Nuclear Magnetic Resonance Study on Multiple Superconducting Phases in UTe₂

概要

Summary of thesis: Nuclear Magnetic Resonance Studies on
Multiple Superconducting Phases in UTe2
Katsuki Kinjo
In this thesis, I report the superconducting properties in UTe2 studied with
nuclear magnetic resonance measurements. In superconductivity, two electrons form a
pair called the “Cooper pair”. Hence, superconductivity is classified into two categories:
spin-singlet or spin-triplet superconductivity. In spin-triplet superconductors, the total
spin angular momentum S is 1, resulting in the remaining of spin and orbital degrees of
freedom. Due to these degrees of freedom, unusual behaviors such as multiple
superconducting (SC) phases and the SC-SC transition induced by the magnetic field
(H) are expected. However, there are few candidates for spin-triplet superconductors,
and thus, spin-triplet multiple SC phases have not been intensively investigated so far.
In 2018, S. Ran et al. discovered the superconductivity in UTe2 with its
superconducting critical temperature Tc ~ 1.6 K[1]. UTe2 is a leading candidate for the
spin-triplet superconductor because the physical properties are very similar to those in
the U-based ferromagnetic superconductors: magnetic properties and the anisotropy of
the upper critical field (Hc2) of superconductivity, especially H-boosted
superconductivity under the b-axis field, and the extremely large upper critical field
(Hc2)[1]. Furthermore, recent nuclear magnetic resonance (NMR) measurements
revealed the tiny change or absence of the change in the spin susceptibility, supporting
the spin-triplet scenario[2-4].
In UTe2, multiple SC
phases induced by the b-axis
high magnetic field above 16
T (HFSC) or pressure above
0.3 GPa (SC2) were observed
as shown in Fig. 1. To
investigate
their
SC
properties, especially the spin
Fig. 1 Schematic image of multiple SC phases in UTe2.
state in these phases, we
performed the 125Te-NMR measurements under a high H[5] and high pressure[6].
Chapter 6: Change of superconducting character in UTe2 induced by magnetic
field
For high-H measurement, above 20 T, the change in the NMR Knight shift (ΔK)
below Tc(H), which is proportional to the change in the spin susceptibility in the SC

state, becomes zero, evidencing the SC spin rotation induced by the high H.
Chapter 7: Drastic change in magnetic anisotropy of UTe2 under pressure revealed
by 125Te-NMR
and
Chapter 8: Superconducting-spin rotation in spin-triplet multiple superconducting
phases in UTe2
Under high pressure, the new SC phases are observed above 0.3 GPa (SC2, SC3
in Fig. 1), and superconductivity suddenly disappears above 1.6 GPa. Our NMR results
under pressure above Tc revealed that the electronic state dramatically changes above
1.6 GPa, and that superconductivity occurs only in the heavy-fermion state. [6].
In the superconducting state under pressure (SC2), the NMR Knight shift does
not show any anomaly at Tc. After subtracting the temperature dependence of the
normal state, ΔK ascribed to the superconductivity is almost zero, indicating the
superconducting spin is almost aligned to the b axis. In addition, below 0.5 K, near the
transition temperature from SC2 to SC3, the NMR Knight shift shows a clear decrease
and the line width of the NMR spectrum suddenly increases. In contrast to the
conventional superconductivity, the line-width increase in SC3 is proportional to
applied H. These features may come from the time-reversal symmetry-breaking
superconductivity due to the mixture of SC order parameters of SC1 and SC2 in SC3
state[7].

References
[1] S. Ran et al., Science 365, 684-687 (2019).
[2] G. Nakamine et al., J. Phys. Soc. Jpn. 88, 113703 (2019).
[3] G. Nakamine, KK, et al., Phys. Rev. B 103, L100503 (2021).
[4] H. Fujibayashi, KK, et al., J. Phys. Soc. Jpn. 91, 043705 (2022).
[5] K. Kinjo et al., arXiv:2206.02444, (2022). Submitted to the Phys. Rev. B.
[6] K. Kinjo et al., Phys. Rev. B 105, L140502 (2022).
[7] K. Kinjo et al., Submitted to Science Advance. ...

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