Research Group for Materials Physics for Heavy Element systems

Group Leader: KAMBE Shinsaku
Members: HAGA Yoshinori, TATEIWA Naoyuki, TOKUNAGA Yo, SAKAI Hironori,YAMAMOTO Etsuji, KUBO Katsunori

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. Recently, it has become clear that these
exotic behaviors for 5f-electron systems are different from thosefor 4f-electrons. This is because electrons with different spin andorbital character can coexist in 5f actinide systems, in contrast to
the case of 4f electrons. 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.
In this mid-term project, we also try to reclaim new fields suchas topological and spintronic aspects in these compounds.
Two Quantum critical points in Ce3PtIn11 [1]Fig. 1 Crystal structure of Ce3PtIn11. Ce(1) and Ce(2) areconsidered to be paramagnetic and antiferromagneticallyordered below TN, respectively. The arrows indicate thedirection of quantization axis at each In site, i.e. Vzz determinedby local symmetry and LDA calculations.
A heavy fermion compound Ce3PtIn11 harbors two inequivalent
Ce sites, which are considered to have different magnetic
properties. This system exhibits magnetic transitions at T1n = 2.2
K and TN = 2 K, prior to entering a superconducting state below
Tc = 0.32 K. Previous measurements imply that magnetism andsuperconductivity are spatially separated and only the Ce(2)sublattice orders magnetically, with a quantum critical point(QCP) at pressure pc = 1.3 GPa, while the Ce(1) sublattice isparamagnetic. In this study, we have confirmed the difference ofmagnetic characters between the Ce(1) and Ce(2) sites by meansof115In nuclear quadrupole resonance (NQR) measurements. The
nuclear relaxation rate behavior of 1/T1 ∝ √T reveals the presenceof critical spin fluctuations indicative of close proximity toanother quantum critical point, presumably at a small negative
pressure, which is associated solely with the Ce(1) sublattice. Onthe other hand, it is revealed that the internal field due to themagnetic ordering appears only at the Ce(2) site, confirming

microscopically the previous study. In the present study, we haveconfirmed new QCP around p=0 associated with the Ce(1) site,in addition to previously confirmed second QCP aroundp=1.3GPa associated with the Ce(2) site.
Uranium compound with honeycomb layer [2]A series of intermetallic compounds crystallizing in thehexagonal Sc0.67Fe2Si5-type structure is known to form with rareearth, actinides and transition metal elements. Among them,uranium isostructural compounds were discussed in terms of
atomic disorder in the uranium-containing layer suggested fromthe imperfect occupancy of the uranium and neighboring atomsites. It is the origin of the non-integer chemical composition.
We attempted to synthesize U-Pt-Al analogue and investigatedstructural and physical properties. The sample was synthesized
by the arc-melting. A small single crystal piece was extracted
from the ingot and examined by single-crystal X-ray diffraction.
Sharp Bragg peaks were successfully indexed as the hexagonalstructure, confirming the Sc0.67Fe2Si5-type structure. In addition,
weak streak reflections were systematically detected. They canbe indexed as (h/3 k/3) in the basal plane and elongated along thec* direction. This result suggests a superstructure in the basal
plane and stacking disorder along the c* direction. The simplest
basal plane structure satisfying both the streak scattering and cite
occupancies is the honeycomb arrangement having a larger lattice
parameter 𝑎𝑎′ = √3𝑎𝑎 as shown in Fig. 2. Such honeycomb
arrangements actually exist in a series of rare earth compounds
LnNi3X9 (Ln: rare earth, X: Al, Ga). Based on this model, theresulting chemical composition is U2Pt6Al15. Similar
superstructure is suggested for rare-earth analogues.
Magnetization measurement on U2Pt6Al15 revealed a welldefined magnetic transition at low temperature around 26 K
despite the disordered structure. Since each honeycomb layer isseparated more than 8 Å, disorder effect brought about by themodulated interlayer magnetic interaction may not be significant.
The preparation of the single crystal and further measurement are
in progress to clarify the ground state properties.
The present investigation addresses a new family of the layered
structure uranium intermetallic compound with honeycomb
arrangements of uranium atoms. In contrast to previous study, anordered honeycomb layer is established and considered to bequite stable, enabling further material investigation by chemical
substitution.

Fig. 2 Structure model forU2Pt6Al15. Only theuranium-containing layer isshown.
References[1] S. Kambe et al, Phys. Rev. B 101, 081103(R), (2020).
[2] Y. Haga et al., JPS Conf. Ser. 29, 013003 (2020).