6,717 research outputs found
Wave functions in the neighborhood of a toroidal surface; hard vs. soft constraint
The curvature potential arising from confining a particle initially in
three-dimensional space onto a curved surface is normally derived in the hard
constraint limit, with the degree of freedom normal to the
surface. In this work the hard constraint is relaxed, and eigenvalues and wave
functions are numerically determined for a particle confined to a thin layer in
the neighborhood of a toroidal surface. The hard constraint and finite layer
(or soft constraint) quantities are comparable, but both differ markedly from
those of the corresponding two dimensional system, indicating that the
curvature potential continues to influence the dynamics when the particle is
confined to a finite layer. This effect is potentially of consequence to the
modelling of curved nanostructures.Comment: 4 pages, no fig
Towards first-principles understanding of the metal-insulator transition in fluid alkali metals
By treating the electron-ion interaction as perturbation in the
first-principles Hamiltonian, we have calculated the density response functions
of a fluid alkali metal to find an interesting charge instability due to
anomalous electronic density fluctuations occurring at some finite wave vector
{\bi Q} in a dilute fluid phase above the liquid-gas critical point. Since
|{\bi Q}| is smaller than the diameter of the Fermi surface, this instability
necessarily impedes the electric conduction, implying its close relevance to
the metal-insulator transition in fluid alkali metals.Comment: 11 pages, 5 figure
Doping Dependence of Polaron Hopping Energies in La(1-x)Ca(x)MnO(3) (0<= x<= 0.15)
Measurements of the low-frequency (f<= 100 kHz) permittivity at T<= 160 K and
dc resistivity (T<= 430 K) are reported for La(1-x)Ca(x)MnO(3) (0<= x<= 0.15).
Static dielectric constants are determined from the low-T limiting behavior of
the permittivity. The estimated polarizability for bound holes ~ 10^{-22}
cm^{-3} implies a radius comparable to the interatomic spacing, consistent with
the small polaron picture established from prior transport studies near room
temperature and above on nearby compositions. Relaxation peaks in the
dielectric loss associated with charge-carrier hopping yield activation
energies in good agreement with low-T hopping energies determined from
variable-range hopping fits of the dc resistivity. The doping dependence of
these energies suggests that the orthorhombic, canted antiferromagnetic ground
state tends toward an insulator-metal transition that is not realized due to
the formation of the ferromagnetic insulating state near Mn(4+) concentration ~
0.13.Comment: PRB in press, 5 pages, 6 figure
Ferromagnetic imprinting of spin polarization in a semiconductor
We present a theory of the imprinting of the electron spin coherence and
population in an n-doped semiconductor which forms a junction with a
ferromagnet. The reflection of non-equilibrium semiconductor electrons at the
interface provides a mechanism to manipulate the spin polarization vector. In
the case of unpolarized excitation, this ballistic effect produces spontaneous
electron spin coherence and nuclear polarization in the semiconductor, as
recently observed by time-resolved Faraday rotation experiments. We investigate
the dependence of the spin reflection on the Schottky barrier height and the
doping concentration in the semiconductor and suggest control mechanisms for
possible device applications.Comment: 4 pages with 2 figure
External Control of a Metal-Insulator Transition in GaMnAs Wires
Quantum transport in disordered ferromagnetic (III,Mn)V semiconductors is
studied theoretically. Mesoscopic wires exhibit an Anderson disorder-induced
metal-insulator transition that can be controlled by a weak external magnetic
field. This metal-insulator transition should also occur in other materials
with large anisotropic magneto resistance effects. The transition can be useful
for studies of zero-temperature quantum critical phase transitions and
fundamental material properties.Comment: Major revised final versio
A new vibrational level of the H molecular ion
A new state of the H molecular ion with binding energy of
1.09 a.u. below the first dissociation limit is predicted, using
highly accurate numerical nonrelativistic quantum calculations. It is the first
L=0 excited state, antisymmetric with respect to the exchange of the two
protons. It manifests itself as a huge p-H scattering length of
Bohr radii.Comment: 6 pages + 3 figure
Nonlinear Ionic Conductivity of Thin Solid Electrolyte Samples: Comparison between Theory and Experiment
Nonlinear conductivity effects are studied experimentally and theoretically
for thin samples of disordered ionic conductors. Following previous work in
this field the {\it experimental nonlinear conductivity} of sodium ion
conducting glasses is analyzed in terms of apparent hopping distances. Values
up to 43 \AA are obtained. Due to higher-order harmonic current density
detection, any undesired effects arising from Joule heating can be excluded.
Additionally, the influence of temperature and sample thickness on the
nonlinearity is explored. From the {\it theoretical side} the nonlinear
conductivity in a disordered hopping model is analyzed numerically. For the 1D
case the nonlinearity can be even handled analytically. Surprisingly, for this
model the apparent hopping distance scales with the system size. This result
shows that in general the nonlinear conductivity cannot be interpreted in terms
of apparent hopping distances. Possible extensions of the model are discussed.Comment: 7 pages, 6 figure
Interplay of Peltier and Seebeck effects in nanoscale nonlocal spin valves
We have experimentally studied the role of thermoelectric effects in
nanoscale nonlocal spin valve devices. A finite element thermoelectric model is
developed to calculate the generated Seebeck voltages due to Peltier and Joule
heating in the devices. By measuring the first, second and third harmonic
voltage response non locally, the model is experimentally examined. The results
indicate that the combination of Peltier and Seebeck effects contributes
significantly to the nonlocal baseline resistance. Moreover, we found that the
second and third harmonic response signals can be attributed to Joule heating
and temperature dependencies of both Seebeck coefficient and resistivity.Comment: 4 pages, 4 figure
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