High-magnetic-field phases in U_1-xTh_xTe₂.

Abstract

At temperatures <i>T</i> much lower than its superconducting critical temperature <i>T</i>_c = 2.1 K, the heavy fermion superconductor UTe₂ has a unique phase diagram of magnetic field <i>H</i> vs. φ and θ, angles <i>H</i> is tilted from the <i>b</i>-axis toward the <i>a</i>- and <i>c</i>-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 <i>H</i> ≤ ~15 T; SC₂ for φ ≤ ~7°, θ ≤ ~4° and ~15 T ≤ <i>H</i> ≤ <i>H</i>_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 <i>H</i> vs. θ phase diagram for Th concentrations 0.005 ≤ <i>x</i> ≤ 0.047 in the U_1-xTh_xTe₂ system at ~0.6 K. Within this range of <i>x</i> values, SC₁ extends over 0 ≤ θ ≤ 90° and <i>H</i> ≤ ~10 T for <i>x</i> = 0.047, while SC₂ is suppressed. The SC_FP and FP phases are unaffected to <i>x</i> = 0.02 but are completely suppressed in the region <i>x</i> = 0.025 to 0.047 where the residual resistance ratio <i>RRR</i> ~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.