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Spin-dependent energy transport in a superconductor

As conventional superconductors in their ground state are spinless and cannot transport energy, it might be assumed that they would not be of interest for the emerging field of spin caloritronics, or spin- and heat-based electronics. Nevertheless, superconducting excitations (known as quasiparticles) can be spinful and also carry heat. The NS2 group (Nanostructures at the Nanosecond timescale) at the LPS have found that the transport of heat by superconducting quasiparticles can be dependent on their spin ; this means, for example, that spin up and down quasiparticles can have different effective temperatures.

Researchers from the NS2 group studied thin-film superconducting aluminium, patterned into a wire. They generated quasiparticles by driving a current into the superconducting wire from a normal metal electrode across a tunnel barrier (blue electrode in figure). When this is done in the presence of an applied parallel magnetic field, the injected quasiparticles are spin-polarised, i.e. there are more spin down quasiparticles than spin up ones.

A short distance (< 1 µm to several µm) from the injector, are superconducting detector electrodes (pink in figure), also in contact with the wire through a tunnel barrier. Spin-sensitive spectroscopic measurements at these detectors allowed the research team to understand how the quasiparticles are distributed in energy in the superconductor..

When spin up and down quasiparticles carry different energy currents, the ‘spin energy mode’ can be excited. In this new excitation mode, spin up and down quasiparticles are distributed differently in energy. This leads to different energy currents carried by quasiparticles of opposite spins. Unlike in normal metals, the spin energy mode in superconductors gives rise to a charge imbalance (i.e. different numbers of positively- and negatively-charged quasiparticles). This charge imbalance appears in a well-circumscribed region of energy and magnetic field. The NS2 researchers’ spectroscopic measurements also allowed them to observe this phenomenon and thus unambiguously identify the spin energy mode in a superconductor.

Nanofabricated device

Reference :
M Kuzmanović, BY Wu, M Weideneder, CHL Quay & M Aprili. Evidence for spin-dependent energy transport in a superconductor. Nature Communications 11, 4336 (2020).