High-Magnetic Field Phases in U${}_{1-x}$Th${}_x$Te${}_2$

Abstract

At temperatures much lower than its superconducting critical temperature $T_c$ of 2.1 K, the heavy fermion superconductor UTe${}_2$ has a remarkable phase diagram of magnetic field $H$ vs. angles $\varphi$ and $\theta$ at which $H$ is tilted away from the $b$-axis toward the $a$- and $c$-axes, respectively, in the orthorhombic unit cell. The phase diagram appears to contain three superconducting phases: (1) a low field superconducting phase SC${}_{\mathrm{LF}}$ extending over all values of $\varphi$ and $\theta$ with an upper critical field $H_{c2}$ with a maximum value of 15 T at $\varphi =\theta =0^{\circ }$; (2) a high field superconducting phase SC${}_{\mathrm{HF}}$ located in a region between $\varphi \approx 7^{\circ }$ and $\theta \approx 4^{\circ }$ in fields from $H_{c2\mathrm{LF}}$ of the SC${}_{\mathrm{LF}}$ phase and the metamagnetic transition at $H_m$ at $\sim 35$ T marking the onset of the magnetic field polarized FP phase: and (3) a SC${}_{\mathrm{FP}}$ superconducting phase that resides entirely within the FP phase in a pocket of superconductivity extending from $\theta \approx 20^{\circ }$ to $40^{\circ }$ in fields from $\sim 40$ T to above 60 T. In this work, we studied the $H$ vs $\theta$ phase diagram at a base temperature of $\sim 0.6$ K as a function of Th concentration $x$ in U${}_{1-x}$Th${}_x$Te${}_2$ pseudobinary compounds for $0.5\%\lesssim x\lesssim 4.7\%$. We find that for all values of $x$ within this range, the SC${}_{\mathrm{LF}}$ phase is retained with a reduced value of $H_{c2}$ of $\sim 10$ T at $\varphi =\theta =0^{\circ }$ for $x=4.7\%$, while the SC${}_{\mathrm{HF}}$ phase is suppressed. The SC${}_{\mathrm{FP}}$ and FP phases are unaffected to values of $x=2\%$ but are completely suppressed in the region $x=2.5$ to $4.7\%$ where the residual resistance ratio RRR has decreased from $\sim 14$ at $x=1.5\%$ to values of $\sim 3$, indicating a significant increase in disorder.