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Experimental evidence for Zeeman spin-orbit coupling
par Revaz Ramazashvili - 12 mars 2021
In recent years, an enormous research effort has been underway to build fundamentally new electronic devices (often called "spintronic devices") that would use the electron spin and charge on an equal footing. A substantial part of this effort has been focusing on understanding and making use of spin-orbit coupling, responsible for the interplay between the electron spin and its orbital motion. Even though spin-orbit coupling has long become a textbook subject, its exploration continues to bring surprises.
For instance, some time ago an LPT physicist showed that, in an external magnetic field, Néel order may give rise to a spin-orbit coupling of an unusual nature, unrelated to the effects known previously. The coupling arises from the Zeeman effect, and thus was termed Zeeman spin-orbit coupling. Being proportional to the applied magnetic field, the coupling is tunable. It emerges from the momentum dependence of the g-tensor, and was predicted to produce a number of spectacular effects, such as excitation of spin resonance transitions by an AC electric rather than magnetic field, a possibility much sought after in spintronics.
However, the Zeeman spin-orbit coupling has not received experimental confirmation yet. That is, not until recently, when the collaboration of an LPT physicist with colleagues from Germany, Russia and Japan produced unequivocal experimental evidence of Zeeman spin-orbit coupling in two very different layered conductors : an organic antiferromagnetic superconductor к-BETS, and a prominent electron-doped superconductor NCCO.
In an earlier work, it was predicted that, in the presence of Zeeman spin-orbit coupling, transverse field (with respect to the staggered magnetization) leaves the electron Landau levels spin-degenerate. And this is precisely what the experiment demonstrated by focusing on the angle dependence of the quantum oscillation amplitude. In addition to its fundamental importance, the Zeeman spin-orbit coupling opens new possibilities for spin manipulation, much sought after in the current effort to harness electron spin for future spintronic applications.
To find out more, the reader is invited to consult the paper that recently appeared in npj Quantum Materials.
The figure : (a) Fermi surface of κ-BETS in the paramagnetic (PM) phase. (b) Interlayer magnetoresistance of the κ-BETS sample, recorded at T = 0.5 K. The vertical dashed line indicates the transition between the low-field antiferromagnetic (AF) and high-field paramagnetic (PM) phases. The insets show the fast Fourier transforms (FFT) of the quantum oscillations. (c) The small reconstructed Fermi surface δ (shaded) in the antiferromagnetic (AF) phase.