260 research outputs found
Anomalous Hall conductivity of clean Sr2RuO4 at finite temperatures
Building on previous work, we calculate the temperature- and
frequency-dependent {\it anomalous} Hall conductivity for the putative
multiband chiral superconductor \Sr using a simple microscopic two-orbital
model without impurities. A Hall effect arises in this system without the
application of an external magnetic field due to the time-reversal-symmetry
breaking chiral superconducting state. The anomalous Hall conductivity is
nonzero only when there is more than one superconducting order parameter,
involving inter- as well as intra-band Cooper pairing. We find that such a
multiband superconducting state gives rise to a distinctive resonance in the
frequency-dependence of the Hall conductivity at a frequency close to the
inter-orbital hopping energy scale that describes hopping between Ru
and orbitals. The detection of this feature, robust to temperature and
impurity effects in the superconducting phase, would thus constitute compelling
evidence in favour of a multiband origin of superconductivity in \Sr, with
strong superconductivity on the and bands. The temperature
dependence of the Hall conductivity and Kerr rotation angle are studied within
this model at the one-loop approximation.Comment: 14 pages, 8 figures. Invited submission, proceedings of M2S 2012.
Published versio
Single-particle Excitation Spectra of C Molecules and Monolayers
In this paper we present calculations of single-particle excitation spectra
of neutral and three-electron-doped Hubbard C molecules and monolayers
from large-scale quantum Monte Carlo simulations and cluster perturbation
theory. By a comparison to experimental photoemission, inverse photoemission,
and angle-resolved photoemission data, we estimate the intermolecular hopping
integrals and the C molecular orientation angle, finding agreement with
recent X-ray photoelectron diffraction (XPD) experiments. Our results
demonstrate that a simple effective Hubbard model, with intermediate coupling,
, provides a reasonable basis for modeling the properties of C
compounds.Comment: 6 page
Identifying spin-triplet pairing in spin-orbit coupled multi-band superconductors
We investigate the combined effect of Hund's and spin-orbit (SO) coupling on
superconductivity in multi-orbital systems. Hund's interaction leads to
orbital-singlet spin-triplet superconductivity, where the Cooper pair wave
function is antisymmetric under the exchange of two orbitals. We identify three
d-vectors describing even-parity orbital-singlet spin-triplet pairings among
t2g-orbitals, and find that the three d-vectors are mutually orthogonal to each
other. SO coupling further assists pair formation, pins the orientation of the
d-vector triad, and induces spin-singlet pairings with a relative phase
difference of \pi/2. In the band basis the pseudospin d-vectors are aligned
along the z-axis and correspond to momentum-dependent inter- and intra-band
pairings. We discuss quasiparticle dispersion, magnetic response, collective
modes, and experimental consequences in light of the superconductor Sr2RuO4.Comment: 6 pages, 5 figure
An Anderson-Fano Resonance and Shake-Up Processes in the Magneto-Photoluminescence of a Two-Dimensional Electron System
We report an anomalous doublet structure and low-energy satellite in the
magneto-photoluminescence spectra of a two-dimensional electron system. The
doublet structure moves to higher energy with increasing magnetic field and is
most prominent at odd filling factors 5 and 3. The lower-energy satellite peak
tunes to lower energy for increasing magnetic field between filling factor 6
and 2. These features occur at energies below the fundamental band of
recombination originating from the lowest Landau level and display striking
magnetic field and temperature dependence that indicates a many-body origin.
Drawing on a recent theoretical description of Hawrylak and Potemski, we show
that distinct mechanisms are responsible for each feature.Comment: 14 pages including 5 figures. To appear in the April 15th edition of
Phy. Rev. B. rapid com
Observation of Collective Excitations of the Dilute 2D Electron System
We report inelastic light scattering measurements of dispersive spin and
charge density excitations in dilute 2D electron systems reaching densities
less than 10^{10} cm^{-2}. In the quantum Hall state at nu=2, roton critical
points in the spin inter--Landau level mode show a pronounced softening as r_s
is increased. Instead of a soft mode instability predicted by Hartree--Fock
calculations for r_s ~ 3.3, we find evidence of multiple rotons in the
dispersion of the softening spin excitations. Extrapolation of the data
indicates the possibility of an instability for r_s >~ 11.Comment: Submitted to Physical Review Letter
Upper limit on spontaneous supercurrents in SrRuO
It is widely believed that the perovskite SrRuO is an unconventional
superconductor with broken time reversal symmetry. It has been predicted that
superconductors with broken time reversal symmetry should have spontaneously
generated supercurrents at edges and domain walls. We have done careful imaging
of the magnetic fields above SrRuO single crystals using scanning Hall
bar and SQUID microscopies, and see no evidence for such spontaneously
generated supercurrents. We use the results from our magnetic imaging to place
upper limits on the spontaneously generated supercurrents at edges and domain
walls as a function of domain size. For a single domain, this upper limit is
below the predicted signal by two orders of magnitude. We speculate on the
causes and implications of the lack of large spontaneous supercurrents in this
very interesting superconducting system.Comment: 9 page
Single-mode approximation and effective Chern-Simons theories for quantum Hall systems
A unified description of elementary and collective excitations in quantum
Hall systems is presented within the single-mode approximation (SMA) framework,
with emphasis on revealing an intimate link with Chern-Simons theories. It is
shown that for a wide class of quantum Hall systems the SMA in general yields,
as an effective theory, a variant of the bosonic Chern-Simons theory. For
single-layer systems the effective theory agrees with the standard Chern-Simons
theory at long wavelengths whereas substantial deviations arise for collective
excitations in bilayer systems. It is suggested, in particular, that Hall-drag
experiments would be a good place to detect out-of-phase collective excitations
inherent to bilayer systems. It is also shown that the intra-Landau-level modes
bear a similarity in structure (though not in scale) to the inter-Landau-level
modes, and its implications on the composite-fermion and composite-boson
theories are discussed.Comment: 9 pages, Revtex
Skyrmions in Higher Landau Levels
We calculate the energies of quasiparticles with large numbers of reversed
spins (``skyrmions'') for odd integer filling factors 2k+1, k is greater than
or equals 1. We find, in contrast with the known result for filling factor
equals 1 (k = 0), that these quasiparticles always have higher energy than the
fully polarized ones and hence are not the low energy charged excitations, even
at small Zeeman energies. It follows that skyrmions are the relevant
quasiparticles only at filling factors 1, 1/3 and 1/5.Comment: 10 pages, RevTe
Proximity Effect and Josephson Coupling in the SO(5) Theory of High-Tc Superconductivity
We consider proximity effect coupling in
Superconducting/Antiferromagnetic/Superconducting (S-A-S) sandwiches using the
recently developed SO(5) effective theory of high temperature
superconductivity. We find that, for narrow junctions, the A region acts like a
strong superconductor, and that there is a critical junction thickness which
depends on the effective SO(5) coupling constants and on the phase difference
across the junction, at which the A region undergoes a Freedericksz-like
transition to a state which is intermediate between superconductor and
antiferromagnet. For thick junctions, the current-phase relation is sinusoidal,
as in standard S-N-S and S-I-S junctions, but for thin junctions it shows a
sharp break in slope at the Freedericksz point.Comment: 4 pages, LATEX, 5 eps fig
SU(4) Skyrmions and Activation Energy Anomaly in Bilayer Quantum Hall Systems
The bilayer QH system has four energy levels in the lowest Landau level,
corresponding to the layer and spin degrees of freedom. We investigate the
system in the regime where all four levels are nearly degenerate and equally
active. The underlying group structure is SU(4). At the QH state is a
charge-transferable state between the two layers and the SU(4) isospin
coherence develops spontaneously. Quasiparticles are isospin textures to be
identified with SU(4) skyrmions. The skyrmion energy consists of the Coulomb
energy, the Zeeman energy and the pseudo-Zeeman energy. The Coulomb energy
consists of the self-energy, the capacitance energy and the exchange energy. At
the balanced point only pseudospins are excited unless the tunneling gap is too
large. Then, the SU(4) skyrmion evolves continuously from the
pseudospin-skyrmion limit into the spin-skyrmion limit as the system is
transformed from the balanced point to the monolayer point by controlling the
bias voltage. Our theoretical result explains quite well the experimental data
due to Murphy et al. and Sawada et al. on the activation energy anomaly induced
by applying parallel magnetic field.Comment: 22 pagets, 6 figures, the final version to be published in PR
- …