Thermal Effects on the Microscopic Properties of 4He Drops

We present path integral Monte Carlo results of small ⁴He drops at very low temperatures around the transition temperature to the normal phase. The results obtained are compared with the ones obtained at zero temperature using the diffusion Monte Carlo method. The analysis is carried out for drops of different size and it is focused on the determination of their energetic and structural properties. Particular emphasis is devoted to the temperature dependence of the density profile, mainly to determine if any sign of the λ-transition could be observed on them.     

Atomic and Molecular Hydrogen Impurities in Liquid 4He

The static properties of a single hydrogen atom and an hydrogen molecule in bulk superfluid ⁴ He are studied by means of the diffusion Monte Carlo method. The analysis includes atomic hydrogen, deuterium and tritium, and molecular hydrogen and deuterium. Our results, which show some differences with previous variational calculations, are in agreement with the only available experimental data corresponding to atomic deuterium. In contrast to the unbinding presented by the atomic impurities, the molecular ones do bind with energies higher than the chemical potential of pure ⁴ He.     

Inter-destination multimedia synchronization: schemes, use cases and standardization

Traditionally, the media consumption model has been a passive and isolated activity. However, the advent of media streaming technologies, interactive social applications, and synchronous communications, as well as the convergence between these three developments, point to an evolution towards dynamic shared media experiences. In this new model, geographically distributed groups of consumers, independently of their location and the nature of their end-devices, can be immersed in a common virtual networked environment in which they can share multimedia services, interact and collaborate in real-time within the context of simultaneous media content consumption. In most of these multimedia services and applications, apart from the well-known intra and inter-stream synchronization techniques that are important inside the consumers’ playout devices, also the synchronization of the playout processes between several distributed receivers, known as multipoint, group or Inter-destination multimedia synchronization (IDMS), becomes essential. Due to the increasing popularity of social networking, this type of multimedia synchronization has gained in popularity in recent years. Although Social TV is perhaps the most prominent use case in which IDMS is useful, in this paper we present up to 19 use cases for IDMS, each one having its own synchronization requirements. Different approaches used in the (recent) past by researchers to achieve IDMS are described and compared. As further proof of the significance of IDMS nowadays, relevant organizations’ (such as ETSI TISPAN and IETF AVTCORE Group) efforts on IDMS standardization (in which authors have been and are participating actively), defining architectures and protocols, are summarized.     

Two-Dimensional Spin-Polarized Hydrogen at Zero Temperature

The ground-state properties of spin polarized hydrogen H↓ in two dimensions (2D) are obtained by means of diffusion Monte Carlo calculations. Using the most accurate to date ab initio H↓–H↓ interatomic potential we have studied hydrogen gas phase, from the very dilute regime until densities above its freezing point. For very low densities, the equation of state of the gas can be described in terms of the gas parameter na ², where a is the s-wave scattering length in 2D. The solid phase in 2D has also been studied up to high pressures and the gas-solid phase transition determined using the double-tangent Maxwell construction.     

Condensate Fraction in Liquid 4He at Zero Temperature

We present results of the one-body density matrix ρ ₁(r) and the condensate fraction n ₀ of liquid ⁴He calculated at zero temperature by means of the Path Integral Ground State Monte Carlo method. This technique allows to generate a highly accurate approximation for the ground state wave function Ψ ₀ in a totally model-independent way, that depends only on the Hamiltonian of the system and on the symmetry properties of Ψ ₀. With this unbiased estimation of ρ ₁(r), we obtain precise results for the condensate fraction n ₀ and the kinetic energy K of the system. The dependence of n ₀ with the pressure shows an excellent agreement of our results with recent experimental measurements. Above the melting pressure, overpressurized liquid ⁴He shows a small condensate fraction that has dropped to 0.8% at the highest pressure of p=87?bar.