1,047 research outputs found
Heat capacity of a thin membrane at very low temperature
We calculate the dependence of heat capacity of a free standing thin membrane
on its thickness and temperature. A remarkable fact is that for a given
temperature there exists a minimum in the dependence of the heat capacity on
the thickness. The ratio of the heat capacity to its minimal value for a given
temperature is a universal function of the ratio of the thickness to its value
corresponding to the minimum. The minimal value of the heat capacitance for
given temperature is proportional to the temperature squared. Our analysis can
be used, in particular, for optimizing support membranes for microbolometers
Characteristics of phonon transmission across epitaxial interfaces: a lattice dynamic study
Phonon transmission across epitaxial interfaces is studied within the lattice
dynamic approach. The transmission shows weak dependence on frequency for the
lattice wave with a fixed angle of incidence. The dependence on azimuth angle
is found to be related to the symmetry of the boundary interface. The
transmission varies smoothly with the change of the incident angle. A critical
angle of incidence exists when the phonon is incident from the side with large
group velocities to the side with low ones. No significant mode conversion is
observed among different acoustic wave branches at the interface, except when
the incident angle is near the critical value. Our theoretical result of the
Kapitza conductance across the Si-Ge (100) interface at temperature
K is 4.6\times10^{8} {\rm WK}^{-1}{\rmm}^{-2}. A scaling law at low temperature is also reported. Based on the features of
transmission obtained within lattice dynamic approach, we propose a simplified
formula for thermal conductanceacross the epitaxial interface. A reasonable
consistency is found between the calculated values and the experimentally
measured ones.Comment: 8 figure
Anomalous quantum chaotic behavior in nanoelectromechanical structures
It is predicted that for sufficiently strong electron-phonon coupling an
anomalous quantum chaotic behavior develops in certain types of suspended
electro-mechanical nanostructures, here comprised by a thin cylindrical quantum
dot (billiard) on a suspended rectangular dielectric plate. The deformation
potential and piezoelectric interactions are considered. As a result of the
electron-phonon coupling between the two systems the spectral statistics of the
electro-mechanic eigenenergies exhibit an anomalous behavior. If the center of
the quantum dot is located at one of the symmetry axes of the rectangular
plate, the energy level distributions correspond to the Gaussian Orthogonal
Ensemble (GOE), otherwise they belong to the Gaussian Unitary Ensemble (GUE),
even though the system is time-reversal invariant.Comment: 4 pages, pdf forma
Thermo-mechanical behavior of surface acoustic waves in ordered arrays of nanodisks studied by near infrared pump-probe diffraction experiments
The ultrafast thermal and mechanical dynamics of a two-dimensional lattice of
metallic nano-disks has been studied by near infrared pump-probe diffraction
measurements, over a temporal range spanning from 100 fs to several
nanoseconds. The experiments demonstrate that, in these systems, a
two-dimensional surface acoustic wave (2DSAW), with a wavevector given by the
reciprocal periodicity of the array, can be excited by ~120 fs Ti:sapphire
laser pulses. In order to clarify the interaction between the nanodisks and the
substrate, numerical calculations of the elastic eigenmodes and simulations of
the thermodynamics of the system are developed through finite-element analysis.
At this light, we unambiguously show that the observed 2DSAW velocity shift
originates from the mechanical interaction between the 2DSAWs and the
nano-disks, while the correlated 2DSAW damping is due to the energy radiation
into the substrate.Comment: 13 pages, 10 figure
Quantum chaos in nanoelectromechanical systems
We present a theoretical study of the electron-phonon coupling in suspended
nanoelectromechanical systems (NEMS) and investigate the resulting quantum
chaotic behavior. The phonons are associated with the vibrational modes of a
suspended rectangular dielectric plate, with free or clamped boundary
conditions, whereas the electrons are confined to a large quantum dot (QD) on
the plate's surface. The deformation potential and piezoelectric interactions
are considered. By performing standard energy-level statistics we demonstrate
that the spectral fluctuations exhibit the same distributions as those of the
Gaussian Orthogonal Ensemble (GOE) or the Gaussian Unitary Ensemble (GUE),
therefore evidencing the emergence of quantum chaos. That is verified for a
large range of material and geometry parameters. In particular, the GUE
statistics occurs only in the case of a circular QD. It represents an anomalous
phenomenon, previously reported for just a small number of systems, since the
problem is time-reversal invariant. The obtained results are explained through
a detailed analysis of the Hamiltonian matrix structure.Comment: 14 pages, two column
Excitation Theory for Space-Dispersive Active Media Waveguides
A unified electrodynamic approach to the guided-wave excitation theory is
generalized to the waveguiding structures containing a hypothetical
space-dispersive medium with drifting charge carriers possessing simultaneously
elastic, piezoelectric and magnetic properties. Substantial features of our
electrodynamic approach are: (i) the allowance for medium losses and (ii) the
separation of potential fields peculiar to the slow quasi-static waves. It is
shown that the orthogonal complementary fields appearing inside the external
source region are just associated with a contribution of the potential fields
inherent in exciting sources. Taking account of medium losses converts the
usual orthogonality relation into a novel form called the quasi-orthogonality
relation. It is found that the separation of potential fields reveals the fine
structure of interaction between the exciting sources and mode eigenfields: in
addition to the exciting currents interacting with the curl fields, the
exciting charges and the double charge (surface dipole) layers appear to
interact with the quasi-static potentials and the displacement currents,
respectively.Comment: LaTeX 2.09, 28 pages with mathematical appendi
Role of confined phonons in thin film superconductivity
We calculate the critical temperature and the superconducting energy
gaps of a thin film superconductor system, where is the
superconducting energy gap of the -th subband. Since the quantization of
both the electron energy and phonon spectrum arises due to dimensional
confinement in one direction, the effective electron-electron interaction
mediated by the quantized confined phonons is different from that mediated by
the bulk phonon, leading to the modification of in the thin film system.
We investigate the dependence of and on the film thickness
with this modified interaction.Comment: 4 pages, 2 figure
On the low-temperature lattice thermal transport in nanowires
We propose a theory of low temperature thermal transport in nano-wires in the
regime where a competition between phonon and flexural modes governs the
relaxation processes. Starting with the standard kinetic equations for two
different types of quasiparticles we derive a general expression for the
coefficient of thermal conductivity. The underlying physics of thermal
conductance is completely determined by the corresponding relaxation times,
which can be calculated directly for any dispersion of quasiparticles depending
on the size of a system. We show that if the considered relaxation mechanism is
dominant, then at small wire diameters the temperature dependence of thermal
conductivity experiences a crossover from to -dependence.
Quantitative analysis shows reasonable agreement with resent experimental
results.Comment: 12 pages, 3 eps figure
Energy levels in polarization superlattices: a comparison of continuum strain models
A theoretical model for the energy levels in polarization superlattices is
presented. The model includes the effect of strain on the local
polarization-induced electric fields and the subsequent effect on the energy
levels. Two continuum strain models are contrasted. One is the standard strain
model derived from Hooke's law that is typically used to calculate energy
levels in polarization superlattices and quantum wells. The other is a
fully-coupled strain model derived from the thermodynamic equation of state for
piezoelectric materials. The latter is more complete and applicable to strongly
piezoelectric materials where corrections to the standard model are
significant. The underlying theory has been applied to AlGaN/GaN superlattices
and quantum wells. It is found that the fully-coupled strain model yields very
different electric fields from the standard model. The calculated intersubband
transition energies are shifted by approximately 5 -- 19 meV, depending on the
structure. Thus from a device standpoint, the effect of applying the
fully-coupled model produces a very measurable shift in the peak wavelength.
This result has implications for the design of AlGaN/GaN optical switches.Comment: Revtex
Moving frames applied to shell elasticity
Exterior calculus and moving frames are used to describe curved elastic
shells. The kinematics follow from the Lie-derivative on forms whereas the
dynamics via stress-forms.Comment: 20 pages, 1 figur
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