Coexistence of ferromagnetic fluctuations and superconductivity in the actinide superconductor UTe2

Abstract

We report low-temperature muon spin relaxation/rotation (µSR) measurements on single crystals of the actinide superconductor UTe2. Below 5 K we observe a continuous slowing down of magnetic fluctuations, which persists through the superconducting transition temperature (Tc = 1.6 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. However, we find no evidence of long-range or local magnetic order down to 0.025 K. Weak transverse-field µSR measurements indicate that the superconductivity coexists with the magnetic fluctuations.

Orientation of point nodes and nonunitary triplet pairing tuned by the easy-axis magnetization in UTe2

“…In order to lift the degeneracy of the multiple order parameters in zero field, a spontaneous magnetization or very slow longitudinal spin fluctuations are needed. This requirement suggests a possibility that a shortrange magnetic order of M a developing in UTe 2 above T c [9,10,12,19] breaks the degenerate order parameters; this is similar to the case of UPt 3 [43,44] which shows a double superconducting transition coupled with a short-range antiferromagnetic order [45,46]. One may suspect that the small γ 0 , less than half of the normal-state value [47], is inconsistent with the nonunitary pairing scenario.…”

“…The presence of linear point nodes in the superconducting gap has been suggested from specific heat [9], nuclear spinlattice relaxation rate 1/T 1 [13], penetration depth [17], and thermal conductivity [17] measurements. Although the results of recent scanning tunneling microscopy (STM) experiments suggest a chiral order parameter [18], broken time-reversal symmetry in the superconducting state has not yet been detected from muon-spin-relaxation measurements [19]. Meanwhile, a recent surface impedance measurement suggests the presence of point nodes somewhere within the ab plane [20].…”

Recent progress on superconductors with time-reversal symmetry breaking

“…1(b), and very recently, UTe 2 using the same technique 23,41 . In the case of the UTe 2 , µSR measurements exhibited a strong temperature dependent relaxation corresponding to ferromagnetic fluctuations, and therefore whether TRS is preserved in the superconducting state could not be probed 42 . For Kerr rotation measurements of UPt 3 , the onset of TRS breaking could be shown to occur at the lower superconducting transition.…”

Tuning magnetic confinement of spin-triplet superconductivity

“…Previous work on uranium-based compounds, such as UGe 2 , URhGe, and UCoGe, has unearthed a rich interplay between superconductivity and ferromagnetism in this family of materials 1 , with suggestions that ferromagnetic spin fluctuations can act to enhance pairing 2 . The recent discovery of superconductivity in UTe 2 has drawn strong attention owing to a fascinating list of properties-including the absence of magnetic order at ambient pressure 3 , Kondo correlations, and extremely high upper critical fields 4 -that have led to proposals of spin-triplet pairing [4][5][6][7] , and a chiral order parameter 8,9 .…”