12,097 research outputs found
Electron-electron interactions in decoupled graphene layers
Multi-layer graphene on the carbon face of silicon carbide is an intriguing
electronic system which typically consists of a stack of ten or more layers.
Rotational stacking faults in this system dramatically reduce inter-layer
coherence. In this article we report on the influence of inter-layer
interactions, which remain strong even when coherence is negligible, on the
Fermi liquid properties of charged graphene layers. We find that inter-layer
interactions increase the magnitudes of correlation energies and decrease
quasiparticle velocities, even when remote-layer carrier densities are small,
and that they lessen the influence of exchange and correlation on the
distribution of carriers across layers.Comment: 8 pages, 4 figures, submitte
Center-commissioned external review of International Water Management Institute: Consolidated report, 19-29 May 2003
Agricultural research / Research institutes / Research policy / Research priorities / Planning / Monitoring / Evaluation / Financial resources / Gender
Value-at-Risk and Tsallis statistics: risk analysis of the aerospace sector
In this study, we analyze the aerospace stocks prices in order to
characterize the sector behavior. The data analyzed cover the period from
January 1987 to April 1999. We present a new index for the aerospace sector and
we investigate the statistical characteristics of this index. Our results show
that this index is well described by Tsallis distribution. We explore this
result and modify the standard Value-at-Risk (VaR), financial risk assessment
methodology in order to reflect an asset which obeys Tsallis non-extensive
statistics.Comment: 10 pages, 4 figures, 1 table, to appear in Physica
Fully Frustrated Cold Atoms
Fully frustrated Josephson Junction arrays (FF-JJA's) exhibit a subtle
compound phase transition in which an Ising transition associated with discrete
broken translational symmetry and a Berezinskii-Kosterlitz-Thouless (BKT)
transition associated with quasi-long-range phase coherence occur nearly
simultaneously. In this Letter we discuss a cold atom realization of the FF-JJA
system. We demonstrate that both orders can be studied by standard
momentum-distribution-function measurements and present numerical results,
based on a successful self-consistent spin-wave approximation, that illustrate
the expected behavior of observables.Comment: 5 pages, 3 figures, submitte
Mesoscopic supercurrent transistor controlled by nonequilibrium cooling
The distinctive quasiparticle distribution existing under nonequilibrium in a
superconductor-insulator-normal metal-insulator-superconductor (SINIS)
mesoscopic line is proposed as a novel tool to control the supercurrent
intensity in a long Josephson weak link. We present a description of this
system in the framework of the diffusive-limit quasiclassical Green-function
theory and take into account the effects of inelastic scattering with arbitrary
strength. Supercurrent enhancement and suppression, including a marked
transition to a -junction are striking features leading to a fully tunable
structure. The role of the degree of nonequilibrium, temperature, and materials
choice as well as features like noise, switching time, and current and power
gain are also addressed.Comment: 8 pages, 9 figures, submitted to Journal of Low Temperature Physic
Phase Diagram of the Bose-Hubbard Model with T_3 symmetry
In this paper we study the quantum phase transition between the insulating
and the globally coherent superfluid phases in the Bose-Hubbard model with T_3
structure, the "dice lattice". Even in the absence of any frustration the
superfluid phase is characterized by modulation of the order parameter on the
different sublattices of the T_3 structure. The zero-temperature critical point
as a function of a magnetic field shows the characteristic "butterfly" form. At
fully frustration the superfluid region is strongly suppressed. In addition,
due to the existence of the Aharonov-Bohm cages at f=1/2, we find evidence for
the existence of an intermediate insulating phase characterized by a zero
superfluid stiffness but finite compressibility. In this intermediate phase
bosons are localized due to the external frustration and the topology of the
T_3 lattice. We name this new phase the Aharonov-Bohm (AB) insulator. In the
presence of charge frustration the phase diagram acquires the typical
lobe-structure. The form and hierarchy of the Mott insulating states with
fractional fillings, is dictated by the particular topology of the T_3 lattice.
The results presented in this paper were obtained by a variety of analytical
methods: mean-field and variational techniques to approach the phase boundary
from the superconducting side, and a strongly coupled expansion appropriate for
the Mott insulating region. In addition we performed Quantum Monte Carlo
simulations of the corresponding (2+1)D XY model to corroborate the analytical
calculations with a more accurate quantitative analysis. We finally discuss
experimental realization of the T_3 lattice both with optical lattices and with
Josephson junction arrays.Comment: 16 pages, 17 figure
Two-qubit entanglement dynamics for two different non-Markovian environments
We study the time behavior of entanglement between two noninteracting qubits
each immersed in its own environment for two different non-Markovian
conditions: a high- cavity slightly off-resonant with the qubit transition
frequency and a nonperfect photonic band-gap, respectively. We find that
revivals and retardation of entanglement loss may occur by adjusting the
cavity-qubit detuning, in the first case, while partial entanglement trapping
occurs in non-ideal photonic-band gap.Comment: 8 pages, 2 figure
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