Insulator Regime Research Articles

Spectral statistics of driven Bose-Hubbard models

We study the spectral statistics of a one-dimensional Bose–Hubbard model subjected to kinetic driving; a form of Floquet engineering where the kinetic energy is periodically driven in time with a zero time-average. As the amplitude of the driving is increased, the ground state of the resulting flat-band system passes from the Mott insulator regime to an exotic superfluid. We show that this transition is accompanied by a change in the system’s spectral statistics from Poisson to GOE-type. Remarkably, and unlike in the conventional Bose–Hubbard model which we use as a benchmark, the details of the GOE statistics are sensitive to the parity of both the particle number and the lattice sites. We show how this effect arises from a hidden symmetry of the Hamiltonian produced by this form of Floquet driving.

The Mott transition in the 5d[formula omitted] compound Ba[formula omitted]NaOsO[formula omitted]: A DFT+DMFT study with PAW spinor projectors

Spin–orbit coupling has been reported to be responsible for the insulating nature of the 5d1 osmate double perovskite Ba2NaOsO6 (BNOO). However, whether spin–orbit coupling indeed drives the metal-to-insulator transition (MIT) in this compound is an open question. In this work we investigate the impact of relativistic effects on the electronic properties of BNOO via density functional theory plus dynamical mean-field theory calculations in the paramagnetic regime, where the insulating phase is experimentally observed. The correlated subspace is modeled with spinor projectors of the projector augmented wave method (PAW) employed in the Vienna Ab Initio Simulation Package (VASP), suitably interfaced with the TRIQS package. The inclusion of PAW spinor projectors in TRIQS enables the treatment of spin–orbit coupling effects fully ab-initio within the dynamical mean-field theory framework. In the present work, we show that spin–orbit coupling, although assisting the MIT in BNOO, is not the main driving force for its gapped spectra, placing this material in the Mott insulator regime. Relativistic effects primarily impact the correlated states’ character, excitations, and magnetic ground-state properties.

Electromagnetic signatures of a chiral quantum spin liquid

Quantum spin liquids (QSL) have emerged as a captivating subject within interacting spin systems that exhibit no magnetic ordering even at the lowest temperature accessible experimentally. However, definitive experimental evidence remains elusive. In light of the recent surge in theoretical and experimental interest in the half-filled Hubbard model on a triangular lattice, which offers the potential for stabilizing a chiral QSL, we investigate the electromagnetic signatures of this phase to facilitate experimental detection. Utilizing a combination of parton mean-field theory and unbiased density-matrix renormalization group calculations, we systematically examine the electrical charge and orbital electrical current associated with a spinon excitation in the chiral QSL. Additionally, we calculate the longitudinal and transverse optical conductivities below the Mott gap. Furthermore, employing quantum field theory analysis, we unravel the connection between spinon excitations and emergent as well as physical gauge fields. Our results demonstrate that the chiral QSL phase exhibits a distinct electromagnetic response, even within a Mott insulator regime. This finding holds great potential for enabling the experimental detection of this long-sought-after phase.

Thermodynamic and electrical transport properties of CeRhSb[formula omitted]Te[formula omitted] systems: Transition from Kondo insulating to the Griffiths and non-Fermi liquid states

It follows from our analysis of CeRhSb that the formation of Kondo insulator state due to the presence of the collective spin singlet state is strongly reduced by its doping with various dopants when their amount exceeds 8%–10%, regardless of whether they are substitute Ce, Rh or Sb. A wide variety of experimental results (electrical resistivity ρ, magnetic susceptibility χ, specific heat C, x-ray photoelectron spectroscopy) and theoretical investigations have convincingly demonstrated the proposed earlier scaling law χ×ρ=const. in the Kondo insulator regime, which is universal for all known Kondo insulators. We also analyze the properties of the Griffiths phase for CeRhSb when Pd substitutes Rh, or Sb is fractionally replaced by Te or Sn, whereas doping of Ce with La leads to the formation of magnetic cluster structure as a result of the Kondo hole effect. Magnetoresistance of CeRhSb as a function of the field B shows a negative B2 behavior, which is characteristic of the Kondo–lattice systems, it is also documented for various topological materials. Electrical resistivity of CeRhSb, as well as of CeRhSb1−xTex (x≤0.6) and CeRh1−xPdxSb (x≤0.04), follows ρ=ρ0−AT law between 0.4 and ∼2.5 K, which suggests a non-Fermi liquid-like behavior due to presence of the Griffiths phase.

Evidence for a Pressure-Induced Phase Transition in the Highly Distorted TlNiO3 Nickelate

Rare-earth perovskite nickelates RNiO3 (R stands for Y, rare earths, Tl or Bi) have attracted wide attention in the past few decades because of their unique electronic properties that emerge from the insulator–metal transition (IMT). This transition can be tuned by chemical substitutions or by external variables, like pressure and temperature, but the mechanism of this transition still remains debated. While most previous studies focused on nickelates with partially filled 4f cations on the R site, here we studied the mechanism of the pressure-induced phase transition on a rare Tl3+ nickelate, which comprises fully filled 4f orbitals. We probed the pressure-induced structural and electronic changes taking place at the bulk, medium, and local atomic level at room temperature by using synchrotron X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) as probing techniques, respectively. High-resolution XRD data under ambient conditions show evidence for the monoclinic phase of TlNiO3, as characterized by the charge ordering between Ni3−δ and Ni3+δ in the insulator regime. High-pressure XRD data elucidated an anomaly in the evolution of the lattice parameters at 10.8 GPa, which we assigned to the structural transition P21/n → Pbnm. High-pressure EXAFS data at the Ni K-edge provide a solid proof for a short-order charge disproportionation under near ambient conditions. The Ni–O pair-bond compression curves unveiled an isotropic overlapping Ni t2g6eg1 → O 2p, with an anomaly at 10.8 GPa. The pressure evolution of the bond-distance variance (Debye–Waller exponents) lodged a hardening at this pressure point, suggesting constrained atomic displacements in the average range of ±0.04(5) Å. We propose that this reduced motional freedom could be linked to an electron–phonon coupling, which is the driving force behind the mechanisms of the IMT. We demonstrated that the XANES technique can be applied to ascertain the pressure-induced transition, by tracking the anomalous behavior of the position and width (full width at half-maximum) of the XANES features.

Thermal fluctuations of the extended Bose-Hubbard model at finite temperature

We consider the finite-temperature effects of the extended Bose-Hubbard model, especially in the Haldane insulator regime. We show the quantum-to-thermal (normal) crossover temperatures reveal signatures of the zero-temperature quantum phase transition. By studying the heat capacity of the system, we find a qualitative difference between the Mott insulator regimes and the other two regimes for small system sizes. We attribute these effects to the thermal energy fluctuations and find characteristic behaviors of this fluctuation in the Haldane insulator regime. We hope these results will provide insights into further research in this system.

Robustness of negative low-field magnetoconductance in ultrathin topological insulator films

In the ultrathin films of three-dimensional topological insulators (TIs), modification of the Berry phase caused by surface hybridization was predicted to allow for a crossover between weak antilocalization (WAL) and weak localization (WL). Here we report on a systematic study of the magnetotransport properties of ${({\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x})}_{2}{\mathrm{Te}}_{3}$ (BST) thin films with well-controlled thickness, doping level, and chemical potential. We found that the sign of magnetoconductance in perpendicular magnetic field remains negative in all circumstances, suggesting absence of global WL in the ultrathin BST films. This is attributed to long-range disorder, as well as conductance corrections by quantum interference and electron-electron interaction. Our work shows that the electron transport in ultrathin TIs is distinctively different from the WAL regime in the systems with symplectic symmetry, and the WL or weak insulator regime associated with the orthogonal symmetry class.

Interplay of quantum spin Hall effect and spontaneous time-reversal symmetry breaking in electron-hole bilayers. I. Transport properties

The band-inverted electron-hole bilayers, such as InAs/GaSb, are an interesting playground for the interplay of quantum spin Hall effect and correlation effects because of the small density of electrons and holes and the relatively small hybridization between the electron and hole bands. It has been proposed that Coulomb interactions lead to a time-reversal symmetry broken phase when the electron and hole densities are tuned from the trivial to the quantum spin Hall insulator regime. We show that the transport properties of the system in the time-reversal symmetry broken phase are consistent with recent experimental observations in InAs/GaSb. Moreover, we carry out a quantum transport study on a Corbino disk where the bulk and edge contributions to the conductance can be separated. We show that the edge becomes smoothly conducting and the bulk is always insulating when one tunes the system from the trivial to the quantum spin Hall insulator phase, providing unambiguous transport signatures of the time-reversal symmetry broken phase.

Exchange interactions in d5 Kitaev materials: From Na2IrO3 to α−RuCl3

We present an analytical study of the exchange interactions between pseudospin one-half ${d}^{5}$ ions in honeycomb lattices with edge-shared octahedra. Various exchange channels involving Hubbard $U$, charge-transfer excitations, and cyclic exchange are considered. Hoppings within ${t}_{2g}$ orbitals as well as between ${t}_{2g}$ and ${e}_{g}$ orbitals are included. Special attention is paid to the trigonal crystal field $\mathrm{\ensuremath{\Delta}}$ effects on the exchange parameters. The obtained exchange Hamiltonian is dominated by ferromagnetic Kitaev interaction $K$ within a wide range of $\mathrm{\ensuremath{\Delta}}$. It is found that a parameter region close to the charge-transfer insulator regime and with a small $\mathrm{\ensuremath{\Delta}}$ is most promising to realize the Kitaev spin liquid phase. Two representative honeycomb materials ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$ and $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ are discussed based on our theory. We have found that both materials share dominant ferromagnetic $K$ and positive nondiagonal $\mathrm{\ensuremath{\Gamma}}$ values. However, their Heisenberg $J$ terms have opposite signs: AFM $J>0$ in ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$ and FM $J<0$ in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$. This brings different magnetic fluctuations and results in their different magnetization behaviors and spin excitation spectra. Proximity to FM state due to the large FM $J$ is emphasized in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$. The differences between the exchange couplings of these two materials originate from the opposite $\mathrm{\ensuremath{\Delta}}$ values, indicating that the crystal field can serve as an efficient control parameter to tune the magnetic properties of ${d}^{5}$ spin-orbit Mott insulators.

Electronic structure models for lead chalcogenide system

Pb1-x Sn x Te is a topological crystalline insulator system with an even number of band Dirac cone at 001,110 and 111 orientations, which represent a trivial system under Kane Mele classification. However, in the past few years, the topological surface states feature in Pb1-x Sn x Te that protected by mirror symmetry drew attention. In contrast to Kane Mele’s topological material that protects by time-reversal symmetry. Pb1-x Sn x Te system has a crystal structure phase transition that can drive the topological features of the system. A systematic study on the energy band gap versus temperature and composition shows an interesting regime at x = (0.3–0.7) and T = 70–130 K. (i.e. the topological crystalline insulator regime, phase transition regime). In this paper, we focused on how the following parameters (energy bandgap, temperature T, composition x) connect by three stander models.

Exploration of trivial and nontrivial electronic phases and of collinear and noncollinear magnetic phases in low-spin d5 perovskites

The $4d$ and $5d$ transition metal oxides have become important members of the emerging quantum materials family due to competition between onsite Coulomb repulsion ($U$) and spin-orbit coupling (SOC). Specifically, the systems with $d^5$ electronic configuration in an octahedral environment are found to be capable of posessing invariant semimetallic state and perturbations can lead to diverse magnetic phases. In this work, by formulating a multi-band Hubbard model and performing SOC tunable DFT+$U$ calculations on a prototype SrIrO$_3$ and extending the analysis to other iso-structural and isovalent compounds, we present eight possible electronic and magnetic configurations in the $U$-SOC phase diagram that can be observed in the family of low-spin $d^5$ perovskites. They include the protected Dirac semimetal state, metal and insulator regimes, collinear and noncollinear spin ordering. The latter is explained through connecting hopping interactions to the rotation and tilting of the octahedra as observed in GdFeO$_3$. Presence of several soft phase boundaries makes the family of $d^5$ perovskites an ideal platform to study electronic and magnetic phase transitions under external stimuli.

Mott insulator of strongly interacting two-dimensional semiconductor excitons

In condensed-matter physics, Mott insulators are an important phase involving strongly interacting electrons because of their intricate relationship with high-temperature superconductors1,2. Mott phases were recently observed for both bosonic and fermionic species in atomic systems3–9. However, in the solid state, the fingerprint of a Mott insulator implemented with bosons has yet to be found. Here we demonstrate such signature by exploring the Bose–Hubbard model using semiconductor excitons confined in a two-dimensional lattice. We emphasize the regime where on-site interactions are comparable to the energy separation between lattice-confined states. We then observe that a Mott phase is accessible, with at most two excitons uniformly occupying each lattice site. The technology introduced here allows us to programme the geometry of the lattice that confines the excitons. This versatility, combined with the long-range nature of dipolar interactions between excitons, provides a route to explore many-body phases that spontaneously break the lattice symmetry10,11. A semiconductor platform for experimentally investigating the multiorbital Bose–Hubbard model with long-range interactions is demonstrated. The interactions between the excitons are strong enough to reach the Mott insulator regime.

Spectral properties of strongly correlated multi-impurity models in the Kondo insulator regime: Emergent coherence, metallic surface states, and quantum phase transitions

We investigate the real-space spectral properties of strongly-correlated multi-impurity arrays in the Kondo insulator regime. Employing a recently developed mapping onto an effective correlated cluster problem makes the problem accessible to the numerical renormalization group. The evolution of the spectrum as function of cluster size and cluster site is studied. We applied the extended Lieb-Mattis theorem to predict whether the spectral function must vanish at the Fermi energy developing a true pseudo-gap or whether the spectral function remains finite at $\w=0$. Our numerical renormalization group spectra confirm the predictions of the theorem and shows a metallic behavior at the surface of a cluster prevailing in arbitrary spatial dimensions. We present a conventional minimal extension of a particle-hole symmetric Anderson lattice model at $U=0$ that leads to a gapped bulk band but a surface band with mainly $f$-orbital character for weak and moderate hybridization strength. The change in the site-dependent spectra upon introducing a Kondo hole in the center of the cluster are presented as a function of the hole-orbital energy. In particular the spectral signatures across the Kosterlitz-Thouless type quantum phase transition from a singlet to a local moment fixed point are discussed.

Majorana zero modes in the interacting fermion chain without pairing

The ground state behavior of the spinless fermion chain with an interaction between fermions at neighboring sites is studied for free open boundaries. For the strong enough repulsion boundary gapless states (bound state of Majorana operators from opposite sites of the chain) can exist inside the gap for bulk excitations, i.e., in the topological insulator regime. We propose to use those Majorana zero modes as topological qubits, similar to the ones in one-dimensional topological superconductors. Possible physical realizations of the considered model are discussed.

The single and coupled BHZ quantum dots with spin–orbit interaction

We investigate theoretically electronic structures of the Bernevig–Hughes–Zhang (BHZ) quantum dots (QDs) with both Rashba spin–orbit interaction (RSOI) and Dresselhaus spin–orbit interaction (DSOI) in the topological insulator regime. For a single QD, the ground state’s electron distribution shows a crescent shape along with specific crystallographic directions ±π∕4 due to the competition between RSOI and DSOI. The single BHZ QD can be tuned to coupled QDs with increasing the gate voltage, and the competition between SOIs and double parabolic confinement potential could rotate the electron distribution of the ground state. Furthermore, we find that the BHZ QDs with SOIs produce similar electronic density distribution to the non-topological quantum ring with SOIs. The BHZ QDs with SOIs could be utilized to explore the evolutionary properties of the BHZ model and may implement some topological electronics applications.

Effect of surface disorder on the chiral surface states of a three-dimensional quantum Hall system

We investigate the effect of surface disorder on the chiral surface states of a three-dimensional quantum Hall system. Utilizing a transfer-matrix method, we find that the localization length of the surface state along the magnetic field decreases with the surface disorder strength in the weak disorder regime but increases anomalously in the strong disorder regime. In the strong disorder regime, the surface states mainly locate at the first inward layer to avoid the strong disorder in the outmost layer. The anomalous increase of the localization length can be explained by an effective model, which maps the strong disorder on the surface layer to the weak disorder on the first inward layer. Our work demonstrates that surface disorder can be an effective way to control the transport behavior of the surface states along the magnetic field. We also investigate the effect of surface disorder on the full distribution of conductances $P(g)$ of the surface states in the quasi-one-dimensional (1D) regime for various surface disorder strengths. In particular, we find that $P(g)$ is Gaussian in the quasi-1D metal regime and log-normal in the quasi-1D insulator regime. In the crossover regime, $P(g)$ exhibits highly nontrivial forms, whose shapes coincide with the results obtained from the Dorokhov-Mello-Pereyra-Kumar equation of a weakly disordered quasi-1D wire in the absence of time-reversal symmetry. Our results suggest that $P(g)$ is fully determined by the average conductance, independent of the details of the system, in agreement with the single-parameter scaling hypothesis.

Comments on the electronic transport mechanisms in the crystalline state of Ge—Sb—Te phase-change materials

It is known that phase-change materials, such as Ge–Sb—Te ternary system, are promising resistive non-volatile random-access memory applications with ultra-rapid reversible transformations between the crystalline and amorphous phases. This class of electronic transition is categorized to be the metal-insulator transition (MIT). The Anderson-type MIT has been discussed extensively in phase-change materials (PCMs) and isothermal annealing of amorphous PCMs (a-PCMs) which, above a certain temperature, leads to the metallic (crystalline) phase. In the insulator regime near the MI transition, Mott-type variable-range hopping (VRH) and/or Efros-Shkolvskii hopping (ESH) at low temperatures below 20 K (and down to 1 K) in Ge1Sb2Te4 (GST124) have been discussed extensively, however, we criticize the above argument through a detailed discussion of physical parameters that support the VRH mechanism. It is not clear whether or not the density-of-states (DOS) near the Fermi level is localized (like the Fermi glass) in the crystalline phase. It is also suggested that grain boundaries are expected to interfere with the electronic transport in the crystalline state. We should take into account the grain boundary effects on the electronic transport in the crystalline phase of Ge—Sb—Te system.

Distinct Quantum Anomalous Hall Ground States Induced by Magnetic Disorders

The quantum anomalous Hall (QAH) effect in a magnetic topological insulator (TI) represents a new state of matter originating from the interplay between topology and magnetism. The defining characteristics of the QAH ground state are the quantized Hall resistivity (ρyx) and vanishing longitudinal resistivity (ρxx) in the absence of an external magnetic field. A fundamental question concerning the QAH effect is whether it is merely a zero-magnetic-field quantum Hall (QH) effect or if it can host unique quantum phases and phase transitions that are unavailable elsewhere. The most dramatic departure of the QAH systems from other QH systems lies in the magnetic disorders that induce spatially random magnetization. Because disorder and magnetism play pivotal roles in the phase diagram of two-dimensional electron systems, the high degree of magnetic disorders in QAH systems may create novel phases and quantum critical phenomena. In this work, we perform systematic transport studies of a series of magnetic TIs with varied strength of magnetic disorders. We find that the ground state can be categorized into two distinct classes: the QAH phase and the anomalous Hall (AH) insulator phase, as the zero-magnetic-field counterparts of the QH liquid and Hall insulator phases in the QH systems. In the low-disorder limit of the QAH phase, we observe a universal quantized longitudinal resistance ρxx=h/e2 at the coercive field. In the AH insulator regime, we find that a magnetic field can drive it to the QAH phase through a quantum critical point with scaling behaviors that are distinct from those in the QH phase transition. We propose that the transmission between chiral edge states at domain boundaries, tunable by magnetic disorder and magnetic fields, is the key for determining the QAH ground state.Received 24 March 2020Accepted 29 October 2020DOI:https://doi.org/10.1103/PhysRevX.10.041063Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasQuantum anomalous Hall effectPhysical SystemsTopological insulatorsCondensed Matter, Materials & Applied Physics

Superconductor-like effects in an ac-driven normal Mott-insulating quantum dot array

We study the current response of an AC driven dissipative Mott insulator system, a normal quantum dot array, using an analytical Keldysh field theory approach. Deep in the Mott insulator regime, the nonequilibrium steady state (NESS) response resembles a resistively shunted Josephson array, with a nonequilibrium Mott insulating to conductor transition as the drive frequency {\Omega} is increased. The diamagnetic component of the NESS in the conducting phase is anomalous, implying negative inductance, strikingly reminiscent of the {\eta}-pairing phase of a Josephson array with negative phase stiffness. However in the presence of an additional DC field the signature of supercurrent - Shapiro steps - is completely absent. We interpret these properties as number-phase fluctuation effects shared with Josephson systems rather than superconductivity.

Thouless conductances of a three-dimensional quantum Hall system

We investigate the longitudinal conductance of a disordered three-dimensional (3D) quantum Hall system within a tight-binding lattice model using numerical Thouless conductance calculations. For the bulk, we confirm that the mobility edges are independent of the propagating directions in this anisotropic system. As disorder increases, the conductance peak of the lowest subband in the horizontal direction floats to the central subband as in the two-dimensional (2D) case, while there is no clear evidence of floating in the vertical direction. We thus conclude that for extended states, the longitudinal conductance in the vertical direction behaves like a quasi-one-dimensional (1D) normal metal, while the longitudinal conductance in the horizontal direction is controlled by layered conducting states stacked coherently. Inside the quantum Hall gap, we study the novel 2D chiral surface states at the sidewalls of the sample. We demonstrate the crossover of the surface states between the quasi-1D metal and insulator regimes, which can be achieved by modifying the interlayer hopping strength and the disorder strength in the model. The typical behaviors of the Thouless conductance and the wave functions of the surface states in these two regimes are investigated. Finally, in order to predict the regime of the surface states for arbitrary parameters, we determine an explicit relationship between the localization length of surface states and the microscopic parameters of the model.