We report on coupled heat and particle transport measurements through a quantum point contact (QPC) connecting two reservoirs of resonantly interacting, finite temperature Fermi gases. After heating one of them, we observe a particle current flowing from cold to hot. We monitor the temperature evolution of the reservoirs and find that the system evolves after an initial response into a nonequilibrium steady state with finite temperature and chemical potential differences across the QPC. In this state any relaxation in the form of heat and particle currents vanishes. From our measurements we extract the transport coefficients of the QPC and deduce a Lorenz number violating the Wiedemann-Franz law by one order of magnitude, a characteristic persisting even for a wide contact. In contrast, the Seebeck coefficient takes a value close to that expected for a noninteracting Fermi gas and shows a smooth decrease as the atom density close to the QPC is increased beyond the superfluid transition. Our work represents a fermionic analog of the fountain effect observed with superfluid helium and poses challenges for microscopic modeling of the finite temperature dynamics of the unitary Fermi gas.
We report on coupled heat and particle transport measurements through a quantum point contact (QPC) connecting two reservoirs of resonantly interacting, finite temperature Fermi gases. After heating one of them, we observe a particle current flowing from cold to hot. We monitor the temperature evolution of the reservoirs and find that the system evolves after an initial response into a nonequilibrium steady state with finite temperature and chemical potential differences across the QPC. In this state any relaxation in the form of heat and particle currents vanishes. From our measurements we extract the transport coefficients of the QPC and deduce a Lorenz number violating the Wiedemann-Franz law by one order of magnitude, a characteristic persisting even for a wide contact. In contrast, the Seebeck coefficient takes a value close to that expected for a noninteracting Fermi gas and shows a smooth decrease as the atom density close to the QPC is increased beyond the superfluid transition. Our work represents a fermionic analog of the fountain effect observed with superfluid helium and poses challenges for microscopic modeling of the finite temperature dynamics of the unitary Fermi gas.
We theoretically investigate the thermal-transport properties of a unitary Fermi gas between two reservoirs connected by a quantum point contact. We find several distinguished properties that are qualitatively different from those of weak or noninteracting gas systems. The particle transport figure of merit is drastically enhanced in the unitary regime. The Lorentz number violates the Wiedemann-Franz law, demonstrating the breakdown of Fermi liquid. These transport properties are the hallmarks of the unitary Fermi gas and are attributed to the existence of preformed Cooper pairs. The variation of the thermal-transport coefficients due to the changing of the transmission coefficient is also studied.
We study energy and particle transport for one-dimensional strongly interacting bosons through a ballistic single channel connecting two atomic reservoirs. We show the emergence of particle- and energy-current separation, leading to the violation of the Wiedemann-Franz law. As a consequence, we predict different time scales for the equilibration of temperature and particle imbalances between the reservoirs. Going beyond the linear spectrum approximation, we show the emergence of thermoelectric effects, which could be controlled by either tuning interactions or the temperature. Our results describe, in a unified picture, fermions in condensed-matter devices and bosons in ultracold atom setups. We conclude by discussing the effects of a controllable disorder.
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.
In this thesis we report on measurements of particle and heat transport through a mesoscopic constriction with a two-component Fermi gas featuring tunable interactions.To this end we realize the ultracold atom equivalent of a two-terminal transport setup by connecting two reservoirs of fermionic lithium 6 by a quantum point contact.This mesoscopic structure is shaped by introducing strong confinement in the center of a cigar-shaped atomic cloud, splitting it into two separated clouds that act as thermodynamic reservoirs.Transport of particles and heat is induced by applying a difference in particle number or temperature.The hereby driven currents through the contact are observed by measuring the time evolution of the spatially resolved density distribution in the reservoirs.Our system offers high control and tunability, allowing access to a broad parameter regime.A high-resolution microscope enables us to project precise mesoscopic potential structure onto the cloud, such as a quantum point contact with one or more channels accessible.Using the specific properties of lithium 6 we can tune the interaction strength between the atoms from non-interacting to strongly correlated regimes, where the superfluid character of the constituents dominates the transport properties.We characterize the quantum point contact in a setting of weakly interacting reservoirs and show the first observation of quantized conductance in neutral matter, where each transverse energy mode in the channel contributes an amount of 1/h to the total conductance.Then we access a regime where only up to few modes are occupied and affirm the universal character of conductance quantization by showing stability towards changes in the channel geometry.The data shows good agreement with the Landauer-Büttiker formula without free parameters.Using the broad Feshbach resonance of lithium 6 we then tune the interaction strength to unitarity, a regime in which the reservoirs are strongly correlated superfluids.Here we observe a non-linear concave current-bias characteristics for a few-mode quantum point contact.This behavior is successfully modeled in a non-equilibrium Keldysh-Green formalism, where the large current at small biases is interpreted in terms of multiple Andreev reflections at a superfluid-normal-superfluid junction.With increasing temperature of the reservoirs towards the critical temperature of the superfluid transition, a linear current-bias relation is restored with conductances that are order of magnitude larger than what is expected from Fermi liquid theory.We then extend this study to the measurement of the thermoelectric response upon imposing a temperature bias.Here we find suppressed heat diffusion due to a suppressed Lorenz number and finite Seebeck coefficient, which brings the system towards a non-equilibrium steady-state with finite temperature and chemical potential differences.i ZUSAMMENFASSUNG In dieser Arbeit untersuchen wir Teilchen-und Wärme-Transport durch mesoskopische Strukturen zwischen zwei Reservoiren, welche aus ultrakalten, Zweikomponenten-Fermigasen mit verstellbaren Wechselwirkungen bestehen.Dazu realisieren wir eine Zweipunkt-Transportanordnung für ultrakalte Atome, indem wir zwei Reservoire bestehend aus fermionischem Lithium 6 mit einem Quantenpunktkontakt verbinden.Wir formen den Quantenpunktkontakt durch starke Einschnürung im Zentrum einer Zigarren-förmigen Atomwolke, welche diese in zwei separierte Wolken teilt.Diese agieren als thermodynamische Reservoire.Um Wärme-und Teilchen-Transport zu messen, präparieren wir eine Differenz in Teilchenzahl oder Temperatur zwischen den Reservoirs induziert.Den dadurch induzierten Strom durch den Kontakt messen wir durch die zeitliche Evolution der räumlich aufgelösten Dichteverteilung der Reservoire.Unser System bietet hohe Kontrolle und Verstellbarkeit und bietet Zugang zu einem breiten Parameterregime.Durch ein hochauflösendes Mikroskop können wir präzise mesoskopische Strukturen auf die Wolke projizieren.Ein Beispiel dazu ist der Quantenpunktkontakt mit ein oder mehreren offenen Kanälen.Die spezifischen Eigenschaften von Lithium 6 erlauben es, die Wechselwirkung zwischen den Atomen zu verstellen, von einem nicht-wechselwirkenden zu einem stark korrelierten Regime, in welchem die superfluide Phase der Teilchen das Transportverhalten dominiert.Wir charakterisieren den Quantenpunktkontakt mit schwachen Wechselwirkungen in den Reservoiren und zeigen erstmalig quantisierte Leitfähigkeit mit neutralen Teilchen.Weiter gehen wir in ein Regime wo nur wenige Moden besetzt sind und bestätigen den universellen Charakter der Quantisierung der Leitfähigkeit, indem wir ihre Stabilität gegen Änderungen in der Geometrie des Kanals zeigen.Unsere Daten stimmen gut mit dem Landauer-Büttiker Formalismus ohne freie Parameter überein.Wir benutzen eine breite Feshbach-Resonanz in Lithium 6, um die Wechselwirkung zum unitären Limit einzustellen.In diesem Regime sind die Reservoire stark korrelierte Superfluide.Hier messen wir eine nichtlineare konkave Strom-Bias-Kennlinie in einem Quantenpunktkontakt mit wenigen Moden.Dieses Verhalten wird erfolgreich mit einem Nicht-Gleichgewichts Keldysh-Green Formalismus modelliert.Darin werden die starken Ströme bei schwachem Bias als mehrfache Andreev Reflexionen an einem Superfluid-Normal-Superfluid Kontakt interpretiert.Mit steigender Temperatur nahe des superfluiden Übergangs in den Reservoiren sehen wir einen Rückgang zu linearem Storm-Bias Verhalten.Die dabei gemessenen Leitfähigkeiten sind um Größenordnungen höher als was von einer Fermi-Flüssigkeit erwartet würde.Im nächsten Schritt erweitern wir diese Studie um die thermoelektrische Reaktion, indem wir eine Temperatur-Differenz induzieren.Wir finden hier unterdrückte Wärme-Diffusion stammend von einer kleinen Lorenz-Zahl, und einen endlichen Seebeckiii Koeffizienten.Diese führen das System zu einem Nichtgleichgewichtszustand mit endlichen Differenzen in Temperatur und chemischem Potential.Im letzten Experiment verlassen wir das ballistische Regime, indem wir ein modifizierbares eindimensionales Gitter endlicher Länge mit dem mesoskopischen Kanal überlagern.Hier sehen wir die Entstehung einer Bandlücke im Transmissionsspektrum, wenn das Gitter Schritt für Schritt aufgebaut wird.Die Bandlücke ist robust gegen Wechselwirkungen, was wir mit der Formierung einer Luther-Emery-Flüssigkeit erklären.Unsere Implementation eines Transport-Aufbaus für kalte Atome schafft einen neue Art, Phänomene auf einer mesoskopischen Längenskala zu studieren; der Aufbau bietet hohe Einstellbarkeit sowohl in der Geometrie als auch in Wechselwirkungsstärke.Die experimentellen Ergebnisse zeigen faszinierende Eigenschaften des unitären Fermigases, deren Verständnis nach mikroskopischem Modellen verlangen.In nächste Schritten könnte man die Effekte der Wechselwirkunsstärke kombinieren mit Spin-abhängigen Potentialen, womit die Realisierung von exotischen Quanten-Vielteilchenzuständen in Griffweite rückt.
We analyze the charge and thermal transport at a junction of interacting quantum wires close to equilibrium. Within the framework of Tomonaga-Luttinger liquids, we compute the thermal conductance for a wide class of boundary conditions and detail the physical processes leading to the breakdown of the Wiedemann-Franz law at the junction. We show how connecting external reservoirs to the quantum wires affects the conductance tensors close to the various fixed points of the phase diagram of the junction. We therefore distinguish two types of violation of the Wiedemann-Franz law: a ``trivial'' one, independent of the junction dynamics and arising from the breakdown of the Fermi-liquid picture in the wire, and a junction-related counterpart, arising from multiparticle scattering processes at the junction.