4,560 research outputs found
Supersolid Bose-Fermi Mixtures in Optical Lattices
We study a mixture of strongly interacting bosons and spinless fermions with
on-site repulsion in a three-dimensional optical lattice. For this purpose we
develop and apply a generalized DMFT scheme, which is exact in infinite
dimensions and reliably describes the full range from weak to strong coupling.
We restrict ourselves to half filling. For weak Bose-Fermi repulsion a
supersolid forms, in which bosonic superfluidity coexists with charge-density
wave order. For stronger interspecies repulsion the bosons become localized
while the charge density wave order persists. The system is unstable against
phase separation for weak repulsion among the bosons.Comment: 4 pages, 5 pictures, Published versio
N\'{e}el transition of lattice fermions in a harmonic trap: a real-space DMFT study
We study the magnetic ordering transition for a system of harmonically
trapped ultracold fermions with repulsive interactions in a cubic optical
lattice, within a real-space extension of dynamical mean-field theory (DMFT).
Using a quantum Monte Carlo impurity solver, we establish that
antiferromagnetic correlations are signaled, at strong coupling, by an enhanced
double occupancy. This signature is directly accessible experimentally and
should be observable well above the critical temperature for long-range order.
Dimensional aspects appear less relevant than naively expected.Comment: 4 pages, 4 figure
Magnetically Stabilized Nematic Order I: Three-Dimensional Bipartite Optical Lattices
We study magnetically stabilized nematic order for spin-one bosons in optical
lattices. We show that the Zeeman field-driven quantum transitions between
non-nematic Mott states and quantum spin nematic states in the weak hopping
limit are in the universality class of the ferromagnetic XXZ (S=1/2) spin
model. We further discuss these transitions as condensation of interacting
magnons. The development of O(2) nematic order when external fields are applied
corresponds to condensation of magnons, which breaks a U(1) symmetry.
Microscopically, this results from a coherent superposition of two non-nematic
states at each individual site. Nematic order and spin wave excitations around
critical points are studied and critical behaviors are obtained in a dilute gas
approximation. We also find that spin singlet states are unstable with respect
to quadratic Zeeman effects and Ising nematic order appears in the presence of
any finite quadratic Zeeman coupling. All discussions are carried out for
states in three dimensional bipartite lattices.Comment: 16 pages, 3 figure
Generalized Dynamical Mean-Field Theory for Bose-Fermi Mixtures in Optical Lattices
We give a detailed discussion of the recently developed Generalized Dynamical
Mean-Field Theory (GDMFT) for a mixture of bosonic and fermionic particles. We
show that this method is non-perturbative and exact in infinite dimensions and
reliably describes the full range from weak to strong coupling. Like in
conventional Dynamical Mean-Field Theory, the small parameter is 1/z, where z
is the lattice coordination number. We apply the GDMFT scheme to a mixture of
spinless fermions and bosons in an optical lattice. We investigate the
ossibility of a supersolid phase, focussing on the case of 1/2 filling for the
fermions and 3/2 filling for the bosons.Comment: 12+ pages 6 figure
Online Meta-learning by Parallel Algorithm Competition
The efficiency of reinforcement learning algorithms depends critically on a
few meta-parameters that modulates the learning updates and the trade-off
between exploration and exploitation. The adaptation of the meta-parameters is
an open question in reinforcement learning, which arguably has become more of
an issue recently with the success of deep reinforcement learning in
high-dimensional state spaces. The long learning times in domains such as Atari
2600 video games makes it not feasible to perform comprehensive searches of
appropriate meta-parameter values. We propose the Online Meta-learning by
Parallel Algorithm Competition (OMPAC) method. In the OMPAC method, several
instances of a reinforcement learning algorithm are run in parallel with small
differences in the initial values of the meta-parameters. After a fixed number
of episodes, the instances are selected based on their performance in the task
at hand. Before continuing the learning, Gaussian noise is added to the
meta-parameters with a predefined probability. We validate the OMPAC method by
improving the state-of-the-art results in stochastic SZ-Tetris and in standard
Tetris with a smaller, 1010, board, by 31% and 84%, respectively, and
by improving the results for deep Sarsa() agents in three Atari 2600
games by 62% or more. The experiments also show the ability of the OMPAC method
to adapt the meta-parameters according to the learning progress in different
tasks.Comment: 15 pages, 10 figures. arXiv admin note: text overlap with
arXiv:1702.0311
Resonant Superfluidity in an Optical Lattice
We study a system of ultracold fermionic Potassium (40K) atoms in a
three-dimensional optical lattice in the vicinity of an s-wave Feshbach
resonance. Close to resonance, the system is described by a multi-band
Bose-Fermi Hubbard Hamiltonian. We derive an effective lowest-band Hamiltonian
in which the effect of the higher bands is incorporated by a self-consistent
mean-field approximation. The resulting model is solved by means of Generalized
Dynamical Mean-Field Theory. In addition to the BEC/BCS crossover we find a
phase transition to a fermionic Mott insulator at half filling, induced by the
repulsive fermionic background scattering length. We also calculate the
critical temperature of the BEC/BCS-state and find it to be minimal at
resonance.Comment: 19 pages, 3 figure
Antiferromagnetic Order of Strongly Interacting Fermions in a Trap: Real-Space Dynamical Mean-Field Analysis
We apply Dynamical Mean-Field Theory to strongly interacting fermions in an
inhomogeneous environment. With the help of this Real-Space Dynamical
Mean-Field Theory (R-DMFT) we investigate antiferromagnetic states of
repulsively interacting fermions with spin 1/2 in a harmonic potential. Within
R-DMFT, antiferromagnetic order is found to be stable in spatial regions with
total particle density close to one, but persists also in parts of the system
where the local density significantly deviates from half filling. In systems
with spin imbalance, we find that antiferromagnetism is gradually suppressed
and phase separation emerges beyond a critical value of the spin imbalance.Comment: 4 pages 5 figures, published versio
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