The Winners of the SPS Awards 2009

The SPS award committee, presided by Prof. Hans Beck (Uni Neuchâtel) had this year again a hard job to decide who of the 12 candidates should win the renowned awards. All the works submitted showed an excellent quality and were of high scientific level.
The three finalists, who had this year for the first time the opportunity to give a 30 min talk at the annual meeting in order to present their outstanding work in more detail, are presented below.

(Laudationes written by Hans Beck, abstracts written by the respective authors)

Camille Bonvin studied physics at the Ecole Polytechnique Fédérale in Lausanne, getting a diploma in mesoscopic physics. She then turned from semiconductor quantum dots to cosmology. Her PhD work at the University of Geneva, devoted to questions concerning dark energy, was completed in 2008. Camille is now a postdoctoral researcher in cosmology at the CEA Saclay in Paris. She has an impressive list of talks on all kinds of fascinating topics addressed not only to specalists in her field, but also to a wider public with the aim of giving them a better understanding of the secrets of our universe.

One of the puzzles of cosmology is the fact that our universe is expanding more and more rapidly, contrary to what one would expect given the gravitational attraction between matter. In the framework of general relativity this can be explained by assuming the existence of repulsive dark energy dominating the energy of the universe.

Camille’s PhD work was devoted to the study of the accelerated expansion by combining experimental observations and theoretical considerations. She has investigated three aspects of dark energy physics:

• Fluctuations of the luminosity distance of supernovae can be used to obtain information about the speed of the expansion, since this distance is influenced by density perturbations in the universe. Camille’s calculations have shown that luminosity distance fluctuations are indeed a promising new observational tool allowing to determine cosmological parameters.
• The expansion of our universe has only recently turned from decelerated into accelerated. K-essence has been proposed for understanding this surprising transition. Camille has shown that the scalar field, representing the dark energy in this framework, has to go through a phase in which its sound velocity exceeds the speed of light. This clearly leads to interesting problems: either one has to accept the violation of causality or one must exclude the existence of closed signal curves – contrary to the principle of Lorentz invariance.
• Another way of generating acceleration of the universe expansion consists in modifiying the law of gravity at cosmological scales. Camille has derived constraints for the parameters showing up in the Lagrangian containing a new time like vector field modifying the gravitational force. She has tested the consequences of this approach by comparing it to the perihelion shift of the planet Mercury and to the deflection of light observed, for instance, during solar eclipses. She has shown that terms in the gravity potential increasing with distance have to be taken into account, even though such a development will then not converge at large distances.

In all this work, which has been executed in collaboration with other specialists in cosmology, Camille has contributed important and crucial ideas and has shown her capacity to perform long and complicated calculations. Mastering perfectly her research field she has made an important contribution to a better understanding of the dark energy problem.

Theoretical and Observational Aspects of dark Energy

Cosmology is the study of the Universe as a whole. It combines theoretical calculations with precise observations. During the last decades, the confrontation between theory and experiments has triggered fundamental questions regarding our understanding of the Universe. In my thesis, I have studied different aspects of one of the most challenging of these problems: the accelerated expansion of our Universe. This behaviour, observed for the first time in 1998, is in complete contradiction with our theoretical predictions. The gravitational interaction, that acts on matter and radiation, should indeed lead to a decelerated expansion. Two directions are currently explored in order to solve this problem: either one invokes the presence of a new exotic form of energy, called dark energy, or one attempts to modify Einstein’s theory of gravity at large scales. In my research, I address the problem of the acceleration of the Universe from two sides: one theoretical and the other more observational. From a theoretical point of view, I am interested in constraining cosmological models designed to explain the acceleration of the Universe, either by using fundamental theoretical principles or by confronting them with observations. Such studies are necessary in order to understand the key features that render a cosmological model viable or not. In this context, I have demonstrated that the k-essence dark energy model is problematic since it allows information to propagate through the Universe with a velocity higher than the speed of light [1]. Therefore, this model violates the fundamental principle of causality and has to be rejected. Secondly, I have worked on a model of modified gravity, called generalized Einstein-Aether theory.
Extremely stringent tests of gravity exist within the Solar System. Therefore every new model that modifies the laws of gravitation has to be compatible with these constraints. In my work, I have shown that the generalized Einstein-Aether theory passes the Solar System tests. From a more observational point of view, I am interested in methods allowing to extract information about our Universe from observations. In order to find the solution to the problem of the accelerated expansion, it is indeed essential to have an accurate knowledge of the Universe expansion rate, as well as of its content. During my thesis, I have investigated a new method that uses the light from distant exploding stars (supernovae) to obtain information on the Universe evolution [2]. Indeed, the light-path between a supernova and an observer is modified by the inhomogeneous distribution of matter in the Universe. Hence, by measuring these modifications, we have access to important information regarding the Universe content and its evolution. Therefore this method will help us to discriminate between different cosmological models.

[1] No-go theorem for k-essence dark energy, C. Bonvin, C. Caprini and R. Durrer, Phys. Rev. Lett.97, 081303 (2006).
[2] Fluctuations of the luminosity distance, C. Bonvin, R. Durrer and A. Gasparini, Phys Rev. D73, 023523 (2006).

This prize is shared by three collaborators of the University of Geneva:

Andrea Caviglia (Italian) obtained his BSc and his MSc – with honors – at the University of Genoa. He then came to the Condensed Matter Physics Department at the University of Geneva for his PhD work dedicated to the study of interfacial phenomena in complex oxide heterostructures.

Stefano Gariglio (Italian) has the same trajectory: physics studies in Genoa and PhD in Geneva on "Transport properties of LaTiO_3+d and REBa₂Cu₃O_7-d thin films: a study of correlation effects", completed in 2003. Since then Stefano has been a post-doctoral fellow at the Condensed Matter Department in Geneva.

Nicolas Reyren (Swiss) has obtained his MSc in Geneva and is working for his PhD in the same group. He also worked in the Geneva University hospital where he developed experiments to learn about the localization capabilities of patients wearing cochlear implants.

This prize is given to the three young physicists for their contribution to the field of oxide interface engineering, in particular the amazing control of superconductivity by using the electric field effect (A. Caviglia, S.Gariglio, N. Reyren et al. Nature 456, 624 (2008)). Using this approach, that reveals the phase diagram of the system, they have succeeded, for the first time, to establish a reversible, field controlled, on and off switching of superconductivity. Before they had discovered superconductivity at the LaAlO₃/SrTiO₃ interface (N. Reyren et al. Science 317, 1196 (2007)).

Oxide interface engineering and the discovery of superconductivity at the LaAlO₃/SrTiO₃ interface has been cited as one of the 10 breakthroughs of 2007 by Science Magazine. The unprecedented control of superconductivity, which has been the dream of many researchers for more than 20 years, provides a route to the development of novel superconducting circuits. Using this principle, novel quantum electronic devices can indeed be envisaged, where superconductivity will be dynamically defined and controlled using local electric fields. The interface engineering approach, widely used by the semiconductor industry for the fabrication of lasers and transistors, reveals novel and surprising features when applied to oxides.

This very promising field of research is developing very rapidly worldwide. It establishes a fascinating link between fundamental physics of superconductivity and its industrial application. The three Geneva specialists in superconductivity therefore fully merit our award for applied physics.

Electric field control of the LaAlO₃/SrTiO₃ interface ground state

Charge transfer in semiconductor 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 broader 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 with competing ground states. The electric field effect can be an efficient tool to modulate the carrier density, a fundamental parameter, and thus possibly tune the ground state of these systems. Recent advances in growth methods have allowed the fabrication of atomically abrupt interfaces between complex oxides where novel electronic phases might be expected.

A particularly interesting system is the interface between band insulators LaAlO₃ and SrTiO₃, which was reported to be conducting in 2004 by Othomo and Hwang. This result is indeed amazing: by depositing on top of an insulating crystal of 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 in III-V semiconductor heterostructures. Correlated oxide systems are however more complex than semiconductors and, in fact, we discovered in 2007 that this metallic interface undergoes a 2D superconducting transition at around 200 mK [1]. The low density superconducting sheet is 10 nm thick and confined between two dielectrics. This situation is a perfect opportunity to try modulating the superconducting state by applying an external electric field.

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 [2]. For large negative voltages 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 ground state of matter driven not by a variation of temperature but by the application of an electric field.

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.

[1] "Superconducting Interfaces Between Insulating Oxides", N. Reyren, S. Thiel, A. D. Caviglia, L. Fitting Kourkoutis, G. Hammerl, C. Richter, C. W. Schneider, T. Kopp, A.-S. Rüetschi, D. Jaccard, M. Gabay, D. A. Muller, J.-M. Triscone, J. Mannhart, Science 317, 1196-1199 (2007).
[2] "Electric field control of the LaAlO₃/SrTiO₃ interface ground state", A. D. Caviglia, S. Gariglio, N. Reyren, D. Jaccard, T. Schneider, M. Gabay, S. Thiel, G. Hammerl, J. Mannhart & J.-M. Triscone, Nature 456, 624-627 (2008).