Mechanism of the insulator-to-metal transition and superconductivity in the spin liquid candidate NaYbSe$_2$ under pressure

TL;DR

This study investigates how pressure induces an insulator-to-metal transition and superconductivity in NaYbSe$_2$, revealing a two-stage transition involving charge transfer, Kondo effects, and heavy fermion behavior, with implications for Yb-based superconductors.

Contribution

The paper combines density functional theory and dynamical mean-field theory to elucidate the electronic phase evolution of NaYbSe$_2$ under pressure, highlighting a two-stage transition and potential high-$T_c$ superconductivity.

Findings

NaYbSe$_2$ is a charge-transfer insulator at ambient pressure.

Under pressure, it transitions to a semi-metallic and then a heavy fermion metallic state.

Superconductivity appears in the heavy fermion phase with nested Yb-4$f$ Fermi surfaces.

Abstract

The quantum spin liquid candidate NaYbSe$_2$ was recently reported to exhibit a Mott transition under pressure. Superconductivity was observed in the high-pressure metallic phase, raising the question concerning its relation with the low-pressure quantum spin liquid ground state. Here we combine the density functional theory and the dynamical mean-field theory to explore the underlying mechanism of the insulator-to-metal transition and superconductivity and establish an overall picture of its electronic phases under pressure. Our results suggest that NaYbSe$_2$ is a charge-transfer insulator at ambient pressure. Upon increasing pressure, however, the system first enters a semi-metallic state with incoherent Kondo scattering against coexisting localized Yb-$4f$ moments, and then turns into a heavy fermion metal. In between, there may exist a delocalization quantum critical point responsible for the observed non-Fermi liquid region with linear-in-$T$ resistivity. The insulator-to-metal transition is therefore a two-stage process. Superconductivity emerges in the heavy fermion phase with well-nested Yb-4$f$ Fermi surfaces, suggesting that spin fluctuations may play a role in the Cooper pairing. NaYbSe$_2$ might therefore be the 3rd Yb-based heavy-fermion superconductor with a very "high" $T_c$ than most heavy fermion superconductors.