Signatures of a surface spin-orbital chiral metal

Nature 626, 752-758 (2024)

Dichroic and spin-dichroic effect in presence of spin-orbital current. (a) Mirror-symmetry preserving spin current (top) and mirror-symmetry breaking spin-orbital current (bottom). The inset shows an impulse of left or right circularly polarized light used in ARPES. (b) Energy-dependent circular dichroism for a fixed k-point. (c) Circular dichroism summed over opposite k-points for spin up, spin down and the sum of both.

In an article published in the journal Nature, signatures of spin-orbital currents breaking both time-reversal and all crystalline symmetries were confirmed in an exotic surface chiral phase of Sr₂RuO₄. It was possible to find signatures of the sought for spin-orbital currents thanks to the use of the circularly-polarized angular- and spin-resolved photoemission technique CP-spin ARPES through contributions of the IF PAN scientist Wojciech Brzezicki.

Chiral phenomena are situations where things "seen in the mirror" appear differently than they do in reality. For example, the direction of circular polarization of a photon observed in a mirror will be opposite to what is directly seen by an observer. Chirality is also an important concept in biochemistry because molecules and molecular structures found in organisms are chiral, and typically only one of the chiral forms is present in living organisms. Because of this, organisms that ingest chiral compounds usually can metabolize only one correct form, and different chiral forms of a pharmaceutical substance typically have varying strength or mode of action on the organism. In condensed matter, chiral properties of elementary excitations of the electronic gas, quasi-particles, typically occur in connection with low symmetry of crystal structure and relativistic effects of electron interactions. The ability to analyze and control these phenomena appears currently intriguing for potential applications of chiral phases in processing information in quantum computing devices resilient to fluctuations and disturbances.

The Nature publication concerned the identification and study of the exotic surface chiral phase of the currently intensively researched low temperature semiconductor Sr₂RuO₄. This hypothetical phase contains ground-state spin-orbital currents that break both time-reversal and all crystalline symmetries, hence its naming as chiral. The method used to observe such symmetry-broken states is circularly-polarized angular- and spin-resolved photoemission (CP-spin-ARPES). In this context, the difference of measured intensity between left and right polarizations gives the circular dichroism measure (CD). Its intensity can be related to the electron’s average angular momentum and typically takes opposite signs for opposite quasimomenta. The important contribution of Prof. Brzezicki to the work was the theoretical finding that in the presence of ground-state spin-orbital currents, standard CD exhibits this usual opposite sign behavior cleanly whereas spin-resolved CD does not. Therefore the CP-spin-ARPES technique was demonstrated to have the capacity to detect this unusual phase and similar phenomena.

Publications

Signatures of a surface spin-orbital chiral metal

Federico Mazzola, Wojciech Brzezicki, Maria Teresa Mercaldo, Anita Guarino, Chiara Bigi, Jill A. Miwa, Domenico De Fazio, Alberto Crepaldi, Jun Fujii, Giorgio Rossi, Pasquale Orgiani, Sandeep Kumar Chaluvadi, Shyni Punathum Chalil, Giancarlo Panaccione, Anupam Jana, Vincent Polewczyk, Ivana Vobornik, Changyoung Kim, Fabio Miletto-Granozio, Rosalba Fittipaldi, Carmine Ortix, Mario Cuoco & Antonio Vecchione

Nature 626, 752-758 (2024)

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