1,066 research outputs found
Dynamical critical exponent of the Jaynes-Cummings-Hubbard model
An array of high-Q electromagnetic resonators coupled to qubits gives rise to
the Jaynes-Cummings-Hubbard model describing a superfluid to Mott insulator
transition of lattice polaritons. From mean-field and strong coupling
expansions, the critical properties of the model are expected to be identical
to the scalar Bose-Hubbard model. A recent Monte Carlo study of the superfluid
density on the square lattice suggested that this does not hold for the
fixed-density transition through the Mott lobe tip. Instead, mean-field
behavior with a dynamical critical exponent z=2 was found. We perform
large-scale quantum Monte Carlo simulations to investigate the critical
behavior of the superfluid density and the compressibility. We find z=1 at the
tip of the insulating lobe. Hence the transition falls in the 3D XY
universality class, analogous to the Bose-Hubbard model.Comment: 4 pages, 4 figures. To appear as a Rapid Communication in Phys. Rev.
Consequences of the Pauli exclusion principle for the Bose-Einstein condensation of atoms and excitons
The bosonic atoms used in present day experiments on Bose-Einstein
condensation are made up of fermionic electrons and nucleons. In this Letter we
demonstrate how the Pauli exclusion principle for these constituents puts an
upper limit on the Bose-Einstein-condensed fraction. Detailed numerical results
are presented for hydrogen atoms in a cubic volume and for excitons in
semiconductors and semiconductor bilayer systems. The resulting condensate
depletion scales differently from what one expects for bosons with a repulsive
hard-core interaction. At high densities, Pauli exclusion results in
significantly more condensate depletion. These results also shed a new light on
the low condensed fraction in liquid helium II.Comment: 4 pages, 2 figures, revised version, now includes a direct comparison
with hard-sphere QMC results, submitted to Phys. Rev. Let
Relaxation and thermalization in the one-dimensional Bose-Hubbard model: A case study for the interaction quantum quench from the atomic limit
Motivated by recent experiments, we study the relaxation dynamics and
thermalization in the one-dimensional Bose-Hubbard model induced by a global
interaction quench. Specifically, we start from an initial state that has
exactly one boson per site and is the ground state of a system with infinitely
strong repulsive interactions at unit filling. Using exact diagonalization and
the density matrix renormalization group method, we compute the time dependence
of such observables as the multiple occupancy and the momentum distribution
function. Typically, the relaxation to stationary values occurs over just a few
tunneling times. The stationary values are identical to the so-called diagonal
ensemble on the system sizes accessible to our numerical methods and we further
observe that the micro-canonical ensemble describes the steady state of many
observables reasonably well for small and intermediate interaction strength.
The expectation values of observables in the canonical ensemble agree
quantitatively with the time averages obtained from the quench at small
interaction strengths, and qualitatively provide a good description of
steady-state values even in parameter regimes where the micro-canonical
ensemble is not applicable due to finite-size effects. We discuss our numerical
results in the framework of the eigenstate thermalization hypothesis. Moreover,
we also observe that the diagonal and the canonical ensemble are practically
identical for our initial conditions already on the level of their respective
energy distributions for small interaction strengths. Finally, we discuss
implications of our results for the interpretation of a recent sudden expansion
experiment [Phys. Rev. Lett. 110, 205301 (2013)], in which the same interaction
quench was realized.Comment: 19 pages, 22 figure
Ultracold atoms in one-dimensional optical lattices approaching the Tonks-Girardeau regime
Recent experiments on ultracold atomic alkali gases in a one-dimensional
optical lattice have demonstrated the transition from a gas of soft-core bosons
to a Tonks-Girardeau gas in the hard-core limit, where one-dimensional bosons
behave like fermions in many respects. We have studied the underlying many-body
physics through numerical simulations which accommodate both the soft-core and
hard-core limits in one single framework. We find that the Tonks-Girardeau gas
is reached only at the strongest optical lattice potentials. Results for
slightly higher densities, where the gas develops a Mott-like phase already at
weaker optical lattice potentials, show that these Mott-like short range
correlations do not enhance the convergence to the hard-core limit.Comment: 4 pages, 3 figures, replaced with published versio
Thermodynamics of the 3D Hubbard model on approach to the Neel transition
We study the thermodynamic properties of the 3D Hubbard model for
temperatures down to the Neel temperature using cluster dynamical mean-field
theory. In particular we calculate the energy, entropy, density, double
occupancy and nearest-neighbor spin correlations as a function of chemical
potential, temperature and repulsion strength. To make contact with cold-gas
experiments, we also compute properties of the system subject to an external
trap in the local density approximation. We find that an entropy per particle
at is sufficient to achieve a Neel state in the
center of the trap, substantially higher than the entropy required in a
homogeneous system. Precursors to antiferromagnetism can clearly be observed in
nearest-neighbor spin correlators.Comment: 4 pages, 6 figure
Evidence for hard and soft substructures in thermoelectric SnSe
SnSe is a topical thermoelectric material with a low thermal conductivity
which is linked to its unique crystal structure. We use low-temperature heat
capacity measurements to demonstrate the presence of two characteristic
vibrational energy scales in SnSe with Debye temperatures thetaD1 = 345(9) K
and thetaD2 = 154(2) K. These hard and soft substructures are quantitatively
linked to the strong and weak Sn-Se bonds in the crystal structure. The heat
capacity model predicts the temperature evolution of the unit cell volume,
confirming that this two-substructure model captures the basic thermal
properties. Comparison with phonon calculations reveals that the soft
substructure is associated with the low energy phonon modes that are
responsible for the thermal transport. This suggests that searching for
materials containing highly divergent bond distances should be a fruitful route
for discovering low thermal conductivity materials.Comment: Accepted by Applied Physics Letter
The Higgs mode in a two-dimensional superfluid
We present solid evidence for the existence of a well-defined Higgs amplitude
mode in two-dimensional relativistic field theories based on analytically
continued results from quantum Monte Carlo simulations of the Bose-Hubbard
model in the vicinity of the superfluid-Mott insulator quantum critical point,
featuring emergent particle-hole symmetry and Lorentz-invariance. The Higgs
boson, seen as a well-defined low-frequency resonance in the spectral density,
is quickly pushed to high energies in the superfluid phase and disappears by
merging with the broad secondary peak at the characteristic interaction scale.
Simulations of a trapped system of ultra-cold Rb atoms demonstrate that
the low-frequency resonance feature is lost for typical experimental
parameters, while the characteristic frequency for the onset of strong response
is preserved.Comment: 9 pages, 13 figures; replaced with published versio
Quantum Monte Carlo simulation in the canonical ensemble at finite temperature
A quantum Monte Carlo method with non-local update scheme is presented. The
method is based on a path-integral decomposition and a worm operator which is
local in imaginary time. It generates states with a fixed number of particles
and respects other exact symmetries. Observables like the equal-time Green's
function can be evaluated in an efficient way. To demonstrate the versatility
of the method, results for the one-dimensional Bose-Hubbard model and a nuclear
pairing model are presented. Within the context of the Bose-Hubbard model the
efficiency of the algorithm is discussed.Comment: 11 pages, 8 figure
The ‘strength of weak ties’ among female baboons : fitness-related benefits of social bonds
Thanks to Cape Nature Conservation for permission to work at De Hoop, and to all the graduate students and field assistants who contributed to our long-term data-base. LB was supported by NSERC Canada Research Chair and Discovery Programs; SPH was supported by the NRF (South Africa) and NSERC Discovery Grants during the writing of this manuscript. We are grateful to one anonymous reviewer and, in particular, Lauren Brent for invaluable feedback on earlier drafts of our manuscript.Peer reviewedPostprin
Bosons Confined in Optical Lattices: the Numerical Renormalization Group revisited
A Bose-Hubbard model, describing bosons in a harmonic trap with a
superimposed optical lattice, is studied using a fast and accurate variational
technique (MF+NRG): the Gutzwiller mean-field (MF) ansatz is combined with a
Numerical Renormalization Group (NRG) procedure in order to improve on both.
Results are presented for one, two and three dimensions, with particular
attention to the experimentally accessible momentum distribution and possible
satellite peaks in this distribution. In one dimension, a comparison is made
with exact results obtained using Stochastich Series Expansion.Comment: 10 pages, 15 figure
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