1,042 research outputs found
Pseudogap of Color Superconductivity in Heated Quark Matter
We show that the pseudogap of the quark density of states is formed in hot
quark matter as a precursory phenomenon of the color superconductivity on the
basis of a low-energy effective theory. We clarify that the decaying process of
quarks near Fermi surface to a hole and the diquark soft mode (qq)_{soft} is
responsible for the formation of the pseudogap. Our result suggests that the
pseudogap is a universal phenomenon in strong coupling superconductors.Comment: Introduction is largely rewritten and minor changes are made in other
parts of the text. Some referenes with comments are added. Numerical errors
in the figures are corrected. To appear in Phys. Rev.
Disordered Fulde-Ferrel-Larkin-Ovchinnikov State in d-wave Superconductors
We study the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) superconducting state in
the disordered systems. We analyze the microscopic model, in which the d-wave
superconductivity is stabilized near the antiferromagnetic quantum critical
point, and investigate two kinds of disorder, namely, box disorder and point
disorder, on the basis of the Bogoliubov-deGennes (BdG) equation. The spatial
structure of modulated superconducting order parameter and the magnetic
properties in the disordered FFLO state are investigated. We point out the
possibility of "FFLO glass" state in the presence of strong point disorders,
which arises from the configurational degree of freedom of FFLO nodal plane.
The distribution function of local spin susceptibility is calculated and its
relation to the FFLO nodal plane is clarified. We discuss the NMR measurements
for CeCoIn_5.Comment: Submitted to New. J. Phys. a focus issue on "Superconductors with
Exotic Symmetries
Effects of Fermi surface and superconducting gap structure in the field-rotational experiments: A possible explanation of the cusp-like singularity in YNiBC
We have studied the field-orientational dependence of zero-energy density of
states (FODOS) for a series of systems with different Fermi surface and
superconducting gap structures. Instead of phenomenological Doppler-shift
method, we use an approximate analytical solution of Eilenberger equation
together with self-consistent determination of order parameter and a
variational treatment of vortex lattice. First, we compare zero-energy density
of states (ZEDOS) when a magnetic field is applied in the nodal direction
() and in the antinodal direction (), by taking
account of the field-angle dependence of order parameter. As a result, we found
that there exists a crossover magnetic field so that for for , consistent with our previous analyses. Next, we showed that and the
shape of FODOS are determined by contribution from the small part of Fermi
surface where Fermi velocity is parallel to field-rotational plane. In
particular, we found that is lowered and FODOS has broader minima, when a
superconducting gap has point nodes, in contrast to the result of the
Doppler-shift method. We also studied the effects of in-plane anisotropy of
Fermi surface. We found that in-plane anisotropy of quasi-two dimensional Fermi
surface sometimes becomes larger than the effects of Doppler-shift and can
destroy the Doppler-shift predominant region. In particular, this tendency is
strong in a multi-band system where superconducting coherence lengths are
isotropic. Finally, we addressed the problem of cusp-like singularity in
YNiBC and present a possible explanation of this phenomenon.Comment: 13pages, 23figure
Reduction of Tc due to Impurities in Cuprate Superconductors
In order to explain how impurities affect the unconventional
superconductivity, we study non-magnetic impurity effect on the transition
temperature using on-site U Hubbard model within a fluctuation exchange (FLEX)
approximation. We find that in appearance, the reduction of Tc roughly
coincides with the well-known Abrikosov-Gor'kov formula. This coincidence
results from the cancellation between two effects; one is the reduction of
attractive force due to randomness, and another is the reduction of the damping
rate of quasi-particle arising from electron interaction. As another problem,
we also study impurity effect on underdoped cuprate as the system showing
pseudogap phenomena. To the aim, we adopt the pairing scenario for the
pseudogap and discuss how pseudogap phenomena affect the reduction of Tc by
impurities. We find that 'pseudogap breaking' by impurities plays the essential
role in underdoped cuprate and suppresses the Tc reduction due to the
superconducting (SC) fluctuation.Comment: 14 pages, 28 figures To be published in JPS
Random Spin-orbit Coupling in Spin Triplet Superconductors: Stacking Faults in Sr_2RuO_4 and CePt_3Si
The random spin-orbit coupling in multicomponent superconductors is
investigated focusing on the non-centrosymmetric superconductor CePt_3Si and
the spin triplet superconductor Sr_2RuO_4. We find novel manifestations of the
random spin-orbit coupling in the multicomponent superconductors with
directional disorders, such as stacking faults. The presence of stacking faults
is indicated for the disordered phase of CePt_3Si and Sr_2RuO_4. It is shown
that the d-vector of spin triplet superconductivity is locked to be d = k_y x -
k_x y with the anisotropy \Delta T_c/T_c0 \sim \bar{\alpha}^2/T_c0 W_z, where
\bar{\alpha}, T_c0, and W_z are the mean square root of random spin-orbit
coupling, the transition temperature in the clean limit, and the kinetic energy
along the c-axis, respectively. This anisotropy is much larger (smaller) than
that in the clean bulk Sr_2RuO_4 (CePt_3Si). These results indicate that the
helical pairing state d = k_y x - k_x y in the eutectic crystal
Sr_2RuO_4-Sr_3Ru_2O_7 is stabilized in contrast to the chiral state d = (k_x
\pm i k_y) z in the bulk Sr_2RuO_4. The unusual variation of T_c in CePt_3Si is
resolved by taking into account the weak pair-breaking effect arising from the
uniform and random spin-orbit couplings. These superconductors provide a basis
for discussing recent topics on Majorana fermions and non-Abelian statistics.Comment: J. Phys. Soc. Jpn. 79 (2010) 08470
Band structures of P-, D-, and G-surfaces
We present a theoretical study on the band structures of the electron
constrained to move along triply-periodic minimal surfaces. Three well known
surfaces connected via Bonnet transformations, namely P-, D-, and G-surfaces,
are considered. The six-dimensional algebra of the Bonnet transformations [C.
Oguey and J.-F. Sadoc, J. Phys. I France 3, 839 (1993)] is used to prove that
the eigenstates for these surfaces are interrelated at a set of special points
in the Brillouin zones. The global connectivity of the band structures is,
however, different due to the topological differences of the surfaces. A
numerical investigation of the band structures as well as a detailed analysis
on their symmetry properties is presented. It is shown that the presence of
nodal lines are closely related to the symmetry properties. The present study
will provide a basis for understanding further the connection between the
topology and the band structures.Comment: 21 pages, 8 figures, 3 tables, submitted to Phys. Rev.
Testing Higgs models via the vertex by a recoil method at the International Linear Collider
In general, charged Higgs bosons appear in non-minimal Higgs models.
The vertex is known to be related to the violation of the
global symmetry (custodial symmetry) in the Higgs sector. Its magnitude
strongly depends on the structure of the exotic Higgs models which contain
higher isospin representations such as triplet Higgs bosons. We study
the possibility of measuring the vertex via single charged
Higgs boson production associated with the boson at the International
Linear Collider (ILC) by using the recoil method. The feasibility of the signal
is analyzed assuming the polarized
electron and positron beams and the expected detector performance for the
resolution of the two-jet system at the ILC. The background events can be
reduced to a considerable extent by imposing the kinematic cuts even if we take
into account the initial state radiation. For a relatively light charged Higgs
boson whose mass is in the region of 120-130 GeV , the vertex would be precisely testable especially
when the decay of is lepton specific. The exoticness of the extended
Higgs sector can be explored by using combined information for this vertex and
the rho parameter.Comment: 22 pages, 23 figure
Analysis of Superconductivity in d-p Model on Basis of Perturbation Theory
We investigate the mass enhancement factor and the superconducting transition
temperature in the d-p model for the high-\Tc cuprates. We solve the
\'Eliashberg equation using the third-order perturbation theory with respect to
the on-site Coulomb repulsion . We find that when the energy difference
between d-level and p-level is large, the mass enhancement factor becomes large
and \Tc tends to be suppressed owing to the difference of the density of
state for d-electron at the Fermi level. From another view point, when the
energy difference is large, the d-hole number approaches to unity and the
electron correlation becomes strong and enhances the effective mass. This
behavior for the electron number is the same as that of the f-electron number
in the heavy fermion systems. The mass enhancement factor plays an essential
role in understanding the difference of \Tc between the LSCO and YBCO
systems.Comment: 4pages, 9figures, to be published in J. Phys. Soc. Jp
Ginzburg-Landau Analysis for the Antiferromagnetic Order in the Fulde-Ferrell-Larkin-Ovchinnikov Superconductor
Incommensurate antiferromangetic (AFM) order in the
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductor is investigated on the
basis of the Ginzburg-Landau theory. We formulate the two component
Ginzburg-Landau model to discuss two degenerate incommensurate AFM states in
the tetragonal crystal structure. Owing to the broken translation symmetry in
the FFLO state, a multiple phase diagram of single-q phase and double-q phase
is obtained under the magnetic field along [100] or [010] direction. Magnetic
properties in each phase are investigated and compared with the neutron
scattering and NMR measurements for a heavy fermion superconductor CeCoIn_5. An
ultrasonic measurement is proposed for a future experimental study to identify
the AFM-FFLO state. The field orientation dependence of the AFM order in
CeCoIn_5 is discussed.Comment: 8 page
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