Study

Our goals

We study the physical properties of magnetism and superconductivity using the technique of nuclear magnetic resonance. We mainly focus on the novel phenomena including the unconventional superconductivity and quantum criticality realized under high pressure. One of the goals of the study is identifying the symmetry of the superconductivity and its pairing mechanism. We are also interested in the multipolar ordering in low temperature.

Belows are the examples of the research topics.

• Superconductivity mediated by valence fluctuation in the heavy-fermion superconductor CeCu₂Si₂
• Pressure-induced superconductivity in CeCoGe₃
• Relationship between the structural instability and superconductivity in Cd₂Re₂O₇
• Pressure-induced physical properties of frustrated oxides
• Pressure-change of the Ce valence state in CeAl₂
• Odd-parity multipolar state in Ce-based antiferromagnets without local inversion symmetry
• Magnetism and quantum critical behavior of α-Mn
• Development of high-pressure measurement technique (NMR, electric resistivity, and Hall effect)

Collaborative researches

We study in cooperation with various groups in Japan and overseas. We have mainly collaborated with groups in Okayama Univ., Osaka Univ. (Japan), and Max Planck Institute (Dresden, Germany). We also constantly work together with Miyoshi and Motoyama groups in Shimane University.

High-pressure NMR

Indenter-type pressure cell

We mainly use the indenter-type pressure cell for the high-pressure NMR measurements. This pressure cell has been developed Kobayashi laboratory in Okayama University (Japan) and is a small clump-type cell made from the Ni-Cr-Al alloy metal. The maximum pressure of this cell is about 5 GPa.

Anvil cell

We also use an anvil-type cell to reach higher pressures above 5 GPa.

Topics

NQR study on CeCu₂Si₂ under pressure

CeCu₂Si₂ is the first heavy-fermion superconductor reported in 1979 and has been studied even 2020s to reveal its superconducting symmetry and pairing mechanism. When increasing pressure, the superconducting phase is reinforced at around 4 GPa and disappears above. This pressure response has been expected to relate the superconducting mechanism in this system, but its origin had been unclarified.

In this study, we performed ⁶³Cu NQR measurements under pressures using an indenter-type cell upto 5 GPa. We revealed that the nuclear relaxation rate shows normal metal (Fermi liquid) behavior around the pressure where the superconductivity is reinforced. No quantum critical magnetic fluctuations were detected under pressures. From this result, we cannot simply think that the superconductivity in CeCu₂Si₂ arises from the quantum critical point of the antiferromagnetism.

We also found that the pressure dependence of the nuclear quadrupolar frequency ν_Q shoed an anomalous behavior where the superconductivity is reinforced. Because ν_Q is sensitive to the electric state around the nuclei, this result suggests that the valence state of Ce ion may relate to the superconductivity reinforcement.

This study is a collaborative research with Osaka University (Japan) and Max Planck Institute for Chemical Physics of Solids, Dresden (Germany). This paper has been selected as the Editors' choice of the Journal of the Physical Society of Japan (JPSJ) in December 2008.