フラストレーションを内在する物質において、自由度の複雑な絡み合いの結果生じる新奇な物性・機能の開拓を目指している。 フラストレーションを内在する物質では巨視的な縮重度が低温まで残るが、多くの系ではスピン・電荷・格子・軌道の自由度のうちの複数の自由度が結合することで縮重を解き、秩序化に至る。 しかし、自由度の結合によるフラストレーション抑制は微妙なバランスの上に成り立っているため、通常の秩序系に比べて電場・磁場などの外場に対する応答が非常に顕著である。 このような自由度の絡み合いと「やわらかさ」の協奏現象が本研究のターゲットである。
現在は、幾何学的フラストレーション物質の代表であるスピネル化合物を中心に、帯磁率、磁化、比熱などの物性の磁場下での応答について研究を進めている。
図1: 三角格子反強磁性体におけるスピンフラストレーション
図2: スピネル化合物CdCr2O4の強磁場磁化過程と磁化プラトー領域での磁気構造
H. Ueda, H. Aruga Katori, H. Mitamura, T. Goto and H. Takagi: Phys. Rev. Lett. 94, 047202 (2005).
(香取浩子)
The interaction between a superconductor (S) and a ferromagnet (F), mediated by the conduction electrons of a normal metal (N), has not yet been clarified [Fig. 1]. In an epitaxial Nb(110)/Au(111)/Fe(110) trilayers of high quality [Fig. 2], the interplay between Nb and Fe, mediated by the conduction electrons of Au, was studied. As a function of the Au-layer thickness tAu, a marked oscillation in Tc with a period of 2.1 nm (~9 atomic monolayer) was observed. This suggests that a new and subtle form of quantum interference occurs in the very clean S/N/F trilayer system.
For more information, see the article:
H. Yamazaki, N. Shannon, and H. Takagi, Phys. Rev. B 73, 094507 (2006)
(or The March 15, 2006 issue of Virtual Journal of Applications of Superconductivity)
(Hiroki YAMAZAKI)
Using scanning tunneling microscopy/spectroscopy (STM/STS),we discovered a 4×4 'checkerboard' spatial modulation of the electronic states in the 'pseudo-gap' phase of Ca2-xNaxCuO2Cl2. 'Pseudo-gap' is a mysterious electronic state that is closely related to high-Tc superconductivity. Our observation reveals the 'hidden electronic order' in the pseudo-gap phase and gives an important hint in understanding the mechanism of high-Tc superconductivity.
This work has been done in collaboration with J. C. Davis Group, Cornell University(STM/STS) and M. Takano Group, Kyoto University(Crystal growth under high-pressure).
T. Hanaguri et al., Nature 430, 1001-1005, 26 August 2004.
(Tetsuo HANAGURI)
Impurity substitution (doping) is the essence of the electronic phase control of correlated electron systems. I focus on electronic structure of the impurities mainly by spectroscopy using synchrotron radiation.
(1) Measurement of dielectric function by nonresonant inelastic x-ray scattering (NRIXS)--Advanced Photon Source (Argonne, US)
In collaboration with Prof. Abbamonte's group (UIUC)
(2) Local structure analysis of impurities by x-ray absorption fine structure (XAFS)--SPring-8 (Harima, Japan)
In collaboration with Dr. Mizuki's group (JAEA)
(Jobu MATSUNO)