Internships and PhDs offers

2016-2017 : Superfluid stiffness and superconducting gap of a two-dimensional electron gas

Two-dimensional (2D) superconductors confront the physicists with completely new challenges by exhibiting a large diversity of exotic electronic phases. In this context, the discovery of a 2D superconducting electron gas confined between two insulating oxides layers LaAlO3 and SrTiO3 has raised a considerable interest [1,2] (fig. 1a). A metallic gate enables to control electrostatically the filling of the interfacial quantum well and thus tune the 2D electron gas properties, including the superconducting transition temperature Tc. The resulting phase diagram has the shape of a dome that collapses into a quantum critical point where the Tc is reduced to zero, (fig. 1b) [3]. The physics of 2D superconductors, is expected to be strongly influenced by the superfluid stiffness Js, an important scale which determines the energy cost to twist the phase of the superconducting condensate. When the stiffness is low, the superconducting Tc should be controlled by the loss of phase coherence instead of electron pairing as in the standard BCS theory.
The goal of this project is to address this issue by measuring both quantities, i.e. the electrons pairing gap and the superfluid stiffness, in the whole phase diagram of a LaAlO3/SrTiO3 interface. The first one can be accessed through a tunneling experiment that measures directly the pairing gap in the single particle density of states [4]. The second one (Js) requires measuring the inductive response of the 2D superconductor at finite frequency, which can be done in the microwave frequency range as we demonstrated recently. The project involves the microfabrication (optical and e-beam lithography technics) and the measurement of tunnel junctions as well as microwave reflection measurements (fig1c,d), both at ultra low temperature (10 mK).

[1] Reyren et al Nature (2008) [2] Biscaras et al Nature Commun. 1,89 (2010) ) ) [3] A. Cavilgia et al. Nature (2008), J. Biscaras et al. Phys. Rev. Lett. (2012), S. Hurand et al. Sci. Rep. (2015) , Biscaras et al. Nature Mat. (2013) [4] Richter et al. Nature (2015)

Contact : Nicolas Bergeal
nicolas.bergeal (arobase)

Figure 1 : a) TEM picture of a LaAlO3/SrTiO3 interface. b) Phase diagram Tc(VG). c and d) Sample and microwave set-up for reflection measurement.

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