Interacting lattice electrons with disorder in two dimensions: Numerical evidence for a metal-insulator transition with a universal critical conductivity
Prabuddha B. Chakraborty, Krzysztof Byczuk, Dieter Vollhardt
The dc-conductivity of electrons on a square lattice interacting with a local repulsion in the presence of disorder is computed by means of quantum Monte Carlo simulations. We provide evidence for the existence of a transition from an Anderson insulator to a correlated disordered metal with a universal value of the critical dc-conductivity σ_{dc,crit} = (1.18 \pm 0.06) e^{2}/h at the transition.
Using uniaxial stress to tune Si:B through the metal-insulator transition we find the conductivity at low temperatures shows an excellent fit to scaling with temperature and stress on both sides of the transition. The scaling functions yield the conductivity in the metallic and insulating phases, and allow a reliable determination of the temperature dependence in the critical regions on both sides of the transition.
We calculate the optical conductivity of one-dimensional Mott insulators at low energies using a field theory description. The square root singularity at the optical gap, characteristic of band insulators, is generally absent and appears only at the Luther-Emery point. We also show that only few particle processes contribute significantly to the optical conductivity over a wide range of frequencies and that the bare perturbative regime is recovered only at very large energies. We discuss possible applications of our results to quasi one-dimensional organic conductors.
The established scaling laws of the conductivity with temperature and doping are strong indications for the quantum nature of the metal-insulator transition in YH$_x$. Here we report the first results on the frequency scaling of the conductivity. Samples were brought from the insulating to the metallic phase by carrier doping via illumination. In the metallic phase, the sub-terahertz conductivity coincides with the dc data. These results do not agree with the simplest picture of a quantum-phase transition.
A recent paper on the insulating state of monolayer WTe2 reported the observation of large oscillations in the conductivity that are periodic in 1/B, resembling quantum oscillations in metals. This remarkable observation has inspired suggestions of exotic physics such as spin-charge separation. We show that a rather more conventional but still nontrivial explanation in terms of gap modulation may be possible in a model of excitonic insulator subject to a magnetic field.
This paper reports thermopower and conductivity measurements through the metal-insulator transition for 2-dimensional electron gases in high mobility Si-MOSFETs. At low temperatures both thermopower and conductivity show critical behaviour as a function of electron density. When approaching the critical density from the metallic side the diffusion thermopower appears to diverge and the conductivity vanishes. On the insulating side the thermopower shows an upturn with decreasing temperature. These features have much in common with those expected for an Anderson transition.
We study the thermal conductivity in the excitonic insulator using a simple quasi one-dimensional two-band model consisting of electron and hole bands with the Coulomb interactions between these bands. Based on the linear response theory within a mean-field scheme, we develop a method to identify the contributions to thermal conductivity driven by excitonic insulator. It is found that there is an additional heat current operator owing to the excitonic phase transition, and that it gives contributions to thermal conductivity which are not expressed in the form of Sommerfeld-Bethe relations writ…
The degenerate free Fermi gas coupled to a random potential is used to compute a.c. conductivity in various dimensions. We first formally diagonalise the hamiltonian using an appropriate basis that is a functional of the disorder potential. Then we compute the a.c. conductivity at zero temperature using the Kubo formula. This a.c. conductivity is a functional of the disordered potential. The wavefunction of extended states is written as exponential of the logarithm. We use the cumulant expansion to compute the disordered averaged a.c. conductivity for Gaussian disorder. The formula is valid if…
Applying newly developed dynamical density matrix renormalization group techniques at zero and finite temperatures to a Hubbard-Holstein model at half-filling, we examine the optical conductivity of a typical one-dimensional Mott insulator Sr_2CuO_3. We find a set of parameters in the Hubbard-Holstein model, which can describe optical conductivity for both Mott-gap excitation in the high-energy region and phonon-assisted spin excitation in the low-energy region. We also find that electron-phonon interaction gives additional broadening in the temperature dependence of the Mott-gap excitation.
We study the optical response of a strongly correlated electron system near the metal-insulator transition using a mapping to the sine-Gordon model. With semiclassical quantization, the spectral weight is distributed between a Drude peak and absorption lines due to breathers. We calculate the Drude weight, the optical gap, and the lineshape of breather absorption.
We investigate the low-temperature electrical and thermal transport properties in atomically precise metallic heterostructures involving strongly-correlated electron systems. The model of the Mott-insulator/ band-insulator superlattice was discussed in the framework of the slave-boson mean-field approximation and transport quantities were derived by use of the Boltzmann transport equation in the relaxation-time approximation. The results for the optical conductivity are in good agreement with recently published experimental data on (LaTiO$_3)_N$/(SrTiO$_3)_M$ superlattices and allow us to esti…
The effect of disorder on a class of transition metal oxides described by a single orbital Hubbard model at half filling is investigated. The phases are characterized by the nature of the electronic and spin excitations. The frequency and temperature-dependent conductivity and spin susceptibility as functions of disorder are calculated. The interplay of disorder and electron-electron interaction produces unusual behavior in this system. For example, the dc conductivity, which is vanishingly small at low disorder in the Mott phase and at high disorder in the localized phase, gets surprisingly e…
We consider two-dimensional system of particles localized on randomly distributed sites of squared lattice with anisotropic transfer matrix elements between localized sites. By summing of "diffusion ladder" and "cooperon ladder" type vertices we calculated the conductivity for various sites and particles densities.
The ground state phase diagrams of the Falicov--Kimball model with local disorder is derived within the dynamical mean--field theory and using the geometrically averaged (''typical'') local density of states. Correlated metal, Mott insulator and Anderson insulator phases are identified. The metal--insulator transitions are found to be continuous. The interaction and disorder compete with each other stabilizing the metallic phase against occurring one of the insulators. The Mott and Anderson insulators are found to be continuously connected.
We investigate the metal insulator transitions at finite temperature for the Hubbard model with diagonal alloy disorder. We solve the dynamical mean field theory equations with the non crossing approximation and we use the coherent potential approximation to handle disorder. The excitation spectrum is given for various correlation strength $U$ and disorder. Two successive metal insulator transitions are observed at integer filling values as $U$ is increased. An important selective transfer of spectral weight arises upon doping. The strong influence of the temperature on the low energy dynamics…
Dynamical conductivity in a disordered one-dimensional model of interacting fermions is studied numerically at high temperatures and in the weak-interaction regime in order to find a signature of many-body localization and vanishing d.c. transport coefficients. On the contrary, we find in the regime of moderately strong local disorder that the d.c. conductivity sigma0 scales linearly with the interaction strength while being exponentially dependent on the disorder. According to the behavior of the charge stiffness evaluated at the fixed number of particles, the absence of the many-body localiz…
Kondo insulators and in particular their non-cubic representatives have remained poorly understood. Here we report on the development of an anisotropic energy pseudogap in the tetragonal compound CeRu$_4$Sn$_6$ employing optical reflectivity measurements in broad frequency and temperature ranges, and local density approximation plus dynamical mean field theory calculations. The calculations provide evidence for a Kondo insulator-like response within the $a-a$ plane and a more metallic response along the c axis and qualitatively reproduce the experimental observations, helping to identify their…
It is well established that for non-interacting electrons, increasing disorder drives a metal into a gapless localized Anderson insulator. While in three dimensions a threshold in disorder must be crossed for the transition, in two dimensions and lower, the smallest amount of disorder destabilizes the metal. The nature of the metal-insulator transition in an interacting system remains unresolved. Here we explore the effect of disorder on a strongly correlated Mott insulator without changing the carrier concentration. Angle resolved photoemission spectroscopy (ARPES) measurements on copper inte…
We report the frequency-dependent conductivity of the manganite system La1-xSrxMnO3 (x <= 0.2) when approaching the metal-insulator transition from the insulating side. Results from low-frequency dielectric measurements are combined with spectra in the infrared region. For low doping levels the behavior is dominated by hopping transport of localized charge carriers at low frequencies and by phononic and electronic excitations in the infrared region. For the higher Sr contents the approach of the metallic state is accompanied by the successive suppression of the hopping contribution at low freq…
We analyze optical conductivity with the goal to demonstrate experimental manifestation of a new state of matter, the so-called fermion condensate. Fermion condensates are realized in quantum spin liquids, exhibiting typical behavior of heavy fermion metals. Measurements of the low-frequency optical conductivity collected on the geometrically frustrated insulator herbertsmithite provide important experimental evidence of the nature of its quantum spin liquid composed of spinons. To analyze recent measurements of the herbertsmithite optical conductivity at different temperatures, we employ a mo…
Metal-insulator transitions driven by disorder (Delta) and/or by electron correlations (U) are investigated within the Anderson-Hubbard model with local binary-alloy disorder using a simple but consistent mean-field approach. The Delta-U phase diagram is derived and discussed for T=0 and finite temperatures.
Phonons are known to generate a thermal Hall effect in certain insulators, including oxides with rare-earth impurities, quantum paraelectrics, multiferroic materials and cuprate Mott insulators. In each case, a special feature of the material is presumed relevant for the underlying mechanism that confers chirality to phonons in a magnetic field. The question is whether a phonon Hall effect is an unusual occurrence - linked to special characteristics such as skew scattering off rare-earth impurities, structural domains, ferroelectricity, ferromagnetism - or a much more common property of insula…
Samarium hexaboride (SmB6) has recently been considered to be a topological Kondo insulator (TKI), the first strongly correlated electron system to exhibit topological surface conduction states. In this contribution, results of electrical resistivity measurements between 80 K and 0.08 K of various SmB6 single crystalline samples are presented, analyzed and discussed. The received results imply that the residual conductivity of SmB6 below about 4 K is of non-activated (metallic-like) nature. It is shown that this metallic-like behavior can be attributed both to surface (2D) conduction states, a…
A new thermodynamic phase resulting from the competition between a commensurate potential and disorder in interacting fermionic or bosonic systems is predicted. It requires interactions of finite extent. This phase, intermediate between the Mott insulator and the Anderson insulator, is both incompressible and has no gap in the conductivity. The corresponding phase is also predicted for commensurate classical elastic systems in presence of correlated disorder.
We apply the dynamical mean field theory (DMFT) in the iterative perturbation theory(IPT) to doubly degenerate eg bands and triply degenerate tg bands on a simple cubic lattice and calculate the spectrum and optical conductivity in arbitrary electron occupation. The spectrum simultaneously shows the effects of multiplet structure and DMFT together with the electron ionization and affinity levels of different electron occupations, coherent peaks at the Fermi energy in the metallic phase and a gap at an integer filling of electrons for sufficiently large Coulomb U. We also calculate the critical…
The electronic response of doped manganites at the transition from the paramagnetic insulating to the ferromagnetic metallic state in $\rm La_{1-x}Ca_{x}MnO_3$ for $\rm (x=0.3,0.2)$ was investigated by dc conductivity, ellipsometry, and VUV reflectance for energies between 0 and 24 eV. A stablized Kramers-Kronig transformation yields the optical conductivity and reveals changes in the optical spectral weight up to 24 eV at the metal to insulator transition. In the observed energy range, the spectral weight is conserved within $\rm 0.3 %$. The redistribution of spectral weight between low and h…
The optical properties of a V4O7 single crystal have been investigated from the high temperature metallic phase down to the low temperature antiferromagnetic insulating one. The temperature dependent behavior of the optical conductivity across the metal-insulator transition (MIT) can be explained in a polaronic scenario. Charge carriers form strongly localized polarons in the insulating phase as suggested by a far-infrared charge gap abruptly opening at T_MIT = 237 K. In the metallic phase instead the presence of a Drude term is indicative of fairly delocalized charges with a moderately renorm…
The spin-frustrated conductor Mn$_3$Pt exhibits a characteristic magnetic structure called partial disorder in which some spin sites can form magnetic order through the generation of non-ordered sites that locally relieve the frustration. Here we report the results of a single-crystal inelastic neutron scattering study of this compound. The measured momentum $\vec{Q}$ correlations of diffusive magnetic scattering reveal that the paramagnetic phase exhibits short-range spin fluctuations with the same type of partial disorder. Its relation to conductivity is also discussed.
We study the effect of disorder and doping on the metal-insulator transition in a repulsive Hubbard model on a square lattice using the determinant quantum Monte Carlo method. First, with the aim of making our results reliable, we compute the sign problem with various parameters such as temperature, disorder, on-site interactions, and lattice size. We show that in the presence of randomness in the hopping elements, the metal-insulator transition occurs and the critical disorder strength differs at different fillings. We also demonstrate that doping is a driving force behind the metal-insulator…
We examined the characteristics of a metal-insulator transition on a two-dimensional non-bipartite lattice when both disorders and electron interactions are present. Using a real-space renormalization group method and finite-size scaling analysis we have found that after the disordered system transits from the insulating state to the metal one, it can reenter an insulating state if there is a further increase in the electron-electron interactions.
A brief review of metal-insulator transitions in three-dimensional doped semiconductors, with emphasis on the "critical exponent puzzle"; and new experimental findings that signal the possibility of an unexpected metal-insulator transition in dilute two-dimensional systems, such as silicon MOSFETs and GaAs/AlGaAs heterostructures.
The effect of binary alloy disorder on the ferromagnetic phases of f-electron materials is studied within the periodic Anderson model. We find that disorder in the conduction band can drastically enhance the Curie temperature due to an increase of the local f-moment. The effect may be explained qualitatively and even quantitatively by a simple theoretical ansatz. The emergence of an alloy Kondo insulator at non-integer filling is also pointed out.
We discuss a semiclassical calculation of low energy charge transport in one-dimensional (1d) insulators with a focus on Mott insulators, whose charge degrees of freedom are gapped due to the combination of short range interactions and a periodic lattice potential. Combining RG and instanton methods, we calculate the nonlinear ac conductivity and interpret the result in terms of multi-photon absorption. We compare the result of the semiclassical calculation for interacting systems to a perturbative, fully quantum mechanical calculation of multi-photon absorption in a 1d band insulator and find…
The scaling of the conductivity at the superfluid-insulator quantum phase transition in two dimensions is studied by numerical simulations of the Bose-Hubbard model. In contrast to previous studies, we focus on properties of this model in the experimentally relevant thermodynamic limit at finite temperature T. We find clear evidence for deviations from w_k-scaling of the conductivity towards w_k/T-scaling at low Matsubara frequencies w_k. By careful analytic continuation using Pade approximants we show that this behavior carries over to the real frequency axis where the conductivity scales wit…
Magnetic field and temperature dependence of the Terahertz conductivity and permittivity of the colossal magnetoresistance manganite Pr_{0.65}Ca_{0.28}Sr_{0.07}MnO_3 (PCSMO) is investigated approaching the metal-to-insulator transition (MIT) from the insulating side. In the charge-ordered state of PCSMO both conductivity and dielectric permittivity increase as function of magnetic field and temperature. Universal scaling relationships between the changes in permittivity and conductivity are observed in a broad range of temperatures and magnetic fields. Similar scaling is also seen in La_{1-x}S…
We study the dc conductivity of a weakly disordered 2d Dirac electron gas with two bands and two spectral nodes, employing a field theoretical version of the Kubo--Greenwood conductivity formula. In this paper we are concerned with the question how the internode scattering affects the conductivity. We use and compare two established techniques for treating the disorder scattering: The perturbation theory, there ladder and maximally crossed diagrams are summed up, and the functional integral approach. Both turn out to be entirely equivalent. For a large number of random potential configurations…
Mean-field theory of non-interacting disordered electron systems is widely and successfully used to describe equilibrium properties of alloys in the whole range of disorder strengths. It, however, fails to take into account effects of quantum coherence and localizing back-scattering effects when applied to transport phenomena. We present an approximate scheme extending the mean-field theory for one-electron properties in that it offers a formula for the two-particle vertex and the electrical conductivity non-perturbatively including the leading-order vertex corrections in a way that the approx…
Recent experiments on Kondo insulator SmB6 have revealed bulk excitations that could be Fermions without charge. This would lead to a residual linear term in the thermal conductivity that was not observed universally in experiments. To solve this mystery, we introduce a novel symmetric geometry for performing thermal transport measurements under strains. Magnetic-field-independent residual linear terms were found in flux-grown SmB6 at nominal zero strain, and they grew under a tensile strain, which is known to modify Sm valence. A small growth-dependent difference in the Sm valence might expla…
When insulator LaAlO3 is grown by epitaxy onto a TiO2-terminated {100} surface of insulator SrTiO3, the resulting system has a metallic character. This phenomenon has been associated with an electrostatic frustration at the interface, as {100} surfaces of SrTiO3 are neutral while those of LaAlO3 are polar, but its microscopic mechanism is not quite understood. Here, we present a structural characterisation of this interface by aberration-corrected transmission electron microscopy. The unit cells at the interface appear elongated: we discuss this distortion in terms of electrostatic charge and …
We have completed a numerical investigation of the Anderson-Hubbard model for three-dimensional simple cubic lattices using a real-space self-consistent Hartree-Fock decoupling approximation for the Hubbard interaction. In this formulation we treat the spatial disorder exactly, and therefore we account for effects arising from localization physics. We have examined the model for electronic densities well away 1/2 filling, thereby avoiding the physics of a Mott insulator. Several recent studies have made clear that the combined effects of electronic interactions and spatial disorder can give ri…
We performed optical-pump terahertz-probe measurements of a Mott insulator YTiO$_{3}$ and a band semiconductor Si using a laser diode (1.47 eV) and a femtosecond pulse laser (1.55 eV). Both samples possess long energy-relaxation times (1.5 ms for YTiO$_{3}$ and 15 $μ$s for Si); therefore, it is possible to extract terahertz complex conductivities of photoinduced carriers under equilibrium. We observed highly contrasting behavior - Drude conductivity in Si and localized conductivity possibly obeying the Jonscher law in YTiO$_{3}$. The carrier number at the highest carrier-concentration layer in…
The electrical {\em dc}-conductivity of disordered, non-interacting electrons is calculated in the asymptotic limit of high lattice dimensions $d\to \infty$. To go beyond the lowest-order contribution in the expansion parameter $1/d$ of the single bubble diagram, vertex corrections are calculated from an asymptotic expression for the two-particle vertex. A mean-field approximation for the dc-conductivity containing the leading high-dimensional vertex corrections is proposed which is free of spurious non-analyticities, i.e. the conductivity is non-negative and shows no unphysical behavior in $d…
In a high mobility two-dimensional electron system in Si, near the critical density, $n_c=0.32\times10^{11}$cm$^{-2}$, of the apparent metal-to-insulator transition, the conductivity displays a linear temperature ($T$) dependence around the Fermi temperature. When $σ_0$, the extrapolated T=0 conductivity from the linear T-dependence, is plotted as a function of density, two regimes with different $σ_0(n)$ relations are seen, suggestive of two different phases. Interestingly, a sharp transition between these two regimes coincides with $n_c$, and $σ_0$ of the transition is $\sim$ $e^2/h$, the qu…
The role of orbital ordering on metal-insulator transition of transition-metal oxides is investigated by the cluster self-consistent field approach in the strong correlation regime. A clear dependence of the insulating gap on the orbital order parameter is found in the single-particle excitation spectra. The thermal fluctuation drives the orbital order-disorder transition, diminishes the gap and leads to the metal-insulator transition. The interplay between spins and orbits results in unusual temperature dependence of the orbital polarization in the orbital insulator, which can be seen in the …
We consider the effect of electron-electron interactions on the current-in-plane (CIP) conductivity and exchange coupling energy of a disordered metallic magnetic multilayer. We analyze its dependence on the value of ferromagnetic splitting of conducting electrons and ferromagnetic layers relative magnetizations orientation. We show that contribution to the CIP conductivity and exchange coupling energy as a periodic function of the angle of magnetizations relative orientation experience $ 2π\to π$ transition depending on the characteristic energies: ferromagnetic splitting of the conducting el…
Three-dimensional topological insulators feature Dirac-like surface states which are topologically protected against the influence of weak quenched disorder. Here we investigate the effect of surface disorder beyond the weak-disorder limit using large-scale numerical simulations. We find two qualitatively distinct regimes: Moderate disorder destroys the Dirac cone and induces diffusive metallic behavior at the surface. Even more remarkably, for strong surface disorder a Dirac cone reappears, as new weakly disordered "surface" states emerge in the sample beneath the disordered surface layer, wh…
The optical response of a system of two-dimensional Bose condensate droplets with an internal charge structure interacting with impurities is considered within the mode-coupling theory. It is found, that the glass transition in the system of droplets occurs at much stronger disorder as compared to the case of stuctureless point particles. The optical conductivity of the system is found to exhibit a complex oscillating structure, intimately related to the charge distribution within the droplets. The application of the model to the problem of the far IR optical conductivity of doped cuprates (in…
For most metals, increasing temperature (T) or disorder will quicken electron scattering. This hypothesis informs the Drude model of electronic conductivity. However, for so-called bad metals this predicts scattering times so short as to conflict with Heisenberg's uncertainty principle. Here we introduce the rare-earth nickelates (RNiO_3, R = rare earth) as a class of bad metals. We study SmNiO_3 thin films using infrared spectroscopy while varying T and disorder. We show that the interaction between lattice distortions and Ni-O bond covalence explains both the bad metal conduction and the ins…
We have conducted temperature and frequency dependent transport measurements in amorphous Nb_x Si_{1-x} samples in the insulating regime. We find a temperature dependent dc conductivity consistent with variable range hopping in a Coulomb glass. The frequency dependent response in the millimeter-wave frequency range can be described by the expression $sigma(omega) \propto (-\imath omega)^alpha$ with the exponent somewhat smaller than one. Our ac results are not consistent with extant theories for the hopping transport.
We study one-dimensional disordered fermions that either undergo metal-insulator transitions or topological phase transitions to become trivial Anderson insulators. We focus on using entanglement to elucidate how the spatial, momentum, and internal degrees of freedom of fermions are affected by the presence of disorder in such cases. We develop entanglement tools that reveal the existence of metallic states in the presence of disorder and further show clear signatures of the corresponding localization transition even in the presence of interactions. In systems where the internal degrees of fre…
We examine the transport properties of magnetically doped topological insulator (TI) thin films subject to correlated non-magnetic disorder. For the disorder we choose a quasi-periodic potential with a random phase. We restrict the disorder to a central region, which is coupled to two leads in a clean quantum spin Hall insulator (QSHI) state and concentrate on different orientations of the quasi-periodicity in the two-dimensional central region. In the case of a diagonally oriented or purely longitudinal quasi-periodicity we find different topological Anderson insulator (TAI) phases, with a qu…
We reinvestigate the behavior of the conductivity of several disordered quantum lattice models at infinite temperature using exact diagonalization. Contrary to the conclusion drawn in a recent investigation of similar quantities in identical systems, we find evidence of a localized regime for strong random fields. We estimate the location of the critical field for the many-body localization transition for the random-field XXZ spin chain, and compare our findings with recent investigations in related systems.
The mesoscopic fluctuations of the Density of electronic States (DoS) and of the conductivity of two- and three- dimensional lattices with randomly distributed substitutional impurities are studied. Correlations of the levels lying above (or below) the Fermi surface, in addition to the correlations of the levels lying on opposite sides of the Fermi surface, take place at half filling due to nesting. The Bragg reflections mediate to increase static fluctuations of the conductivity in the middle of the band which change the distribution function of the conductivity at half- filling.
We elucidate the mechanism by which a Mott insulator transforms into a non-Fermi liquid metal upon increasing disorder at half filling. By correlating maps of the local density of states, the local magnetization and the local bond conductivity, we find a collapse of the Mott gap toward a V-shape pseudogapped density of states that occurs concomitantly with the decrease of magnetism around the highly disordered sites but an increase of bond conductivity. These metallic regions percolate to form an emergent non-Fermi liquid phase with a conductivity that increases with temperature. Bond conducti…
The heat carriers responsible for the unexpectedly large thermal Hall conductivity of the cuprate Mott insulator La$_2$CuO$_4$ were recently shown to be phonons. However, the mechanism by which phonons in cuprates acquire chirality in a magnetic field is still unknown. Here, we report a similar thermal Hall conductivity in two cuprate Mott insulators with significantly different crystal structures and magnetic orders - Nd$_2$CuO$_4$ and Sr$_2$CuO$_2$Cl$_2$ - and show that two potential mechanisms can be excluded - the scattering of phonons by rare-earth impurities and by structural domains. Ou…
We present a theory of the metal-insulator transition in a disordered two-dimensional electron gas. A quantum critical point, separating the metallic phase which is stabilized by electronic interactions, from the insulating phase where disorder prevails over the electronic interactions, has been identified. The existence of the quantum critical point leads to a divergence in the density of states of the underlying collective modes at the transition, causing the thermodynamic properties to behave critically as the transition is approached. We show that the interplay of electron-electron interac…
We study a two dimensional, two-band double-exchange model for $e_g$ electrons coupled to Jahn-Teller distortions in the presence of quenched disorder using a recently developed Monte-Carlo technique. In the absence of disorder the half-filled system at low temperatures is an orbitally ordered ferromagnetic insulator with a staggered pattern of Jahn-Teller distortions. We examine the finite temperature transition to the orbitally disordered phase and uncover a qualitative difference between the intermediate and strongly coupled systems, including a thermally driven insulator to metal crossover…
Local magnetic moments in disordered sytems can be described in terms of annealed magnetic disorder, in addition to the underlying quenched disorder. It is shown that for noninteracting electron systems at zero temperature, the annealed disorder leads to a new mechanism, and a new universality class, for a metal-insulator transition. The transition is driven by a vanishing of the thermodynamic density susceptibility rather than by localization effects. The critical behavior near two-dimensions is determined, and the underlying physics is discussed.
The fluctuation conductivity $σ_{\rm s}$ in bulk superconductors with non s-wave pairing and with nonmagnetic disorder of strength $D$ is studied at low $T$ and within the Gaussian approximation. It is shown by assuming a quasi two-dimensional (2D) electronic state that, only if the gap function $d_μ({\p})$ is, as in a 2D p-wave pairing state, linear in the in-plane (relative) momentum ${\p}_\perp$, the in-plane fluctuation conductivity on the line $D=D_c$ is weakly divergent in low $T$ limit. The present result may be useful in clarifying the true gap function of spin-triplet ${\rm Sr_2RuO_4}…
The disorder-driven metal-insulator transition in the quantum spin Hall systems is studied by scaling analysis of the Thouless conductance $g$. Below a critical disorder strength, the conductance is independent of the sample size $M$, an indication of critically delocalized electron states. The calculated beta function $β=d\ln g/d\ln M$ indicates that the metal-insulator transition is Kosterlitz-Thouless (KT) type, which is characterized by bounding and unbounding of vortex-antivortex pairs of the local currents. The KT like metal-insulator transition is a basic characteristic of the quantum s…
