861 research outputs found
Superconductivity in a Ferromagnetic Layered Compound
We examine superconductivity in layered systems with large Fermi-surface
splitting due to coexisting ferromagnetic layers. In particular, the hybrid
ruthenate-cuprate compound RuSr_2GdCu_2O_8 is examined on the coexistence of
the superconductivity and the ferromagnetism, which has been observed recently.
We calculate critical fields of the superconductivity taking into account the
Fulde-Ferrell-Larkin-Ovchinnikov state in a model with Fermi-surfaces which
shapes are similar to those obtained by a band calculation. It is shown that
the critical field is enhanced remarkably due to a Fermi-surface effect, and
can be high enough to make the coexistence possible in a microscopic scale. We
also clarify the direction of the spatial oscillation of the order parameter,
which may be observed by scanning tunneling microscope experiments.Comment: 4 pages, 4 figures, (Latex, revtex.sty, epsf.sty
Upper critical fields of quasi-low-dimensional superconductors with coexisting singlet and triplet pairing interactions in parallel magnetic fields
Quasi-low-dimensional type II superconductors in parallel magnetic fields are
studied when singlet pairing interactions and relatively weak triplet pairing
interactions coexist. Singlet and triplet components of order parameter are
mixed at high fields, and at the same time an inhomogeneous superconducting
state called a Fulde-Ferrell-Larkin-Ovchinnikov state occurs. As a result, the
triplet pairing interactions enhance the upper critical field of
superconductivity remarkably even at temperatures far above the transition
temperature of parallel spin pairing. It is found that the enhancement is very
large even when the triplet pairing interactions are so weak that a high field
phase of parallel spin pairing may not be observed in practice. A possible
relvance of the result in organic superconductors and a
hybrid-ruthenate-cuprate superconductor is discussed.Comment: 4 pages, 5 figures, (Latex, revtex.sty, epsf.sty
Effects of short-range correlations on the Coulomb screening and the pairing interactions in electron-phonon systems - triplet pairing mediated by phonons
Effects of short-range correlations on the Coulomb screening, the phonons,
and the pairing interactions are examined in electron-phonon systems. First, we
derive a model Hamiltonian of Coulomb interactions which includes both the
long-range part v_q and the short-range part U. It is found from the expression
of the dielectric function that the strong on-site correlations weaken the
Coulomb screening. Secondly, we examine the screened phonons and the
interaction mediated by phonons. In a consistent picture, we derive an
expression of the effective interaction which includes (1) the screened Coulomb
interactions, (2) the pairing interactions mediated by phonons, and (3) the
effective interactions mediated by spin and charge fluctuations. It is
rewritten in a form of a summation of (a) the effective interactions of the
pure Hubbard model without the long-range Coulomb interactions, and (b) the
phonon-mediated interactions plus screened Coulomb interactions with
corrections due to both U and v_q. Thirdly, we derive an effective Hamiltonian
analogous to the BCS Hamiltonian. Fourthly, for some typical values of
parameters, we obtain the ground state phase diagrams. It is found that
spin-triplet superconductivity mediated by phonons occurs when the short-range
electron correlations are sufficiently strong, and the Coulomb screening is
sufficiently weak. We estimate the orders of the transition temperatures when
the triplet superconductivity occurs. The obtained values are realistic for
existing candidates of the triplet superconductors as the order of the
magnitudes. The possible relevance of the phonon-mediated interactions to the
heavy fermion superconductor UPt_3 and the layered superconductors such
(TMTSF)_2X and Sr_2RuO_4 are briefly discussed.Comment: 19 pages, 16 figures, jpsj2.cl
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
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
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
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