Magnetic Methods in Studies of New Superconducting Hydrides in a Diamond Anvil Cell

In recent years, we witnessed a great success in discoveries of the high-Tc hydrides in high pressure experiments. This was achieved by creating metallic hydrogen ‘alloys’ with other elements across the periodic table. Although the original idea suggested by Neil Aschcroft [1] was to use alloying to reduce the pressure required for producing pure metallic hydrogen, the pressures required to stabilize these hydrides are still so high that in-situ measurements of the complete expulsion of the magnetic field (the Meissner state), a defining character of superconductivity, is difficult to obtain. Such deficit of the Meissner effect studies has provoked multiple critiques on claims of superconductivity in these new hydride materials, and stimulated new technical developments to meet these challenges. In this perspective article, we will provide a short summary of the results obtained with four alternative techniques which were used to probe the magnetic field expulsion and the Meissner state in a diamond-avnvil cell (DAC), which is the only high-pressure vessel capable of studying the high Tc superconducting hydrides at very high pressures.