497 research outputs found
Angular Fulde-Ferrell-Larkin-Ovchinnikov state in cold fermion gases in a toroidal trap
We study the possibility of angular Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)
state, in which the rotation symmetry is spontaneously broken, in population
imbalanced fermion gases near the BCS-BEC crossover. We investigate the
superfluid gases at low temperatures on the basis of the Bogoliubov-de Gennes
equation, and examine the stability against thermal fluctuations using the
T-matrix approach beyond the local-density approximation (LDA). We find that
the angular FFLO state is stabilized in the gases confined in the toroidal trap
but not in the harmonic trap. The angular FFLO state is stable near the BCS-BEC
crossover owing to the formation of pseudogap. Spatial dependences of number
density and local population imbalance are shown for an experimental test.Comment: final version for publication in Phys. Rev. B Rapid Communicatio
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
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
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
Single Crystal Growth of Skutterudite CoP3 under High Pressure
A new method to grow single crystals of skutterudite compounds is examined.
Using a wedge-type, cubic-anvil, high-pressure apparatus, single crystals of
CoP3 were grown from stoichiometric melts under a pressure of 3.5 GPa. Powder
x-ray diffraction and electron probe microanalysis measurements indicate that
the as-grown boules are a single phase of CoP3. The results suggest that CoP3
is a congruent melting compound under high pressure.Comment: 6pages,5 figures, J. Crystal Growth (in press
Bogoliubov-de Gennes study of trapped spin-imbalanced unitary Fermi gases
It is quite common that several different phases exist simultaneously in a
system of trapped quantum gases of ultra-cold atoms. One such example is the
strongly-interacting Fermi gas with two imbalanced spin species, which has
received a great amount of attention due to the possible presence of exotic
superfluid phases. By employing novel numerical techniques and algorithms, we
self-consistently solve the Bogoliubov de-Gennes equations, which describe
Fermi superfluids in the mean-field framework. From this study, we investigate
the novel phases of spin-imbalanced Fermi gases and examine the validity of the
local density approximation (LDA), which is often invoked in the extraction of
bulk properties from experimental measurements within trapped systems. We show
how the validity of the LDA is affected by the trapping geometry, number of
atoms and spin imbalance.Comment: 15 pages, 5 figures, to be published in New J. Phys. (focus issue on
"Strongly Correlated Quantum Fluids: From Ultracold Quantum Gases to QCD
Plasmas"
Novel phase diagram of superconductor NaxCoO2-yH2O in a 75 % relative humidity
We succeeded in synthesizing the powder samples of bilayer-hydrate sodium
cobalt oxide superconductors NaxCoO2-yH2O with Tc = 0 ~ 4.6 K by systematically
changing the keeping duration in a 75 % relative humidity atmosphere after
intercalation of water molecules. From the magnetic measurements, we found that
the one-day duration sample does not show any superconductivity down to 1.8 K,
and that the samples kept for 2 ~ 7 days show superconductivity, in which Tc
increases up to 4.6 K with increasing the duration. Tc and the superconducting
volume fraction are almost invariant between 7 days and 1month duration. The
59Co NQR spectra indicate a systematic change in the local charge distribution
on the CoO2 plane with change in duration.Comment: 4 pages, 5 figures, submitted to Journal of the Physical Society of
Japa
Charge and orbital order in Fe_3O_4
Charge and orbital ordering in the low-temperature monoclinic structure of
magnetite (Fe_3O_4) is investigated using LSDA+U. While the difference between
t_{2g} minority occupancies of Fe^{2+}_B and Fe^{3+}_B cations is large and
gives direct evidence for charge ordering, the screening is so effective that
the total 3d charge disproportion is rather small. The charge order has a
pronounced [001] modulation, which is incompatible with the Anderson criterion.
The orbital order agrees with the Kugel-Khomskii theory.Comment: 4 pages, 2 figure
Perturbation Theory for a Repulsive Hubbard Model in Quasi-One-Dimensional Superconductors
We investigate pairing symmetry and a transition temperature in a
quasi-one-dimensional repulsive Hubbard model. We solve the Eliashberg equation
using the third-order perturbation expansion with respect to the on-site
repulsion . We find that when the electron number density is shifted from
the half-filled, a transition into unconventional superconductivity is
expected. When one dimensionality is weak, a spin-singlet state is favorable.
By contrast, when one dimensionality is strong and electron number density is
far from the half-filled, a spin-triplet state is stabilized. Finally, we
discuss the possibility of unconventional superconductivity caused by the
on-site Coulomb repulsion in -NaVO.Comment: 4 pages, 7 figure
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