Abstract
The crystal structure of a new superconductor UTe2 has been investigated using single-crystal neutron diffraction for the first time at the low temperature (LT) of 2.7 K, just above the superconducting transition temperature of ∼1.6 K, in order to clarify whether the orthorhombic structure of type Immm (No. 71), reported for the room-temperature (RT) structure persists down to the superconducting phase and can be considered as a parent symmetry for the development of spin-triplet superconductivity. In contrast to the previously reported phase transition at about 100 K [Stöwe (1996). J. Solid State Chem. 127, 202-210], our high-precision LT neutron diffraction data show that the body-centred RT symmetry is indeed maintained down to 2.7 K. No sign of a structural change from RT down to 2.7 K was observed. The most significant change depending on temperature was observed for the U ion position and the U-U distance along the c direction, implying its potential importance as a magnetic interaction path. No magnetic order could be deduced from the neutron diffraction data refinement at 2.7 K, consistent with bulk magnetometry. Assuming normal thermal evolution of the lattice parameters, moderately large linear thermal expansion coefficients of about α = 2.8 (7) × 10-5 K-1 are estimated.
Highlights
Very recently, unconventional spin-triplet superconductivity has been reported in UTe2 below 1.6 K (Ran, Eckberg et al, 2019; Aoki et al, 2019)
Starting from the orthorhombic structural model of space group Immm at RT, assuming a one-step symmetry lowering at an intermediate temperature [in agreement with findings by Stowe (1996)] and considering the group–subgroup relationship, a few possible subgroups can be identified as potential candidate structures for the refinement of our low temperature (LT) data
Our single-crystal neutron diffraction results are consistent with previously measured electrical resistivity, magnetization and specific heat data over a wide temperature range (Ran, Eckberg et al, 2019)
Summary
Unconventional spin-triplet superconductivity has been reported in UTe2 below 1.6 K (Ran, Eckberg et al, 2019; Aoki et al, 2019). The single-crystal data did not reveal any anomalies; powder diffraction showed a clear phase transition between 110 and 92 K where a significant change in all three lattice parameters occurs. Ran, Eckberg et al (2019) performed the first attempt to determine the crystal structure of UTe2 at LT (5 K) using cold neutron powder diffraction. No additional peaks in the LT neutron powder diffraction pattern, compared with the RT X-ray one, were observed in discrepancy with the previous findings; no structural refinement was performed (Ran, Eckberg et al, 2019). In order to check these scenarios and rule out possible ambiguity in structural determination and to provide structural details on UTe2 close to the superconducting phase transition, we performed a single-crystal neutron diffraction experiment at LT. The obtained data were refined starting from the RT orthorhombic structural model (space group Immm) and following the possible highest group–subgroup symmetry lowering relation paths
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