Research Group for Materials Physics for Heavy Element systems
Group Leader: KAMBE Shinsaku
Members: HAGA Yoshinori, TOKUNAGA Yo, SAKAI Hironori, YAMAMOTO Etsuji, KUBO
Katsunori, TOKIWA Yoshifumi, Petr Opletal

In heavy element systems, valence fluctuations, the Kondo
effect, and the RKKY interaction compete with one another.
Because of this, exotic behaviors such as quantum critical points,
heavyfermions,
non-Fermi
liquids,
anisotropic
superconductivity, and multipolar ordering appear when suchcompetition is strong. By means of advanced experimental andtheoretical approaches, our research group tries to clarify these
exotic behaviors due to the “many-fold” character of both 4 f and
5f compounds, including transuranium.
Spin fluctuations as driving force of superconductivity inuranium-based ferromagnetic superconductors [1]Attractive interaction responsible for superconducting (SC)pairs is mediated by lattice vibrations (phonons) in conventional
spin-singlet superconductors (Fig. 1). The strength of thisinteraction is generally assumed to be field-independent, since thelattice vibrations are not directly coupled to applied magnetic
field. In the case of Uranium-based ferromagnetic (FM)superconductors UGe2, URhGe, and UCoGe, however, theinteraction of spin-triplet pairs is supposed to be mediated by theexchange of FM spin fluctuations, whose excitation spectrum canbe modified strongly by magnetic field. This suggests thepossibility that their pairing interaction is also strongly affectedby magnetic field.
In this study, using NMR technique, we have investigated thefield effect on the FM spin fluctuations and thus the pairing
interactions in URhGe and UCoGe. These two superconductors
have the same crystal structure of the orthorhombic TiNiSi typeand are both weak itinerant ferromagnet with Ising anisotropy
along the same crystal axis. The compounds thus offer similarbases for the SC mechanism. However, their upper critical field
Hc2, one of the most fundamental SC parameters, exhibits some

FM spin fluctuations by a tiny applied field along the easymagnetization axis has been observed. We show that such a
difference is indeed reflected in the above mentioned two distinct
phase diagrams of the Hc2 and hence provides further experimentsupport for the mechanism of the superconductivity mediated by
the FM spin fluctuations.
Universal scaling behavior under pressure in a prototypical
4f heavy fermion antiferromagnet CeRh2Si2 [2]The case of an antiferromagnetic (AFM) heavy fermion (HF) is
considered as a standard example to realize a quantum phasetransition where the AFM transition occurs at TN = 0 by applying
pressure. At high temperatures, the system is in the fully
incoherent regime with local moments. As the temperature
decreases below ~T0, which is the energy scale of Kondointeractions, the local moments are gradually screened andcounted in the Fermi volume. Conversely, in the local quantum
criticality framework, the Kondo singlet is supposed to be
critically destroyed at the quantum critical point along with asudden change in the f character. The pressure variation of T0 in
the prototypical heavy fermion antiferromagnet CeRh2Si2 hasbeen evaluated using the 29Si NMR technique. A universal
scaling behavior (Fig. 2) observed on the energy scale (Γ0) of thelocal fluctuations in the entire pressure region demonstrates that
the Kondo energy scale T0 monotonically increases by applying
pressure without a critical anomaly around the quantum criticalpressure Pc~1.2 GPa of antiferromagnetism (the inset of Fig. 2).
Our NMR result indicates that the progressive delocalization of
the 4f electrons occurs across Pc, accompanied by thedevelopment of antiferromagnetic correlations among the 4felectrons, i.e., no local quantum criticality occurs in this material.
References[1] Y. Tokunaga et al., JPS Conf. Proc. 30, 011037 (2020).
[2] H. Sakai et al., Phys. Rev. B 103, 085114 (2021).
[3] D. L. Cox et al., J. Appl. Phys. 57, 3166 (1985).

Fig. 1 Two types of superconducting electron pairs. In general,superconducting pairs adopt a spin-singlet state, in which thespins of the pairs are all antiparallel (a). However, in someuranium compounds, superconducting pairs adopt a spintriplet state, in which the spins of the pairs are all parallel (b).

remarkable difference.
Our measurements of the field-angular dependence of NMR
spin-lattice relaxation rate 1/T1 in URhGe revealed that spin
fluctuations in FM state are robust against magnetic field applied
any directions. This possess a strong contrast to the case of its
sister compound UCoGe, in which the rapid suppression of theFig. 2 Scaling plot of the characteristic energy Γ0/T0 vs T/T0 in CeRh2Si2
under various pressures. The dotted curve represents the universal
curve calculated for independently screened local moments basedon a single Kondo impurity model [3] The inset shows the T-P
phase diagram for CeRh2Si2.