Research Group for Materials Physics for Heavy Element systems
Group Leader: Shinsaku KAMBE
Members: Yoshinori Haga, Naoyuki Tateiwa, Yo Tokunaga, Hironori Sakai, Etsuji Yamamoto,Katsunori Kubo, Jiri Pospisil and Taisuke Hattori

Gradual localization of 5f states in orthorhombic UTGe
ferromagnets [1]Unusual domes of stable ferromagnetism (FM) appear in themagnetic phase diagrams of substituted orthorhombic UTGe
compounds. Within this scope we have investigated FM domein FM SC UCoGe substituted by Ru by XMCD and polarized
neutron diffraction (PND). UCoGe is unique material wherevery unusual compensation of the antiparallel U and large Co
moments was proposed by PND [2]. Our PND results carried

Fig.1 Magnetization density maps for UCo0.88Ru0.12Ge; projection onto
the a-b plane in applied magnetic field 1 T and 9 T in µB/Å2 unit.

out on UCo0.97Ru0.03Ge and UCo0.88Ru0.12Ge single crystals
showed parallel arrangement of both U and Co magnetic
moment (Fig. 1). In addition, extrapolation of our results to thelimit of pure UCoGe predicted identical arrangement. Toconfirm our controversial result, we have performed XMCD
experiment at the JAEA beamline in SPring-8. The XMCD
spectrum of UCo0.88Ru0.12Ge indicates that the direction of themagnetic moment at U site is parallel to that at the Co site
consistently with the PND experiments. We have also
investigated the U magnetic moment components µL/µS ratio
carrying indirect information about hybridization strength in thesystem. 1.9 and 2.3 values were found for UCo0.97Ru0.03Ge andUCo0.88Ru0.12Ge, respectively. As these are smaller than 3.2 ofthe localized case, a strong hybridization is indicated here. We
have recognized that FM dome is connected with weak
localization of the U 5f states which strong itinerant character
remains however still conserve. We found general rule for U 5f
states character for all ferromagnetic UTGe compounds basedon our results together with scaling of heat capacity data jumps
at TC. We also extrapolated value of µL/µS = 1.5 for UCoGe
signalizing strong itinerant character of 5f states in thiscompound. Our results point to necessity of another deep
research of FM state in UTGe compounds to understandunconventional SC in FM SC UCoGe and URhGe [1].
Itinerant-localized transitions in magnetic phases of theperiodic Anderson model [3]In heavy element systems, since the characteristic energy
scale can be reduced by the strong electron correlations, external
perturbations such as pressure and a magnetic field are able tochange the Fermi-surface structure, which is an important
ingredient to characterize a metal. In particular, such Fermisurface reconstructions, called Lifshitz transitions [4], areobserved in magnetically ordered states, such as in UGe2.
To clarify the characteristics of the Lifshitz transitions
theoretically, we have investigated magnetically ordered statesof the periodic Anderson model, which is a typical model todescribe the heavy element systems. Around half-filling, i.e., theelectron number per site n is nearly two, we find an
antiferromagnetic (AF) phase, and far away from half-filling,
we find a ferromagnetic phase. For example, we show the AF
moment mAF as a function of the energy level of the f electronsef at n=1.917 in Fig. 2. Inside the AF phase, a Lifshitz transition
takes place, and the size of the AF moment changes there. Inorder to understand this transition further, we have also analysed
the f-electron contribution to the Fermi surface by evaluating themomentum distribution function at the Fermi momentum. Then,we find that, in the large ordered-moment state (AF2), the felectron contribution to the Fermi surface becomes small. Thisobservation clearly shows that this Lifshitz transition is anitinerant-localized transition of the f electrons. For theferromagnetically ordered case, we have reached a similarconclusion.
n = 1.917, U /t = 8, V /t = 1
10.8

AF10.6mAFIn 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 spinand orbital character can coexist in 5f actinide systems, incontrast to the case of 4f electrons. By means of advanced
experimental and theoretical approaches, our research grouptries 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.

PM0.4
AF20.20−3−2.5−2−1.5−1 −0.5
ε f /t00.5

1Fig. 2 AF moment mAF as a function of ef. Fermi-surface structures ineach state are also shown. There are two AF states, AF1 and AF2, andthe transition between them is a Lifshitz transition.

References[1] M. Valiska, J. Pospisil et al., JPSJ. 84, 084707 (2015).
[2] K. Prokes et al., Phys. Rev B. 81, 180407(R) (2010).
[3] K. Kubo, J. Phys. Soc. Jpn. 84, 094702 (2015).
[4] I. M. Lifshitz, Sov. Phys. JETP 11, 1130 (1960).