Field Effect Tuning of Superconductivity at Oxide Interfaces
Photo of the Geneva Group: (from Left to Right) Stefano Gariglio, Andrea Caviglia, Claudia Cancellieri, Nicolas Reyren and Jean-Marc Triscone
[This is an invited article based on recent work of this collaboration -- 2Physics.com]
Authors: A.D. Caviglia1 , S. Gariglio1, N. Reyren1, C. Cancellieri1, D. Jaccard1, S. Thiel2, G. Hammerl2, J. Mannhart2, J.-M. Triscone1
Affiliation: 1Département de Physique de la Matière Condensée, University of Geneva, Genève, Switzerland, >>Link to Group Homepage
2Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, Germany, >>Link to Group Homepage
Charge transfer in semiconductors interfaces has brought about exceptional technological progress, one of the best examples being the development of the Field Effect Transistor (FET). Applying the same principle to materials with a wider spectrum of electronic properties, such as complex oxides, is an exciting opportunity both for fundamental and applied physics. These oxide compounds often exhibit strong electronic correlations and complex phase diagrams. In such systems, the electric field effect can be used to tune the ground state of the system . These materials also display a broad range of functional properties, such as high dielectric permittivity, piezoelectricity and ferroelectricity, superconductivity, spin polarised current, colossal magnetoresistance and ferromagnetism. Recent advances in growth methods have allowed the fabrication of atomically abrupt interfaces between these materials where novel electronic phases are created. Indeed the emerging field of complex oxide interfaces has a high potential impact for applications  and has been classified as one of the 10 breakthroughs of 2007 by the journal Science .
Fig.1 Photo of the device (courtesy of J. Mannhart)
The LaAlO3/SrTiO3 interface
A particularly interesting system is the interface between band insulators LaAlO3 and SrTiO3, which was reported to be conducting in 2004 in a seminal publication . This result is indeed amazing: by depositing on top of an insulating crystal (SrTiO3) a thin film of a good insulator (LaAlO3), a metallic interface is generated. This immediately calls to mind the two dimensional (2D) electron gas generated by modulation doping in III-V semiconductors. Correlated oxide systems are however more complex than semiconductors and in fact, in 2007 we discovered that this metallic interface undergoes a 2D superconducting transition at around 200 mK . The superconducting sheet is 10 nm thick and confined between two dielectrics. What a perfect opportunity to try modulating the superconducting state by applying an external electric field!
Fig.2: Atomic view of the interface (courtesy of J. Mannhart)
A complex phase diagram uncovered
Hence a gate electrode has been deposited on the backside of the SrTiO3 crystal and the sheet resistance as a function of temperature for different applied gate voltages has been measured down to 20 mK. For large negative voltages (typically less than -200 V), corresponding to the smallest accessible electron densities, the sheet resistance increases as the temperature is decreased, indicating an insulating ground state. No traces of superconductivity are left! As the electron density is increased the system becomes a superconductor. A further increase in the electron density produces first a rise of the critical temperature to a maximum of 310 mK. For larger voltages the critical temperature decreases again. This is a beautiful example of a quantum phase transition: a change of the electronic phase of matter driven not by a variation of temperature but by the application of an electric field. These findings have been reported recently in the journal Nature .
A bright future
This fascinating interface offers many possibilities, among them, fundamental studies of quantum phase transitions in low dimensions. This discovery also opens the way to the fabrication of new mesoscopic devices based on the ability to switch on and off the superconducting state at the nanoscale.
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