317 research outputs found
Mixed-State Quasiparticle Spectrum for d-wave Superconductors
Controversy concerning the pairing symmetry of high- materials has
motivated an interest in those measurable properties of superconductors for
which qualitative differences exist between the s-wave and d-wave cases. We
report on a comparison between the microscopic electronic properties of d-wave
and s-wave superconductors in the mixed state. Our study is based on
self-consistent numerical solutions of the mean-field Bogoliubov-de Gennes
equations for phenomenological BCS models which have s-wave and d-wave
condensates in the absence of a magnetic field. We discuss differences between
the s-wave and the d-wave local density-of-states, both near and away from
vortex cores. Experimental implications for both scanning-tunneling-microscopy
measurements and specific heat measurements are discussed.Comment: 10 pages, REVTEX3.0, 3 figures available upon reques
Quantum Maxwell-Bloch equations for spatially inhomogeneous semiconductor lasers
We present quantum Maxwell-Bloch equations (QMBE) for spatially inhomogeneous
semiconductor laser devices. The QMBE are derived from fully quantum mechanical
operator dynamics describing the interaction of the light field with the
quantum states of the electrons and the holes near the band gap. By taking into
account field-field correlations and field-dipole correlations, the QMBE
include quantum noise effects which cause spontaneous emission and amplified
spontaneous emission. In particular, the source of spontaneous emission is
obtained by factorizing the dipole-dipole correlations into a product of
electron and hole densities. The QMBE are formulated for general devices, for
edge emitting lasers and for vertical cavity surface emitting lasers, providing
a starting point for the detailed analysis of spatial coherence in the near
field and far field patterns of such laser diodes. Analytical expressions are
given for the spectra of gain and spontaneous emission described by the QMBE.
These results are applied to the case of a broad area laser, for which the
frequency and carrier density dependent spontaneous emission factor beta and
the evolution of the far field pattern near threshold are derived.Comment: 22 pages RevTex and 7 figures, submitted to Phys.Rev.A, revisions in
abstract and in the discussion of temporal coherenc
Vortex structure in chiral p-wave superconductors
We investigate the vortex structure in chiral p-wave superconductors by the
Bogoliubov-de Gennes theory on a tight-binding model. We calculate the spatial
structure of the pair potential and electronic state around a vortex, including
the anisotropy of the Fermi surface and superconducting gap structure. The
differences of the vortex structure between -wave
and -wave superconductors are clarified in the
vortex lattice state. We also discuss the winding case of the
-wave superconductivity.Comment: 10 pages, 8 figure
SO(5) theory of insulating vortex cores in high- materials
We study the fermionic states of the antiferromagnetically ordered vortex
cores predicted to exist in the superconducting phase of the newly proposed
SO(5) model of strongly correlated electrons. Our model calculation gives a
natural explanation of the recent STM measurements on BSCCO, which in
surprising contrast to YBCO revealed completely insulating vortex cores.Comment: 4 pages, 1 figur
Elasticity-driven interaction between vortices in type-II superconductors
The contribution to the vortex lattice energy which is due to the
vortex-induced strains is calculated covering all the magnetic field range
which defines the vortex state. This contribution is compared with previously
reported ones what shows that, in the most part of the vortex state, it has
been notably underestimated until now. The reason of such underestimation is
the assumption that only the vortex cores induce strains. In contrast to what
is generally assumed, both core and non-core regions are important sources of
strains in high- superconductors.Comment: 10 pages, 1 figure, revtex
The selectivity of responses to red-green colour and achromatic contrast in the human visual cortex: An fMRI adaptation study
© 2015 Federation of European Neuroscience Societies and John Wiley and Sons Ltd. There is controversy as to how responses to colour in the human brain are organized within the visual pathways. A key issue is whether there are modular pathways that respond selectively to colour or whether there are common neural substrates for both colour and achromatic (Ach) contrast. We used functional magnetic resonance imaging (fMRI) adaptation to investigate the responses of early and extrastriate visual areas to colour and Ach contrast. High-contrast red-green (RG) and Ach sinewave rings (0.5 cycles/degree, 2 Hz) were used as both adapting stimuli and test stimuli in a block design. We found robust adaptation to RG or Ach contrast in all visual areas. Cross-adaptation between RG and Ach contrast occurred in all areas indicating the presence of integrated, colour and Ach responses. Notably, we revealed contrasting trends for the two test stimuli. For the RG test, unselective processing (robust adaptation to both RG and Ach contrast) was most evident in the early visual areas (V1 and V2), but selective responses, revealed as greater adaptation between the same stimuli than cross-adaptation between different stimuli, emerged in the ventral cortex, in V4 and VO in particular. For the Ach test, unselective responses were again most evident in early visual areas but Ach selectivity emerged in the dorsal cortex (V3a and hMT+). Our findings support a strong presence of integrated mechanisms for colour and Ach contrast across the visual hierarchy, with a progression towards selective processing in extrastriate visual areas. We measured responses to red-green color and achromatic contrast in human visual cortex with fMRI adaptation. Cross adaptation between the two contrast types occurred across the visual hierarchy indicating integrated color and achromatic responses. Selective adaptation for color (i.e. greater adaptation to color than achromatic contrast) emerged in ventral cortex, particularly in areas V4 and VO, whereas selective adaptation to achromatic contrast (i.e. greater adaptation to achromatic than color contrast) was evident in dorsal cortex (V3a, hMT+). Our findings suggest a progression from integrated color and achromatic responses in early visual cortex to more selective processing in extrastriate visual areas.Link_to_subscribed_fulltex
Absence of Dipole Transitions in Vortices of Type II Superconductors
The response of a single vortex to a time dependent field is examined
microscopically and an equation of motion for vortex motion at non-zero
frequencies is derived. Of interest are frequencies near ,
where is the bulk energy gap and is the fermi energy. The low
temperature, clean, extreme type II limit and maintaining of equilibrium with
the lattice are assumed. A simplification occurs for large planar mass
anisotropy. Thus the results may be pertinent to materials such as and
high temperature superconductors. The expected dipole transition between core
states is hidden because of the self consistent nature of the vortex potential.
Instead the vortex itself moves and has a resonance at the frequency of the
transition.Comment: 12 pages, no figure
Atomic Bose Gas with Negative Scattering Length
We derive the equation of state of a dilute atomic Bose gas with an
interatomic interaction that has a negative scattering length and argue that
two continuous phase transitions, occuring in the gas due to quantum degeneracy
effects, are preempted by a first-order gas-liquid or gas-solid transition
depending on the details of the interaction potential. We also discuss the
consequences of this result for future experiments with magnetically trapped
spin-polarized atomic gasses such as lithium and cesium.Comment: 16 PAGES, REVTEX 3.0, ACCEPTED FOR PUBLICATION IN PHYS. REV.
Localization length and impurity dielectric susceptibility in the critical regime of the metal-insulator transition in homogeneously doped p-type Ge
We have determined the localization length \xi and the impurity dielectric
susceptibility \chi_{\rm imp} as a function of Ga acceptor concentrations (N)
in nominally uncompensated ^{70}Ge:Ga just below the critical concentration
(N_c) for the metal-insulator transition. Both \xi and \chi_{\rm imp} diverge
at N_c according to the functions \xi\propto(1-N/N_c)^{-\nu} and \chi_{\rm
imp}\propto(N_c/N-1)^{-\zeta}, respectively, with \nu=1.2\pm0.3 and
\zeta=2.3\pm0.6 for 0.99N_c< N< N_c. Outside of this region (N<0.99N_c), the
values of the exponents drop to \nu=0.33\pm0.03 and \zeta=0.62\pm0.05. The
effect of the small amount of compensating dopants that are present in our
nominally uncompensated samples, may be responsible for the change of the
critical exponents at N\approx0.99N_c.Comment: RevTeX, 4 pages with 5 embedded figures, final version (minor
changes
Low field vortex matter in YBCO: an atomic beam magnetic resonance study
We report measurements of the low field structure of the magnetic vortex
lattice in an untwinned YBCO single-crystal platelet. Measurements were carried
out using a novel atomic beam magnetic resonance (ABMR) technique. For a 10.7 G
field applied parallel to the c-axis of the sample, we find a triangular
lattice with orientational order extending across the entire sample. We find
the triangular lattice to be weakly distorted by the a-b anisotropy of the
material and measure a distortion factor, f = 1.16. Model-experiment
comparisons determine a penetration depth, lambda_ab = 140 (+-20) nm. The paper
includes the first detailed description of the ABMR technique. We discuss both
technical details of the experiment and the modeling used to interpret the
measurements.Comment: 44 pages, 13 figures, submitted to Phys. Rev. B Revision includes
Postscript wrapped figures + minor typo
- …