845 research outputs found

### First Principles Study of CaFe2As2 "Collapse" Under Pressure

We perform first principles calculations on CaFe2As2 under hydrostatic
pressure. Our total energy calculations show that though the striped
antiferromagnetic (AFM) orthorhombic (OR) phase is favored at P=0, a
non-magnetic collapsed tetragonal (cT) phase with diminished c-parameter is
favored for P > 0.36 GPa, in agreement with experiments. Rather than a
mechanical instability, this is an enthalpically driven transition from the
higher volume OR phase to the lower volume cT phase. Calculations of electronic
density of states reveal pseudogaps in both OR and cT phases, though As(p)
hybridization with Fe(d) is more pronounced in the OR phase. We provide an
estimate for the inter-planar magnetic coupling. Phonon entropy considerations
provide an interpretation of the finite temperature phase boundaries of the cT
phase.Comment: 4 pages, 4 figures, 1 Tabl

### Resonant atom-field interaction in large-size coupled-cavity arrays

We consider an array of coupled cavities with staggered inter-cavity
couplings, where each cavity mode interacts with an atom. In contrast to
large-size arrays with uniform-hopping rates where the atomic dynamics is known
to be frozen in the strong-hopping regime, we show that resonant atom-field
dynamics with significant energy exchange can occur in the case of staggered
hopping rates even in the thermodynamic limit. This effect arises from the
joint emergence of an energy gap in the free photonic dispersion relation and a
discrete frequency at the gap's center. The latter corresponds to a bound
normal mode stemming solely from the finiteness of the array length. Depending
on which cavity is excited, either the atomic dynamics is frozen or a
Jaynes-Cummings-like energy exchange is triggered between the bound photonic
mode and its atomic analogue. As these phenomena are effective with any number
of cavities, they are prone to be experimentally observed even in small-size
arrays.Comment: 12 pages, 4 figures. Added 5 mathematical appendice

### The most probable wave function of a single free moving particle

We develop the most probable wave functions for a single free quantum
particle given its momentum and energy by imposing its quantum probability
density to maximize Shannon information entropy. We show that there is a class
of solutions in which the quantum probability density is self-trapped with
finite-size spatial support, uniformly moving hence keeping its form unchanged.Comment: revtex, 4 page

### Electrical manipulation of an electronic two-state system in Ge/Si quantum dots

We calculate that the electron states of strained self-assembled Ge/Si
quantum dots provide a convenient two-state system for electrical control. An
electronic state localized at the apex of the quantum dot is nearly degenerate
with a state localized at the base of the quantum dot. Small electric fields
shift the electronic ground state from apex-localized to base-localized, which
permits sensitive tuning of the electronic, optical and magnetic properties of
the dot. As one example, we describe how spin-spin coupling between two Ge/Si
dots can be controlled very sensitively by shifting the individual dot's
electronic ground state between apex and base

### Electron quantum dynamics in closed and open potentials at high magnetic fields: Quantization and lifetime effects unified by semicoherent states

We have developed a Green's function formalism based on the use of an
overcomplete semicoherent basis of vortex states, specially devoted to the
study of the Hamiltonian quantum dynamics of electrons at high magnetic fields
and in an arbitrary potential landscape smooth on the scale of the magnetic
length. This formalism is used here to derive the exact Green's function for an
arbitrary quadratic potential in the special limit where Landau level mixing
becomes negligible. This solution remarkably embraces under a unified form the
cases of confining and unconfining quadratic potentials. This property results
from the fact that the overcomplete vortex representation provides a more
general type of spectral decomposition of the Hamiltonian operator than usually
considered. Whereas confining potentials are naturally characterized by
quantization effects, lifetime effects emerge instead in the case of
saddle-point potentials. Our derivation proves that the appearance of lifetimes
has for origin the instability of the dynamics due to quantum tunneling at
saddle points of the potential landscape. In fact, the overcompleteness of the
vortex representation reveals an intrinsic microscopic irreversibility of the
states synonymous with a spontaneous breaking of the time symmetry exhibited by
the Hamiltonian dynamics.Comment: 19 pages, 4 figures ; a few typos corrected + some passages in Sec. V
rewritte

### Spin Polarization via Electron Tunneling through an Indirect-Gap Semiconductor Barrier

We study the spin dependent tunneling of electrons through a zinc-blende
semiconductor with the indirect X (or D) minimum serving as the tunneling
barrier. The basic difference between tunneling through the G vs. the X barrier
is the linear-k spin-orbit splitting of the two spin bands at the X point, as
opposed to the k3 Dresselhaus splitting at the G point. The linear coefficient
of the spin splitting b at the X point is computed for several semiconductors
using density-functional theory and the transport characteristics are
calculated using the barrier tunneling model. We show that both the
transmission coefficient as well as the spin polarization can be large,
suggesting the potential application of these materials as spin filters.Comment: 9 page

### Electronic structure of crystalline binary and ternary Cd-Te-O compounds

The electronic structure of crystalline CdTe, CdO, $\alpha$-TeO$_2$,
CdTeO$_3$ and Cd$_3$TeO$_6$ is studied by means of first principles
calculations. The band structure, total and partial density of states, and
charge densities are presented. For $\alpha$-TeO$_2$ and CdTeO$_3$, Density
Functional Theory within the Local Density Approximation (LDA) correctly
describes the insulating character of these compounds. In the first four
compounds, LDA underestimates the optical bandgap by roughly 1 eV. Based on
this trend, we predict an optical bandgap of 1.7 eV for Cd$_3$TeO$_6$. This
material shows an isolated conduction band with a low effective mass, thus
explaining its semiconducting character observed recently. In all these oxides,
the top valence bands are formed mainly from the O 2p electrons. On the other
hand, the binding energy of the Cd 4d band, relative to the valence band
maximum, in the ternary compounds is smaller than in CdTe and CdO.Comment: 13 pages, 15 figures, 2 tables. Accepted in Phys Rev

### Electric field driven donor-based charge qubits in semiconductors

We investigate theoretically donor-based charge qubit operation driven by
external electric fields. The basic physics of the problem is presented by
considering a single electron bound to a shallow-donor pair in GaAs: This
system is closely related to the homopolar molecular ion H_2^+. In the case of
Si, heteropolar configurations such as PSb^+ pairs are also considered. For
both homopolar and heteropolar pairs, the multivalley conduction band structure
of Si leads to short-period oscillations of the tunnel-coupling strength as a
function of the inter-donor relative position. However, for any fixed donor
configuration, the response of the bound electron to a uniform electric field
in Si is qualitatively very similar to the GaAs case, with no valley quantum
interference-related effects, leading to the conclusion that electric field
driven coherent manipulation of donor-based charge qubits is feasible in
semiconductors

### Hydrodynamic View of Wave-Packet Interference: Quantum Caves

Wave-packet interference is investigated within the complex quantum
Hamilton-Jacobi formalism using a hydrodynamic description. Quantum
interference leads to the formation of the topological structure of quantum
caves in space-time Argand plots. These caves consist of the vortical and
stagnation tubes originating from the isosurfaces of the amplitude of the wave
function and its first derivative. Complex quantum trajectories display
counterclockwise helical wrapping around the stagnation tubes and hyperbolic
deflection near the vortical tubes. The string of alternating stagnation and
vortical tubes is sufficient to generate divergent trajectories. Moreover, the
average wrapping time for trajectories and the rotational rate of the nodal
line in the complex plane can be used to define the lifetime for interference
features.Comment: 4 pages, 3 figures (major revisions with respect to the previous
version have been carried out

### On the stable configuration of ultra-relativistic material spheres. The solution for the extremely hot gas

During the last stage of collapse of a compact object into the horizon of
events, the potential energy of its surface layer decreases to a negative value
below all limits. The energy-conservation law requires an appearance of a
positive-valued energy to balance the decrease. We derive the internal-state
properties of the ideal gas situated in an extremely strong, ultra-relativistic
gravitational field and suggest to apply our result to a compact object with
the radius which is slightly larger than or equal to the Schwarzschild's
gravitational radius. On the surface of the object, we find that the extreme
attractivity of the gravity is accompanied with an extremely high internal,
heat energy. This internal energy implies a correspondingly high pressure, the
gradient of which has such a behavior that it can compete with the gravity. In
a more detail, we find the equation of state in the case when the magnitude of
the potential-type energy of constituting gas particles is much larger than
their rest energy. This equation appears to be identical with the
general-relativity condition of the equilibrium between the gravity and
pressure gradient. The consequences of the identity are discussed.Comment: 12 pages (no figure, no table) Changes in 3-rd version: added an
estimate of neutrino cooling and relative time-scale of the final stage of
URMS collaps

- …