Theoretical studies on off-axis phase diagrams and Knight shifts in UTe$_2$ -- Tetra-critical point, d-vector rotation, and multiple phases

Abstract

Inspired by recent remarkable sets of experiments on UTe$_2$: discoveries of the fourth horizontal internal transition line running toward a tetra-critical point (TCP) at $H$=15T, the off-axis high field phases, and abnormally large Knight shift (KS) drop below $T_{\rm c}$ for $H$$\parallel$$a$-magnetic easy axis, we advance further our theoretical work on the field ($H$)-temperature ($T$) phase diagram for $H$$\parallel$$b$-magnetic hard axis which contains a positive sloped $H_{\rm c2}$ departing from TCP. A nonunitary spin-triplet pairing with three components explains these experimental facts simultaneously and consistently by assuming that the underlying normal electron system with a narrow bandwidth characteristic to the Kondo temperature $\sim$60K unsurprisingly breaks the particle-hole symmetry. This causes a special invariant term in Ginzburg-Landau (GL) free energy functional which couples directly with the 5f magnetic system, giving rise to the $T_{\rm c}$ splitting and ultimately to the positive sloped $H_{\rm c2}$ and the horizontal internal transition line connected to TCP. The large KS drop can be understood in terms of this GL invariance whose coefficient is negative and leads to a diamagnetic response where the Cooper pair spin is antiparallel to the applied field direction. The present scenario also accounts for the observed d-vector rotation phenomena and off-axis phase diagrams with extremely high $H_{\rm c2}$$\gtrsim$70T found at angles in between the $b$ and $c$-axes and between the $bc$ plane and $a$-axis, making UTe$_2$ a fertile playground for a topological superconductor.