721 research outputs found
Josephson effect in a weak link between borocarbides
A stationary Josephson effect is analyzed theoretically for a weak link
between borocarbide superconductors. It is shown that different models of the
order parameter result in qualitatively different current-phase relations
Violation of the isotropic- approximation in overdoped La_{2-x}Sr_xCuO_4
Magnetotransport measurements on the overdoped cuprate La_{1.7}Sr_{0.3}CuO_4
are fitted using the Ong construction and band parameters inferred from
angle-resolved photoemission. Within a band picture, the low temperature Hall
data can only be fitted satisfactorily by invoking strong basal-plane
anisotropy in the mean-free-path . This violation of the isotropic-
approximation supports a picture of dominant small-angle elastic scattering in
cuprates due to out-of-plane substitutional disorder. We show that both band
anisotropy and anisotropy in the elastic scattering channel strongly
renormalize the Hall coefficient in overdoped La_{2-x}Sr_xCuO_4 over a wide
doping and temperature range.Comment: 4 pages, 4 figure
Fulde-Ferrell-Larkin-Ovchinnikov state in a perpendicular field of quasi two-dimensional CeCoIn5
A Fulde-Ferrell-Larkin-Ovchinnkov (FFLO) state was previously reported in the
quasi-2D heavy fermion CeCoIn5 when a magnetic field was applied parallel to
the ab-plane. Here, we conduct 115^In NMR studies of this material in a
PERPENDICULAR field, and provide strong evidence for FFLO in this case as well.
Although the topology of the phase transition lines in the H-T phase diagram is
identical for both configurations, there are several remarkable differences
between them. Compared to H//ab, the FFLO region for H perpendicular to the
ab-plane shows a sizable decrease, and the critical field separating the FFLO
and non-FFLO superconducting states almost ceases to have a temperature
dependence. Moreover, directing H perpendicular to the ab-plane results in a
notable change in the quasiparticle excitation spectrum within the planar node
associated with the FFLO transition.Comment: 5 pages, 3 figure
BCS theory for s+g-wave superconductivity borocarbides Y(Lu)NiBC
The s+g mixed gap function \Delta_k=\Delta {[(1-x)-x\sin^4\theta\cos4\phi]}
(x: weight of g-wave component) has been studied within BCS theory. By suitable
consideration of the pairing interaction, we have confirmed that the
coexistence of s- and g-wave, as well as the state with equal s and g
amplitudes (i.e., x=1/2) may be stable. This provides the semi-phenomenological
theory for the s+g-wave superconductivity with point nodes which has been
observed experimentally in borocarbides YNi_2B_2C and possibly in LuNi_2B_2C.Comment: 5 pages, 3 figure
Spins in the Vortices of a High Temperature Superconductor
Neutron scattering is used to characterise the magnetism of the vortices for
the optimally doped high-temperature superconductor La(2-x)Sr(x)CuO(4)
(x=0.163) in an applied magnetic field. As temperature is reduced, low
frequency spin fluctuations first disappear with the loss of vortex mobility,
but then reappear. We find that the vortex state can be regarded as an
inhomogeneous mixture of a superconducting spin fluid and a material containing
a nearly ordered antiferromagnet. These experiments show that as for many other
properties of cuprate superconductors, the important underlying microscopic
forces are magnetic
On the Ginzburg-Landau Analysis of the Upper Critical Field Hc2 in MgB2
Temperature dependence of the upper critical field Hc2 (T) for the
superconducting magnesium diboride, MgB2, is studied in the vicinity of Tc by
using a two-band Ginzburg-Landau (G-L) theory. The temperature dependence of
Hc2 (T) near Tc exhibits a positive curvature. In addition, the calculated
temperature dependence and its higher order derivatives are also shown to be in
a good agreement with the experimental data. In analogy with the multi-band
character of Eliashberg microscopic theory, the positive curvature of Hc2 (T)
is described reasonably by solving the two-band of G-L theory.Comment: 14 pages, 2 figures, submitted to SUST November 200
Dispersive Gap Mode of Phonons in Anisotropic Superconductors
We estimate the effect of the superconducting gap anisotropy in the
dispersive gap mode of phonons, which is observed by the neutron scattering on
borocarbide superconductors. We numerically analyze the phonon spectrum
considering the electron-phonon coupling, and examine contributions coming from
the gap suppression and the sign change of the pairing function on the Fermi
surface. When the sign of the pairing function is changed by the nesting
translation, the gap mode does not appear. We also discuss the suppression of
the phonon softening of the Kohn anomaly due to the onset of superconductivity.
We demonstrate that observation of the gap dispersive mode is useful for
sorting out the underlying superconducting pairing function.Comment: 7 pages, 12 figures, to be published in J. Phys. Soc. Jp
Phase separation and phase transitions in undoped and Rh3+ doped iron pnictide CaFe2As2: a Raman scattering study
Iron-pnictides Ca(Fe1-xRhx)2As2 (x = 0, 0.035, and 0.19) were studied across
the tetragonal-orthorhombic and uncollapsed to collapsed tetragonal phase
transitions using Raman spectroscopy. The effect of the phase separation was
observed in the high-temperature phase for the first time. Two phases with the
low-spin and high-spin states of the Fe2+ ions coexist in the undoped (x = 0)
and doped (x = 0.035) samples at ambient conditions. These two phases are
present on a larger length scale and they are not limited to the local
distortions. Both phases in the undoped sample successfully undergo
tetra-to-ortho phase transitions approximately at the same temperature T0 = 160
K. In doped samples, a cascade of phase transitions is observed at temperatures
T* = 100 K and Tc = 72 K with cooling for the first time. A complex type of the
re-entrant magnetic structure is realized in the x = 0.035 sample at
temperatures below Tc, which is distinct from the usual orthorhombic twofold
one. The overdoped sample (x = 0.19) also shows phase separation at
temperatures 7 - 295 K in Raman spectra similar to these of the remnant phase.Comment: 18 pages, 8 figure
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