Scientific profile

Here below, my main scientific research activities are briefly described. The key publications are also listed.

Superconductivity at interfaces between insulating oxides

In 2007, we have discovered that the ground state of the conducting interface found at the LaAlO₃/SrTiO₃ interface is a superconducting condensate. The critical temperature is about 200mK. Signatures of 2D superconductivity have been found and analyses of the anisotropic transport properties allow the superconducting thickness to be estimated to be about 10nm. Recently, we succeeded tuning superconductivity with an electric field. These experiments allow superconductivity to be switched on and off and the system phase diagram, which reveals a quantum critical point separating a weakly localized regime from a superconducting pocket, to be mapped out.

• Science 317, 1196 (2007)
• Nature 456, 624 (2008)
• Physical Review Letters 102, 046809 (2009)
• Applied Physics Letters 94, 112506 (2009)
• Physical Review Letters 104, 126803 (2010)

Improper ferroelectricity at oxide interfaces

In ferroelectric/paraelectric PbTiO₃/SrTiO₃, we have shown that the ferroelectric properties can be tuned as a function of the amount of PbTiO₃ in the system and that, at very short wavelengths, for layer thicknesses of one or two unit cells, a new type of improper ferroelectricity appears. This ferroelectricity is related to the very unusual coupling of three order parameters, two antiferrodistortive ones which, together, break inversion symmetry allowing the further coupling to a polar mode. This new phenomena is opening the route for designing novel functionalities using order parameter coupling at interfaces.

• Physical Review Letters 95, 177601 (2005)
• Advanced Materials 19, 4153 (2007)
• Nature 452, 732 (2008)
• Physical Review Letters 104, 187601 (2010)

Field effect and ferroelectric field effect

We have been developing during the years a large expertise on field effect and ferroelectric field effect experiments. On high T_c materials, we have used these techniques to modulate the system carrier density without modifying the background disorder. This approach allows a detailed study of the normal state and superconducting properties as a function of the carrier density to be realized. In doped SrTiO₃ and in SrRuO₃, we have developed the idea of the local ferroelectric field effect which allows non-volatile, reversible, electronic nanofeatures to be realized. This technique uses an atomic force microscope to control the ferroelectric polarization at nanoscale which, via the ferroelectric field effect, permits to modulate the carrier density of the adjacent layer on the same lengthscale.

• Science 269, 373 (1995)
• Science 276, 1100 (1997)
• Science 284, 1152 (1999)
• Physical Review Letters 88, 67002 (2002)
• Nature 424, 1015 (2003)
• Nature 441, 195 (2006)
• Physical Review Letters 98, 057002 (2007)

Size effects in ferroelectrics

In ferroelectric materials, we have studied using different techniques including x-ray diffraction, atomic force microscopy, and x-ray photoelectron diffraction the long standing problem of size effects in ferroelectrics: Is there a minimum thickness below which ferroelectricity disappears? -A very important question for several applications. We have shown that very thin films, as thin as 3 unit-cells, of PbTiO₃ remain ferroelectric and that the depolarizing field, due to the imperfect screening of the polarization, is controlling the change in polarization in thin films.

• Applied Physics Letters 75, 856 (1999).
• Science 303, 488 (2004)
• Physical Review Letters 94, 047603 (2005)

Control and imaging of ferroelectric domains at nanoscale

We have used atomic force microscopy to study the ferroelectric domain structure in thin epitaxial ferroelectric films. Applying a voltage between a metallic tip and an electrode, across a thin epitaxial ferroelectric film, allows the domain structure to be modified and controlled at nanoscale. Imaging of ferroelectric domains is obtained using piezoresponse force microscopy.

• Applied Physics Letters 72, 1454 (1998)
• Applied Physics Letters 79, 530 (2001)

Domain wall dynamics in ferroelectric thin films

Using an atomic force microscope to write and read ferroelectric domain structures, varying the writing parameters, we have been able to extract the domain wall velocity as a function of the applied electric field. We have discovered that domain wall motion is a creep process. These results combined with analyses of domain walls roughness have allowed us to show that the elastic walls are two-dimensional and that the creep process is controlled by random bond disorder.

• Physical Review Letters 89, 097601 (2002)
• Physical Review Letters 94, 197601 (2005)

High T_c superlattices

In the field of high T_c, we were the first to realize high T_c superlattices based on superconducting YBa₂Cu₃O₇ and insulating PrBa₂Cu₃O₇. These materials allowed us to study several important problems including the dependence of the critical temperature on the YBa₂Cu₃O₇ thickness, vortex dynamics in thin layers, and more generally to use these c- and a-axis superlattices as model systems.

• Physical Review Letters 63, 1016 (1989)
• Physical Review Letters, 64, 804 (1990)
• Science 251, 780 (1991)
• Physical Review Letters 67, 1354 (1991)
• Physical Review Letters 73, 328 (1994)
• Physical Review Letters 74, 3041 (1995)
• Reports on Progress in Physics 60, 1673 (1997)

Selection of 10 publications

1. Tunable Rashba spin-orbit interaction at oxide interfaces, A.D. Caviglia, M. Gabay, S. Gariglio, N. Reyren, C. Cancellieri, and J.-M. Triscone, Physical Review Letters 104, 126803 (2010).
   A strong tunable Rashba spin orbit coupling is observed in the LaAlO₃/SrTiO₃ system demonstrating the interfacial nature of the electron gas.
   
2. Electric field control of the LaAlO₃/SrTiO₃ interface ground state, A. Caviglia, S. Gariglio, N. Reyren, D. Jaccard, T. Schneider, M. Gabay, S. Thiel, G. Hammerl, J. Mannhart, and J.-M. Triscone, Nature 456, 624 (2008).
   On and off switching of superconductivity using the electric field effect is demonstrated and the LaAlO₃/SrTiO₃ phase diagram revealed.
   
3. Improper ferroelectricity in perovskite oxide artificial superlattices, E. Bousquet, M. Dawber, N. Stucki, C. Lichtensteiger, P. Hermet, J.-M. Triscone, and Ph. Ghosez, Nature 452, 732 (2008).
   A novel type of improper ferroelectricity is observed at the interface between PbTiO₃ and SrTiO₃ resulting from the unusual coupling of three order parameters.
   
4. 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. Ruetschi, D. Jaccard, M. Gabay, D. A. Muller, J.-M. Triscone and J. Mannhart, Science 317, 1196 (2007).
   Discovery of superconductivity at the interface between two excellent insulators : SrTiO₃ and LaAlO₃.
   
5. Local on-off switching of 2-D superconductivity with ferroelectric field effect, K. Takahashi, D. Jaccard, M. Gabay, K. Shibuya, T. Ohnishi, M. Lippmaa, J.-M. Triscone, Nature 441, 195 (2006).
   The ferroelectric field effect in a thin doped SrTiO₃ layer allows the non-volatile on and off switching of superconductivity.
   
6. Unusual behaviour of the ferroelectric polarization in PbTiO₃/SrTiO₃ superlattices, M. Dawber, C. Lichtensteiger, M. Cantoni, M. Veithen, P. Ghosez, K. Johnston, K.M. Rabe, and J.-M. Triscone, Phys. Rev. Lett. 95, 177601 (2005).
   In superlattices made of ferroelectric PbTiO₃ and paraelectric SrTiO₃, the ferroelectric properties can be tuned. At short wavelength, an anomalous recovery of ferroelectricity is observed.
   
7. Domain wall roughness in epitaxial ferroelectric PbZr_0.2Ti_0.8O₃ thin films, P. Paruch, T. Giamarchi, and J.-M. Triscone, Phys. Rev. Lett. 94, 197601 (2005).
   Atomic force microscopy is used to study ferroelectric domain wall roughness which allows the type of disorder controlling domain wall motion to be identified.
   
8. Ferroelectricity and tetragonality in ultrathin PbTiO₃ films, C. Lichtensteiger, J.-M. Triscone, J. Junquera, and P. Ghosez, Phys. Rev. Lett. 94, 047603 (2005).
   Ultrathin PbTiO₃ films have been grown and studied using x-ray diffraction. It is shown that films as thin as 20Å are ferroelectric.
   
9. Domain wall creep in epitaxial Pb(Zr_0.2Ti_0.8)O₃ thin films, T. Tybell, P. Paruch, T. Giamarchi, and J.-M. Triscone, Phys. Rev. Lett. 89, 097601 (2002).
   Atomic force microscopy is used to study ferroelectric domain walls dynamics. It is shown that domain wall motion is a creep process.
   
10. Electrostatic modulation of superconductivity in ultra-thin GdBa₂Cu₃O₇ films, C.H. Ahn, S. Gariglio, P. Paruch, T. Tybell, L. Antognazza, and J.-M. Triscone, Science 284, 1152 (1999).
    High T_c superconductivity is modulated using the ferroelectric field effect.
    

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