arXiv subjects

Arun Paramekanti

Publications and source records attributed to Arun Paramekanti.

At least 73 records · Page 4Linked to original sources

arXiv2017

Determination of Hund's coupling in 5d Oxides using Resonant Inelastic X-ray Scattering

We report resonant inelastic X-ray scattering (RIXS) measurements on ordered double perovskite samples containing Re$^{5+}$ and Ir$^{5+}$ with $5d^2$ and $5d^4$ electronic configurations respectively. In particular, the observed RIXS spectra of Ba$_2$YReO$_6$ and Sr$_2$MIrO$_6$ (M=Y, Gd) show sharp intra-t$_{2g}$ transitions, which can be quantitatively understood using a minimal `atomic' Hamiltonian incorporating spin-orbit coupling ($λ$) and Hund's coupling ($J_H$). Our analysis yields $λ=0.38(2)$eV with $J_H=0.26(2)$eV for Re$^{5+}$, and $λ=0.42(2)$eV with $J_H=0.25(4)$eV for Ir$^{5+}$. Our results provide the first sharp estimates for the Hund's coupling in $5d$ oxides, and suggest that it should be treated on equal footing with spin-orbit interaction in multi-orbital $5d$ transition metal compounds.

cond-mat.str-el↗

arXiv2017

Emergence of Chiral Spin Liquids via Quantum Melting of Non-Coplanar Magnetic Orders

Quantum spin liquids (QSLs) are long-range entangled states of quantum magnets which lie beyond the Landau paradigm of classifying phases of matter via broken symmetries. A physical route to arriving at QSLs is via frustration-induced quantum melting of ordered states such as valence bond crystals or magnetic orders. Here, we show, using extensive exact diagonalization (ED) and density-matrix renormalization group (DMRG) studies of concrete $SU(2)$ invariant spin models on honeycomb, triangular and square lattices, that chiral spin liquids (CSLs) emerge as descendants of triple-$Q$ spin crystals with tetrahedral magnetic order and a large scalar spin chirality. Such ordered-to-CSL melting transitions may yield lattice realizations of effective Chern-Simons-Higgs field theories. Our work provides a distinct unifying perspective on the emergence of CSLs, and suggests that materials with magnetic skyrmion crystal order might provide a good starting point to search for CSLs.

cond-mat.str-el↗

arXiv2017

Kitaev materials beyond iridates: order by quantum disorder and Weyl magnons in rare-earth double perovskites

Motivated by the experiments on the rare-earth double perovskites, we propose a generalized Kitaev-Heisenberg model to describe the generic interaction between the spin-orbit-entangled Kramers' doublets of the rare-earth moments. We carry out a systematic analysis of the mean-field phase diagram of this new model. In the phase diagram, there exist large regions with a continuous U(1) or O(3) degeneracy. Since no symmetry of the model protects such a continuous degeneracy, we predict that the quantum fluctuation lifts the continuous degeneracy and favors various magnetic orders in the phase diagram. From this order by quantum disorder mechanism, we further predict that the magnetic excitations of the resulting ordered phases are characterized by nearly gapless pseudo-Goldstone modes. We find that there exist Weyl magnon excitations for certain magnetic orders. We expect our prediction to inspire further study of Kitaev physics, the order by quantum disorder phenomenon and topological spin wave modes in the rare-earth magnets and the systems alike.

cond-mat.str-el↗

arXiv2016

High-temperature large-gap quantum anomalous Hall insulator in ultrathin double perovskite films

Motivated by the goal of realizing topological phases in thin films and heterostructures of correlated oxides, we propose here a quantum anomalous Hall insulator (QAHI) in ultrathin films of double perovskites based on mixed 3d-5d or 3d-4d transition metal ions, grown along the [111] direction. Considering the specific case of ultrathin Ba2FeReO6, we present a theoretical analysis of an effective Hamiltonian derived from first-principles. We establish that a strong spin-orbit coupling at Re site, t2g symmetry of the low-energy d-bands, polarity of its [111] orientation of perovskite structure, and mixed 3d-5d chemistry results in room temperature magnetism with a robust QAHI state of Chern number C=1 and a large band-gap. We uncover and highlight a non-relativistic orbital-Rashba-type effect in addition to the spin-orbit coupling, that governs this QAHI state. Our prediction of a large topological band-gap of ~100 meV in electronic structure, and a magnetic transition temperature Tc~300K estimated by Monte Carlo simulations, is expected to stimulate experimental efforts at synthesis of such films and enable possible practical applications of its dissipationless edge currents.

cond-mat.str-el↗

arXiv2016

Haldane-Hubbard Mott Insulator: From Tetrahedral Spin Crystal to Chiral Spin Liquid

Motivated by cold atom experiments on Chern insulators, we study the honeycomb lattice Haldane-Hubbard Mott insulator of spin-$1/2$ fermions using exact diagonalization and density matrix renormalization group methods. We show that this model exhibits various chiral magnetic orders including a wide regime of triple-Q tetrahedral order. Incorporating third-neighbor hopping frustrates and ultimately melts this tetrahedral spin crystal. From analyzing low energy spectra, many-body Chern numbers, entanglement spectra, and modular matrices, we identify the molten state as a chiral spin liquid (CSL) with gapped semion excitations. We formulate and study the Chern-Simons-Higgs field theory of the exotic CSL-to-tetrahedral spin crystallization transition.

cond-mat.str-el↗

arXiv2015

Controlled confinement of half-metallic two-dimensional electron gas in BaTiO3/Ba2FeReO6/BaTiO3 heterostructures: A first-principles study

Using density functional theory calculations, we establish that the half-metallicity of bulk Ba2FeReO6 survives down to 1 nm thickness in BaTiO3/Ba2FeReO6/BaTiO3 heterostructures grown along the (001) and (111) directions. The confinement of the two-dimensional (2D) electron gas in this quantum well structure arises from the suppressed hybridization between Re/Fe d states and unoccupied Ti d states, and it is further strengthened by polar fields for the (111) direction. This mechanism, distinct from the polar catastrophe, leads to an order of magnitude stronger confinement of the 2D electron gas than that at the LaAlO3/SrTiO3 interface. We further show low-energy bands of (111) heterostructure display nontrivial topological character. Our work opens up the possibility of realizing ultrathin spintronic devices.

cond-mat.mtrl-sci↗

arXiv2015

Competing Chiral Orders in the Topological Haldane-Hubbard Model of Spin-1/2 Fermions and Bosons

Motivated by experiments on ultracold atoms which have realized the Haldane model for a Chern insulator, we consider its strongly correlated Mott limit with spin-$1/2$ fermions. We find that slave rotor mean field theory yields gapped or gapless chiral spin liquid Mott insulators. To study competing magnetic orders, we consider the strong coupling effective spin Hamiltonian which includes chiral three-spin exchange. We obtain its classical phase diagram, uncovering various chiral magnetic orders including tetrahedral, cone, and noncoplanar spiral states which can compete with putative chiral quantum spin liquids. We study the effect of thermal fluctuations on these states, identifying crossovers in the spin chirality, and phase transitions associated with lattice symmetry breaking. We also discuss analogous effective spin Hamiltonians for correlated spin-$1/2$ bosons. Finally, we point out possible experimental implications of our results for cold atom experiments.

cond-mat.quant-gas↗

arXiv2014

Thermal Phase Transitions of Strongly Correlated Bosons with Spin-Orbit Coupling

Experiments on ultracold atomic gases have begun to explore lattice effects and thermal fluctuations for two-component bosons with spin-orbit coupling (SOC). Motivated by this, we study a $tJ$ model of strongly correlated lattice bosons, with equal Rashba-Dresselhaus SOC and a uniform magnetic field. At zero temperature, a Gutzwiller ansatz is shown to capture lattice variants of stripe superfluid (SF) ground states. We formulate a finite temperature generalization of the Gutzwiller approach and show that thermal fluctuations in the doped Mott insulator drive a two-step melting of the stripe SF, revealing a wide intermediate regime of a normal fluid with stripe order.

cond-mat.quant-gas↗

arXiv2014

Nematic and spin-charge orders driven by hole-doping a charge-transfer insulator

Recent experimental discoveries have brought a diverse set of broken symmetry states to the center stage of research on cuprate superconductors. Here, we focus on a thematic understanding of the diverse phenomenology by exploring a strong-coupling mechanism of symmetry breaking driven by frustration of antiferromagnetic order. We achieve this through a variational study of a three-band model of the CuO$_2$ plane with Kondo-type exchange couplings between doped oxygen holes and classical copper spins. Two main findings from this strong-coupling multi-band perspective are 1) that the symmetry hierarchy of spin stripe, charge stripe, intra-unit-cell nematic order and isotropic phases are all accessible microscopically within the model, 2) many symmetry-breaking patterns compete with energy differences within a few meV per Cu atom to produce a rich phase diagram. These results indicate that the diverse phenomenology of broken-symmetry states in hole-doped antiferromagnetic charge-transfer insulators may indeed arise from hole-doped frustration of antiferromagnetism.

cond-mat.str-el↗

arXiv2014

Spin-orbit Coupling in Optical lattices

In this review, we discuss the physics of spin-orbit coupled quantum gases in optical lattices. After reviewing some relevant experimental techniques, we introduce the basic theoretical model and discuss some of its generic features. In particular, we concentrate on the interplay between spin-orbit coupling and strong interactions and show how it leads to various exotic quantum phases in both the Mott insulating and superfluid regimes. Phase transitions between the Mott and superfluid states are also discussed.

cond-mat.quant-gas↗

arXiv2014

Optical gyrotropy in quadrupolar Kondo systems

Recent experiments point to a variety of intermetallic systems which exhibit exotic quadrupolar orders driven by the Kondo coupling between conduction electrons and localized quadrupolar degrees of freedom. Using a Luttinger k.p Hamiltonian for the conduction electrons, we study the impact of such quadrupolar order on their energies and wave functions. We discover that such quadrupolar orders can induce a nontrivial Berry curvature for the conduction electron bands, leading to a nonvanishing optical gyrotropic effect. We estimate the magnitude of the gyrotropic response in a candidate quadrupolar material, PrPb3, and discuss the resulting Faraday rotation in thin films.

cond-mat.str-el↗

arXiv2014

Chiral spin superfluidity and spontaneous spin Hall effect of interacting bosons

Recent experiments on ultracold atoms in optical lattices have synthesized a variety of tunable bands with degenerate double-well structures in momentum space. Such degeneracies in the single particle spectrum strongly enhance quantum fluctuations, and may lead to exotic many-body ground states. Here we consider weakly interacting spinor Bose gases in such bands, and discover a universal quantum "order by disorder" phenomenon which selects a novel chiral spin superfluid with remarkable properties such as spontaneous anomalous spin Hall effect and momentum space antiferromagnetism. For bosons in the excited Dirac band of a hexagonal lattice, such a state supports staggered spin loop currents in real space. We show that Bloch oscillations provide a powerful dynamical route to quantum state preparation of such a chiral spin superfluid. Our predictions can be readily tested in spin resolved time-of-flight experiments.

cond-mat.quant-gas↗

arXiv2013

Order-by-disorder and magnetic field response in Heisenberg-Kitaev model on a hyperhoneycomb lattice

We study the finite temperature phase diagram of the Heisenberg-Kitaev model on a three dimensional hyperhoneycomb lattice. Using semiclassical analysis and classical Monte-Carlo simulations, we investigate quantum and thermal order-by-disorder, as well as the magnetic ordering temperature. We find the parameter regime where quantum and thermal fluctuations favor different magnetic orders, which leads to an additional finite temperature phase transition within the ordered phase. This transition, however, occurs at a relatively low temperature and the entropic effects may dominate most of the finite temperature region below the ordering temperature. In addition, we explore the magnetization process in the presence of a magnetic field and discover spin-flop transitions which are sensitive to the applied-field direction. We discuss implications of our results for future experiments on a hyperhoneycomb lattice system such as the recently discovered β-Li2IrO3.

cond-mat.str-el↗

arXiv2013

Phases and phase transitions of frustrated hard-core bosons on a triangular ladder

We study hardcore bosons on a triangular ladder at half filling in the presence of a frustrating hopping term and a competing nearest neighbor repulsion $V$ which promotes crystallization. Using the finite-size density-matrix renormalization group method, we obtain the phase diagram which contains three phases: a uniform superfluid (SF), an insulating charge density wave (CDW) crystal and a bond ordered insulator (BO). We find that the transitions from SF to CDW and SF to BO are continuous in nature, with critical exponents varying continously along the phase boundaries, while the transition from CDW to BO is found to be first order. The phase diagram is also shown to contain an exactly solvable Majumdar Ghosh point, and re-entrant SF to CDW phase transitions.

cond-mat.str-el↗

arXiv2013

Formation and detection of a chiral orbital Bose liquid in an optical lattice

Recent experiments on $p$-orbital atomic bosons have suggested the emergence of a spectacular ultracold superfluid with staggered orbital currents in optical lattices. This raises fundamental questions like the effects of collective thermal fluctuations, and how to directly observe such chiral order. Here, we show via Monte Carlo simulations that thermal fluctuations destroy this superfluid in an unexpected two-step process, unveiling an intermediate normal phase with spontaneously broken time-reversal symmetry, dubbed "chiral Bose liquid". For integer fillings ($n\geq 2$) in the chiral Mott regime, thermal fluctuations are captured by an effective orbital Ising model, and Onsager's powerful exact solution is adopted to determine the transition from this intermediate liquid to the para-orbital normal phase at high temperature. A suitable lattice quench is designed to convert the staggered angular momentum, previously thought by experts difficult to directly probe, into coherent orbital oscillations, providing a smoking-gun signature of chiral order.

cond-mat.quant-gas↗

arXiv2013

RKKY interactions and anomalous Hall effect in metallic rare-earth pyrochlores

Motivated by experiments on Pr2Ir2O7, we consider metallic pyrochlore systems A2B2O7, where the A-sites are occupied by rare-earth local moments and the B-sites host 5d transition metal ions with itinerant strongly spin-orbit coupled electrons. Assuming non-Kramers doublets on the A-site, we derive the RKKY interaction between them mediated by the B-site itinerant electrons and find extended non-Heisenberg interactions. Analyzing a simplified model of the RKKY interaction, we uncover a local moment phase with coexisting spiral Ising-like magnetic dipolar and XY-like quadrupolar ordering. This state breaks time-reversal and lattice symmetries, and reconstructs the B-site electronic band structure, producing a Weyl Metallic phase with an intrinsic anomalous Hall effect and an undetectably small magnetization. We discuss implications of our results for Pr2Ir2O7.

cond-mat.str-el↗

arXiv2012

Anisotropic quantum quench in the presence of frustration or background gauge fields: A probe of bulk currents and topological chiral edge modes

Bosons and fermions, in the presence of frustration or background gauge fields, can form manybody ground states that support equilibrium 'charge' or 'spin' currents. Motivated by the experimental creation of frustration or artificial gauge fields in ultracold atomic systems, we propose a general scheme by which making a sudden anisotropic quench of the atom tunneling across the lattice and tracking the ensuing density modulations provides a powerful and gauge invariant route to visualizing diverse equilibrium current patterns. Using illustrative examples of trapped superfluid Bose and normal Fermi systems in the presence of artificial magnetic fluxes on square lattices, and frustrated bosons in a triangular lattice, we show that this scheme to probe equilibrium bulk current order works independent of particle statistics. We also show that such quenches can detect chiral edge currents in gapped topological states, such as quantum Hall or quantum spin Hall insulators.

cond-mat.quant-gas↗

arXiv2012

Superfluidity of Dirac Fermions in a Tunable Honeycomb Lattice: Cooper Pairing, Collective Modes, and Critical Currents

Motivated by recent experiments on atomic Dirac fermions in a tunable honeycomb optical lattice, we study the attractive Hubbard model of superfluidity in the anisotropic honeycomb lattice. At weak-coupling, we find that the maximum mean field pairing transition temperature, as a function of density and interaction strength, occurs for the case with isotropic hopping amplitudes. In this isotropic case, we go beyond mean field theory and study collective fluctuations, treating both pairing and density fluctuations for interaction strengths ranging from weak to strong coupling. We find evidence for a sharp sound mode, together with a well-defined Leggett mode over a wide region of the phase diagram. We also calculate the superfluid order parameter and collective modes in the presence of nonzero superfluid flow. The flow-induced softening of these collective modes leads to dynamical instabilities involving stripe-like density modulations as well as a Leggett-mode instability associated with the natural sublattice symmetry breaking charge-ordered state on the honeycomb lattice. The latter provides a non-trivial test for the experimental realization of the one-band Hubbard model. We delineate regimes of the phase diagram where the critical current is limited by depairing or by such collective instabilities, and discuss experimental implications of our results.

cond-mat.quant-gas↗