Articles de revues : « Insulating-To-Metal transition »

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CHEN, DONG-MENG, et LIANG-JIAN ZOU. « ORBITAL INSULATORS AND ORBITAL ORDER–DISORDER INDUCED METAL–INSULATOR TRANSITION IN TRANSITION-METAL OXIDES ». International Journal of Modern Physics B 21, n^o 05 (20 février 2007) : 691–706. http://dx.doi.org/10.1142/s0217979207036618.

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The role of orbital ordering on metal–insulator transition in transition-metal oxides is investigated by the cluster self-consistent field approach in the strong correlation regime. A clear dependence of the insulating gap of single-particle excitation spectra on the orbital order parameter is found. The thermal fluctuation drives the orbital order–disorder transition, diminishes the gap and leads to the metal–insulator transition. The unusual temperature dependence of the orbital polarization in the orbital insulator is also manifested in the resonant X-ray scattering intensity.

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Pustogow, A., A. S. McLeod, Y. Saito, D. N. Basov et M. Dressel. « Internal strain tunes electronic correlations on the nanoscale ». Science Advances 4, n^o 12 (décembre 2018) : eaau9123. http://dx.doi.org/10.1126/sciadv.aau9123.

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In conventional metals, charge carriers basically move freely. In correlated electron materials, however, the electrons may become localized because of strong Coulomb interactions, resulting in an insulating state. Despite considerable progress in the last decades, elucidating the driving mechanisms that suppress metallic charge transport, the spatial evolution of this phase transition remains poorly understood on a microscopic scale. Here, we use cryogenic scanning near-field optical microscopy to study the metal-to-insulator transition in an electronically driven charge-ordered system with a 20-nm spatial resolution. In contrast to common mean-field considerations, we observe pronounced phase segregation with a sharp boundary between metallic and insulating regions evidencing its first-order nature. Considerable strain in the crystal spatially modulates the effective electronic correlations within a few micrometers, leading to an extended “zebra” pattern of metallic and insulating stripes. We can directly monitor the spatial strain distribution via a gradual enhancement of the optical conductivity as the energy gap is depressed. Our observations shed new light on previous analyses of correlation-driven metal-insulator transitions.

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ASOKAMANI, R., CH U. M. TRINADH, G. PARI et S. NATARAJAN. « INSULATOR-TO-METAL TRANSITION IN LaRhO3 UNDER HIGH PRESSURE ». Modern Physics Letters B 09, n^o 11n12 (20 mai 1995) : 701–9. http://dx.doi.org/10.1142/s0217984995000644.

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The band structure calculations of perovskite transition metal compound LaRhO 3 performed using 'tight binding linear muffin tin orbital' (TB-LMTO) method within local density approximation (LDA) under ambient and high pressures are reported here. Our calculations are able to successfully explain the insulating nature of the system and the insulator-to-metal transition (IMT) is observed for the reduced volume of 0.90. The first electronic structure calculation reported here for LaRhO 3 enables us to compare it with that of LaCoO 3 which brings out the role played by the d bands. These studies lead to distinguish between these two insulating systems and LaCoO 3 is found to be a charge transfer (CT) insulator which is in agreement with the recent experimental observations whereas LaRhO 3 is a conventional band insulator. Further, the equilibrium lattice constant, bulk modulus, its first derivative, and metallization volume obtained from the total energy calculations for expanded and reduced cell volumes are also reported for LaRhO 3.

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Driscoll, D. C., M. Hanson, C. Kadow et A. C. Gossard. « Transition to insulating behavior in the metal-semiconductor digital composite ErAs:InGaAs ». Journal of Vacuum Science & ; Technology B : Microelectronics and Nanometer Structures 19, n^o 4 (2001) : 1631. http://dx.doi.org/10.1116/1.1388211.

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Liang, Yongcheng, Ping Qin, Zhiyong Liang, Lizhen Zhang, Xun Yuan et Yubo Zhang. « Identification of a monoclinic metallic state in VO2 from a modified first-principles approach ». Modern Physics Letters B 33, n^o 12 (30 avril 2019) : 1950148. http://dx.doi.org/10.1142/s0217984919501483.

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Metal-insulator transition (MIT) underlies many remarkable and technologically important phenomena in VO2. Even though its monoclinic structure had before been the reserve of the insulating state, recent experiments have observed an unexpected monoclinic metallic state. Here, we use a modified approach combining first-principles calculations with orbital-biased potentials to reproduce the correct stability ordering and electronic structures of different phases of VO2. We identify a ferromagnetic monoclinic metal that is likely to be the experimentally observed mysterious metastable state. Furthermore, the calculations show that an isostructural insulator-metal electronic transition is followed by the lattice distortion from the monoclinic structure to the rutile one. These results not only explain the experimental observations of the monoclinic metallic state and the decoupled structural and electronic transitions of VO2, but also provide a useful understanding for the metal-insulator transition in other strongly correlated d electron systems.

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Allain, Adrien, Zheng Han et Vincent Bouchiat. « Electrical control of the superconducting-to-insulating transition in graphene–metal hybrids ». Nature Materials 11, n^o 7 (20 mai 2012) : 590–94. http://dx.doi.org/10.1038/nmat3335.

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Lee, Seung-Yong, Hyuneui Lim, Gyoung-Rin Choi, Jan-Di Kim, Eun-Kyung Suh et Sang-Kwon Lee. « Metal-to-Insulating Transition of Single Polyaniline (PANI) Nanowire : A Dedoping Effect ». Journal of Physical Chemistry C 114, n^o 27 (22 juin 2010) : 11936–39. http://dx.doi.org/10.1021/jp101424b.

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Morrow, Ryan, et Patrick Woodward. « Competing Superexchange Interactions in Double Perovskite Osmates ». Acta Crystallographica Section A Foundations and Advances 70, a1 (5 août 2014) : C1366. http://dx.doi.org/10.1107/s2053273314086331.

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Double perovskites, A2BB'O6, containing mixed transition metal ions have exhibited numerous desirable properties such as colossal magnetoresistance, half metallic transport, and high temperature ferrimagnetism. However, a predictive understanding of the superexchange mechanisms which control the magnetism of these materials when they are insulating and B is 3d transition metal and B' is a 4d or 5d transition metal has remained elusive. In this work, a number of insulating double perovskite osmates, A2BOsO6 (A=Sr,Ca,La; B=Cr,Fe,Co,Ni) have been chosen and studied using magnetometry, specific heat, XMCD, and neutron powder diffraction techniques in order to systematically probe the effects of electronic configuration and bonding geometry on the magnetic ground state. It is concluded that the magnetic properties of these materials are controlled by a competition between short range B–O–Os and long range superexchange interactions which are sensitive to bonding geometry resulting in tunability of the magnetic ground state.

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Cohen, Oded, et Zvi Ovadyahu. « 1/f NOISE NEAR THE METAL-INSULATOR TRANSITION ». International Journal of Modern Physics B 08, n^o 07 (30 mars 1994) : 897–903. http://dx.doi.org/10.1142/s0217979294000440.

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The 1/f noise level in polycrystalline indium oxide thin films and of zinc oxide accumulation layers is found to be much higher than that usually observed in metals. A systematic study of the flicker noise properties in these systems reveals a correlation between the 1/f noise magnitude and the proximity of the system to the insulating phase. In fact, the noise appears to increase dramatically close to the Anderson transition but when the average transport properties exhibited by the system are still diffusive. For static disorder that exceeds the critical value characterized by KFl≃1 the system exhibits insulating behavior and the noise level saturates at a rather high, but disorder independent value. The similarity of these findings to the behavior of the magnetic-field-induced Conductance Fluctuations in this system will be pointed out to suggest a common physical origin. This leads to the prediction of high levels of 1/f in all electronic systems that are close to the metal-insulator transition.

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Jager, Marieke F., Christian Ott, Peter M. Kraus, Christopher J. Kaplan, Winston Pouse, Robert E. Marvel, Richard F. Haglund, Daniel M. Neumark et Stephen R. Leone. « Tracking the insulator-to-metal phase transition in VO2with few-femtosecond extreme UV transient absorption spectroscopy ». Proceedings of the National Academy of Sciences 114, n^o 36 (21 août 2017) : 9558–63. http://dx.doi.org/10.1073/pnas.1707602114.

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Coulomb correlations can manifest in exotic properties in solids, but how these properties can be accessed and ultimately manipulated in real time is not well understood. The insulator-to-metal phase transition in vanadium dioxide (VO2) is a canonical example of such correlations. Here, few-femtosecond extreme UV transient absorption spectroscopy (FXTAS) at the vanadiumM2,3edge is used to track the insulator-to-metal phase transition in VO2. This technique allows observation of the bulk material in real time, follows the photoexcitation process in both the insulating and metallic phases, probes the subsequent relaxation in the metallic phase, and measures the phase-transition dynamics in the insulating phase. An understanding of the VO2absorption spectrum in the extreme UV is developed using atomic cluster model calculations, revealing V3+/d2character of the vanadium center. We find that the insulator-to-metal phase transition occurs on a timescale of 26 ± 6 fs and leaves the system in a long-lived excited state of the metallic phase, driven by a change in orbital occupation. Potential interpretations based on electronic screening effects and lattice dynamics are discussed. A Mott–Hubbard-type mechanism is favored, as the observed timescales and d2nature of the vanadium metal centers are inconsistent with a Peierls driving force. The findings provide a combined experimental and theoretical roadmap for using time-resolved extreme UV spectroscopy to investigate nonequilibrium dynamics in strongly correlated materials.

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Ильинский, А. В., et Е. Б. Шадрин. « Корреляционная природа фазового перехода изолятор--металл в пленках V-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=- ». Физика твердого тела 62, n^o 8 (2020) : 1284. http://dx.doi.org/10.21883/ftt.2020.08.49616.081.

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Abstract A complex character of the mechanism of thermal phase transformations from the insulating phase to the metallic phase has been revealed in thin V_2O_3 films. The insulator–metal phase transition in V_2O_3 is shown to consist of two stages: the hysteresis-less temperature-extended electron Mott transition extended in temperature and the stepwise structural Peierls transition with temperature hysteresis. The features of the insulator–metal phase transition revealed for V_2O_3 are discussed. These features are analyzed on the base of their comparison with characteristic features of analogous phase transition in VO_2 films.

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Li, Fengmiao, Yuting Zou, Myung-Geun Han, Kateryna Foyevtsova, Hyungki Shin, Sangjae Lee, Chong Liu et al. « Single-crystalline epitaxial TiO film : A metal and superconductor, similar to Ti metal ». Science Advances 7, n^o 2 (janvier 2021) : eabd4248. http://dx.doi.org/10.1126/sciadv.abd4248.

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Titanium monoxide (TiO), an important member of the rock salt 3d transition-metal monoxides, has not been studied in the stoichiometric single-crystal form. It has been challenging to prepare stoichiometric TiO due to the highly reactive Ti2+. We adapt a closely lattice-matched MgO(001) substrate and report the successful growth of single-crystalline TiO(001) film using molecular beam epitaxy. This enables a first-time study of stoichiometric TiO thin films, showing that TiO is metal but in proximity to Mott insulating state. We observe a transition to the superconducting phase below 0.5 K close to that of Ti metal. Density functional theory (DFT) and a DFT-based tight-binding model demonstrate the extreme importance of direct Ti–Ti bonding in TiO, suggesting that similar superconductivity exists in TiO and Ti metal. Our work introduces the new concept that TiO behaves more similar to its metal counterpart, distinguishing it from other 3d transition-metal monoxides.

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Wang, Yan, et John F. Muth. « Expanded Thermochromic Color Changes in VO2 Thin Film Devices Using Structured Plasmonic Metal Layers ». MRS Proceedings 1494 (2013) : 171–77. http://dx.doi.org/10.1557/opl.2013.158.

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ABSTRACTWe investigate metallic thin films on VO2 and show that the magnitude of the reflected color change in that visible portion of the spectrum as VO2 undergoes the insulating to metallic phase transition can be controlled by changing the type of metal, the thickness of the metal and by patterning the metal at the nano scale. We consider the role of surface plasmas in the metal film and show that in the near infrared, the magnitude of the reflectivity increase for metal coated VO2 films, but decrease for uncoated VO2 thin films. This is explained in the context of Fresnel equations and considering the large change in the imaginary part of the dielectric constant as the VO2 changes state from the insulating to metallic phase.

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HORIBA, K., K. ONO, M. OSHIMA, Y. AIURA et O. SHIINO. « ELECTRONIC STRUCTURES OF THE METAL-TO-INSULATOR TRANSITION SYSTEM 1T-TaSxSe2-x STUDIED BY ANGLE-RESOLVED PHOTOEMISSION SPECTROSCOPY ». Surface Review and Letters 09, n^o 02 (avril 2002) : 1085–89. http://dx.doi.org/10.1142/s0218625x0200338x.

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We have investigated the electronic structures of metal-to-insulator transitions (MIT) due to charge-density-wave phase transition in 1T- TaS x Se 2-x by angle-resolved photoemission spectroscopy. In the insulating phase of the sample with x = 1.5, we observe a gap formation at the Fermi energy (E F ), while in the metallic phase of the sample with x = 1.2, we do not observe a drastic change in the spectra between at room temperature and at low temperature. We observe a peak originating from the lower Hubbard band in the insulating phase of the sample with x = 1.5. In the metallic phase of the sample with x = 1.2, the peak intensity is smaller and the peak energy shifts near E F , but the lower Hubbard band peak still remains. These results suggest that MIT in 1T- TaS x Se 2-x is due to the Mott localization.

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YU, GANG, et ALAN J. HEEGER. « PHOTOINDUCED CHARGE CARRIERS IN INSULATING CUPRATES : FERMI GLASS INSULATOR, METAL-INSULATOR TRANSITION AND SUPERCONDUCTIVITY ». International Journal of Modern Physics B 07, n^o 22 (10 octobre 1993) : 3751–815. http://dx.doi.org/10.1142/s0217979293003498.

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The results of photoexcitation experiments in partially doped, insulating cuprates are reviewed, and the physical implications of these “photodoping” experiments are discussed. The existence of a Fermi-glass insulating state at intermediate doping levels is thoroughly explored. The localized electronic states near the Fermi energy (EF) are characterized through transport, steady-state photoconductivity (including spectral response) and time resolved transient photodoping experiments (sub-nanosecond through microsecond). The experimental results indicate an Anderson-type metal-insulator transition in the doped cuprates; i.e., the transition from metal to insulator is dominated by disorder-induced localization. The Fermi energy can be shifted across the mobility edge either by increasing the doping level or by transient photoexcitation at high pump intensities, thereby causing the metal-insulator transition. The high-Tc superconductors, therefore, can be characterized as disordered metals with the Fermi energy relatively close to the mobility edge (Ec). Although the photo-generated carriers are generated homogeneously, the data indicate electronic phase separation into metallic “droplets”. The temperature dependence of the photoinduced conductivity implies that these droplets become superconducting below the intrinsic transition temperature observed in the heavily doped metallic regime.

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ROZENBERG, MARCELO J., GOETZ MOELLER et GABRIEL KOTLIAR. « THE METAL–INSULATOR TRANSITION IN THE HUBBARD MODEL AT ZERO TEMPERATURE II ». Modern Physics Letters B 08, n^o 08n09 (20 avril 1994) : 535–43. http://dx.doi.org/10.1142/s0217984994000571.

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We study the metal-to-insulator transition of the Hubbard model at zero temperatures in infinite dimensions. The coexistence of metallic and insulating solutions for a finite range of the interaction is established. It is shown that the metallic solution is lower in energy for any interaction in the coexistence region and that the transition is of second order.

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SHUKLA, SHAILESH, DEEPAK KUMAR, NITYA NATH SHUKLA et RAJENDRA PRASAD. « METAL–INSULATOR TRANSITIONS IN TETRAHEDRAL SEMICONDUCTORS UNDER LATTICE CHANGE ». International Journal of Modern Physics B 18, n^o 07 (20 mars 2004) : 975–88. http://dx.doi.org/10.1142/s0217979204024525.

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Although most insulators are expected to undergo insulator to metal transition on lattice compression, tetrahedral semiconductors Si, GaAs and InSb can become metallic on compression as well as by expansion. We focus on the transition by expansion which is rather peculiar; in all cases the direct gap at Γ point closes on expansion and thereafter a zero-gap state persists over a wide range of lattice constant. The solids become metallic at an expansion of 13% to 15% when an electron Fermi surface around L-point and a hole Fermi surface at Γ-point develop. We provide an understanding of this behavior in terms of arguments based on symmetry and simple tight-binding considerations. We also report results on the critical behavior of conductivity in the metal phase and the static dielectric constant in the insulating phase and find common behavior. We consider the possibility of excitonic phases and distortions which might intervene between insulating and metallic phases.

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Ziegler, Klaus. « Metal–Insulator Transition in Three-Dimensional Semiconductors ». Symmetry 11, n^o 11 (1 novembre 2019) : 1345. http://dx.doi.org/10.3390/sym11111345.

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We use a random gap model to describe a metal–insulator transition in three-dimensional semiconductors due to doping, and find a conventional phase transition, where the effective scattering rate is the order parameter. Spontaneous symmetry breaking results in metallic behavior, whereas the insulating regime is characterized by the absence of spontaneous symmetry breaking. The transition is continuous for the average conductivity with critical exponent equal to 1. Away from the critical point, the exponent is roughly 0.6, which may explain experimental observations of a crossover of the exponent from 1 to 0.5 by going away from the critical point.

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Li, Da, Qilang Wang et Xiangfan Xu. « Thermal Conductivity of VO2 Nanowires at Metal-Insulator Transition Temperature ». Nanomaterials 11, n^o 9 (17 septembre 2021) : 2428. http://dx.doi.org/10.3390/nano11092428.

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Vanadium dioxide (VO2) nanowires endowed with a dramatic metal−insulator transition have attracted enormous attention. Here, the thermal conductance of VO2 nanowires with different sizes, measured using the thermal bridge method, is reported. A size-dependent thermal conductivity was observed where the thicker nanowire showed a higher thermal conductivity. Meanwhile, the thermal conductivity jump at metal−insulator transition temperature was measured to be much higher in the thicker samples. The dominant heat carriers were phonons both at the metallic and the insulating regimes in the measured samples, which may result from the coexistence of metal and insulator phases at high temperature. Our results provide a window into exploring the mechanism of the metal−insulator transition of VO2 nanowires.

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Kondratyev, A. M., V. N. Korobenko et A. D. Rakhel. « Metal–non-metal transition in lead–bismuth eutectic ». Journal of Physics : Condensed Matter 34, n^o 19 (14 mars 2022) : 195601. http://dx.doi.org/10.1088/1361-648x/ac553d.

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Abstract Thermodynamic functions and electrical resistivity of Pb–Bi eutectic alloy have been measured over wide ranges of specific volume and pressure in the liquid and gaseous state. The experimental data show a crossover from metallic to insulating behavior in the electrical resistivity of the alloy when its specific volume increases. It is found that in the crossover region the constant volume temperature coefficient of resistivity changes sign from positive to negative and passes through zero at a value of the specific volume which is 2.4 times larger than that in normal state. The second salient feature of the alloy revealed by these experiments is that the isochores plotted in the specific internal energy—pressure plane are straight lines. Based on these experimental data and using an earlier developed approach, an equation of state (EOS) of the alloy has been constructed whose accuracy is determined mainly by the errors of the measurements. It is shown that this EOS can be used to obtain direct estimates of the specific volume and pressure at the critical point of the liquid–gas transition as well as the critical volume for the metal–non-metal (M–NM) transition observed in this eutectic. The results indicate that the critical specific volumes for these two transitions are equal, and the M–NM transition can be described by the classical percolation theory.

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ARAPOV, YU G., S. V. GUDINA, G. I. HARUS, V. N. NEVEROV, N. G. SHELUSHININA, M. V. YAKUNIN, S. M. PODGORNYH, E. A. USKOVA et B. N. ZVONKOV. « TRANSPORT PROPERTIES OF 2D ELECTRON GAS IN AN n-InGaAs/GaAs DQW IN A VICINITY OF LOW MAGNETIC-FIELD-INDUCED HALL INSULATOR–QUANTUM HALL LIQUID TRANSITION ». International Journal of Nanoscience 06, n^o 03n04 (juin 2007) : 173–77. http://dx.doi.org/10.1142/s0219581x07004523.

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The resistivity (ρ) of low mobility dilute 2D electron gas in an n- InGaAs / GaAs double quantum well (DQW) exhibits the monotonic "insulating-like" temperature dependence (dρ/dT < 0) at T = 1.8–70 K in zero magnetic field. This temperature interval corresponds to a ballistic regime (kBTτ/ħ > 0.1–3.5) for our samples, and the electron density is on an "insulating" side of the so-called B = 0 2D metal–insulator transition. We show that the observed features of localization and Landau quantization in a vicinity of the low magnetic-field-induced insulator–quantum Hall liquid transition is due to the σxy(T) anomalous T-dependence.

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Kurbanov, U. T. « EW METAL/SUPERCONDUCTOR-INSULATOR TRANSITIONS AND THEIR EFFECTS ON HIGH-TCSUPERCONDUCTIVITY INUNDERDOPED AND OPTIMALLY DOPED CUPRATES ». Eurasian Physical Technical Journal 21, n^o 1 (47) (29 mars 2024) : 21–27. http://dx.doi.org/10.31489/2024no1/21-27.

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Abstract.Anew approach to the metal/superconductor-insulator transition in doped cuprates by studying the polaron formation and localization of doped charge carriers (holes) in them and the possibility of transforming a metallic or superconducting system into an insulatorwas developed. Amore suitable criterion for such a phase transition by comparing the bandwidth (or Fermi energy) of large polarons with their binding energies in the cuprateswas derived. The possibility of the metal/superconductor-insulator transition and phase separation in doped cuprates resulting in the formation of competing metallic/superconducting and insulating phases in underdoped, optimally doped and even in overdoped high-Tccuprateswas predicted. Then the possible detrimental and beneficial effects of the different disorders (e.g. polaron formation and charge-density-wave transition) and the coexisting insulating and superconducting phases on the critical temperature 𝑇𝑐of the superconducting transition of underdoped and optimally doped cuprateswas examined. The actual superconducting transition temperature 𝑇𝑐in these materials using the theory of Bose-liquid superconductivity, and not theBardeen-Cooper-Schrieffer-like theory of Fermi-liquid superconductivity, which is incapable of predicting the relevant value of 𝑇𝑐in high-𝑇𝑐cuprateswas determined. We find thatthe suppressing of the polaronic and charge-density-waveeffects in optimally doped cuprates results in the enhancement of 𝑇𝑐, while some lattice defects (e.g., anion vacancies) in the cuprates may strongly affect, on 𝑇𝑐and enhance high-𝑇𝑐superconductivity in them.

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Han, Tianxue. « The Research on the Complexity of 1T-TaS2 at Ultra-low Temperatures ». Journal of Physics : Conference Series 2152, n^o 1 (1 janvier 2022) : 012002. http://dx.doi.org/10.1088/1742-6596/2152/1/012002.

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Abstract Graphene, as a successfully industrialized two-dimensional material, has greatly promoted the development of other two-dimensional materials, such as transition metal dichalcogenide (TMDs). 1T-TaS2 is a classical TMDs material, which presents metallicity at high temperature. It undergoes a variety of charge density wave (CDW) phase transitions during the temperature declining process, and presents insulating properties at low temperature. During the temperature rise period, 1T-TaS2 goes through a phase transition, from an energy band insulator to Mott insulator, followed by an insulation-metal phase transition. The complexity of 1T-TaS2 phase diagram encourages researchers to conduct extensive research on it. This paper, via means of resistance, magnetic susceptibility and other technical methods, finds out that the ultra-low temperature of 1T-TaS2 suggests additional complexity. In addition, with the angle resolved photoemission spectroscopy (ARPES) technique of in-situ alkali metal evaporation, this paper proposes that the 1T-TaS2 ultra-low temperature ground state may exist a combination of state and surface state. Our findings provide more experimental evidence for the physical mechanism of this system.

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PIAMONTEZE, CÍNTHIA, HÉLIO C. N. TOLENTINO, FLÁVIO C. VICENTIN, ALINE Y. RAMOS, NESTOR E. MASSA, JOSE A. ALONSO, MARIA J. MARTINEZ-LOPE et M. T. CASAIS. « ELECTRONIC CHANGES RELATED TO THE METAL-TO-INSULATOR PHASE TRANSITION IN RNiO3 ». Surface Review and Letters 09, n^o 02 (avril 2002) : 1121–25. http://dx.doi.org/10.1142/s0218625x02003615.

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Rare earth nickel oxide perovskites (R NiO 3, R=rare earth) have, except for LaNiO 3, a metal–insulator (MI) phase transition as temperature decreases. The transition temperature (T MI ) increases as the R-ion becomes smaller. They present also, at low temperatures, a complex antiferromagnetic order. For lighter R-ions (e.g. Pr and Nd), the antiferromagnetic transition temperature (T N ) is close to T MI , while for heavier R-ions (e.g. Eu, Sm), T MI and T N are very far apart, suggesting that the magnetic and electronic behaviors are not directly coupled. Although R NiO 3 perovskites are placed in the boundary of the Mott–Hubbard and charge transfer regimes, there are several evidences pointing to a charge transfer gap, mainly controlled by ligand-to-metal charge transfer energy, and thus strongly dependent on hybridization. Ni L-edge absorption spectroscopy (transition 2p → 3d) gives direct information on the density of Ni 3d empty states, and in particular on the multiplet splitting and hybridization between Ni 3d and O 2p bands. Here we present Ni L3 and L2 absorption spectra measured for NdNiO 3 and EuNiO 3 (T MI = 200 and 480 K). At room temperature, dramatic differences are observed between EuNiO 3 (insulating) and NdNiO 3 (metallic). The normalized spectra give evidence for a higher density of 3d unoccupied states and a larger multiplet splitting in EuNiO 3. Both effects might be correlated to a decrease in hybridization. The same behavior is observed for NdNiO 3 as it is cooled down to the insulating phase (T < 200 K), revealing that in these compounds the opening of the gap is directly related to the degree of hybridization.

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Nazir, S. « Jeff=12 Mott-insulating to S = 0 metal transition in Ce@Y-doped Y2NiIrO6 ». Materials Science in Semiconductor Processing 179 (août 2024) : 108488. http://dx.doi.org/10.1016/j.mssp.2024.108488.

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Meer, H., O. Gomonay, A. Wittmann et M. Kläui. « Antiferromagnetic insulatronics : Spintronics in insulating 3d metal oxides with antiferromagnetic coupling ». Applied Physics Letters 122, n^o 8 (20 février 2023) : 080502. http://dx.doi.org/10.1063/5.0135079.

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Antiferromagnetic transition metal oxides are an established and widely studied materials system in the context of spin-based electronics, commonly used as passive elements in exchange bias-based memory devices. Currently, major interest has resurged due to the recent observation of long-distance spin transport, current-induced switching, and THz emission. As a result, insulating transition metal oxides are now considered to be attractive candidates for active elements in future spintronic devices. Here, we discuss some of the most promising materials systems and highlight recent advances in reading and writing antiferromagnetic ordering. This article aims to provide an overview of the current research and potential future directions in the field of antiferromagnetic insulatronics.

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Gobelko, I. E., A. V. Rozhkov et D. N. Dresvyankin. « Metal–Insulator Transition and Other Electronic Properties of AB-Stacked Bilayer Graphene Deposited on a Ferromagnetic Substrate ». JETP Letters 118, n^o 9 (novembre 2023) : 676–83. http://dx.doi.org/10.1134/s0021364023603068.

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Using a simple theoretical model, AB-stacked bilayer graphene deposited on a ferromagnetic insulating substrate is studied. In addition to the exchange Zeeman field induced by the substrate, the model allows one to take into account the effective external electric field perpendicular to the graphene sample plane (such field arises due to the contact with the substrate and can also be induced by applying a gate voltage). It has been demonstrated that AB-stacked graphene in zero electric field is in a metallic state. As the field increases, a transition to the insulating phase occurs. The spectrum of electron states, the band gap, and other characteristics of the phases on both sides of the metal−insulator transition have been calculated. Our results are consistent with density functional theory calculations and can be useful for spintronics.

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MOLLAH, S. « THERMAL HYSTERESIS IN RESISTIVITY AND MAGNETIZATION OF PrCa(Sr)MnO ». Modern Physics Letters B 22, n^o 32 (30 décembre 2008) : 3241–48. http://dx.doi.org/10.1142/s021798490801762x.

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Thermal hysteresis in resistivity and magnetization of Pr 0.65 Ca 0.35-x Sr x MnO 3(x = 0–0.35) manganites has been systematically studied to establish the interplay of their charge and spin-ordering. The increasing Sr concentration (x) transforms the charge-ordered (CO)/antiferromagnetic (AFM) insulating system (for x = 0) into a mixed-phased one (for x = 0.1) with sharp metal–insulator (MI) transition and finally leads to a ferromagnetic (FM) metallic (for x = 0.35) system. It has been found that the interplay of charge and spin-ordering is higher in mixed-phased state and the thermal hysteresis loop area is bigger. It increases with the increase of sharpness of MI transition. Interplay of charge and spin-ordering decreases with the increase of either CO/AFM insulating or FM metallic phase and is almost absent in completely CO/AFM insulating (with x = 0) or FM metallic (with x = 0.35) states bringing about zero thermal hysteresis loop.

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Gunanto, Yohanes Edi, K. Sinaga, B. Kurniawan, S. Poertadji, H. Tanaka, T. Ono, E. Steven et J. S. Brooks. « Transition Insulator-Metal and Antiferromagnetic-Paramagnetic Cu Doped in La_0.47Ca_0.53MnO₃ ». Advanced Materials Research 1123 (août 2015) : 73–77. http://dx.doi.org/10.4028/www.scientific.net/amr.1123.73.

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The study of the perovskite manganites La0.47Ca0.53Mn1-xCuxO3 with x = 0, 0.06, 0.09, and 0.13 has been done. The magnetic structure was determined using high-resolution neutron scattering at room temperature and low temperature. All samples were paramagnetic at room temperature and antiferromagnetic at low temperature. Using the SQUID Quantum Design, the samples showed that the doping of the insulating antiferromagnetic phase La0.47Ca0.53MnO3 with Cu doping resulted in the temperature transition from an insulator to metal state, and an antiferromagnetic to paramagnetic phase. The temperature transition from an insulator to metal state ranged from 23 to 100 K and from 200 to 230 K for the transition from an antiferromagnetic to paramagnetic phase.

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MONTORSI, ARIANNA, et MARIO RASETTI. « METAL-INSULATOR TRANSITION IN THE HUBBARD MODEL ». Modern Physics Letters B 10, n^o 18 (10 août 1996) : 863–71. http://dx.doi.org/10.1142/s0217984996000985.

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We study the metal-insulator transition of the d-dimensional Hubbard model by treating the hopping term between adjacent 1-d chains in the frame of a Clifford linearization scheme, and keeping the full model along the chains. A general equation for the critical point is worked out in terms of the correlation functions of the one-dimensional model, assuming that the transition is of the second order. The equation, which holds at any temperature, is here investigated especially at T=0, where the latter condition holds true. The solution shows the existence of an insulating ground state in any d only at half-filling for U strictly positive, as in the exact 1-d case. The transition is found to be related to a parameter reminiscent of entropy.

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Gobelko, I. E., A. V. Rozhkov et D. N. Dresvyankin. « Perekhod metall-dielektrik i drugie elektronnye svoystva dvukhsloynogo AV-grafena na ferromagnitnoy podlozhke ». Письма в Журнал экспериментальной и теоретической физики 118, n^o 9-10 (11) (15 décembre 2023) : 689–96. http://dx.doi.org/10.31857/s1234567823210103.

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Using a simple theoretical model, AB-stacked bilayer graphene deposited on a ferromagnetic insulating substrate is studied. In addition to the exchange Zeeman field induced by the substrate, the model allows one to take into account the effective external electric field perpendicular to the graphene sample plane (such field arises due to the contact with the substrate and can also be induced by applying a gate voltage). It has been demonstrated that AB-stacked graphene in zero electric field is in a metallic state. As the field increases, a transition to the insulating phase occurs. The spectrum of electron states, the band gap, and other characteristics of the phases on both sides of the metal−insulator transition have been calculated. Our results are consistent with density functional theory calculations and can be useful for spintronics.

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Kürthy, G. Mayer-von, W. Wischert et S. Kemmler-Sack. « Der Halbleiter-Metall-Übergang in Pyrochloren des Systems Ln2-xCuxRu2O7-z (Ln = Pr, Sm, Eu, Gd, Dy) / The Semiconductor-Metal Transition in Pyrochlors of the System Ln2-xCuxRu2O7-z (Ln = Pr, Sm, Eu, Gd, Dy) ». Zeitschrift für Naturforschung B 44, n^o 7 (1 juillet 1989) : 750–60. http://dx.doi.org/10.1515/znb-1989-0706.

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In the pyrochlor system Ln2-xCuxRu2O7-z a partial substitution of Pr bv Cu is observed for x ≾ 0.375. With increasing x a gradual semiconductor-metal transition occurs. The change of sign of TCR from negative to positive is observed for ρ ∾ 6 × 10-3 Ω cm (x = 0.2). The presence of variable range hopping in the insulating regime strongly suggests that the metal-semiconductor transition is of Anderson type. For the pyrochlores of type Ln1.625Cu0.375Ru7-z with the smaller rare earth ions Sm, Eu, Gd, Dy the metallic conductivity is absent and localization of Anderson type occurs.

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Baldini, Edoardo, Michael A. Sentef, Swagata Acharya, Thomas Brumme, Evgeniia Sheveleva, Fryderyk Lyzwa, Ekaterina Pomjakushina et al. « Electron–phonon-driven three-dimensional metallicity in an insulating cuprate ». Proceedings of the National Academy of Sciences 117, n^o 12 (11 mars 2020) : 6409–16. http://dx.doi.org/10.1073/pnas.1919451117.

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The role of the crystal lattice for the electronic properties of cuprates and other high-temperature superconductors remains controversial despite decades of theoretical and experimental efforts. While the paradigm of strong electronic correlations suggests a purely electronic mechanism behind the insulator-to-metal transition, recently the mutual enhancement of the electron–electron and the electron–phonon interaction and its relevance to the formation of the ordered phases have also been emphasized. Here, we combine polarization-resolved ultrafast optical spectroscopy and state-of-the-art dynamical mean-field theory to show the importance of the crystal lattice in the breakdown of the correlated insulating state in an archetypal undoped cuprate. We identify signatures of electron–phonon coupling to specific fully symmetric optical modes during the buildup of a three-dimensional (3D) metallic state that follows charge photodoping. Calculations for coherently displaced crystal structures along the relevant phonon coordinates indicate that the insulating state is remarkably unstable toward metallization despite the seemingly large charge-transfer energy scale. This hitherto unobserved insulator-to-metal transition mediated by fully symmetric lattice modes can find extensive application in a plethora of correlated solids.

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Harrison, N. M., V. R. Saunders, R. Dovesi et W. C. Mackrodt. « Transition metal materials : a first principles approach to the electronic structure of the insulating phase ». Philosophical Transactions of the Royal Society of London. Series A : Mathematical, Physical and Engineering Sciences 356, n^o 1735 (15 janvier 1998) : 75–88. http://dx.doi.org/10.1098/rsta.1998.0150.

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Ma, Chung T., Salinporn Kittiwatanakul, Apiprach Sittipongpittaya, Yuhan Wang, Md Golam Morshed, Avik W. Ghosh et S. Joseph Poon. « Phase Change-Induced Magnetic Switching through Metal–Insulator Transition in VO2/TbFeCo Films ». Nanomaterials 13, n^o 21 (27 octobre 2023) : 2848. http://dx.doi.org/10.3390/nano13212848.

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The ability to manipulate spins in magnetic materials is essential in designing spintronics devices. One method for magnetic switching is through strain. In VO2 on TiO2 thin films, while VO2 remains rutile across the metal–insulator transition, the in-plane lattice area expands going from a low-temperature insulating phase to a high-temperature conducting phase. In a VO2/TbFeCo bilayer, the expansion of the VO2 lattice area exerts tension on the amorphous TbFeCo layer. Through the strain effect, magnetic properties, including the magnetic anisotropy and magnetization, of TbFeCo can be changed. In this work, the changes in magnetic properties of TbFeCo on VO2/TiO2(011) are demonstrated using anomalous Hall effect measurements. Across the metal–insulator transition, TbFeCo loses perpendicular magnetic anisotropy, and the magnetization in TbFeCo turns from out-of-plane to in-plane. Using atomistic simulations, we confirm these tunable magnetic properties originating from the metal–insulator transition of VO2. This study provides the groundwork for controlling magnetic properties through a phase transition.

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DAS, PROLOY TARAN, Arun Kumar Nigam et Tapan Kumar Nath. « Emergence of size induced metallic state in the ferromagnetic insulating Pr0.8Sr0.2MnO3 manganite : Breaking of surface polarons ». JOURNAL OF ADVANCES IN PHYSICS 8, n^o 2 (15 avril 2015) : 2084–93. http://dx.doi.org/10.24297/jap.v8i2.1517.

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Nano-dimensional effects on electronic-, magneto-transport properties of granular ferromagnetic insulating (FMI) Pr0.8Sr0.2MnO3 (PSMO) manganite (down to 40 nm) have been investigated in details. From the electronic and magnetic transport properties, a metallic state has been observed in grain size modulation by suppressing the ferromagnetic insulating state of PSMO bulk system. A distinct metal-insulator transition (MIT) temperature around 150 K has been observed in all nanometric samples. The observed insulator to metallic transition with size reduction can be explained with surface polaron breaking model, originates due to enhanced grain surface disorder. This proposed phenomenological polaronic model plays a significant role to understand the polaronic destabilization process on the grain surface regime of these phase separated nano-mangnatie systems. Temperature dependent resistivity and magnetoresistance data in presence of external magnetic fields are investigated in details with various compatible models.

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Qiu, Richard, Chieh-Wen Liu, Shuhao Liu et Xuan Gao. « New Reentrant Insulating Phases in Strongly Interacting 2D Systems with Low Disorder ». Applied Sciences 8, n^o 10 (14 octobre 2018) : 1909. http://dx.doi.org/10.3390/app8101909.

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The metal-insulator transition (MIT) in two-dimension (2D) was discovered by Kravchenko et al. more than two decades ago in strongly interacting 2D electrons residing in a Si-metal-oxide-semiconductor field-effect transistor (Si-MOSFET). Its origin remains unresolved. Recently, low magnetic field reentrant insulating phases (RIPs), which dwell between the zero-field (B = 0) metallic state and the integer quantum Hall (QH) states where the Landau-level filling factor υ > 1, have been observed in strongly correlated 2D GaAs hole systems with a large interaction parameter, rs, (~20–40) and a high purity. A new complex phase diagram was proposed, which includes zero-field MIT, low magnetic field RIPs, integer QH states, fractional QH states, high field RIPs and insulating phases (HFIPs) with υ < 1 in which the insulating phases are explained by the formation of a Wigner crystal. Furthermore, evidence of new intermediate phases was reported. This review article serves the purpose of summarizing those recent experimental findings and theoretical endeavors to foster future research efforts.

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TRAVĚNEC, IGOR. « METAL–INSULATOR TRANSITION IN 3D QUANTUM PERCOLATION ». International Journal of Modern Physics B 22, n^o 29 (20 novembre 2008) : 5217–27. http://dx.doi.org/10.1142/s0217979208049388.

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We present the metal–insulator transition study of a quantum site percolation model on simple cubic lattice. Transfer matrix method is used to calculate transport properties — Landauer conductance — for the binary distribution of energies. We calculate the mobility edge in disorder (ratio of insulating sites) — energy plane in detail and we find the extremal critical disorder somewhat closer to the classical percolation threshold than formerly reported. We calculate the critical exponent ν along the mobility edge and find it constant and equal to the one of 3D Anderson model, confirming common universality class. Possible exception is the center of the conduction band, where either the single-parameter scaling is not valid anymore, or finite size effects are immense. One of the reasons for such statement is the difference between results from arithmetic and geometric averaging of conductance at special energies. Only the geometric mean yields zero critical disorder in band center, which was theoretically predicted.

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Jin, Feng, Le Wang, Anmin Zhang, Jianting Ji, Youguo Shi, Xiaoqun Wang, Rong Yu, Jiandi Zhang, E. W. Plummer et Qingming Zhang. « Raman interrogation of the ferroelectric phase transition in polar metal LiOsO3 ». Proceedings of the National Academy of Sciences 116, n^o 41 (23 septembre 2019) : 20322–27. http://dx.doi.org/10.1073/pnas.1908956116.

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Ferroelectric (FE) distortions in a metallic material were believed to be experimentally inaccessible because itinerant electrons would screen the long-range Coulomb interactions that favor a polar structure. It has been suggested by Anderson and Blount [P. W. Anderson, E. I. Blount, Phys. Rev. Lett. 14, 217−219 (1965)] that a transition from paraelectric phase to FE phase is possible for a metal if, in the paraelectric phase, the electrons at the Fermi level are decoupled from the soft transverse optical phonons, which lead to ferroelectricity. Here, using Raman spectroscopy combined with magnetotransport measurements on a recently discovered FE metal LiOsO3, we demonstrate active interplay of itinerant electrons and the FE order: Itinerant electrons cause strong renormalization of the FE order parameter, leading to a more gradual transition in LiOsO3 than typical insulating FEs. In return, the FE order enhances the anisotropy of charge transport between parallel and perpendicular to the polarization direction. The temperature-dependent evolution of Raman active in-plane 3Eg phonon, which strongly couples to the polar-active out-of-the-plane A2u phonon mode in the high-temperature paraelectric state, exhibits a deviation in Raman shift from the expectation of the pseudospin−phonon model that is widely used to model many insulating FEs. The Curie−Weiss temperature (θ ≈ 97 K) obtained from the optical susceptibility is substantially lower than Ts, suggesting a strong suppression of FE fluctuations. Both line width and Fano line shape of 3Eg Raman mode exhibit a strong electron−phonon coupling in the high-temperature paraelectric phase, which disappears in the FE phase, challenging Anderson/Blount’s proposal for the formation of FE metals.

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Nishikawa, Kazutaka, Masamichi Yoshimura et Yoshihide Watanabe. « Phase transition behavior in nanostructured VO₂ with M1, M2, and R phases observed via temperature-dependent XRD measurements ». Journal of Vacuum Science & ; Technology A 40, n^o 3 (mai 2022) : 033401. http://dx.doi.org/10.1116/6.0001705.

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Vanadium dioxide (VO2) exhibits a metal-insulator phase transition at approximately 70 °C. Two different crystal structures, M1 and M2 phases, in the insulating state have been reported. The M2 phase is expected to be a Mott insulator, whereas the M1 phase is expected to act as a band insulator. It is important to clarify the origin and transition properties of the M2 phase. In this study, we fabricated VO2 nanostructures via sputtering deposition and subsequent lamp annealing at various O2 pressures. Temperature-dependent XRD measurements revealed that the nanostructured VO2 exhibits the M2 phase in temperatures just before the phase transition from M1 to R (metal state) during the heating process. The ratio of V5+/(V4+ + V5+) in nanostructured VO2 was estimated from XPS results; an increase in this ratio resulted in the broadening of the temperature range of the M2 phase. Furthermore, in the cooling process, direct phase transitions from R to M1 were observed. Structural changes are likely to occur in different pathways during heating and cooling. Our findings should contribute to the identification of the phase transition mechanism and to the development of a Mott field-effect transistor that utilizes nanostructured VO2 in the M2 phase.

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Pesotskii, Sergei I., Rustem B. Lyubovskii, Gennady V. Shilov, Vladimir N. Zverev, Svetlana A. Torunova, Elena I. Zhilyaeva et Enric Canadell. « Effect of External Pressure on the Metal–Insulator Transition of the Organic Quasi-Two-Dimensional Metal κ-(BEDT-TTF)2Hg(SCN)2Br ». Magnetochemistry 8, n^o 11 (8 novembre 2022) : 152. http://dx.doi.org/10.3390/magnetochemistry8110152.

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The metal–insulator transition in the organic quasi-two-dimensional metal κ-(BEDT-TTF)2Hg(SCN)2Br at TMI ≈ 90 K has been investigated. The crystal structure changes during this transition from monoclinic above TMI to triclinic below TMI. A theoretical study suggested that this phase transition should be of the metal-to-metal type and brings about a substantial change of the Fermi surface. Apparently, the electronic system in the triclinic phase is unstable toward a Mott insulating state, leading to the growth of the resistance when the temperature drops below TMI ≈ 90 K. The application of external pressure suppresses the Mott transition and restores the metallic electronic structure of the triclinic phase. The observed quantum oscillations of the magnetoresistance are in good agreement with the calculated Fermi surface for the triclinic phase, providing a plausible explanation for the puzzling behavior of κ-(BEDT-TTF)2Hg(SCN)2Br as a function of temperature and pressure around 100 K. The present study points out interesting differences in the structural and physical behaviors of the two room temperature isostructural salts of κ-(BEDT-TTF)2Hg(SCN)2X with X = Br, Cl.

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DIDUKH, L., YU DOVHOPYATY et YU SKORENKYY. « ENERGY GAP IN THE HUBBARD MODEL ». International Journal of Modern Physics B 14, n^o 07 (20 mars 2000) : 729–35. http://dx.doi.org/10.1142/s0217979200000613.

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A new variant of the generalized Hartree–Fock approximation for calculation of single-particle Green function in the Hubbard model is proposed. The calculated single-particle energy spectrum allows to study metal–insulator transition. Dependences of the energy gap width and the polar states concentration on model parameters are obtained. Conditions of a metallic and an insulating state realisation are found.

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YAGIL, YOAD, GUY DEUTSCHER et DAVID J. BERGMAN. « THE ROLE OF MICROGEOMETRY IN THE ELECTRICAL BREAKDOWN OF METAL-INSULATOR MIXTURES ». International Journal of Modern Physics B 07, n^o 19 (30 août 1993) : 3353–74. http://dx.doi.org/10.1142/s0217979293003267.

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The breakdown phenomena of percolative metal-insulator composites and their extreme sensitivity to fine details of the microgeometry are discussed, for three different cases: the critical current of superconductor-insulator mixtures (or superconductor-normal metal); the dielectric breakdown of metal-insulator composites below the percolation threshold (insulating regime); and the electrical breakdown above the metal-insulator transition (metallic regime). Two experimental techniques for characterizing the microgeometry are described: (a) 1/f noise measurements, which provide the fourth moment of the current distribution; (b) the harmonic generation method, where the weakly nonlinear electrical response due to local Joule heating provides information on the fourth (and higher) moment of the current distribution.

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Liu, Peitao, et Cesare Franchini. « Advanced First-Principle Modeling of Relativistic Ruddlesden—Popper Strontium Iridates ». Applied Sciences 11, n^o 6 (11 mars 2021) : 2527. http://dx.doi.org/10.3390/app11062527.

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In this review, we provide a survey of the application of advanced first-principle methods on the theoretical modeling and understanding of novel electronic, optical, and magnetic properties of the spin-orbit coupled Ruddlesden–Popper series of iridates Srn+1IrnO3n+1 (n = 1, 2, and ∞). After a brief description of the basic aspects of the adopted methods (noncollinear local spin density approximation plus an on-site Coulomb interaction (LSDA+U), constrained random phase approximation (cRPA), GW, and Bethe–Salpeter equation (BSE)), we present and discuss select results. We show that a detailed phase diagrams of the metal–insulator transition and magnetic phase transition can be constructed by inspecting the evolution of electronic and magnetic properties as a function of Hubbard U, spin–orbit coupling (SOC) strength, and dimensionality n, which provide clear evidence for the crucial role played by SOC and U in establishing a relativistic (Dirac) Mott–Hubbard insulating state in Sr2IrO4 and Sr3Ir2O7. To characterize the ground-state phases, we quantify the most relevant energy scales fully ab initio—crystal field energy, Hubbard U, and SOC constant of three compounds—and discuss the quasiparticle band structures in detail by comparing GW and LSDA+U data. We examine the different magnetic ground states of structurally similar n = 1 and n = 2 compounds and clarify that the origin of the in-plane canted antiferromagnetic (AFM) state of Sr2IrO4 arises from competition between isotropic exchange and Dzyaloshinskii–Moriya (DM) interactions whereas the collinear AFM state of Sr3Ir2O7 is due to strong interlayer magnetic coupling. Finally, we report the dimensionality controlled metal–insulator transition across the series by computing their optical transitions and conductivity spectra at the GW+BSE level from the the quasi two-dimensional insulating n = 1 and 2 phases to the three-dimensional metallic n=∞ phase.

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RAJ, SATYABRATA, TAKAFUMI SATO, SEIGO SOUMA, TAKASHI TAKAHASHI, D. D. SARMA et PRIYA MAHADEVAN. « METAL-INSULATOR TRANSITION OF NaxWO3 STUDIED BY ANGLE-RESOLVED PHOTOEMISSION SPECTROSCOPY ». Modern Physics Letters B 23, n^o 24 (20 septembre 2009) : 2819–46. http://dx.doi.org/10.1142/s0217984909021004.

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The electronic structure of sodium tungsten bronzes Na x WO 3 is investigated by high-resolution angle-resolved photoemission spectroscopy (ARPES). The ARPES spectra measured in both insulating and metallic phases of Na x WO 3 reveals the origin of metal-insulator transition (MIT) in sodium tungsten bronze system. It is found that in insulating Na x WO 3 the states near the Fermi level (EF) are localized due to the strong disorder caused by the random distribution of Na + ions in WO 3 lattice. Due to the presence of disorder and long-range Coulomb interaction of conduction electrons, a soft Coulomb gap arises, where the density of states vanishes exactly at EF. In the metallic regime the states near EF are populated and the Fermi level shifts upward rigidly with increasing electron doping (x). Volume of electron-like Fermi surface (FS) at the Γ(X) point of the Brillouin zone gradually increases with increasing Na concentration due to W 5d t2g band filling. A rigid shift of the Fermi energy is found to give a qualitatively good description of the Fermi surface evolution. As we move from bulk-sensitive to more surface sensitive photon energy, we found the emergence of Fermi surfaces at X(M) and M(R) points similar to the one at the Γ(X) point in the metallic regime, suggesting that the reconstruction of surface was due to rotation/deformation of WO 6 octahedra.

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Xia, Chengliang, Yue Chen et Hanghui Chen. « Pressure-induced metal–insulator transition in oxygen-deficient LiNbO₃-type ferroelectrics ». Journal of Physics : Condensed Matter 34, n^o 2 (28 octobre 2021) : 025501. http://dx.doi.org/10.1088/1361-648x/ac2e30.

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Abstract Hydrostatic pressure and oxygen vacancies usually have deleterious effects on ferroelectric materials because both tend to reduce their polarization. In this work we use first-principles calculations to study an important class of ferroelectric materials—LiNbO3-type ferroelectrics (LiNbO3 as the prototype), and find that in oxygen-deficient LiNbO3−δ , hydrostatic pressure induces an unexpected metal–insulator transition between 8 and 9 GPa. Our calculations also find that strong polar displacements persist in both metallic and insulating oxygen-deficient LiNbO3−δ and the size of polar displacements is comparable to pristine LiNbO3 under the same pressure. These properties are distinct from widely used perovskite ferroelectric oxide BaTiO3, whose polarization is quickly suppressed by hydrostatic pressure and/or oxygen vacancies. The anomalous pressure-driven metal–insulator transition in oxygen-deficient LiNbO3−δ arises from the change of an oxygen vacancy defect state. Hydrostatic pressure increases the polar displacements of oxygen-deficient LiNbO3−δ , which reduces the band width of the defect state and eventually turns it into an in-gap state. In the insulating phase, the in-gap state is further pushed away from the conduction band edge under hydrostatic pressure, which increases the fundamental gap. Our work shows that for LiNbO3-type strong ferroelectrics, oxygen vacancies and hydrostatic pressure combined can lead to new phenomena and potential functions, in contrast to the harmful effects occurring to perovskite ferroelectric oxides such as BaTiO3.

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Wang, Qi, Yaomi Itoh, Tohru Tsuruoka, Tsuyoshi Hasegawa, Satoshi Watanabe, Shu Yamaguchi, Toshiro Hiramoto et Masakazu Aono. « Two Types of On-State Observed in the Operation of a Redox-Based Three-Terminal Device ». Key Engineering Materials 596 (décembre 2013) : 111–15. http://dx.doi.org/10.4028/www.scientific.net/kem.596.111.

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A redox-based three-terminal device was fabricated using Ta2O5 as the ionic transfer material, and its operation was investigated. We found that application of a negative polarity gate bias, which increases oxygen anions in the channel regions, can make a conductive path between a source electrode and a drain electrode. The insulating state of the pristine device is turned on to a semiconductor state by the application of a negative polarity gate bias. Since turning off to the insulating state could not be achieved, the switching process resembles the soft breakdown of the first turning-on process of oxygen vacancy controlled resistive random access memories, although the polarity of the bias is opposite to that used in the first turning-on process. Further application of a gate bias causes a transition from the semiconductor state to a metal state. Accordingly, there are two types of on-state. It is possible to switch between the semiconductor and metal states.

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Gorbenko, O. Yu, A. R. Kaul, N. A. Babushkina, L. M. Belova et B. Guettler. « Colossal Isotope Shift of the Metal-Insulator Transition Temperature in Epitaxial Thin Films of (La1-y Pry)0·7Ca0·3MnO3 ». Australian Journal of Physics 52, n^o 2 (1999) : 269. http://dx.doi.org/10.1071/p98066.

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A colossal shift of the maximum resistivity temperature induced by oxygen isotope exchange was registered for 60 nm thick films of the solid solution La0·35Pr0·35Ca0·3MnO3 on perovskite substrates. The magnitude of the effect depends on lattice strain resulting in the largest shift for the film on LaAlO3: the 16O sample showed a metal-insulator transition at 182 K, whereas the 18O sample was insulating down to 4·2 K, which is the highest difference ever reported for CMR manganites. An XRD and Raman spectrometry study indicates no difference in chemical composition or structure for the 18O and 16O samples at room temperature. The results are consistent with the model coupling the metal-insulator transition in the perovskite manganites to the lattice dynamics.

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Wang, Shanmin, Jinlong Zhu, Yi Zhang, Xiaohui Yu, Jianzhong Zhang, Wendan Wang, Ligang Bai et al. « Unusual Mott transition in multiferroic PbCrO3 ». Proceedings of the National Academy of Sciences 112, n^o 50 (24 novembre 2015) : 15320–25. http://dx.doi.org/10.1073/pnas.1510415112.

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The Mott insulator in correlated electron systems arises from classical Coulomb repulsion between carriers to provide a powerful force for electron localization. Turning such an insulator into a metal, the so-called Mott transition, is commonly achieved by “bandwidth” control or “band filling.” However, both mechanisms deviate from the original concept of Mott, which attributes such a transition to the screening of Coulomb potential and associated lattice contraction. Here, we report a pressure-induced isostructural Mott transition in cubic perovskite PbCrO3. At the transition pressure of ∼3 GPa, PbCrO3 exhibits significant collapse in both lattice volume and Coulomb potential. Concurrent with the collapse, it transforms from a hybrid multiferroic insulator to a metal. For the first time to our knowledge, these findings validate the scenario conceived by Mott. Close to the Mott criticality at ∼300 K, fluctuations of the lattice and charge give rise to elastic anomalies and Laudau critical behaviors resembling the classic liquid–gas transition. The anomalously large lattice volume and Coulomb potential in the low-pressure insulating phase are largely associated with the ferroelectric distortion, which is substantially suppressed at high pressures, leading to the first-order phase transition without symmetry breaking.

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Pan, Yong. « Insight into sulfur vacancy-induced insulator to metal transition of Li2S ». Functional Materials Letters 10, n^o 05 (octobre 2017) : 1750067. http://dx.doi.org/10.1142/s1793604717500679.

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Sulfur vacancy-induced charge carrier trap is quite important for improving the insulating characteristic of Li2S. In this work, we apply the first-principle calculations to investigate the influence of sulfur intrinsic vacancy and sulfur vacancy concentration on the structure and conductivity of Li2S. Sulfur intrinsic vacancy is dynamically stable. Importantly, sulfur vacancy reduces the band gap of Li2S. The calculated band gap of sulfur intrinsic vacancy is 0.700[Formula: see text]eV. We suggest that the removed sulfur atom leads to Li-2[Formula: see text] state shift from conduction band to Fermi level and improves the charge overlap between the top of valence band (VB) and the bottom of conduction band (CB). The strong charge interaction of Li pairing atoms forms the Li–Li metallic bond. Finally, we predict that sulfur vacancy gives rise to the insulator to metal transition of Li2S. Our works open up a new possibility for improving the conductivity of Li2S.

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