495 research outputs found
Comparison of s- and d-wave gap symmetry in nonequilibrium superconductivity
Recent application of ultrafast pump/probe optical techniques to
superconductors has renewed interest in nonequilibrium superconductivity and
the predictions that would be available for novel superconductors, such as the
high-Tc cuprates. We have reexamined two of the classical models which have
been used in the past to interpret nonequilibrium experiments with some
success: the mu* model of Owen and Scalapino and the T* model of Parker.
Predictions depend on pairing symmetry. For instance, the gap suppression due
to excess quasiparticle density n in the mu* model, varies as n^{3/2} in d-wave
as opposed to n for s-wave. Finally, we consider these models in the context of
S-I-N tunneling and optical excitation experiments. While we confirm that
recent pump/probe experiments in YBCO, as presently interpreted, are in
conflict with d-wave pairing, we refute the further claim that they agree with
s-wave.Comment: 14 pages, 11 figure
High Pressure Thermoelasticity of Body-centered Cubic Tantalum
We have investigated the thermoelasticity of body-centered cubic (bcc)
tantalum from first principles by using the linearized augmented plane wave
(LAPW) and mixed--basis pseudopotential methods for pressures up to 400 GPa and
temperatures up to 10000 K. Electronic excitation contributions to the free
energy were included from the band structures, and phonon contributions were
included using the particle-in-a-cell (PIC) model. The computed elastic
constants agree well with available ultrasonic and diamond anvil cell data at
low pressures, and shock data at high pressures. The shear modulus and
the anisotropy change behavior with increasing pressure around 150 GPa because
of an electronic topological transition. We find that the main contribution of
temperature to the elastic constants is from the thermal expansivity. The PIC
model in conjunction with fast self-consistent techniques is shown to be a
tractable approach to studying thermoelasticity.Comment: To be appear in Physical Review
Dynamics of Tachyon and Phantom Field beyond the Inverse Square Potentials
We investigate the cosmological evolution of the tachyon and phantom-tachyon
scalar field by considering the potential parameter () as a function of another potential parameter
(), which correspondingly extends the
analysis of the evolution of our universe from two-dimensional autonomous
dynamical system to the three-dimension. It allows us to investigate the more
general situation where the potential is not restricted to inverse square
potential and .One result is that, apart from the inverse square potential,
there are a large number of potentials which can give the scaling and dominant
solution when the function equals for one or some
values of as well as the parameter satisfies
condition Eq.(18) or Eq.(19). We also find that for a class of different
potentials the dynamics evolution of the universe are actually the same and
therefore undistinguishable.Comment: 8 pages, no figure, accepted by The European Physical Journal
C(2010), online first,
http://www.springerlink.com/content/323417h708gun5g8/?p=dd373adf23b84743b523a3fa249d51c7&pi=
Fractional Quantum Hall Effect via Holography: Chern-Simons, Edge States, and Hierarchy
We present three holographic constructions of fractional quantum Hall effect
(FQHE) via string theory. The first model studies edge states in FQHE using
supersymmetric domain walls in N=6 Chern-Simons theory. We show that D4-branes
wrapped on CP^1 or D8-branes wrapped on CP^3 create edge states that shift the
rank or the level of the gauge group, respectively. These holographic edge
states correctly reproduce the Hall conductivity. The second model presents a
holographic dual to the pure U(N)_k (Yang-Mills-)Chern-Simons theory based on a
D3-D7 system. Its holography is equivalent to the level-rank duality, which
enables us to compute the Hall conductivity and the topological entanglement
entropy. The third model introduces the first string theory embedding of
hierarchical FQHEs, using IIA string on C^2/Z_n.Comment: 36 pages, 6 figures; v2: with an improved derivation of Hall
conductivity in section 3.2, typo corrections, and additional references; v3:
explanations and comments adde
Melting of Charge/Orbital Ordered States in NdSrMnO: Temperature and Magnetic Field Dependent Optical Studies
We investigated the temperature ( 15 290 K) and the magnetic
field ( 0 17 T) dependent optical conductivity spectra of a
charge/orbital ordered manganite, NdSrMnO. With variation
of and , large spectral weight changes were observed up to 4.0 eV. These
spectral weight changes could be explained using the polaron picture.
Interestingly, our results suggested that some local ordered state might remain
above the charge ordering temperature, and that the charge/orbital melted state
at a high magnetic field (i.e. at 17 T and 4.2 K) should be a three
dimensional ferromagnetic metal. We also investigated the first order phase
transition from the charge/orbital ordered state to ferromagnetic metallic
state using the - and % -dependent dielectric constants . In
the charge/orbital ordered insulating state, was positive and
. With increasing and , was
increased up to the insulator-metal phase boundaries. And then,
abruptly changed into negative and , which was
consistent with typical responses of a metal. Through the analysis of using an effective medium approximation, we found that the melting
of charge/orbital ordered states should occur through the percolation of
ferromagnetic metal domains.Comment: submitted to Phys. Rev.
Recent experimental results in sub- and near-barrier heavy ion fusion reactions
Recent advances obtained in the field of near and sub-barrier heavy-ion
fusion reactions are reviewed. Emphasis is given to the results obtained in the
last decade, and focus will be mainly on the experimental work performed
concerning the influence of transfer channels on fusion cross sections and the
hindrance phenomenon far below the barrier. Indeed, early data of sub-barrier
fusion taught us that cross sections may strongly depend on the low-energy
collective modes of the colliding nuclei, and, possibly, on couplings to
transfer channels. The coupled-channels (CC) model has been quite successful in
the interpretation of the experimental evidences. Fusion barrier distributions
often yield the fingerprint of the relevant coupled channels. Recent results
obtained by using radioactive beams are reported. At deep sub-barrier energies,
the slope of the excitation function in a semi-logarithmic plot keeps
increasing in many cases and standard CC calculations over-predict the cross
sections. This was named a hindrance phenomenon, and its physical origin is
still a matter of debate. Recent theoretical developments suggest that this
effect, at least partially, may be a consequence of the Pauli exclusion
principle. The hindrance may have far-reaching consequences in astrophysics
where fusion of light systems determines stellar evolution during the carbon
and oxygen burning stages, and yields important information for exotic
reactions that take place in the inner crust of accreting neutron stars.Comment: 40 pages, 63 figures, review paper accepted for EPJ
Interwell coupling effect in Si/SiGe quantum wells grown by ultra high vacuum chemical vapor deposition
Si/Si0.66Ge0.34coupled quantum well (CQW) structures with different barrier thickness of 40, 4 and 2 nm were grown on Si substrates using an ultra high vacuum chemical vapor deposition (UHV-CVD) system. The samples were characterized using high resolution x-ray diffraction (HRXRD), cross-sectional transmission electron microscopy (XTEM) and photoluminescence (PL) spectroscopy. Blue shift in PL peak energy due to interwell coupling was observed in the CQWs following increase in the Si barrier thickness. The Si/SiGe heterostructure growth process and theoretical band structure model was validated by comparing the energy of the no-phonon peak calculated by the 6 + 2-bandk·pmethod with experimental PL data. Close agreement between theoretical calculations and experimental data was obtained
Transcriptional regulation of the AP-1 and Nrf2 target gene sulfiredoxin
“Two-cysteine” peroxiredoxins are antioxidant enzymes that exert a cytoprotective effect in many models of oxidative stress. However, under highly oxidizing conditions they can be inactivated through hyperoxidation of their peroxidatic active site cysteine residue. Sulfiredoxin can reverse this hyperoxidation, thus, reactivating peroxiredoxins. Here we review recent investigations that have shed further light on sulfiredoxin’s role and regulation. Studies have revealed sulfiredoxin to be a dynamically regulated gene whose transcription is induced by a variety of signals and stimuli. Sulfiredoxin expression is regulated by the transcription factor AP-1, which mediates its up-regulation by synaptic activity in neurons, resulting in protection against oxidative stress. Furthermore, sulfiredoxin has been identified as a new member of the family of genes regulated by Nuclear factor erythroid 2-related factor (Nrf2) via a conserved cis-acting antioxidant response element (ARE). As such, sulfiredoxin is likely to contribute to the net antioxidative effect of small molecule activators of Nrf2. As discussed here. the proximal AP-1 site of the sulfiredoxin promoter is embedded within the ARE, as is common with Nrf2 target genes. Other recent studies have shown that sulfiredoxin induction via Nrf2 may form an important part of the protective response to oxidative stress in the lung, preventing peroxiredoxin hyperoxidation and, in certain cases, subsequent degradation. We illustrate here that sulfiredoxin can be rapidly induced in vivo by administration of CDDO-TFEA, a synthetic triterpenoid inducer of endogenous Nrf2, which may offer a way of reversing peroxiredoxin hyperoxidation in vivo following chronic or acute oxidative stress
Theory of charge transport in diffusive normal metal / conventional superconductor point contacts
Tunneling conductance in diffusive normal metal / insulator / s-wave
superconductor (DN/I/S) junctions is calculated for various situations by
changing the magnitudes of the resistance and Thouless energy in DN and the
transparency of the insulating barrier. The generalized boundary condition
introduced by Yu. Nazarov [Superlattices and Microstructures 25 1221 (1999)] is
applied, where the ballistic theory by Blonder Tinkham and Klapwijk (BTK) and
the diffusive theory by Volkov Zaitsev and Klapwijk based on the boundary
condition of Kupriyanov and Lukichev (KL) are naturally reproduced. It is shown
that the proximity effect can enhance (reduce) the tunneling conductance for
junctions with a low (high) transparency. A wide variety of dependencies of
tunneling conductance on voltage bias is demonstrated including a -shaped
gap like structure, a zero bias conductance peak (ZBCP) and a zero bias
conductance dip (ZBCD)
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