6,160 research outputs found
Exciting a d-density wave in an optical lattice with driven tunneling
Quantum phases with unusual symmetries may play a key role for the
understanding of solid state systems at low temperatures. We propose a
realistic scenario, well in reach of present experimental techniques, which
should permit to produce a stationary quantum state with -symmetry
in a two-dimensional bosonic optical square lattice. This state, characterized
by alternating rotational flux in each plaquette, arises from driven tunneling
implemented by a stimulated Raman scattering process. We discuss bosons in a
square lattice, however, more complex systems involving other lattice
geometries appear possible.Comment: 4 pages, 3 figure
Creep of current-driven domain-wall lines: intrinsic versus extrinsic pinning
We present a model for current-driven motion of a magnetic domain-wall line,
in which the dynamics of the domain wall is equivalent to that of an overdamped
vortex line in an anisotropic pinning potential. This potential has both
extrinsic contributions due to, e.g., sample inhomogeneities, and an intrinsic
contribution due to magnetic anisotropy. We obtain results for the domain-wall
velocity as a function of current for various regimes of pinning. In
particular, we find that the exponent characterizing the creep regime depends
strongly on the presence of a dissipative spin transfer torque. We discuss our
results in the light of recent experiments on current-driven domain-wall creep
in ferromagnetic semiconductors, and suggest further experiments to corroborate
our model.Comment: For figure in GIF format, see
http://www.phys.uu.nl/~duine/mapping.gif v2: (hopefully) visible EPS figure
added. v2: expanded new versio
Current-driven and field-driven domain walls at nonzero temperature
We present a model for the dynamics of current- and field-driven domain-wall
lines at nonzero temperature. We compute thermally-averaged drift velocities
from the Fokker-Planck equation that describes the nonzero-temperature dynamics
of the domain wall. As special limits of this general description, we describe
rigid domain walls as well as vortex domain walls. In these limits, we
determine also depinning times of the domain wall from an extrinsic pinning
potential. We compare our theory with previous theoretical and experimental
work
Vortex-lattice pinning in two-component Bose-Einstein condensates
We investigate the vortex-lattice structure for single- and two-component
Bose-Einstein condensates in the presence of an optical lattice, which acts as
a pinning potential for the vortices. The problem is considered in the
mean-field quantum-Hall regime, which is reached when the rotation frequency
of the condensate in a radially symmetric trap approaches the (radial)
trapping frequency and the interactions between the atoms are weak. We
determine the vortex-lattice phase diagram as a function of optical-lattice
strength and geometry. In the limit of strong pinning the vortices are always
pinned at the maxima of the optical-lattice potential, similar to the
slow-rotation case. At intermediate pinning strength, however, due to the
competition between interactions and pinning energy, a structure arises for the
two-component case where the vortices are pinned on lines of minimal potential
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