Working on functional oxide heterostructures.
Doctor of Philosophy, Geneva University (2018)
Master of Science, University of Saint Andrews (2013)
MPhys, University of St Andrews
PhD, University of Geneva
Harold Hwang, Postdoctoral Faculty Sponsor
- Intrinsic magnetism in superconducting infinite-layer nickelates NATURE PHYSICS 2022
- Machines for Materials and Materials for Machines: Metal-Insulator Transitions and Artificial Intelligence FRONTIERS IN PHYSICS 2021; 9
- Dynamics of the electrically induced insulator-to-metal transition in rare-earth nickelates PHYSICAL REVIEW B 2021; 104 (16)
- Crossover between distinct symmetries in solid solutions of rare earth nickelates APL MATERIALS 2021; 9 (8)
Near-Atomic-Scale Mapping of Electronic Phases in Rare Earth Nickelate Superlattices
2021; 21 (6): 2436-2443
Nanoscale mapping of the distinct electronic phases characterizing the metal-insulator transition displayed by most of the rare-earth nickelate compounds is fundamental for discovering the true nature of this transition and the possible couplings that are established at the interfaces of nickelate-based heterostructures. Here, we demonstrate that this can be accomplished by using scanning transmission electron microscopy in combination with electron energy-loss spectroscopy. By tracking how the O K and Ni L edge fine structures evolve across two different NdNiO3/SmNiO3 superlattices, displaying either one or two metal-insulator transitions depending on the individual layer thickness, we are able to determine the electronic state of each of the individual constituent materials. We further map the spatial configuration associated with their metallic/insulating regions, reaching unit cell spatial resolution. With this, we estimate the width of the metallic/insulating boundaries at the NdNiO3/SmNiO3 interfaces, which is measured to be on the order of four unit cells.
View details for DOI 10.1021/acs.nanolett.0c04538
View details for Web of Science ID 000634766600013
View details for PubMedID 33685129
View details for PubMedCentralID PMC7995248
- Optical properties of LaNiO3 films tuned from compressive to tensile strain PHYSICAL REVIEW B 2020; 102 (15)
Length scales of interfacial coupling between metal and insulator phases in oxides
2020; 19 (11): 1182-+
Controlling phase transitions in transition metal oxides remains a central feature of both technological and fundamental scientific relevance. A well-known example is the metal-insulator transition, which has been shown to be highly controllable. However, the length scale over which these phases can be established is not yet well understood. To gain insight into this issue, we atomically engineered an artificially phase-separated system through fabricating epitaxial superlattices that consist of SmNiO3 and NdNiO3, two materials that undergo a metal-to-insulator transition at different temperatures. We demonstrate that the length scale of the interfacial coupling between metal and insulator phases is determined by balancing the energy cost of the boundary between a metal and an insulator and the bulk phase energies. Notably, we show that the length scale of this effect exceeds that of the physical coupling of structural motifs, which introduces a new framework for interface-engineering properties at temperatures against the bulk energetics.
View details for DOI 10.1038/s41563-020-0757-x
View details for Web of Science ID 000558140100006
View details for PubMedID 32778815
- Vibrational properties of LaNiO3 films in the ultrathin regime APL MATERIALS 2020; 8 (6)
Coupling Lattice Instabilities Across the Interface in Ultrathin Oxide Heterostructures
ACS MATERIALS LETTERS
2020; 2 (4): 389-394
Oxide heterointerfaces constitute a rich platform for realizing novel functionalities in condensed matter. A key aspect is the strong link between structural and electronic properties, which can be modified by interfacing materials with distinct lattice symmetries. Here, we determine the effect of the cubic-tetragonal distortion of SrTiO3 on the electronic properties of thin films of SrIrO3, a topological crystalline metal hosting a delicate interplay between spin-orbit coupling and electronic correlations. We demonstrate that below the transition temperature at 105 K, SrIrO3 orthorhombic domains couple directly to tetragonal domains in SrTiO3. This forces the in-phase rotational axis to lie in-plane and creates a binary domain structure in the SrIrO3 film. The close proximity to the metal-insulator transition in ultrathin SrIrO3 causes the individual domains to have strongly anisotropic transport properties, driven by a reduction of bandwidth along the in-phase axis. The strong structure-property relationships in perovskites make these compounds particularly suitable for static and dynamic coupling at interfaces, providing a promising route towards realizing novel functionalities in oxide heterostructures.
View details for DOI 10.1021/acsmaterialslett.9b00540
View details for Web of Science ID 000526398200012
View details for PubMedID 32478332
View details for PubMedCentralID PMC7254603
Thickness-Dependent Perovskite Octahedral Distortions at Heterointerfaces
2019; 19 (6): 4188-4194
In this study, we analyze how the octahedral tilts and rotations of thin films of LaNiO3 and LaAlO3 grown on different substrates, determined using synchrotron X-ray diffraction-measured half-integer Bragg peaks, depend upon the total film thickness. We find a striking difference between films grown on SrTiO3 and LaAlO3 substrates which appears to stem not only from the difference in epitaxial strain state but also from the level of continuity at the heterointerface. In particular, the chemically and structurally discontinuous LaNiO3/SrTiO3 and LaAlO3/SrTiO3 interfaces cause a large variation in the octahedral network as a function of film thickness whereas the rather continuous LaNiO3/LaAlO3 interface seems to allow from just a few unit cells the formation of a stable octahedral pattern corresponding to that expected only given the applied biaxial strain.
View details for DOI 10.1021/acs.nanolett.9b01772
View details for Web of Science ID 000471834900107
View details for PubMedID 31117765
View details for PubMedCentralID PMC6595436
Rare-earth nickelates RNiO3: thin films and heterostructures
REPORTS ON PROGRESS IN PHYSICS
2018; 81 (4): 046501
This review stands in the larger framework of functional materials by focussing on heterostructures of rare-earth nickelates, described by the chemical formula RNiO3 where R is a trivalent rare-earth R = La, Pr, Nd, Sm, …, Lu. Nickelates are characterized by a rich phase diagram of structural and physical properties and serve as a benchmark for the physics of phase transitions in correlated oxides where electron-lattice coupling plays a key role. Much of the recent interest in nickelates concerns heterostructures, that is single layers of thin film, multilayers or superlattices, with the general objective of modulating their physical properties through strain control, confinement or interface effects. We will discuss the extensive studies on nickelate heterostructures as well as outline different approaches to tuning and controlling their physical properties and, finally, review application concepts for future devices.
View details for DOI 10.1088/1361-6633/aaa37a
View details for Web of Science ID 000425123800001
View details for PubMedID 29266004
Conductivity and Local Structure of LaNiO3 Thin Films
2017; 29 (18)
A marked conductivity enhancement is reported in 6-11 unit cell LaNiO3 thin films. A maximal conductivity is also observed in ab initio calculations for films of the same thickness. In agreement with results from state of the art scanning transmission electron microscopy, the calculations also reveal a differentiated film structure comprising characteristic surface, interior, and heterointerface structures. Based on this observation, a three-element parallel conductor model is considered and leads to the conclusion that the conductivity enhancement for films of 6-11 unit cells, stems from the onset of intercompetition between the three local structures in the film depth.
View details for DOI 10.1002/adma.201605197
View details for Web of Science ID 000400636400005
View details for PubMedID 28262988
Interfacial Control of Magnetic Properties at LaMnO3/LaNiO3 Interfaces
2015; 15 (11): 7355-7361
The functional properties of oxide heterostructures ultimately rely on how the electronic and structural mismatches occurring at interfaces are accommodated by the chosen materials combination. We discuss here LaMnO3/LaNiO3 heterostructures, which display an intrinsic interface structural asymmetry depending on the growth sequence. Using a variety of synchrotron-based techniques, we show that the degree of intermixing at the monolayer scale allows interface-driven properties such as charge transfer and the induced magnetic moment in the nickelate layer to be controlled. Further, our results demonstrate that the magnetic state of strained LaMnO3 thin films dramatically depends on interface reconstructions.
View details for DOI 10.1021/acs.nanolett.5b02720
View details for Web of Science ID 000364725400028
View details for PubMedID 26484628