153 research outputs found
Possible spin-triplet -wave pairing due to disconnected Fermi surfaces in NaCoOHO
We propose that spin-triplet pairing mechanism due to disconnected Fermi
surfaces proposed in our previous study [Phys. Rev. B {\bf 63} 174507 (2001)]
may be at work in a recently discovered superconductor NaCoO
HO. We introduce a single band effective model that takes into account the
pocket-like Fermi surfaces along with the van Hove singularity near the K point
found in the band calculation results. Applying fluctuation exchange method and
solving the linearized {\'E}liashberg equation, the most dominant pairing is
found to have spin-triplet -wave symmetry, where the nodes of the gap
function do not intersect the pocket Fermi surfaces. Presence of finite
is suggested in sharp contrast with cases when the gap nodes intersect the
Fermi surface.Comment: A revised version of the paper, which has been accepted for
publication in Physical Reiview Letters. Some sentences and paragraphs added
in the discussion part (page 4
Local density of states as a probe for tunneling magnetoresistance effect: application to ferrimagnetic tunnel junctions
We investigate the tunneling magnetoresistance (TMR) effect using the lattice
models which describe the magnetic tunnel junctions (MTJ). First, taking a
conventional ferromagnetic MTJ as an example, we show that the product of the
local density of states (LDOS) at the center of the barrier traces the TMR
effect qualitatively. The LDOS inside the barrier has the information on the
electrodes and the electron tunneling through the barrier, which enables us to
easily evaluate the tunneling conductance more precisely than the conventional
Julliere's picture. We then apply this method to the MTJs with collinear
ferrimagnets and antiferromagnets. We find that the TMR effect in the
ferrimagnetic and antiferromagnetic MTJs changes depending on the interfacial
magnetic structures originating from the sublattice structure, which can also
be captured by the LDOS. Our findings will reduce the computational cost for
the qualitative evaluation of the TMR effect, and be useful for a broader
search for the materials which work as the TMR devices showing high
performance.Comment: 11 pages, 11 figure
Beckner's inequality and its application to Banach spaces (Research on structures of operators via methods in geometry and probability theory)
A unified origin for the 3D magnetism and superconductivity in NaCoO
We analyze the origin of the three dimensional (3D) magnetism observed in
nonhydrated Na-rich NaCoO within an itinerant spin picture using a 3D
Hubbard model. The origin is identified as the 3D nesting between the inner and
outer portions of the Fermi surface, which arise due to the local minimum
structure of the band at the -A line. The calculated spin wave
dispersion strikingly resembles the neutron scattering result. We argue that
this 3D magnetism and the spin fluctuations responsible for superconductivity
in the hydrated systems share essentially the same origin.Comment: 5pages, 6figure
Competition between singlet and triplet pairings in Na_xCoO_2 yH_2O
We discuss the pairing symmetry of a cobaltate superconductor
NaCoO HO by adopting an effective single band model that
takes into account the hole pockets, as discussed in our previous paper
[to appear in Phys. Rev. Lett.] Here we consider the off-site repulsions in
addition to the on-site repulsion considered in our previous study. We show
that the spin-triplet f-wave pairing proposed in our previous study is robust
to some extent even in the presence of off-site repulsions. However, f-wave
pairing gives way to singlet pairings for sufficiently large values of off-site
repulsions. Among the singlet pairings, i-wave and extended s-wave pairings are
good candidates which do not break time reversal symmetry below in
agreement with the experiments.Comment: 12 page
Algorithm for spin symmetry operation search
A spin space group provides a suitable way to fully exploit the symmetry of a
spin arrangement with a negligible spin-orbit coupling. There has been a
growing interest in applying spin symmetry analysis with the spin space group
in the field of magnetism. However, there is no established algorithm to search
for spin symmetry operations of the spin space group. This paper presents an
exhaustive algorithm for determining spin symmetry operations of commensurate
spin arrangements. The present algorithm searches for spin symmetry operations
from the symmetry operations of a corresponding nonmagnetic crystal structure
and determines their spin-rotation parts by solving a Procrustes problem. An
implementation is distributed under a permissive free software license in
spinspg v0.1.1: https://github.com/spglib/spinspg.Comment: The implementation will be released after publicatio
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