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High-Pressure High-Temperature Superconductivity

Periodic Reporting for period 2 - HPSuper (High-Pressure High-Temperature Superconductivity)

Reporting period: 2018-08-01 to 2020-01-31

Superconductors facilitate electrical currents without losses. This is used for applications like very strong magnets for imagining in medicine and for power transmission. First prototypes of superconducting motors in cargo ships are operational and airplaines are being developed. All these applications, however, are limited by the need to cool superconductors below a transition temperature. Most applications use niobium based superconductors which require cooling to below -250 °C.

New superconductors with a higher transition temperature are thus very desirable for applications. It has been predicted that hydrogen and hydrogen compounds will allow to achieve high transition temperatures. Indeed, hydrogen compounds with new record transition temperatures have been discovered very recently. For hydrogen sulphide H3S a transition temperature of -70°C has been found at a very high pressure in 2015 while transition temperatures of -15°C and -45°C were found for lanthanum hydride LaH10 and yttrium hydride YH6. All these compounds are superconductors at very high pressures of 1-2 million bar only. Such high pressures are achieved with diamond anvil pressure cells where only micrometre-sized samples are produced. Whilst such high pressures are not suitable for applications, these materials provide the opportunity to study high-transition-temperature superconductors. This insight will help to design superconductors for use at room temperature and ambient pressure.

This research project has two main objectives. First, it aims to reveal the microscopic mechanism of superconductivity in hydrogen compounds at high pressures and second it will search for new superconductors with higher transition temperature and at lower pressure. So far, work has focused towards the first objective, understanding the mechanism of high-temperature superconductivity in hydrogen sulphide. Work towards the second objective will start in 2020.

Understanding the mechanism of high-temperature superconductivity is achieved through detailed studies of the properties of hydrogen-based superconductors such as H3S, LaH10 and YH6 at high pressures. We focus on several key parameters of the superconducting and normal metal states including the charge carrier concentration, strength of the electron-phonon coupling, London penetration depth, superconducting coherence length, superconducting gap, and Fermi surface topology as well as isotope effect on these. Together these properties provide crucial insight into the mechanism of superconductivity which is not yet available. Electrical transport and magnetic measurements are used in this project to extract the above key parameters.

Electrical transport studies include Hall effect and resistivity as well as tunnelling and quantum oscillation measurements. Such measurements require electrical contacts to the micrometre-sized sample inside the diamond anvil pressure cell. The sample is enclosed between two diamonds and a metal gasket. Thus, access to the sample is very difficult. The project benefits from long-term expertise of the researchers in producing metal tracks on anvils as a replacement for conventional wires. The development of simple methods to produce these tracks in the small size necessary for the diamond anvils at 1 million bar has been successfully completed. A picture of such a microscopic arrangement of the electrodes is shown in Figure 1. This configuration can be used for resistivity, Hall effect, and quantum oscillation measurements. Tunnelling measurements will require additional insulation layers which will be implemented in the near future. As a next step, the project will focus on producing samples of hydrogen sulphide at high pressures.
Producing high-pressure samples of H3S, LaH10, and YH6 inside a diamond anvil cell requires several apparati. As a first step, diamond anvil cells compatible with the low-temperature resistivity and Hall effect measurements were developed (Figure 2). Both diamond anvil cells for measurements in lab-based cryostats with commercial magnets as well as for measurements at international high-magnetic-field facilities have been successfully tested. Results from these measurements are currently being analysed and we are preparing several manuscripts to publish this work shortly.
We have studied the formation of H3S via different synthesis routes. The results obtained so far demonstrate the delicacy of preparing H3S, LaH10, and YH6 for high-pressure studies. This highlights that a big challenge on the way towards applications of superconductivity lie with the materials preparation. Our results on the synthesis of H3S will be important for the community in the global effort to synthesise and test hydrogen compounds at high pressures.

Our test measurements at low pressures and the training projects for PhD students engaged in the project have delivered results on the high-pressure superconductivity of NbSe2, cuprate superconductor YBa2Cu3O7−δ, and TiSe2. These results highlight the interplay of competing orders in these materials. Publications for these results are in preparation and will certainly contribute to the understanding of the superconductivity in these materials and in general.
Diamond anvil with gold tracks and sulphur sample for electrical transport measurements