Research Group for Strongly Correlated Actinide ScienceGroup Leader: TOKUNAGA YoMembers: HAGA Yoshinori, SAKAI Hironori, YAMAMOTO Etsuji, KUBO Katsunori,TOKIWA Yoshifumi, Petr OPLETAL, KITAZAWA Takafumi, KAMBE Shinsaku, KANEKO
Koji, TABATA Chihiro
Materials contain a huge number of electrons, typically in theorder of Avogadro's number. In actinide materials, the electrons
are strongly correlated with each other, and various exotic
phenomena, such as spin-triplet topological superconductivity(SC), novel magnetism, and higher-rank multipolar ordering,
emerge from the strong interactions. By means of advanced
experimental and theoretical approaches, our research group tries
to understand the rich physics of correlated actinide materials.
Ferromagnetic crossover observed inside ferromagnetic
order of U7Te12 [1]U7Te12 is ferromagnet with three non-equivalent uranium
crystallographic positions, two distinctive ferromagnetic (FM)orderings at different temperatures perpendicular to each other.
In ordinary ferromagnets at temperatures below the Curie
temperature (TC) magnetic moments point in the same direction
called easy magnetization axis. By application of external
magnetic field, the FM second order phase transition at TC
changes to crossover due to external magnetic field breaking
time-reversal symmetry.

phenomena due to lack of investigation into materials withmultiple crystallographic positions of magnetic ions and thecomplexity of such investigation.
Change of superconducting character in UTe2 induced bymagnetic field [2]SC occurs when a coherent quantum fluid is formed fromelectron pairs. In most superconductors, although the total spin(S) of the pairs is in the singlet state (S =0), it is also possible tobe in the triplet state (S=1). Such superconductors, called spintriplet superconductors, are coherent quantum fluids with spinand orbital degrees of freedom. Spin-triplet SC would involve
rich physics but are very rare. The recent discovery of FM
superconductors, in which the ferromagnetism and SC arise fromthe same electrons, has made it possible to study the spin-triplet
pairing state in detail. Additionally, a spin-triplet SC candidate
UTe2 has been more recently discovered with a relatively high SC
transition temperature of 1.6–2.1 K.
One of the most characteristic features of UTe2 is a magnetic
field (H)-boosted SC, when the H (>16 T) is applied exactly
parallel to the b axis (Fig. 2). In this study, we performed
magnetic susceptibility and nuclear magnetic resonancemeasurements to understand the SC properties as well as the spinstate of the high-field SC (HHSC) phase. We found that, up to
24.8 T, the HHSC state has bulk nature and is quite sensitive tothe H angle and that its SC character is different from that in thelow-field SC (LHSC) state. We also revealed that the dominant
spin component of the spin-triplet pair is along the a axis in theLHSC state but is changed in the HHSC state along the b axis.
Our results indicate that the H-induced multiple SC phases
originate from the remaining spin degrees of freedom.

Fig. 1 Temperature dependence of magnetic susceptibility of two
perpendicular axes a and c for U7Te12. The data for a-axis (black)show FM transition at 48 K while data for c-axis (white) show
FM transition at 26 K. In the inset we show crystal structure ofU7Te12 with distinguished nonequal U positions.

In U7Te12 at TC = 48 K magnetic moments order along the aaxis, while at 26 K magnetic moments order along the c-axis (Fig.
1) without any sign of reorientation [1]. The different ordering
direction and temperatures are results of different nonequaluranium positions creating two semi-independent magnetic
sublattices. No sign of entropy change is found around 26 K,excluding any phase transition and suggests that ordering at 26 K
is FM crossover in zero external magnetic field induced by timereversal symmetry breaking at TC. The uranium magneticallyordering at TC creates internal magnetic field, which breaks timereversal symmetry and leads to crossover at 26 K.
Interestingly, the local symmetry at TC is lowered much morethan required for FM ordering but the lower symmetry ensuresthe crossover nature of FM transition at 26 K, suggesting anothermechanism might be in play at TC.
The case of U7Te12 is unique and interesting as two separate
magnetic orderings with perpendicular easy magnetization axes
coexisting in one material. It is unusual and not well investigatedFig. 2 (left) H-T phase diagram of UTe2 under H applied exactly parallelto the b axis. The SC transition temperature (Tc) and the upper
critical field (Hc2) are determined from the T and H dependencesof magnetic susceptibility, respectively. Anomalies are also
observed in the susceptibility measurements at Hkink and H* [2].
(right) Schematic view of the crystal structure of UTe2.

References[1] P. Opletal et al., J. Phys. Soc. Jpn. 92, 034704 (2023).
[2] K. Kinjo et al., Phys. Rev. B 107, L060502 (2023).