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Maximal Rashba-Like Spin Splitting via Kinetic-Energy-Coupled Inversion-Symmetry Breaking

Date: 
Monday, October 23, 2017
Contact: 

Ivana Vobornik: ivana.vobornik@elettra.eu

Research collaboration led by Professor Philip King from University of St. Andrews, and comprising the researchers from Max Planck Institute for Chemical Physics of Solids in Dresden, Institute for Theoretical Physics of the University of Heidelberg and researchers from I05 beamline at Diamond Light Source and APE beamline at Elettra, described a new route to maximise the spin-splitting of surface states.

(a) Bulk and surface Fermi surfaces of PtCoO2 measured by ARPES; (b) Expected spin texture of the surface states; (c) Spin-resolved ARPES measurements of an in-plane spin polarization (⟨Sy⟩) of the Fermi surface for the cut along kx

The electronic properties of surfaces are often different from those of the bulk. In particular, the intrinsically broken symmetries of the surface compared with the bulk of the material allow for appearance of the new electronic surface states. For the systems in which spin-orbit interaction is strong, a non-negligible separation of these states according to their spin takes place. The spin splitting of surface- or interface-localized two-dimensional electron gases is characterized by a locking of the electron spin perpendicular to its momentum.

The spin splitting is at the heart of novel spintronic devices, where both spin and charge degrees of freedom can be utilized, resulting in more efficient data transfer and data storage. For this reason, increasing the spin splitting of the electronic states of interest represents a major challenge. In order to increase the spin splitting two routes are possible: (i) increasing the spin-orbit coupling and (ii) maximizing the influence of inversion symmetry breaking on surfaces.

The magnitude of the spin-orbit coupling increases with the atomic number. For this reason the surfaces of heavy materials, such as lead and bismuth, received a major attention over the last years. However, the observed spin-splittings in these materials remain significantly reduced with respect to what is expected from their atomic spin-orbit interaction, thus (besides their toxicity) not rendering them good candidates for future utilization in spintronics.

King’s team opted for the second route and presented a mechanism for increasing the spin splitting by finding a way for realizing a much larger coupling of inversion-symmetry. The key element that they identified is a pronounced asymmetry of surface hopping energies— that is, a kinetic-energy-coupled inversion-symmetry breaking. Using spin- and angle-resolved photoemission spectroscopy, it was demonstrated that such a strong inversion-symmetry breaking, when combined with spin–orbit interaction, can mediate Rashba-like spin splittings that become much larger than typically expected. The energy scale of the inversion-symmetry breaking is so large that the spin splitting in the CoO2- and RhO2- derived surface states of delafossite oxides becomes controlled by the full atomic spin–orbit coupling of the 3d and 4d transition metals (Co and Rh, respectively), resulting in some of the largest known Rashba-like spin splittings. The structural building blocks that facilitate the bandwidth-scaled inversion-symmetry breaking are common to numerous materials. These findings therefore provide opportunities for creating spin-textured states and suggest routes to interfacial control of inversion-symmetry breaking in desired heterostructures of oxides and other material classes.

Figure (a) represents bulk and surface Fermi surfaces of PtCoO2 measured by ARPES. Spin-resolved ARPES measurements of an in-plane spin polarization (⟨Sy⟩) of the Fermi surface for the cut along kx shown in (a) are shown in (c) and confirm the chiral in-plane spin texture of the surface states predicted by theoretical calculations shown in (b). This spin texture is the consequence of the enhanced spin-splitting of the electronic states revealed on the surface of this material.

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