36 37 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 2 / 2 3 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 2 / 2 3 a) Monoclinic distortion . Cartoon of the distortion of the hexagonal lattice along one of the three symmetry axes. Above and below the insulator to metal transition temperature, T IMT b) Experimental setup . X-rays bunches with tuneable energy resonant with the vanadium L 2,3 edge impinge on the sample. The electrons emitted from the sample are collected and imaged through electron lenses. In the time-resolved configuration, the signal originated by isolated X-rays pulses with linear horizontal (LH) polarization is collected by suitable synchronized gating of the detection apparatus. The pump infrared laser is synchronized to the synchrotron pulses. Using light pulses to control electronic properties in vanadiumoxides Quantum Materials – Hard Condensed Matter – Electronic Properties – Physics – Materials Science – Imaging Vanadium(III) oxide (V 2 O 3 ) is an interesting material that has been extensively studied due to its unique properties. At low temperatures, V 2 O 3 is a Mott insulator, which is an insulator that is driven by strong electronic correlations. However, at approximately 170 K, V 2 O 3 undergoes a transition from an insulator to a metal, both structurally and electronically. This transition is fascinating as it raises questions about the fundamental mechanisms behind the change. Is it purely electronic, or does a change in symmetry drive it? Answering these questions is crucial to understand the properties of V 2 O 3 fully. One of the exciting things about the insulator to metal transition is that it can also be induced by applying an electric field, which is known as resistive switching, or by exciting the material with ultrashort light pulses. This opens up the possibility of manipulating the conductivity of V 2 O 3 on demand, which is essential for developing non-linear devices and neuromorphic materials. However, the transition speed is limited by the fact that, in quasi-equilibrium conditions, the electronic transition is always accompanied by a slower lattice change. Recent research has proposed that ultrafast light excitation can drive a non-thermal transition that decouples the lattice change from the electronic one. This would allow researchers to control the phase transition on extremely fast timescales, enabling the material to transform from an insulator to a metal at frequencies as high as a fewTHz. This possibility offers tremendous potential for developing ultrafast Mottronics capable of operating at extremely high frequencies. To study this phenomenon, an international team of researchers first demonstrated that the low-temperature insulating state of V 2 O 3 is strongly inhomogenous. In contrast, themetalic phase is homogeneous at thenanoscale due to the lack of monoclinic distortion in the a-b plane. The team then took snapshots of the inhomogeneous nanotexture during the photo-induced insulator-to-metal phase transition using time-resolved X-ray Photoemission Electron Microscopy (PEEM) measurements at the I06 beamline. The experiment demonstrated that the excitation with ultrashort near-infrared light pulses triggers the formation of a non-thermal state characterised by the electronic properties of the metallic phase, but retaining the same monoclinic distortion and nanotexture of the insulating state. Their results unveil a profound and general link between the real-space topology, the transition dynamics and the emergence of non-thermal electronic states in quantum materials. Furthermore, the team’s findings suggest possible routes to control metastable metallicity via the topology of the nanotexture. By combining real-space morphology control via interface engineering, electric fields, or pressure with novel excitation schemes to coherently manipulate insulator-to-metal phase transitions, researchers hope to achieve full and reversible control of the electronic properties of correlated oxides. In addition, the existence of light-induced non-thermal states in quantum materials is of great interest for resistive switching and neuromorphic computing applications. The ultimate goal is achieving all-electronic switching for ultrafast Mottronics capable of operating at frequencies as high as a few THz. Related publication: Ronchi, A. I Nanoscale self-organization and metastable non-thermal metallicity in Mott insulators. Nature Communications 13 , 3730 (2022). DOI: 10.1038/s41467-022-31298-0 Funding acknowledgement: Italian Ministry of University and Research: PRIN 2015 (Prot. 2015C5SEJJ001) and PRIN 2017 (Prot. 20172H2SC4_005) Università Cattolica del Sacro Cuore: D.1, D.2.2 and D.3.1 grants KU Leuven Research Funds: Project No. KAC24/18/056, No. C14/17/080 and iBOF/21/084 European Union: INTERREG-E-TEST Project (EMR113) and INTERREG-VL-NL- ETPATHFINDER Project (0559) “Severo Ochoa”Programme for Centres of Excellence in R&D: project MINCINN, Grant SEV-2016-0686 Corresponding authors: Prof Claudio Giannetti, Università Cattolica del Sacro Cuore, claudio.
[email protected] Prof Michele Fabrizio, Scuola Internazionale Superiore di Studi Avanzati (SISSA),
[email protected] (a) Schematics of soft x-ray resonant scattering from polar vortex array. X-ray circular dichroism (XCD) intensities were measured along the qz direction at satellite peaks diffracted from the vortex array period. (b) XCD asymmetry ratios as a function of q z , proportional to the incident angle. Black and red symbols (lines) represent the measurements (calculations) at the +1 and -1 satellite peaks, respectively. X-ray resonant scattering reveals chiral structures of polar vortices in ferroelectric superlattices Quantum Materials – Hard Condensed Matter – Electronic Properties – Physics – Materials Science – Multiferroics Recent research discovered polar vortex domains consisting of polar dipole vectors in ferroelectric superlattices. Because of their high potential as next-generation memory devices or functional devices, it is important to understand in detail the structure of polar vortices because it affects their properties and how they can be utilised in devices. Resonant Elastic X-ray Scattering (REXS) offers a non-destructive method to understand the complete 3D structure of the vortex. An international team of researchers used the soft X-ray scattering setup at Diamond’s I10 REXS experiment to determine the distribution of the three-dimensional polar vectors formed in a PbTiO 3 /SrTiO 3 superlattice. This setup allows the X-ray energy to be resonantly tuned to the L absorption edge of titanium, as well as providing polarisation control of the X-rays. The handedness of the circular polarisation of incident X-rays must be freely changed to measure X-ray Circular Dichroism (XCD), which is the difference between X-ray scattering intensities for left- and right-handed polarisation and allows X-ray resonant scattering to distinguish chiral structures. The research team observed that the sign of XCD varies depending on the incident angle of the X-rays. A new resonant scattering theory for quantitative analysis of the results was also required. Using a newly developed quantitative calculation, they revealed that this result meant a three-dimensional vortex array structure, not a simple one-dimensional helix. Their research results are expected to play a significant role in the understanding and application of ferroelectric chiral domain structures. This method is non-destructive and uses resonant scattering, which enables real- time observation of subtle changes due to external stimuli such as electric fields or high-power laser pumping at ultrafast time scales. This research on the polar structure of ferroelectrics corresponds to the counterpart of X-ray resonant magnetic scattering for the magnetic structure, which has already been extensively studied. These similarities naturally trigger X-ray resonant scattering studies on multiferroics that exhibit both properties at the same time. There is still no way to simultaneously measure two properties of the same atom or system, and X-ray resonant scattering is likely to be the answer. Related publication: Kim, KT. et al. Chiral structures of electric polarization vectors quantified by X-ray resonant scattering. Nature Communications 13 , 1769 (2022). DOI: 10.1038/s41467-022-29359-5 Funding acknowledgement: National Research Foundation of Korea (NRF-2020R1A2C1009597) US Department of Energy (DE-AC02-05CH11231) Corresponding authors: Prof Dong Ryeol Lee, Soongsil University,
[email protected] Dr Margaret R. McCarter, Lawrence Berkeley National Laboratory,
[email protected] MagneticMaterials Group Beamline I10 MagneticMaterials Group Beamline I06