arXiv subjects

Leon Balents

Publications and source records attributed to Leon Balents.

At least 127 records · Page 7Linked to original sources

arXiv2013

Ordering and criticality in an underscreened Kondo chain

Motivated by the nickel valence controversy in the perovskite nickelate RNiO_3, we consider a one-dimensional underscreened Kondo chain consisting of alternating spin-1 ("nickel") and electron ("oxygen") sites, which in addition to the usual electron hopping and spin-spin interaction between the S=1 spin and the electron also contains a spin mediated electron hopping term. Using the density-matrix renormalization group (DMRG), we obtained the zero temperature phase diagram of the model, as well as various correlation functions in each phase. Importantly, for a certain range of parameters the model exhibits a quasi-long-range spiral (QS) order. To understand the DMRG results, we construct a mean-field theory based on a Schwinger fermion decomposition of the S=1 spins, from which we argue that the QS phase corresponds to a phase in proximity to the spin Bose metal state proposed by D. N. Sheng, O. I. Motrunich, and M. P. A. Fisher [Phys. Rev. B 79, 205112 (2009)]. Notably, we find no evidence for a phase with the symmetry of "nickel"-centered charge order, which has been argued to arise due to site-selective Kondo screening of half the S=1 spins, and suggest that order of this type occurs only due to an additional energy gain from spontaneous lattice distortions, not present in this model.

cond-mat.str-el↗

arXiv2013

Ground States of Spin-1/2 Triangular Antiferromagnets in a Magnetic Field

We use a combination of density matrix renormalization group calculations and analytical approaches to study a simplified model for a spatially anisotropic spin-1/2 triangular lattice Heisenberg antiferromagnet: the three-leg triangular spin tube (TST). The model is described by three Heisenberg chains, with exchange constant J, coupled antiferromagnetically with exchange constant J' along the diagonals of the ladder system, with periodic boundary conditions in the shorter direction. We determine the full phase diagram of this model as a function of spatial anisotropy, J'/J, and magnetic field. We find a rich phase diagram, which is dominated by quantum states - phases corresponding to the classical ground state appears only in a small region. Among the dominant phases generated by quantum effects are commensurate and incommensurate coplanar quasi-ordered states, which appear in the vicinity of the isotropic region for most fields, and in the high field region for most anisotropies. The coplanar states, while not classical ground states, can be understood semiclassically. Even more strikingly, the largest region of phase space is occupied by a spin density wave phase, which has incommensurate collinear correlations along the field. This phase has no semiclassical analog, and may be ascribed to enhanced one-dimensional (1d) fluctuations due to frustration. Cutting across the phase diagram is a magnetization plateau, with a gap to all excitations and up up down spin order, with a quantized magnetization equal to 1/3 of the saturation value. In the TST, this plateau extends almost but not quite to the decoupled chains limit. Most of the above features are expected to carry over to the two dimensional system, which we also discuss. At low field, a dimerized phase appears, which is particular to the 1d nature of the TST, and which can be understood from quantum Berry phase arguments.

cond-mat.str-el↗

arXiv2013

Ferromagnetism in itinerant two-dimensional t2g systems

Motivated by the recent indications of ferromagnetism in transition metal oxide heterostructures, we propose a possible mechanism to generate ferromagnetism for itinerant t2g systems in two spatial dimensions that does not rely on the coupling between local moments and conduction electrons. We particularly emphasize the orbital nature of different bands and show that, when the Fermi level lies near the bottom of the upper bands, a non-perturbative interaction effect due to the quasi-one-dimensional nature of the upper bands may drive a transition to a state in which the upper bands are ferromagnetically polarized. In the quasi-one-dimensional limit, the full thermodynamics may be obtained exactly. We discuss the connection between our mechanism with several itinerant t2g systems that may have ferromagnetic instabilities.

cond-mat.str-el↗

arXiv2013

Accuracy of topological entanglement entropy on finite cylinders

Topological phases are unique states of matter which support non-local excitations which behave as particles with fractional statistics. A universal characterization of gapped topological phases is provided by the topological entanglement entropy (TEE). We study the finite size corrections to the TEE by focusing on systems with $Z_2$ topological ordered state using density-matrix renormalization group (DMRG) and perturbative series expansions. We find that extrapolations of the TEE based on the Renyi entropies with Renyi index $n\geq 2$ suffer from much larger finite size corrections than do extrapolations based on the von Neumann entropy. In particular, when the circumference of the cylinder is about ten times the correlation length, the TEE obtained using von Neumann entropy has an error of order $10^{-3}$, while for Renyi entropies it can even exceed 40%. We discuss the relevance of these findings to previous and future searches for topological ordered phases, including quantum spin liquids.

cond-mat.str-el↗

arXiv2013

Finite size effects in the Z_2 spin liquid on the kagome lattice

Motivated by the recent discovery of the Z_2 quantum spin liquid state in the nearest neighbor Heisenberg model on the kagome lattice, we investigate the "even-odd" effect occuring when this state is confined to infinitely long cylinders of finite circumference. We pursue a dual analysis, where we map the effective Z_2 gauge theory from the kagome lattice to a frustrated Ising model on the dice lattice. Unexpectedly, we find that the latter theory, if restricted to nearest neighbor interactions, is insufficient to capture this effect. We provide an explanation of why further neighbor interactions are needed via a high-temperature expansion of the effective Hamiltonian. We then carry out projective symmetry group analysis to understand which second neighbor interactions can be introduced while respecting the lattice symmetries. Finally, we qualitatively compare our results to numerics by computing the dimerization operator within our theory. Systems with odd circumferences exhibit a non-vanishing dimerization that decays exponentially with circumference.

cond-mat.str-el↗

arXiv2013

Spin Liquid Regimes at Nonzero Temperature in Quantum Spin Ice

Quantum spin liquids are highly entangled ground states of quantum systems with emergent gauge structure, fractionalized spinon excitations, and other unusual properties. While these features clearly distinguish quantum spin liquids from conventional, mean-field-like states at zero temperature (T), their status at T>0 is less clear. Strictly speaking, it is known that most quantum spin liquids lose their identity at non-zero temperature, being in that case adiabatically transformable into a trivial paramagnet. This is the case for the U(1) quantum spin liquid states recently proposed to occur in the quantum spin ice pyrochlores. Here we propose, however, that in practical terms, the latter quantum spin liquids can be regarded as distinct phases from the high temperature paramagnet. Through a combination of gauge mean field theory calculations and physical reasoning, we argue that these systems sustain both quantum spin liquid and thermal spin liquid phases, dominated by quantum fluctuations and entropy, respectively. These phases are separated by a first order "thermal confinement" transition, such that for temperatures below the transition, spinons and emergent photons are coherently propagating excitations, and above it the dynamics is classical. Even for parameters for which the ground state is magnetically ordered and not a quantum spin liquid, this strong first order transition occurs, pre-empting conventional Landau-type criticality. We argue that this picture explains the anomalously low temperature phase transition observed in the quantum spin ice material Yb2Ti2O7.

cond-mat.str-el↗

arXiv2013

Dimer Mott Insulator in an Oxide Heterostructure

We study the problem of designing an artificial Mott insulator in a correlated oxide heterostructure. We consider the extreme limit of quantum confinement based on ionic discontinuity doping, and argue that a unique dimer Mott insulator can be achieved for the case of a single SrO layer in a GdTiO$_3$ matrix. In the dimer Mott insulator, electrons are localized not to individual atoms but to bonding orbitals on molecular dimers formed across a bilayer of two TiO$_2$ planes, and is analogous to the Mott insulating state of Hubbard ladders, studied in the 1990s. We verify the existence of the dimer Mott insulator through both ab initio and model Hamiltonian studies, and find for reasonable values of Hubbard $U$ that it is stable and ferromagnetic with a clear bonding/anti-bonding splitting of order 0.65eV, and a significant smaller Mott gap whose size depends upon $U$. The combined effects of polar discontinuity, strong structural relaxation and electron correlations all contribute to the realization of this unique ground state.

cond-mat.str-el↗

arXiv2012

Non-Fermi liquid and topological states with strong spin-orbit coupling

We argue that a class of strongly spin-orbit coupled materials, including some pyrochlore iridates and the inverted band gap semiconductor HgTe, may be described by a minimal model consisting of the Luttinger Hamiltonian supplemented by Coulomb interactions, a problem studied by Abrikosov and collaborators. It contains two-fold degenerate conduction and valence bands touching quadratically at the zone center. Using modern renormalization group methods, we update and extend Abrikosov's classic work and show that interactions induce a quantum critical non-Fermi liquid phase, stable provided time-reversal and cubic symmetries are maintained. We determine the universal power-law exponents describing various observables in this "Luttinger Abrikosov Beneslavskii" state, which include conductivity, specific heat,non-linear susceptibility and magnetic Gruneisen number. Furthermore, we determine the phase diagram in the presence of cubic and/or time-reversal symmetry breaking perturbations, which includes topological insulator and Weyl semi-metal phases. Many of these phases possess an extraordinarily large anomalous Hall effect, with the Hall conductivity scaling sub-linearly with magnetization.

cond-mat.str-el↗

arXiv2012

Toward an artificial Mott insulator: Correlations in confined, high-density electron liquids in SrTiO3

We investigate correlation physics in high-density, two-dimensional electron liquids that reside in narrow SrTiO3 quantum wells. The quantum wells are remotely doped via an interfacial polar discontinuity and the three-dimensional (3D) carrier density is modulated by changing the width of the quantum well. It is shown that even at 3D densities well below one electron per site, short-range Coulomb interactions become apparent in transport, and an insulating state emerges at a critical density. We also discuss the role of disorder in the insulating state.

cond-mat.str-el↗

arXiv2012

Quantum oscillations from a two-dimensional electron gas at a Mott/band insulator interface

We report on the magnetotransport properties of a prototype Mott insulator/band insulator perovskite heterojunction in magnetic fields up to 31 T and at temperatures between 360 mK and 10 K. Shubnikov-de Haas oscillations in the magnetoresistance are observed. The oscillations are two-dimensional in nature and are interpreted as arising from either a single, spin-split subband or two subbands. In either case, the electron system that gives rise to the oscillations represents only a fraction of the electrons in the space charge layer at the interface. The temperature dependence of the oscillations are used to extract an effective mass of ~ 1 me for the subband(s). The results are discussed in the context of the t2g-states that form the bottom of the conduction band of SrTiO3.

cond-mat.mes-hall↗

arXiv2012

Spin Liquid Ground State of the Spin-1/2 Square $J_1$-$J_2$ Heisenberg Model

We perform highly accurate density matrix renormalization group (DMRG) simulations to investigate the ground state properties of the spin-1/2 antiferromagnetic square lattice Heisenberg $J_1$-$J_2$ model. Based on studies of numerous long cylinders with circumferences of up to 14 lattice spacings, we obtain strong evidence for a topological quantum spin liquid state in the region $0.41\leq J_2/J_1\leq 0.62$, separating conventional Néel and striped antiferromagnetic states for smaller and larger $J_2/J_1$, respectively. The quantum spin liquid is characterized numerically by the absence of magnetic or valence bond solid order, and non-zero singlet and triplet energy gaps. Furthermore, we positively identify its topological nature by measuring a non-zero topological entanglement entropy $γ=0.70\pm 0.02$, extremely close to $γ=\ln(2) \approx 0.69$ (expected for a $Z_2$ quantum spin liquid) and a non-trivial finite size dimerization effect depending upon the parity of the circumference of the cylinder. We also point out that a valence bond solid, and indeed any discrete symmetry breaking state, would be expected to show a constant correction to the entanglement entropy of {\sl opposite} sign to the topological entanglement entropy.

cond-mat.str-el↗

arXiv2012

Identifying Topological Order by Entanglement Entropy

Topological phases are unique states of matter incorporating long-range quantum entanglement, hosting exotic excitations with fractional quantum statistics. We report a practical method to identify topological phases in arbitrary realistic models by accurately calculating the Topological Entanglement Entropy (TEE) using the Density Matrix Renormalization Group (DMRG). We argue that the DMRG algorithm naturally produces a minimally entangled state, from amongst the quasi-degenerate ground states in a topological phase. This proposal both explains the success of this method, and the absence of ground state degeneracy found in prior DMRG sightings of topological phases. We demonstrate the effectiveness of the calculational procedure by obtaining the TEE for several microscopic models, with an accuracy of order $10^{-3}$ when the circumference of the cylinder is around ten times the correlation length. As an example, we definitively show the ground state of the quantum $S=1/2$ antiferromagnet on the kagomé lattice is a topological spin liquid, and strongly constrain the full identification of this phase of matter.

cond-mat.str-el↗

arXiv2012

Generic quantum spin ice

We consider possible exotic ground states of quantum spin ice as realized in rare earth pyrochlores. Prior work in Phys. Rev. Lett. 108, 037202 introduced a gauge mean field theory (gMFT) to treat spin or pseudospin Hamiltonians for such systems, reformulated as a problem of bosonic spinons coupled to a U(1) gauge field. We extend gMFT to treat the most general, nearest neighbor exchange Hamiltonian, which contains a further exchange interaction, not considered previously. This term leads to interactions between spinons, and requires a significant extension of gMFT, which we provide. As an application, we focus especially on the non-Kramers materials Pr2TM2O7 (TM=Sn, Zr, Hf, and Ir), for which the additional term is especially important, but for which an Ising-planar exchange coupling discussed previously is forbidden by time-reversal symmetry. In this case, when the planar XY exchange is unfrustrated, we perform a full analysis and find three quantum ground states: a U(1) quantum spin liquid (QSL), an antiferro-quadrupolar ordered state and a non-coplanar ferro-quadrupolar ordered one. We also consider the case of frustrated XY exchange, and find that it favors a pi-flux QSL, with an emergent line degeneracy of low energy spinon excitations. This feature greatly enhances the stability of the QSL with respect to classical ordering.

cond-mat.str-el↗

arXiv2012

Definitive Evidence for Order-by-Quantum-Disorder in Er2Ti2O7

Here we establish the systematic existence of a U(1) degeneracy of all symmetry-allowed Hamiltonians quadratic in the spins on the pyrochlore lattice, at the mean-field level. By extracting the Hamiltonian of Er2Ti2O7 from inelastic neutron scattering measurements, we then show that the U(1)-degenerate states of Er2Ti2O7 are its classical ground states, and unambiguously show that quantum fluctuations break the degeneracy in a way which is confirmed by experiment. This is the first definitive observation of order-by-disorder in any material. We provide further verifiable consequences of this phenomenon, and several additional comparisons between theory and experiment.

cond-mat.str-el↗

arXiv2012

Time-reversal invariant realization of the Weyl semimetal phase

We propose a realization of the Weyl semimetal phase that is invariant under time reversal and occurs due to broken inversion symmetry. We consider both a simple superlattice model and a more realistic tight-binding model describing an experimentally reasonable HgTe/CdTe multilayer structure. The two models have the same underlying symmetry, therefore their low-energy features are equivalent. We find a Weyl semimetal phase between the normal insulator and the topological insulator phases that exists for a finite range of the system parameters and exhibits a finite number of Weyl points with robust band touching at the Fermi level. This phase is experimentally characterized by a strong conductivity anisotropy and topological surface states. The principal conductivities change in a complementary fashion as the system parameters are varied, and the surface states only exist in a region of momentum space that is determined by the positions of the Weyl points.

cond-mat.mes-hall↗

arXiv2011

Topological nodal semimetals

We present a study of "nodal semimetal" phases, in which non-degenerate conduction and valence bands touch at points (the "Weyl semimetal") or lines (the "line node semimetal") in three-dimensional momentum space. We discuss a general approach to such states by perturbation of the critical point between a normal insulator (NI) and a topological insulator (TI), breaking either time reversal (TR) or inversion symmetry. We give an explicit model realization of both types of states in a NI--TI superlattice structure with broken TR symmetry. Both the Weyl and the line-node semimetals are characterized by topologically-protected surface states, although in the line-node case some additional symmetries must be imposed to retain this topological protection. The edge states have the form of "Fermi arcs" in the case of the Weyl semimetal: these are chiral gapless edge states, which exist in a finite region in momentum space, determined by the momentum-space separation of the bulk Weyl nodes. The chiral character of the edge states leads to a finite Hall conductivity. In contrast, the edge states of the line-node semimetal are "flat bands": these states are approximately dispersionless in a subset of the two-dimensional edge Brillouin zone, given by the projection of the line node onto the plane of the edge. We discuss unusual transport properties of the nodal semimetals, and in particular point out quantum critical-like scaling of the DC and optical conductivity of the Weyl semimetal, and similarities to the conductivity of graphene in the line node case.

cond-mat.mes-hall↗

arXiv2011

Probing the metal-insulator transition of NdNiO3 by electrostatic doping

Modulation of the charge carrier density in a Mott material by remote doping from a highly doped conventional band insulator is proposed to test theoretical predictions of band filling control of the Mott metal-insulator transition without introducing lattice distortions or disorder, as is the case for chemical doping. The approach is experimentally tested using ultrathin (2.5 nm) NdNiO3 films that are epitaxially grown on La-doped SrTiO3 films. We show that remote doping systematically changes the charge carrier density in the NdNiO3 film and causes a moderate shift in the metal-insulator transition temperature. These results are discussed in the context of theoretical models of this class of materials exhibiting a metal-insulator transition.

cond-mat.mtrl-sci↗

arXiv2011

Spin Liquid Phases for Spin-1 systems on the Triangular lattice

Motivated by recent experiments on material Ba3NiSb2O9, we propose two novel spin liquid phases (A and B) for spin-1 systems on a triangular lattice. At the mean field level, both spin liquid phases have gapless fermionic spinon excitations with quadratic band touching, thus in both phases the spin susceptibility and C_v/T saturate to a constant at zero temperature, which are consistent with the experimental results on Ba3NiSb2O9. On the lattice scale, these spin liquid phases have Sp(4) ~ SO(5) gauge fluctuation; while in the long wavelength limit this Sp(4) gauge symmetry is broken down to U(1)xZ_2 in type A spin liquid phase, and broken down to Z_4 in type B phase. We also demonstrate that the $A$ phase is the parent state of the ferro-quadrupole state, nematic state, and the noncollinear spin density wave state.

cond-mat.str-el↗