Disorder-driven Metal-insulator Transition Research Articles

Disorder-induced Anderson-like localization for bidimensional thermoelectrics optimization

Disorder-induced Anderson-like localization for bidimensional thermoelectrics optimization

In Situ Study of Vacancy Disordering in Crystalline Phase-Change Materials Under Electron Beam Irradiation

A disorder-driven metal insulator transition has been reported in crystalline Ge-Sb-Te phase-change materials (PCMs). The high concentration and statistical distribution of atomic vacancies were identified as the key factors in shaping the localization properties of electrons and, thus, the electrical transport. Vacancy ordering has been consistently observed in crystalline Ge-Sb-Te thin films upon thermal annealing, triggering a structural transition from a cubic rocksalt structure to a layered hexagonal structure and an insulator to metal transition. In this work, we demonstrate an opposite vacancy disordering process upon extensive electron beam irradiation, which is accompanied by the reverse transition from the stable hexagonal phase to the metastable cubic phase. The combined in situ transmission electron microscopy experiments and density functional theory nudged elastic band calculations reveal three transition stages, including (I) the vacancy diffusion in the hexagonal phase, (II) the change in atomic stacking, and (III) the disappearance of vacancy-rich planes. The mechanism of vacancy disordering is attributed to kinetic knock-on collision effects of the high-energy electron beam, which prevail over the heating effects.

A Review on Disorder-Driven Metal-Insulator Transition in Crystalline Vacancy-Rich GeSbTe Phase-Change Materials.

Metal–insulator transition (MIT) is one of the most essential topics in condensed matter physics and materials science. The accompanied drastic change in electrical resistance can be exploited in electronic devices, such as data storage and memory technology. It is generally accepted that the underlying mechanism of most MITs is an interplay of electron correlation effects (Mott type) and disorder effects (Anderson type), and to disentangle the two effects is difficult. Recent progress on the crystalline Ge1Sb2Te4 (GST) compound provides compelling evidence for a disorder-driven MIT. In this work, we discuss the presence of strong disorder in GST, and elucidate its effects on electron localization and transport properties. We also show how the degree of disorder in GST can be reduced via thermal annealing, triggering a disorder-driven metal–insulator transition. The resistance switching by disorder tuning in crystalline GST may enable novel multilevel data storage devices.

Disorder-Driven Metal-Insulator Transitions in Deformable Lattices.

We show that, in the presence of a deformable lattice potential, the nature of the disorder-driven metal-insulator transition is fundamentally changed with respect to the noninteracting (Anderson) scenario. For strong disorder, even a modest electron-phonon interaction is found to dramatically renormalize the random potential, opening a mobility gap at the Fermi energy. This process, which reflects disorder-enhanced polaron formation, is here given a microscopic basis by treating the lattice deformations and Anderson localization effects on the same footing. We identify an intermediate "bad insulator" transport regime which displays resistivity values exceeding the Mott-Ioffe-Regel limit and with a negative temperature coefficient, as often observed in strongly disordered metals. Our calculations reveal that this behavior originates from significant temperature-induced rearrangements of electronic states due to enhanced interaction effects close to the disorder-driven metal-insulator transition.

On holographic disorder-driven metal-insulator transitions

We give a minimal holographic model of a disorder-driven metal-insulator transition. It consists in a CFT with a charge sector and a translation-breaking sector that interact in the most generic way allowed by the symmetries and by dynamical consistency. In the gravity dual, it reduces to a Massive Gravity-Maxwell model with a new direct coupling between the gauge field and the metric that is allowed when gravity is massive. We show that the effect of this coupling is to decrease the DC electrical conductivity generically. This gives a nontrivial check that holographic massive gravity can be consistently interpreted as disorder from the CFT perspective. The suppression of the conductivity happens to such an extent that it does not obey any lower bound and it can be very small in the insulating phase. In some cases, the large disorder limit produces gradient instabilities that hint at the formation of modulated phases.

Dilute magnetic topological semiconductors

Replacing semiconductors with topological insulators, we propose the problem of dilute magnetic topological semiconductors. Performing the renormalization group analysis for an effective field theory, where doped magnetic impurities give rise to a spatially modulated random axion term, we find a novel insulator-metal transition from either a topological or band insulating phase to an inhomogeneously distributed Weyl metallic state with such insulating islands, where extremely broad distributions of ferromagnetic clusters combined with strong spin-orbit interactions are responsible for the emergence of randomly distributed Weyl metallic islands. Since electromagnetic properties in a Weyl metal are described by axion electrodynamics, the role of random axion electrodynamics in transport phenomena casts an interesting problem beyond the physics of percolation in conventional disorder-driven metal-insulator transitions.

Kosterlitz–Thouless transition in disordered two-dimensional topological insulators

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 = dlng/dlnM indicates that the metal–insulator transition is of Kosterlitz–Thouless (KT) type, which is characterized by binding and unbinding of vortex–antivortex pairs of the local currents. The KT-like metal–insulator transition is a basic characteristic of the quantum spin Hall state, being independent of the time-reversal symmetry.

Localization and one-parameter scaling in hydrogenated graphene

We report a metal-insulator transition in disordered graphene with low coverages of hydrogen atoms. Hydrogen interacting with graphene creates short-range disorder and localizes states near the neutrality point. The energy range of localization grows with increasing of H concentration. Calculations show that the conductances through low-energy propagating channels decay exponentially with sample size and are well fitted by one-parameter scaling function, similar to a disorder-driven metal-insulator transition in 2-dimensional disordered systems.

Disorder-type-dependent phase diagram in a CsCl-type complex Anderson lattice model

The investigation on disorder-driven metal-insulator transition of the Anderson lattice model has been extended to a CsCl-type complex lattice to accommodate for the hybridized bands originating from atomic orbitals of inequivalent lattice sites. Using the standard transfer-matrix method and finite-size scaling an asymmetrical mobility edge is found in the energy-disorder phase diagram under site-selective disorder. The critical disorder is larger in the less disorder-affected band than its counterpart in the more disorder-affected band. Moreover, a unique type of delocalized state appears at the edge of the less disorder-affected band. The asymptotic localization length ${\ensuremath{\lambda}}_{M}(W)$ increases linearly with the longitudinal sample size, which depends neither on the lateral size $M$ nor on the disorder strength $W$. Except for this particular energy, metal-insulator transitions exist for all other energies and reduced localization lengths satisfy the single-parameter scaling law. The critical exponent, $\ensuremath{\nu}\ensuremath{\approx}1.5\char21{}1.6$, is in good agreement with the previous results. The critical disorder is larger than that of a simple cubic lattice due to increased coordination number. The complex Anderson lattice model considered here has the advantage of simulating the localization disparity between electron- and hole-doped bands in the presence of site-selective disorder.

Ultrafast terahertz conductivity of photoexcited nanocrystalline silicon

The ultrafast transient ac conductivity of nanocrystalline silicon films is investigated using time-resolved terahertz spectroscopy. While epitaxial silicon on sapphire exhibits a free carrier Drude response, silicon nanocrystals embedded in glass show a response that is best described by a classical Drude–Smith model, suitable for disorder-driven metal–insulator transitions. In this work, we explore the time evolution of the frequency dependent complex conductivity after optical injection of carriers on a picosecond time scale. Furthermore, we show the lifetime of photoconductivity in the silicon nanocrystal films is dominated by trapping at the Si/SiO2 interface states, occurring on a 1–100 ps time scale depending on particle size and hydrogen passivation.

Effective model of the electronic Griffiths phase

We present simple analytical arguments explaining the universal emergence of electronic Griffiths phases as precursors of disorder-driven metal-insulator transitions in correlated electronic systems. A simple effective model is constructed and solved within dynamical mean field theory. It is shown to capture all the qualitative and even quantitative aspects of such Griffiths phases.

Renormalization Group and Ward Identities in Quantum Liquid Phases and in Unconventional Critical Phenomena

By reviewing the application of the renormalization group to different theoretical problems, we emphasize the role played by the general symmetry properties in identifying the relevant running variables describing the behavior of a given physical system. In particular, we show how the constraints due to the Ward identities, which implement the conservation laws associated with the various symmetries, help to minimize the number of independent running variables. This use of the Ward identities is examined both in the case of a stable phase and of a critical phenomenon. In the first case we consider the problems of interacting fermions and bosons. In one dimension general and specific Ward identities are sufficient to show the non-Fermi-liquid character of the interacting fermion system, and also allow to describe the crossover to a Fermi liquid above one dimension. This crossover is examined both in the absence and presence of singular interaction. On the other hand, in the case of interacting bosons in the superfluid phase, the implementation of the Ward identities provides the asymptotically exact description of the acoustic low-energy excitation spectrum, and clarifies the subtle mechanism of how this is realized below and above three dimensions. As a critical phenomenon, we discuss the disorder-driven metal-insulator transition in a disordered interacting Fermi system. In this case, through the use of Ward identities, one is able to associate all the disorder effects to renormalizations of the Landau parameters. As a consequence, the occurrence of a metal-insulator transition is described as a critical breakdown of a Fermi liquid.

Glassy behavior of electrons as a precursor to the localization transition

A theoretical model is presented, describing the glassy freezing of electrons in the vicinity of disorder-driven metal–insulator transitions. Our results indicate that the onset of glassy dynamics should emerge before the localization transition is reached, thus predicting the existence of an intermediate metallic glass phase between the normal metal and the insulator.

Transport around criticalities: Localization-delocalization and paramagnetic-ferromagnetic transitions

We study two examples of transport properties of electron systems in the vicinity of a critical point. The first example, a one-body problem, is the disorder-driven metal-insulator transition in the integer quantum Hall system. We have numerically examined the multifractality of the critical wavefunction, which is relevant to an anomalous diffusion of electrons. The second example, a many-body problem, is the paramagnetic-ferromagnetic transition in a strongly correlated clean electron system. We show that the proximity to ferromagnetism can give rise to a negative magnetoresistance. These exemplify how a criticality manifests itself in transport properties.

Localization-Induced Griffiths Phase of Disordered Anderson Lattices

We demonstrate that local density of states fluctuations in disordered Anderson lattice models universally lead to the emergence of non-Fermi liquid (NFL) behavior. The NFL regime appears at moderate disorder ( W = Wc) and is characterized by power-law anomalies, e.g., C/T approximately 1/T((1-alpha)), where the exponent alpha varies continuously with disorder, as in other Griffiths phases. This Griffiths phase is not associated with the proximity to any magnetic ordering, but reflects the approach to a disorder-driven metal-insulator transition (MIT). Remarkably, the MIT takes place only at much larger disorder W(MIT) approximately 12Wc, resulting in an extraordinarily robust NFL metallic phase.

Microstructure and thermoelectric properties of rapidly quenched BixSb100-x thin film alloys

The electric resistivity, p, and the Seebeck coefficient, S, of amorphous and polycrystalline (annealed) Bi x Sb 100-x films have been measured in the temperature range between 5 K and 350 K. The amorphous films were prepared in situ by the flash evaporation technique at low temperature (T 10 K). Values of S(T) of the amorphous samples show pronounced dependences on the bismuth content as far as x < 20 at% Bi and in the region 75 < x/at% Bi < 90. The behaviour of S(T) in the region x < 20 at% Bi is dominated by the disorder-driven metal-insulator transition which appears at approximately 10-15 at% Bi. On the other hand, between 75 and 90 at% Bi very small crystalline clusters which are embedded in the amorphous matrix even after vapour quenching are responsible for the large variation of S(T) when the composition of the thin films is changed.

Scaling theory of the Hall coefficient near the metal-insulator transition, a renormalization-group approach

We investigate the critical properties of the Hall conduxtivity in weak magetic fields near the disorder driven metal-insulator transition. We derive an effective lagrangian governing the interactions of the diffusive modes in the presence of time-reversal and particle-hole symmetry breaking and identify a new operator relevant to the scaling behavior of the weak-field Hall conductivity. A Wilson-Polyakov renormalization-group analysis is carried out and the flow equations are obtained near two dimensions. We show that the perturbative behavior of the Hall conductivity in the weakly localized regime, in two dimensions, is not simply related to the critical behavior of the Hall conductivity, due to the role played by a new scaling variable. Solving the renormalization-group equations close to the zero-field fixed point, our theory predicts that the Hall number vanishes at the mobility edge with an exponent g = 1 in agreement with several experimental observations. We clarify the connection between the field-theoretic approach and the conventional perturbative approach and elucidate why perturbation in the disorder, which is valid in the weak localization regime fails to predict the correct critical behavior.

Superconductivity and disorder-driven metal-insulator transition in quench-condensed tin films

The authors have studied the effects of disorder in a series of Sn films quench-condensed onto cryogenically cooled substrates. They find a simple scaling of the conductivity with film thickness sigma approx. (d-d/sub c/)/sup ..nu../, with ..nu.. /approx equal/ 0.85, and a metal-insulator transition near d/sub c/ /approx equal/ 40-50 /angstrom/. The superconducting transition temperatures depend linearly on 1/d, provided d is not too small, and the films exhibit strong annealing effects. These features are explained using a model in which grain boundary scattering dominates intragrain scattering.