Showing papers on "Mott insulator published in 2013"

Strong Correlations from Hund’s Coupling

Abstract: Strong electronic correlations are often associated with the proximity of a Mott-insulating state. In recent years however, it has become increasingly clear that the Hund’s rule coupling (intra-atomic exchange) is responsible for strong correlations in multiorbital metallic materials that are not close to a Mott insulator. Hund’s coupling has two effects: It influences the energetics of the Mott gap and strongly suppresses the coherence scale for the formation of a Fermi liquid. A global picture has emerged recently, which emphasizes the importance of the average occupancy of the shell as a control parameter. The most dramatic effects occur away from half-filling or single occupancy. We review the theoretical understanding and physical properties of these Hund’s metals, together with the relevance of this concept to transition-metal oxides (TMOs) of the 3d, and especially 4d, series (such as ruthenates), as well as to the iron-based superconductors (iron pnictides and chalcogenides).

Zigzag magnetic order in the iridium oxide Na2IrO3.

Abstract: We explore the phase diagram of spin-orbit Mott insulators on a honeycomb lattice, within the Kitaev-Heisenberg model extended to its full parameter space. Zigzag-type magnetic order is found to occupy a large part of the phase diagram of the model, and its physical origin is explained as due to interorbital t(2g)-e(g) hopping. The magnetic susceptibility, spin wave spectra, and zigzag order parameter are calculated and compared to the experimental data, obtaining thereby the spin coupling constants in Na(2)IrO(3) and Li(2)IrO(3).

Superconductivity and the pseudogap in the two-dimensional Hubbard model.

Abstract: Recently developed numerical methods have enabled the explicit construction of the superconducting state of the Hubbard model of strongly correlated electrons in parameter regimes where the model also exhibits a pseudogap and a Mott insulating phase. ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ symmetry superconductivity is found to occur in proximity to the Mott insulator, but separated from it by a pseudogapped nonsuperconducting phase. The superconducting transition temperature and order parameter amplitude are found to be maximal at the onset of the normal-state pseudogap. The emergence of superconductivity from the normal state pseudogap leads to a decrease in the excitation gap. All of these features are consistent with the observed behavior of the copper-oxide superconductors.

Pinning the Order: The Nature of Quantum Criticality in the Hubbard Model on Honeycomb Lattice

Abstract: A new algorithm of quantum Monte Carlo simulations designed to detect very weak magnetic order allows high-resolution studies of the correlation between magnetic order and electrical insulation in Mott insulators. Specific investigations of the Hubbard model on honeycomb lattice---a paradigmatic model for Mott insulators---show that the correlation emerges through a continuous quantum phase transition belonging to a new universality class.

Correlation effects in two-dimensional topological insulators

Abstract: Topological insulators have become one of the most active research areas in condensed matter physics. This article reviews progress on the topic of electronic correlation effects in the two-dimensional case, with a focus on systems with intrinsic spin-orbit coupling and numerical results. Topics addressed include an introduction to the noninteracting case, an overview of theoretical models, correlated topological band insulators, interaction-driven phase transitions, topological Mott insulators and fractional topological states, correlation effects on helical edge states, and topological invariants of interacting systems.

Local Temperature Redistribution and Structural Transition During Joule‐Heating‐Driven Conductance Switching in VO2

Abstract: Joule-heating induced conductance-switching is studied in VO2 , a Mott insulator. Complementary in situ techniques including optical characterization, blackbody microscopy, scanning transmission X-ray microscopy (STXM) and numerical simulations are used. Abrupt redistribution in local temperature is shown to occur upon conductance-switching along with a structural phase transition, at the same current.

Density Functional Theory Studies of the Electronic Structure of Solid State Actinide Oxides

Abstract: The actinide oxides have been extensively studied in the context of the nuclear fuel cycle. They are also of fundamental interest as members of a class of strongly correlated materials, the Mott insulators. Their complex physical and chemical properties make them challenging systems to characterize, both experimentally and theoretically. Chiefly, this is because actinide oxides can exhibit both electronic localization and electronic delocalization and have partially occupied f orbitals, which can lead to multiple possibilities for ground states. Of particular concern for theoretical work is that the large number of competing states display strong correlations which are dffcult to capture with computationally tractable methods.

Polariton condensation in solitonic gap states in a one-dimensional periodic potential

Abstract: Manipulation of nonlinear waves in artificial periodic structures leads to spectacular spatial features, such as generation of gap solitons or onset of the Mott insulator phase transition. Cavity exciton–polaritons are strongly interacting quasiparticles offering large possibilities for potential optical technologies. Here we report their condensation in a one-dimensional microcavity with a periodic modulation. The resulting mini-band structure dramatically influences the condensation process. Contrary to non-modulated cavities, where condensates expand, here, we observe spontaneous condensation in localized gap soliton states. Depending on excitation conditions, we access different dynamical regimes: we demonstrate the formation of gap solitons either moving along the ridge or bound to the potential created by the reservoir of uncondensed excitons. We also find Josephson oscillations of gap solitons triggered between the two sides of the reservoir. This system is foreseen as a building block for polaritonic circuits, where propagation and localization are optically controlled and

Topological order and absence of band insulators at integer filling in non-symmorphic crystals

Abstract: A crystal is a band insulator if the energy bands are filled with electrons. Partially filled bands result in a metal, or sometimes a Mott insulator when interactions are strong. A study now shows that for many crystalline structures, the Mott insulator is the only possible insulating state, even for filled bands.

Correlated quantum phenomena in the strong spin-orbit regime

Abstract: We discuss phenomena arising from the combined influence of electron correlation and spin-orbit coupling, with an emphasis on emergent quantum phases and transitions in heavy transition metal compounds with 4d and 5d elements. A common theme is the influence of spin-orbital entanglement produced by spin-orbit coupling, which influences the electronic and magnetic structure. In the weak-to-intermediate correlation regime, we show how non-trivial band-like topology leads to a plethora of phases related to topological insulators. We expound these ideas using the example of pyrochlore iridates, showing how many novel phases such as the Weyl semi-metal, axion insulator, topological Mott insulator, and topological insulators may arise in this context. In the strong correlation regime, we argue that spin-orbital entanglement fully or partially removes orbital degeneracy, reducing or avoiding the normally ubiquitous Jahn-Teller effect. As we illustrate for the honeycomb lattice iridates and double perovskites, this leads to enhanced quantum fluctuations of the spin-orbital entangled states and the chance to promote exotic quantum spin liquid and multipolar ordered ground states. Connections to experiments, materials, and future directions are discussed.

Gapped symmetry preserving surface state for the electron topological insulator

Abstract: It is well known that the three-dimensional (3D) electronic topological insulator (TI) with charge-conservation and time-reversal symmetry cannot have a trivial insulating surface that preserves symmetry. It is often implicitly assumed that if the TI surface preserves both symmetries then it must be gapless. Here we show that it is possible for the TI surface to be both gapped and symmetry preserving, at the expense of having surface-topological order. In contrast to analogous bosonic topological insulators, this symmetric surface topological order is intrinsically non-Abelian. We show that the surface-topological order provides a complete nonperturbative definition of the electron TI that transcends a free-particle band-structure picture, and could provide a useful perspective for studying strongly correlated topological Mott insulators.

Topological states in a ladder-like optical lattice containing ultracold atoms in higher orbital bands

Abstract: Topological insulators are classified according to their symmetries. Discovery of them in electronic solids is thus restricted by orbital and crystalline symmetries available in nature. Synthetic quantum matter, such as the recent double-well optical lattices loaded with s and p orbital ultracold atoms, can exploit symmetries and interaction beyond natural conditions. Here we unveil a topological phase of interacting fermionic atoms on a two-leg ladder derived from the above experimental optical lattice by dimension reduction. The topological band structure originates from the staggered phases of sp orbital tunnelling, requiring neither spin-orbit coupling nor other known mechanisms like p-wave pairing, artificial gauge field or rotation. Upon crossing over to two-dimensional coupled ladders, the edge modes from individual ladder form a parity-protected flat band at zero energy. Experimental signatures are found in density correlations and phase transitions to trivial band and Mott insulators.

Universal Electric‐Field‐Driven Resistive Transition in Narrow‐Gap Mott Insulators

Abstract: One of today's most exciting research frontier and challenge in condensed matter physics is known as Mottronics, whose goal is to incorporate strong correlation effects into the realm of electronics. In fact, taming the Mott insulator-to-metal transition (IMT), which is driven by strong electronic correlation effects, holds the promise of a commutation speed set by a quantum transition, and with negligible power dissipation. In this context, one possible route to control the Mott transition is to electrostatically dope the systems using strong dielectrics, in FET-like devices. Another possibility is through resistive switching, that is, to induce the insulator-to-metal transition by strong electric pulsing. This action brings the correlated system far from equilibrium, rendering the exact treatment of the problem a difficult challenge. Here, we show that existing theoretical predictions of the off-equilibrium manybody problem err by orders of magnitudes, when compared to experiments that we performed on three prototypical narrow gap Mott systems V2-xCrxO3, NiS2-xSex and GaTa4Se8, and which also demonstrate a striking universality of this Mott resistive transition (MRT). We then introduce and numerically study a model based on key theoretically known physical features of the Mott phenomenon in the Hubbard model. We find that our model predictions are in very good agreement with the observed universal MRT and with a non-trivial timedelay electric pulsing experiment, which we also report. Our study demonstrates that the MRT can be associated to a dynamically directed avalanche.

Na$_2$IrO$_3$ as a Novel Relativistic Mott Insulator with a 340\,meV Gap

Abstract: We study Na2IrO3 by angle-resolved photoemission spectroscopy, optics, and band structure calculations in the local-density approximation (LDA). The weak dispersion of the Ir 5d-t(2g) manifold highlights the importance of structural distortions and spin-orbit (SO) coupling in driving the system closer to a Mott transition. We detect an insulating gap Δ(gap)≃340 meV which, at variance with a Slater-type description, is already open at 300 K and does not show significant temperature dependence even across T(N)≃15 K. An LDA analysis with the inclusion of SO and Coulomb repulsion U reveals that, while the prodromes of an underlying insulating state are already found in LDA+SO, the correct gap magnitude can only be reproduced by LDA+SO+U, with U=3 eV. This establishes Na2IrO3 as a novel type of Mott-like correlated insulator in which Coulomb and relativistic effects have to be treated on an equal footing.

Electric-field-induced metal maintained by current of the Mott insulator Ca 2 RuO 4

Abstract: Recently, “application of electric field (E-field)” has received considerable attention as a new method to induce novel quantum phenomena since application of E-field can tune the electronic states directly with obvious scientific and industrial advantages over other turning methods However, E-field-induced Mott transitions are rare and typically require high E-field and low temperature Here we report that the multiband Mott insulator Ca2RuO4 shows unique insulator-metal switching induced by applying a dry-battery level voltage at room temperature The threshold field Eth ~40 V/cm is much weaker than the Mott gap energy Moreover, the switching is accompanied by a bulk structural transition Perhaps the most peculiar of the present findings is that the induced metal can be maintained to low temperature by a weak current

Resistive switching induced by electronic avalanche breakdown in GaTa$_4$Se$_{8-x}$Te$_x$ narrow gap Mott Insulators

Abstract: Mott transitions induced by strong electric fields are receiving a growing interest. Recent theoretical proposals have focused on the Zener dielectric breakdown in Mott insulators, however experimental studies are still too scarce to conclude about the mechanism. Here we report a study of the dielectric breakdown in the narrow gap Mott insulators GaTa$_4$Se$_{8-x}$Te$_x$. We find that the I-V characteristics and the magnitude of the threshold electric field (E$_{th}$) do not correspond to a Zener breakdown, but rather to an avalanche breakdown. E$_{th}$ increases as a power law of the Mott Hubbard gap (E$_g$), in surprising agreement with the universal law E$_{th}$ $\propto$E$_g$$^{2.5}$ reported for avalanche breakdown in semiconductors. However, the delay time for the avalanche that we observe in Mott insulators is over three orders of magnitude longer than in conventional semiconductors. Our results suggest that the electric field induces local insulator-to-metal Mott transitions that create conductive domains which grow to form filamentary paths across the sample.

Robustness of basal-plane antiferromagnetic order and the J(eff)=1/2 state in single-layer iridate spin-orbit Mott insulators.

Abstract: The magnetic structure and electronic ground state of the layered perovskite Ba(2)IrO(4) have been investigated using x-ray resonant magnetic scattering. Our results are compared with those for Sr(2)IrO(4), for which we provide supplementary data on its magnetic structure. We find that the dominant, long-range antiferromagnetic order is remarkably similar in the two compounds and that the electronic ground state in Ba(2)IrO(4), deduced from an investigation of the x-ray resonant magnetic scattering L(3)/L(2) intensity ratio, is consistent with a J(eff)=1/2 description. The robustness of these two key electronic properties to the considerable structural differences between the Ba and Sr analogues is discussed in terms of the enhanced role of the spin-orbit interaction in 5d transition metal oxides.

Locking of iridium magnetic moments to the correlated rotation of oxygen octahedra in Sr2IrO4 revealed by x-ray resonant scattering

Abstract: Sr2IrO4 is a prototype of the class of Mott insulators in the strong spin–orbit interaction (SOI) limit described by a Jeff = 1/2 ground state. In Sr2IrO4, the strong SOI is predicted to manifest itself in the locking of the canting of the magnetic moments to the correlated rotation by 11.8(1)° of the oxygen octahedra that characterizes its distorted layered perovskite structure. Using x-ray resonant scattering at the Ir L3 edge we have measured accurately the intensities of Bragg peaks arising from different components of the magnetic structure. From a careful comparison of integrated intensities of peaks due to basal-plane antiferromagnetism, with those due to b-axis ferromagnetism, we deduce a canting of the magnetic moments of 12.2(8)°. We thus confirm that in Sr2IrO4 the magnetic moments rigidly follow the rotation of the oxygen octahedra, indicating that, even in the presence of significant non-cubic structural distortions, it is a close realization of the Jeff = 1/2 state.

Photoinduced states in a Mott insulator.

Abstract: We investigate the properties of the metallic state obtained by photodoping carriers into a Mott insulator. In a strongly interacting system, these carriers have a long lifetime, so that they can dissipate their kinetic energy to a phonon bath. In the relaxed state, the scattering rate saturates at a nonzero temperature-independent value, and the momentum-resolved spectral function features broad bands which differ from the well-defined quasiparticle bands of a chemically doped system. Our results indicate that a photodoped Mott insulator behaves as a bad metal, in which strong scattering between doublons and holes inhibits Fermi-liquid behavior down to low temperature.

Topological Bose-Mott Insulators in a One-Dimensional Optical Superlattice

Abstract: We study topological properties of the Bose-Hubbard model with repulsive interactions in a one-dimensional optical superlattice. We find that the Mott insulator states of the single-component (two-component) Bose-Hubbard model under fractional fillings are topological insulators characterized by a nonzero charge (or spin) Chern number with nontrivial edge states. For ultracold atomic experiments, we show that the topological Chern number can be detected through measuring the density profiles of the bosonic atoms in a harmonic trap.

Dimensionality-controlled Mott transition and correlation effects in single-layer and bilayer perovskite iridates

Abstract: We studied Sr${}_{2}$IrO${}_{4}$ and Sr${}_{3}$Ir${}_{2}$O${}_{7}$ using angle-resolved photoemission spectroscopy, making direct experimental determinations of intra- and intercell coupling parameters as well as Mott correlations and gap sizes. The results are generally consistent with LDA+$U$+spin-orbit coupling calculations, though the calculations missed the momentum positions of the dominant electronic states and neglected the importance of intercell coupling on the size of the Mott gap. The calculations also ignore the correlation-induced spectral peak widths, which are critical for making a connection to activation energies determined from transport experiments. The data indicate a dimensionality-controlled Mott transition in these 5$d$ transition-metal oxides.

Superconductivity and bandwidth-controlled Mott metal-insulator transition in 1T-TaS2−xSex

Abstract: We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) of layered chalcogenide 1$T$-TaS${}_{2\ensuremath{-}x}$Se${}_{x}$ to elucidate the electronic states especially relevant to the occurrence of superconductivity. We found a direct evidence for a Ta-5$d$-derived electron pocket associated with the superconductivity, which is fragile against a Mott-gap opening observed in the insulating ground state for S-rich samples. In particular, a strong electron-electron interaction-induced Mott gap driven by a Ta 5$d$ orbital also exists in the metallic ground state for Se-rich samples, while finite ARPES intensity near the Fermi level likely originating from a Se 4$p$ orbital survives, indicative of the orbital-selective nature of the Mott transition. Present results suggest that effective electron correlation and $p$-$d$ hybridization play a crucial role to tune the superconductivity and Mott metal-insulator transition.

Bosonic Mott insulator with Meissner currents.

Abstract: We introduce a generic bosonic model exemplifying that (spin) Meissner currents can persist in insulating phases of matter. We consider two species of interacting bosons on a lattice. Our model exhibits separation of charge (total density) and spin (relative density): the charge sector is gapped in a bosonic Mott insulator phase with total density one, while the spin sector remains superfluid due to interspecies conversion. Coupling the spin sector to the gauge fields yields a spin Meissner effect reflecting the long-range spin superfluid coherence. We investigate the resulting phase diagram and describe other possible spin phases of matter in the Mott regime possessing chiral currents as well as a spin-density wave phase. The model presented here is realizable in Josephson junction arrays and in cold atom experiments.

Scaling of the gap, fidelity susceptibility, and Bloch oscillations across the superfluid-to-Mott-insulator transition in the one-dimensional Bose-Hubbard model

Abstract: We investigate the interaction-induced superfluid-to-Mott-insulator transition in the one-dimensional Bose-Hubbard model (BHM) for fillings $n=1$, $n=2$, and $n=3$ by studying the single-particle gap, the fidelity susceptibility, and the amplitude of Bloch oscillations via density-matrix renormalization-group methods. We apply a generic scaling procedure for the gap, which allows us to determine the critical points with very high accuracy. We also study how the fidelity susceptibility behaves across the phase transition. Furthermore, we show that in the BHM, and in a system of spinless fermions, the amplitude of Bloch oscillations after a tilt of the lattice vanishes at the critical points. This indicates that Bloch oscillations can serve as a tool to detect the transition point in ongoing experiments with ultracold gases.

Visualizing the atomic-scale electronic structure of the Ca2CuO2Cl2 Mott insulator

Abstract: Understanding high temperature superconductivity in the cuprates is one of the hardest problems in physics to date. Wang et al. use state-of-the-art scanning tunnelling spectroscopy to visualize the atomic-scale electronic structure of the Mott insulator phase from which this elusive state emerges.

Chiral Mott insulator with staggered loop currents in the fully frustrated Bose-Hubbard model

Abstract: Motivated by experiments on Josephson junction arrays in a magnetic field and ultracold interacting atoms in an optical lattice in the presence of a ``synthetic'' orbital magnetic field, we study the ``fully frustrated'' Bose-Hubbard model and quantum XY model with half a flux quantum per lattice plaquette. Using Monte Carlo simulations and the density matrix renormalization group method, we show that these kinetically frustrated boson models admit three phases at integer filling: a weakly interacting chiral superfluid phase with staggered loop currents which spontaneously break time-reversal symmetry, a conventional Mott insulator at strong coupling, and a remarkable ``chiral Mott insulator'' (CMI) with staggered loop currents sandwiched between them at intermediate correlation. We discuss how the CMI state may be viewed as an exciton condensate or a vortex supersolid, study a Jastrow variational wave function which captures its correlations, present results for the boson momentum distribution across the phase diagram, and consider various experimental implications of our phase diagram. Finally, we consider generalizations to a staggered flux Bose-Hubbard model and a two-dimensional (2D) version of the CMI in weakly coupled ladders.

Epitaxy-distorted spin-orbit Mott insulator in Sr 2 IrO 4 thin films

Abstract: High-quality epitaxial thin films of ${J}_{\mathrm{eff}}$ $=$ 1/2 Mott insulator Sr${}_{2}$IrO${}_{4}$ with increasing in-plane tensile strain have been grown on top of SrTiO${}_{3}$(001) substrates. Increasing the in-plane tensile strain up to \ensuremath{\sim}0.3$%$ was observed to drop the $c$/$a$ tetragonality by 1.2$%$. X-ray absorption spectroscopy detected a strong reduction of the linear dichroism upon increasing in-plane tensile strain towards a reduced anisotropy in the local electronic structure. While the most relaxed thin film shows a consistent dependence with previously reported single crystal bulk measurements, electrical transport reveals a charge gap reduction from 200 meV down to 50 meV for the thinnest and most epitaxy-distorted film. We argue that the reduced tetragonality plays a major role in the change of the electronic structure, which is reflected in the change of the transport properties. Our work opens the possibility for exploiting epitaxial strain as a tool for both structural and functional manipulation of spin-orbit Mott systems.

A new route to the Mott-Hubbard metal-insulator transition: Strong correlations effects in Pr0.7Ca0.3MnO3

Abstract: Resistive random access memory based on the resistive switching phenomenon is emerging as a strong candidate for next generation non-volatile memory. So far, the resistive switching effect has been observed in many transition metal oxides, including strongly correlated ones, such as, cuprate superconductors, colossal magnetoresistant manganites and Mott insulators. However, up to now, no clear evidence of the possible relevance of strong correlation effects in the mechanism of resistive switching has been reported. Here, we study Pr0.7Ca0.3MnO3, which shows bipolar resistive switching. Performing micro-spectroscopic studies on its bare surface we are able to track the systematic electronic structure changes in both, the low and high resistance state. We find that a large change in the electronic conductance is due to field-induced oxygen vacancies, which drives a Mott metal-insulator transition at the surface. Our study demonstrates that strong correlation effects may be incorporated to the realm of the emerging oxide electronics.

Theory of a competitive spin liquid state for weak Mott insulators on the triangular lattice.

Abstract: We propose a novel quantum spin liquid state that can explain many of the intriguing experimental properties of the low-temperature phase of the organic spin liquid candidate materials κ-(BEDT-TTF)2Cu2(CN)3 and EtMe3Sb[Pd(dmit)2]2. This state of paired fermionic spinons preserves all symmetries of the system, and it has a gapless excitation spectrum with quadratic bands that touch at momentum k[over →]=0. This quadratic band touching is protected by symmetries. Using variational Monte Carlo techniques, we show that this state has highly competitive energy in the triangular lattice Heisenberg model supplemented with a realistically large ring-exchange term.