743 research outputs found
Superconductivity without Local Inversion Symmetry; Multi-layer Systems
While multi-layer systems can possess global inversion centers, they can have
regions with locally broken inversion symmetry. This can modify the
superconducting properties of such a system. Here we analyze two dimensional
multi-layer systems yielding spatially modulated antisymmetric spin-orbit
coupling (ASOC) and discuss superconductivity with mixed parity order
parameters. In particular, the influence of ASOC on the spin susceptibility is
investigated at zero temperature. For weak inter-layer coupling we find an
enhanced spin susceptibility induced by ASOC, which hints the potential
importance of this aspect for superconducting phase in specially structured
superlattices.Comment: 4 pages, 2 figures, proceedings of the 26th International Conference
on Low Temperature Physics (LT26
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
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
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
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
Fourth Order Perturbation Theory for Normal Selfenergy in Repulsive Hubbard Model
We investigate the normal selfenergy and the mass enhancement factor in the
Hubbard model on the two-dimensional square lattice. Our purpose in this paper
is to evaluate the mass enhancement factor more quantitatively than the
conventional third order perturbation theory. We calculate it by expanding
perturbatively up to the fourth order with respect to the on-site repulsion
. We consider the cases that the system is near the half-filling, which are
similar situations to high- cuprates. As results of the calculations, we
obtain the large mass enhancement on the Fermi surface by introducing the
fourth order terms. This is mainly originated from the fourth order
particle-hole and particle-particle diagrams. Although the other fourth order
terms have effect of reducing the effective mass, this effect does not cancel
out the former mass enhancement completely and there remains still a large mass
enhancement effect. In addition, we find that the mass enhancement factor
becomes large with increasing the on-site repulsion and the density of
state (DOS) at the Fermi energy . According to many current reseaches,
such large and enhance the effective interaction between
quasiparticles, therefore the superconducting transition temperature
increases. On the other hand, the large mass enhancement leads the reduction of
the energy scale of quasiparticles, as a result, is reduced. When we
discuss , we have to estimate these two competitive effects.Comment: 6pages,8figure
Effects of proximity to an electronic topological transition on normal state transport properties of the high-Tc superconductors
Within the time dependent Ginzburg-Landau theory, the effects of the
superconducting fluctuations on the transport properties above the critical
temperature are characterized by a non-zero imaginary part of the relaxation
rate gamma of the order parameter. Here, we evaluate Im gamma for an
anisotropic dispersion relation typical of the high-Tc cuprate superconductors
(HTS), characterized by a proximity to an electronic topological transition
(ETT). We find that Im gamma abruptly changes sign at the ETT as a function of
doping, in agreement with the universal behavior of the HTS. We also find that
an increase of the in-plane anisotropy, as is given by a non-zero value of the
next-nearest to nearest hopping ratio r=t'/t, increases the value of | Im gamma
| close to the ETT, as well as its singular behavior at low temperature,
therefore enhancing the effect of superconducting fluctuations. Such a result
is in qualitative agreement with the available data for the excess Hall
conductivity for several cuprates and cuprate superlattices.Comment: to appear in Phys. Rev.
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