5,025 research outputs found
Chromomagnetic instability in two-flavor quark matter at nonzero temperature
We calculate the effective potential of the 2SC/g2SC phases including vector
condensates () and study the gluonic phase and the
single plane-wave Larkin-Ovchinnikov-Fulde-Ferrell state at nonzero
temperature. Our analysis is performed within the framework of the gauged
Nambu--Jona-Lasinio model. We compute potential curvatures with respect to the
vector condensates and investigate the temperature dependence of the Meissner
masses squared of gluons of color 4--7 and 8 in the neutral 2SC/g2SC phases.
The phase diagram is presented in the plane of temperature and coupling
strength. The unstable regions for gluons 4--7 and 8 are mapped out on the
phase diagram. We find that, apart from the case of strong coupling, the
2SC/g2SC phases at low temperatures are unstable against the vector
condensation until the temperature reaches tens of MeV.Comment: 10 pages, 10 figures, revisions to text, published in Phys. Rev.
Abrikosov flux-lines in two-band superconductors with mixed dimensionality
We study vortex structure in a two-band superconductor, in which one band is
ballistic and quasi-two-dimensional (2D), and the other is diffusive and
three-dimensional (3D). A circular cell approximation of the vortex lattice
within the quasiclassical theory of superconductivity is applied to a recently
developed model appropriate for such a two-band system [Tanaka et al 2006 Phys.
Rev. B 73, 220501(R); Tanaka et al 2007 Phys. Rev. B 75, 214512]. We assume
that superconductivity in the 3D diffusive band is "weak", i.e., mostly
induced, as is the case in MgB. Hybridization with the "weak" 3D diffusive
band has significant and intriguing influence on the electronic structure of
the "strong" 2D ballistic band. In particular, the Coulomb repulsion and the
diffusivity in the "weak" band enhance suppression of the order parameter and
enlargement of the vortex core by magnetic field in the "strong" band,
resulting in reduced critical temperature and field. Moreover, increased
diffusivity in the "weak" band can result in an upward curvature of the upper
critical field near the transition temperature. A particularly interesting
feature found in our model is the appearance of additional bound states at the
gap edge in the "strong" ballistic band, which are absent in the single-band
case. Furthermore, coupling with the "weak" diffusive band leads to reduced
band gaps and van Hove singularities of energy bands of the vortex lattice in
the "strong" ballistic band. We find these intriguing features for parameter
values appropriate for MgB.Comment: 11 pages, 14 figure
Neutral Larkin--Ovchinnikov--Fulde--Ferrell state and chromomagnetic instability in two-flavor dense QCD
In two-flavor dense quark matter, we describe the dynamics in the single
plane wave Larkin--Ovchinnikov--Fulde--Ferrell (LOFF) state satisfying the
color and electric neutrality conditions. We find that because the neutral LOFF
state itself suffers from a chromomagnetic instability in the whole region
where it coexists with the (gapped/gapless) two-flavor superconducting
(2SC/g2SC) phases, it cannot cure this instability in those phases. This is
unlike the recently revealed gluonic phase which seems to be able to resolve
this problem.Comment: Revtex4, 5 pages, 3 figures, clarifications added, to appear in
Phys.Rev.Let
Low-lying excitations around a single vortex in a d-wave superconductor
A full quantum-mechanical treatment of the Bogoliubov-de Gennes equation for
a single vortex in a d-wave superconductor is presented. First, we find
low-energy states extended in four diagonal directions, which have no
counterpart in a vortex of s-wave superconductors. The four-fold symmetry is
due to 'quantum effect', which is enhanced when is small. Second,
for , a peak with a large energy gap is
found in the density of states, which is due to the formation of the lowest
bound states.Comment: 7pages, Revte
Discrete transverse superconducting modes in nano-cylinders
Spatial variation in the superconducting order parameter becomes significant
when the system is confined at dimensions well below the typical
superconducting coherence length. Motivated by recent experimental success in
growing single-crystal metallic nanorods, we study quantum confinement effects
on superconductivity in a cylindrical nanowire in the clean limit. For large
diameters, where the transverse level spacing is smaller than superconducting
order parameter, the usual approximations of Ginzburg-Landau theory are
recovered. However, under external magnetic field the order parameter develops
a spatial variation much stronger than that predicted by Ginzburg-Landau
theory, and gapless superconductivity is obtained above a certain field
strength. At small diameters, the discrete nature of the transverse modes
produces significant spatial variations in the order parameter with increased
average magnitude and multiple shoulders in the magnetic response.Comment: 10 pages, 8 figure
Detection of Striped Superconductors Using Magnetic Field Modulated Josephson Effect
In a very interesting recent Letter\cite{berg}, the authors suggested that a
novel form of superconducting state is realized in LaBaCuO with
close to 1/8. This suggestion was based on experiments\cite{li} on this
compound which found predominantly two-dimensional (2D) characters of the
superconducting state, with extremely weak interplane coupling. Later this
specific form of superconducting state was termed striped
superconductors\cite{berg08}. The purpose of this note is to point out that the
suggested form\cite{berg} of the superconducting order parameter can be
detected directly using magnetic field modulated Josephson effect.Comment: Expanded version as appeared in prin
Nuclear Magnetic Relaxation Rate in a Noncentrosymmetric Superconductor
For a noncentrosymmetric superconductor such as CePt3Si, we consider a Cooper
pairing model with a two-component order parameter composed of spin-singlet and
spin-triplet pairing components.
We demonstrate that such a model on a qualitative level accounts for
experimentally observed features of the temperature dependence of the nuclear
spin-lattice relaxation rate 1/T1, namely a peak just below Tc and a line-node
gap behavior at low temperatures.Comment: 4 page
Josephson effect in thin-film superconductor/insulator/superconductor junctions with misaligned in-plane magnetic fields
We study a tunnel junction consisting of two thin-film s-wave superconductors
separated by a thin, insulating barrier in the presence of misaligned in-plane
exchange fields. We find an interesting interplay between the superconducting
phase difference and the relative orientation of the exchange fields,
manifested in the Josephson current across the junction. Specifically, this may
be written , where
I_0 and I_m are constants, and is the relative orientation of the
exchange fields while is the superconducting phase difference.
Similar results have recently been obtained in other S/I/S junctions coexisting
with helimagnetic or ferromagnetic order. We calculate the superconducting
order parameter self-consistently, and investigate quantitatively the effect
which the misaligned exchange fields constitute on the Josephson current, to
see if I_m may have an appreciable effect on the Josephson current. It is found
that I_0 and I_m become comparable in magnitude at sufficiently low
temperatures and fields close to the critical value, in agreement with previous
work. From our analytical results, it then follows that the Josephson current
in the present system may be controlled in a well-defined manner by a rotation
of the exchange fields on both sides of the junction. We discuss a possible
experimental realization of this proposition.Comment: 8 pages, 8 figures. Accepted for publication in Phys. Rev.
Positions of Point-Nodes in Borocarbide Superconductor YNi2B2C
To determine the superconducting gap function of YNi2B2C, we calculate the
local density of states (LDOS) around a single vortex core with the use of
Eilenberger theory and the band structure calculated by local density
approximation assuming various gap structures with point-nodes at different
positions. We also calculate the angular-dependent heat capacity in the vortex
state on the basis of the Doppler-Shift method. Comparing our results with the
STM/STS experiment, the angular-dependent heat capacity and thermal
conductivity, we propose the gap-structure of YNi2B2C, which has the
point-nodes and gap minima along . Our gap-structure is consistent with
all results of angular-resolved experiments.Comment: 7 pages, 5 figure
Critical phenomena in a highly constrained classical spin system: Neel ordering from the Coulomb phase
Many classical, geometrically frustrated antiferromagnets have
macroscopically degenerate ground states. In a class of three-dimensional
systems, the set of degenerate ground states has power-law correlations and is
an example of a Coulomb phase. We investigate Neel ordering from such a Coulomb
phase, induced by weak additional interactions that lift the degeneracy. We
show that the critical point belongs to a universality class that is different
from the one for the equivalent transition out of the paramagnetic phase, and
that it is characterised by effective long-range interactions; alternatively,
ordering may be discontinuous. We suggest that a transition of this type may be
realised by applying uniaxial stress to a pyrochlore antiferromagnet.Comment: 4 pages, 3 figure
- …