101,954 research outputs found
Coherent and dissipative transport in a Josephson junction between fermionic superfluids of Li-6 atoms
No full textQuantum systems out of equilibrium offer the possibility of understanding intriguing and challenging problems in modern physics. Studying transport properties is not only valuable to unveil fundamental properties of quantum matter but it is also an excellent tool for developing new quantum devices which inherently employ quantum-mechanical effects. In this contribution, we present our experimental studies on quantum transport using ultracold Fermi gases of 6Li atoms. We realize the analogous of a Josephson junction by bisecting fermionic superfluids by a thin optical barrier. We observe coherent dynamics in both the population and in the relative phase between the two reservoirs. For critical parameters, the superfluid dynamics exhibits both coherent and resistive flow due to phase-slippage events manifesting as vortices propagating into the bulk. We uncover also a regime of strong dissipation where the junction operation is irreversibly affected by vortex proliferation. Our studies open new directions for investigating dissipation and superfluid transport in strongly correlated fermionic systems
Tunneling transport of unitary fermions across the superfluid transition
No full textWe investigate the transport of a Fermi gas with unitarity-limited interactions across the superfluid phase transition, probing its response to a direct current (dc) drive through a tunnel junction. As the superfluid critical temperature is crossed from below, we observe the evolution from a highly nonlinear to an Ohmic conduction characteristic, associated with the critical breakdown of the Josephson dc current induced by pair condensate depletion. Moreover, we reveal a large and dominant anomalous contribution to resistive currents, which reaches its maximum at the lowest attained temperature, fostered by the tunnel coupling between the condensate and phononic Bogoliubov-Anderson excitations. Increasing the temperature, while the zeroing of supercurrents marks the transition to the normal phase, the conductance drops considerably but remains much larger than that of a normal, uncorrelated Fermi gas tunneling through the same junction. We attribute such enhanced transport to incoherent tunneling of sound modes, which remain weakly damped in the collisional hydrodynamic fluid of unpaired fermions at unitarity
Dynamical phase diagram of ultracold Josephson junctions
No full text\ua9 2020 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.We provide a complete study of the phase diagram characterising the distinct dynamical regimes emerging in a three-dimensional Josephson junction in an ultracold quantum gas. Considering trapped ultracold superfluids separated into two reservoirs by a barrier of variable height and width, we analyse the population imbalance dynamics following a variable initial population mismatch. We demonstrate that as the chemical potential difference is increased, the system transitions from Josephson plasma oscillations to either a dissipative (in the limit of low and narrow barriers) or a self-trapped regime (for large and wider barriers), with a crossover between the dissipative and the self-trapping regimes which we explore and characterize for the first time. This work, which extends beyond the validity of the standard two-mode model, connects the role of the barrier width, vortex rings and associated acoustic emission with different regimes of the superfluid dynamics across the junction, establishing a framework for its experimental observation, which is found to be within current experimental reach
Decay of persistent currents in annular atomic superfluids
No full textWe investigate the role of vortices in the decay of persistent current states
of annular atomic superfluids by solving numerically the Gross-Pitaevskii
equation, and we directly compare our results with experimental data from Ref.
[1]. We theoretically model the optical phase-imprinting technique employed to
experimentally excite finite-circulation states in Ref. [1] in the
Bose-Einstein condensation regime, accounting for imperfections of the optical
gradient imprinting profile. By comparing simulations of this realistic
protocol to an ideal imprinting, we show that the introduced density
excitations arising from imperfect imprinting are mainly responsible for
limiting the maximum reachable winding number in the
superfluid ring. We also investigate the effect of a point-like obstacle with
variable potential height onto the decay of circulating supercurrents.
For a given obstacle height, a critical circulation exists, such that for
an initial circulation larger than the supercurrent decays through
the emission of vortices, which cross the superflow and thus induce phase
slippage. Higher values of the obstacle height further favour the
entrance of vortices, thus leading to lower values of . Furthermore, the
stronger vortex-defect interaction at higher leads to vortices that
propagate closer to the center of the ring condensate. The combination of both
these effects leads to an increase of the supercurrent decay rate for
increasing , in agreement with experimental observations.
[1]: G. Del Pace, et al., Phys. Rev. X 12, 041037 (2022
Josephson effect in fermionic superfluids across the BEC-BCS crossove
No full textThe Josephson effect is a macroscopic quantum phenomenon that reveals the broken symmetry associated with any superfluid state. Here we report on the observation of the Josephson effect between two fermionic superfluids coupled through a thin tunneling barrier. We show that the relative population and phase are canonically conjugate dynamical variables throughout the crossover from the molecular Bose-Einstein condensate (BEC) to the Bardeen-Cooper-Schrieffer (BCS) superfluid regime. For larger initial excitations from equilibrium, the dynamics of the superfluids become dissipative, which we ascribe to the propagation of vortices through the superfluid bulk. Our results highlight the robust nature of resonant superfluids
Giant reed (Arundo donax L.) as a sustainable Energy crop for 2nd generation bio-ethanol production.
No full textThe production of new generation biofuels, made from lignocellulosic feedstock, is a fundamental and unavoidable step towards the achievement of the EU GHG emission reduction targets. Significantly less land is required per unit of product with 2nd gen. compared to traditional 1st gen. biofuels. Because massive import of low-density feedstock is not feasible, 2nd gen. biofuels will stimulate and activate EU internal production/recovery of biomass crops for energy, with minimal competition with food/feed production. An agro-industrial project, with an estimated output of 160,000 mT/y cellulosic bioethanol, is in progress at Crescentino (Piedmont – Italy), based on Chemtex cellulosic bioethanol technology Pro.E.SaTM. M&G Group and its subsidiary, Chemtex, have been conducting extensive multidisciplinary research to select and characterize a set of energy crop species. This work, to date, has demonstrated the economic viability of growing these crops for conversion to liquid transportation fuels and developed some of the specific data for designing and planning the supply chain system in a wide set of agronomic conditions and cropping schemes. Arundo donax L. (Adx) will represent the main input feedstock to the process, together with corn stalks and cereal straw, collected from nearby areas.
Adx has been indicated as one of the most promising lignocellulosic herbaceous energy crop in the Mediterranean area, due to its desirable traits: a geophytes and sinantropic, perennial C3 grass (high photosynthetic rates and little photoinhibition were observed), which showed a strong metal tolerance, making it suitable for ecoremediation purposes. Pure stands of improved strains exhibited a longevity of +10 years, with a high annual yield of biomass (35 t ha-1 DM). Reduced input requirements (tillage, fertilizers, agrochemicals) and favorable harvest logistics increase the energy production efficiency of this sub-cosmopolite species. Some agronomic and ecological aspects of Adx require further evaluation before spreading it on larger areas. A field trial was established to investigate the effect of irrigation and nitrogen fertilization on biomass yield and energy use efficiency in Adx production
Sound emission and annihilations in a programmable quantum vortex collider
No full textIn quantum fluids, the quantization of circulation forbids the diffusion of a vortex swirling flow seen in classical viscous fluids. Yet, accelerating quantum vortices may lose their energy into acoustic radiations1,2, similar to the way electric charges decelerate on emitting photons. The dissipation of vortex energy underlies central problems in quantum hydrodynamics3, such as the decay of quantum turbulence, highly relevant to systems as varied as neutron stars, superfluid helium and atomic condensates4,5. A deep understanding of the elementary mechanisms behind irreversible vortex dynamics has been a goal for decades3,6, but it is complicated by the shortage of conclusive experimental signatures7. Here we address this challenge by realizing a programmable vortex collider in a planar, homogeneous atomic Fermi superfluid with tunable inter-particle interactions. We create on-demand vortex configurations and monitor their evolution, taking advantage of the accessible time and length scales of ultracold Fermi gases8,9. Engineering collisions within and between vortex–antivortex pairs allows us to decouple relaxation of the vortex energy due to sound emission and that due to interactions with normal fluid (that is, mutual friction). We directly visualize how the annihilation of vortex dipoles radiates a sound pulse. Further, our few-vortex experiments extending across different superfluid regimes reveal non-universal dissipative dynamics, suggesting that fermionic quasiparticles localized inside the vortex core contribute significantly to dissipation, thereby opening the route to exploring new pathways for quantum turbulence decay, vortex by vortex
Strongly correlated superfluid order parameters from dc Josephson supercurrents
No full textThe direct-current (dc) Josephson effect provides a phase-sensitive tool for investigating superfluid order parameters. We report on the observation of dc Josephson supercurrents in strongly interacting fermionic superfluids across a tunneling barrier in the absence of any applied potential difference. For sufficiently strong barriers, we observed a sinusoidal current-phase relation, in agreement with Josephson's seminal prediction. We mapped out the zero-resistance state and its breakdown as a function of junction parameters, extracting the Josephson critical current behavior. By comparing our results with an analytic model, we determined the pair condensate fraction throughout the Bardeen-Cooper-Schrieffer-Bose-Einstein condensation crossover. Our work suggests that coherent Josephson transport may be used to pin down superfluid order parameters in diverse atomic systems, even in the presence of strong correlations
QTh7A.17
No full textWe propose to use ultracold-fermionic atoms in optical lattices to quantum-simulate electronic transport in quantum-cascade-laser structures. The parallelism between the two systems is discussed
Λ-enhanced grey molasses on the D 2 transition of Rubidium-87 atoms
No full textLaser cooling based on dark states, i.e. states decoupled from light, has proven to be effective to increase the phase-space density of cold trapped atoms. Dark-states cooling requires open atomic transitions, in contrast to the ordinary laser cooling used for example in magneto-optical traps (MOTs), which operate on closed atomic transitions. For alkali atoms, dark-states cooling is therefore commonly operated on the D 1 transition nS 1/2 → nP 1/2. We show that, for 87Rb, thanks to the large hyperfine structure separations the use of this transition is not strictly necessary and that "quasi-dark state" cooling is efficient also on the D 2 line, 5S 1/2 → 5P 3/2. We report temperatures as low as (4.0 ± 0.3) μK and an increase of almost an order of magnitude in the phase space density with respect to ordinary laser sub-Doppler cooling
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