105 research outputs found
Spin Fluctuation-Induced Superconductivity in Organic Compounds
Spin fluctuation-induced superconductivity in two-dimensional organic
compounds such as \kappa-(ET)_2-X is investigated by using a simplified dimer
Hubbard model with right-angled isosceles triangular lattice (transfer matrices
-\tau, -\tau^\prime). The dynamical susceptiblity and the self-energy are
calculated self-consistently within the fluctuation exchange approximation and
the value for T_c as obtained by solving the linearized Eliashberg-type
equations is in good agreement with experiment. The pairing symmetry is of
d_{x^2-y^2} type. The calculated (U/\tau)-dependence of T_c compares
qualitatively well with the observed pressure dependence of T_c. Varying the
value for \tau^\prime/\tau from 0 to 1 we interpolate between the square
lattice and the regular triangular lattice and find firstly that values of T_c
for \kappa-(ET)_2-X and cuprates scale well and secondly that T_c tends to
decrease with increasing \tau^\prime/\tau and no superconductivity is found for
\tau^\prime/\tau=1, the regular triangular lattice.Comment: 4 pages, 6 eps figures, uses jpsj.st
Comparative magnetotransport and Tc measurements on kappa-(BEDT-TTF)2Cu(SCN)2 under pressure
We compare magnetotransport measurements under pressure on the organic
superconductor -(BEDT-TTF)Cu(SCN) with different
pressure-media and discover that the results are pressure media dependent. This
pressure-medium dependence is thought to originate from the difference in
thermal contraction between the very soft and highly anisotropic sample and the
isotropically contracting, but solid pressure medium, thus resulting in
non-hydrostatic pressure on the sample. However, comparison of pressure
measurements with different media reveals a pressure-medium independent
correlation between the superconducting transition temperature, T,
and the size of the quasi 2-dimensional Fermi surface pocket and thus the quasi
2-dimensional carrier density in -(BEDT-TTF)Cu(SCN). The
observed pressure-induced increase in the quasi 2-dimensional carrier density
can be interpreted as a transfer of carriers from quasi 1-dimensional Fermi
surface sections, reminiscent of a mechanism in cuprate superconductors, where
pressure is known to transfer carriers from the insulating charge reservoir
layers into the conducting cuprate sheets.Comment: 11 pages, 6 figure
Hyperconductivity in chilled beryllium metal
It is shown that in the vicinity of 77 K beryllium has a superior specific conductance compared with the nominally excellent metallic conductors aluminum and copper. It is concluded that beryllium should be considered for some conduction applications, despite its well known toxicity problems
Neutron Scattering Study of Spin Density Wave Order in the Superconducting State of Excess-Oxygen-Doped La2CuO4+y
We report neutron scattering measurements of spin density wave order within
the superconducting state of a single crystal of predominately stage-4
La2CuO4+y with a Tc(onset) of 42 K. The low temperature elastic magnetic
scattering is incommensurate with the lattice and is characterized by
long-range order in the copper-oxide plane with the spin direction identical to
that in the insulator. Between neighboring planes, the spins exhibit
short-range correlations with a stacking arrangement reminiscent of that in the
undoped antiferromagnetic insulator. The elastic magnetic peak intensity
appears at the same temperature within the errors as the superconductivity,
suggesting that the two phenomena are strongly correlated. These observations
directly reveal the persistent influence of the antiferromagnetic order as the
doping level increases from the insulator to the superconductor. In addition,
our results confirm that spin density wave order for incommensurabilities near
1/8 is a robust feature of the La2CuO4-based superconductors.Comment: 14 pages, LaTeX, includes 8 figure
Superconductivity in Fullerides
Experimental studies of superconductivity properties of fullerides are
briefly reviewed. Theoretical calculations of the electron-phonon coupling, in
particular for the intramolecular phonons, are discussed extensively. The
calculations are compared with coupling constants deduced from a number of
different experimental techniques. It is discussed why the A_3 C_60 are not
Mott-Hubbard insulators, in spite of the large Coulomb interaction. Estimates
of the Coulomb pseudopotential , describing the effect of the Coulomb
repulsion on the superconductivity, as well as possible electronic mechanisms
for the superconductivity are reviewed. The calculation of various properties
within the Migdal-Eliashberg theory and attempts to go beyond this theory are
described.Comment: 33 pages, latex2e, revtex using rmp style, 15 figures, submitted to
Review of Modern Physics, more information at
http://radix2.mpi-stuttgart.mpg.de/fullerene/fullerene.htm
Suppression of superconductivity by non-magnetic disorder in organic superconductor -(BEDT-TTF)Cu(NCS)
The suppression of superconductivity by nonmagnetic disorder is investigated
systematically in the organic superconductor
-(BEDT-TTF)Cu(NCS). We introduce a nonmagnetic disorder arising
from molecule substitution in part with deuterated BEDT-TTF or BMDT-TTF for
BEDT-TTF molecules and molecular defects introduced by X-ray irradiation. A
quantitative evaluation of the scattering time is carried out
by de Haas-van Alphen (dHvA) effect measurement. A large reduction in with a linear dependence on is found in the
small-disorder region below 1 10
s in both the BMDT-TTF molecule-substituted and X-ray-irradiated
samples. The observed linear relation between and is in agreement with the Abrikosov-Gorkov (AG) formula, at least in the
small-disorder region. This observation is reasonably consistent with the
unconventional superconductivity proposed thus far for the present organic
superconductor. A deviation from the AG formula, however, is observed in the
large-disorder region above 1 10
s, which reproduces the previous transport study (J. G. Analytis {\it et
al.}: Phys. Rev. Lett. {\bf 96} (2006) 177002). We present some interpretations
of this deviation from the viewpoints of superconductivity and the inherent
difficulties in the evaluation of scattering time.Comment: 11 pages, 6 figure
High Pressure Effects on Superconductivity
The review is devoted to a discussion of the effects of high pressure imposed
on superconducting materials. Low-temperature superconductors, high-temperature
superconducting cuprates, and some unconventional superconducting compounds are
investigated. Experimental as well as theoretical results regarding the
pressure effects on Tc and other interesting properties are summarized.Comment: To be published in: "Frontiers in Superconducting Materials", Edt. A.
Narlikar, Springer Verla
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