13,519 research outputs found
Transport in Bilayer Graphene: Calculations within a self-consistent Born approximation
The transport properties of a bilayer graphene are studied theoretically
within a self-consistent Born approximation. The electronic spectrum is
composed of -linear dispersion in the low-energy region and -square
dispersion as in an ordinary two-dimensional metal at high energy, leading to a
crossover between different behaviors in the conductivity on changing the Fermi
energy or disorder strengths. We find that the conductivity approaches
per spin in the strong-disorder regime, independently of the
short- or long-range disorder.Comment: 8 pages, 5 figure
Ando, Lavrov, and Segawa Reply
Authors' Reply to the Comment by Janossy et al. [cond-mat/0005275] on our
article, "Magnetoresistance Anomalies in Antiferromagnetic
YBa_{2}Cu_{3}O_{6+x}: Fingerprints of Charged Stripes" [cond-mat/9905071, Phys.
Rev. Lett. 83, 2813 (1999)].Comment: 1 page, 1 figure, accepted for publication in PR
Conductance of Disordered Wires with Symplectic Symmetry: Comparison between Odd- and Even-Channel Cases
The conductance of disordered wires with symplectic symmetry is studied by
numerical simulations on the basis of a tight-binding model on a square lattice
consisting of M lattice sites in the transverse direction. If the potential
range of scatterers is much larger than the lattice constant, the number N of
conducting channels becomes odd (even) when M is odd (even). The average
dimensionless conductance g is calculated as a function of system length L. It
is shown that when N is odd, the conductance behaves as g --> 1 with increasing
L. This indicates the absence of Anderson localization. In the even-channel
case, the ordinary localization behavior arises and g decays exponentially with
increasing L. It is also shown that the decay of g is much faster in the
odd-channel case than in the even-channel case. These numerical results are in
qualitative agreement with existing analytic theories.Comment: 4 page
Quantum oscillations in a topological insulator Bi_{1-x}Sb_{x}
We have studied transport and magnetic properties of Bi_{1-x}Sb_x, which is
believed to be a topological insulator - a new state of matter where an
insulating bulk supports an intrinsically metallic surface. In nominally
insulating Bi_{0.91}Sb_{0.09} crystals, we observed strong quantum oscillations
of the magnetization and the resistivity originating from a Fermi surface which
has a clear two-dimensional character. In addition, a three-dimensional Fermi
surface is found to coexist, which is possibly due to an unusual coupling of
the bulk to the surface. This finding demonstrates that quantum oscillations
can be a powerful tool to directly probe the novel electronic states in
topological insulators.Comment: 4 pages, 4 figure
Doping n-type carriers by La-substitution for Ba in YBa_2Cu_3O_y system
Thus far, there is no cuprate system where both n-type and p-type charge
carriers can be doped without changing the crystallographic structure. For
studying the electron-hole symmetry in an identical structure, we try to dope
n-type carriers to YBa2Cu3Oy system by reducing oxygen content and substituting
La3+ ions for Ba2+. Single crystals of La-doped YBa2Cu3Oy are grown by a flux
method with Y2O3 crucibles and it is confirmed that La actually substitutes
\~13% of Ba. The oxygen content y can be varied between 6.21 and 6.95 by
annealing the crystals in an atmosphere with controlled oxygen partial
pressure. The in-plane resistivity rho_ab at room temperature was found to
increase with decreasing oxygen content y down to 6.32, but interestingly
further decrease in y results in a decrease in rho_ab. The most reduced samples
with y = 6.21 show rho_ab of ~30 mOhm cm at room temperature, which is as much
as seven orders-of-magnitude smaller than the maximum value at y = 6.32.
Furthermore, both the Hall coefficient and the Seebeck coefficient of the y =
6.21 samples are found to be negative at room temperatures. The present results
demonstrate that the non-doped Mott-insulating state has been crossed upon
reducing y and n-type carriers are successfully doped in this material.Comment: 4 pages, 4 figures, 1 table, accepted for publication in Phys. Rev.
Can Dark Matter Annihilation Dominate the Extragalactic Gamma-Ray Background?
Annihilating dark matter (DM) has been discussed as a possible source of
gamma-rays from the galactic center (GC) and contributing to the extragalactic
gamma-ray background (EGB). Assuming universality of the density profile of DM
halos, we show that it is quite unlikely that DM annihilation is a main
constituent of EGB, without exceeding the observed gamma-ray flux from the GC.
This argument becomes stronger when we include enhancement of the density
profiles by supermassive black holes or baryon cooling. The presence of
substructure may loosen the constraint, but only if a very large cross section
as well as the rather flat profile are realized.Comment: 4 pages, 2 figures, accepted by PR
Magnetic shape-memory effects in La2-xSrxCuO4 crystals
The magnetic field affects the motion of electrons and the orientation of
spins in solids, but it is believed to have little impact on the crystal
structure. This common perception has been challenged recently by ferromagnetic
shape-memory alloys, where the spin-lattice coupling is so strong that
crystallographic axes even in a fixed sample are forced to rotate, following
the direction of moments. One would, however, least expect any structural
change to be induced in antiferromagnets where spins are antiparallel and give
no net moment. Here we report on such unexpected magnetic shape-memory effects
that take place ironically in one of the best-studied 2D antiferromagnets,
La2-xSrxCuO4 (LSCO). We find that lightly-doped LSCO crystals tend to align
their b axis along the magnetic field, and if the crystal orientation is fixed,
this alignment occurs through the generation and motion of crystallographic
twin boundaries. Both resistivity and magnetic susceptibility exhibit curious
switching and memory effects induced by the crystal-axes rotation; moreover,
clear kinks moving over the crystal surfaces allow one to watch the crystal
rearrangement directly with a microscope or even bare eyes.Comment: 3 pages, 4 figures; shortend version of this paper has been published
in Nature as a Brief Communicatio
Coulomb drag in high Landau levels
Recent experiments on Coulomb drag in the quantum Hall regime have yielded a
number of surprises. The most striking observations are that the Coulomb drag
can become negative in high Landau levels and that its temperature dependence
is non-monotonous. We develop a systematic diagrammatic theory of Coulomb drag
in strong magnetic fields explaining these puzzling experiments. The theory is
applicable both in the diffusive and the ballistic regimes; we focus on the
experimentally relevant ballistic regime (interlayer distance smaller than
the cyclotron radius ). It is shown that the drag at strong magnetic
fields is an interplay of two contributions arising from different sources of
particle-hole asymmetry, namely the curvature of the zero-field electron
dispersion and the particle-hole asymmetry associated with Landau quantization.
The former contribution is positive and governs the high-temperature increase
in the drag resistivity. On the other hand, the latter one, which is dominant
at low , has an oscillatory sign (depending on the difference in filling
factors of the two layers) and gives rise to a sharp peak in the temperature
dependence at of the order of the Landau level width.Comment: 26 pages, 13 figure
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