32,413 research outputs found
Optical properties of TlNi2Se2: Observation of pseudogap formation
The quasi-two-dimensional nickel chalcogenides is a newly
discovered superconductor. We have performed optical spectroscopy study on
single crystals over a broad frequency range at various
temperatures. The overall optical reflectance spectra are similar to those
observed in its isostructure . Both the suppression in
and the peaklike feature in suggest the progressive
formation of a pseudogap feature in the midinfrared range with decreasing
temperatures, which might be originated from the dynamic local fluctuation of
charge-density-wave (CDW) instability. We propose that the CDW instability in
is driven by the saddle points mechanism, due to the existence of
van Hove singularity very close to the Fermi energy.Comment: 5 pages, 4 figure
Inhomogeneity Induces Resonance Coherence Peaks in Superconducting BSCCO
In this paper we analyze, using scanning tunneling spectroscopy, the density
of electronic states in nearly optimally doped BSCCO in zero field. Focusing on
the superconducting gap, we find patches of what appear to be two different
phases in a background of some average gap, one with a relatively small gap and
sharp large coherence peaks and one characterized by a large gap with broad
weak coherence peaks. We compare these spectra with calculations of the local
density of states for a simple phenomenological model in which a 2 xi_0 * 2
xi_0 patch with an enhanced or supressed d-wave gap amplitude is embedded in a
region with a uniform average d-wave gap.Comment: 4 pages, 3 figure
Is the cosmic UV background fluctuating at redshift z ~ 6 ?
We study the Gunn-Peterson effect of the photo-ionized intergalactic
medium(IGM) in the redshift range 5< z <6.4 using semi-analytic simulations
based on the lognormal model. Assuming a rapidly evolved and spatially uniform
ionizing background, the simulation can produce all the observed abnormal
statistical features near redshift z ~ 6. They include: 1) rapidly increase of
absorption depths; 2) large scatter in the optical depths; 3) long-tailed
distributions of transmitted flux and 4) long dark gaps in spectra. These
abnormal features are mainly due to rare events, which correspond to the
long-tailed probability distribution of the IGM density field, and therefore,
they may not imply significantly spatial fluctuations in the UV ionizing
background at z ~ 6.Comment: 12 pages, 4 figs, accepted by ApJ
The coexistence of superconductivity and ferromagnetism in nano-scale metallic grains
A nano-scale metallic grain in which the single-particle dynamics are chaotic
is described by the so-called universal Hamiltonian. This Hamiltonian includes
a superconducting pairing term and a ferromagnetic exchange term that compete
with each other: pairing correlations favor minimal ground-state spin, while
the exchange interaction favors maximal spin polarization. Of particular
interest is the fluctuation-dominated regime where the bulk pairing gap is
comparable to or smaller than the single-particle mean level spacing and the
Bardeen-Cooper-Schrieffer theory of superconductivity breaks down.
Superconductivity and ferromagnetism can coexist in this regime. We identify
signatures of the competition between superconductivity and ferromagnetism in a
number of quantities: ground-state spin, conductance fluctuations when the
grain is weakly coupled to external leads and the thermodynamic properties of
the grain, such as heat capacity and spin susceptibility.Comment: 13 pages, 13 figures, Proceedings of the Conference on the Frontiers
of Quantum and Mesoscopic Thermodynamics (FQMT11
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