Experimental investigation of quantised vortices using grid and quartz tuning forks in superfluid helium-4 in the zero temperature limit“. Thesis, Lancaster University, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.547953.

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Wirths, Eva-Maria Verfasser], Alkwin [Akademischer Betreuer] Slenczka und Bernhard [Akademischer Betreuer] [Dick. „High resolution electronic spectroscopy of van der Waals clusters formed in superfluid helium nanodroplets / Eva-Maria Wirths. Betreuer: Alkwin Slenczka ; Bernhard Dick“. Regensburg : Universitätsbibliothek Regensburg, 2015. http://d-nb.info/1080609717/34.

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Langbehn, Bruno [Verfasser], Thomas [Akademischer Betreuer] Möller, Daniela [Akademischer Betreuer] Rupp, Thomas [Gutachter] Möller, Daniela [Gutachter] Rupp und Karl-Heinz [Gutachter] Meiwes-Broer. „Imaging the shapes and dynamics of superfluid helium nanodroplets / Bruno Langbehn ; Gutachter: Thomas Möller, Daniela Rupp, Karl-Heinz Meiwes-Broer ; Thomas Möller, Daniela Rupp“. Berlin : Technische Universität Berlin, 2021. http://d-nb.info/1238140696/34.

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NAVA, MARCO. „TWO DIMENSIONAL AND NOVEL QUASI TWO DIMENSIONAL QUANTUM LIQUIDS“. Doctoral thesis, Università degli Studi di Milano, 2013. http://hdl.handle.net/2434/216307.

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In this thesis we have used Quantum Monte Carlo techniques to study two systems that can be regarded as the archetype for neutral strongly interacting systems: 4He, and its fermionic counterpart 3He.More specifically, we have used the Path Integral Ground State and the Path Integral Monte Carlo methods to study a system of two dimensional 3He (2d-3He) and a system of 4He adsorbed on Graphene-Fluoride (GF) and Graphane (GH) at both zero and finite temperature. The purpose of the study of 4He on GF (GH) was the research of new physical phenomena, whereas in the case of 2d-3He it was the application of novel methodologies for the ab-initio study of static and dynamic properties of Fermi systems. In the case of 2d-3He we have computed the spin susceptibility as function of density which turned out to be in very good agreement with experimental data; we have also obtained the first ab-initio evaluation of the zero-sound mode and the dynamic structure factor of 2d-3He that is in remarkably good agreement with experiments. In the case of 4He adsorbed on GF (GH), we determined the zero temperature equilibrium density of the first monolayer of 4He showing also that the commensurate sqrt(3) x sqrt(3) R30 phase is unstable on both substrates; at equilibrium density we found that 4He on GF (GH) is a modulated superfluid with an anisotropic phono-rotonic spectrum; at high coverages we found an incommensurate triangular solid and, on both GF and GH, a commensurate phase at filling factor x= 2/7 that is locally stable or at least metastable. Remarkably, in this commensurate solid phase and for both GF and GH, our computations show preliminary evidence of the presence of a superfluid fraction.

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Lantz, Jonas. „Heat Transfer Correlations Between a Heated Surface and Liquid & Superfluid Helium : For Better Understanding of the Thermal Stability of the Superconducting Dipole Magnets in the LHC at CERN“. Thesis, Linköping University, Department of Management and Engineering, 2007. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-10124.

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This thesis is a study of the heat transfer correlations between a wire and liquid helium cooled to either 1.9 or 4.3 K. The wire resembles a part of a superconducting magnet used in the Large Hadron Collider (LHC) particle accelerator currently being built at CERN. The magnets are cooled to 1.9 K and using helium as a coolant is very efficient, especially at extremely low temperatures since it then becomes a superfluid with an apparent infinite thermal conductivity. The cooling of the magnet is very important, since the superconducting wires need to be thermally stable.

Thermal stability means that a superconductive magnet can remain superconducting, even if a part of the magnet becomes normal conductive due to a temperature increase. This means that if heat is generated in a wire, it must be transferred to the helium by some sort of heat transfer mechanism, or along the wire or to the neighbouring wires by conduction. Since the magnets need to be superconductive for the operation of the particle accelerator, it is crucial to keep the wires cold. Therefore, it is necessary to understand the heat transfer mechanisms from the wires to the liquid helium.

The scope of this thesis was to describe the heat transfer mechanisms from a heater immersed in liquid and superfluid helium. By performing both experiments and simulations, it was possible to determine properties like heat transfer correlations, critical heat flux limits, and the differences between transient and steady-state heat flow. The measured values were in good agreement with values found in literature with a few exceptions. These differences could be due to measurement errors. A numerical program was written in Matlab and it was able to simulate the experimental temperature and heat flux response with good accuracy for a given heat generation.

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Dissertationen zum Thema „Helium Superfluid“

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