arXiv subjects

Olexei I. Motrunich

Publications and source records attributed to Olexei I. Motrunich.

At least 73 records · Page 4Linked to original sources

arXiv2011

Variational study of J1-J2 Heisenberg model on Kagome lattice using projected Schwinger boson wave functions

Motivated by the unabating interest in the spin-1/2 Heisenberg antiferromagnetic model on the Kagome lattice, we investigate the energetics of projected Schwinger boson (SB) wave functions in the $J_1$--$J_2$ model with antiferromagnetic $J_2$ coupling. Our variational Monte Carlo results show that Sachdev's $Q_1=Q_2$ SB ansatz has a lower energy than the Dirac spin liquid for $J_2\gtrsim 0.08 J_1$ and the ${\bf q=0}$ Jastrow type magnetically ordered state. This work demonstrates that the projected SB wave functions can be tested on the same footing as their fermionic counterparts.

cond-mat.str-el↗

arXiv2011

Exotic Gapless Mott Insulators of Bosons on Multi-Leg Ladders

We present evidence for an exotic gapless insulating phase of hard-core bosons on multi-leg ladders with a density commensurate with the number of legs. In particular, we study in detail a model of bosons moving with direct hopping and frustrating ring exchange on a 3-leg ladder at $ν=1/3$ filling. For sufficiently large ring exchange, the system is insulating along the ladder but has two gapless modes and power law transverse density correlations at incommensurate wave vectors. We propose a determinantal wave function for this phase and find excellent comparison between variational Monte Carlo and density matrix renormalization group calculations on the model Hamiltonian, thus providing strong evidence for the existence of this exotic phase. Finally, we discuss extensions of our results to other $N$-leg systems and to $N$-layer two-dimensional structures.

cond-mat.str-el↗

arXiv2010

Failure of Gutzwiller-type wave function to capture gauge fluctuations: Case study in the Exciton Bose Liquid context

Slave particle approaches are widely used in studies of exotic quantum phases. A complete description beyond mean field also contains dynamical gauge fields, while a simplified procedure considers Gutzwiller-projected trial states. We apply this in the context of bosonic models with ring exchanges realizing so-called Exciton Bose Liquid (EBL) phase and compare a Gutzwiller wave function against an accurate EBL wave function. We solve the parton-gauge theory and show that dynamical fluctuations of the spatial gauge fields are necessary for obtaining qualitatively accurate EBL description. On the contrary, just the Gutzwiller projection leads to a state with subtle differences in the long-wavelength properties, thus suggesting that Gutzwiller wave functions may generally fail to capture long-wavelength physics.

cond-mat.str-el↗

arXiv2010

Insulating phases of electrons on a zigzag strip in the orbital magnetic field

We consider electrons on a two-leg triangular ladder at half-filling and in an orbital magnetic field. In a two-band regime in the absence of the field, the electronic system remains conducting for weak interactions since there is no four-fermion Umklapp term. We find that in the presence of the orbital field there is a four-fermion Umklapp and it is always relevant for repulsive interactions. Thus in this special ladder, the combination of the orbital magnetic field and interactions provides a mechanism to drive metal-insulator transition already at weak coupling. We discuss properties of the possible resulting phases C0S2 and various C0S1 and C0S0.

cond-mat.str-el↗

arXiv2010

Possible realization of the Exciton Bose Liquid phase in a hard-core boson model with ring-only exchange interactions

We investigate a hard-core boson model with ring-only exchanges on a square lattice, where a $K_1$ term acts on 1$\times$1 plaquettes and a $K_2$ term acts on 1$\times$2 and 2$\times$1 plaquettes, with a goal of realizing a novel Exciton Bose Liquid (EBL) phase first proposed by Paramekanti, et al [Phys. Rev. B {\bf 66}, 054526 (2002)]. We construct Jastrow-type variational wave functions for the EBL, study their formal properties, and then use them as seeds for a projective Quantum Monte Carlo study. Using Green's Function Monte Carlo, we obtain an unbiased phase diagram which at half-filling reveals CDW for small $K_2$, valence bond solid for intermediate $K_2$, and possibly for large $K_2$ the EBL phase. Away from half-filling, the EBL phase is present for intermediate $K_2$ and remains stable for a range of densities below 1/2 before phase separation occurs at lower densities.

cond-mat.str-el↗

arXiv2010

Study of a hard-core boson model with ring-only interactions

We present a Quantum Monte Carlo study of a hardcore boson model with ring-only exchanges on a square lattice, where a $K_1$ term acts on 1$\times$1 plaquettes and a $K_2$ term acts on 1$\times$2 and 2$\times$1 plaquettes. At half-filling, the phase diagram reveals charge density wave for small $K_2$, valence bond solid for intermediate $K_2$, and possibly for large $K_2$ the novel Exciton Bose Liquid (EBL) phase first proposed by Paramekanti, et al[Phys. Rev. B {\bf 66}, 054526 (2002)]. Away from half-filling, the EBL phase is present already for intermediate $K_2$ and remains stable for a range of densities below 1/2 before phase separation sets in at lower densities

cond-mat.str-el↗

arXiv2010

Effects of impurities in Spin Bose-Metal phase on a two-leg triangular strip

We study effects of nonmagnetic impurities in a Spin Bose-Metal (SBM) phase discovered in a two-leg triangular strip spin-1/2 model with ring exchanges (D. N. Shenget al, arXiv:0902.4210). This phase is a quasi-1D descendant of a 2D spin liquid with spinon Fermi sea, and the present study aims at interpolating between the 1D and 2D cases. Different types of defects can be treated as local energy perturbations, which we find are always relevant. As a result, a nonmagnetic impurity generically cuts the system into two decoupled parts. We calculate bond energy and local spin susceptibility near the defect, both of which can be measured in experiments. The Spin Bose-Metal has dominant correlations at characteristic incommensurate wavevectors that are revealed near the defect. Thus, the bond energy shows a static texture oscillating as a function of distance from the defect and decaying as a slow power law. The local spin susceptibility also oscillates and actually {\it increases} as a function of distance from the defect, similar to the effect found in the 1D chain [S. Eggert and I. Affleck, Phys. Rev. Lett. {\bf 75}, 934 (1995)]. We calculate the corresponding power law exponents for the textures as a function of one Luttinger parameter of the SBM theory.

cond-mat.str-el↗

arXiv2010

Effects of Zeeman field on a Spin Bose-Metal phase

We consider Zeeman field effects on a Spin Bose-Metal (SBM) phase on a two-leg triangular ladder. This phase was found in a spin-1/2 model with ring exchanges [D. N. Sheng et. al., Phys. Rev. B {\bf 79}, 205112 (2009)], and was also proposed to appear in an interacting electronic model with longer-ranged repulsion [Lai et. al., Phys. Rev. B {\bf 81}, 045105 (2010)]. Using bosonization of a spinon-gauge theory, we study the stability of the SBM phase and its properties under the field. We also explore phases arising from potential instabilities of the SBM; in all cases, we find a gap to spin-1 excitations while spin-nematic correlations are power law. We discuss two-dimensional analogues of these phases where spinons can pair with their own species.

cond-mat.str-el↗

arXiv2010

Variational Studies of Triangular Heisenberg Antiferromagnet in Magnetic Field

We present a variational study of the Heisenberg antiferromagnet on the spatially anisotropic triangular lattice in magnetic field. First we construct a simple yet accurate wavefunction for the 1/3-magnetization plateau uud phase on the isotropic lattice. Beginning with this state, we obtain natural extensions to nearby commensurate coplanar phases on either side of the plateau. The latter occur also for low lattice anisotropy, while the uud state extends to much larger anisotropy. Far away from the 1/3 plateau and for significant anisotropy, incommensurate states have better energetics, and we address competition between coplanar and non-coplanar states in the high field regime. For very strong anisotropy, our study is dominated by quasi-1d physics. The variational study is supplemented by exact diagonalization calculations which provide a reference for testing the energetics of our trial wavefunctions as well as helping to identify candidate phases.

cond-mat.str-el↗

arXiv2009

Two-band electronic metal and neighboring spin liquid (spin Bose-metal) on a zigzag strip with longer-ranged repulsion

We consider an electronic model for realizing the Spin Bose-metal (SBM) phase on a 2-leg triangular strip --a spin liquid phase found by D. N. Sheng et al [Phys. Rev. B {\bf 79}, 205112 (2009)] in a spin-1/2 model with ring exchanges. The SBM can be viewed as a "C1S2" Mott insulator of electrons where the overall charge transporting mode is gapped out. We start from a two-band "C2S2" metal and consider extended repulsion motivated by recent ab initio derivation of electronic model for $κ$-ET spin liquid material [K. Nakamura et al, J. Phys. Soc. Jpn. {\bf 78}, 083710(2009)]. Using weak coupling renormalization group analysis, we find that the extended interactions allow much wider C2S2 metallic phase than in the Hubbard model with on-site repulsion only. An eight-fermion Umklapp term plays a crucial role in producing a Mott insulator but can not be treated in weak coupling. We use Bosonization to extend the analysis to intermediate coupling and study phases obtained out of the C2S2 metal upon increasing overall repulsion strength, finding that the SBM phase is a natural outcome for extended interactions.

cond-mat.str-el↗

arXiv2009

Interlayer coherent composite Fermi liquid phase in quantum Hall bilayers

Composite fermions have played a seminal role in understanding the quantum Hall effect, particularly the formation of a compressible `composite Fermi liquid' (CFL) at filling factor nu = 1/2. Here we suggest that in multi-layer systems interlayer Coulomb repulsion can similarly generate `metallic' behavior of composite fermions between layers, even if the electrons remain insulating. Specifically, we propose that a quantum Hall bilayer with nu = 1/2 per layer at intermediate layer separation may host such an interlayer coherent CFL, driven by exciton condensation of composite fermions. This phase has a number of remarkable properties: the presence of `bonding' and `antibonding' composite Fermi seas, compressible behavior with respect to symmetric currents, and fractional quantum Hall behavior in the counterflow channel. Quantum oscillations associated with the Fermi seas give rise to a new series of incompressible states at fillings nu = p/[2(p \pm 1)] per layer (p an integer), which is a bilayer analogue of the Jain sequence.

cond-mat.str-el↗

arXiv2009

Spin Bose-Metal phase in a spin-1/2 model with ring exchange on a two-leg triangular strip

Recent experiments on triangular lattice organic Mott insulators have found evidence for a 2D spin liquid in proximity to the metal-insulator transition. A Gutzwiller wavefunction study of the triangular lattice Heisenberg model with appropriate four-spin ring exchanges has found that the projected spinon Fermi sea state has a low variational energy. This wavefunction, together with a slave particle gauge theory, suggests that such spin liquid possesses spin correlations that are singular along surfaces in momentum space ("Bose surfaces"). Signatures of this state, which we refer to as a "Spin Bose-Metal" (SBM), are expected to be manifest in quasi-1D ladder systems: The discrete transverse momenta cut through the 2D Bose surface leading to a distinct pattern of 1D gapless modes. Here we search for a quasi-1D descendant of the triangular lattice SBM state by exploring the Heisenberg plus ring model on a two-leg strip (zigzag chain). Using DMRG, variational wavefunctions, and a Bosonization analysis, we map out the full phase diagram. Without ring exchange the model is equivalent to the J_1 - J_2 Heisenberg chain, and we find the expected Bethe-chain and dimerized phases. Remarkably, moderate ring exchange reveals a new gapless phase over a large swath of the phase diagram. Spin and dimer correlations possess particular singular wavevectors and allow us to identify this phase as the hoped for quasi-1D descendant SBM state. We derive a low energy theory and find three gapless modes and one Luttinger parameter controlling all power laws. Potential instabilities out of the zigzag SBM give rise to other interesting phases such as a period-3 VBS or a period-4 Chirality order, which we discover in the DMRG; we also find an interesting SBM state with partial ferromagnetism.

cond-mat.str-el↗

arXiv2008

Spin, Bose, and Non-Fermi Liquid Metals in Two Dimensions: Accessing via Multi-Leg Ladders

Characterizing and accessing quantum phases of itinerant bosons or fermions in two dimensions (2D) that exhibit singular structure along surfaces in momentum space but have no quasi-particle description remains as a central challenge in the field of strongly correlated physics. Fortuitously, signatures of such 2D strongly correlated phases are expected to be manifest in quasi-one-dimensional "$N$-leg ladder" systems. The ladder discretization of the transverse momentum cuts through the 2D surface, leading to a quasi-1D descendant state with a set of low-energy modes whose number grows with the number of legs and whose momenta are inherited from the 2D surfaces. These multi-mode quasi-1D liquids constitute a new and previously unanticipated class of quantum states interesting in their own right. But more importantly they carry a distinctive quasi-1D "fingerprint" of the parent 2D quantum fluid state. This can be exploited to access the 2D phases from controlled numerical and analytical studies in quasi-1D models. The preliminary successes and future prospects in this endeavor will be briefly summarized.

cond-mat.str-el↗

arXiv2008

Comparative study of Higgs transition in one-component and two-component lattice superconductor models

Using Monte Carlo simulations, we study a Higgs transition in several three-dimensional lattice realizations of the noncompact CP$^1$ model (NCCP$^1$), a gauge theory with two complex matter fields with SU(2) invariance. By comparing with a one-component theory, which is well understood and has continuous transition in the inverted XY universality class, we argue that the two-component case also has continuous Higgs transition with a larger correlation length exponent (i.e., it is ``more continuous''). The transition can become first order in the vicinity of a new ``Molecular'' phase, which occurs in one of our models, but is continuous in a wide range of parameters away from this phase. The situation is significantly clarified by studying a model where the Molecular phase is entirely absent, and a wide regime with a continuous transition can be readily established. The two-component theory is also an effective description of the hedgehog-suppressed O(3) universality, and results are relevant for the recently discussed ``deconfined quantum criticality'' scenario for the continuous Valence Bond Solid to Neel quantum phase transition.

cond-mat.stat-mech↗

arXiv2008

Impurities in triangular lattice spin 1/2 antiferromagnet

We study effects of nonmagnetic impurities in a spin-1/2 frustrated triangular antiferromagnet with the aim of understanding the observed broadening of $^{13}$C NMR lines in the organic spin liquid material $κ$-(ET)$_2$Cu$_2$(CN)$_3$. For high temperatures down to $J/3$, we calculate local susceptibility near a nonmagnetic impurity and near a grain boundary for the nearest neighbor Heisenberg model in high temperature series expansion. We find that the local susceptibility decays to the uniform one in few lattice spacings, and for a low density of impurities we would not be able to explain the line broadening present in the experiments already at elevated temperatures. At low temperatures, we assume a gapless spin liquid with a Fermi surface of spinons. We calculate the local susceptibility in the mean field and also go beyond the mean field by Gutzwiller projection. The zero temperature local susceptibility decays as a power law and oscillates at $2 k_F$. As in the high temperature analysis we find that a low density of impurities is not able to explain the observed broadening of the lines. We are thus led to conclude that there is more disorder in the system. We find that a large density of point-like disorder gives broadening that is consistent with the experiment down to about 5K, but that below this temperature additional mechanism is likely needed.

cond-mat.str-el↗

arXiv2008

Strong-Coupling Phases of Frustrated Bosons on a 2-leg Ladder with Ring Exchange

Developing a theoretical framework to access quantum phases of itinerant bosons or fermions in two dimensions (2D) that exhibit singular structure along surfaces in momentum space but have no quasi-particle description remains as a central challenge in the field of strongly correlated physics. In this paper we propose that distinctive signatures of such 2D strongly correlated phases will be manifest in quasi-one-dimensional "N-leg ladder" systems. Characteristic of each parent 2D quantum liquid would be a precise pattern of 1D gapless modes on the N-leg ladder. These signatures could be potentially exploited to approach the 2D phases from controlled numerical and analytical studies in quasi-1D. As a first step we explore itinerant boson models with a frustrating ring exchange interaction on the 2-leg ladder, searching for signatures of the recently proposed two-dimensional d-wave correlated Bose liquid (DBL) phase. A combination of exact diagonalization, density matrix renormalization group, variational Monte Carlo, and bosonization analysis of a quasi-1D gauge theory, all provide compelling evidence for the existence of a new strong-coupling phase of bosons on the 2-leg ladder which can be understood as a descendant of the two-dimensional DBL. We suggest several generalizations to quantum spin and electron Hamiltonians on ladders which could likewise reveal fingerprints of such 2D non-Fermi liquid phases.

cond-mat.str-el↗

arXiv2008

Studies of non-magnetic impurities in the spin-1/2 Kagome Antiferromagnet

Motivated by recent experiments on ZnCu$_3$(OH)$_6$Cl$_2$, we study the inhomogeneous Knight shifts (local susceptibilities) of spin 1/2 Kagome antiferromagnet in the presence of nonmagnetic impurities. Using high temperature series expansion, we calculate the local susceptibility and its histogram down to about T=0.4J. At low temperatures, we explore a Dirac spin liquid proposal and calculate the local susceptibility in the mean field and beyond mean field using Gutzwiller projection, finding the overall picture to be consistent with the NMR experiments.

cond-mat.str-el↗

arXiv2007

Spin solid phases of spin 1 and spin 3/2 antiferromagnets on a cubic lattice

We study spin S=1 and S=3/2 Heisenberg antiferromagnets on a cubic lattice focusing on spin solid states. Using Schwinger boson formulation for spins, we start in a U(1) spin liquid phase proximate to Neel phase and explore possible confining paramagnetic phases as we transition away from the spin liquid by the process of monopole condensation. Electromagnetic duality is used to rewrite the theory in terms of monopoles. For spin 1 we find several candidate phases of which the most natural one is a phase with spins organized into parallel Haldane chains. For spin 3/2 we find that the most natural phase has spins organized into parallel ladders. As a by-product, we also write a Landau theory of the ordering in two special classical frustrated XY models on the cubic lattice, one of which is the fully frustrated XY model. In a particular limit our approach maps to a dimer model with 2S dimers coming out of every site, and we find the same spin solid phases in this regime as well.

cond-mat.str-el↗