NMR study of spin-triplet superconductivity in uranium-based materials

Y. Tokunaga1), H. Sakai1), S. Kambe1), G. Nakamine2) S. Kitagawa2), K. Ishida2),
A. Nakamura3), Y. Shimizu3), Y. Homma3), D.-X. Li3), F. Honda3), and D. Aoki3)
1) : Materials Physics for Heavy Element Systems Gr., JAEA 2): Kyoto Univ. 3): Tohoku Univ.

While ferromagnetism and superconductivity are
generally exclusive to each other, these two phenomena
coexist on a microscopic scale in some uranium-basedmaterials, so-called “ferromagnetic superconductors”
UGe2, UCoGe and URhGe. A characteristic feature of thematerials is that they exhibit very large upper critical fields
of superconductivity, by far exceeding the ordinary Pauliparamagnetic limit for spin-singlet pair, providing a strong
indication of an exotic, spin-triplet pairing (Fig.1).
In November 2018, S. Ran et al. in University ofMaryland reported evidence for superconductivity in auranium-based material UTe2, exhibiting rather hightransition temperature of 1.6 K [1]. Their finding was soonafter confirmed by D. Aoki et al in Tohoku University [2].
While the ground state of UTe2 is paramagnetic, notferromagnetic, the material still exhibits a very large upper
critical field of the superconductivity, similar to theferromagnetic superconductors. UTe2 is thus suggested tohost the spin-triplet pairing. Further discovery of fieldinduced superconductivity above 40 T highlights a closeinterplay between superconductivity and magnetism in
UTe2 [3].

Fig.1 In the superconducting state, two electrons form the Cooper pairwith making collective motion. In conventional spin-singlet state, theelectron spins of the pairs are all antiparallel and the total spin moment
is zero. On the other hand, in the spin-triplet state, the spins are parallel
and the total spin moment is one.

In this study, in order to elucidate the mechanism of theunconventional superconductivity in UTe2, we have
performed 125Te-NMR experiments using a high-quality
single crystal [4, 5]. The crystal structure of UTe2 is thebody-centered orthorhombic structure, where Te atoms
occur at two different sites: Te1 and Te2 sites with point
symmetries mm2 and m2m, respectively. The inset of Fig.2

shows an example of NMR spectrum measured with fieldalong the crystal b-axis [4]. The spectrum consists of two
distinct peaks arising from the Te1 and Te2 sites. Thenarrow NMR lines point to the high quality of our singlecrystal.

Fig.2 The temperature dependence of the spin-spin relaxation rate,
1/T2 obtained for fields applied along the a-axis [4]. The inset shows
an example of NMR spectrum measured with field along the b-axis.

In the paramagnetic state, the NMR Knight shift K andthe spin-lattice relaxation rate, 1/T1T confirmed a moderate
Ising-type anisotropy for both the static and dynamicalsusceptibilities above 20 K [4]. On the other hand, a sudden
increase of the spin-spin relaxation rate, 1/T2 toward 20 K
demonstrates the development of strong longitudinalmagnetic fluctuations along the a-axis at very low
frequency (Fig.2). The NMR data suggests that UTe2 isclose to a ferromagnetic instability, where a characteristic
crossover temperature for spin fluctuations exists around
20 K.
Theoretically, the spin-triplet superconductivity is
tightly connected to an exotic pairing mechanism forsuperconductivity: the superconducting pairs are mediatedvia spin fluctuations rather than by ordinary electron–phonon coupling [6]. Our results imply that the spinfluctuations near a ferromagnetic quantum critical point
are at the origin of the spin-triplet pairing in UTe2.
References[1] S. Ran et al., Science, 365 684 (2019).
[2] D. Aoki et al., J. Phys. Soc. Jpn., 88 043702 (2019).
[3] S. Ran et al. Nat. Phys. 15, 1250 (2019).
[4] Y. Tokunaga et al., J. Phys. Soc. Jpn., 88 073701 (2019).
[5] G. Nakamine et al., J. Phys. Soc. Jpn., 88 113703 (2019).
[6] D. Fay and J. Appel, Phys. Rev. B 22, 3173 (1980).