Excitonic Insulator to Superconductor Phase Transition in Ultra-Compressed Helium

TL;DR

This study predicts that under extreme compression, helium transitions from an excitonic insulator to a superconductor, revealing novel quantum phases and enhanced electron-phonon interactions at terapascal pressures.

Contribution

It uncovers the excitonic insulator phase in helium before metallization and demonstrates the potential for high-temperature superconductivity at ultra-high pressures.

Findings

Helium becomes an excitonic insulator prior to metallicity.

Electron-phonon coupling is significantly enhanced during the phase transition.

Predicted superconducting critical temperatures are approximately 30 K at 20 TPa and 100 K at 100 TPa.

Abstract

Helium, the second most abundant element in the universe, exhibits an extremely large electronic band gap of about $20$ eV at low pressures ($\le 0.1$ GPa). While the metallization pressure of hcp helium has been accurately predicted, thus far little attention has been paid to the specific mechanisms driving the band-gap closure and electronic properties of this quantum crystal in the terapascal regime (1 TPa $= 1,000$ GPa). Here, we employ state-of-the-art density functional theory and many-body perturbation theory calculations to fill up this knowledge gap. It is found that prior to reaching metallicity bulk solid helium becomes an excitonic insulator (EI), an exotic state of matter typically observed in low-dimensional systems in which electrostatically bound electron-hole pairs form spontaneously. Furthermore, it is shown that electron-phonon coupling (EPC) is significantly enhanced across the EI to metal phase transition as signaled by prominent phonon softening and giant EPC strength values ($\lambda \sim 10-100$) estimated at specific reciprocal space points. Accordingly, we predict metallic helium to be a superconductor with a critical temperature of $\approx 30$ K at $20$ TPa and of $\approx 100$ K at $100$ TPa. These unforeseen phenomena have important consequences on the elastic, thermodynamic and transport properties of metallic helium hence may be critical for improving our fundamental understanding and modelling of celestial bodies.