589 research outputs found
Large thermopower in the antiferromagnetic semiconductor BaMnBi
We report electrical and thermal transport properties of Mn-based material
BaMnBi with ThCrSi structure. The resistivity of the
antiferromagnetic BaMnBi shows a metal-semiconductor transition at
K with decreasing temperature. Correspondingly, the thermopower
shows a peak at the same temperature, approaching ~150 V/K. With
increasing temperature decreases to about 125 V/K at the room
temperature. The magnetic field enhances the peak value to 210 V/K. The
Hall resistivity reveals an abrupt change of the carrier density close to the
metal-semiconductor transition temperature.Comment: 3 pages, 3 figure
Modular invariance and anomaly cancellation formulas in odd dimension
By studying modular invariance properties of some characteristic forms, we
get some new anomaly cancellation formulas on dimensional manifolds.
As an application, we derive some results on divisibilities of the index of
Toeplitz operators on dimensional spin manifolds and some congruent
formulas on characteristic number for dimensional spin manifolds.Comment: 16 pages. arXiv admin note: text overlap with arXiv:1109.5794,
arXiv:0812.0201, arXiv:0811.4460; text overlap with arXiv:math/0605366 by
other author
Large linear magnetoresistance and magnetothermopower in layered SrZnSb
We report the large linear magnetoresistance ( in 9 T field at 2
K) and magnetothermopower in layered SrZnSb crystal with
quasi-two-dimensional Sb layers. A crossover from the semiclassical parabolic
field dependent magnetoresistance to linear field dependent magnetoresistance
with increasing magnetic field is observed. The magnetoresistance behavior can
be described very well by combining the semiclassical cyclotron contribution
and the quantum limit magnetoresistance. Magnetic field also enhances the
thermopower. Our results can be well understood by the magnetotransport of
Dirac states in the bulk band structure.Comment: 3 pages, 3 figure
Multiband effects and the possible Dirac states in LaAgSb
Here we report the possible signature of Dirac fermions in the
magnetoresistance, Hall resistivity and magnetothermopower of LaAgSb. The
opposite sign between Hall resistivity and Seebeck coefficient indicates the
multiband effect. Electronic structure calculation reveals the existence of the
linear bands and the parabolic bands crossing the Fermi level. The large linear
magnetoresistance was attributed to the quantum limit of the possible Dirac
fermions or the breakdown of weak-field magnetotransport at the charge density
wave phase transition. Analysis of Hall resistivity using two-band model
reveals that Dirac holes which dominate the electronic transport have much
higher mobility and larger density than conventional electrons. Magnetic field
suppresses the apparent Hall carrier density, and also induces the sign change
of the Seebeck coefficient from negative to positive. These effects are
possibly attributed to the magnetic field suppression of the density of states
at the Fermi level originating from the quantum limit of the possible Dirac
holes.Comment: 7 pages, 7 figure
Adiabatic Limits, Vanishing Theorems and the Noncommutative Residue
In this paper, we compute the adiabatic limit of the scalar curvature and
prove several vanishing theorems, we also derive a Kastler-Kalau-Walze type
theorem for the noncommutative residue in the case of foliations
CaIrSn: A weakly correlated nodeless superconductor
We report detailed Seebeck coefficient, Hall resistivity as well as specific
heat measurement on CaIrSn single crystals. The Seebeck
coefficient exhibits a peak corresponding to the anomaly in resistivity at
, and the carrier density is suppressed significantly below . This
indicates a significant Fermi surface reconstruction and the opening of the
charge density wave gap at the supperlattice transition. The magnetic field
induced enhancement of the residual specific heat coefficient
exhibits a nearly linear dependence on magnetic field, indicating a nodeless
gap. In the temperature range close to the Seebeck coefficient can be
described well by the diffusion model. The zero-temperature extrapolated
thermoelectric power is very small, implying large normalized Fermi
temperature. Consequently the ratio is very small. Our
results indicate that CaIrSn is a weakly correlated nodeless
superconductor.Comment: 5 pages, 5 figure
Large magnetothermopower effect in Dirac materials (Sr/Ca)MnBi2
We report temperature and magnetic field dependence of the thermal transport
properties in single crystals of (Sr/Ca)MnBi with linear energy dispersion.
In SrMnBi thermopower is positive, indicating hole-type carriers and the
magnetic field enhances the thermopower significantly. The maximum change of
thermopower is about 1600% in 9 T field and at 10 K. A negative thermopower is
observed in CaMnBi with dominant electron-type carriers and, in contrast,
the magnetic field suppresses the absolute value of thermopower.
First-principle band structure shows that the chemical potential is close to
the Dirac-cone-like points in linear bands. The magnetic field suppresses the
apparent Hall carrier density of CaMnBi below 50 K. The large
magnetothermopower effect in (Sr/Ca)MnBi is attributed to the magnetic
field shift of chemical potentialComment: 3 papges, 3 figure
The smallest one-realization of a given set
For any set of positive integers, a mixed hypergraph is a
realization of if its feasible set is , furthermore, is a
one-realization of if it is a realization of and each entry of its
chromatic spectrum is either 0 or 1. Jiang et al. \cite{Jiang} showed that the
minimum number of vertices of realization of with
is . Krl \cite{Kral} proved that there exists a
one-realization of with at most vertices. In this
paper, we improve Krl's result, and determine the size of the
smallest one-realization of a given set. As a result, we partially solve an
open problem proposed by Jiang et al. in 2002 and by Krl in
2004
Significant suppression of thermal conductivity in FeSb2 by Te doping
Kondo insulator like material FeSb2 was found to exhibit colossal Seebeck
coefficient. It would have had huge potential in thermoelectric applications in
cryogenic temperature range if it had not been for the large thermal
conductivity. Here we studied the influence of Te doping at Sb site on thermal
conductivity and thermoelectric effect in high quality single crystals.
Surprisingly, only 5% Te doping suppresses thermal conductivity by two orders
of magnitude, which may be attributed to the substitution disorder. Te doping
also results in transition from an semiconductor to a metal. Consequently
thermoelectric figure of merit (ZT ? 0:05) in Fe(Sb0:9Te0:1)2 at ? 100K was
enhanced by about one order of magnitude when compared to ZT < 0:005 in undoped
FeSb2.Comment: 3 Pages, 3 figures. Submitted to Physical Review B for Rapid
Communicatio
Quasi-two-dimensional Dirac fermions and quantum magnetoresistance in LaAgBi
We report quasi-two-dimensional Dirac fermions and quantum magnetoresistance
in LaAgBi. The band structure shows several narrow bands with nearly linear
energy dispersion and Dirac-cone-like points at the Fermi level. The quantum
oscillation experiments revealed one quasi-two-dimensional Fermi pocket and
another complex pocket with small cyclotron resonant mass. The in-plane
transverse magnetoresistance exhibits a crossover at a critical field
from semiclassical weak-field dependence to the high-field unsaturated
linear magnetoresistance which is attributed to the quantum limit of the Dirac
fermions. Our results suggest the existence of quasi 2D Dirac fermions in
rare-earth based layered compounds with two-dimensional double-sized Bi square
nets, similar to (Ca,Sr)MnBi, irrespective of magnetic order.Comment: 7 pages, 7 figures. arXiv admin note: text overlap with
arXiv:1204.133
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