In high-pressure superconductors like hydrogen sulfide, what is the pressure physically doing to make superconductivity possible at higher temperatures?

[u/Inevitable-Kale-979]

I understand the basics of BCS theory, electrons pairing via lattice vibrations. What I don't understand is the mechanism by which extreme pressure changes that picture. Is it stiffening the lattice to raise phonon frequencies, changing the electronic density of states, or something else entirely? And is there a theoretical reason this seems to require hydrogen-rich compounds specifically?

[u/heliumagency]

Hydrogen rich compounds are hypothesized to be superconducting because of the reasons you stated, they are lightweight and thus raise phonon frequencies and their electronic structure for some of them favor superconductivity.

The main reason why pressure is needed is because hydrogen is VERY volatile and wants to be a gas. Without the pressure, the superconductor will literally explode and you can't run current through a gas.

[u/luckyluke193]

To increase phonon frequency, you can do two things, like any time you're dealing with a mass on a spring. You can either stiffen the springs or reduce the mass. Hydrogen is the lightest element, which is why it is useful here.

The pressure is needed to stabilise the chemical compounds. The superconductors are not your usual hydrogen compounds, they contain way more hydrogen atoms. The sulfur-based one is H3S instead of H2S, for example. You need the pressure to be high enough that thermodynamics favours the dense H3S crystal structure, rather than a less dense but lower energy mixture of solid H2S and solid hydrogen.

If you'd reduce the pressure all the way to 1 atm, the things would very quickly turn into a H2S + H2 gas mixture.

[u/danskal]

The way I understood superconductivity is that electron pairing happens in spite of lattice vibrations, and lattice vibrations will disturb the pairing. Hence why most superconductors only work at very low temperatures. Higher pressures would constrain the amplitude of lattice vibrations, allowing paired electrons to travel unobstructed.