Superconductors allow to transmit electrical currents without losses. This is used for applications including very strong magnets for imagining in medicine and for power transmission. First prototypes of superconducting motors in cargo ships are operational and airplanes 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. Hydrogen compounds have recently achieved high transition temperatures close to 0°C. This includes hydrogen sulphide H3S with a transition temperature of -70°C found 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 aims to reveal the microscopic mechanism of superconductivity in hydrogen compounds at high pressures.
Understanding the mechanism of high-temperature superconductivity is achieved through detailed studies of the properties of hydrogen-based superconductors such as H3S, LaH10 and related compounds 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. 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. In addition, we search for novel superconductors with high transition temperature at lower pressures.
Our electrical transport studies require 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 various measurements including resistivity and Hall effect.
We have developed diamond-anvil cells to generate high pressures and to allow a wide range of measurements on individual samples. This allowed us to correlate comprehensive experimental results to obtain as clear as possible insight into the high-temperature superconductivity. In the case of hydrogen sulphide, we find that the superconductivity is represented by clean behaviour. This in turn enables us to identify with high certainty that superconductivity is driven by strong interactions of electrons with lattice vibrations and we quantify the strength of this interaction.
We conclude that the strong interactions are required to enable high-temperature superconductivity. Together with theory colleagues, we have been able to identify routes to realise strong interactions at lower pressures for instance in lanthanum hydride La4H23 where we indeed discovered superconductivity. This paves a way for the search of novel high-temperature superconductors at ambient pressure.
