2,596 research outputs found
Instanton Calculus of Lifshitz Tails
For noninteracting particles moving in a Gaussian random potential, there
exists a disagreement in the literature on the asymptotic expression for the
density of states in the tail of the band. We resolve this discrepancy. Further
we illuminate the physical facet of instantons appearing in replica and
supersymmetric derivations with another derivation employing a Lagrange
multiplier field.Comment: 5 page
Glucose modulation of ATP-sensitive K-currents in wild-type, homozygous and heterozygous glucokinase knock-out mice
Strain Modulated Electronic Properties of Ge Nanowires - A First Principles Study
We used density-functional theory based first principles simulations to study
the effects of uniaxial strain and quantum confinement on the electronic
properties of germanium nanowires along the [110] direction, such as the energy
gap and the effective masses of the electron and hole. The diameters of the
nanowires being studied are up to 50 {\AA}. As shown in our calculations, the
Ge [110] nanowires possess a direct band gap, in contrast to the nature of an
indirect band gap in bulk. We discovered that the band gap and the effective
masses of charge carries can be modulated by applying uniaxial strain to the
nanowires. These strain modulations are size-dependent. For a smaller wire (~
12 {\AA}), the band gap is almost a linear function of strain; compressive
strain increases the gap while tensile strain reduces the gap. For a larger
wire (20 {\AA} - 50 {\AA}), the variation of the band gap with respect to
strain shows nearly parabolic behavior: compressive strain beyond -1% also
reduces the gap. In addition, our studies showed that strain affects effective
masses of the electron and hole very differently. The effective mass of the
hole increases with a tensile strain while the effective mass of the electron
increases with a compressive strain. Our results suggested both strain and size
can be used to tune the band structures of nanowires, which may help in design
of future nano-electronic devices. We also discussed our results by applying
the tight-binding model.Comment: 1 table, 8 figure
Effective medium theory of elastic waves in random networks of rods
We formulate an effective medium (mean field) theory of a material consisting
of randomly distributed nodes connected by straight slender rods, hinged at the
nodes. Defining novel wavelength-dependent effective elastic moduli, we
calculate both the static moduli and the dispersion relations of ultrasonic
longitudinal and transverse elastic waves. At finite wave vector the waves
are dispersive, with phase and group velocities decreasing with increasing wave
vector. These results are directly applicable to networks with empty pore
space. They also describe the solid matrix in two-component (Biot) theories of
fluid-filled porous media. We suggest the possibility of low density materials
with higher ratios of stiffness and strength to density than those of foams,
aerogels or trabecular bone.Comment: 14 pp., 3 fig
Netons: Vibrations of Complex Networks
We consider atoms interacting each other through the topological structure of
a complex network and investigate lattice vibrations of the system, the quanta
of which we call {\em netons} for convenience. The density of neton levels,
obtained numerically, reveals that unlike a local regular lattice, the system
develops a gap of a finite width, manifesting extreme rigidity of the network
structure at low energies. Two different network models, the small-world
network and the scale-free network, are compared: The characteristic structure
of the former is described by an additional peak in the level density whereas a
power-law tail is observed in the latter, indicating excitability of netons at
arbitrarily high energies. The gap width is also found to vanish in the
small-world network when the connection range .Comment: 9 pages, 6 figures, to appear in JP
Cooling of cryogenic electron bilayers via the Coulomb interaction
Heat dissipation in current-carrying cryogenic nanostructures is problematic
because the phonon density of states decreases strongly as energy decreases. We
show that the Coulomb interaction can prove a valuable resource for carrier
cooling via coupling to a nearby, cold electron reservoir. Specifically, we
consider the geometry of an electron bilayer in a silicon-based
heterostructure, and analyze the power transfer. We show that across a range of
temperatures, separations, and sheet densities, the electron-electron
interaction dominates the phonon heat-dissipation modes as the main cooling
mechanism. Coulomb cooling is most effective at low densities, when phonon
cooling is least effective in silicon, making it especially relevant for
experiments attempting to perform coherent manipulations of single spins.Comment: 9 pages, 5 figure
Tilt-angle landscapes and temperature dependence of the conductance in biphenyl-dithiol single-molecule junctions
Using a density-functional-based transport method we study the conduction
properties of several biphenyl-derived dithiol (BPDDT) molecules wired to gold
electrodes. The BPDDT molecules differ in their side groups, which control the
degree of conjugation of the pi-electron system. We have analyzed the
dependence of the low-bias zero-temperature conductance on the tilt angle phi
between the two phenyl ring units, and find that it follows closely a
cos^2(phi) law, as expected from an effective pi-orbital coupling model. We
show that the tilting of the phenyl rings results in a decrease of the
zero-temperature conductance by roughly two orders of magnitude, when going
from a planar conformation to a configuration in which the rings are
perpendicular. In addition we demonstrate that the side groups, apart from
determining phi, have no influence on the conductance. All this is in agreement
with the recent experiment by Venkataraman et al. [Nature 442, 904 (2006)].
Finally, we study the temperature dependence of both the conductance and its
fluctuations and find qualitative differences between the examined molecules.
In this analysis we consider two contributions to the temperature behavior, one
coming from the Fermi functions and the other one from a thermal average over
different contact configurations. We illustrate that the fluctuations of the
conductance due to temperature-induced changes in the geometric structure of
the molecule can be reduced by an appropriate design.Comment: 9 pages, 6 figures; submitted to Phys. Rev.
Ground-plane screening of Coulomb interactions in two-dimensional systems: How effectively can one two-dimensional system screen interactions in another?
The use of a nearby metallic ground-plane to limit the range of the Coulomb
interactions between carriers is a useful approach in studying the physics of
two-dimensional (2D) systems. This approach has been used to study Wigner
crystallization of electrons on the surface of liquid helium, and most
recently, the insulating and metallic states of semiconductor-based
two-dimensional systems. In this paper, we perform calculations of the
screening effect of one 2D system on another and show that a 2D system is at
least as effective as a metal in screening Coulomb interactions. We also show
that the recent observation of the reduced effect of the ground-plane when the
2D system is in the metallic regime is due to intralayer screening.Comment: 14 pages, 7 figures Accepted in PR
Probing Ion-Ion and Electron-Ion Correlations in Liquid Metals within the Quantum Hypernetted Chain Approximation
We use the Quantum Hypernetted Chain Approximation (QHNC) to calculate the
ion-ion and electron-ion correlations for liquid metallic Li, Be, Na, Mg, Al,
K, Ca, and Ga. We discuss trends in electron-ion structure factors and radial
distribution functions, and also calculate the free-atom and metallic-atom
form-factors, focusing on how bonding effects affect the interpretation of
X-ray scattering experiments, especially experimental measurements of the
ion-ion structure factor in the liquid metallic phase.Comment: RevTeX, 19 pages, 7 figure
Fractional-Period Excitations in Continuum Periodic Systems
We investigate the generation of fractional-period states in continuum
periodic systems. As an example, we consider a Bose-Einstein condensate
confined in an optical-lattice potential. We show that when the potential is
turned on non-adiabatically, the system explores a number of transient states
whose periodicity is a fraction of that of the lattice. We illustrate the
origin of fractional-period states analytically by treating them as resonant
states of a parametrically forced Duffing oscillator and discuss their
transient nature and potential observability.Comment: 10 pages, 6 figures (some with multiple parts); revised version:
minor clarifications of a couple points, to appear in Physical Review
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