Throughout the history of modern physics, major discoveries are often directly linked to instrumental advances. For example, the discovery of superconductivity by Kammerlingh-Onnes was directly linked to his ability to liquify helium. Although today many physical measurements can be performed on commercial apparatus, it remains true that most “state of the art” measurements rely on instruments developed within laboratories. “Extreme conditions” of very low temperatures, very high magnetic field, and very high pressure, have brought many of the recent breakthroughs in solid state physics, allowing new states of matter as well as helping to understand material properties in their normal state. In IMAPEC particular emphasis has been put on developing new instrumentation for high pressure studies.

High pressure is an extremely valuable tool in solid state physics. Very high pressures can completely change the nature of matter. Recently the discovery of very high temperature superconductivity at pressures of several megabars has fired the interest of even the general public ^[1]. In the IMAPEC team we focus on the study of strongly correlated electron systems. These require much lower pressures, of the order of 10 GPa. This is sufficient to reduce the interatomic distances by as much as several percent and significantly change the microscopic interactions. Importantly, at such “moderate” pressures, we are able to perform a multitude of precise measurements of various physical properties. Monitoring how these change with pressure can greatly aid the understanding of the unique phenomena in these systems. Furthermore, by tuning these interactions to some pertinent value, in some cases the ground state can be modified, allowing us to obtain new fragile states of matter, that could be extremely difficult to obtain at ambient pressure.

Our developments are mainly based on the “diamond anvil cell”, a marvelous device for submitting a sample to hydrostatic pressures easily in excess of 10 GPa (10⁵ atmospheres), with very good hydrostatic conditions through the use of rare gasses, like helium or argon, as the pressure transmitting medium. We have adapted this technique over more than 20 years in order to allow various kinds of measurements to be made in the diamond anvil cell. We developed the possibility to introduce wires inside the pressure chamber, and make contacts on tiny samples, allowing resistivity measurements ^[2]. We also invented a technique, by welding a thermocouple to the sample, to perform calorimetry measurements under pressure ^[3]. and susceptibility, with in addition the possibility of fine tuning the pressure at low temperature. All these measurements can be performed in extreme conditions of very low temperatures and high magnetic field. Such measurements are crucial to search for transitions within a superconducting phase, such as the recently discovered multiple superconducting phases in UTe₂^[4]. We also perform AC calorimetry measurements by introducing a small pick-up coil inside the pressure chamber, allowing the detection of a superconducting transition as we showed in the iron based superconductor FeSe ^[5]. More recently we have also become interested in the study of some families of insulating materials, like Mott insulators ^[6], mulitferroics ^[7], or quantum spin systems. For such studies, in addition to calorimetry measurements, we have developed dielectric constant measurements in the diamond anvil cell ^[8].

We have also developed a system to directly change the pressure at low temperature, using helium bellows and a home-designed force amplification device ^[9]. This system is now installed on a dilution refrigerator, allowing a precise control of the pressure, without the necessity to warm up the pressure cell to change the pressure.

Measurements in the lab using the diamond anvil cell are performed at pressures up to 15 GPa, temperatures down to 20mK, and in magnetic fields up to 16T. Our cell can also be used in the Laboratoire Nationale des Champs Magnétiques Intenses (LNCMI) in magnetic fields up to 30T.
In addition to the diamond anvil cell, we have also developed other high-pressure techniques. Our in-situ pressure tuning device also allows the very smooth continuous application of uniaxial pressure. Uniaxial pressure can sometimes give very complementary information to hydrostatic pressure. For example, in the ferromagnetic superconductor URhGe, uniaxial pressure drives the Curie temperature in the opposite direction to hydrostatic pressure ^[10]. For measurements in even higher magnetic fields than those available in LNCMI Grenoble, we have developed a pressure cell adapted to measurements in pulsed magnetic fields in the LNCMI Toulouse. This cell now allows measurements up to 60 T at pressures up to 6 GPa, and temperatures down to 1.4 K ^[11].
This arsenal makes the IMAPEC team one of leaders worldwide for the study of strongly correlated systems under pressure. Our main focus today is on the fascinating uranium based superconductors, but the unique capacity and versatility of our techniques draws multiple collaborations on neighboring topics, from excellent groups worldwide.

^[1] A. P. Drozdov et al., Nature 569, 528 (2019)
^[2] J. Thomasson et al., Solid State Comm. 106, 637 (1998)
^[3] A. Demuer et al., J. Low Temp. Phys. 120, 245 (2000)
^[4] D. Braithwaite et al., Communications Physics 2, 147 (2019)
^[5] D. Braithwaite et al., Journal of Physics: Condensed Matter 21, 232202 (2009)
^[6] J. Mokdad et al., Physical Review B 100, 245101 (2019)
^[7] W. Lafargue-Dit-Hauret et al., Physical Review B 103, 214432 (2021)
^[8] J. Mokdad et al., Review of Scientific Instruments 91, 093902 (2020)
^[9] B. Salce et al., Rev. Sci. Instrum. 71, 2461 (2000)
^[10] D. Braithwaite et al., Physical Review Letters 120, 037001 (2018)
^[11] D. Braithwaite et al., Review of Scientific Instruments 87, 023907 (2016)