Universal dynamics on the way to thermalisation

It is demonstrated how a many-body system far from thermal equilibrium can exhibit universal dynamics in passing a non-thermal fixed point. As an example, the process of Bose-Einstein (BE) condensation of a dilute cold gas is considered. If the particle flux into the low-energy modes, induced, e.g., by a cooling quench, is sufficiently strong, the Bose gas develops a characteristic power-law single-particle spectrum $n(k)\sim k^{-5}$, and critical slowing down in time occurs. The fixed point is shown to be marked by the creation and dilution of tangled vortex lines. Alternatively, for a weak cooling quench and particle flux, the condensation process runs quasi adiabatically, passing by the fixed point in far distance, and signatures of critical scaling remain absent.Comment: 5+2 pages, 8 figure

Critical Dynamics of a Two-dimensional Superfluid near a Non-Thermal Fixed Point

Critical dynamics of an ultracold Bose gas far from equilibrium is studied in two spatial dimensions. Superfluid turbulence is created by quenching the equilibrium state close to zero temperature. Instead of immediately re-thermalizing, the system approaches a meta-stable transient state, characterized as a non-thermal fixed point. A focus is set on the vortex density and vortex-antivortex correlations which characterize the evolution towards the non-thermal fixed point and the departure to final (quasi-)condensation. Two distinct power-law regimes in the vortex-density decay are found and discussed in terms of a vortex binding-unbinding transition and a kinetic description of vortex scattering. A possible relation to decaying turbulence in classical fluids is pointed out. By comparing the results to equilibrium studies of a two-dimensional Bose gas, an intuitive understanding of the location of the non-thermal fixed point in a reduced phase space is developed.Comment: 11 pages, 13 figures; PRA versio

On-site number statistics of ultracold lattice bosons

We study on-site occupation number fluctuations in a system of interacting bosons in an optical lattice. The ground-state distribution is obtained analytically in the limiting cases of strong and weak interaction, and by means of exact Monte Carlo simulations in the strongly correlated regime. As the interaction is increased, the distribution evolves from Poissonian in the non-interacting gas to a sharply peaked distribution in the Mott-insulator (MI) regime. In the special case of large occupation numbers, we demonstrate analytically and check numerically that there exists a wide interval of interaction strength, in which the on-site number fluctuations remain Gaussian and are gradually squeezed until they are of order unity near the superfluid (SF)-MI transition. Recently, the on-site number statistics were studied experimentally in a wide range of lattice potential depths [Phys. Rev. Lett. \textbf{96}, 090401 (2006)]. In our simulations, we are able to directly reproduce experimental conditions using temperature as the only free parameter. Pronounced temperature dependence suggests that measurements of on-site atom number fluctuations can be employed as a reliable method of thermometry in both SF and MI regimes.Comment: 9 pages, 4 figure

An effective potential for one-dimensional matter-wave solitons in an axially inhomogeneous trap

We demonstrate that a tight transverse trap with the local frequency, $\omega_{\perp}$, gradually varying in the longitudinal direction ($x$) induces an effective potential for one-dimensional solitons in a self-attractive Bose-Einstein condensate. An analytical approximation for this potential is derived by means of a variational method. In the lowest approximation, the potential is $N(S+1)\omega_{\perp}(x)$, with $N$ the soliton's norm (number of atoms), and $S$ its intrinsic vorticity (if any). The results can be used to devise nonuniform traps helping to control the longitudinal dynamics of the solitons. Numerical verification of the analytical predictions will be presented elsewhere.Comment: to be published in Physics Letters

Modelling in the Strait of Gibraltar: from Operational Oceanography to Scale Interactions.

We make a review on the modelling efforts devoted to better understand the complex oceanography of the Strait of Gibraltar, where Atlantic waters enter the Mediterranean Sea as a surface flow, and Mediterranean outflowing waters spread into the interior of the North Atlantic forming a prominent basin-scale termohaline anomaly at mid-depths. Besides the mean exchange flows relevant phenomena include tides, high amplitude internal waves, meteorologically forced subinertial oscillations, mixing, and involve a wide-range of spatio-temporal scales. The remarkable progress achieved in understanding and modelling the ocean processes in the Strait of Gibraltar allows now undertaking new societal demands and scientific challenges. One societal demand is given by the increasing need of operational oceanographic information as a support tool for decision-makers in an area considered as one of world's busiest shipping lanes, with an increased risk of maritime accidents and environmental pollution. We present an Operational Oceanography system for the Strait of Gibraltar responding to that demand. On the other hand, new scientific challenges call for the need of developing perspective-modelling studies accounting for process and scale interactions. Using a global ocean general circulation model with regional high resolution around the Iberian Peninsula we are able to resolve the local-scale at the Strait of Gibraltar and the Gulf of Cádiz while focusing on the basin scale. As a result, we find that tidally-induced local-scale processes in the Strait and in the Gulf of Cádiz appear to have a drastic impact on the distribution of Mediterranean outflow waters in the Atlantic basin

Detecting Super-Counter-Fluidity by Ramsey Spectroscopy

Spatially selective Ramsey spectroscopy is suggested as a method for detecting the super-counter-fluidity of two-component atomic mixture in optical lattice.Comment: 3pages, no figures, replaced with revised version accepted by PRA. Discussion of the Ramsey pattern specific for topological excitations is adde

Controlling collapse in Bose-Einstein condensates by temporal modulation of the scattering length

We consider, by means of the variational approximation (VA) and direct numerical simulations of the Gross-Pitaevskii (GP) equation, the dynamics of 2D and 3D condensates with a scattering length containing constant and harmonically varying parts, which can be achieved with an ac magnetic field tuned to the Feshbach resonance. For a rapid time modulation, we develop an approach based on the direct averaging of the GP equation,without using the VA. In the 2D case, both VA and direct simulations, as well as the averaging method, reveal the existence of stable self-confined condensates without an external trap, in agreement with qualitatively similar results recently reported for spatial solitons in nonlinear optics. In the 3D case, the VA again predicts the existence of a stable self-confined condensate without a trap. In this case, direct simulations demonstrate that the stability is limited in time, eventually switching into collapse, even though the constant part of the scattering length is positive (but not too large). Thus a spatially uniform ac magnetic field, resonantly tuned to control the scattering length, may play the role of an effective trap confining the condensate, and sometimes causing its collapse.Comment: 7 figure

Russia and the diffusion of political norms: the perfect rival?

Russian norm diffusion has been studied mainly as the mirror image of the EU’s, but deserves to be studied in its own right and complexity. Three core categories of political norms are explored: sovereign choice, regime and conservative ideas. It is argued that Russia does not promote a coherent political model, by lack of one, but rather diffuses a disparate set of conservative ideas and non-democratic practices. Russia’s normative positioning is equivocal. It champions established international norms like sovereignty, placing itself within the dominant normative community, but contesting the application by the West. When it comes to certain liberal political norms (often reduced to a strawman version), it questions their validity and rejects them, placing itself outside the dominant normative community, but claiming to defend ‘genuine’ European values. This makes Russia an ambiguous norm contester, rather than the perfect normative rival of the EU. The complexity of its norm contestation follows from exogenous motives: it is predominantly an anti-hegemonic reaction against what it perceives as the Western imposition of norms, harming vital Russian interests

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements

Molecular mechanisms of cell death: recommendations of the Nomenclature Committee on Cell Death 2018.

Over the past decade, the Nomenclature Committee on Cell Death (NCCD) has formulated guidelines for the definition and interpretation of cell death from morphological, biochemical, and functional perspectives. Since the field continues to expand and novel mechanisms that orchestrate multiple cell death pathways are unveiled, we propose an updated classification of cell death subroutines focusing on mechanistic and essential (as opposed to correlative and dispensable) aspects of the process. As we provide molecularly oriented definitions of terms including intrinsic apoptosis, extrinsic apoptosis, mitochondrial permeability transition (MPT)-driven necrosis, necroptosis, ferroptosis, pyroptosis, parthanatos, entotic cell death, NETotic cell death, lysosome-dependent cell death, autophagy-dependent cell death, immunogenic cell death, cellular senescence, and mitotic catastrophe, we discuss the utility of neologisms that refer to highly specialized instances of these processes. The mission of the NCCD is to provide a widely accepted nomenclature on cell death in support of the continued development of the field