63,308 research outputs found
Presure-Induced Superconducting State of Antiferromagnetic CaFeAs
The antiferromagnet CaFeAs does not become superconducting when
subject to ideal hydrostatic pressure conditions, where crystallographic and
magnetic states also are well defined. By measuring electrical resistivity and
magnetic susceptibility under quasi-hydrostatic pressure, however, we find that
a substantial volume fraction of the sample is superconducting in a narrow
pressure range where collapsed tetragonal and orthorhombic structures coexist.
At higher pressures, the collapsed tetragonal structure is stabilized, with the
boundary between this structure and the phase of coexisting structures strongly
dependent on pressure history. Fluctuations in magnetic degrees of freedom in
the phase of coexisting structures appear to be important for
superconductivity.Comment: revised (6 pages, 5 figures) - includes additional experimental
result
Anomalous Pressure Dependence of Kadowaki-Woods ratio and Crystal Field Effects in Mixed-valence YbInCu4
The mixed-valence (MV) compound YbInCu4 was investigated by electrical
resistivity and ac specific heat at low temperatures and high pressures. At
atmospheric pressure, its Kadowaki-Woods (KW) ratio, A/\gamma ^2, is 16 times
smaller than the universal value R_{KW}(=1.0 x 10^-5 \mu \Omega cm mol^2 K^2
mJ^-2), but sharply increases to 16.5R_{KW} at 27 kbar. The pressure-induced
change in the KW ratio and deviation from R_{KW} are analyzed in terms of the
change in f-orbital degeneracy N and carrier density n. This analysis is
further supported by a dramatic change in residual resistivity \rho_0 near 25
kbar, where \rho_0 jumps by a factor of 7.Comment: 4pages, 3figure
Pressure effects on the heavy-fermion antiferromagnet CeAuSb2
The f-electron compound CeAuSb2, which crystallizes in the ZrCuSi2-type
tetragonal structure, orders antiferromagnetically between 5 and 6.8 K, where
the antiferromagnetic transition temperature T_N depends on the occupancy of
the Au site. Here we report the electrical resistivity and heat capacity of a
high-quality crystal CeAuSb2 with T_N of 6.8 K, the highest for this compound.
The magnetic transition temperature is initially suppressed with pressure, but
is intercepted by a new magnetic state above 2.1 GPa. The new phase shows a
dome shape with pressure and coexists with another phase at pressures higher
than 4.7 GPa. The electrical resistivity shows a T^2 Fermi liquids behavior in
the complex magnetic state, and the residual resistivity and the T^2
resistivity coefficient increases with pressure, suggesting the possibility of
a magnetic quantum critical point at a higher pressure.Comment: 5 pages, 5 firure
Crossover from weak to strong pairing in unconventional superconductors
Superconductors are classified by their pairing mechanism and the coupling
strength, measured as the ratio of the energy gap to the critical temperature,
Tc. We present an extensive comparison of the gap ratios among many single- and
multiband superconductors from simple metals to high-Tc cuprates and iron
pnictides. Contrary to the recently suggested universality of this ratio in
Fe-based superconductors, we find that the coupling in pnictides ranges from
weak, near the BCS limit, to strong, as in cuprates, bridging the gap between
these two extremes. Moreover, for Fe- and Cu-based materials, our analysis
reveals a universal correlation between the gap ratio and Tc, which is not
found in conventional superconductors and therefore supports a common
unconventional pairing mechanism in both families. An important consequence of
this result for ferropnictides is that the separation in energy between the
excitonic spin-resonance mode and the particle-hole continuum, which determines
the resonance damping, no longer appears independent of Tc.Comment: 15 pages, 3 figures, 5 tables with an exhaustive overview of the
published gap and spin-resonance measurements in Fe-based superconductors.
New in V3: updated references. To be published in Phys. Rev.
The Gluon Spin in the Chiral Bag Model
We study the gluon polarization contribution at the quark model
renormalization scale to the proton spin, , in the chiral bag model. It
is evaluated by taking the expectation value of the forward matrix element of a
local gluon operator in the axial gauge . It is shown that the confining
boundary condition for the color electric field plays an important role. When a
solution satisfying the boundary condition for the color electric field, which
is not the conventionally used but which we favor, is used, the has a
positive value for {\it all} bag radii and its magnitude is comparable to the
quark spin polarization. This results in a significant reduction in the
relative fraction of the proton spin carried by the quark spin, which is
consistent with the small flavor singlet axial current measured in the EMC
experiments.Comment: Corrections to figure
Quasi-dark Mode in a Metamaterial for Analogous Electromagnetically-induced Transparency
We study a planar metamaterial supporting electromagnetically-induced
transparency (EIT)-like effect by exploiting the coupling between bright and
quasi-dark eigenmodes. The specific design of such a metamaterial consists of a
cut-wire (CW) and a single-gap split-ring resonator (SRR). From the numerical
and the analytical results we demonstrate that the response of SRR, which is
weakly excited by external electric field, is mitigated to be a quasi-dark
eigenmode in the presence of strongly radiative CW. This result suggests more
relaxed conditions for the realization of devices utilizing the EIT-like
effects in metamaterial, and thereby widens the possibilities for many
different structural implementations.Comment: 11 pages, 4 figure
Effects of pressure on the ferromagnetic state of the CDW compound SmNiC2
We report the pressure response of charge-density-wave (CDW) and
ferromagnetic (FM) phases of the rare-earth intermetallic SmNiC2 up to 5.5 GPa.
The CDW transition temperature (T_{CDW}), which is reflected as a sharp
inflection in the electrical resistivity, is almost independent of pressure up
to 2.18 GPa but is strongly enhanced at higher pressures, increasing from 155.7
K at 2.2 GPa to 279.3 K at 5.5 GPa. Commensurate with the sharp increase in
T_{CDW}, the first-order FM phase transition, which decreases with applied
pressure, bifurcates into the upper (T_{M1}) and lower (T_c) phase transitions
and the lower transition changes its nature to second order above 2.18 GPa.
Enhancement both in the residual resistivity and the Fermi-liquid T^2
coefficient A near 3.8 GPa suggests abundant magnetic quantum fluctuations that
arise from the possible presence of a FM quantum critical point.Comment: 5 pages, 5 figure
Scaling of cluster heterogeneity in percolation transitions
We investigate a critical scaling law for the cluster heterogeneity in
site and bond percolations in -dimensional lattices with . The
cluster heterogeneity is defined as the number of distinct cluster sizes. As an
occupation probability increases, the cluster size distribution evolves
from a monodisperse distribution to a polydisperse one in the subcritical
phase, and back to a monodisperse one in the supercritical phase. We show
analytically that diverges algebraically approaching the percolation
critical point as with the critical exponent
associated with the characteristic cluster size. Interestingly, its
finite-size-scaling behavior is governed by a new exponent where is the fractal dimension of the critical percolating
cluster and is the correlation length exponent. The corresponding scaling
variable defines a singular path to the critical point. All results are
confirmed by numerical simulations.Comment: 4 pages, 4 figure
Inflation by non-minimal coupling
Inflationary scenarios based on simple non-minimal coupling and its
generalizations are studied. Generalizing the form of non-minimal coupling to
"K(phi)R" with an arbitrary function K(phi), we show that the flat potential
still is obtainable when V(phi)/K^2(phi) is asymptotically constant. Very
interestingly, if the ratio of the dimensionless self-coupling constant of the
inflaton field and the non-minimal coupling constant is small the cosmological
observables for general monomial cases are in good agreement with recent
observational data.Comment: 9 pages, 1 figur
Tunable far infrared studies of molecular parameters in support of stratospheric measurements
Lab studies were made in support of far infrared spectroscopy of the stratosphere using the Tunable Far InfraRed (TuFIR) method of ultrahigh resolution spectroscopy and, more recently, spectroscopic and retrieval calculations performed in support of satellite-based atmospheric measurement programs: the Global Ozone Monitoring Experiment (GOME), and the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY)
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