110 research outputs found
Nonmagnetic impurities in a S=(1/2) frustrated triangular antiferromagnet: Broadening of 13C NMR lines in κ-(ET)2Cu2(CN)3
We study effects of nonmagnetic impurities in a spin-1/2 frustrated triangular antiferromagnet with the aim of understanding the observed broadening of 13C NMR lines in the organic spin liquid material κ-(ET)2Cu2(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 2kF. 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 pointlike disorder gives broadening that is consistent with the experiment down to about 5 K, but that below this temperature additional mechanism is likely needed
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 term acts on 11 plaquettes and a term acts
on 12 and 21 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 , valence bond solid for intermediate , and possibly for
large the EBL phase. Away from half-filling, the EBL phase is present for
intermediate and remains stable for a range of densities below 1/2 before
phase separation occurs at lower densities.Comment: 18 pages, 15 figure
Variational study of J_(1)-J_(2) 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) ≳ 0.08J_(1) and the 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
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
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 term acts on 11
plaquettes and a term acts on 12 and 21 plaquettes. At
half-filling, the phase diagram reveals charge density wave for small ,
valence bond solid for intermediate , and possibly for large 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 and remains stable for a range of
densities below 1/2 before phase separation sets in at lower densitiesComment: 4 page
Explicit construction of quasi-conserved local operator of translationally invariant non-integrable quantum spin chain in prethermalization
We numerically construct translationally invariant quasi-conserved operators
with maximum range M which best-commute with a non-integrable quantum spin
chain Hamiltonian, up to M = 12. In the large coupling limit, we find that the
residual norm of the commutator of the quasi-conserved operator decays
exponentially with its maximum range M at small M, and turns into a slower
decay at larger M. This quasi-conserved operator can be understood as a dressed
total "spin-z" operator, by comparing with the perturbative Schrieffer-Wolff
construction developed to high order reaching essentially the same maximum
range. We also examine the operator inverse participation ratio of the
operator, which suggests its localization in the operator Hilbert space. The
operator also shows almost exponentially decaying profile at short distance,
while the long-distance behavior is not clear due to limitations of our
numerical calculation. Further dynamical simulation confirms that the
prethermalization-equilibrated values are described by a generalized Gibbs
ensemble that includes such quasi-conserved operator.Comment: 22 pages with 13 pages of main text, 9 figures and 5 appendices
(published version
Exact Quantum Many-Body Scar States in the Rydberg-Blockaded Atom Chain
A recent experiment in the Rydberg atom chain observed unusual oscillatory
quench dynamics with a charge density wave initial state, and theoretical works
identified a set of many-body "scar states" showing nonthermal behavior in the
Hamiltonian as potentially responsible for the atypical dynamics. In the same
nonintegrable Hamiltonian, we discover several eigenstates at \emph{infinite
temperature} that can be represented exactly as matrix product states with
finite bond dimension, for both periodic boundary conditions (two exact
states) and open boundary conditions (two states and one each ). This discovery explicitly demonstrates violation of strong
eigenstate thermalization hypothesis in this model and uncovers exact quantum
many-body scar states. These states show signatures of translational symmetry
breaking with period-2 bond-centered pattern, despite being in one dimension at
infinite temperature. We show that the nearby many-body scar states can be well
approximated as "quasiparticle excitations" on top of our exact scar
states, and propose a quasiparticle explanation of the strong oscillations
observed in experiments.Comment: Published version. In addition to (v2): (1) Add additional proofs to
the exact scar states and intuitions behind SMA and MMA to the appendices.
(2) Add entanglement scaling of SMA and MMA to the appendice
Possible Exciton Bose Liquid in a Hard-Core Boson Ring Model
We present a quantum Monte Carlo study of a hard-core boson model with ring-only exchanges on a square lattice, where a K_1 term acts on 1×1 plaquettes and a K_2 term acts on 1×2 and 2×1 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 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
Phases and phase transitions in a U(1) × U(1) system with θ = 2π/3 mutual statistics
We study a U(1) × U(1) system with short-range interactions and mutual θ = 2π/3 statistics in (2+1)
dimensions. We are able to reformulate the model to eliminate the sign problem and perform a Monte Carlo
study. We find a phase diagram containing a phase with only small loops and two phases with one species of
proliferated loop. We also find a phase where both species of loop condense, but without any gapless modes.
Lastly, when the energy cost of loops becomes small, we find a phase that is a condensate of bound states, each
made up of three particles of one species and a vortex of the other. We define several exact reformulations of the
model that allow us to precisely describe each phase in terms of gapped excitations. We propose field-theoretic
descriptions of the phases and phase transitions, which are particularly interesting on the “self-dual” line where
both species have identical interactions. We also define irreducible responses useful for describing the phases
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