High-magnetic-field phases in U_1-xTh_xTe₂

At temperatures T much lower than its superconducting critical temperature T_c = 2.1 K, the heavy fermion superconductor UTe₂ has a unique phase diagram of magnetic field H vs. φ and θ, angles H is tilted from the b-axis toward the a- and c-axes, respectively, of its orthorhombic unit cell. The phase diagram contains three distinct superconducting phases: SC₁ in which φ and θ extend from 0 to 90° and H ≤ ~15 T; SC₂ for φ ≤ ~7°, θ ≤ ~4° and ~15 T ≤ H ≤ H_m = ~35 T, the onset of the magnetic field polarized (FP) phase, and SC_FP which resides entirely within the FP phase in a pocket of superconductivity extending from θ ≈ 20° to 40° and from ~40 T to above 60 T. We studied the evolution of the H vs. θ phase diagram for Th concentrations 0.005 ≤ x ≤ 0.047 in the U_1-xTh_xTe₂ system at ~0.6 K. Within this range of x values, SC₁ extends over 0 ≤ θ ≤ 90° and H ≤ ~10 T for x = 0.047, while SC₂ is suppressed. The SC_FP and FP phases are unaffected to x = 0.02 but are completely suppressed in the region x = 0.025 to 0.047 where the residual resistance ratio RRR ~3 indicates a significant amount of disorder. These results complement recent studies of nonsuperconducting disordered UTe₂ single crystals in which the SC₁ and SC₂ phases are absent, but the FP and so-called "orphan" SC_FP phases are retained.