Charge Transfer Effect under Odd-Parity Crystalline Electric Field

TL;DR

This paper theoretically analyzes how odd-parity crystalline electric fields in $eta$-YbAlB$_4$ induce charge transfer effects that activate magnetic-toroidal and electric-dipole fluctuations, which diverge at the quantum critical point.

Contribution

It reveals the role of odd-parity CEF in activating MT and ED degrees of freedom and their critical enhancement at the valence transition in $eta$-YbAlB$_4$.

Findings

MT and ED fluctuations diverge at the quantum critical point

Odd-parity CEF induces admixture of 4f and 5d wave functions

Critical valence fluctuations are enhanced simultaneously

Abstract

Charge-transfer effect under odd-parity crystalline electric field (CEF) is analyzed theoretically. In quantum-critical metal $\beta$-YbAlB$_4$, seven-fold configuration of B atoms surrounding Yb atom breaks local inversion symmetry at the Yb site, giving rise to the odd-parity CEF. Analysis of the CEF on the basis of hybridization picture shows that admixture of 4f and 5d wave functions at Yb with pure imaginary coefficient occurs, which makes magnetic-toroidal (MT) and electric-dipole (ED) degrees of freedom active. By constructing the minimal model for periodic crystal $\beta$-YbAlB$_4$, we show that the MT as well as ED fluctuation is divergently enhanced at the quantum critical point of valence transition simultaneously with critical valence fluctuations.