The United Control over Charge Density and Spin State of Low Dimensional Electron System at Titanates Project Objectives
The interplay between orbital ordering, charge correlation, spin–orbit coupling, and lattice distortion in Transition Metal Oxides (TMOs) leads to the complex two-dimensional electron states (2DES) typified with fascinating properties. Understanding, controlling and tailoring electronic properties of TMOs surfaces and interfaces, in general, are mandatory for further development of novel quantum devices.
Project Funding
This research program is supported by the Swiss National Science Foundation (SNSF) through following grants: 1. The United Control over Charge Density and Spin State of Low Dimensional Electron System at Titanates (SNF grant 200021_182695). 2. In-situ spectroscopy of oxide heterostructures (SNF grant 206021_177006). 3. Experimental realization of novel quantum materials with MBE/PLD+STM+ARPES (Grant No. IZLCZ2-170075.)
Building a picture of transition metal oxides Novel electronic and magnetic properties can emerge in low-dimensional systems, where electrons are confined to a narrow area. We spoke to Professor Milan Radovic about his research into titanates, and how their properties can be tailored and controlled in lowdimensional systems, which could then open up wider possibilities in oxide electronics.
Project Partners
DTU-Denmark supports this research through a staff-exchange collaboration.
The modern electronics industry uses large quantities of silicon in device production embedded in the current technology. However, transition metal oxides (TMOs) also have a range of interesting properties which are way more versatile than silicon. “Transition metal oxides can be insulating, metallic, magnetic, super-conducting, etc., and can also be used for optoelectronics,” says Professor Milan Radovic, a scientist in the Spectroscopy of Novel Materials Group at the Paul Scherrer Institute (PSI) in Switzerland. The main problem in terms of using transition metal oxides for industrial applications is that they are essentially very expensive as raw materials. “Silicon can be found pretty much everywhere and its big crystal can be grown easily,” points out Professor Radovic. “The transition metal oxides, however, include some rare earth elements which are “rare” and therefore very expensive and it will be hard to imagine that devices based on such elements will be produced on a large industrial scale. They are however very relevant for certain special needs and purposes, perhaps in novel quantum computing technology.”
Stronthium titanate As the Principal Investigator of several research projects based at the PSI, Professor Radovic and his team are now investigating how to control electron behaviour in titanates such as strontium titanate (SrTiO3), which is among the most widely used of the transition metal oxides. Strontium titanate has some very interesting electrical properties, now Professor Radovic aims to gain deeper
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insights into this compound, which will help lay the foundations for further development in oxide electronics. “We want to understand it in order to control and further improve its properties,” he says. The electronic structure is the key consideration in terms of understanding the properties and behaviour of the system. “If the system is metallic than it is usually simple, while if there is a gap between conduction and valence bands, then you may aim to modify system to generate more exotic properties.” This research includes investigation into titanium dioxide (TiO2), which can be thought of as the parent compound of the titanates. Professor Radovic’s research is largely focused on artificial materials in the form of
thin films with a thickness of just a few unit cells. “Strontium is an element that tends to diffuse, segregate or intercalate in the oxide matrix. Therefore we decided to use titanium dioxide, which is a simpler compound, and to investigate the effect of Sr contents, hoping to create a system with novel properties,” outlines Professor Radovic. The focus here is on low-dimensional electron systems, where electrons freely move in the confined area, leading to the emergence of new physics and properties that many scientists are now working to harness. In their research, Professor Radovic and his colleagues are able to transform a 3-dimensional transition metal oxide into a 2-dimensional system. “In this 2-D system the electrons propagate only in plane. This dramatically affects the electronic properties of the system,” he outlines. A pulse laser deposition system is used to make a 2-D system (physically, it is an utra- thin film) in what is essentially a bottom-up process, an approach which gives researchers a high degree of freedom. “We have more parameters to play with,” explains Professor Radovic. “During the growth of the thin film you can incorporate substitutions or defects which consequently affect a crystal lattice and charge carrier concentration.” Researchers are investigating how they can then control and tune electron behaviour. This essentially involves putting two oxides in close proximity to each other, with a 2-D electron gas at the interface between them, the structure of which is
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then affected by changes of the parent materials. “Where two materials are in close proximity to each other, it may be possible to transfer certain properties from one material into the other which you maybe would like to alter,” says Professor Radovic. This opens up the possibility of effectively tailoring the electronic and magnetic properties of a system, which is an exciting prospect relevant to a wide range of technological applications. “If, for example, a ferromagnetic material is put in close proximity to a non-magnetic one, then additional properties emerge in the compound of interest. So we’re able to
Industrial applications There are a wide range of potential applications of these transition metal oxides, with researchers investigating how novel quantum materials can be used in the next generation of novel devices. New quantum technology using spin and charge promises a revolutionary advancement in electronics. “Topological TMOs with their resolute electronic states are up-and-coming platforms for quantum computers” says Professor Radovic. The goal for Professor Radovic and his colleagues will not be to actually produce new devices however, but rather to develop a deeper understanding of these transitional metal oxides
Transition metal oxides can be metalic, insulating, or both. They can be magnetic, superconducting, and can also be used for optoelectronics. influence the behaviour of the neighbouring compound,” explains Professor Radovic. The main aspects that can be changed or tuned in a system are electronic density, electronic correlations and bandwidth. Electron density relates to how many electrons can cross the fermi layer of a material, yet Professor Radovic says this is not enough in terms of the project’s overall agenda. “This gives the electron density, but not the space where electrons can live. The electronic structure is actually important to know because it tells you how many possible states can be in a system, which electrons can jump over, which is a topic we are very interested in,” he outlines.
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and their potential. “If somebody wants to use these titanates as a 2-D electron gas platform, then we can describe what is useful, what are the drawback, and the wider possibilities. When you effectively have a recipe for thin films, then it’s easier to produce devices,” he says. The research agenda in Professor Radovic’s team also includes investigation into other materials aside from titanates, such as nickelates, manganates, cuprates and iridates with a view to harnessing their technological potential. Many of the concepts that Professor Radovic is investigating are relevant to a variety of different oxides. “Manipulation with these in thin films form can significantly alter the critical properties of the transitional metal oxides,” he stresses.
Contact Details
Project Coordinator, Milan Radovic Swiss Light Source, Paul Scherrer Institut WSLA 123 Forschungstrasse 111 CH-5232 Villigen PSI Switzerland T: +41 (0)56 310 5565 E: milan.radovic@psi.ch W: https://www.psi.ch/en/lsc/people/ milan-radovic 1. A. F. Santander-Syro, F. Fortuna, C. Bareille, T. C. Rodel, G. Landolt, N. C. Plumb, J. H. Dil, and M. Radovic, Giant spin splitting of the two-dimensional electron gas at the surface of SrTiO3, Nature Materials, 13, 1085–1090 doi:10.1038/nmat4107 (2014). 2. N. C. Plumb, M. Salluzzo, E. Razzoli, M. Månsson, M. Falub, J. Krempasky, C.Matt, J. Chang, J. Minar, J. Braun, H. Ebert, B. Delley, K.-J. Zhou, C. Monney, T.Schmitt, M. Shi, J. Mesot, C. Quitmann, L. Patthey, M. Radovic, Mixed dimensionality of confined conducting electrons in the surface region of SrTiO3, Phys. Rev. Lett. 113, 086801 (2014).
Milan Radovic
Milan Radovic started his research career at Institute of Nuclear sciences “Vinča”- University of Belgrade in 2000. In 2005 he moved to Italy where he obtained a PhD at Università degli Studi di Napoli Federico II, Naples. In 2009, he was invited to engage a joined position at EPFL- Ecole Polytechnique Fédérale de Lausanne and Paul Scherrer Institute. Since 2013 he has been a staff scientist at Paul Scherrer Institute, Switzerland.
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