A new phase of quantum matter: a surface spin-orbital chiral metal

Chirality is a property of extreme importance in science. For example, it is fundamental to understand DNA, thus the origin of life as we know it. When talking about chirality, one refers to a property of an object with a well-defined handiness, thus distinguishable from its mirror image. For examples, many molecules, such as amino acids, are chiral, just like our hands are.

In condensed matter physics, electrons are “chiral” in the sense that their spin and orbital degrees of freedom are not conserved upon mirror operation and possess a defined handiness. When this happens, such electrons may promote the appearance of novel quantum phenomena. For example, they can generate forms of magnetism that cannot be described by classical rules and that are very elusive to be detected by using standard probes.

Over the last few decades, bulk materials with chiral electrons have been discovered and they have been immediately thought to offer a platform to develop novel spintronics. However, their physics has been elusive in low dimensional systems, such as at the surfaces of compounds or in layered materials. The present study fills this gap. Driven by a theoretical prediction, the researchers have been able to detect signatures attributable to spin-orbital chirality in the surface of the metal oxide Sr₂RuO₄, meaning that both time and mirror symmetry are broken as result of such an effect. Here, chirality is responsible for breaking the spin, orbital, and mainly the spin-orbital component upon mirror symmetry. This stabilizes unconventional magnetism at the surface and opens potential pathways to look for similar effects in other low dimensional systems.

In the quantum phenomenon that was discovered, the chirality of so-called spin-orbital currents was detected by studying the interaction between light and matter, in which a suitably polarized photon can emit an electron from the surface of the material with a well-defined orbital and angular momentum state. The experiments were carried out at the APE-LE beamline of the Elettra synchrotron, exploiting the ability of polarizing the light-probe circularly, and the spin-resolution available at the experimental end-station. Researchers combined circularly polarized light, able to couple to the orbital and angular momentum, with a spin-detector, able to filter the electrons according to their spin.

Figure 1: a) The possible charge, spin and orbital currents that can be created in a material; b) examples of mirror-preserving (top) and mirror-broken (bottom) configurations for the case of an orbital current; c,d) ARPES data collected at the APE-LE circular dichroism (CD). The total signal is shown as (c) Fermi surface and (d) energy versus momentum of the investigated material.

As one can see in Figure 1a, as a charge gives rise to a current, and a spin gives rise to a spin-current, more complex quantities can be associated to a current as well. Some of these quantities, such as a spin-orbital quadrupole, give rise to a spin-orbital quadruple current, which breaks the time-reversal and mirror symmetries. The combination of circular light from the synchrotron and spin-selective photoelectron spectroscopy (Figure 2) can effectively detect these symmetry breakings and detect asymmetries in the spin channels. Ultimately, such asymmetries are the signature of the spin-orbital chiral nature of our system.

Figure 2: Circular dichroism and spin-resolved photoemission data showing the amplitude of the spin-asymmetries. The data, collected for three momentum values (a,b,c) shows the total value of the spin-up (red), spin-down (blue) and spin-integrated (gray) signal collected by using circular dichroism. As one ca see, the red and blue curves, which in a completely standard system should be equal and opposite, here are significantly different, demonstrating the spin-orbital chiral nature of the system.

This work is of fundamental importance to shed light on hidden and elusive phenomena in condensed matter physics and paves the way towards the opportunity of exploiting unconventional forms of magnetism in low dimensions even in ultrathin applications.

This research was conducted by the following research team:

Federico Mazzola^1,2, Wojciech Brzezicki^3,4, Maria Teresa Mercaldo⁵, Anita Guarino⁶, Chiara Bigi⁷, Jill A. Miwa⁸, Domenico De Fazio¹, Alberto Crepaldi⁹, Jun Fujii², Giorgio Rossi^2,10, Pasquale Orgiani², Sandeep Kumar Chaluvadi², Shyni Punathum Chalil², Giancarlo Panaccione², Anupam Jana², Vincent Polewczyk², Ivana Vobornik², Changyoung Kim¹¹, Fabio Miletto-Granozio¹², Rosalba Fittipaldi⁶, Carmine Ortix⁵, Mario Cuoco⁶ and Antonio Vecchione^6
1 Department of Molecular Sciences and Nanosystems, Ca’ Foscari University of Venice, Venice, Italy.
² Istituto Officina dei Materiali, Consiglio Nazionale delle Ricerche, Trieste, Italy.
³ Institute of Theoretical Physics, Jagiellonian University, Kraków, Poland.
⁴ International Centre for Interfacing Magnetism and Superconductivity with Topological Matter, Institute of Physics, Polish Academy of Sciences, Warsaw, Poland.
⁵ Dipartimento di Fisica “E. R. Caianiello”, Università di Salerno, Fisciano, Italy.
⁶ Istituto SPIN, Consiglio Nazionale delle Ricerche, Fisciano, Italy.
⁷ Synchrotron SOLEIL, Saint-Aubin, France.
⁸ Department of Physics and Astronomy, Interdisciplinary Nanoscience Center, Aarhus University, Aarhus, Denmark.
⁹ Dipartimento di Fisica, Politecnico di Milano, Italy.
¹⁰ Dipartimento di Fisica, Università degli Studi di Milano, Italy.
¹¹ Department of Physics and Astronomy, Seoul National University, Seoul, South Korea.
¹² Istituto SPIN, Consiglio Nazionale delle Ricerche, Naples, Italy.

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