arXiv subjects

Jeffrey Mohan

Publications and source records attributed to Jeffrey Mohan.

9 recordsLinked to original sources

arXiv2024

Saturation of thermal and spin conductances in a dissipative superfluid junction

Fermionic superfluid junctions typically exhibit suppressed thermal and spin transport due to the presence of a pairing gap but allow coherent particle transport. While dissipation generally weakens coherent transport, it can also induce excitations that open other transport channels. In this work, we experimentally study a one-dimensional superfluid junction of strongly interacting fermions with local particle loss and observe dissipation-induced thermal and spin transport that appear to saturate at strong dissipation. Notably, in this regime, the measured thermal and spin conductances are comparable to the universal quantized conductance of one-dimensional ideal Fermi gas. Qualitatively similar behavior is observed for two dissipation mechanisms, either spin-imbalanced or pairwise losses. Our findings provide new insights into transport in interacting open quantum systems and suggest possibilities of dissipative control of spin and thermoelectric transport.

cond-mat.quant-gas↗

arXiv2024

Dark state transport between unitary Fermi superfluids

The formation of dark states is an important concept in quantum sciences, but its compatibility with strong interparticle interactions -- for example, in a quantum degenerate gas -- is hardly explored. Here, we realize a dark state in one of the spins of a two-component, resonantly interacting Fermi gas using a $\Lambda$ system within the $D_2$ transitions of $^6$Li at high magnetic field. The dark state is created in a micrometer-sized region within a one-dimensional channel connecting two superfluid reservoirs. The particle transport between the reservoirs is used as a probe. We observe that atoms are transported in the dark state and the superfluid-assisted fast current is preserved. If the dark state resonant condition is not met, the transport is suppressed by the spontaneous emission. We also uncover an asymmetry in the transport timescale across the two-photon resonance, which is absent in the non-interacting regime and diminished at higher temperatures. This work raises questions on the interplay of dark states with interparticle interactions and opens up perspectives for optical manipulation of fermionic pairing.

cond-mat.quant-gas↗

arXiv2024

Universal entropy transport far from equilibrium across the BCS-BEC crossover

The transport properties of strongly interacting fermionic systems can reveal exotic states of matter, but experiments and theory have predominantly focused on bulk systems in the hydrodynamic limit describable with linear response coefficients such as electrical and thermal conductivity. In a ballistic channel connecting two superfluid reservoirs, recent experiments revealed a far-from-equilibrium regime beyond linear hydrodynamics where particle and entropy currents respond nonlinearly to biases of chemical potential and temperature, and their ratio is robust to the channel geometry. However, the origin of this robustness and its relation to the strong interparticle interactions remain unknown. Here, we study the coupled transport of particles and entropy tuning the interaction across the Bardeen-Cooper-Schrieffer to Bose-Einstein condensate (BCS-BEC) crossover, the reservoir degeneracy across the superfluid phase transition, as well as the local potentials and confinement of the channel. Surprisingly, the entropy advectively transported per particle depends only on the interactions and reservoir degeneracy and not on the details of the channel, suggesting that this property has its origin in the universal equilibrium properties of the reservoirs. In contrast, the magnitudes of the advective and diffusive entropy currents vary significantly with the channel details. The advective current increases monotonically towards the BEC side, which can be largely explained by the estimated superfluid gap in the channel. The Wiedemann-Franz law that links the advective and diffusive currents in Fermi liquids is most egregiously violated at unitarity, suggesting a change in the nature of the excitations responsible for entropy diffusion near unitarity. These observations pose fundamental questions regarding transport phenomena in strongly interacting Fermi systems far from equilibrium.

cond-mat.quant-gas↗

arXiv2023

Irreversible entropy transport enhanced by fermionic superfluidity

The nature of particle and entropy flow between two superfluids is often understood in terms of reversible flow carried by an entropy-free, macroscopic wavefunction. While this wavefunction is responsible for many intriguing properties of superfluids and superconductors, its interplay with excitations in non-equilibrium situations is less understood. Here, we observe large concurrent flows of both particles and entropy through a ballistic channel connecting two strongly interacting fermionic superfluids. Both currents respond nonlinearly to chemical potential and temperature biases. We find that the entropy transported per particle is much larger than the prediction of superfluid hydrodynamics in the linear regime and largely independent of changes in the channel's geometry. In contrast, the timescales of advective and diffusive entropy transport vary significantly with the channel geometry. In our setting, superfluidity counterintuitively increases the speed of entropy transport. Moreover, we develop a phenomenological model describing the nonlinear dynamics within the framework of generalised gradient dynamics. Our approach for measuring entropy currents may help elucidate mechanisms of heat transfer in superfluids and superconducting devices.

cond-mat.quant-gas↗

arXiv2023

DC transport in a dissipative superconducting quantum point contact

We study the current-voltage characteristics of a superconducting junction with particle losses at the contacts. We adopt the Keldysh formalism to compute the steady-state current for varying transmission of the contact. In the low transmission regime, the dissipation leads to an enhancement of the current at low bias, a nonmonotonic dependence of current on dissipation, and the emergence of new structures in the current-voltage curves. The effect of dissipation by particle loss is found to be qualitatively different from that of a finite temperature and a finite inelastic scattering rate in the reservoirs.

cond-mat.quant-gas↗

arXiv2022

Superfluid signatures in a dissipative quantum point contact

We measure superfluid transport of strongly interacting fermionic lithium atoms through a quantum point contact with local, spin-dependent particle loss. We observe that the characteristic non-Ohmic superfluid transport enabled by high-order multiple Andreev reflections transitions into an excess Ohmic current as the dissipation strength exceeds the superfluid gap. We develop a model with mean-field reservoirs connected via tunneling to a dissipative site. Our calculations in the Keldysh formalism reproduce the observed nonequilibrium particle current, yet do not fully explain the observed loss rate or spin current.

cond-mat.quant-gas↗

arXiv2021

Flat band transport and Josephson effect through a finite-size sawtooth lattice

We study theoretically the transport through a finite-size sawtooth lattice coupled to two fermionic reservoirs kept in the superfluid state. We focus on the DC Josephson effect and find that the flat band states of the sawtooth lattice can support larger critical current and at higher temperature than the dispersive band states. However, for this to occur the boundary states of the finite-size lattice need to be tuned at resonance with the bulk flat band states by means of additional boundary potentials. We show that transport in a two-terminal configuration can reveal the salient features of the geometric contribution of flat band superconductivity, namely the linear dependence of key quantities, such as the critical current and critical temperature, on the interaction. Our results are based on parameters of a realistic experimental lattice potential, and we discuss the conditions one needs to reach to observe the predicted effects experimentally.

cond-mat.quant-gas↗

arXiv2020

Interaction-Assisted Reversal of Thermopower with Ultracold Atoms

We study thermoelectric currents of neutral, fermionic atoms flowing through a mesoscopic channel connecting a hot and a cold reservoir across the superfluid transition. The thermoelectric response results from a competition between density-driven diffusion from the cold to the hot reservoir and the channel favoring transport of energetic particles from hot to cold. We control the relative strength of both contributions to the thermoelectric response using an external optical potential in a nearly non-interacting and a strongly-interacting system. Without interactions, the magnitude of the particle current can be tuned over a broad range but is restricted to flow from hot to cold in our parameter regime. Strikingly, strong interparticle interactions additionally reverse the direction of the current. We quantitatively model ab initio the non-interacting observations and qualitatively explain the interaction-assisted reversal by the reduction of entropy transport due to pairing correlations. Our work paves the way to studying the coupling of spin and heat in strongly correlated matter using spin-dependent optical techniques with cold atoms.

cond-mat.quant-gas↗

arXiv2019

Quantized conductance through a dissipative atomic point contact

Signatures of quantum transport are expected to quickly vanish as dissipation is introduced in a system. This dissipation can take several forms, including that of particle loss, which has the consequence that the total probability current is not conserved. Here, we study the effect of such losses at a quantum point contact (QPC) for ultracold atoms. Experimentally, dissipation is provided by a near-resonant optical tweezer whose power and detuning control the loss rates for the different internal atomic states as well as their effective Zeeman shifts. We theoretically model this situation by including losses in the Landauer-B\"uttiker formalism over a wide range of dissipative rates. We find good agreement between our measurements and our model, both featuring robust conductance plateaus. Finally, we are able to map out the atomic density by varying the position of the near-resonant tweezer inside the QPC, realizing a dissipative scanning gate microscope for cold atoms.

cond-mat.quant-gas↗