7,244 research outputs found
Pressure-induced enhancement of superconductivity and superconducting-superconducting transition in CaC
We measured the electrical resistivity, , of superconducting
CaC at ambient and high pressure up to 16 GPa. For 8 GPa, we found
a large increase of with pressure from 11.5 up to 15.1 K. At 8 GPa,
drops and levels off at 5 K above 10 GPa. Correspondingly, the residual
increases by 200 times and the behavior
becomes flat. The recovery of the pristine behavior after depressurization is
suggestive of a phase transition at 8 GPa between two superconducting phases
with good and bad metallic properties, the latter with a lower and more
static disorder
Effects of Umklapp Scattering on Electronic States in One Dimension
The effects of Umklapp scattering on electronic states are studied in one
spatial dimension at absolute zero. The model is basically the Hubbard model,
where parameters characterizing the normal () and Umklapp () scattering
are treated independently. The density of states is calculated in the t-matrix
approximation by taking only the forward and Umklapp scattering into account.
It is found that the Umklapp scattering causes the global splitting of the
density of states. In the presence of sufficiently strong Umklapp scattering, a
pole in the t-matrix appears in the upper half plane, signalling an instability
towards the 'pairing' ordered state ( is the reciprocal lattice
vector), whose consequences are studied in the mean field approximation. It
turns out that this ordered state coexists with spin-density-wave state and
also brings about Cooper-pairs. A phase diagram is determined in the plane of
and electron filling .Comment: 22 pages, LaTeX, 17 figures included, uses jpsj.st
Rotationally resolved spectroscopy of (20000) Varuna in the near-infrared
Models of the escape and retention of volatiles by minor icy objects exclude
any presence of volatile ices on the surface of TNOs smaller than ~1000km in
diameter at the typical temperature in this region of the solar system, whereas
the same models show that water ice is stable on the surface of objects over a
wide range of diameters. Collisions and cometary activity have been used to
explain the process of surface refreshing of TNOs and Centaurs. These processes
can produce surface heterogeneity that can be studied by collecting information
at different rotational phases. The aims of this work are to study the surface
composition of (20000)Varuna, a TNO with a diameter ~650km and to search for
indications of rotational variability. We observed Varuna during two
consecutive nights in January 2011 with NICS@TNG obtaining a set of spectra
covering the whole rotation period of Varuna. After studying the spectra
corresponding to different rotational phases, we did not find any indication of
surface variability. In all the spectra, we detect an absorption at 2{\mu}m,
suggesting the presence of water ice on the surface. We do not detect any other
volatiles on the surface, although the S/N is not high enough to discard their
presence. Based on scattering models, we present two possible compositions
compatible with our set of data and discuss their implications in the frame of
the collisional history of the Kuiper Belt. We find that the most probable
composition for the surface of Varuna is a mixture of amorphous silicates,
complex organics, and water ice. This composition is compatible with all the
materials being primordial. However, our data can also be fitted by models
containing up to a 10% of methane ice. For an object with the characteristics
of Varuna, this volatile could not be primordial, so an event, such as an
energetic impact, would be needed to explain its presence on the surface.Comment: 6 pages, 5 figures, to be published in A&
Anisotropy in the helicity modulus of a 3D XY-model: application to YBCO
We present a Monte Carlo study of the helicity moduli of an anisotropic
classical three-dimensional (3D) XY-model of YBCO in superconducting state. It
is found that both the ab-plane and the c-axis helicity moduli, which are
proportional to the inverse square of the corresponding magnetic field
penetration depth, vary linearly with temperature at low temperatures. The
result for the c-axis helicity modulus is in disagreement with the experiments
on high quality samples of YBCO. Thus we conclude that purely classical phase
fluctuations of the superconducting order parameter cannot account for the
observed c-axis electrodynamics of YBCO.Comment: 7 pages, 1 figur
Fibrillar templates and soft phases in systems with short-range dipolar and long-range interactions
We analyze the thermal fluctuations of particles that have a short-range
dipolar attraction and a long-range repulsion. In an inhomogeneous particle
density region, or "soft phase," filamentary patterns appear which are
destroyed only at very high temperatures. The filaments act as a fluctuating
template for correlated percolation in which low-energy excitations can move
through the stable pattern by local rearrangements. At intermediate
temperatures, dynamically averaged checkerboard states appear. We discuss
possible implications for cuprate superconducting and related materials.Comment: 4 pages, 4 postscript figures. Discussion of implications for
experiment and theory has been expande
Public servants' expectations and perceptions of their managers' role and behaviour in post-bureaucratic organisations
Scanning Tunneling Spectroscopy on the novel superconductor CaC6
We present scanning tunneling microscopy and spectroscopy of the newly
discovered superconductor CaC. The tunneling conductance spectra, measured
between 3 K and 15 K, show a clear superconducting gap in the quasiparticle
density of states. The gap function extracted from the spectra is in good
agreement with the conventional BCS theory with = 1.6 0.2
meV. The possibility of gap anisotropy and two-gap superconductivity is also
discussed. In a magnetic field, direct imaging of the vortices allows to deduce
a coherence length in the ab plane 33 nm
Colossal Magnetoresistance in the Mn2+ Oxypnictides NdMnAsO1-xFx
Colossal magnetoresistance (CMR) is a rare phenomenon in which the electronic
resistivity of a material can be decreased by orders of magnitude upon
application of a magnetic field. Such an effect could be the basis of the next
generation of magnetic memory devices. Here we report CMR in the
antiferromagnetic oxypnictide NdMnAsO1-xFx as a result of competition between
an antiferromagnetic insulating phase with strong electron correlations and a
paramagnetic semiconductor upon application of a magnetic field. The discovery
of CMR in antiferromagnetic Mn2+ oxypnictide materials could open up an array
of materials for further investigation and optimisation for technological
applications
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