17,226 research outputs found
Electronic Structures of Antiperovskite Superconductor MgCNi and Related Compounds
Electronic structure of a newly discovered antiperovskite superconductor
MgCNi is investigated by using the LMTO band method. The main contribution
to the density of states (DOS) at the Fermi energy comes from Ni
3 states which are hybridized with C 2 states. The DOS at is
varied substantially by the hole or electron doping due to the very high and
narrow DOS peak located just below . We have also explored
electronic structures of C-site and Mg-site doped MgCNi systems, and
described the superconductivity in terms of the conventional phonon mechanism.Comment: 3 pages, presented at ORBITAL2001 September 11-14, 2001 (Sendai,
JAPAN
Electronic structures of antiperovskite superconductors: MgXNi (X=B,C,N)
We have investigated electronic structures of a newly discovered
antiperovskite superconductor MgCNi and related compounds MgBNi and
MgNNi. In MgCNi, a peak of very narrow and high density of states is
located just below , which corresponds to the antibonding
state of Ni-3d and C- but with the predominant Ni-3d character. The
prominent nesting feature is observed in the -centered electron Fermi
surface of an octahedron-cage-like shape that originates from the 19th band.
The estimated superconducting parameters based on the simple rigid-ion
approximation are in reasonable agreement with experiment, suggesting that the
superconductivity in MgCNi is described well by the conventional phonon
mechanism.Comment: 5 pages, 5 figure
Electronic structure of metallic antiperovskite compound GaCMn
We have investigated electronic structures of antiperovskite GaCMn and
related Mn compounds SnCMn, ZnCMn, and ZnNMn. In the paramagnetic
state of GaCMn, the Fermi surface nesting feature along the
direction is observed, which induces the antiferromagnetic (AFM) spin ordering
with the nesting vector {\bf Q} . Calculated
susceptibilities confirm the nesting scenario for GaCMn and also explain
various magnetic structures of other antiperovskite compounds. Through the band
folding effect, the AFM phase of GaCMn is stabilized. Nearly equal
densities of states at the Fermi level in the ferromagnetic and AFM phases of
GaCMn indicate that two phases are competing in the ground state.Comment: 4 pages, 5 figure
Thermal activation energy of 3D vortex matter in NaFe1-xCoxAs (x=0.01, 0.03 and 0.07) single crystals
We report on the thermally activated flux flow dependency on the doping
dependent mixed state in NaFe1-xCoxAs (x=0.01, 0.03, and 0.07) crystals using
the magnetoresistivity in the case of B//c-axis and B//ab-plane. It was found
clearly that irrespective of the doping ratio, magnetoresistivity showed a
distinct tail just above the Tc, offset associated with the thermally activated
flux flow (TAFF) in our crystals. Furthermore, in TAFF region the temperature
dependence of the activation energy follows the relation U(T, B)=U_0 (B)
(1-T/T_c )^q with q=1.5 in all studied crystals. The magnetic field dependence
of the activation energy follows a power law of U_0 (B)~B^(-{\alpha}) where the
exponent {\alpha} is changed from a low value to a high value at a crossover
field of B=~2T, indicating the transition from collective to plastic pinning in
the crystals. Finally, it is suggested that the 3D vortex phase is the dominant
phase in the low-temperature region as compared to the TAFF region in our
series samples
Anomalous double peak structure in Nb/Ni superconductor/ferromagnet tunneling DOS
We have experimentally investigated the density of states (DOS) in Nb/Ni
(S/F) bilayers as a function of Ni thickness, . Our thinnest samples show
the usual DOS peak at , whereas intermediate-thickness samples
have an anomalous ``double-peak'' structure. For thicker samples ( nm), we see an ``inverted'' DOS which has previously only been reported in
superconductor/weak-ferromagnet structures. We analyze the data using the
self-consistent non-linear Usadel equation and find that we are able to
quantitatively fit the features at if we include a large amount
of spin-orbit scattering in the model. Interestingly, we are unable to
reproduce the sub-gap structure through the addition of any parameter(s).
Therefore, the observed anomalous sub-gap structure represents new physics
beyond that contained in the present Usadel theory.Comment: 4 pages, 3 figure
A self-tuning mechanism in (3+p)d gravity-scalar theory
We present a new type of self-tuning mechanism for ()d brane world
models in the framework of gravity-scalar theory. This new type of self-tuning
mechanism exhibits a remarkable feature. In the limit , being
the string coupling, the geometry of bulk spacetime remains virtually unchanged
by an introduction of the Standard Model(SM)-brane, and consequently it is
virtually unaffected by quantum fluctuations of SM fields with support on the
SM-brane. Such a feature can be obtained by introducing Neveu-Schwarz(NS)-brane
as a background brane on which our SM-brane is to be set. Indeed, field
equations naturally suggest the existence of the background NS-brane. Among the
given such models, of the most interest is the case with , where
represents the bulk cosmological constant. This model contains a pair
of coincident branes (of the SM- and the NS-branes), one of which is a
codimension-2 brane placed at the origin of 2d transverse space (), another a codimension-1 brane placed at the edge of .
These two branes are (anti) T-duals of each other, and one of them may be
identified as our SM-brane plus the background NS-brane. In the presence of the
background NS-brane (and in the absence of ), the 2d transverse space
becomes an orbifold with an appropriate deficit angle.
But this is only possible if the ()d Planck scale and the string
scale () are of the same order, which
accords with the hierarchy assumption \cite{1,2,3} that the electroweak scale
is the only short distance scale existing in nature
Charge and Orbital Ordering and Spin State Transition Driven by Structural Distortion in YBaCo_2O_5
We have investigated electronic structures of antiferromagnetic YBaCo_2O_5
using the local spin-density approximation (LSDA) + U method. The charge and
orbital ordered insulating ground state is correctly obtained with the strong
on-site Coulomb interaction. Co^{2+} and Co^{3+} ions are found to be in the
high spin (HS) and intermediate spin (IS) state, respectively. It is considered
that the tetragonal to orthorhombic structural transition is responsible for
the ordering phenomena and the spin states of Co ions. The large contribution
of the orbital moment to the total magnetic moment indicates that the
spin-orbit coupling is also important in YBaCo_2O_5.Comment: 4 pages including 4 figures, Submitted to Phys. Rev. Let
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