Quantum criticality between topological and band insulators in 3+1 dimensions

Four-component massive and massless Dirac fermions in the presence of long range Coulomb interaction and chemical potential disorder exhibit striking fermionic quantum criticality. For an odd number of flavors of Dirac fermions, the sign of the Dirac mass distinguishes the topological and the trivial band insulator phases, and the gapless semimetallic phase corresponds to the quantum critical point that separates the two. Up to a critical strength of disorder, the semimetallic phase remains stable, and the universality class of the direct phase transition between two insulating phases is unchanged. Beyond the critical strength of disorder the semimetallic phase undergoes a phase transition into a disorder controlled diffusive metallic phase, and there is no longer a direct phase transition between the two types of insulating phases.     

Coexistence of Charge Order and Antiferromagnetic Order in an Extended Periodic Anderson Model

The competition between the RKKY interaction and the Kondo effect leads to a magnetic phase transition,which occurs ubiquitously in heavy fermion materials.However,there are more and more experimental evidences indicating that the valence fluctuation plays an essential role in the Ce-and Y-based compounds.We study an extended periodic Anderson model(EPAM) which includes the onsite Coulomb repulsion Ucf between the localized electrons and conduction electrons.By employing the density matrix embed...     

Surface phase transitions of multiple-site associating fluids

Surface phase transitions of Lennard–Jones (LJ) based two- and four-site associating fluids have been studied for various associating strengths using grand-canonical transition matrix Monte Carlo simulations. Our results suggest that, in the case of a smooth surface, represented by a LJ 9-3-type potential, multiple-site associating fluids display a prewetting transition within a certain temperature range. However, the range of the prewetting transition decreases with increasing associating strength and increasing number of sites on the fluid molecules. With the addition of associating sites on the surface, a quasi-2D vapor–liquid transition may appear, which is observed at a higher surface site density for weaker associating fluids. The prewetting transition at lower associating strength is found to shift towards the quasi-2D vapor–liquid transition with increasing surface site density. However, for highly associating fluids, the prewetting transition is still intact, but shifts slightly towards the lower temperature range. Adsorption isotherms, chemical potentials and density profiles are used to characterize surface phase transitions.     

Charged particles on a two-dimensional lattice subject to anisotropic Jahn-Teller interactions

The properties of a system of charged particles on a 2D lattice, subject to an anisotropic Jahn-Teller-type interaction and 3D Coulomb repulsion, are investigated. In the mean-field approximation without Coulomb interaction, the system displays a phase transition of first order. When the long-range Coulomb interaction is included, Monte Carlo simulations show that the system displays very diverse mesoscopic textures, ranging from spatially disordered pairs to ordered arrays of stripes, or charged clusters, depending only on the ratio of the two interactions (and the particle density). Remarkably, charged objects with an even number of particles are more stable than with an odd number of particles. We suggest that the diverse functional behavior-including superconductivity-observed in oxides can be thought to arise from the self-organization of this type.     

Ising analogue to compact-star matter

By constructing an Ising analogue of compact-star matter at subsaturation density we explored the effect of Coulomb frustration on the nuclear liquid-gas phase transition. Our conclusion is twofold. First, the range of temperatures where inhomogeneous phases form expands with increasing Coulomb-field strength. Second, within the approximation of uniform electron distribution, the limiting point upon which the phase-coexistence region ends does not exhibit any critical behavior. Possible astrophysics consequences and thermodynamical connections are discussed.     

Transport in two dimensional electronic micro-emulsions

In two dimensional electron systems with Coulomb or dipolar interactions, a direct transition, whether first or second order, from a liquid to a crystalline state is forbidden. As a result, between these phases there must be other (micro-emulsion) phases which can be viewed as a meso-scale mixture of the liquid and crystalline phases. We investigate the transport properties of these new electronic phases and present arguments that they are responsible for the various transport anomalies that have been seen in experiments on the strongly correlated 2DEG in high mobility semiconductor devices with low electron densities.     

Exploration of the phase diagram of 5D anisotropic SU(2) gauge theory

In this paper we attempt a non-perturbative study of the five-dimensional, anisotropic SU(2) gauge theory on the lattice using Monte Carlo techniques. Our goal is the exploration of the phase diagram, defining the various phases and the critical boundary lines. Three phases appear, two of them are continuations of the Strong and the Weak coupling phases of pure 4d SU(2) to non-zero coupling β^′${\beta }^{\prime }$ in the fifth transverse direction and they are separated by a crossover transition, while the third phase is a 5D Coulombic phase. We provide evidence that the phase transition between the 5D Coulomb phase and the Weak coupling phase is a second order phase transition. Assuming that this result is not altered when increasing the lattice volume we give a first estimate of the associated critical exponents. This opens the possibility for a continuum effective five-dimensional field theory.     

Catalysis of the 〈b̄b〉 Condensate in the Composite Higgs Model

Increasing external pressure gives rise to s–d electron transfer in calcium that results in the localization of the charge density in the interstices of the crystal structure, i.e., the formation of an electride. The corresponding electronic states are partially filled and localized and, hence, electronic correlations could arise. We have carried out theoretical calculations for the high-pressure phases of Ca taking into account the Coulomb interactions between the electronic states centered on the interstitial site. The results of our calculations and proposed microscopic model showed that the structural phase transition under high pressure is due to an interplay of hybridization and correlation effects. Furthermore, it was found that the Coulomb repulsion can explain the experimentally observed anomalous increase in resistivity of the simple cubic phase of calcium under pressure.

     

Tunable charge density wave in TiS_3 nanoribbons

Recently, modifications of charge density wave(CDW) in two-dimensional(2D) show intriguing properties in quasi-2D materials such as layered transition metal dichalcogenides(TMDCs). Optical, electrical transport measurements and scanning tunneling microscopy uncover the enormous difference on the many-body states when the thickness is reduced down to monolayer. However, the CDW in quasi-one-dimensional(1D) materials like transition metal trichalcogenides(TMTCs) is yet to be explored in low dimension whose mechanism is likely distinct from their quasi-2D counterparts.Here, we report a systematic study on the CDW properties of titanium trisulfide(TiS_3). Two phase transition temperatures were observed to decrease from 53 K(103 K) to 46 K(85 K) for the bulk and 15-nm thick nanoribbon, respectively,which arises from the increased fluctuation effect across the chain in the nanoribbon structure, thereby destroying the CDW coherence. It also suggests a strong anisotropy of CDW states in quasi-1D TMTCs which is different from that in TMDCs.Remarkably, by using back gate of-30 V ～ 70 V in 15-nm device, we can tune the second transition temperature from110 K(at-30 V) to 93 K(at 70 V) owing to the altered electron concentration. Finally, the optical approach through the impinging of laser beams on the sample surface is exploited to manipulate the CDW transition, where the melting of the CDW states shows a strong dependence on the excitation energy. Our results demonstrate TiS_3 as a promising quasi-1D CDW material and open up a new window for the study of collective phases in TMTCs.     

Nonexponential relaxations in a two-dimensional electron system in silicon

The relaxations of conductivity have been studied in a strongly disordered two-dimensional (2D) electron system in Si after excitation far from equilibrium by a rapid change of carrier density ns at low temperatures T. The dramatic and precise dependence of the relaxations on ns and T strongly suggests (a) the transition to a glassy phase as T-->0, and (b) the Coulomb interactions between 2D electrons play a dominant role in the observed out-of-equilibrium dynamics.     

Ground states of the one-dimensional anisotropic extended hubbard model

We investigate a one-dimensional half-filled electron system with on-site and spin-dependent nearest-neighbor-site Coulomb interactions. Non-Tomonaga-Luttinger-type scatterings bring about Gaussian and hidden SU(2) Berezinskii-Kosterlitz-Thouless transitions in the charge and spin parts. We accurately determine these critical points using the level-spectroscopy method. The boundaries deviate from those given by bosonization prediction with the increase of interactions. In particular, in the easy-plain anisotropy region, we find a crossing point of transition lines in the charge and spin parts, which is a multicritical point of four phases. We also check consistencies among excitation levels.     

The Critical Properties of One—Dimensional Extended Hubbard Model

In the framework of nonperturbative quantum field theory,the critical phenomena of one-dimensional extended Hubbard model (EHM) at half-filling are discussed from weak to intermediate interactions.After the EHM being mapped into two decoupled sine-Gordon models,the ground state phase diagram of the system is derived in an explicit way.It is confirmed that the coexisting phases appear in different interaction regimes which cannot be found by conventional theoretical methods.The diagram shows that there are seven different phase regions in the ground state,which seems not to be the same as previous discussions,especially the boundary between the phase separation and condensed phase regions.The phase transition properties of the model between various phase regions are studied in detail.     

Reduced effective spin-orbital degeneracy and spin-orbital ordering in paramagnetic transition-metal oxides: Sr2IrO4 versus Sr2RhO4

We discuss the notions of spin-orbital polarization and ordering in paramagnetic materials, and address their consequences in transition-metal oxides. Extending the combined density functional and dynamical mean field theory scheme to the case of materials with large spin-orbit interactions, we investigate the electronic excitations of the paramagnetic phases of Sr(2)IrO(4) and Sr(2)RhO(4). We show that the interplay of spin-orbit interactions, structural distortions and Coulomb interactions suppresses spin-orbital fluctuations. As a result, the room temperature phase of Sr(2)IrO(4) is a paramagnetic spin-orbitally ordered Mott insulator. In Sr(2)RhO(4), the effective spin-orbital degeneracy is reduced, but the material remains metallic, due to both, smaller spin-orbit and smaller Coulomb interactions. The corresponding spectra are in excellent agreement with photoemission data. Finally, we make predictions for the spectra of paramagnetic Sr(2)IrO(4).     

Magnetic properties and critical behavior of quasi-2D layered Cr4Te5 thin film

Quasi-2D layered Cr₄Te₅ thin film has attracted great attention because it possesses the high Curie temperature close to room temperature and relatively large saturation magnetization. However, the magnetic interactions and the nature of magnetic phase transition in the Cr₄Te₅ film have not been explored thoroughly. In this paper, we focused on the critical behavior of its magnetic phase transition through the epitaxial Cr₄Te₅ film fabricated by pulsed laser deposition (PLD). The final critical exponents β = 0.359(2) and γ = 1.54(2) were obtained by linear extrapolation together with Arrott-Noakes equation of state, and their accuracy was confirmed by using the Widom scaling relation and scaling hypothesis. We find that some magnetic disorders exist in the Cr₄Te₅ film system, which is related to Cr₄Te₅ critical behavior why its critical behavior is quite far from any conventional universality class. Furthermore, we also determined that the Cr₄Te₅ film exhibits a quasi-2D long-range magnetic interaction. Finally, the itinerant ferromagnets of Cr₄Te₅ films were confirmed by the Takahashi’s self-consistent renormalization theory of spin fluctuations. Our work provides a new idea for understanding the mechanism of magnetic interactions in similar 2D layered films.     

Coulomb interaction and semimetal-insulator transition in graphene

The strong Coulomb interaction between massless Dirac fermions can drive a semimetal-insulator transition in single-layer graphene by dynamically generating an excitonic fermion gap. There is a critical interaction strength λc that separates the semimetal phase from the insulator phase. We calculate the specific heat and susceptibility of the system and show that they exhibit distinct behaviors in the semimetal and insulator phases.     

Effects of Thermally Induced Roton-Like Excitation on the Superfluid Density of a Quasi-2D Dipolar Bose Condensed Gas

Using the Green's function approach, the density–density correlation function and the dielectric function in the random-phase approximation for a quasi-two-dimensional (quasi-2D) dipolar Bose gas are derived. From the pole of the density correlation function, by considering thermally induced roton-like excitations, the excitation spectrum of the system is calculated. It is shown that the position and depth of the roton minimum of the excitation spectrum are tunable by changing the temperature. To show how the position of the roton minimum influences the phenomenon of superfluidity, the superfluid density of the system is obtained and it is shown that the interplay of the thermal rotonization, contact and dipole–dipole interaction (DDI) can affect the superfluid fraction of a quasi-2D Bose gas. It is found that contact, DDI interactions, and thermally induced rotons enhance the fluctuations and reduce the superfluid density. In the absence of DDI and thermally induced rotons, the usual T³ dependence of superfluid density in 2D is obtained and the correction T⁴ term arises from DDI. It is shown that if the roton minimum is close to zero, the thermally induced rotons change the linear temperature dependence of the superfluid fraction, leading to a transition to nontrivial supersolid phase.     

Cavity‐Photon‐Induced High‐Order Transitions between Ground States of Quantum Dots

It is shown that quantum electromagnetic transitions to high orders are essential to describe the time‐dependent path of a nanoscale electron system in a Coulomb blockade regime when coupled to external leads and placed in a 3D rectangular photon cavity. The electronic system consists of two quantum dots embedded asymmetrically in a short quantum wire. The two lowest in energy spin degenerate electron states are mostly localized in each dot with only a tiny probability in the other dot. In the presence of the leads, a slow high‐order transition between the ground states of the two quantum dots is identified. The Fourier power spectrum for photon–photon correlations in the steady state shows a Fano type of resonance for the frequency of the slow transition. Full account is taken of the geometry of the multilevel electronic system, and the electron–electron Coulomb interactions together with the para‐ and diamagnetic electron–photon interactions are treated with step‐wise exact numerical diagonalization and truncation of appropriate many‐body Fock spaces. The matrix elements for all interactions are computed analytically or numerically exactly.     

Quantum phase transition in quasi-one-dimensional BaRu6O12

We report the first systematic study of the electrical transport and magnetic properties of BaRu6O12, which has a quasi-one-dimensional (quasi-1D) hollandite structure. We show that BaRu6O12 is quasi-1D electronically as well. Its physical properties were found to be extremely sensitive to disorder. Furthermore, a transition from being metallic with a resistance drop around 2 K to being weakly insulating as the applied magnetic field was increased was also found. We propose that these two features are related to the possible presence of a quantum phase transition in this material system.     

Bose-einstein condensation in quasi-2D trapped gases

We discuss Bose-Einstein condensation (BEC) in quasi-2D trapped gases and find that well below the transition temperature T(c) the equilibrium state is a true condensate, whereas at intermediate temperatures T相似文献   

Dynamical singlets and correlation-assisted Peierls transition in VO2

A theory of the metal-insulator transition in vanadium dioxide from the high- temperature rutile to the low- temperature monoclinic phase is proposed on the basis of cluster dynamical mean-field theory, in conjunction with the density functional scheme. The interplay of strong electronic Coulomb interactions and structural distortions, in particular, the dimerization of vanadium atoms in the low-temperature phase, plays a crucial role. We find that VO2 is not a conventional Mott insulator, but that the formation of dynamical V-V singlet pairs due to strong Coulomb correlations is necessary to trigger the opening of a Peierls gap.