Thèses sur le sujet « Chiral currents »

Topologische Invarianten sind von zentraler Bedeutung für die Interpretation vieler Phänomene kondensierter Materie. In dieser Arbeit wird die erste Messung einer solchen Invarianten vorgestellt. Dazu wird ein neu entwickeltes Messprotokoll mit ultrakalten bosonischen Atomen in einem eindimensionalen optischen Gitter verwendet. Außerdem wird die Messung chiraler Meissner-Ströme in einer Leitergeometrie in einem künstlichen Magnetfeld sowie die Präparation sogenannter "Resonating Valence Bond"-Zustände (RVB) in vier Gitterplätze umfassenden Plaketten präsentiert. Das Hauptmerkmal des experimentellen Aufbaus ist ein Paar orthogonaler Übergitter-Potentiale, die es ermöglichen eine Vielzahl verschiedener Systeme zu simulieren. Die Modulation des Übergitters mit einem weiteren Paar interferierender Strahlen ermöglicht zu dem die Realisierung eines künstlichen Magnetfelds. Die Zak-Phase ist eine Invariante, welche die topologischen Eigenschaften eines Energiebandes charakterisiert. Sie ist definiert als die Berry-Phase eines Teilchens bei adiabatischem Durchlaufen eines Pfades im Quasiimpulsraum durch die Brillouinzone. Ein einfaches Beispiel für ein System mit zwei verschiedenen topologischen Klassen ist eine eindimensionale Kette mit alternierender Tunnelkopplungsstärke. Im Experiment können diese Klassen durch Messung der Differenz zwischen ihren Zak-Phasen $\Deta\Phi_\text{Zak}\approx\pi$ unter Verwendung von Bloch-Oszillationen und Ramsey-Interferometrie in Übergittern unterschieden werden. Der zweite Teil dieser Arbeit befasst sich mit der Messung chiraler Meissner-Ströme von Bosonen in einer Leitergeometrie mit magnetischem Fluss, welche eines der einfachsten Modelle zur Beobachtung von Orbitaleffekten ist. Obwohl die Atome ladungsneutral sind und daher keine Lorentzkraft auf sie wirkt, kann durch eine externe Modulation im Übergitter ein künstliches Magnetfeld erzeugt werden. Die dadurch hervorgerufenen Wahrscheinlichkeitsströme auf beiden Seiten der Leiter wurden separat mit einer Projektionsmethode gemessen. Beim Ändern der Tunnelkopplung entlang der Leitersprossen wurde, in Analogie zu einem Typ-II Supraleiter, ein Übergang zwischen einer Meissner-artigen Phase mit gesättigtem maximalen chiralen Strom und einer Vortex-Phase mit abnehmendem Strom beobachtet. Dieses System mit ultrakalten Atomen kann auch als Analogon zur Spin-Bahn-Kopplung betrachtet werden. RVB-Zustände gelten als fundamental für das Verständnis von Hochtemperatursupraleitern. Der dritte Teil der Arbeit widmet sich mit der Realisierung eines Minimalbeispiels solcher Zustände auf einer Plakette bei halber Füllung. In diesem System wurden die zwei RVB-Zustände mit s- und d-Wellen-Symmetrie sowie Superpositionen der beiden Zustände präpariert. Die in dieser Arbeit vorgestellten Experimente stellen einen neuen Ansatz dar, die topologischen Eigenschaften von Bloch-Bändern in optischen Gittern zu untersuchen; sie öffnen die Türen zur Erforschung von wechselwirkenden Teilchen in niedrigdimensionalen Systemen in einem homogenen Magnetfeld sowie der Eigenschaften des Grundzustandes des Heisenberg-Modells.
The determination of topological invariants is of fundamental importance to interpret many condensed-matter phenomena. This thesis reports on the implementation of a newly developed protocol to measure these invariants for the first time, using ultracold bosonic atoms in one-dimensional optical lattices. In addition, it deals with the measurement of chiral Meissner currents in a ladder-like lattice geometry exposed to an artificial magnetic field, and presents results on the preparation of Resonating Valence Bond (RVB) states on plaquettes. The key feature of the experimental setup is a pair of orthogonal superlattice potentials that permit a rich variety of systems to be simulated, and that when combined with a pair of interfering beams which periodically modulate the lattice allow the realization of artificial magnetic fields. The Zak phase is an invariant that characterizes the topological properties of an energy band, and is defined as the Berry phase that a particle acquires as it adiabatically moves in the quasimomentum space across the Brillouin zone. A dimerized lattice -- a one-dimensional chain with alternating couplings -- is a simple example of a system that possesses two different topological classes. Using a combination of Bloch oscillations and Ramsey interferometry in superlattices we measured the difference of the Zak phase $\approx\pi$ for the two possible polyacetylene phases, which directly indicates that they belong to different topological classes. The second part of this thesis deals with the measurement of chiral Meissner currents in bosonic ladders with magnetic flux, one of the simplest models to observe orbital effects. Although charge neutrality prevents atoms from experiencing the Lorentz force when they are exposed to a magnetic field, employing lattice modulation techniques we implemented an artificial magnetic field on a ladder created with optical lattices. By using a projection technique, we were able to measure the probability currents on each side of the ladder. When changing the coupling strengths along the rungs of the ladder, we found, in analogy to type-II superconductors, a transition between a Meissner-like phase with saturated maximum chiral current and a vortex phase with decreasing currents. Additionally, the flux ladder realizes spin-orbit coupling with ultracold atoms. It is believed that RVB states are fundamental for the understanding of high-T$_c$ superconductivity. The third part of this work describes our measurements on the preparation of minimum instances of RVB states with bosonic atoms in isolated four-site plaquettes at half filling. These small systems possess two RVB states with $s$- and $d$-wave symmetry. Using atom manipulation techniques we prepared these two states, as well as a quantum resonance between them. The experiments in this thesis establish a new general approach for probing the topological structure of Bloch bands in optical lattices. Moreover, they open up the pathway to exploring interacting particles in low dimensions exposed to uniform magnetic fields and to studying ground state properties of the Heisenberg Hamiltonian.

The potential of countercurrent chromatography (CCC) as a small footprint bioreactor/separator for manufacture of enantiopure chiral molecules was explored, using as a model reaction the isolation of L-amino butyric acid (L-ABA) from a DL-ABA racemate and the enantioselectivity of D-amino acid oxidase (DAAO). Bioconversion of D-ABA to ketobutyric acid (KBA) by DAAO, immobilised by selective partitioning in the stationary phase of the CCC centrifuge, was accompanied by separation of unreacted L-ABA from KBA by the countercurrent action of the centrifuge. For effective bioreactor/separator action, a high partition of the biocatalyst to the stationary phase was required in order to retain the biocatalyst in the coil, with differing partitions of substrates and products between the stationary phase (SP) and mobile phase (MP) so that these could be separated. Aqueous two-phase systems (ATPS) were the major two-phase systems used to provide SP and MP, as these are well reported to be effective in preserving enzyme activity. The distribution ratios of DL-ABA, KBA and DAAO were measured in a range of phases with polyethylene glycols (PEGs) of different molecular weights, different salts, and different compositions of PEG and salt, using an automated robotic method, developed for the purpose. A system of 14% w/w PEG 1000/ 14% w/w potassium phosphate, pH 7.6, gave the best combination of distributions ratios (CPEG phase/Csalt phase = CSP/CMP) for ABA, KBA and biocatalyst (DAAO) of 0.6, 2.4 and 19.6 respectively. A limited number of aqueous-organic and ionic liquid two-phase systems were also reviewed, but found unsatisfactory. CCC operating conditions such as substrate concentration, biocatalyst concentration, the mobile phase flow rate (residence time in the CCC coil), temperature, rotational speed and operational modes (single flow and multiple-dual flow) and types of mixing (cascade and wave-like) were optimised to produce total conversion of D-ABA to KBA, which was then completely separated from unreacted, enantiomerically pure (>99% ee), LABA. Advantages of the CCC bioreactor over conventional technology include reduced equipment footprint, cheaper running costs, and faster purifications. However, in its current format the drawbacks, such as enzyme instability and excessive optimisation time, reduce its commercial appeal. Additional investigations into the use of whole cell preparations of biocatalyst in the CCC bioreactor showed potential to overcome the problem of enzyme instability and this may in the future give the CCC bioreactor a place in the enantioseparation field.

In this work we consider the axial-vector form factors of the nucleon in flavor-SU(2) and of the baryon octet in flavor-SU(3) Chiral Perturbation Theory. We include the Δ-isobar and the baryon decuplet as explicit degrees of freedom and focus on their consistent treatment in terms of chiral power counting. We employ the use of on-shell meson and baryon masses in the one-loop contributions to the axial-vector form factors. The convergence properties of such an approach are scrutinized. Our results are compared to the available flavor-SU(2) QCD lattice data.

The ability to describe electromagnetic properties of nuclei is fundamental to our understand- ing of nuclear structure and dynamics. Experimental methods that measure these properties enable a clean way to isolate the nuclear physics content, because the relatively weak and well understood electromagnetic interaction is perturbative in nature and thus appropriately described. In this thesis we study electromagnetic properties of light nuclei within the framework of chiral effective field theory (EFT). The modern approach to low-energy nuclear physics is formulated by chiral EFT which describes the nucleus in terms of nucleon and pion degrees of freedom based on the symmetries of the underlying fundamental theory of quantum chromo- dynamics. It provides a systematically improvable calculation scheme and permits a unified description of the strong-interaction dynamics between nucleons and the interaction with an external probe. The nuclear component of such an interaction is described by nuclear currents. Both nuclear interactions and currents are consistently derived within chiral EFT and exhibit a naturally emerging many-body operator structure. Recent progress on the development of nuclear interactions and nuclear currents have set the stage for high-precision calculations complemented with systematic truncation uncertainty estimates. We study the deuteron, the triton, and the helion electromagnetic form factors with two- and three-nucleon chiral interactions developed in an order-by-order manner which allows us to compute the associated truncation uncertainty estimates. We find good agreement at low momentum transfers for the charge form factors and a consistent description of the experimental first minimum once the uncertainty estimates are incorporated. For the tri- nucleon magnetic form factors we find that leading two-body currents (2BCs), which arise from the exchange of a pion between a pair of nucleons, lead to better agreement with data over the entire momentum-transfer region. To obtain insights into the effect of various chiral interactions with and without three-nucleon forces and to quantify the impact of 2BCs on the zero-momentum-transfer region, we analyze the magnetic moments and the electromagnetic radii of these light nuclei. We observe that three-nucleon forces reduce the radii slightly and have a negligible effect on the magnetic moment, while 2BCs significantly modify both the magnetic radius and magnetic moment indicating that the exchange dynamics between the nucleons are essential for magnetic observables. As a first step towards a consistent study of other light nuclei, we examine the magnetic moment and a magnetic transition of 6Li which is the next light nucleus after the three- nucleon nuclei with nonvanishing magnetic ground-state properties. To achieve this, we include contributions to the magnetic dipole operator beyond leading order which arise from the leading 2BCs and we employ similarity renormalization group evolved chiral interactions to enhance the many-body convergence. Our results are in remarkable agreement with a new precision experiment after consistently evolving and including 2BCs to the magnetic dipole operator, thus advancing our understanding of nuclear interactions and electromagnetic currents in many-nucleon systems.

In this thesis I report on the experimental results obtained during the years of my PhD in the laboratory of the University of Florence devoted to the investigation of quantum degenerate gases of Ytterbium. I discuss the main results that we achieved, focusing the attention on the experiments concerning two main research lines, the first related to the quantum simulation of synthetic gauge fields with ultracold Yb atoms and the second one to the investigation of two-orbital quantum physics exploiting the 1S0->3P0 clock transition. In particular we have been able to unify these fields of research simulating for the first time a synthetic gauge field for neutral atoms exploiting the orbital degree of freedom offered by two-electron atoms. The realization of artificial gauge fields for neutral atoms is a current trend in the context of quantum simulation and several techniques have been proposed and experimentally realized. Here we adopt a recently proposed quantum simulation scheme which relies on the concept of synthetic dimension. In this scheme an internal degree of freedom of the atom is interpreted as an extra dimension of the system and a hybrid 2D ladder is realized combining this synthetic dimension with a real one-dimensional optical lattice. An artificial magnetic field naturally arises in this hybrid 2D lattice as a consequence of the phase imprinted on the atoms by the laser coupling between the synthetic sites. We exploited this scheme in two different experiments with fermionic 173Yb, in which we map the synthetic dimension in the first case on the ground and clock states of the atom and in the second case on the nuclear spin states of the ground level. Couplings between synthetic sites are realized exploiting single-photon clock transitions and two-photon Raman transitions in the first and second experiment respectively. Despite their simplicity these systems feature some fundamental properties of larger quantum Hall bars, one of which is the presence of chiral currents counter-propagating on the synthetic edges. We have been able to induce and detect these chiral currents in ladders characterized by two and three (only in the Raman case) legs. In the case of the clock approach, for which the experimental realization is simpler, we have also been able to tune the artificial magnetic field and characterized for the first time the strength of the currents as a function of the synthetic flux, a result impossible to achieve in real solid-state systems where magnetic fields of the order of several thousand of Tesla would be required. In the three-leg Raman case we have also investigated the dynamics of the system observing the skipping-orbit-like trajectories performed by fermions in the hybrid space after a quenching of the synthetic tunnelling. In another experiment we used the orbital degree of freedom of 173Yb to demonstrate the possibility to implement Spin-Orbit Coupling (SOC) with single-photon clock transitions in a system of fermionic atoms trapped in a one-dimensional optical lattice, using as pseudospin states the fundamental level 1S0 and the clock state 3P0. This orbital approach to the synthesis of SOC in ultracold gases allows us to overcome some of the limitations imposed by Raman schemes in alkali atoms, where heating due to the presence of intermediate levels has detrimental effects in the observation of many-body processes. The emergence of SOC is detected by evaluating the broadening of the clock spectroscopic response which results from transitions connecting states with different lattice quasimomentum. Our ability to observe these narrow features relies on the high spectroscopic resolution of our clock laser system and is enabled by the long-term stabilization of the laser frequency on the metrological reference delivered by INRiM (the Italian metrological institute) from Turin to Florence through a 642-km-long fiber link. Remarkably, exploiting the long term accuracy provided by the fiber link, we have been able to improve the absolute value of the clock transition in 173Yb by two orders of magnitude with respect to the value previously reported in literature. We exploited the orbital degree of freeedom of 173Yb also to realize a new kind of Feshbach resonance which allows for the tuning of the scattering properties in a mixture of atoms in different orbital states. The possibility to tune interactions by means of standard Feshbach resonances lacked in two-electron atoms due to the absence of a hyperfine structure in the fundamental state. We instead experimentally demonstrated how a similar mechanism is possible also for this class of elements provided that atoms in two different electronic states are considered. In particular, we exploited the orbital Feshbach resonance mechanism to realize a strongly interacting two-orbital gas of 173Yb and characterized the resonance position evaluating the hydrodynamic expansion of the gas. The last part of the thesis reports, instead, some results in which the properties of clock excitation in bosonic 174Yb have been investigated. By means of high resolution spectroscopic measurements on particles confined in 3D optical lattices, the scattering lengths and loss rate coefficients for atoms in different collisional channels involving the ground level 1S0 and the metastable state 3P0 are derived. These quantities, that at our knowledge were still unreported in literature before our work, set important constraints for future experimental studies of two-electron atoms for quantum-technological applications.

The Chiral Effective Field Theory (ChEFT) is a modern framework to analyze properties of few-nucleon systems in the low-energy regime. It is based on the most general elfective Lagrangian for pions and nucleons consistent with the chiral symmetry of QCD. For energies below the pion-production threshold it is possible to eliminate the pionie degrees of freedom and derive nuclear potentials and nuclear current operators solely in terms of nucleonic degrees of freedom. This is very important because, despite many elforts in the past, the consistence between two-nucleon forces, many-nucleon forces and corresponding current operators has not been achieved yet. In this thesis, we consider recently derived long-range two-pion exchange (TPE) contributions to the nuclear current operator which appear at higher order chiral expansions. These operators do not contain any new free parameters. Based on ChEFT dynamical picture, we study their role in the electron and photon scattering reactions and compare chiral predictions with those obtained in the conventional framework and to the existing experimental data. The bound and scattering states are calculated with five different chiral nucleon-nucleon potentials leading to the so-called theoretical uncertainty bands for the predicted observables.

The professionalization of political parties has significantly altered the means by which parties interact with voters and supporters. The current study is an attempt to examine what these changes in political communication mean for the ability of parties to organize supporters and mobilize them both in a campaign setting and in the longer-term struggle. Habermasian and Gramscian perspectives on the relational aspects of political communication highlight the challenges presented by the growing unidirectionality of communication and the concomitant atrophying of intermediary institutions. Beyond this, the work of Bottici and McLuhan is used to expose the effects of the 'arational' aspects of these changes in both form and content. To test the plausibility of the theoretical insights obtained, the case of the New Democratic Party of Canada is considered. The study concludes by considering the potential of new technological developments for resolving or mitigating concerns identified throughout the thesis.
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