Low-temperature variation of magnetic order in a nonmagnetic n-Ge:As semiconductor in the vicinity of the metal-insulator phase transition

The phenomenon of the low-temperature transition from antiferro- to ferromagnetic ordering of impurity spins in a nonmagnetic compensated n-Ge:As semiconductor near the metal-insulator phase transformation has been experimentally observed. The effect is manifested by rather sharp changes in the spin density and g-factor in the electron spin resonance spectra. As the relative content of a compensating impurity (gallium) is reduced below 0.7, the transition temperature begins to decrease and, at a degree of compensation below 0.3, drops below the studied temperature range (i.e., below 2 K).     

On the Optical Conductivity of Kondo Insulators

The temperature-dependent optical conductivity of a Kondo insulator for different temperatures with impurity doping on the conduction band is studied within the U = ∞ Anderson lattice model in the framework of a slave-boson mean-field theory under the coherent potential approximation. The results show that the depletion of the optical conductivity in low-frequency region decreases with increasing of the temperature, and the system exhibits a transition between a low-temperature insulating phase and a high-temperature metallic phase. With increasing of the impurity concentration, the effect of the impurity scattering could result in a nearly temperature-independent gap behavior. The numerical results on the localization of the electron and its variation with temperature are also discussed with the help of optical sum rule.     

Effect of charge carriers on the magnetic properties of the CdCr<Subscript>2</Subscript>Se<Subscript>4</Subscript> ferromagnetic semiconductor

The results of an investigation into how the composition of annealing mixtures influences the Curie temperature of single crystals of the CdCr₂Se₄ ferromagnetic semiconductor are reported. The mechanism of doping of single crystals is analyzed, and the role played by each of the components of the annealing batch is established. It is concluded that the indirect exchange involving n-type carriers near impurity gallium ions leads to a sharp increase in the phase transition temperature (from 130 to 172 K) of the samples. The dependences of the magnetization of samples with different phase transition temperatures on the temperature and magnetic field are compared.     

Quantum phase transition of cold atoms trapped in optical lattices

We review our recent theoretical advances in phase transition of cold atoms in optical lattices, such as triangular lattice, honeycomb lattice, and Kagomé lattice. By employing the new developed numerical methods called dynamical cluster approximation and cellular dynamical mean-field theory, the properties in different phases of cold atoms in optical lattices are studied, such as density of states, Fermi surface and double occupancy. On triangular lattice, a reentrant behavior of phase translation line between Fermi liquid state and pseudogap state is found due to the Kondo effect. We find the system undergoes a second order Mott transition from a metallic state into a Mott insulator state on honeycomb lattice and triangular Kagomé lattice. The stability of quantum spin Hall phase towards interaction on honeycomb lattice with spin-orbital coupling is systematically discussed. And we investigate the transition from quantum spin Hall insulator to normal insulator in Kagomé lattice which includes a nearest-neighbor intrinsic spin-orbit coupling and a trimerized Hamiltonian. In addition, we propose the experimental protocols to observe these phase transition of cold atoms in optical lattices.     

Insulator-semimetallic transition in quasi-1D charged impurity-infected armchair boron-nitride nanoribbons

We address control of electronic phase transition in charged impurity-infected armchair-edged boron-nitride nanoribbons (ABNNRs) with the local variation of Fermi energy. In particular, the density of states of disordered ribbons produces the main features in the context of pretty simple tight-binding model and Green's functions approach. To this end, the Born approximation has been implemented to find the effect of π-band electron-impurity interactions. A modulation of the π-band depending on the impurity concentrations and scattering potentials leads to the phase transition from insulator to semimetallic. We present here a detailed physical meaning of this transition by studying the treatment of massive Dirac fermions. From our findings, it is found that the ribbon width plays a crucial role in determining the electronic phase of disordered ABNNRs. The obtained results in controllable gap engineering are useful for future experiments. Also, the observations in this study have also fueled interest in the electronic properties of other 2D materials.     

Spectroscopic manifestations of orientational phase transition in adsorbate monolayer

In the framework of the virtual crystal approximation, the temperature dependences of frequencies and integral intensities of infrared absorption spectral lines are established for an adsorbate monolayer near the point of the order-to-disorder orientational phase transition. The results obtained are consistent with the experimentally observed temperature dependences of the vibrational spectra for the 2 × 1 monolayer CO/NaCl (100). For this system, the Davydov splitting does not vanish on the phase transition due to a nonzero value of the average dipole moment (unlike molecular crystals and orientationally planar monolayers in which the average dipole moment becomes zero at the phase transition temperature).     

Conductivity of the Two-Dimensional Rayleigh Model Near the Percolation Threshold: A Subthreshold Region of Concentrations

The conductivity of the two-dimensional Rayleigh model near a critical point, the percolation threshold, is investigated. The effective conductivity of the model with a metal–prefect conductor phase transition is calculated in the binary (pair) approximation. For the alternative model with a metal–insulator phase transition the corresponding effective conductivity is determined from the Keller–Dykhne reciprocity relation.     

"Metallic" and "insulating" behavior of the two-dimensional electron gas on a vicinal surface of Si MOSFET'S

The resistance R of the 2DEG on the vicinal Si surface shows unusual behavior which is very different from that in Si (100) MOSFET's studied earlier. The low-temperature crossover from dR/dT<0 ("insulator") to dR/dT>0 ("metal") occurs at a low resistance of R(c)square approximately 0.04xh/e2. This crossover, which we attribute to the existence of a narrow impurity band at the interface, is accompanied by a distinct hysteresis in the resistance. At higher temperatures, another change in the sign of dR/dT is seen. We describe it by temperature dependent impurity scattering of the 2DEG near the transition from the degenerate to nondegenerate state.     

Quantum phase transition and entanglement in Heisenberg XX spin chain with impurity

In this paper,we study the quantum phase transition and the effect of impurity on the thermal entanglement between any two lattices in three-qubit Heisenberg XX chain in a uniform magnetic field.We show that the quantum phase transition always appears when impurity parameter is an arbitrary constant and unequal to zero,the external magnetic field and impurity parameters have a great effect on it.Also,there exists a relation between the quantum phase transition and the entanglement.By modulating the temperature,magnetic field and the impurity parameters,the entanglement between any two lattices can exhibit platform-like behaviour,which can be used to realize entanglement switch.     

Metal–insulator transition in boron-doped amorphous carbon films

Boron-doped amorphous graphite-like carbon (GLC) films have been prepared with different boron concentrations. Electrical transport measurements in the temperature range 1.3–300?K on the films shows a doping-induced metal–insulator (MI) transition. On the metallic side of the transition, the experimental data are interpreted in terms of weak localization and the effect of electron–electron interactions. Data on the insulator side of transition are analyzed in terms of hopping conduction. Critical behaviour is observed near the transition, with the resistivity obeying a power-law temperature dependence.     

CPA density of states and conductivity in a double-exchange system containing impurities

We study density of states and conductivity of the doped double-exchange system, treating interaction of charge carriers both with the localized spins and with the impurities in the coherent potential approximation. It is shown that under appropriate conditions there is a gap between the conduction band and the impurity band in paramagnetic phase, while the density of states is gapless in ferromagnetic phase. This can explain metal-insulator transition frequently observed in manganites and magnetic semiconductors. Activated conductivity in the insulator phase is numerically calculated. Received 13 June 2000 and Received in final form 5 January 2001     

Dynamics of a locally distorted impurity in a host crystal with displacive phase transition

We study the dynamic behaviour of a soft displacive impurity in a host crystal undergoing a displacive phase transition. The impurity-induced localized mode and the dynamic autocorrelation function of the impurity below the local freezing temperature are investigated in mean-field approximation (MFA). Furthermore, we give a physical interpretation of the MFA result of the local freezing-out, and discuss the fluctuation behaviour of various types of impurities in relation to recent experiments.     

Low-field carrier transport properties in biased bilayer graphene

Based on a semiclassical Boltzmann transport equation in random phase approximation, we develop a theoretical model to understand low-field carrier transport in biased bilayer graphene, which takes into account the charged impurity scattering, acoustic phonon scattering, and surface polar phonon scattering as three main scattering mechanisms. The surface polar optical phonon scattering of carriers in supported bilayer graphene is thoroughly studied using the Rode iteration method. By considering the metal–BLG contact resistance as the only one free fitting parameter, we find that the carrier density dependence of the calculated total conductivity agrees well with that observed in experiment under different temperatures. The conductivity results also suggest that in high carrier density range, the metal–BLG contact resistance can be a significant factor in determining the BLG conductivity at low temperature, and both acoustic phonon scattering and surface polar phonon scattering play important roles at higher temperature, especially for BLG samples with a low doping concentration, which can compete with charged impurity scattering.     

Impurity-induced anisotropic semiconductor-semimetal transition in monolayer biased black phosphorus

Taking into account the electron-impurity interaction within the continuum approximation of tight-binding model, the Born approximation, and the Green's function method, the main features of anisotropic electronic phase transition are investigated in monolayer biased black phosphorus (BP). To this end, we concentrated on the disordered electronic density of states (DOS), which gives useful information for electro-optical devices. Increasing the impurity concentration in both unbiased and biased impurity-infected single-layer BP, in addition to the decrease of the band gap, independent of the direction, leads to the midgap states and an extra Van Hove singularity inside and outside of the band gap, respectively. Furthermore, strong impurity scattering potentials lead to a semiconductor-semimetal transition and one more Van Hove singularity in x-direction of unbiased BP and surprisingly, this transition does not occur in biased BP. We found that there is no phase transition in y-direction. Since real applications require structures with modulated band gaps, we have studied the influence of different bias voltages on the disordered DOS in both directions, resulting in the increase of the band gap.     

氧化钒晶体的半导体至金属相变的理论研究

本文利用第一性原理方法研究了金红石相和单斜相VO₂晶体的电子结构和热力学性质.在计算中采用局域密度近似结合Hubbard U模型(LDA+U)描述电子的局域强关联效应,同时也利用微扰密度泛函方法计算了两种相结构的声子谱.计算结果表明V原子3d电子轨道中x²-y²轨道能级分裂决定了VO₂晶体在不同相结构下的金属和绝缘体特性.零温状态方程计算揭示了在68 GPa时可以发生从单斜结构 关键词： 2')" href="#">VO₂ 相变 第一性原理     

Topological Phase Transition with Larger Winding Number in the Non-Hermitian Dimerized Lattice

The topological phase transitions among normal insulator phase, two kinds of topological insulator phases, and topological semimetal phase are shown based on the non-Hermitian dimerized Su–Schrieffer–Heeger (SSH) model with the nonreciprocal intercell and long-range hopping. In contrast to the previous work, it is found that the topological insulator phase in the present SSH model can hold the larger non-Bloch winding number accompanied by exceptional winding of the generalized Brillouin zone around the gap-closing points. Compared with the usual topological insulator phase in non-Hermitian SSH model, the topological insulator with the larger winding number owns two pairs of zero energy modes with a distinct form of edge localization in the gap. The physical mechanism of the distinct edge localization for zero energy modes via a equivalent Hermitian version of the non-Hermitian SSH model is revealed. Additionally, the process of the phase transition is visualized among normal insulator phase, topological insulator phases, and topological semimetal phase in detail via the evolution of the gap-closing points on the plane of generalized Brillouin zone. This work further verifies the non-Bloch theory and enrich the investigation about the topologically nontrivial phase with the larger topological invariant in the non-Hermitian SSH model.     

Interacting bosons in an optical lattice

A strongly interacting Bose gas in an optical lattice is studied using a hard‐core interaction. Two different approaches are introduced, one is based on a spin‐1/2 Fermi gas with attractive interaction, the other one on a functional integral with an additional constraint (slave‐boson approach). The relation between fermions and hard‐core bosons is briefly discussed for the case of a one‐dimensional Bose gas. For a three‐dimensional gas we identify the order parameter of the Bose‐Einstein condensate through a Hubbard‐Stratonovich transformation and treat the corresponding theories within a mean‐field approximation and with Gaussian fluctuations. This allows us to evaluate the phase diagram, including the Bose‐Einstein condensate and the Mott insulator, the density‐density correlation function, the static structure factor, and the quasiparticle excitation spectrum. The role of quantum and thermal fluctuations are studied in detail for both approaches, where we find good agreement with the Gross‐Pitaevskii equation and with the Bogoliubov approach in the dilute regime. In the dense regime, which is characterized by the phase transition between the Bose‐Einstein condensate and the Mott insulator, we discuss a renormalized Gross‐Pitaevskii equation. This equation can describe the macroscopic wave function of the Bose‐Einstein condensate in the dilute regime as well as close to the transition to the Mott insulator. Finally, we compare the results of the attractive spin‐1/2 Fermi gas and those of the slave‐boson approach and find good agreement for all physical quantities.     

Ferrimagnetism and metal–insulator transition in an organic polymer chain

The ferrimagnetism and quantum phase transition of a bipartite lozenge periodic Anderson-like organic polymer, in which the localized f electrons hybridize with the odd site conduction orbitals, are investigated by means of Green＇s function theory. The ground state turns out to be gapless ferrimagnetism. At a finite temperature, the ferrimagnetic-to- paramagnetic phase transition takes place. The Kondo screenings and Ruderman-Kittel-Kasuya-Yosida （RKKY） inter- action can reduce and increase the transition temperature, respectively. Two Kondo screenings compete with each other, giving rise to the localized f electron spin screened antiferromagnetically. Accordingly, in a magnetic field, all spins are aligned along the chain easily, which is associated with metal-insulator transition. Furthermore, in a temperature-field plane, we reveal the gapless and spin polarized phases, which are characterized by susceptibility and specific heat, and whose behaviours are determined by the competition between the up-spin and down-spin hole excitations.     

液体-液体相变与反常特性

绝大多数物质的液态密度随温度降低而增大,即常见的热胀冷缩现象.但存在一类物质,如水及第四主族的硅、锗等,其液态密度在一定温度范围内随温度的升高而增大,即密度反常现象.此外,该类物质还存在动力学反常(密度越大粒子运动越快)、热力学反常(热力学量的涨落随温度降低而升高)等其他反常特性.这类材料的化学性质千差万别,但却具有相似的物理反常特性.进一步的理论研究发现部分材料具有两种液态,即高密度液态和低密度液态,两者之间存在一级相变.因此,反常特性与液体-液体相变是否有直接关联是一个值得深入研究的课题.本文主要介绍了具有液体-液体相变的一类材料及其反常特性,包括高温高压下氢的液体-液体相变及其超临界现象,镓的反常特性及其与液体-液体相变的关联等.     

Detection of topological transitions by transport through molecules and nanodevices

We analyze the phase transitions of an interacting electronic system weakly coupled to free-electron leads by considering its zero-bias conductance. This is expressed in terms of two effective impurity models for the cases with and without spin degeneracy. Using the half-filled ionic Hubbard ring, we demonstrate that the weight of the first conductance peak as a function of external flux or of the difference in gate voltages between even and odd sites allows one to identify the topological charge transition between a correlated insulator and a band insulator.