196 research outputs found
Temperature dependence of the upper critical field of an anisotropic singlet superconductivity in a square lattice tight-binding model in parallel magnetic fields
Upper critical field parallel to the conducting layer is studied in
anisotropic type-II superconductors on square lattices. We assume enough
separation of the adjacent layers, for which the orbital pair-breaking effect
is suppressed for exactly aligned parallel magnetic field. In particular, we
examine the temperature dependence of the critical field H_c(T) of the
superconductivity including the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO or LOFF)
state, in which the Cooper pairs have non-zero center-of-mass momentum q. In
the system with the cylindrically symmetric Fermi-surface, it is known that
H_c(T) of the d-wave FFLO state exhibits a kink at a low temperature due to a
change of the direction of q in contrast to observations in organic
superconductors. It is shown that the kink disappears when the Fermi-surface is
anisotropic to some extent, since the direction of q is locked in an optimum
direction independent of the temperature.Comment: 5 pages, 5 figures, revtex.sty, submitted to J.Phys.Soc.Jp
Phonon Dynamics and Multipolar Isomorphic Transition in beta-pyrochlore KOs2O6
We investigate with a microscopic model anharmonic K-cation oscillation
observed by neutron experiments in beta-pyrochlore superconductor KOs2O6, which
also shows a mysterious first-order structural transition at Tp=7.5 K. We have
identified a set of microscopic model parameters that successfully reproduce
the observed temperature dependence and the superconducting transition
temperature. Considering changes in the parameters at Tp, we can explain
puzzling experimental results about electron-phonon coupling and neutron data.
Our analysis demonstrates that the first-order transition is multipolar
transition driven by the octupolar component of K-cation oscillations. The
octupole moment does not change the symmetry and is characteristic to
noncentrosymmetric K-cation potential.Comment: 5 pages, 4 figures, submitted to J. Phys. Soc. Jp
Superconductivity and Rattling under High Pressure in the beta-Pyrochlore Oxide RbOs2O6
Rattling-induced superconductivity in the beta-pyrochlore oxide RbOs2O6 is
investigated under high pressures up to 6 GPa. Resistivity measurements in a
high-quality single crystal show that the superconducting transition
temperature Tc increases gradually from 6.3 K at ambient pressure to 8.8 K at
3.5 GPa, surprisingly remains almost constant at 8.8 \pm 0.1 K in a wide
pressure range between 3.5 (Po) and 4.8 GPa, and suddenly drops to 6.3 K at Ps
= 4.9 GPa, followed by a gradual decrease with further pressure increase. Two
anomalies in the temperature dependence of the normal-state resistivity are
observed at Po Ps, revealing the presence of two high-pressure
phases corresponding to the changes in Tc. The rattling of the Rb ion inside a
cage made of Os and O atoms may be slightly and seriously modified in these
high-pressure phases that probably have cages of reduced symmetry,
respectively, so that electron-rattler interactions that govern the
superconducting and transport properties of beta-RbOs2O6 are significantly
affected.Comment: arXiv admin note: text overlap with arXiv:1009.035
Glial-Specific Deletion of Med12 Results in Rapid Hearing Loss via Degradation of the Stria Vascularis
Mediator protein complex subunit 12 (Med12) is a core component of the basal transcriptional apparatus and plays a critical role in the development of many tissues. Mutations in Med12 are associated with X-linked intellectual disability syndromes and hearing loss; however, its role in nervous system function remains undefined. Here, we show that temporal conditional deletion of Med12 in astrocytes in the adult central nervous system results in region specific alterations in astrocyte morphology. Surprisingly, behavioral studies revealed rapid hearing loss after adult deletion of Med12 that was confirmed by a complete abrogation of auditory brainstem responses. Cellular analysis of the cochlea revealed degeneration of the stria vascularis, in conjunction with disorganization of basal cells adjacent to the spiral ligament and downregulation of key cell adhesion proteins. Physiological analysis revealed early changes in endocochlear potential, consistent with strial-specific defects. Together, our studies reveal that Med12 regulates auditory function in the adult by preserving the structural integrity of the stria vascularis
A Two-dimensional Superconductor in a Tilted Magnetic Field - new states with finite Cooper-pair momentum
Varying the angle Theta between applied field and the conducting planes of a
layered superconductor in a small interval close to the plane-parallel field
direction, a large number of superconducting states with unusual properties may
be produced. For these states, the pair breaking effect of the magnetic field
affects both the orbital and the spin degree of freedom. This leads to pair
wave functions with finite momentum, which are labeled by Landau quantum
numbers 0<n<\infty. The stable order parameter structure and magnetic field
distribution for these states is found by minimizing the quasiclassical free
energy near H_{c2} including nonlinear terms. One finds states with coexisting
line-like and point-like order parameter zeros and states with coexisting
vortices and antivortices. The magnetic response may be diamagnetic or
paramagnetic depending on the position within the unit cell. The structure of
the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states at Theta=0 is reconsidered.
The transition n->\infty of the paramagnetic vortex states to the FFLO-limit is
analyzed and the physical reason for the occupation of higher Landau levels is
pointed out.Comment: 24 pages, 11 figure
Enhancement of the upper critical field and a field-induced superconductivity in antiferromagnetic conductors
We propose a mechanism by which the paramagnetic pair-breaking effect is
largely reduced in superconductors with coexisting antiferromagnetic long-
range and short-range orders. The mechanism is an extension of the Jaccarino
and Peter mechanism to antiferromagnetic conductors, but the resultant phase
diagram is quite different. In order to illustrate the mechanism, we examine a
model which consists of mobile electrons and antiferromagnetically correlated
localized spins with Kondo coupling between them. It is found that for weak
Kondo coupling, the superconductivity occurs over an extraordinarily wide
region of the magnetic field including zero field. The critical field exceeds
the Chandrasekhar and Clogston limit, but there is no lower limit in contrast
to the Jaccarino and Peter mechanism. On the other hand, for strong Kondo
coupling, both the low-field superconductivity and a field-induced
superconductivity occur. Possibilities in hybrid ruthenate cuprate
superconductors and some organic superconductors are discussed.Comment: 5 pages, 1 figure, revtex.sty, to be published in J.Phys.Soc.Jpn.
Vol.71, No.3 (2002
Seismic Vulnerability Assessment of Priority Cultural Heritage Structures in the Philippines
At the end of 2013 two catastrophic events occurred in the Philippines: the M 7.2 earthquake
in Bohol and the strongest ever recorded Typhoon Haiyan, causing destruction across the islands of Cebu,
Bohol and the Visayas region. These events raised the need to carry out a multi-hazard risk assessment of
heritage buildings, many of which were irretrievably lost in the disasters. Philippines’ Department of
Tourism engaged ARS Progetti S.P.A., Rome, Italy, and the Center for Conservation of Cultural Property
and Environment in the Tropics (CCCPET), University of Sto. Tomas, Manila, to undertake the
“Assessment of the Multi-Hazard Vulnerability of Priority Cultural Heritage Structures in the
Philippines”, with experts from University College London, UK, and De La Salle University.
The main objective of the project was to reduce the vulnerability of cultural heritage structures to multiple
natural hazards, including earthquake, typhoon, flood, by: (i) prioritizing of specific structures based on
hazard maps and historical records; (ii) assessing their vulnerability; and (iii) recommending options to
mitigate the impacts on them. The paper presents the methodology introduced to determine the seismic
risk these heritage buildings are exposed to. All the selected cultural heritage structures are under the
jurisdiction of the National Museum Commission of Philippines and of the National Commission for
Culture and Arts
Infrared and optical properties of pure and cobalt-doped LuNi_2B_2C
We present optical conductivity data for Lu(NiCo)BC over
a wide range of frequencies and temperatures for x=0 and x=0.09. Both materials
show evidence of being good Drude metals with the infrared data in reasonable
agreement with dc resistivity measurements at low frequencies. An absorption
threshold is seen at approximately 700 cm-1. In the cobalt-doped material we
see a superconducting gap in the conductivity spectrum with an absorption onset
at 24 +/- 2 cm-1 = 3.9$ +/- 0.4 k_BT_c suggestive of weak to moderately strong
coupling. The pure material is in the clean limit and no gap can be seen. We
discuss the data in terms of the electron-phonon interaction and find that it
can be fit below 600 cm-1 with a plasma frequency of 3.3 eV and an
electron-phonon coupling constant lambda_{tr}=0.33 using an alpha^{2}F(omega)
spectrum fit to the resistivity.Comment: 10 pages with 10 embedded figures, submitted to PR
Magnetic field - temperature phase diagram of quasi-two-dimensional organic superconductor lambda-(BETS)_2 GaCl_4 studied via thermal conductivity
The thermal conductivity kappa of the quasi-two-dimensional (Q2D) organic
superconductor lambda-(BETS)_2 GaCl_4 was studied in the magnetic field H
applied parallel to the Q2D plane. The phase diagram determined from this bulk
measurement shows notable dependence on the sample quality. In dirty samples
the upper critical field H_{c2} is consistent with the Pauli paramagnetic
limiting, and a sharp change is observed in kappa(H) at H_{c2 parallel}. In
contrast in clean samples H_{c2}(T) shows no saturation towards low
temperatures and the feature in kappa(H) is replaced by two slope changes
reminiscent of second-order transitions. The peculiarity was observed below ~
0.33T_c and disappeared on field inclination to the plane when the orbital
suppression of superconductivity became dominant. This behavior is consistent
with the formation of a superconducting state with spatially modulated order
parameter in clean samples.Comment: 10 pages, 8 figures, new figure (Fig.5) and references added, title
change
Phonon-mediated anisotropic superconductivity in the Y and Lu nickel borocarbides
We present scanning tunneling spectroscopy and microscopy measurements at low
temperatures in the borocarbide materials RNi2B2C (R=Y, Lu). The characteristic
strong coupling structure due to the pairing interaction is unambiguously
resolved in the superconducting density of states. It is located at the
superconducting gap plus the energy corresponding to a phonon mode identified
in previous neutron scattering experiments. These measurements also show that
this mode is coupled to the electrons through a highly anisotropic
electron-phonon interaction originated by a nesting feature of the Fermi
surface. Our experiments, from which we can extract a large electron-phonon
coupling parameter lambda (between 0.5 and 0.8), demonstrate that this
anisotropic electron-phonon coupling has an essential contribution to the
pairing interaction. The tunneling spectra show an anisotropic s-wave
superconducting gap function.Comment: 5 pages, 3 figure
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