3,979 research outputs found
Electronic and vibrational properties of in the charge-density wave phase from first principles
We study the charge-density wave phase in by using first
principle calculations. We show that, regardless of the local functional used
and as long as the cell parameters are in agreement with the experiment,
density-functional calculations are able to reproduce not only the structural
instability of , but also the effective distortion observed in
the experiments. We study the electronic structure evolution of the system
under the charge-density wave deformation. In particular, we show that the
energy bands for the distorted superstructure, unfolded into the original
Brillouin zone, are in reasonable agreement with angle-resolved photoemission
spectroscopy (ARPES) data taken at low temperature. On the contrary, the energy
bands for the undistorted structure are not in good agreement with ARPES at
high temperature. Motivated by these results, we investigate the effect of the
correlation on the electrons of the localized Ti- orbitals by using the
LDA+ method. We show that within this approximation the electronic bands for
both the undistorted and distorted structure are in very good agreement with
ARPES. On the other hand, the eliminates the phonon instability of the
system. Some possible explanations for this counter intuitive result are
proposed. Particularly, the possibility of taking into account the dependence
of the parameter from the atomic positions is suggested.Comment: 21 pages, 16 figures, 3 pages of Supplementary materia
High-pressure phase diagram of hydrogen and deuterium sulfides from first principles: structural and vibrational properties including quantum and anharmonic effects
We study the structural and vibrational properties of the high-temperature
superconducting sulfur trihydride and trideuteride in the high-pressure
and phases by first-principles density-functional-theory
calculations. On lowering pressure, the rhombohedral transition is expected, with hydrogen bond desymmetrization and
occurrence of trigonal lattice distortion. In hydrostatic conditions we find
that, contrary to what suggested in some recent experiments, if the
rhombohedral distortion exists it affects mainly the hydrogen-bonds, whereas
the resulting cell distortion is minimal. We estimate that the occurrence of a
stress anisotropy of approximately could explain this discrepancy.
Assuming hydrostatic conditions, we calculate the critical pressure at which
the rhombohedral transition occurs. Quantum and anharmonic effects, which are
relevant in this system, are included at nonperturbative level with the
stochastic self-consistent harmonic approximation (SSCHA). Within this
approach, we determine the transition pressure by calculating the free energy
Hessian. We find that quantum anharmonic effects are responsible for a strong
reduction of the critical pressure with respect to the one obtained with the
classical harmonic approach. Moreover, we observe a prominent isotope effect,
as we estimate higher pressure transition for DS than for HS.
Finally, within SSCHA we calculate the anharmonic phonon spectral functions in
the phase. The strong anharmonicity of the system is confirmed by
the occurrence of very large anharmonic broadenings leading to complex
non-Lorentzian line shapes. However, for the vibrational spectra at zone
center, accessible e.g. by infrared spectroscopy, the broadenings are very
small (linewidth at most around 2~meV) and anharmonic phonon quasiparticles are
well defined
Second order structural phase transitions, free energy curvature, and temperature-dependent anharmonic phonons in the self-consistent harmonic approximation: theory and stochastic implementation
The self-consistent harmonic approximation is an effective harmonic theory to
calculate the free energy of systems with strongly anharmonic atomic
vibrations, and its stochastic implementation has proved to be an efficient
method to study, from first-principles, the anharmonic properties of solids.
The free energy as a function of average atomic positions (centroids) can be
used to study quantum or thermal lattice instability. In particular the
centroids are order parameters in second-order structural phase transitions
such as, e.g., charge-density-waves or ferroelectric instabilities. According
to Landau's theory, the knowledge of the second derivative of the free energy
(i.e. the curvature) with respect to the centroids in a high-symmetry
configuration allows the identification of the phase-transition and of the
instability modes. In this work we derive the exact analytic formula for the
second derivative of the free energy in the self-consistent harmonic
approximation for a generic atomic configuration. The analytic derivative is
expressed in terms of the atomic displacements and forces in a form that can be
evaluated by a stochastic technique using importance sampling. Our approach is
particularly suitable for applications based on first-principles
density-functional-theory calculations, where the forces on atoms can be
obtained with a negligible computational effort compared to total energy
determination. Finally we propose a dynamical extension of the theory to
calculate spectral properties of strongly anharmonic phonons, as probed by
inelastic scattering processes. We illustrate our method with a numerical
application on a toy model that mimics the ferroelectric transition in
rock-salt crystals such as SnTe or GeTe
Web User Session Characterization via Clustering Techniques
We focus on the identification and definition of "Web user-sessions", an aggregation of several TCP connections generated by the same source host on the basis of TCP connection opening time. The identification of a user session is non trivial; traditional approaches rely on threshold based mechanisms, which are very sensitive to the value assumed for the threshold and may be difficult to correctly set. By applying clustering techniques, we define a novel methodology to identify Web user-sessions without requiring an a priori definition of threshold values. We analyze the characteristics of user sessions extracted from real traces, studying the statistical properties of the identified sessions. From the study it emerges that Web user-sessions tend to be Poisson, but correlation may arise during periods of network/hosts anomalous functioning
Anharmonic phonon spectra of PbTe and SnTe in the self-consistent harmonic approximation
At room temperature, PbTe and SnTe are efficient thermoelectrics with a cubic
structure. At low temperature, SnTe undergoes a ferroelectric transition with a
critical temperature strongly dependent on the hole concentration, while PbTe
is an incipient ferroelectric. By using the stochastic self-consistent harmonic
approximation, we investigate the anharmonic phonon spectra and the occurrence
of a ferroelectric transition in both systems. We find that vibrational spectra
strongly depends on the approximation used for the exchange-correlation kernel
in density functional theory. If gradient corrections and the theoretical
volume are employed, then the calculation of the free energy Hessian leads to
phonon spectra in good agreement with experimental data for both systems. In
PbTe, we reproduce the transverse optical mode phonon satellite detected in
inelastic neutron scattering and the crossing between the transverse optical
and the longitudinal acoustic modes along the X direction. In the case
of SnTe, we describe the occurrence of a ferroelectric transition from the high
temperature Fmm structure to the low temperature R3m one.Comment: 12 pages, 15 Picture
Interference at the Single Photon Level Along Satellite-Ground Channels
Quantum interference arising from superposition of states is a striking
evidence of the validity of Quantum Mechanics, confirmed in many experiments
and also exploited in applications. However, as for any scientific theory,
Quantum Mechanics is valid within the limits in which it has been
experimentally verified. In order to extend such limits, it is necessary to
observe quantum interference in unexplored conditions such as moving terminals
at large distance in Space. Here we experimentally demonstrate single photon
interference at a ground station due to the coherent superposition of two
temporal modes reflected by a rapidly moving satellite thousand kilometers
away. The relative speed of the satellite induces a varying modulation in the
interference pattern. The measurement of the satellite distance in real time by
laser ranging allowed us to precisely predict the instantaneous value of the
interference phase. We then observed the interference patterns with visibility
up to with three different satellites and with path length up to 5000
km. Our results attest the viability of photon temporal modes for fundamental
tests of Physics and Quantum Communications in Space.Comment: Version accepted for publication in Phys. Rev. Let
Using Autoregressive Models for Real-Time Packet Loss Concealment in Networked Music Performance Applications
In Networked Music Performances (NMP), concealing the effects of lost/late packets on the quality of the playback audio stream is of pivotal importance to mitigate the impact of the resulting audio artifacts. Traditional packet loss concealment techniques implemented in standard audio codecs can be leveraged only at the price of an increased mouth-to-ear latency, which may easily exceed the strict delay requirements of NMP interactions.
This paper investigates the adoption of a low-complexity prediction technique based on autoregressive models to fill audio gaps caused by missing packets. Numerical results show that the proposed approach outperforms packet loss concealment methods normally implemented in NMP systems, typically based on filling audio gaps with silence or repetition of the last received audio segment
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