53 research outputs found
Absence of long-range order in a triangular spin system with dipolar interactions
Antiferromagnetic Heisenberg model on the triangular lattice is perhaps the
best known example of frustrated magnets, but it orders at low temperatures.
Recent density matrix renormalization group (DMRG) calculations find that next
nearest neighbor interaction enhances the frustration and leads to a spin
liquid for . In addition, DMRG study of a dipolar
Heisenberg model with longer range interactions gives evidence for a spin
liquid at small dipole titling angle . In both cases,
the putative spin liquid region appears to be small. Here, we show that for the
triangular lattice dipolar Heisenberg model, a robust quantum paramagnetic
phase exists in a surprisingly wide region, , for
dipoles tilted along the lattice diagonal direction. We obtain the phase
diagram of the model by functional renormalization group (RG) which treats all
magnetic instabilities on equal footing. The quantum paramagnetic phase is
characterized by a smooth continuous flow of vertex functions and spin
susceptibility down to the lowest RG scale, in contrast to the apparent
breakdown of RG flow in phases with stripe or spiral order. Our finding points
to a promising direction to search for quantum spin liquids in ultracold
dipolar molecules.Comment: 5 pages, 2 figures and the supplementary material
Weyl nodes in periodic structures of superconductors and spin active materials
Motivated by recent progress in epitaxial growth of proximity structures of
s-wave superconductors (S) and spin-active materials (M), we show that the
periodic structure of S and M can behave effectively as a superconductor with
pairs of point nodes, near which the low energy excitations are Weyl fermions.
A simple toy model, where M is described by a Kronig-Penney potential with both
spin-orbit coupling and exchange field, is proposed and solved to obtain the
phase diagram of the nodal structure, the spin texture of the Weyl fermions, as
well as the zero energy surface states in the form of open Fermi lines ("Fermi
arcs"). Going beyond the simple model, a lattice model with alternating layers
of S and magnetic topological insulators (M) is solved. The calculated
spectrum confirms previous prediction of Weyl nodes based on tunneling
Hamiltonian of Dirac electrons. Our results provide further evidence that
periodic structures of S and M are well suited for engineering gapless
topological superconductors.Comment: Research article, contribution to theme issue "Andreev bound states"
ed. Laura H. Greene and James A. Saul
Nonequilibrium spin-transfer torque in SFNFS junctions
We report theoretical results for the nonequilibrium spin current and
spin-transfer torque in voltage-biased SFNFS Josephson structures. The
subharmonic gap structures and high voltage asymptotic behaviors of the dc and
ac components of the spin current are analyzed and related to the
spin-dependent inelastic scattering of quasiparticles at both F layers.Comment: minor changes, published versio
Scrambling dynamics and many-body chaos in a random dipolar spin model
Is there a quantum many-body system that scrambles information as fast as a
black hole? The Sachev-Ye-Kitaev model can saturate the conjectured bound for
chaos, but it requires random all-to-all couplings of Majorana fermions that
are hard to realize in experiments. Here we examine a quantum spin model of
randomly oriented dipoles where the spin exchange is governed by dipole-dipole
interactions. The model is inspired by recent experiments on dipolar spin
systems of magnetic atoms, dipolar molecules, and nitrogen-vacancy centers. We
map out the phase diagram of this model by computing the energy level
statistics, spectral form factor, and out-of-time-order correlation (OTOC)
functions. We find a broad regime of many-body chaos where the energy levels
obey Wigner-Dyson statistics and the OTOC shows distinctive behaviors at
different times: Its early-time dynamics is characterized by an exponential
growth, while the approach to its saturated value at late times obeys a power
law. The temperature scaling of the Lyapunov exponent shows that
while it is well below the conjectured bound at high temperatures,
approaches the bound at low temperatures and for large number of
spins.Comment: 7 pages, 8 figures with updated reference
Effective action approach to the p-band Mott insulator and superfluid transition
Motivated by the recent experiment on p-orbital band bosons in optical
lattices, we study theoretically the quantum phases of Mott insulator and
superfluidity in two-dimensions. The system features a novel superfluid phase
with transversely staggered orbital current at weak interaction, and a Mott
insulator phase with antiferro-orbital order at strong coupling and
commensurate filling. We go beyond mean field theory and derive from a
microscopic model an effective action that is capable of describing both the
p-band Mott insulating and superfluid phases in strong coupling. We further
calculate the excitation spectra near the quantum critical point and find two
gapless modes away from the tip of the Mott lobe but four gapless modes at the
tip. Our effective theory reveals how the phase coherence peak builds up in the
Mott regime when approaching the critical point. We also discuss the finite
temperature phase transition of p-band superfluidity.Comment: 9+epsilon pages, 7 figures, one appendix added, accepted by Phys.
Rev.
Theory of interacting fermions in shaken square optical lattice
We develop a theory of weakly interacting fermionic atoms in shaken optical
lattices based on the orbital mixing in the presence of time-periodic
modulations. Specifically, we focus on fermionic atoms in circularly shaken
square lattice with near resonance frequencies, i.e., tuned close to the energy
separation between -band and the -bands. First, we derive a
time-independent four-band effective Hamiltonian in the non-interacting limit.
Diagonalization of the effective Hamiltonian yields a quasi-energy spectrum
consistent with the full numerical Floquet solution that includes all higher
bands. In particular, we find that the hybridized -band develops multiple
minima and therefore non-trivial Fermi surfaces at different fillings. We then
obtain the effective interactions for atoms in the hybridized -band
analytically and show that they acquire momentum dependence on the Fermi
surface even though the bare interaction is contact-like. We apply the theory
to find the phase diagram of fermions with weak attractive interactions and
demonstrate that the pairing symmetry is -wave. Our theory is valid for a
range of shaking frequencies near resonance, and it can be generalized to other
phases of interacting fermions in shaken lattices.Comment: 12 pages with 5 figures. Comments and reference suggestions are
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