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High Magnetic Field Investigation of Transport and Thermodynamic Properties of Metallic Oxide Hetrointerfaces

Final Report Summary - OXIDES-AMCCOLLAM2009 (High Magnetic Field Investigation of Transport and Thermodynamic Properties of Metallic Oxide Hetrointerfaces)

This project focused on perovskite oxide heterostructures, in which thin films of different transition metal oxides are grown in alternating layers to form an artificial structure. These materials have been shown to support interface properties which differ dramatically from those of the bulk components, and can be tuned by external influences such as electric or magnetic fields. The rapidly growing worldwide research effort to develop and understand these structures is motivated both by their enormous potential for multifunctional electronic devices, and the unique opportunity they provide to study the complex fundamental physics of 2-dimensional electron systems in materials with strong electron-electron interactions.

A striking example of an oxide heterostructure with highly desirable and interesting properties is the LaAlO3/SrTiO3 system. Here, a highly conducting 2-dimensional electron gas forms at the interface between two band insulators and, moreover, exhibits a number of effects not previously observed in 2D electron systems, such as superconductivity and magnetism. Understanding and controlling the LaAlO3/SrTiO3 2D electron gas (2DEG) is currently the central problem in this field.

The primary aim of this project was to investigate the fundamental properties of the LaAlO3/SrTiO3 2DEG with emphasis on discovering its electronic bandstructure and Fermi surface, and characterising the charge carriers responsible for conduction. Samples grown under systematically varied conditions and with different components and structures were also to be investigated, in order to gain an understanding of the factors influencing the 2D conduction.

This type of study requires high quality samples, and experimental conditions of low temperatures and very high magnetic fields. For these reasons, this project was carried out within a unique collaboration between research groups at the MESA+ Institute of the University of Twente, in the Netherlands, where some of the best quality LaAlO3/SrTiO3 samples in the world are being produced, and the High Field Magnet Laboratory (HFML) in Nijmegen, the Netherlands, where the researcher coordinating this project was based, and where a combination of millikelvin temperatures and magnetic fields up to 33 T can be achieved.

During the course of the project, high sensitivity transport measurements were carried out, at low temperatures and high magnetic fields, on heterostructures of LaAlO3/SrTiO3, SrTiO3/LaAlO3/SrTiO3 , and SrTiO3/SrCuO2/LaAlO3/SrTiO3. It was found that SrTiO3/SrCuO2/LaAlO3SrTiO3 structures had an exceptionally high-mobility interface 2DEG, and quantum oscillations of the magnetoresistance (the Shubnikov-de Haas effect) were observed in this material. Comprehensive measurement and analysis of these quantum oscillations allowed the direct experimental determination of the 2DEG conduction subband structure, revealing multiple, high-mobility, 2-dimensional electronic subbands with a small energy splitting of a few millielectronvolts. The effective masses of the charge carriers were measured to be in the range 1 - 3 times the electron mass, with differences between masses and mobilities in different subbands. When a magnetic field was applied in the plane of the 2DEG, a diamagnetic shift of the subband energies and depopulation of the highest subband was observed. This latter result is widely observed in semiconductor 2DEGs and strengthens the analogy between LaAlO3/SrTiO3-based systems and conventional semiconductor heterostructures, leading to a deeper understanding of how the occupation of subbands can be tuned in a measureable way to increase the mobility of the 2DEG. This will be important for further investigation of the fundamental physics of these 2DEGs, as well as in the development of high quality oxide electronic devices.

These results from SrTiO3/SrCuO2/LaAlO3/SrTiO3 samples represent a major success of this project and have already provided a basis for both theoretical and other experimental work in the field. Previous theoretical work predicted that multiple electronic subbands should contribute to transport in LaAlO3/SrTiO3 systems, but only one subband had been observed experimentally. Our results confirm that multiple high-mobility subbands are active, and also reveal a much smaller energy scale than was predicted theoretically. New theoretical work, based directly on the results of this project, has calculated a bandstructure in good agreement with our experimental results, and a significantly greater understanding of the LaAlO3/SrTiO3 interface bandstructure has thus been achieved.

LaAlO3/SrTiO3 and SrTiO3/LaAlO3/SrTiO3 samples were found to have lower mobility conducting channels than the samples containing a SrCuO2 layer, but a systematic magnetotransport study in high magnetic fields showed that under certain conditions, depending on thermal or photo-excitation of the charge carriers, and on the thickness of the LaAlO3 layer, they exhibit two-subband conduction and/or strongly negative magnetoresistance indicative of magnetic scattering. These results are in keeping with the results from the higher mobility samples, and an overall understanding of the electronic behaviour of LaAlO3/SrTiO3-based systems in different structural and electronic configurations is now definitely emerging from this work. This kind of understanding, particularly the understanding of magnetic effects and when they arise, is very important in developing the types of oxide structure suitable for electronic devices.

Commercial oxide electronics is one of the ultimate aims of this research field, with low-energy/high-speed switching, spin-polarised 2DEG devices, and the creation of memory-functionality being realisable aims. The potential multi-functionality of oxide devices, due to the complex and tunable properties of transition metal oxides, and the high carrier densities in oxide 2DEGs, promise versatile and compact devices which will considerably extend the capabilities currently possible with semiconductor devices. Oxide electronics is expected to widely impact society, from hi-tech industries to everyday electronic devices in our homes and businesses, and significant steps towards understanding the basic structures, such as have been made during this project, are an essential part of progress towards this aim.

An additional objective of this project was to develop the experimental capabilities and infrastructure at the HFML, which is an international user-facility, to allow high quality magnetisation and transport measurements of low-level signals in the extreme conditions of low temperature, high magnetic field and high pressure. This aspect of the project has also been very successful, and resulted in unique measurement possibilities for guest scientists.

The introduction of specialised low-noise amplification devices, and improvements in wiring and measurement set-ups have led to several important results from torque magnetometry measurements of the Fermi surface of iron pnictide superconductors. In particular, a new type of magnetometer was developed to measure the magnetisation of very small samples, and this has led to magnetisation measurements of the magnetically frustrated bulk oxide material AgNiO2 that would not have been possible anywhere else in the world. Under certain conditions of high magnetic field this material is believed to support an electronic supersolid phase, and our high sensitivity results provided important evidence for this scenario. This magnetometer has attracted a lot of interest from researchers in the field of strongly correlated electron systems, where magnetic effects are almost ubiquitous, but the required single crystal samples are often extremely small, and many new measurements have been planned and funded within new or existing scientific collaborations.

Alongside work on oxide heterostructures, several studies of bulk oxide materials have been carried out. Hall effect and high pressure studies of the electron-nematic phase near quantum criticality in Sr3Ru2O7, and a high pressure study of the metal-insulator transition in Eu2IrO7 have been highly successful. These measurements, and the other results described in this summary have led to several publications in high-profile physics journals, with several additional publications in preparation. The results of this project have already provided motivation for other researchers in the fields of oxide heterostructures and strongly correlated electron systems, and a number of scientific proposals to extend and build on the work described in this summary have been funded.

A final important aspect of this project has been the development of strong international collaborations with other researchers working in the same or related fields. Fruitful collaborations have been established with world-leading groups and experts in Canada, Japan, and Argentina, as well as within Europe. These collaborations have led to travel of researchers between groups, and stimulating exchanges of expertise and ideas, which are planned to continue in the longer term.

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