Normal and superfluid fractions of inhomogeneous nonequilibrium quantum fluids

We present a theoretical analysis of the normal and superfluid fractions of quantum fluids described by a nonequilibrium extension of the Gross-Pitaevskii equation in the presence of an external potential. Both disordered and regular potentials are considered. The normal and superfluid fractions are defined by the response of the nonequilibrium quantum fluid to a vector potential, in analogy with the equilibrium case. We find that the physical meaning of these definitions breaks down out of equilibrium. The normal and superfluid fractions no longer add up to one and for some types of external potentials, they can even become negative.Comment: 6 pages, 5 figure

The spatial coherence of weakly interacting one-dimensional non-equilibrium Bosonic quantum fluids

We present a theoretical analysis of spatial correlations in a one-dimensional driven-dissipative non-equilibrium condensate. Starting from a stochastic generalized Gross-Pitaevskii equation, we derive a noisy Kuramoto-Sivashinsky equation for the phase dynamics. For sufficiently strong interactions, the coherence decays exponentially in close analogy to the equilibrium Bose gas. When interactions are small on a scale set by the nonequilibrium condition, we find through numerical simulations a crossover between a Gaussian and exponential decay with peculiar scaling of the coherence length on the fluid density and noise strength.Comment: 5 pages, 2 figures, supplemental materia

Finite-temperature Wigner solid and other phases of ripplonic polarons on a helium film

Electrons on liquid helium can form different phases depending on density, and temperature. Also the electron-ripplon coupling strength influences the phase diagram, through the formation of so-called "ripplonic polarons", that change how electrons are localized, and that shifts the transition between the Wigner solid and the liquid phase. We use an all-coupling, finite-temperature variational method to study the formation of a ripplopolaron Wigner solid on a liquid helium film for different regimes of the electron-ripplon coupling strength. In addition to the three known phases of the ripplopolaron system (electron Wigner solid, polaron Wigner solid, and electron fluid), we define and identify a fourth distinct phase, the ripplopolaron liquid. We analyse the transitions between these four phases and calculate the corresponding phase diagrams. This reveals a reentrant melting of the electron solid as a function of temperature. The calculated regions of existence of the Wigner solid are in agreement with recent experimental data.Comment: 12 pages, 6 figures. arXiv admin note: text overlap with arXiv:1012.4576, arXiv:0709.4140 by other author

The role of the European Union in the Human Rights Council

The European Union has and continues to place human rights and democracy at the heart of its external relations, namely through its activities in the UN human rights system. With the ongoing criticism about the inadequacies of the UN Commission on Human Rights Kofi Annan recommended that this ailing body be replaced with a new and more effective Human Rights Council. In light of the European Union’s desire to further human rights and democracy across the globe it rapidly got involved in playing an active and visible role in the overarching UN human rights reform process. Negotiations for establishing the Human Rights Council in addition to the deliberations of the mandated working groups outlining the details and modalities of the Council involved making difficult and demanding decisions. The different opinions and positions not only led to a delayed inception but also generated concerns that the new Council would not be any more efficient or credible than its predecessor. Many have claimed that its first operational year has produced disappointing results by not taking more concrete action regarding countries facing dire human rights crises such as Zimbabwe. In contrast, others argue that in its first year the Council did achieve a number of successes including the modalities of the Universal Periodic Review process, the review of UN Special Procedures, the adoption of the codes of conduct for mandate holders in addition to the adoption of a number of resolutions of great importance in the Council’s plenary. The European Union actively participated in each phase and in all areas of the Human Rights Council from the setting up process to engaging in the sometimes arduous negotiations of both procedural and substantive matters. While it has indeed been successful in the workings of some areas the EU’s shortcomings have recently overshadowed its achievements. The following study explores the role of the EU in the Human Rights Council from the setting-up process to its contributions in the review of the UN Special Procedures, the UPR process and the plenary sessions since the Council’s inception. Following the highlighting of both the EU’s achievements and shortcomings the study concludes with five core recommendations that have the potential to help rectify the challenges currently faced by the European Union in the Human Rights Council

Mean-field phase diagram of the 1-D Bose gas in a disorder potential

We study the quantum phase transition of the 1D weakly interacting Bose gas in the presence of disorder. We characterize the phase transition as a function of disorder and interaction strengths, by inspecting the long-range behavior of the one-body density matrix as well as the drop in the superfluid fraction. We focus on the properties of the low-energy Bogoliubov excitations that drive the phase transition, and find that the transition to the insulator state is marked by a diverging density of states and a localization length that diverges as a power-law with power 1. We draw the phase diagram and we observe that the boundary between the superfluid and the Bose glass phase is characterized by two different algebraic relations. These can be explained analytically by considering the limiting cases of zero and infinite disorder correlation length.Comment: 10 pages, 10 figure

Vortices in nonequilibrium photon condensates

We present a theoretical study of vortices in arrays of photon condensates. Even when interactions are negligible, as is the case in current experiments, pumping and losses can lead to a finite vortex core size. While some properties of photon condensate vortices, such as their self-acceleration and the generation of vortex pairs by a moving vortex, resemble those in interacting polariton condensates far from equilibrium, in several aspects they differ from previously studied systems: the vortex core size is determined by the balance between pumping and tunneling, the core appears oblate in the direction of its motion and new vortex pairs can spontaneously nucleate in the core region.Comment: 5 pages plus supplementary figure

Noise-induced transition from superfluid to vortex state in two-dimensional nonequilibrium polariton condensates -- semi-analytical treatment

We develop a semi-analytical description for the Berezinskii-Kosterlitz-Thouless (BKT) like phase transition in nonequilibrium Bose-Einstein condensates. Our theoretical analysis is based on a noisy generalized Gross-Pitaevskii equation. Above a critical strength of the noise, spontaneous vortex-antivortex pairs are generated. We provide a semi-analytical determination of the transition point based on a linearized Bogoliubov analysis, to which some nonlinear corrections are added. We present two different approaches that are in agreement with our numerical calculations in a wide range of system parameters. We find that for small losses and not too small energy relaxation, the critical point approaches that of the equilibrium BKT transition. Furthermore, we find that losses tend to stabilize the ordered phase: keeping the other parameters constant and increasing the losses leads to a higher critical noise strength for the spontaneous generation of vortex-antivortex pairs. Our theoretical analysis is relevant for experiments on microcavity polaritons

The diurnal evolution of the urban heat island of Paris: a model-based case study during Summer 2006

The urban heat island (UHI) over Paris during summer 2006 was simulated using the Advanced Regional Prediction System (ARPS) updated with a simple urban parametrization at a horizontal resolution of 1 km. Two integrations were performed, one with the urban land cover of Paris and another in which Paris was replaced by cropland. The focus is on a five-day clear-sky period, for which the UHI intensity reaches its maximum. The diurnal evolution of the UHI intensity was found to be adequately simulated for this five day period. The maximum difference at night in 2 m temperature between urban and rural areas stemming from the urban heating is reproduced with a relative error of less than 10%. The UHI has an ellipsoidal shape and stretches along the prevailing wind direction. The maximum UHI intensity of 6.1 K occurs at 23:00 UTC located 6 km downstream of the city centre and this largely remains during the whole night. An idealized one-column model study demonstrates that the nocturnal differential sensible heat flux, even though much smaller than its daytime value, is mainly responsible for the maximum UHI intensity. The reason for this nighttime maximum is that additional heat is only affecting a shallow layer of 150 m. An air uplift is explained by the synoptic east wind and a ramp upwind of the city centre, which leads to a considerable nocturnal adiabatic cooling over cropland. The idealized study demonstrates that the reduced vertical adiabatic cooling over the city compared to cropland induces an additional UHI build-up of 25%. The UHI and its vertical extent is affected by the boundary-layer stability, nocturnal low-level jet as well as radiative cooling. Therefore, improvements of representing these boundary-layer features in atmospheric models are important for UHI studies

From single particle to superfuid excitations in a dissipative polariton gas

Using angle-resolved heterodyne four-wave-mixing technique, we probe the low momentum excitation spectrum of a coherent polariton gas. The experimental results are well captured by the Bogoliubov transformation which describes the transition from single particle excitations of a normal fluid to sound-wave-like excitations of a superfluid. In a dense coherent polariton gas, we find all the characteristics of a Bogoliubov transformation, i.e. the positive and negative energy branch with respect to the polariton gas energy at rest, sound-wave-like shapes for the excitations dispersion, intensity and linewidth ratio between the two branches in agreement with the theory. The influence of the non-equilibrium character of the polariton gas is shown by a careful analysis of its dispersion.Comment: 4 pages, 3 figure