282 research outputs found
Antiferromagnetic superconductors with effective mass anisotropy in magnetic fields
We derive critical field H_c2 equations for antiferromagnetic
\textit{s}-wave, d_{x^2-y^2}-wave, and d_{xy}-wave superconductors with
effective mass anisotropy in three dimensions, where we take into account (i)
the Jaccarino-Peter mechanism of magnetic-field-induced superconductivity
(FISC) at high fields, (ii) an extended Jaccarino-Peter mechanism that reduces
the Pauli paramagnetic pair-breaking effect at low fields where
superconductivity and an antiferromagnetic long-range order with a canted spin
structure coexist, and (iii) the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO or
LOFF) state. As an example, experimental phase diagrams observed in organic
superconductor kappa-(BETS)_2FeBr_4 are theoretically reproduced. In
particular, the upper critical field of low-field superconductivity is well
reproduced without any additional fitting parameter other than those determined
from the critical field curves of the FISC at high fields. Therefore, the
extended Jaccarino-Peter mechanism seems to occur actually in the present
compound. It is predicted that the FFLO state does not occur in the FISC at
high fields in contrast to the compound lambda-(BETS)_2FeCl_4, but it may occur
in low-field superconductivity for s-wave and d_{x^2-y^2}-wave pairings. We
also briefly discuss a possibility of compounds that exhibit unconventional
behaviors of upper critical fields.Comment: 11 pages, 9 figures, revtex
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
On the Fulde-Ferrell State in Spatially Isotropic Superconductors
Effects of superconducting fluctuations on the Fulde-Ferrell (FF) state are
discussed in a spatially isotropic three-dimensional superconductor under a
magnetic field. For this system, Shimahara recently showed that within the
phenomenological Ginzburg-Landau theory, the long-range order of the FF state
is suppressed by the phase fluctuation of the superconducting order parameter.
[H. Shimahara: J. Phys. Soc. Jpn. {\bf 67} (1998) 1872, Physica B {\bf 259-261}
(1999) 492] In this letter, we investigate this instability of the FF state
against superconducting fluctuations from the microscopic viewpoint, employing
the theory developed by Nozi\'eres and Schmitt-Rink in the BCS-BEC crossover
field. Besides the absence of the second-order phase transition associated with
the FF state, we show that even if the pairing interaction is weak, the shift
of the chemical potential from the Fermi energy due to the fluctuations is
crucial near the critical magnetic field of the FF state obtained within the
mean-field theory.Comment: 11 pages, 1 figur
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
Reduction of Pauli paramagnetic pair-breaking effect in antiferromagnetic superconductors
Antiferromagnetic superconductors in a magnetic field are studied. We examine
a mechanism which significantly reduces the Pauli paramagnetic pair-breaking
effect. The mechanism is realized even in the presence of the orbital
pair-breaking effect. We illustrate it using a three-dimensional model with an
intercalated magnetic subsystem. The upper critical field is calculated for
various parameters. It is shown that the upper critical field can reach several
times the pure Pauli paramagnetic limit. The possible relevance to the large
upper critical field observed in the heavy fermion antiferromagnetic
superconductor CePt_3Si discovered recently is briefly discussed. We try to
understand the large upper critical field in the compound CePt_3Si and
field-induced superconductivity in the compound CePb_3 within a unified
framework.Comment: 5 pages, 2 figures, revtex4, minor correction
Spin-Triplet Superconductivity Mediated by Phonons in Quasi-One-Dimensional Systems
We investigate the spin-triplet superconductivity mediated by phonons in
quasi-one-dimensional (Q1D) systems with open Fermi surfaces. We obtain the
ground state phase diagrams. It is found that spin-triplet superconductivity
occurs for weak screening and strong on-site Coulomb interaction, even in the
absence of any additional nonphonon pairing interactions. We find that the
nodeless spin-triplet state is more favorable than the spin-triplet state with
line nodes, for the parameter values of the Q1D superconductors (TMTSF)_2X. We
also find that Q1D open Fermi surface, which is the specific feature of this
system, plays an essential role in the pairing symmetry. We discuss the
compatibility of the present results with the experimental results in these
compounds.Comment: 8 pages, 15 figures, with jpsj2.cl
Coexistence of Singlet and Triplet Attractive Channels in the Pairing Interactions Mediated by Antiferromagnetic Fluctuations
We propose a phase diagram of quasi-low-dimensional type II superconductors
in parallel magnetic fields, when antiferromagnetic fluctuations contribute to
the pairing interactions. We point out that pairing interactions mediated by
antiferromagnetic fluctuations necessarily include both singlet channels and
triplet channels as attractive interactions. Usually, a singlet pairing is
favored at zero field, but a triplet pairing occurs at high fields where the
singlet pairing is suppressed by the Pauli paramagnetic pair-breaking effect.
As a result, the critical field increases divergently at low temperatures. A
possible relation to experimental phase diagrams of a quasi-one-dimensional
organic superconductor is briefly discussed. We also discuss a possibility that
a triplet superconductivity is observed even at zero field.Comment: 4 pages, 1 figure (Latex, revtex.sty, epsf.sty
Magnetic properties of the two-dimensional Heisenberg model on a triangular lattice
The spin Green's function of the antiferromagnetic Heisenberg model on a
triangular lattice is calculated using Mori's projection operator technique. At
T=0 the spin excitation spectrum is shown to have gaps at the wave vectors of
the classical Neel ordering. This points to the absence of the
antiferromagnetic long-range order in the ground state. The calculated spin
correlation on the neighboring sites of the same sublattice is in good
agreement with the value derived from exact diagonalization. The temperature
dependencies of the spin correlations and the gaps are calculated.Comment: 5 pages, 3 figure
Josephson Effect in Fulde-Ferrell-Larkin-Ovchinnikov Superconductors
Due to the difference in the momenta of the superconducting order parameters,
the Josephson current in a Josephson junction between a
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductor and a conventional BCS
superconductor is suppressed. We show that the Josephson current may be
recovered by applying a magnetic field in the junction. The field strength and
direction at which the supercurrent recovery occurs depend upon the momentum
and structure of the order parameter in the FFLO state. Thus the Josephson
effect provides an unambiguous way to detect the existence of an FFLO state,
and to measure the momentum of the order parameter.Comment: 4 pages with one embedded eps figur
Isotope effect in superconductors with coexisting interactions of phonon and nonphonon mechanisms
We examine the isotope effect of superconductivity in systems with coexisting
interactions of phonon and nonphonon mechanisms in addition to the direct
Coulomb interaction. The interaction mediated by the spin fluctuations is
discussed as an example of the nonphonon interaction. Extended formulas for the
transition temperature Tc and the isotope-effect coefficient alpha are derived
for cases (a) omega_np omega_D, where omega_np is
an effective cutoff frequency of the nonphonon interaction that corresponds to
the Debye frequency omega_D in the phonon interaction. In case (a), it is found
that the nonphonon interaction does not change the condition for the inverse
isotope effect, i.e., mu^* > lambda_ph/2, but it modifies the magnitude of
alpha markedly. In particular, it is found that a giant isotope shift occurs
when the phonon and nonphonon interactions cancel each other largely. For
instance, strong critical spin fluctuations may give rise to the giant isotope
effect. In case (b), it is found that the inverse isotope effect occurs only
when the nonphonon interaction and the repulsive Coulomb interaction, in total
effect, work as repulsive interactions against the superconductivity. We
discuss the relevance of the present result to some organic superconductors,
such as kappa-(ET)2Cu(NCS)2 and Sr2RuO4 superconductors, in which inverse
isotope effects have been observed, and briefly to high-Tc cuprates, in which
giant isotope effects have been observed.Comment: 4 pages, 2 figures, (with jpsj2.cls, ver.1.2), v2:linguistic
correction
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