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Research: Pressure tuning of strongly correlated electron systems

High pressures are used to tune the electronic properties of correlated electron systems, in particular close to phase transitions.

External pressure is a powerful and in principle clean and reversible control parameter for investigating the condensed matter physics. As an example pressure is an ideal parameter to probe quantum critical points or the non-Fermi liquid behaviour. In general, the aim of research at high pressure is to discover new physical properties that have never been observed at ambient pressure, in order to develop our understanding of the physics and with the hope that the adaptation of such extraordinary properties at normal conditions is possible.

At the experimental level we are permanently trying to increase our know-how by developing new types of pressure experiments (see figure 1 and 2), improving pressure conditions (hydrostaticity) or extending the pressure range of investigations.

Patterned LaAlO3/SrTiO3 interface in the pressure cell for resistivity, thermopower, Hall and Nernst measurements
Fig 1. Pressure cell containing a patterned LaAlO3/SrTiO3 interface. The setup allows the measurement of resistivity, thermopower, Hall effect and Nernst effect. All properties can be tuned by applying a gate voltage.

Set up (detail) of the pressure chamber for AC magnetic susceptibility and specific heat, resistivity and thermopower measurements
Fig.2. Set up (detail) of the pressure chamber for AC magnetic susceptibility and specific heat, resistivity and thermopower measurements.

We use mainly transport properties for probing the high pressure physics of strongly correlated electron systems and focus on a few selected challenging topics. Of particular interest is the study of phase transitions at low temperatures.

One of the main goals of our research is to improve the understanding of some likely unconventional superconductors by tuning their physical properties with pressure. Three examples are mentioned in the following. Cases 1 and 2, CeCu2Si2 and pure Fe respectively, are a continuation of previous fruitful research. Case 3, the LaAlO3/SrTiO3 interface is a recent topic.

  1. Valence-fluctuation mediated superconductivity in CeCu2Si2

    Under pressures of around 45'000 bar, the superconductivity of the heavy fermion compound CeCu2Si2 was ascribed to valence fluctuations of the cerium ion [1]. In order to provide further evidence of such a novel superconducting mechanism, we are using a new technique for multi-probe experiments [2]. The electrical resistivity, the thermopower, the Hall and Nernst effects and the alternative specific heat are investigated simultaneously on well characterized crystals pressurized in a liquid medium. The main idea is to track in the pressure-temperature phase diagram the valence transition line.

  2. Ferromagnetic spin-fluctuation mediated superconductivity in iron

    In the case of iron, the superconductivity observed in the range of 130’000 – 300’000 bar was interpreted as mediated by spin fluctuations, but the role of the concomitant structural change remains unclear [3]. We are searching if superconductivity is intrinsic to the high pressure hcp phase or linked to the martensitic transformation. Investigations in various pressure mediums are under way, especially mediums providing better hydrostaticity, in which a narrowing of the superconducting dome is expected.

  3. Presure dependence of the normal and superconducting properties of Nb-doped SrTiO3 and the LaAlO3/SrTiO3 interface

    We are investigating the pressure response of the LaAlO3/SrTiO3 interface [4] as well as bulk or thin film Nb-doped SrTiO3. At first sight, pressure effect and doping effect are related and we expect that the intriguing normal and superconducting properties of these low carrier metals can be continuously tuned with pressure.

    Using our new multi-probe high pressure cell, the resistivity, the thermopower, the Hall and Nernst effects can now be measured down to the milliKelvin temperature range and also as a function of an external electrical field. This multi-probe investigation provides information on the possible change with pressure of the carrier density and allows us to access unexplored regions of the phase diagram. The thermopower is a sensitive probe of the carrier number and all transport properties should vary consistently versus the gate voltage. The Nernst effect, which is already large in the normal phase, sheds some light about superconductivity. Furthermore, the setup can be easily adapted for thermopower measurements up to ambient temperature and even a bit above. More generally we are taking advantage of nanolithography, using typically STO as a substrate material for the realization of complex measurements in the very restricted space of a pressure chamber. In collaboration with the group of J.-M. Triscone.

References

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