Mar 9, 2023

Quantum crossover: How to distinguish single-particle and pair currents

Posted by in categories: particle physics, quantum physics

If you cool down low-density atomic gas to ultralow temperatures (−273°C), you get a new state of matter called the Bose-Einstein Condensate (BEC). A BEC has strongly coupled two-atom molecules behaving like a collective wave following quantum mechanics. If you reduce the pairing strength between them—for example, by increasing the magnetic field—the atoms form Cooper pairs according to Bardeen-Cooper-Schrieffer (BCS) theory (which won a Nobel Prize).

The process is called BCS-BEC crossover. And the theory forms the basis of superfluids and superconductors, materials that do not display viscosity or . Hiroyuki Tajima and his team from the University of Tokyo proposed a new method to distinguish current carriers in the BCS-BEC crossover. The key is in the fluctuations of current.

Electronic devices display images thanks to electrons moving in a conductor—aka single-particle current. Your device may heat up due to the resistance caused by collisions of electrons in the conductor that dissipate electric energy as heat. But superconductors show zero resistance to current flow, saving lots of energy. This is possible because of paired electrons, which would have otherwise repelled each other due to their negative charge. In other words, the current in superconductors is mainly due to the pair-tunneling transport involving moving paired-current carriers rather than a single-particle current carrier.

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