Coexistence of ferromagnetic fluctuations and superconductivity in the actinide superconductor ${\mathrm{UTe}}_2$

We report low-temperature muon spin relaxation/rotation ($\mu \mathrm{SR}$) measurements on single crystals of the actinide superconductor ${\mathrm{UTe}}_2$. Below 5 K we observe a continuous slowing down of magnetic fluctuations that persists through the superconducting transition temperature ($T_c=1.6$ K), but we find no evidence of long-range or local magnetic order down to 0.025 K. The temperature dependence of the dynamic relaxation rate down to 0.4 K agrees with the self-consistent renormalization theory of spin fluctuations for a three-dimensional weak itinerant ferromagnetic metal. Our $\mu \mathrm{SR}$ measurements also indicate that the superconductivity coexists with the magnetic fluctuations.

Figure 1

Temperature dependence of the ZF exponential relaxation rates obtained from fits of the ZF-$\mu \mathrm{SR}$ asymmetry spectra to Eq. (1). Inset: Representative ZF signals for $T=4.9$ K and $T=0.04$ K. The solid curves are the resultant fits to Eq. (1).

Figure 2

Representative LF-$\mu \mathrm{SR}$ asymmetry spectra at (a) 2.5 K and (b) 0.25 K, for several different values of the applied magnetic field. The solid curves are fits to Eq. (1).

Figure 3

Field dependence of the relaxation rates ${\lambda }_1$ and ${\lambda }_2$ from the fits of the LF-$\mu \mathrm{SR}$ asymmetry spectra at (a) 2.5 K, and (b) 0.25 K. The solid red curves are fits of ${\lambda }_1\left(H_{\mathrm{LF}}\right)$ to Eq. (2).

Figure 4

Temperature dependence of ${\lambda }_1/T$ ($\equiv 1/T_{1}T$) for zero field. The solid blue line is a fit of the data over $0.4\le T\le 4.9$ K to the power-law equation $1/T_{1}T\propto T^{-n}$, which yields the exponent $n=1.35\pm 0.04$. The dashed line is a similar fit over $0.037\le T\le 0.3$ K, yielding $n=1.12\pm 0.14$. The inset shows a plot of $T/{\lambda }_1$ ($\equiv T_{1}T$) versus $T^{1.12}$ with a linear fit that yields the $T=0$ intercept $T/{\lambda }_1=(0.7\pm 4.2)\times {10}^{-3}$ K $\mu \mathrm{s}$.

Figure 5

Weak TF-$\mu \mathrm{SR}$ asymmetry spectra recorded for $H=23$ Oe. The solid curves are fits to Eq. (3).

Figure 6

(a) TF-$\mu \mathrm{SR}$ asymmetry spectra at $T=0.05$ and 5 K for a magnetic field $H=1$ kOe applied parallel to the $z$ direction, displayed in a rotating reference frame frequency of 13.15 MHz. The solid curves are fits to Eq. (4). Temperature dependence of the fitted (b) muon spin precession frequencies, and (c) TF relaxation rates.