3,525 research outputs found
Frustrated quantum Ising spins simulated by spinless bosons in a tilted lattice: from a quantum liquid to antiferromagnetic order
We study spinless bosons in a decorated square lattice with a near-diagonal
tilt. The resonant subspace of the tilted Mott insulator is described by an
effective Hamiltonian of frustrated quantum Ising spins on a non-bipartite
lattice. This generalizes an earlier proposal for the unfrustrated quantum
Ising model in one dimension which was realized in a recent experiment on
ultracold Rb atoms in an optical lattice. Very close to diagonal tilt,
we find a quantum liquid state which is continuously connected to the
paramagnet. Frustration can be reduced by increasing the tilt angle away from
the diagonal, and the system undergoes a transition to an antiferromagnetically
ordered state. Using quantum Monte Carlo simulations and exact diagonalization,
we find that for realistic system sizes the antiferromagnetic order appears to
be quasi-one-dimensional; however, in the thermodynamic limit the order is
two-dimensional.Comment: 27 pages, 14 figure
Metallic spin glasses
Recent work on the zero temperature phases and phase transitions of strongly
random electronic system is reviewed. The transition between the spin glass and
quantum paramagnet is examined, for both metallic and insulating systems.
Insight gained from the solution of infinite range models leads to a quantum
field theory for the transition between a metallic quantum paramagnetic and a
metallic spin glass. The finite temperature phase diagram is described and
crossover functions are computed in mean field theory. A study of fluctuations
about mean field leads to the formulation of scaling hypotheses.Comment: Contribution to the Proceedings of the ITP Santa Barbara conference
on Non-Fermi liquids, 25 pages, requires IOP style file
Quantum criticality of U(1) gauge theories with fermionic and bosonic matter in two spatial dimensions
We consider relativistic U(1) gauge theories in 2+1 dimensions, with N_b
species of complex bosons and N_f species of Dirac fermions at finite
temperature. The quantum phase transition between the Higgs and Coulomb phases
is described by a conformal field theory (CFT). At large N_b and N_f, but for
arbitrary values of the ratio N_b/N_f, we present computations of various
critical exponents and universal amplitudes for these CFTs. We make contact
with the different spin-liquids, charge-liquids and deconfined critical points
of quantum magnets that these field theories describe. We compute physical
observables that may be measured in experiments or numerical simulations of
insulating and doped quantum magnets.Comment: 30 pages, 8 figure
Comment on "Spin Transport properties of the quantum one-dimensional non-linear sigma model"
In a recent preprint (cond-mat/9905415), Fujimoto has used the Bethe ansatz
to compute the finite temperature, zero frequency Drude weight of spin
transport in the quantum O(3) non-linear sigma model in a magnetic field . We show here that, contrary to his claims, the results are in accord
with earlier semiclassical results (Sachdev and Damle, cond-mat/9610115). We
also comment on his 1/N expansion, and show that it does not properly describe
the long-time correlations.Comment: 4 page
Ultracold bosons in a synthetic periodic magnetic field: Mott phases and re-entrant superfluid-insulator transitions
We study Mott phases and superfluid-insulator (SI) transitions of ultracold
bosonic atoms in a two-dimensional square optical lattice at commensurate
filling and in the presence of a synthetic periodic vector potential
characterized by a strength and a period , where is an integer
and is the lattice spacing. We show that the Schr\"odinger equation for the
non-interacting bosons in the presence of such a periodic vector potential can
be reduced to an one-dimensional Harper-like equation which yields energy
bands. The lowest of these bands have either single or double minima whose
position within the magnetic Brillouin zone can be tuned by varying for a
given . Using these energies and a strong-coupling expansion technique, we
compute the phase diagram of these bosons in the presence of a deep optical
lattice. We chart out the and dependence of the momentum distribution
of the bosons in the Mott phases near the SI transitions and demonstrate that
the bosons exhibit several re-entrant field-induced SI transitions for any
fixed period . We also predict that the superfluid density of the resultant
superfluid state near such a SI transition has a periodicity () in
real space for odd (even) and suggest experiments to test our theory.Comment: 8 pages, 11 figures, v
Putting competing orders in their place near the Mott transition
We describe the localization transition of superfluids on two-dimensional
lattices into commensurate Mott insulators with average particle density p/q
(p, q relatively prime integers) per lattice site. For bosons on the square
lattice, we argue that the superfluid has at least q degenerate species of
vortices which transform under a projective representation of the square
lattice space group (a PSG). The formation of a single vortex condensate
produces the Mott insulator, which is required by the PSG to have density wave
order at wavelengths of q/n lattice sites (n integer) along the principle axes;
such a second-order transition is forbidden in the Landau-Ginzburg-Wilson
framework. We also discuss the superfluid-insulator transition in the direct
boson representation, and find that an interpretation of the quantum
criticality in terms of deconfined fractionalized bosons is only permitted at
special values of q for which a permutative representation of the PSG exists.
We argue (and demonstrate in detail in a companion paper: L. Balents et al.,
cond-mat/0409470) that our results apply essentially unchanged to electronic
systems with short-range pairing, with the PSG determined by the particle
density of Cooper pairs. We also describe the effect of static impurities in
the superfluid: the impurities locally break the degeneracy between the q
vortex species, and this induces density wave order near each vortex. We
suggest that such a theory offers an appealing rationale for the local density
of states modulations observed by Hoffman et al. (cond-mat/0201348) in STM
studies of the vortex lattice of BSCCO, and allows a unified description of the
nucleation of density wave order in zero and finite magnetic fields. We note
signatures of our theory that may be tested by future STM experiments.Comment: 35 pages, 16 figures; (v2) part II is cond-mat/0409470; (v3) added
new appendix and clarifying remarks; (v4) corrected typo
Hall conductivity from dyonic black holes
A class of strongly interacting 2+1 dimensional conformal field theories in a
transverse magnetic field can be studied using the AdS/CFT duality. We compute
zero momentum hydrodynamic response functions of maximally supersymmetric 2+1
dimensional SU(N) Yang-Mills theory at the conformal fixed point, in the large
N limit. With background magnetic field B and electric charge density rho, the
Hall conductivity is found to be rho/B. The result, anticipated on kinematic
grounds in field theory, is obtained from perturbations of a four dimensional
AdS black hole with both electric and magnetic charges.Comment: 1+13 pages. TT correlator corrected. Typos corrected and added ref
Non-equilibrium dynamic critical scaling of the quantum Ising chain
We solve for the time-dependent finite-size scaling functions of the 1D
transverse-field Ising chain during a linear-in-time ramp of the field through
the quantum critical point. We then simulate Mott-insulating bosons in a tilted
potential, an experimentally-studied system in the same equilibrium
universality class, and demonstrate that universality holds for the dynamics as
well. We find qualitatively athermal features of the scaling functions, such as
negative spin correlations, and show that they should be robustly observable
within present cold atom experiments.Comment: 4 pages + 2 page supplemen
Purification of genuine multipartite entanglement
In tasks, where multipartite entanglement plays a central role, state
purification is, due to inevitable noise, a crucial part of the procedure. We
consider a scenario exploiting the multipartite entanglement in a
straightforward multipartite purification algorithm and compare it to bipartite
purification procedures combined with state teleportation. While complete
purification requires an infinite amount of input states in both cases, we show
that for an imperfect output fidelity the multipartite procedure exhibits a
major advantage in terms of input states used.Comment: 5 pages, 2 figure
Quantum simulator for the Ising model with electrons floating on a helium film
We propose a physical setup that can be used to simulate the quantum dynamics
of the Ising model with present-day technology. Our scheme consists of
electrons floating on superfluid helium which interact via Coulomb forces. In
the limit of low temperatures, the system will stay near the ground state where
its Hamiltonian is equivalent to the Ising model and thus shows phenomena such
as quantum criticality. Furthermore, the proposed design could be generalized
in order to study interacting field theories (e.g., ) and
adiabatic quantum computers.Comment: 4 page
- …