121HIGHLIGHTS 2020

order of 0.4 in metallic, highly doped LAO/STO multilayer, a value comparable to the bare EPC in this material. At the same time, the ~130 meV feature is observed in the whole doping range, but it disappears at high carrier density, which is an effect predicted within the large polaron theory as a result of the screening of the lattice polarisation from the carriers. Thus, the new composite d-d + LO3 phonon features observed by RIXS is a hallmark of large polarons.

By demonstrating that quasiparticles in STO and STO-based heterostructures are large polarons, the range of theoretical models for the explanation of the normal and the superconducting state of this system is narrowed. In particular, it emerges that in the range of doping for which both bulk STO and two-dimensional electron systems at the STO surface and at the LAO/STO interface are superconducting, quasiparticles are large- polarons.

Large Polarons as Key Quasiparticles in SrTiO3 and SrTiO3-Based Heterostructures, A. Geondzhian (a), A. Sambri (b), G.M. De Luca (b), R. Di Capua (b), E. Di Gennaro (b), D. Betto (a), M. Rossi (c), Y.Y. Peng (c), R. Fumagalli (c),

N.B. Brookes (a), L. Braicovich (a,c), K. Gilmore (a), G. Ghiringhelli (c) and M. Salluzzo (b), Phys. Rev. Lett. 125, 126401 (2020); https://doi.org/10.1103/ PhysRevLett.125.126401. (a) ESRF

(b) CNR-SPIN and Dipartimento di Fisica Ettore Pancini Università di Napoli Federico II, Naples (Italy) (c) CNR-SPIN and Dipartimento di Fisica, Politecnico di Milano, Milan (Italy)

[1] J.L. van Mechelen et al., Phys. Rev. Lett. 100, 226403 (2008). [2] J.T. Devreese et al., Phys. Rev. B 81, 1252 (2010). [3] W. Meevasana et al., New J. Phys. 12, 023004 (2010).

PRINCIPAL PUBLICATION AND AUTHORS

REFERENCES

MECHANISM BEHIND OVONIC THRESHOLD SWITCHING IN CHALCOGENIDE GLASSES REVEALED

In 1968, Ovshinsky reported the unique reversible drop of resistivity observed in chalcogenide glasses upon high electric-field application [1], later renamed ovonic threshold switching (OTS), although the physical mechanism behind this phenomenon remained unknown. X-ray absorption spectroscopy combined with modern atomistic simulation methods reveals the atomic structure of OTS glasses.

Despite wide use in the field of phase-change memories, IR optics, thermoelectrics and many other devices, a basic understanding of some specific behaviours of chalcogenide materials remains incomplete. Much debate surrounds the explanation of the unique contrast of electronic properties between the amorphous and crystalline phases of phase-change materials (PCMs) [2]. Chalcogenide PCMs can be rapidly and reversibly switched between amorphous and crystalline phases with very different optical and electrical properties. This contrast of properties has led to their use in rewritable optical storage products and advanced non-volatile resistive memories (NVMs). In addition, OTS, observed in the amorphous phase of chalcogenide materials upon the application of high electric fields, is at the basis of the advent of high- density 3D NVMs thanks to the combination of a PCM memory element and an OTS selector device, both based on a chalcogenide material (Figure 106). These new memory architectures pave the way for the realisation of storage class memories (SCMs), bridging the performance gap between volatile and non-volatile memory as

well as neuromorphic circuits inspired by human brain functions. The switching mechanism of OTS was recently elucidated using electrical, optical and X-ray absorption spectroscopy (XAS) experiments as well as ab initio molecular dynamics (AIMD) simulations [3]. While the mechanisms of subthreshold conduction in chalcogenide glasses are now well understood thanks to increasingly accurate models to describe experimental observations, the origin of the OTS effect and the underlying physical mechanism involved was still not fully described or understood.

An extended X-ray absorption fine structure (EXAFS) experiment, performed at the Ge, Sb and Se K-edges on state-of-the-art GeSe-based OTS glasses thin films at beamline BM08, paved the way to robust structural models of prototype OTS glasses (Figure 107). These models are the basis of simulation of the impact of applying an electric field on the OTS material using advanced AIMD simulations. Surprisingly, the simulations revealed that the OTS mechanism relies on unexpected and metastable subtle