Temperature Mott Transition Research Articles

Mass and spectral function of scalar and pseudoscalar mesons in a hot and chirally imbalanced medium using the two-flavor NJL model

We explore the properties of neutral mesons within the context of a chirally imbalanced medium, employing the two-flavor Nambu–Jona-Lasinio model. The temperature dependence of the constituent quark mass at finite values of the chiral chemical potential (CCP) demonstrates the well-established phenomena of chiral catalysis at lower temperatures and inverse chiral catalysis at higher temperatures. The polarization functions in both the scalar (σ) and pseudoscalar (π0) channels have been evaluated using the real-time formalism of thermal field theory. These have been used to determine the masses and spectral functions of σ and π mesons. Detailed investigation of the analytic structure of the imaginary part of the polarization function for σ and π mesons results in the emergence of nontrivial Landau-cut contributions due to the presence of chiral imbalance. The multiple solutions for the mass of the π meson for specific values of CCP have been analyzed on the basis of their residue at the pole. Furthermore, we have observed abrupt changes in the masses of both scalar and pseudoscalar mesons at finite CCP values, particularly at higher temperatures. A decreasing trend in the Mott transition temperature is seen with the increase in CCP. Published by the American Physical Society 2024

Towards Unlocking/Tuning the Mott Transition Temperature in Alkaline-Doped Vanadium Oxide Thermochromic Coatings and Potential Green Air-Conditioning via Room Temperature VxOy-V-VxOy Layered Coatings

In this contribution, we validate for the first time that the near infrared-infrared (NIR-IR) modulation of the optical transmission (DTTRANS = T(T<TMIT) - T(T>TMIT)) of vanadium oxide-based nanomaterials can be controlled or tuned via a genuine approach with a simultaneous drastic reduction of its Mott transition temperature TMIT. More accurately, we report a significant thermochromism in multilayered V2O5/V/V2O5 stacks equivalent to that of pure VO2 thin films but with a far lower transition temperature TMIT. Such a multilayered V2O5/V/V2O5 thermochromic system exhibited a net control or tunability of the optical transmission modulation in the NIR-IR (DTTRANS) via the nano-scaled thickness of the intermediate vanadium layer. In addition, the control of DTTRANS is accompanied by a noteworthy diminution of the Mott transition temperature TMIT from the bulk value of 68.8 °C to the range of 27.5–37.5 °C. The observed peculiar thermochromism in the multilayered V2O5/V/V2O5 is likely to be ascribed to a significant interfacial diffusion or an excessive interfacial stress/strain, and/or to an effective halide (Na, K, Ca) doping. This doping is driven by a significant diffusion from the borosilicate substrate surface towards the V2O5/V/V2O5 stacks. If the upscaling of this approach is validated, the current findings would contribute to advancing thermochromic nanomaterials and their applications in smart windows for managing solar heat and green air-conditioning technologies.

Tuning moiré excitons and correlated electronic states through layer degree of freedom

Moiré coupling in transition metal dichalcogenides (TMDCs) superlattices introduces flat minibands that enable strong electronic correlation and fascinating correlated states, and it also modifies the strong Coulomb-interaction-driven excitons and gives rise to moiré excitons. Here, we introduce the layer degree of freedom to the WSe2/WS2 moiré superlattice by changing WSe2 from monolayer to bilayer and trilayer. We observe systematic changes of optical spectra of the moiré excitons, which directly confirm the highly interfacial nature of moiré coupling at the WSe2/WS2 interface. In addition, the energy resonances of moiré excitons are strongly modified, with their separation significantly increased in multilayer WSe2/monolayer WS2 moiré superlattice. The additional WSe2 layers also modulate the strong electronic correlation strength, evidenced by the reduced Mott transition temperature with added WSe2 layer(s). The layer dependence of both moiré excitons and correlated electronic states can be well described by our theoretical model. Our study presents a new method to tune the strong electronic correlation and moiré exciton bands in the TMDCs moiré superlattices, ushering in an exciting platform to engineer quantum phenomena stemming from strong correlation and Coulomb interaction.

Chiral crossover characterized by Mott transition at finite temperature * *Supported by the NSFC (11775165)

We discuss the proper definition for the chiral crossover at finite temperature, based on Goldstone's theorem. Different from the commonly used maximum change in chiral condensate, we propose defining the crossover temperature using the Mott transition of pseudo-Goldstone bosons, which, by definition, guarantees Goldstone's theorem. We analytically and numerically demonstrate this property in the frame of a Pauli-Villars regularized NJL model. In an external magnetic field, we find that the Mott transition temperature shows an inverse magnetic catalysis effect.

Significant control of metal-insulator transition temperature through catalytic excessive oxygen doping in high-performance vanadium dioxide nanobeam channel

The strategy of a reliable transition temperature control of vanadium dioxide (VO2) is reported. Rectangular VO2 nanobeams were synthesized by a thermal chemical vapor deposition (TCVD) system. The metal-insulator transition (MIT) temperature increases to above 380 K when the TiO2 ratio of the source is 5 at.%, although the Ti source is not physically doped into VO2 nanobeams. The XPS spectra of the V 2p orbital reveal the excessive oxidation of V after the TCVD processes with a higher TiO2 ratio, indicating that the TiO2 precursor is important in the O-doping of the surface VO bonds when forming volatile Ti-O gas species. Thus, TiO2 reactants can be used as a VO2 surface chemical modifier to manipulate the MIT transition temperature and maintain a homogenous VO2 phase, which is useful for a Mott device application with a record on/off switching ratio > 104 and Mott transition temperature > 380 K.

Effect of the anomalous magnetic moment of quarks on the phase structure and mesonic properties in the NJL model

Employing a field dependent three-momentum cut-off regularization technique, we study the phase structure and mesonic masses using the $2$-flavour Nambu-Jona Lasinio model at finite temperature and density in presence of arbitrary external magnetic field. This approach is then applied to incorporate the effects of the anomalous magnetic moment(AMM) of quarks on constituent quark mass and thermodynamic observables as a function of temperature/baryonic density. The critical temperature for transition from chiral symmetry broken to the restored phase is observed to decrease with the external magnetic field, which can be classified as inverse magnetic catalysis, while an opposite behaviour is realized in the case of a vanishing magnetic moment, implying magnetic catalysis. These essential features are also reflected in the phase diagram. Furthermore, the properties of the low lying scalar and neutral pseudoscalar mesons are also studied in presence of a hot and dense magnetized medium including AMM of the quarks using random phase approximation. For non-zero values of magnetic field, we notice a sudden jump in the mass of the Goldstone mode at and above the Mott transition temperature which is found to decrease substantially with the increase in magnetic field when the AMM of the quarks are taken into consideration.

Mott dissociation of pions and kaons in hot, dense quark matter

We describe the Mott dissociation of pions and kaons within a Beth-Uhlenbeck approach based on the PNJL model, which allows for a unified description of bound, resonant and scattering states. Within this model we evaluate the temperature and chemical potential dependent modification of the phase shifts both in the pseudoscalar and scalar isovector meson channels for $N_f=2+1$ quark flavors. We show that the character change of the pseudoscalar bound states to resonances in the continuum at the Mott transition temperature is signaled by a jump of the phase shift at the threshold from $\pi$ to zero, in accordance with the Levinson theorem. In particular, we demonstrate the importance of accounting for the scattering continuum states, which ensures that the total phase shift in each of the meson channels vanishes at high energies, thus eliminating mesonic correlations from the thermodynamics at high temperatures. In this way, we prove that the present approach provides a unified description of the transition from a meson gas to a quark-gluon plasma. We discuss the occurrence of an anomalous mode for mesons composed of quarks with unequal masses which is particularly pronounced for $K^+$ and $\kappa^+$ states at finite densities a a possible mechanism to explain the "horn" effect for the $K^+/\pi^+$ ratio in heavy-ion collisions.

Bulk viscosity of two-flavor quark matter from the Kubo formalism

We study the bulk viscosity of quark matter in the strong coupling regime within the two-flavor Nambu--Jona-Lasinio model. The dispersive effects that lead to non-zero bulk viscosity arise from quark-meson fluctuations above the Mott transition temperature, where meson decay into two quarks is kinematically allowed. We adopt the Kubo-Zubarev formalism and compute the equilibrium imaginary-time correlation function for pressure in the $O(1/N_c)$ power counting scheme. The bulk viscosity of matter is expressed in terms of the Lorentz components of the quark spectral function and includes multi-loop contributions which arise via re-summation of infinite geometrical series of loop diagrams. We show that the multi-loop contributions dominate the single-loop contribution close to the Mott line, whereas at high temperatures the one-loop contribution is dominant. The multi-loop bulk viscosity dominates the shear viscosity close to the Mott temperature by factors 5 to 20, but with increasing temperature the shear viscosity becomes the dominant dissipation mechanism of stresses as the one-loop contribution becomes the main source of bulk viscosity.

Effect of discrete quark momenta on the Goldstone mode in a magnetic field

The meson static properties are investigated in Pauli-Villars regularized Nambu--Jona-Lasinio model in strong magnetic field. The quark dimension reduction leads to not only the magnetic catalysis effect on chiral symmetry restoration but also a sudden jump of the mass of the Goldstone mode at the Mott transition temperature.

Estimating transport coefficients in hot and dense quark matter

We compute the transport coefficients, namely, the coefficients of shear and bulk viscosity, as well as thermal conductivity, for hot and dense quark matter. The calculations are performed within the Nambu--Jona-Lasinio (NJL) model. The estimation of the transport coefficients is made using a quasiparticle approach of solving the Boltzmann kinetic equation within the relaxation time approximation. The transition rates are calculated in a manifestly covariant manner to estimate the thermal-averaged cross sections for quark-quark and quark-antiquark scattering. The calculations are performed for finite chemical potential also. Within the parameters of the model, the ratio of shear viscosity to entropy density has a minimum at the Mott transition temperature. At vanishing chemical potential, the ratio of bulk viscosity to entropy density, on the other hand, decreases with temperature, with a sharp decrease near the critical temperature, and vanishes beyond it. At finite chemical potential, however, it increases slowly with temperature beyond the Mott temperature. The coefficient of thermal conductivity also shows a minimum at the critical temperature.

Self-doped nanocolumnar vanadium oxides thin films for highly selective NO2 gas sensing at low temperature

Vanadium dioxide (VO2) as strong correlated oxide has been intensively studied due to the unique properties accompanied by metal-insulator transition (MIT) near room temperature. Their extrinsic electrical and optical transformations on MIT have been greatly attracted in many potential applications. In this study, we propose to apply an intrinsic electrical property of VO2 in metallic phase as doping agent of chemiresistive sensor. Self-doped nanocoulmnar vanadium oxides (SNVO) thin films are simply fabricated by glancing angle deposition without any template and chemical additives. The nanocolumnar VO2 thin films were deposited by e-beam evaporator, and then the SNVO with residual VO2 in V2O5 nanocolums were synthesized by an air atmosphere annealing, which are composed of dominant oxidized V2O5 which plays a sensing role on the surface and the residual VO2 with self-doping effect inside nanocolumns. Especially at low temperature (150°C), SNVO sensor shows an ultra-high performance (Rgas/Rair>100) to NO2 (5ppm) gas. It is supposed that the residual VO2 which shows metallic properties above Mott transition temperature (∼67°C) acts as an intrinsic electron donor by pseudo-free electron, which helps adsorption of oxygen ions on the surface and extends the depletion region of few nanometers of narrow necks between nanocolumns.

Surface charge sensing by altering the phase transition in VO2

Detection of surface charges has various applications in medicine, electronics, biotechnology, etc. The source of surface charge induction may range from simple charge-polarized molecules like water to complicated proteins. It was recently discovered that surface charge accumulation can alter the temperature at which VO2 undergoes a Mott transition. Here, we deposited polar molecules onto the surface of two-terminal thin-film VO2 lateral devices and monitored the joule-heating-driven Mott transition, or conductance switching. We observed that the power required to induce the conductance switching reduced upon treatment with polar molecules and, using in-situ blackbody-emission direct measurement of local temperature, we show that this reduction in power was accompanied by reduction in the Mott transition temperature. Further evidence suggested that this effect has specificity to the nature of the species used to induce surface charges. Using x-ray absorption spectroscopy, we also show that there is no detectable change in oxidation state of vanadium or structural phase in the bulk of the 40 nm VO2 thin-film even as the phase transition temperature is reduced by up to 20 K by the polar molecules. The ability to alter the phase transition parameters by depositing polar molecules suggests a potential application in sensing surface charges of different origins and this set of results also highlights interesting aspects of the phase transition in VO2.

Finite Temperature Mott Transition in a Nonlocal PNJL Model

We provide a novel calculation of the Mott effect in non-local PNJL models. We find that the deconfinement transition temperature in these models is lower than the Mott temperature. Furthermore, the mass and the width of the σ and the π meson modes is calculated with the result that the width in nonlocal models is in general reduced as compared to local models. Difficulties encountered while attempting to Wick rotate covariant models are carefully discussed.

Dynamical Mean-Field Theory on the Basis of the Fourth-Order Perturbation Expansion

Adopting a fourth-order perturbation expansion as a solver for the impurity problem, we develop a dynamical mean-field theory for the Hubbard model. Zero temperature Mott transition is discussed by calculating a quasiparticle weight z in the infinite dimensional Hubbard model at half filling. We obtain fairly good agreement with the result on the basis of the numerical renormalization group as an impurity solver, revising the result based on the second-order perturbation expansion as the impurity solver.

Dielectric Breakdown and Avalanches at Nonequilibrium Metal-Insulator Transitions

Motivated by recent experiments on the finite temperature Mott transition in VO(2) films, we propose a classical coarse-grained dielectric breakdown model where each degree of freedom represents a nanograin which transitions from insulator to metal with increasing temperature and voltage at random thresholds due to quenched disorder. We describe the properties of the resulting nonequilibrium metal-insulator transition and explain the universal characteristics of the resistance jump distribution. We predict that by tuning voltage, another critical point is approached, which separates a phase of boltlike avalanches from percolationlike ones.

Dynamical mean field theory phase-space extension and critical properties of the finite temperature Mott transition

We consider the finite temperature metal-insulator transition in the half filled paramagnetic Hubbard model on the infinite dimensional Bethe lattice. A new method for calculating the Dynamical Mean Field Theory fixpoint surface in the phase diagram is presented and shown to be free from the convergence problems of standard forward recursion. The fixpoint equation is then analyzed using dynamical systems methods. On the fixpoint surface the eigenspectra of its Jacobian is used to characterize the hysteresis boundaries of the first order transition line and its second order critical end point. The critical point is shown to be a cusp catastrophe in the parameter space, opening a pitchfork bifurcation along the first order transition line, while the hysteresis boundaries are shown to be saddle-node bifurcations of two merging fixpoints. Using Landau theory the properties of the critical end point is determined and related to the critical eigenmode of the Jacobian. Our findings provide new insights into basic properties of this intensively studied transition.

Nanostructure-Dependent Metal−Insulator Transitions in Vanadium-Oxide Nanowires

Single-crystal VO2 nanowires were synthesized using atmospheric-pressure and physical vapor deposition and outfitted with electrodes for current−voltage measurements. The Mott insulator-to-metal transition temperatures of several nanowires with varying lateral dimensions were determined by measuring the voltage values at which the sharp current step, signaling that the occurrence of the insulator-to-metal or the reverse transitions, had taken place. The observed Mott transition temperatures, which ranged between 62 and 70 °C for the nanowires measured, trended downward with decreasing nanowire width. We ascribe this to strong interactions between the nanowire and the underlying silica substrate. However, the scatter in the Mott-temperature versus nanowire width exceeded the experimental uncertainty in the values of the Mott temperature, indicating that other parameters also contribute to the precise value of the Mott transition temperature of nanostructured VO2.

Pressure-driven magnetic moment collapse in the ground state of MnO

The zero temperature Mott transition region in antiferromagnetic, spin MnO is probed using the correlated band theory LSDA+U method. The first transition encountered is an insulator–insulator volume collapse within the rocksalt structure that is characterized by an unexpected Hund's rule violating ‘spin-flip’ moment collapse. This spin-flip to takes fullest advantage of the anisotropy of the Coulomb repulsion, allowing gain in the kinetic energy (which increases with decreasing volume) while retaining a sizable amount of the magnetic exchange energy. While transition pressures vary with the interaction strength, the spin-flip state is robust over a range of interaction strengths and for both B1 and B8 structures.

Dynamical mean-field study of the Mott transition in the half-filled Hubbard model on a triangular lattice

We employ dynamical mean field theory (DMFT) with a Quantum Monte Carlo (QMC) atomic solver to investigate the finite temperature Mott transition in the Hubbard model with the nearest neighbor hopping on a triangular lattice at half-filling. We estimate the value of the critical interaction to be $U_c=12.0 \pm 0.5$ in units of the hopping amplitude $t$ through the evolution of the magnetic moment, spectral function, internal energy and specific heat as the interaction $U$ and temperature $T$ are varied. This work also presents a comparison between DMFT and finite size determinant Quantum Monte Carlo (DQMC) and a discussion of the advantages and limitations of both methods.

DC transformer: Dream or reality?

We propose to use the exciton coupling between electrons and holes in different quantum wells to reach a strong Coulomb drag effect. The drag has to be really strong below the Mott transition when the most of the carriers are bound in excitons. We suggest to use the exciton drag for fabrication of DC transformer. Preliminary estimates for Si/SiO 2/Si structure give the Mott transition temperature of the order of 100 K.