221 research outputs found
Magnetotransport of coupled electron-holes
The carriers in InAs-GaSb double quantum wells are hybrid ``electron-holes''.
We study the magnetotransport properties of such particles using a
two-component Keldysh technique, which results in a semi-analytic expression
for the small-field current. We show that zero temperature current can be large
even when the Fermi energy lies within the hybridization gap, a result which
cannot be understood within a semiclassical (Boltzmann) approach. Magnetic
field dependence of the conductance is also affected significantly by the
hybridization of electrons and holes.Comment: 4 pages, 2 figure
Combination quantum oscillations in canonical single-band Fermi liquids
Chemical potential oscillations mix individual-band frequencies of the de
Haas-van Alphen (dHvA) and Shubnikov-de Haas (SdH) magneto-oscillations in
canonical low-dimensional multi-band Fermi liquids. We predict a similar mixing
in canonical single-band Fermi liquids, which Fermi-surfaces have two or more
extremal cross-sections. Combination harmonics are analysed using a single-band
almost two-dimensional energy spectrum. We outline some experimental conditions
allowing for resolution of combination harmonics
Underbarrier nucleation kinetics in a metastable quantum liquid near the spinodal
We develop a theory in order to describe the effect of relaxation in a
condensed medium upon the quantum decay of a metastable liquid near the
spinodal at low temperatures. We find that both the regime and the rate of
quantum nucleation strongly depend on the relaxation time and its temperature
behavior. The quantum nucleation rate slows down with the decrease of the
relaxation time. We also discuss the low temperature experiments on cavitation
in normal He and superfluid He at negative pressures. It is the sharp
distinctions in the high frequency sound mode and in the temperature behavior
of the relaxation time that make the quantum cavitation kinetics in He and
He completely different in kind.Comment: 10 pages, 2 figure
Quantum Statistical Effects on Fusion Dynamics of Heavy-Ions
In order to describe the fusion of two very heavy nuclei at near barrier
energies, a generalized Langevin approach is proposed, which incorporates the
quantum statistical fluctuations in accordance with the fluctuation and
dissipation theorem. It is illustrated that the quantum statistical effects
introduce an enhancement of the formation of compound nucleus, though the
quantum enhancement is somewhat less pronounced as indicated in the previous
calculations.Comment: 17 pages, 6 figure
A Theory of Magnets with Competing Double Exchange and Superexchange Interactions
We study the competition between ferromagnetic double exchange (DE) and
nearest-neighbour antiferromagnetic exchange in CMR materials. Towards this
end, a single site mean field theory is proposed which emphasizes the
hopping-mediated nature of the DE contribution. We find that the competition
between these two exchange interactions leads to ferro- or antiferromagnetic
order with incomplete saturation of the (sub)lattice magnetization. This
conclusion is in contrast to previous results in the literature which find a
canted spin arrangement under similar circumstances. We attribute this
difference to the highly anisotropic exchange interactions used elsewhere. The
associated experimental implications are discussed.Comment: 4 pages, Latex-Revtex, 3 PostScript figures. Please see report
cond-mat/980523
Quantum fluctuations and glassy behavior: The case of a quantum particle in a random potential
In this paper we expand our previous investigation of a quantum particle
subject to the action of a random potential plus a fixed harmonic potential at
a finite temperature T. In the classical limit the system reduces to a
well-known ``toy'' model for an interface in a random medium. It also applies
to a single quantum particle like an an electron subject to random
interactions, where the harmonic potential can be tuned to mimic the effect of
a finite box. Using the variational approximation, or alternatively, the limit
of large spatial dimensions, together with the use the replica method, and are
able to solve the model and obtain its phase diagram in the
plane, where is the particle's mass. The phase diagram is similar to that
of a quantum spin-glass in a transverse field, where the variable
plays the role of the transverse field. The glassy phase is characterized by
replica-symmetry-breaking. The quantum transition at zero temperature is also
discussed.Comment: revised version, 23 pages, revtex, 5 postscript figures in a separate
file figures.u
Multifractals Competing with Solitons on Fibonacci Optical Lattice
We study the stationary states for the nonlinear Schr\"odinger equation on
the Fibonacci lattice which is expected to be realized by Bose-Einstein
condensates loaded into an optical lattice. When the model does not have a
nonlinear term, the wavefunctions and the spectrum are known to show fractal
structures. Such wavefunctions are called critical. We present a phase diagram
of the energy spectrum for varying the nonlinearity. It consists of three
portions, a forbidden region, the spectrum of critical states, and the spectrum
of stationary solitons. We show that the energy spectrum of critical states
remains intact irrespective of the nonlinearity in the sea of a large number of
stationary solitons.Comment: 5 pages, 4 figures, major revision, references adde
Negative Echo in the Density Evolution of Ultracold Fermionic Gases
We predict a nonequilibrium critical phenomenon in the space-time density
evolution of a fermionic gas above the temperature of transition into the
superfluid phase. On the BCS side of the BEC-BCS crossover, the evolution of a
localized density disturbance exhibits a negative echo at the point of the
initial inhomogeneity. Approaching the BEC side, this effect competes with the
slow spreading of the density of bosonic molecules. However, even here the echo
dominates for large enough times. This effect may be used as an experimental
tool to locate the position of the transition.Comment: 4 pages, 2 figure
Extraordinary magnetoresistance in graphite: experimental evidence for the time-reversal symmetry breaking
The ordinary magnetoresistance (MR) of doped semiconductors is positive and
quadratic in a low magnetic field, B, as it should be in the framework of the
Boltzmann kinetic theory or in the conventional hopping regime. We observe an
unusual highly-anisotropic in-plane MR in graphite, which is neither quadratic
nor always positive. In a certain current direction MR is negative and linear
in B in fields below a few tens of mT with a crossover to a positive MR at
higher fields, while in a perpendicular current direction we observe a giant
super-linear and positive MR. These extraordinary MRs are respectively
explained by a hopping magneto-conductance via non-zero angular momentum
orbitals, and by the magneto-conductance of inhomogeneous media. The linear
orbital NMR is a unique signature of the broken time-reversal symmetry (TRS) in
graphite. While some local paramagnetic centers could be responsible for the
broken TRS, the observed large diamagnetism suggests a more intriguing
mechanism of this breaking, involving superconducting clusters with
unconventional (chiral) order parameters and spontaneously generated
normal-state current loops in graphite.Comment: 4 pages, 5 figure
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