34 research outputs found
Magnon thermal Hall effect in kagome antiferromagnets with Dzyaloshinskii-Moriya interactions
We theoretically study magnetic and topological properties of
antiferromagnetic kagome spin systems in the presence of both in- and
out-of-plane Dzyaloshinskii-Moriya interactions. In materials such as the iron
jarosites, the in-plane interactions stabilize a canted noncollinear "umbrella"
magnetic configuration with finite scalar spin chirality. We derive expressions
for the canting angle, and use the resulting order as a starting point for a
spin-wave analysis. We find topological magnon bands, characterized by non-zero
Chern numbers. We calculate the magnon thermal Hall conductivity, and propose
the iron jarosites as a promising candidate system for observing the magnon
thermal Hall effect in a noncollinear spin configuration. We also show that the
thermal conductivity can be tuned by varying an applied magnetic field, or the
in-plane Dzyaloshinskii-Moriya strength. In contrast with previous studies of
topological magnon bands, the effect is found to be large even in the limit of
small canting.Comment: 11 pages, 11 figure
A flow equation approach to periodically driven quantum systems
We present a theoretical method to generate a highly accurate {\em
time-independent} Hamiltonian governing the finite-time behavior of a
time-periodic system. The method exploits infinitesimal unitary transformation
steps, from which renormalization group-like flow equations are derived to
produce the effective Hamiltonian. Our tractable method has a range of validity
reaching into frequency regimes that are usually inaccessible via high
frequency expansions in the parameter , where is the
upper limit for the strength of local interactions. We demonstrate our approach
on both interacting and non-interacting many-body Hamiltonians where it offers
an improvement over the more well-known Magnus expansion and other high
frequency expansions. For the interacting models, we compare our approximate
results to those found via exact diagonalization. While the approximation
generally performs better globally than other high frequency approximations,
the improvement is especially pronounced in the regime of lower frequencies and
strong external driving. This regime is of special interest because of its
proximity to the resonant regime where the effect of a periodic drive is the
most dramatic. Our results open a new route towards identifying novel
non-equilibrium regimes and behaviors in driven quantum many-particle systems.Comment: 25 pages, 14 figure
Spin dynamics of the generalized quantum spin compass chain
We calculate the dynamical spin structure factor of the generalized
spin- compass spin chain using the density matrix renormalization group.
The model, also known as the twisted Kitaev spin chain, was recently proposed
to be relevant for the description of the spin chain compound CoNbO. It
features bond-dependent interactions and interpolates between an Ising chain
and a one-dimensional variant of Kitaev's honeycomb spin model. The structure
factor, in turn, is found to interpolate from gapped and non-dispersive in the
Ising limit to gapless with non-trivial continua in the Kitaev limit. In
particular, the component of the structure factor perpendicular to the Ising
directions changes abruptly at the Kitaev point into a dispersionless continuum
due to the emergence of an extensive groundstate degeneracy. We show this
continuum is consistent with analytical Jordan-Wigner results. We also discuss
implications for future inelastic scattering experiments and applications to
materials, particularly CoNbO.Comment: Main text: 10 pages, 6 figures. Supplemental material: 5 pages, 2
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Luther-Emery liquid and dominant singlet superconductivity in the two-orbital Hubbard chain
We investigate the pairing tendencies in the two-orbital Hubbard chain at
intermediate repulsive interaction strengths , and for degenerate orbitals.
At half-filling and large , the ferromagnetic Hund's coupling,
, generates effective spin- moments, with antiferromagnetic
correlations between sites. Thus the system can be viewed as an electronic
generalization of Haldane's spin- chain in that limit. Using large-scale
density matrix renormalization group calculations, we study the system's
behavior under light hole-doping. For in units of the non-interacting
bandwidth and we find that singlet pairing
dominates the long-distance physics, establishing this system as a promising
platform for repulsively mediated superconductivity. We provide evidence that
the system approaches a Luther-Emery liquid state at large system sizes,
similarly to the behavior of doped one-orbital two-leg ladders at weak
coupling. The numerically calculated central charge approaches one in the
thermodynamic limit, indicating a single gapless mode as is expected for the
Luther-Emery state. Exponents characterizing the power-law decays of singlet
pair-pair and charge density-density correlations are determined, and found to
approximately satisfy the Luther-Emery identity. Candidate materials to realize
this physics are discussed.Comment: Main text: 9 pages, 6 figures. Supplementary: 6 pages, 5 figure
Momentum-space entanglement after a quench in one-dimensional disordered fermionic systems
We numerically investigate the momentum-space entanglement entropy and
entanglement spectrum of the random-dimer model and its generalizations, which
circumvent Anderson localization, after a quench in the Hamiltonian parameters.
The type of dynamics that occurs depends on whether or not the Fermi level of
the initial state is near the energy of the delocalized states present in these
models. If the Fermi level of the initial state is near the energy of the
delocalized states, we observe an interesting slow logarithmic-like growth of
the momentum-space entanglement entropy followed by an eventual saturation.
Otherwise, the momentum-space entanglement entropy is found to rapidly
saturate. We also find that the momentum-space entanglement spectrum reveals
the presence of delocalized states in these models for long times after the
quench and the many-body entanglement gap decays logarithmically in time when
the Fermi level is near the energy of the delocalized states.Comment: 4+e pages, 3 figure
Analogue of Hamilton-Jacobi theory for the time-evolution operator
In this paper we develop an analogue of Hamilton-Jacobi theory for the
time-evolution operator of a quantum many-particle system. The theory offers a
useful approach to develop approximations to the time-evolution operator, and
also provides a unified framework and starting point for many well-known
approximations to the time-evolution operator. In the important special case of
periodically driven systems at stroboscopic times, we find relatively simple
equations for the coupling constants of the Floquet Hamiltonian, where a
straightforward truncation of the couplings leads to a powerful class of
approximations. Using our theory, we construct a flow chart that illustrates
the connection between various common approximations, which also highlights
some missing connections and associated approximation schemes. These missing
connections turn out to imply an analytically accessible approximation that is
the "inverse" of a rotating frame approximation and thus has a range of
validity complementary to it. We numerically test the various methods on the
one-dimensional Ising model to confirm the ranges of validity that one would
expect from the approximations used. The theory provides a map of the relations
between the growing number of approximations for the time-evolution operator.
We describe these relations in a table showing the limitations and advantages
of many common approximations, as well as the new approximations introduced in
this paper.Comment: 17 pages, 5 figures, 1 tabl
Reconstructing the spatial structure of quantum correlations
Quantum correlations are a fundamental property of quantum many-body states.
Yet they remain experimentally elusive, hindering certification of genuine
quantum behavior, especially in quantum materials. Here we show that the
momentum-dependent dynamical susceptibility measured via inelastic neutron
scattering enables the systematic reconstruction of quantum correlation
functions, which express the degree of quantum coherence in the fluctuations of
two spins at arbitrary mutual distance. Using neutron scattering data on the
compound KCuF \unicode{x2014} a system of weakly coupled
Heisenberg chains \unicode{x2014} and of numerically exact quantum Monte
Carlo data, we show that quantum correlations possess a radically different
spatial structure with respect to conventional correlations. Indeed, they
exhibit a new emergent length of quantum-mechanical origin \unicode{x2014}
the quantum coherence length \unicode{x2014} which is finite at any finite
temperature (including when long-range magnetic order develops). Moreover, we
show theoretically that coupled Heisenberg spin chains exhibit a form of
quantum monogamy, with a trade-off between quantum correlations along and
transverse to the spin chains. These results highlight real-space quantum
correlators as an informative, model-independent means of probing the
underlying quantum state of real quantum materials.Comment: Main text: 8 pages, 5 figures. Supplementary information: 4 pages, 5
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