1,045 research outputs found
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
Bounds on Decoherence and Error
When a confined system interacts with its walls (treated quantum
mechanically), there is an intertwining of degrees of freedom. We show that
this need not lead to entanglement, hence decoherence. It will generally lead
to error. The wave function optimization required to avoid decoherence is also
examined.Comment: 10 pages, plain TeX, no figure
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
Spin moment over 10-300 K and delocalization of magnetic electrons above the Verwey transition in magnetite
In order to probe the magnetic ground state, we have carried out temperature
dependent magnetic Compton scattering experiments on an oriented single crystal
of magnetite (FeO), together with the corresponding first-principles
band theory computations to gain insight into the measurements. An accurate
value of the magnetic moment associated with unpaired spins is obtained
directly over the temperature range of 10-300K. is found to be
non-integral and to display an anomalous behavior with the direction of the
external magnetic field near the Verwey transition. These results reveal how
the magnetic properties enter the Verwey energy scale via spin-orbit coupling
and the geometrical frustration of the spinel structure, even though the Curie
temperature of magnetite is in excess of 800 K. The anisotropy of the magnetic
Compton profiles increases through the Verwey temperature and indicates
that magnetic electrons in the ground state of magnetite become delocalized on
Fe B-sites above .Comment: 5 pages, 5 figures, to appear in Journal of Physics and Chemistry of
Solid
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.
Antiferromagnetic order in the FFLO state
We investigate the antiferromagnetic (AF) order in the d-wave superconducting
(SC) state at high magnetic fields. A two-dimensional model with on-site
repulsion U, inter-site attractive interaction V and antiferromagnetic exchange
interaction J is solved using the mean field theory. For finite values of U and
J, a first order transition occurs from the normal state to the FFLO state,
while the FFLO-BCS phase transition is second order, consistent with the
experimental results in CeCoIn_5. Although the BCS-FFLO transition is
continuous, the Ne'el temperature of AF order is discontinuous at the phase
boundary because the AF order in the FFLO state is induced by the Andreev bound
state localized in the zeros of FFLO order parameter, while the AF order hardly
occurs in the uniform BCS state. The spatial structure of the magnetic moment
is investigated for the commensurate AF state as well as for the incommensurate
AF state. The influence of the spin fluctuations is discussed for both states.
Since the fluctuations are enhanced in the normal state for incommensurate AF
order, this AF order can be confined in the FFLO state. The experimental
results in CeCoIn_5 are discussed.Comment: Proceedings of LT25 conference (Amsterdam, 2008
Pseudogap in fermionic density of states in the BCS-BEC crossover of atomic Fermi gases
We study pseudogap behaviors of ultracold Fermi gases in the BCS-BEC
crossover region. We calculate the density of states (DOS), as well as the
single-particle spectral weight, above the superfluid transition temperature
including pairing fluctuations within a -matrix approximation.
We find that DOS exhibits a pseudogap structure in the BCS-BEC crossover
region, which is most remarkable near the unitarity limit. We determine the
pseudogap temperature at which the pseudogap structure in DOS disappears.
We also introduce another temperature at which the BCS-like
double-peak structure disappears in the spectral weight. While one finds
in the BCS regime, becomes higher than in the
crossover and BEC regime. We also determine the pseudogap region in the phase
diagram in terms of temperature and pairing interaction.Comment: 6 pages, 4 figures, Proceedings of QFS 200
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
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