57 research outputs found
Two tricritical lines from a Ginzburg-Landau expansion: application to the LOFF phase
We study the behavior of the two plane waves configuration in the LOFF phase
close to T=0. The study is performed by using a Landau-Ginzburg expansion up to
the eighth order in the gap. The general study of the corresponding grand
potential shows, under the assumption that the eighth term in the expansion is
strictly positive, the existence of two tricritical lines. This allows to
understand the existence of a second tricritical point for two antipodal plane
waves in the LOFF phase and justifies why the transition becomes second order
at zero temperature. The general analysis done in this paper can be applied to
other cases.Comment: LaTex file, 15 pages, 6 figure
Huge metastability in high-T_c superconductors induced by parallel magnetic field
We present a study of the temperature-magnetic field phase diagram of
homogeneous and inhomogeneous superconductivity in the case of a
quasi-two-dimensional superconductor with an extended saddle point in the
energy dispersion under a parallel magnetic field. At low temperature, a huge
metastability region appears, limited above by a steep superheating critical
field (H_sh) and below by a strongly reentrant supercooling field (H_sc). We
show that the Pauli limit (H_p) for the upper critical magnetic field is
strongly enhanced due to the presence of the Van Hove singularity in the
density of states. The formation of a non-uniform superconducting state is
predicted to be very unlikely.Comment: 5 pages, 2 figures; to appear in Phys. Rev.
Homogeneous Fermion Superfluid with Unequal Spin Populations
For decades, the conventional view is that an s-wave BCS superfluid can not
support uniform spin polarization due to a gap in the quasiparticle
excitation spectrum. We show that this is an artifact of the dismissal of
quasiparticle interactions in the conventional approach at the
outset. Such interactions can cause triplet fluctuations in the ground state
and hence non-zero spin polarization at "magnetic field" . The
resulting ground state is a pairing state of quasiparticles on the ``BCS
vacuum". For sufficiently large , the spin polarization of at unitarity
has the simple form . Our study is motivated by the recent
experiments at Rice which found evidence of a homogenous superfluid state with
uniform spin polarization.Comment: 4 pages, 3 figure
Possible symmetries of the superconducting order parameter in a hexagonal ferromagnet
We study the order parameter symmetry in a hexagonal crystal with co-existing
superconductivity and ferromagnetism. An experimental example is provided by
carbon-based materials, such as graphite-sulfur composites, in which an
evidence of such co-existence has been recently discovered. The presence of a
non-zero magnetization in the normal phase brings about considerable changes in
the symmetry classification of superconducting states, compared to the
non-magnetic case.Comment: 4 pages, REVTe
Mass-Induced Crystalline Color Superconductivity
We demonstrate that crystalline color superconductivity may arise as a result
of pairing between massless quarks and quarks with nonzero mass m_s. Previous
analyses of this phase of cold dense quark matter have all utilized a chemical
potential difference \delta\mu to favor crystalline color superconductivity
over ordinary BCS pairing. In any context in which crystalline color
superconductivity occurs in nature, however, it will be m_s-induced. The effect
of m_s is qualitatively different from that of \delta\mu in one crucial
respect: m_s depresses the value of the BCS gap \Delta_0 whereas \delta\mu
leaves \Delta_0 unchanged. This effect in the BCS phase must be taken into
account before m_s-induced and \delta\mu-induced crystalline color
superconductivity can sensibly be compared.Comment: 12 pages, 4 figures. v2: very small change onl
Order parameter symmetry in ferromagnetic superconductors
We analyze the symmetry and the nodal structure of the superconducting order
parameter in a cubic ferromagnet, such as ZrZn. We demonstrate how the
order parameter symmetry evolves when the electromagnetic interaction of the
conduction electrons with the internal magnetic induction and the spin-orbit
coupling are taken into account. These interactions break the cubic symmetry
and lift the degeneracy of the order parameter. It is shown that the order
parameter which appears immediately below the critical temperature has two
components, and its symmetry is described by {\em co-representations} of the
magnetic point groups. This allows us to make predictions about the location of
the gap nodes.Comment: 12 pages, ReVTeX, submitted to PR
Vortex dynamics and upper critical fields in ultrathin Bi films
Current-voltage (I-V) characteristics of quench condensed, superconducting,
ultrathin films in a magnetic field are reported. These I-V's show
hysteresis for all films, grown both with and without thin underlayers.
Films on Ge underlayers, close to superconductor-insulator transition (SIT),
show a peak in the critical current, indicating a structural transformation of
the vortex solid (VS). These underlayers, used to make the films more
homogeneous, are found to be more effective in pinning the vortices. The upper
critical fields (B) of these films are determined from the resistive
transitions in perpendicular magnetic field. The temperature dependence of the
upper critical field is found to differ significantly from Ginzburg-Landau
theory, after modifications for disorder.Comment: Phys Rev B, to be published Figure 6 replaced with correct figur
Mesoscopic interplay of superconductivity and ferromagnetism in ultra-small metallic grains
We review the effects of electron-electron interactions on the ground-state
spin and the transport properties of ultra-small chaotic metallic grains. Our
studies are based on an effective Hamiltonian that combines a superconducting
BCS-like term and a ferromagnetic Stoner-like term. Such terms originate in
pairing and spin exchange correlations, respectively. This description is valid
in the limit of a large dimensionless Thouless conductance. We present the
ground-state phase diagram in the fluctuation-dominated regime where the
single-particle mean level spacing is comparable to the bulk BCS pairing gap.
This phase diagram contains a regime in which pairing and spin exchange
correlations coexist in the ground-state wave function. We discuss the
calculation of the tunneling conductance for an almost-isolated grain in the
Coulomb-blockade regime, and present measurable signatures of the competition
between superconductivity and ferromagnetism in the mesoscopic fluctuations of
the conductance.Comment: 6 pages, 3 figures, To be published in the proceedings of the NATO
Advance Research Workshop "Recent Advances in Nonlinear Dynamics and Complex
System Physics.
Anomalous Superconducting Properties and Field Induced Magnetism in CeCoIn5
In the heavy fermion superconductor CeCoIn5 (Tc=2.3K) the critical field is
large, anisotropic and displays hysteresis. The magnitude of the critical-field
anisotropy in the a-c plane can be as large as 70 kOe and depends on
orientation. Critical field measurements in the (110) plane suggest 2D
superconductivity, whereas conventional effective mass anisotropy is observed
in the (100) plane. Two distinct field-induced magnetic phases are observed: Ha
appears deep in the superconducting phase, while Hb intersects Hc2 at T=1.4 K
and extends well above Tc. These observations suggest the possible realization
of a direct transition from ferromagnetism to Fulde-Ferrel-Larkin-Ovchinnikov
superconductivity in CeCoIn5.Comment: 4 pages, 3 figure
Interface superconductivity in the eutectic Sr2RuO4-Ru: 3-K phase of Sr2RuO4
The eutectic system Sr2RuO4-Ru is referred to as the 3-K phase of the
spin-triplet supeconductor Sr2RuO4 because of its enhanced superconducting
transition temperature Tc of ~3 K. We have investigated the field-temperature
(H-T) phase diagram of the 3-K phase for fields parallel and perpendicular to
the ab-plane of Sr2RuO4, using out-of-plane resistivity measurements. We have
found an upturn curvature in the Hc2(T) curve for H // c, and a rather gradual
temperature dependence of Hc2 close to Tc for both H // ab and H // c. We have
also investigated the dependence of Hc2 on the angle between the field and the
ab-plane at several temperatures. Fitting the Ginzburg-Landau effective-mass
model apparently fails to reproduce the angle dependence, particularly near H
// c and at low temperatures. We propose that all of these charecteric features
can be explained, at least in a qualitative fashion, on the basis of a theory
by Sigrist and Monien that assumes surface superconductivity with a
two-component order parameter occurring at the interface between Sr2RuO4 and Ru
inclusions. This provides evidence of the chiral state postulated for the 1.5-K
phase by several experiments.Comment: 7 pages and 5 figs; accepted for publication in Phys. Rev.
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