Electronic and vibrational properties of TiSe2\textup{TiSe}_2 in the charge-density wave phase from first principles

We study the charge-density wave phase in $\textup{TiSe}_2$ 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 $\textup{TiSe}_2$, 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-$d$ orbitals by using the LDA+$U$ 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 $U$ 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 $U$ 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 $Im\bar{3}m$ and $R3m$ phases by first-principles density-functional-theory calculations. On lowering pressure, the rhombohedral transition $Im\bar{3}m \rightarrow R3m$ 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 $10\%$ 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 D${}_3$S than for H${}_3$S. Finally, within SSCHA we calculate the anharmonic phonon spectral functions in the $Im\bar{3}m$ 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 $\Gamma$X direction. In the case of SnTe, we describe the occurrence of a ferroelectric transition from the high temperature Fm$\overline{3}$m 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 $67\%$ 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