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Connecting strongly correlated superfluids by a quantum point contact

Dominik Husmann, Shun Uchino, Sebastian Krinner, Lebrat, M., Thierry Giamarchi, Tilman Esslinger, Jean-Philippe Brantut

Published Jan 1, 2015• 56 citations
OpenAlex

Abstract

Point contacts provide simple connections between macroscopic particle reservoirs. In electric circuits, strong links between metals, semiconductors, or superconductors have applications for fundamental condensed-matter physics as well as quantum information processing. However, for complex, strongly correlated materials, links have been largely restricted to weak tunnel junctions. We studied resonantly interacting Fermi gases connected by a tunable, ballistic quantum point contact, finding a nonlinear current-bias relation. At low temperature, our observations agree quantitatively with a theoretical model in which the current originates from multiple Andreev reflections. In a wide contact geometry, the competition between superfluidity and thermally activated transport leads to a conductance minimum. Our system offers a controllable platform for the study of mesoscopic devices based on strongly interacting matter.

Related papers

Connecting strongly correlated superfluids by a quantum point contact

Dominik Husmann, Shun Uchino, Sebastian Krinner +4 more

Point contacts provide simple connections between macroscopic particle reservoirs. In electric circuits, strong links between metals, semiconductors, or superconductors have applications for fundamental condensed-matter physics as well as quantum information processing. However, for complex, strongly correlated materials, links have been largely restricted to weak tunnel junctions. We studied resonantly interacting Fermi gases connected by a tunable, ballistic quantum point contact, finding a nonlinear current-bias relation. At low temperature, our observations agree quantitatively with a theoretical model in which the current originates from multiple Andreev reflections. In a wide contact geometry, the competition between superfluidity and thermally activated transport leads to a conductance minimum. Our system offers a controllable platform for the study of mesoscopic devices based on strongly interacting matter.

arXiv:1508.00578•Science•Dec 17, 2015•134 citations

Entropy transport through a superfluid quantum point contact: A Keldysh field-theory approach

Davide Bertolusso, C. J. Bolech, Thierry Giamarchi

We study the matter and entropy transport between two ultracold neutral Fermi-gas reservoirs linked by a quantum point contact under a chemical-potential gradient. We describe the two leads with a BCS mean-field model and derive the current-bias characteristics for both particle and entropy transport. We compute the out-of-equilibrium steady-state currents by using the Keldysh formalism. In accordance with previous works in the literature, we confirm the well-known behavior for the particle current and extend the computation to the entropy current in the BCS regime. The entropy current shows an oscillatory behavior at low voltage in the ballistic junction limit. We analyze the results for a wide range of values of the junction's transparency. We also compare our findings with experimental results in cold atomic gases in the unitary regime.

arXiv:2605.00679•Physical Review Research•Aug 17, 2026•0 citations

Mesoscopic transport phenomena in a unitary Fermi gas

Dominik Husmann

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.

•Jan 1, 2018•2 citations

INTRODUCTION TO THE TRANSPORT OF FERMI GASES THROUGH QUANTUM POINT CONTACTS

Chunyuan SHAN, Xiaoyu Dai, Boyang Liu

Transport measurement is an important tool for studying the fundamental properties of matter. Among them, transport through quantum point contacts is a very interesting phenomenon in mesoscopic physics. In non-interacting or weakly interacting Fermi systems, the conductivity exhibits quantized behavior as a function of gate voltage, revealing the quantum nature of matter. In recent years, transport in cold atomic systems has become one of the frontiers in this field. Cold atomic systems have highly are controllable and exhibit many novel phases, leading to new experimental phenomena that cannot be observed in traditional material physics. This article will review some of the latest experimental progress in transport through quantum point contacts in cold atomic Fermi gases, and will also introduce the non-equilibrium field theoretical methods commonly used to study this system.

Wuli yu gongcheng.•Oct 1, 2023•0 citations

Controlled transport between Fermi superfluids through a quantum point contact

Juan Yao, Boyang Liu, Mingyuan Sun +1 more

Recent advances in experimental techniques allow one to create a quantum point contact between two Fermi superfluids in cold atomic gases with a tunable transmission coefficient. In this Rapid Communication we propose that three distinct behaviors of charge transports between two Fermi superfluids can be realized in this single setup, which are the multiple Andreev reflection, the self-trapping, and the Josephson oscillation. We investigate the dynamics of atom number difference between two reservoirs for different initial conditions and different transmission coefficients, and present a coherent picture of how the crossover between different regimes takes place. Our results can now be directly verified in the current experimental system.

arXiv:1712.07125•Physical Review A•Oct 8, 2018•16 citations

Asymmetry and nonlinearity of current-bias characteristics in superfluid–normal-state junctions of weakly interacting Bose gases

Shun Uchino

We uncover current-bias characteristics of superfluid-normal state junctions with weakly-interacting Bose gases. It is shown that in the presence of a chemical potential bias the characteristics can strongly be asymmetric for origin. The salient feature that is absent in the fermionic counterpart arises from a tunneling process associated with a condensate and a bosonic Andreev reflection process. It turns out that such processes are intrinsically nonlinear and therefore do not obey Ohm's law even at a low bias. In addition, the remaining processes are found to obey Ohm's law and become dominant for transport driven by a temperature bias.

arXiv:2203.06927•Physical Review A•Jul 26, 2022•2 citations
Point contacts provide simple connections between macroscopic particle reservoirs. In electric circuits, strong links between metals, semiconductors, or superconductors have applications for fundamental condensed-matter physics as well as quantum information processing. However, for complex, strongly correlated materials, links have been largely restricted to weak tunnel junctions. We studied resonantly interacting Fermi gases connected by a tunable, ballistic quantum point contact, finding a nonlinear current-bias relation. At low temperature, our observations agree quantitatively with a theoretical model in which the current originates from multiple Andreev reflections. In a wide contact geometry, the competition between superfluidity and thermally activated transport leads to a conductance minimum. Our system offers a controllable platform for the study of mesoscopic devices based on strongly interacting matter.
Point contacts provide simple connections between macroscopic particle reservoirs. In electric circuits, strong links between metals, semiconductors, or superconductors have applications for fundamental condensed-matter physics as well as quantum information processing. However, for complex, strongly correlated materials, links have been largely restricted to weak tunnel junctions. We studied resonantly interacting Fermi gases connected by a tunable, ballistic quantum point contact, finding a nonlinear current-bias relation. At low temperature, our observations agree quantitatively with a theoretical model in which the current originates from multiple Andreev reflections. In a wide contact geometry, the competition between superfluidity and thermally activated transport leads to a conductance minimum. Our system offers a controllable platform for the study of mesoscopic devices based on strongly interacting matter.
We study the matter and entropy transport between two ultracold neutral Fermi-gas reservoirs linked by a quantum point contact under a chemical-potential gradient. We describe the two leads with a BCS mean-field model and derive the current-bias characteristics for both particle and entropy transport. We compute the out-of-equilibrium steady-state currents by using the Keldysh formalism. In accordance with previous works in the literature, we confirm the well-known behavior for the particle current and extend the computation to the entropy current in the BCS regime. The entropy current shows an oscillatory behavior at low voltage in the ballistic junction limit. We analyze the results for a wide range of values of the junction's transparency. We also compare our findings with experimental results in cold atomic gases in the unitary regime.
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.
Transport measurement is an important tool for studying the fundamental properties of matter. Among them, transport through quantum point contacts is a very interesting phenomenon in mesoscopic physics. In non-interacting or weakly interacting Fermi systems, the conductivity exhibits quantized behavior as a function of gate voltage, revealing the quantum nature of matter. In recent years, transport in cold atomic systems has become one of the frontiers in this field. Cold atomic systems have highly are controllable and exhibit many novel phases, leading to new experimental phenomena that cannot be observed in traditional material physics. This article will review some of the latest experimental progress in transport through quantum point contacts in cold atomic Fermi gases, and will also introduce the non-equilibrium field theoretical methods commonly used to study this system.
Recent advances in experimental techniques allow one to create a quantum point contact between two Fermi superfluids in cold atomic gases with a tunable transmission coefficient. In this Rapid Communication we propose that three distinct behaviors of charge transports between two Fermi superfluids can be realized in this single setup, which are the multiple Andreev reflection, the self-trapping, and the Josephson oscillation. We investigate the dynamics of atom number difference between two reservoirs for different initial conditions and different transmission coefficients, and present a coherent picture of how the crossover between different regimes takes place. Our results can now be directly verified in the current experimental system.
We uncover current-bias characteristics of superfluid-normal state junctions with weakly-interacting Bose gases. It is shown that in the presence of a chemical potential bias the characteristics can strongly be asymmetric for origin. The salient feature that is absent in the fermionic counterpart arises from a tunneling process associated with a condensate and a bosonic Andreev reflection process. It turns out that such processes are intrinsically nonlinear and therefore do not obey Ohm's law even at a low bias. In addition, the remaining processes are found to obey Ohm's law and become dominant for transport driven by a temperature bias.