451 research outputs found
Broken rotation symmetry in the fractional quantum Hall system
We demonstrate that the two-dimensonal electron system in a strong
perpendicular magnetic field has stable states which break rotational but not
translational symmetry. The Laughlin fluid becomes unstable to these states in
quantum wells whose thickness exceeds a critical value which depends on the
electron density. The order parameter at 1/3 reduced density resembles that of
a nematic liquid crystal, in that a residual two-fold rotation axis is present
in the low symmetry phase. At filling factors 1/5 and 1/7, there are states
with four- and six-fold axes, as well. We discuss the experimental detection of
these phases.Comment: 8 pages, LaTex 3.1, figures attache
Phase diagram of UPt in the model
The phase diagram of the unconventional superconductor UPt is explained
under the long-standing hypothesis that the pair wavefunction belongs to the
representation of the point group. The main objection to this theory
has been that it disagrees with the experimental phase diagram when a field is
applied along the c-axis. By a careful analysis of the free energy this
objection is shown to be incorrect. This singlet theory also explains the
unusual anisotropy in the upper critical field curves, often thought to
indicate a triplet pair function.Comment: 11 pages, Revtex, 2 figures (uuencoded, gzip'ed Postscript
Theory of Neutron Diffraction from the Vortex Lattice in UPt3
Neutron scattering experiments have recently been performed in the
superconducting state of UPt3 to determine the structure of the vortex lattice.
The data show anomalous field dependence of the aspect ratio of the unit cell
in the B phase. There is apparently also a change in the effective coherence
length on the transition from the B to the C phases. Such observations are not
consistent with conventional superconductvity. A theory of these results is
constructed based on a picture of two-component superconductivity for UPt3. In
this way, these unusual observations can be understood. There is a possible
discrepancy between theory and experiment in the detailed field dependence of
the aspect ratio.Comment: 11 pages; uses REVTEX, APS and PRABIB styles; 2 Postscript figure
files include
Electronic inhomogeneity at magnetic domain walls in strongly-correlated systems
We show that nano-scale variations of the order parameter in
strongly-correlated systems can induce local spatial regions such as domain
walls that exhibit electronic properties representative of a different, but
nearby, part of the phase diagram. This is done by means of a Landau-Ginzburg
analysis of a metallic ferromagnetic system near an antiferromagnetic phase
boundary. The strong spin gradients at a wall between domains of different spin
orientation drive the formation of a new type of domain wall, where the central
core is an insulating antiferromagnet, and connects two metallic ferromagnetic
domains. We calculate the charge transport properties of this wall, and find
that its resistance is large enough to account for recent experimental results
in colossal magnetoresistance materials. The technological implications of this
finding for switchable magnetic media are discussed.Comment: Version submitted to Physical Review Letters, except for minor
revisions to reference
Magnetism of Superconducting UPt3
The phase diagram of superconducting in pressure-temperature
plane, together with the neutron scattering data is studied within a two
component superconducting order parameter scenario. In order to give a
qualitative explanation to the experimental data a set of two linearly
independent antiferromagnetic moments which emerge appropriately at the
temperature \mbox{} and \mbox{} and
couple to superconductivity is proposed. Several constraints on the fourth
order coefficients in the Ginzburg-Landau free energy are obtained.Comment: 17 pages, figures available on request to
[email protected]
Phenomenological noise model for superconducting qubits: two-state fluctuators and 1=f noise
We present a general phenomenological model for superconducting qubits
subject to noise produced by two-state fluctuators whose couplings to the qubit
are all roughly the same. In flux qubit experiments where the working point can
be varied, it is possible to extract both the form of the noise spectrum and
the number of fluctuators. We find that the noise has a broad spectrum
consistent with 1=f noise and that the number of fluctuators with slow
switching rates is surprisingly small: less than 100. If the fluctuators are
interpreted as unpaired surface spins, then the size of their magnetic moments
is surprisingly large.Comment: 7 pages, 2 figures, 1 tabl
Generation and control of resonance states in crossed magnetic and electric fields
A two-dimensional electron system interacting with an impurity and placed in
crossed magnetic and electric fields is under investigation. Since it is
assumed that an impurity center interacts as an attractive -like
potential a renormalization procedure for the retarded Green's function has to
be carried out. For the vanishing electric field we obtain a close analytical
expression for the Green's function and we find one bound state localized
between Landau levels. It is also shown by numerical investigations that
switching on the electric field new long-living resonance states localized in
the vicinity of Landau levels can be generated.Comment: 6 pages, 6 figures, 1 tabl
Cooling of cryogenic electron bilayers via the Coulomb interaction
Heat dissipation in current-carrying cryogenic nanostructures is problematic
because the phonon density of states decreases strongly as energy decreases. We
show that the Coulomb interaction can prove a valuable resource for carrier
cooling via coupling to a nearby, cold electron reservoir. Specifically, we
consider the geometry of an electron bilayer in a silicon-based
heterostructure, and analyze the power transfer. We show that across a range of
temperatures, separations, and sheet densities, the electron-electron
interaction dominates the phonon heat-dissipation modes as the main cooling
mechanism. Coulomb cooling is most effective at low densities, when phonon
cooling is least effective in silicon, making it especially relevant for
experiments attempting to perform coherent manipulations of single spins.Comment: 9 pages, 5 figure
Vortex lattice structures of SrRuO
The vortex lattice structures of SrRuO for the odd parity
representations of the superconducting state are examined for the magnetic
field along the crystallographic directions. Particular emphasis is placed upon
the two dimensional representation which is believed to be relevant to this
material. It is shown that when the zero-field state breaks time reversal
symmetry, there must exist two superconducting transitions when there is a
finite field along a high symmetry direction in the basal plane. Also it is
shown that a square vortex lattice is expected when the field is along the
-axis. The orientation of the square lattice with respect to the underlying
ionic lattice yields information as to which Ru 4d orbitals are relevant to the
superconducting state.Comment: 5 pages, 2 figure
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