How to distinguish single-particle and pair currents

The vertical axis is temperature divided by the Fermi temperature and the horizontal axis is the attractive interaction strength. The red solid curve shows the critical temperature of the superfluid. The blue dashed line shows characteristic temperatures between normal and pseudogap phases. The green dotted line shows the characteristic temperatures between pseudogap and bound dimer phases. (b) Schemes for the tunnel transport process in the normal phase, the pseudogap phase, and the dimer bound phases. The Fano factor, the ratio between current noise and current, helps us distinguish pair current and single particle current. Photo credit: Tajima et. Al. 2023

When you cool low-density atomic gas to ultralow temperatures (−273 °C), you get a new state of matter called Bose-Einstein condensate (BEC). A BEC consists of strongly coupled diatomic molecules that, according to quantum mechanics, behave like a collective wave. If the pairing strength between them is reduced – for example by increasing the magnetic field – the atoms form Cooper pairs according to the Nobel Prize-winning Bardeen-Cooper-Schrieffer (BCS) theory.

The process is called BCS-BEC crossover. And the theory forms the basis for superfluids and superconductors, materials that have neither viscosity nor electrical resistance. Hiroyuki Tajima and his team from the University of Tokyo proposed a new method to distinguish current carriers in BCS-BEC crossover. The key lies in the fluctuations in the current.

Electronic devices display images thanks to electrons moving in a conductor – also known as single particle current. Your device can heat up due to the resistance caused by collisions of electrons in the conductor, which release electrical energy as heat. But superconductors show no resistance to the flow of current and save a lot of energy. This is made possible by paired electrons that would otherwise have repelled each other due to their negative charge. In other words, the current in superconductors is mainly due to pair tunnel transport, which involves moving paired current carriers and not a single particle current carrier.

Tajima and his team studied the quantum transport phenomena using an ultracold atomic Fermi gas. It is an artificial quantum matter that mimics an electron or fermion system with tunable interaction strength. “In order to understand the non-trivial transport, we have to distinguish whether single-particle tunneling or pair tunneling predominates in strongly interacting gases,” Tajima said. “Identifying single particle tunneling and pair tunneling is crucial for understanding quantum transport not only in cold atomic systems but also in high-temperature superconductors.”

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Because the researchers could control the interactions between particles, the atomic gas allowed them to study quantum many-body physics systematically. The gas shows a normal phase when the interaction strength between atoms is weak. At this stage it behaves as a relatively good conductor like a metal with electrical resistance. So one can expect a single particle current (electron tunneling) under a chemical potential bias (voltage).

As you increase the interaction strength, the gas transitions to the bonded dimer phase via an intervening pseudogap phase. In the pseudogap phase, the BCS-BEC crossover takes place at low temperatures. At a critical temperature for a given interaction strength, the atomic gas becomes superfluid with no viscosity. Below the phase transition temperature, Cooper pairs form and lead to pair current. In the Pseudogap phase, non-superfluid Cooper pairs form due to attractive interactions, resulting in an anomalous current in this region. But in the bound dimer phase, the pair current is dominant. Tajima’s team found a way to distinguish the current carriers in each phase by measuring an observable macroscopic property.

The team showed that the current fluctuations, quantified as the Fano factor, can distinguish single-particle and pair currents in a tunnel transport of strongly interacting Fermi gases. The value of the Fano factor is 1 for single particle current and 2 for pair current. In the future, her approach can be applied to other unconventional superconductors and various many-body phenomena realized in cold atoms.

Fano factor (the ratio between current noise and current) in ultracold Fermi gases with controllable attractive interaction strength. Photo credit: Tajima et. Al. 2023

“Our results show that it is possible to identify the microscopic transport carriers from the macroscopic observables (ie current and noise) even in strongly correlated quantum matter,” adds Tajima.

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“This collaboration took place entirely via online discussions, which surprisingly allowed us to share interdisciplinary knowledge, leading to this research.”

The study is published in the journal PNAS Nexus.

More information:
Hiroyuki Tajima et al, Non-equilibrium noise as a probe of pair tunnel transport in BCS-BEC crossover, PNAS Nexus (2023). DOI: 10.1093/pnasnexus/pgad045

Journal Information:
PNAS Nexus

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