Research Group for Actinide Materials ScienceGroup Leader : Zachary FiskMembers : Yoshinori Haga, Etsuji Yamamoto, Naoyuki Tateiwa, Tatsuma D. Matsuda,Yuji Matsumoto, Yuta Yokoyama, Akio Nakamura

5f electrons in actinide compounds play essential roles invarious phase transitions in actinide compounds. The electronic
states can be modified by the chemical or physical environment
around these electrons. It is therefore interesting to investigate
new materials under various conditions such as pressure,magnetic field or low temperature to find new phenomena. The
purification of a known compound also provides new insightparticularly for phenomena occurring at low temperatures,
where impurity disorder can deeply disturb and hence hide theintrinsic behavior. Here we report recent progresses in thestudies of the formation of heavy fermion state, unconventional
superconductivity in UPt3 using a high-quality single crystal,and experimental development in high-pressure techniques.
Unconventional superconductivity of UPt3
UPt3 is a prototypical heavy fermion superconductor. Earlier
studies revealed that an unconventional odd-parity Cooper
pairing is realized. We have investigated the anisotropy ofthermal conductivity in the superconducting state using a highquality single crystal. As a result, it is found that thesuperconducting state has a two-fold rotational symmetry in thebasal plane which is lower than the six-fold symmetry of thecrystal structure. The obtained superconducting symmetry
successfully explains the characteristics of the superconducting
phase diagram [1].
On the other hand, an anomalousenhancement of the electron effective mass has been detected inthis compound depending on the magnetic field direction. Thisobservation confirms the participation of heavy fermion in theunconventional superconductivity [2]. The work has been
conducted in collaboration with Tokyo Institute of Technology
and the University of Tokyo.
Fermi surfaces in the hidden-order state of URu2Si2
The electronic state of URu2Si2 was investigated using
cyclotron resonance in collaboration with Kyoto University.
Using this technique, Fermi surfaces with an extremely large
effective mass were detected, where the conventional de Haasvan Alphen experiments were unable to detect these heavymasses. By measuring the angular dependence, the Fermi
surfaces were shown to have two-fold symmetry, consistent
with the recent observation in magnetic anisotropy [3].
Heavy fermion formation in a dilute magnetic alloy
It is well recognized that strongly correlated heavy fermion
behavior in rare-earth and actinide compounds has its origin in
the Kondo interaction between magnetic moments andconduction electrons.
The experimental proof of theparticipation of magnetic electrons in the Fermi surface has notbeen reported. By detecting the Fermi surfaces in dilute alloys
CexLa1-xRu2Si2, the evolution of electronic state as a function oftemperature was revealed. Here, the magnetic impurity state attemperatures higher than the characteristic temperature TK ischanged into a heavy conduction electron at low temperatures[4].

Fig.1 Magnetization measurement performed at high pressure.
YbCu2Si2 is a paramagnet at ambient pressure. However, aclear ferromagnetic moment has been observed at 10.5 GPa.

Development of a high-pressure cell for magnetization
measurement
5f electrons in actinides respond sensitively to pressure bychanging their ground state. It is therefore interesting to study
magnetic behavior of 5f electrons under pressure.
We
successfully developed a high pressure cell suitable for thispurpose.
By using ceramic anvils with extremely low
background, high-sensitivity and high maximum pressure wereboth achieved. Using the present pressure cell in a commercial
SQUID magnetometer, a pressure-induced ferromagnetism in an
f-electron system was found [5]. Figure 1 demonstrates theobservation of ferromagnetism at 10.5 GPa in a heavy fermion
compound YbCu2Si2.
References[1] Y. Machida et al., Phys. Rev. Lett. 108, 157002 (2012).
[2] S. Kittaka et al., J. Phys. Soc. Jpn. 82, 024707 (2013).
[3] S. Tonegawa et al., Phys. Rev. Lett. 109, 036401 (2012).
[4] Y. Matsumoto et al., J. Phys. Soc. Jpn. 81, 054703 (2012).
[5] N. Tateiwa et al., Rev. Sci. Instrum. 83, 053906 (2012).