9,453 research outputs found
Noncollinear magnetic order in quasicrystals
Based on Monte-Carlo simulations, the stable magnetization configurations of
an antiferromagnet on a quasiperiodic tiling are derived theoretically. The
exchange coupling is assumed to decrease exponentially with the distance
between magnetic moments. It is demonstrated that the superposition of
geometric frustration with the quasiperiodic ordering leads to a
three-dimensional noncollinear antiferromagnetic spin structure. The structure
can be divided into several ordered interpenetrating magnetic supertilings of
different energy and characteristic wave vector. The number and the symmetry of
subtilings depend on the quasiperiodic ordering of atoms.Comment: RevTeX, 4 pages, 5 low-resolution color figures (due to size
restrictions); to appear in Physical Review Letter
Spectrum of a duality-twisted Ising quantum chain
The Ising quantum chain with a peculiar twisted boundary condition is
considered. This boundary condition, first introduced in the framework of the
spin-1/2 XXZ Heisenberg quantum chain, is related to the duality
transformation, which becomes a symmetry of the model at the critical point.
Thus, at the critical point, the Ising quantum chain with the duality-twisted
boundary is translationally invariant, similar as in the case of the usual
periodic or antiperiodic boundary conditions. The complete energy spectrum of
the Ising quantum chain is calculated analytically for finite systems, and the
conformal properties of the scaling limit are investigated. This provides an
explicit example of a conformal twisted boundary condition and a corresponding
generalised twisted partition function.Comment: LaTeX, 7 pages, using IOP style
Probing Phases and Quantum Criticality using Deviations from the Local Fluctuation-Dissipation Theorem
Introduction Cold atomic gases in optical lattices are emerging as excellent
laboratories for testing models of strongly interacting particles in condensed
matter physics. Currently, one of the major open questions is how to obtain the
finite temperature phase diagram of a given quantum Hamiltonian directly from
experiments. Previous work in this direction required quantum Monte Carlo
simulations to directly model the experimental situation in order to extract
quantitative information, clearly defeating the purpose of an optical lattice
emulator. Here we propose a new method that utilizes deviations from a local
fluctuation dissipation theorem to construct a finite temperature phase
diagram, for the first time, from local observables accessible by in situ
experimental observations. Our approach extends the utility of the
fluctuation-dissipation theorem from thermometry to the identification of
quantum phases, associated energy scales and the quantum critical region. We
test our ideas using state-of-the-art large-scale quantum Monte Carlo
simulations of the two-dimensional Bose Hubbard model.Comment: 7 pages; 4 figures; also see supplementary material of 7 pages with 3
figure
On Flux Quantization in F-Theory II: Unitary and Symplectic Gauge Groups
We study the quantization of the M-theory G-flux on elliptically fibered
Calabi-Yau fourfolds with singularities giving rise to unitary and symplectic
gauge groups. We seek and find its relation to the Freed-Witten quantization of
worldvolume fluxes on 7-branes in type IIB orientifold compactifications on
Calabi-Yau threefolds. By explicitly constructing the appropriate four-cycles
on which to calculate the periods of the second Chern class of the fourfolds,
we find that there is a half-integral shift in the quantization of G-flux
whenever the corresponding dual 7-brane is wrapped on a non-spin submanifold.
This correspondence of quantizations holds for all unitary and symplectic gauge
groups, except for SU(3), which behaves mysteriously. We also perform our
analysis in the case where, in addition to the aforementioned gauge groups,
there is also a 'flavor' U(1)-gauge group.Comment: 33 pages, 4 figure
Magneto-optical Feshbach resonance: Controlling cold collision with quantum interference
We propose a method of controlling two-atom interaction using both magnetic
and laser fields. We analyse the role of quantum interference between magnetic
and optical Feshbach resonances in controlling cold collision. In particular,
we demonstrate that this method allows us to suppress inelastic and enhance
elastic scattering cross sections. Quantum interference is shown to modify
significantly the threshold behaviour and resonant interaction of ultracold
atoms. Furthermore, we show that it is possible to manipulate not only the
spherically symmetric s-wave interaction but also the anisotropic higher
partial-wave interactions which are particularly important for high temperature
superfluid or superconducting phases of matter.Comment: 7 pages 3 figures, some minor errors are corrected, Accepted in J.
Phys.
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