Disorder-induced Metal-insulator Transition Research Articles

Concurrent weak localization and double Schottky barrier across a grain boundary in bicrystal SrTiO3

Stoichiometric ${\mathrm{SrTiO}}_{3}$ (STO) is a wide band-gap insulator in which oxygen deficiency generates a low-temperature metallic ferromagnetic state. Here, we report weak ferromagnetic-like and metallic transport in oxygen-deficient bicrystal of $\mathrm{STO}$ with a low-temperature electron mobility and surface carrier density of $2800\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ and $3.500\ifmmode\times\else\texttimes\fi{}{10}^{16}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$, respectively. Moreover, we show that magnetotransport behavior in the bulk and across the grain boundary (GB) in $\mathrm{STO}$ single crystals are significantly different. In the bulk, our magnetotransport results agree with those reported in the literature. Importantly, low-temperature magnetotransport across the GB shows a disorder-induced metal-insulator transition, and it is modeled as a quasi--two-dimensional system below 10 K that accounts for weak localization (WL), supported by electron-electron, electron-phonon, and Hikami-Larkin-Nagaoka (HLN) quantum interference models. The HLN model accounts for realistic length scales: the dephasing length ${L}_{\ensuremath{\phi}}$, and the spin-orbit (SO) scattering length ${L}_{\mathrm{SO}}$, which are comparable to the electron mean-free path, resembling the surface-conducting nature across the GB. Our experimental work shows the importance of structural defects in the interpretation of magnetotransport in ${\mathrm{SrTiO}}_{3}$ and shows that WL is concurrent with a double Schottky barrier at the GB interface.

Two-body mobility edge in the Anderson-Hubbard model in three dimensions: Molecular versus scattering states

Most of our quantitative understanding of disorder-induced metal-insulator transitions comes from numerical studies of simple noninteracting tight-binding models, like the Anderson model in three dimensions. An important outstanding problem is the fate of the Anderson transition in the presence of additional Hubbard interactions of strength $U$ between particles. Based on large-scale numerics, we compute the position of the mobility edge for a system of two identical bosons or two fermions with opposite spin components. The resulting phase diagram in the interaction-energy-disorder space possesses a remarkably rich and counterintuitive structure, with multiple metallic and insulating phases. We show that this phenomenon originates from the molecular or scattering-like nature of the pair states available at given energy $E$ and disorder strength $W$. The disorder-averaged density of states of the effective model for the pair is also investigated. Finally, we discuss the implications of our results for ongoing research on many-body localization.

Enhancing a Near-Term Quantum Accelerator's Instruction Set Architecture for Materials Science Applications

Quantum computers with tens to hundreds of noisy qubits are being developed today. To be useful for real-world applications, we believe that these near-term systems cannot simply be scaled-down non-error-corrected versions of future fault-tolerant large-scale quantum computers. These near-term systems require specific architecture and design attributes to realize their full potential. To efficiently execute an algorithm, the quantum coprocessor must be designed to scale with respect to qubit number and to maximize useful computation within the qubits' decoherence bounds. In this work, we employ an application-system-qubit co-design methodology to architect a near-term quantum coprocessor. To support algorithms from the real-world application area of simulating the quantum dynamics of a material system, we design a (parameterized) arbitrary single-qubit rotation instruction and a two-qubit entangling controlled-Z instruction. We introduce dynamic gate set and paging mechanisms to implement the instructions. To evaluate the functionality and performance of these two instructions, we implement a two-qubit version of an algorithm to study a disorder-induced metal-insulator transition and run 60 random instances of it, each of which realizes one disorder configuration and contains 40 two-qubit instructions (or gates) and 104 single-qubit instructions. We observe the expected quantum dynamics of the time-evolution of this system.

Disorder-induced metal-insulator transition in cooled silver and copper nanoparticles: A statistical study

The existence of a disorder-induced metal-insulator transition (MIT) has been proved in cooled silver and copper nanoparticles by using level spacing statistics. Nanoparticles are obtained by employing molecular dynamics simulations. Results show that structural disorder is not strong enough to affect their electronic character, and it remains in the metallic regime. Whereas, electronic properties cross to the insulating regime after increasing the chemical disorder strength, W/t. Then, based on scaling theory, we have found that the critical chemical disorder WC/t in which MIT happens for silver and copper nanoparticles are 24.0±1.1 and 22.3±0.9, respectively. Its universality has also been studied.

Anderson localization effect on Mott phase in 1T-TaS2

In the layered dichalcogenide 1T-TaS2, whether there is a disorder-driven transition from insulator to metal is still a matter in dispute. It is predicted that the commensurate charge density wave (CCDW) phase at low temperature behaves as a Mott insulator due to the strong correlation of electrons. Meanwhile, the stacking of TaS layers is found to be dislocated along the c axis, which will introduce considerable effect of disorder. Therefore, further theoretical study is needed to show the cooperative effect of correlation and disorder in 1T-TaS2. The statistical dynamical mean-field theory, which treats interactions and disorder on an equal footing, is used to study the effect of disorder on the Mott insulating phase in 1T-TaS2. Two different kinds of disorder effects are considered in the one-dimensional extended Anderson-Hubbard model, where the stacking dislocation of TaS layers is described by the off-diagonal hopping disorder and the diagonal disorder term represents the effect of disorder introduced by impurities. We find that the off-diagonal disorder by itself could not close the Mott gap at Fermi level, suggesting that Mott mechanism should be more dominant in the CCDW phase of 1T-TaS2 with the stacking dislocation of TaS layers. On the other hand, the diagonal disorder introduced by impurities will close the Mott gap when the strength of disorder (W) is larger than the correlation of electrons (U). Proved by the lattice-size scaling of the generalized inverse participation ratio, both the off-diagonal disorder and diagonal disorder can make all states Anderson-localized. As a result, there is no disorder-induced metal-insulator transition in a correlated system with either off-diagonal disorder or diagonal disorder. In addition, an anomalistic state is introduced by the off-diagonal disorder at the center of the energy band of the non-interacting system, which is a special Anderson-localized state with a very larger localization length. In the correlated cases, the electron-electron interactions have strong effect on splitting the anomalistic state into two individual states, which are located symmetrically in both the upper and lower Hubbard subbands with an energy interval U.

Strong interplay between electron-phonon interaction and disorder in low-doped systems

The effects of doping on the spectral properties of low doped systems are investigated by means of Coherent Potential Approximation to describe the distributed disorder induced by the impurities and Phonon-Phonon Non-Crossing Approximation to characterize a wide class of electron-phonon interactions which dominate the low-energy spectral features. When disorder and electron-phonon interaction work on comparable energy scales, a strong interplay between them arises, the effect of disorder can no more be described as a mere broadening of the spectral features and the phonon signatures are still visible despite the presence of strong disorder. As a consequence, the disorder-induced metal-insulator transition, is strongly affected by a weak or moderate electron-phonon coupling which is found to stabilize the insulating phase.

Superconductivity in the system MoxCyGazOδ prepared by focused ion beam induced deposition

We have prepared the new amorphous superconductor MoxCyGazOδ with a maximum critical temperature Tc of 3.8 K by the direct-write nano-patterning technique of focused (gallium) ion beam induced deposition (FIBID) using Mo(CO)6 as precursor gas. From a detailed analysis of the temperature-dependent resistivity and the upper critical field, we found clear evidence for proximity of the samples to a disorder-induced metal-insulator transition. We observed a strong dependence of Tc on the deposition parameters and identified clear correlations between Tc, the localization tendency visible in the resistance data and the sample composition. By an in-situ feedback-controlled optimization process in the FIB-induced growth, we were able to identify the beam parameters which lead to samples with the largest Tc-value and sharpest transition into the superconducting state.

Crystal-Facet-Dependent Metallization in Electrolyte-Gated Rutile TiO2 Single Crystals

The electric-field-induced metallization of insulating oxides is a powerful means of exploring and creating exotic electronic states. Here we show by the use of ionic liquid gating that two distinct facets of rutile TiO2, namely, (101) and (001), show clear evidence of metallization, with a disorder-induced metal-insulator transition at low temperatures, whereas two other facets, (110) and (100), show no substantial effects. This facet-dependent metallization can be correlated with the surface energy of the respective crystal facet and, thus, is consistent with oxygen vacancy formation and diffusion that results from the electric fields generated within the electric double layers at the ionic liquid/TiO2 interface. These effects take place at even relatively modest gate voltages.

Compositional disorder and transport peculiarities in the amorphous indium oxides

We present results of the disorder-induced metal-insulator transition (MIT) in three-dimensional amorphous indium-oxide films. The amorphous version studied here differs from the one reported by Shahar and Ovadyahu [Phys. Rev. B 46, 10917 (1992)] in that it has a much lower carrier concentration. As a measure of the static disorder we use the dimensionless parameter ${k}_{F}\ensuremath{\ell}$. Thermal annealing is employed as the experimental handle to tune the disorder. On the metallic side of the transition, the low temperature transport exhibits weak-localization and electron-electron correlation effects characteristic of disordered electronic systems. These include a fractional power-law conductivity versus temperature behavior anticipated to occur at the critical regime of the transition. The MIT occurs at a ${k}_{F}\ensuremath{\ell}\ensuremath{\approx}0.3$ for both versions of the amorphous material. However, in contrast with the results obtained on the electron-rich version of this system, no sign of superconductivity is seen down to $\ensuremath{\approx}$0.3 K even for the most metallic sample used in the current study. This demonstrates that using ${k}_{F}\ensuremath{\ell}$ as a disorder parameter for the superconductor-insulator transition (SIT) is an ill defined procedure. A microstructural study of the films, employing high resolution chemical analysis, gives evidence for spatial fluctuations of the stoichiometry. This brings to light that, while the films are amorphous and show excellent uniformity in transport measurements of macroscopic samples, they contain compositional fluctuations that extend over mesoscopic scales. These, in turn, reflect prominent variations of carrier concentrations thus introducing an unusual type of disorder. It is argued that this compositional disorder may be the reason for the apparent violation of the Ioffe-Regel criterion in the two versions of the amorphous indium oxide. However, more dramatic effects due to this disorder are expected when superconductivity sets in, which are in fact consistent with the prominent transport anomalies observed in the electron-rich version of indium oxide. The relevance of compositional disorder (or other agents that are effective in spatially modulating the BCS potential) to other systems near their SIT is discussed.

Anderson-Hubbard model with box disorder: Statistical dynamical mean-field theory investigation

Strongly correlated electrons with box disorder in high-dimensional lattices are investigated. We apply a statistical dynamical mean-field theory that treats local correlations nonperturbatively. The incorporation of a finite lattice connectivity allows for the detection of disorder-induced localization via the probability distribution function of the local density of states. We obtain a complete paramagnetic ground-state phase diagram and find correlation-induced as well as disorder-induced metal-insulator transitions. Our results qualitatively confirm predictions obtained by typical medium theory. Moreover, we find that the probability distribution function of the local density of states in the metallic phase strongly deviates from a log-normal distribution as found for the noninteracting case.

Epitaxial suppression of the metal-insulator transition in CrN

Both single- and polycrystalline CrN layers are grown by reactive sputtering on MgO and quartz substrates, respectively. Temperature-dependent x-ray diffraction indicates a phase transition near 280 K to a low-temperature orthorhombic phase for polycrystalline CrN, while epitaxial constraints cause single-crystal CrN(001) and CrN(111) to remain in the cubic high-temperature phase. Electronic transport measurements indicate variable-range-hopping for the cubic phase below \ensuremath{\sim}120 K, a discontinuity at the phase transition for the polycrystalline layers, strongly and weakly disordered metallic conduction for the orthorhombic phase if deposited at 600 and 800 \ifmmode^\circ\else\textdegree\fi{}C, respectively, and a disorder-induced metal-insulator transition in the cubic phase.

External Control of a Metal-Insulator Transition in GaMnAs Wires

Quantum transport in disordered ferromagnetic (III,Mn)V semiconductors is studied theoretically. Mesoscopic wires exhibit an Anderson disorder-induced metal-insulator transition that can be controlled by a weak external magnetic field. This metal-insulator transition should also occur in other materials with large anisotropic magnetoresistance effects. The transition can be useful for studies of zero-temperature quantum critical phase transitions and fundamental material properties.

Critical parameters for the disorder-induced metal-insulator transition in fcc and bcc lattices

We use a transfer-matrix method to study the disorder-induced metal-insulator transition. We take isotropic nearest-neighbor hopping and an onsite potential with uniformly distributed disorder. Following the previous work done on the simple-cubic lattice, we perform numerical calculations for the body-centered cubic and face-centered cubic lattices, which are more common in nature. We obtain the localization length from calculated Lyapunov exponents for different system sizes. This data is analyzed using finite-size scaling to find the critical parameters. We create an energy-disorder phase diagram for both lattice types, noting that it is symmetric about the band center for the body-centered cubic lattice but not for the face-centered cubic lattice. We find a critical exponent of approximately 1.5--1.6 for both lattice types for transitions occurring either at fixed energy or at fixed disorder, agreeing with results previously obtained for other systems belonging to the same orthogonal universality class. We notice an increase in critical disorder with the number of nearest neighbors, which agrees with intuition.

Possibility of metal–insulator transitions in hole-doped cuprates induced by strong electron correlation, disorder and extrinsic carrier self-trapping

The adequate theoretical approaches to the correlation- and disorder-induced metal–insulator transitions (MITs) (i.e., Mott and Anderson MITs) and the new MITs caused by the extrinsic self-trapping of carriers in hole-doped cuprates are developed. Using these approaches we obtain the appropriate criteria for the Mott and Anderson MITs driven by the strong electron correlation (in a narrow tight-binding impurity band) and disorder (in the special distribution of dopants), and the new MITs driven by the strong carrier–phonon–dopant (or impurity) interactions which lead to the self-trapping of carriers near dopants and the formation of large extrinsic (bi)polarons. The relevance of the results to high- T c cuprates is discussed.

POSSIBLE QUANTITATIVE CRITERIA FOR THE MOTT AND ANDERSON TRANSITIONS IN DOPED UNCOMPENSATED SYSTEMS

Metal-insulator transitions (MITs) in doped uncompensated systems are investigated in the Mott–Hubbard and Anderson impurity models by considering the intercarrier correlation and screening effect of carriers in the same hydrogenic impurity center, the formation of the superlattices with different coordination numbers (z=6, 8 and 12) and by studying the effect of randomness in impurity distribution. We have obtained simple and quite general criteria for the Mott and Anderson transitions and used these criteria to describe quantitatively the correlation and disorder-induced MITs in doped semiconductors and high-T c cuprates. We examine the validity of the obtained criteria for the Mott and Anderson MITs in these doped systems. It is found that the newly derived criteria for the Mott MIT are well satisfied in doped semiconductors, but they cannot be used to describe the observed MITs in the hole-doped high-T c cuprates, whereas the newly derived criteria for the Anderson MIT are applicable equally to describe the MITs observed both in doped semiconductors (at weak and intermediate disorders) and in doped cuprates (at intermediate and strong disorders). The new criteria for the Anderson MIT are extended to the polaronic systems in p-type cuprates. Our results are in quantitative agreement with the existing well-established experimental data and shed more light on the different types of MITs that occur in doped uncompensated semiconductors and cuprates.

Metal-to-insulator crossover and pseudogap in single-layerBi2+xSr2−xCu1+yO6+δsingle crystals in high magnetic fields

The in-plane ${\ensuremath{\rho}}_{ab}(H)$ and the out-of-plane ${\ensuremath{\rho}}_{c}(H)$ magneto-transport in magnetic fields up to $28\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ has been investigated in a series of high quality, single crystal, hole-doped La-free Bi2201 cuprates for a wide doping range and over a wide range of temperatures down to $40\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. With decreasing hole concentration going from the overdoped $(p=0.2)$ to the underdoped $(p=0.12)$ regimes, a crossover from a metallic to an insulating behavior of ${\ensuremath{\rho}}_{ab}(T)$ is observed in the low temperature normal state, resulting in a disorder induced metal insulator transition. In the zero temperature limit, the normal state ratio ${\ensuremath{\rho}}_{c}(H)∕{\ensuremath{\rho}}_{ab}(H)$ of the heavily underdoped samples in pure Bi2201 shows an anisotropic 3-D behavior, in striking contrast with that observed in La-doped Bi2201 and LSCO systems. Our data strongly support that the negative out-of-plane magnetoresistance is largely governed by interlayer conduction of quasiparticles in the superconducting state, accompanied by a small contribution of normal state transport associated with the field dependent pseudogap. Both in the optimal and overdoped regimes, the semiconducting behavior of ${\ensuremath{\rho}}_{c}(H)$ persists even for magnetic fields above the pseudogap closing field ${H}_{\mathit{pg}}$. The method suggested by [Shibauchi et al. Phys. Rev. Lett. 86, 5763 (2001)] for evaluating ${H}_{\mathit{pg}}$ is unsuccessful for both under- and overdoped Bi2201 samples. Our findings suggest that the normal state pseudogap is not always a precursor of superconductivity.

Disorder-Induced Metal–Insulator Transition in Conducting Polymers

The metal–insulator (M-I) transition in conducting polymers is particularly interesting; critical behavior has been observed over a relatively wide temperature range in a number of systems, including polyacetylene, polypyrrole, poly (p-phenylene vinylene), and polyaniline. In each case, metallic, critical, and insulating regimes have been identified through Zabrodskii plots of the logarithmic derivative of the conductivity. The critical regime (in which the conductivity varies as Tβ, where β ≈ 1/3) is tunable by varying the extent of disorder and by applying external pressure and/or an external magnetic field. The transitions from metallic to critical behavior and from critical to insulating behavior have been induced with a magnetic field and from insulating to metallic behavior with applied pressure.

Superconductivity and localization

We present a review of theoretical and experimental works on the problem of mutual interplay of Anderson localization and superconductivity in strongly disordered systems. Superconductivity occurs close to the metal-insulator transition in some disordered systems such as amorphous metals, superconducting compounds disordered by fast neutron irradiation, etc. High-temperature superconductors are especially interesting from this point of view. Only bulk systems are considered in this review. The superconductor-insulator transition in purely two-dimensional disordered systems is not discussed. We start with a brief discussion of the modern aspects of localization theory including the basic concept of scaling, self-consistent theory and interaction effects. After that we analyze disorder effects on Cooper pairing and superconducting transition temperature as well as the Ginzburg-Landau equations for superconductors which are close to those for the Anderson transition. A necessary generalization of the usual theory of “dirty” superconductors is formulated which allows to analyze anomalies of the main superconducting properties close to the disorder-induced metal-insulator transition. Under very rigid conditions superconductivity may persist even in the localized phase (Anderson insulator). Strong disordering leads to considerable reduction of superconducting transition temperature T c and to important anomalies in the behavior of the upper critical field H c2 . Fluctuation effects are also discussed. In the vicinity of the Anderson transition, inhomogeneous superconductivity appears due to statistical fluctuations of the local density of states. We briefly discuss a number of experiments demonstrating superconductivity close to the Anderson transition both in traditional and high- T c superconductors. In traditional systems superconductivity is in most cases destroyed before the metal-insulator transition. In the case of high- T c superconductors a number of anomalies show that superconductivity is apparently conserved in the localized phase before it is suppressed by a strong enough disorder.

In-plane transport properties of heavily delta -doped GaAs n-i-p-i superlattices: Metal-insulator transition, weak and strong localization.

We performed intralayer-transport measurements on heavily \ensuremath{\delta}-doped GaAs n-i-p-i superlattices in the temperature range of 4--300 K. The n-layer electron density was varied by applying a bias voltage between the selectively contacted n- and p-type layers. This tunability allowed the observation of the disorder-induced metal-insulator transition in a doping superlattice. We compare the experimental electron-density dependence of the mobility at T=4 K with theoretical predictions. Good agreement is achieved in the metallic regime, solving the multisubband Boltzmann equation in relaxation-time approximation and close to the metal-insulator transition, applying a self-consistent current-relaxation theory. Furthermore, we analyze the electron conductivity data of both the metallic and insulating phases as functions of temperature and magnetic field oriented perpendicular to the doping layers. The magnetic field as well as the temperature dependence of the conductivity give evidence for weak localization of the metallic phase. In the strongly localized insulating regime our data show a transition from activated high temperature to weakly temperature-dependent low-temperature behavior. A detailed model is elaborated which explains this observation as transition from thermally activated transport over critical barriers of the disorder potential to phonon-assisted tunneling. \textcopyright{} 1996 The American Physical Society.

Reflectance of conducting polypyrrole: Observation of the metal-insulator transition driven by disorder.

By controlling the extent of disorder through electrochemical synthesis at reduced temperatures, conducting polypyrrole (PPy) can be obtained in the metallic regime, in the insulating regime, and in the critical regime of the disorder-induced metal-insulator (M-I) transition. We present the results of reflectance measurements (0.002--6 eV) of PPy carried out at room temperature on the metallic side and on the insulating side of the M-I transition. While the reflectance spectra obtained from samples on both sides of the M-I transition exhibit spectral features expected for a partially filled conduction band, the electronic states near the Fermi energy (${\mathit{E}}_{\mathit{F}}$) are different in the two regimes. The data obtained from metallic samples indicate delocalized electronic wave functions in the conduction band, whereas the spectral features which characterize the insulating regime indicate that the states near ${\mathit{E}}_{\mathit{F}}$ are localized. Consistent with theoretical predictions for the metallic and insulating regimes, the optical conductivity \ensuremath{\sigma}(\ensuremath{\omega}) and the real part of the dielectric function ${\mathrm{\ensuremath{\varepsilon}}}_{1}$(\ensuremath{\omega}) each show different frequency dependences in the far infrared. In the metallic regime \ensuremath{\sigma}(\ensuremath{\omega})\ensuremath{\propto}${\mathrm{\ensuremath{\omega}}}^{1/2}$ for \ensuremath{\Elzxh}\ensuremath{\omega}600 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and ${\mathrm{\ensuremath{\varepsilon}}}_{1}$(\ensuremath{\omega}) (g0) increases rapidly as \ensuremath{\omega}\ensuremath{\rightarrow}0, as described by the ``localization-modified Drude model,'' leading to the conclusion that metallic polypyrrole is a disordered metal near the M-I transition. In contrast, the insulating regime is characterized as a Fermi glass as confirmed by \ensuremath{\sigma}(\ensuremath{\omega})\ensuremath{\propto}${\mathrm{\ensuremath{\omega}}}^{2}$ for \ensuremath{\Elzxh}\ensuremath{\omega}600 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$.