12,344 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 $k$-linear dispersion in the low-energy region and $k$-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
$2e^2/\pi^2\hbar$ per spin in the strong-disorder regime, independently of the
short- or long-range disorder.Comment: 8 pages, 5 figure

### Systematic evolution of the magnetotransport properties of Bi_{2}Sr_{2-x}La_{x}CuO_{6} in a wide doping range

Recently we have succeeded in growing a series of high-quality
Bi_{2}Sr_{2-x}La_{x}CuO_{6} crystals in a wide range of carrier concentrations.
The data of \rho_{ab}(T) and R_H(T) of those crystals show behaviors that are
considered to be "canonical" to the cuprates. The optimum zero-resistance T_c
has been raised to as high as 38 K, which is almost equal to the optimum T_c of
La_{2-x}Sr_{x}CuO_{4}.Comment: 2 pages, 2 figures, to be published in Physics C (Proceedings of the
International Conference on Materials and Mechanisms of Superconductivity,
High Temperature Superconductors VI (M2S-HTSC-VI), Houston, Feb 20-25, 2000

### 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

### Conductance plateau transitions in quantum Hall wires with spatially correlated random magnetic fields

Quantum transport properties in quantum Hall wires in the presence of
spatially correlated disordered magnetic fields are investigated numerically.
It is found that the correlation drastically changes the transport properties
associated with the edge state, in contrast to the naive expectation that the
correlation simply reduces the effect of disorder. In the presence of
correlation, the separation between the successive conductance plateau
transitions becomes larger than the bulk Landau level separation determined by
the mean value of the disordered magnetic fields. The transition energies
coincide with the Landau levels in an effective magnetic field stronger than
the mean value of the disordered magnetic field. For a long wire, the strength
of this effective magnetic field is of the order of the maximum value of the
magnetic fields in the system. It is shown that the effective field is
determined by a part where the stronger magnetic field region connects both
edges of the wire.Comment: 7 pages, 10 figure

### 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 $a$ smaller than
the cyclotron radius $R_c$). 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 $T$, 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 $T$ of the order of the Landau level width.Comment: 26 pages, 13 figure

### $^{16}{\rm O} + ^{16}{\rm O}$ nature of the superdeformed band of $^{32}{\rm S}$ and the evolution of the molecular structure

The relation between the superdeformed band of $^{32}{\rm S}$ and $^{16}{\rm
O} + ^{16}{\rm O}$ molecular bands is studied by the deformed-base
antisymmetrized molecular dynamics with the Gogny D1S force. It is found that
the obtained superdeformed band members of $^{32}{\rm S}$ have considerable
amount of the $^{16}{\rm O} + ^{16}{\rm O}$ component. Above the superdeformed
band, we have obtained two excited rotational bands which have more prominent
character of the $^{16}{\rm O} + ^{16}{\rm O}$ molecular band. These three
rotational bands are regarded as a series of $^{16}{\rm O} + ^{16}{\rm O}$
molecular bands which were predicted by using the unique $^{16}{\rm O}$
-$^{16}{\rm O}$ optical potentil. As the excitation energy and principal
quantum number of the relative motion increase, the $^{16}{\rm O} + ^{16}{\rm
O}$ cluster structure becomes more prominent but at the same time, the band
members are fragmented into several states

### Hall plateau diagram for the Hofstadter butterfly energy spectrum

We extensively study the localization and the quantum Hall effect in the
Hofstadter butterfly, which emerges in a two-dimensional electron system with a
weak two-dimensional periodic potential. We numerically calculate the Hall
conductivity and the localization length for finite systems with the disorder
in general magnetic fields, and estimate the energies of the extended levels in
an infinite system. We obtain the Hall plateau diagram on the whole region of
the Hofstadter butterfly, and propose a theory for the evolution of the plateau
structure with increasing disorder. There we show that a subband with the Hall
conductivity $n e^2/h$ has $|n|$ separated bunches of extended levels, at least
for an integer $n \leq 2$. We also find that the clusters of the subbands with
identical Hall conductivity, which repeatedly appear in the Hofstadter
butterfly, have a similar localization property.Comment: 9 pages, 12 figure

### Quantum transport properties of two-dimensional systems in disordered magnetic fields with a fixed sign

Quantum transport in disordered magnetic fields is investigated numerically
in two-dimensional systems. In particular, the case where the mean and the
fluctuation of disordered magnetic fields are of the same order is considered.
It is found that in the limit of weak disorder the conductivity exhibits a
qualitatively different behavior from that in the conventional random magnetic
fields with zero mean. The conductivity is estimated by the equation of motion
method and by the two-terminal Landauer formula. It is demonstrated that the
conductance stays on the order of $e^2/h$ even in the weak disorder limit. The
present behavior can be interpreted in terms of the Drude formula. The
Shubnikov-de Haas oscillation is also observed in the weak disorder regime.Comment: 6 pages, 7 figures, to appear in Phys. Rev.

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