Press Release

Elucidation of Metal–Insulator Transition Mechanism in Ferromagnetic Hollandite
- Formation of Tetramers of Chromium and Peierls Mechanism -

January 16, 2012

Chiba University, Japan
High Energy Accelerator Research Organization (KEK), Japan
The University of Tokyo, Japan

Abstract

Professor Yukinori Ohta, (Graduate School of Science, Chiba University), Associate Professor Takehisa Konishi, (Graduate School of Advanced Integration Science, Chiba University), Associate Professor Hironori Nakao and Assistant Professor Akiko Nakao^* (Condensed Matter Research Center (CMRC), Institute of Materials Structure Science, KEK), Professor Yutaka Ueda and Dr. Masahiko Isobe (Institute for Solid State Physics, The University of Tokyo), Professor D. I. Khomskii (II. Physikalisches Institut, Universität zu Köln) and colleagues were the first to elucidate the mechanism by which a hollandite-type oxide^*1 K₂Cr₈O₁₆ transforms from a metal to an insulator, while maintaining ferromagnetism,^*2 by analyzing its crystal structure using the Photon Factory^*3 at KEK.

This transition is induced by the lattice distortion associated with the formation of tetramers^*4 of chromium (Cr), which leads to the localization of electrons. This is the first example of the Peierls transition^*5 in a system of fully spin-polarized electrons, at which the band structure^*6 changes with a change in a lattice structure; these results are confirmed by both experiment and theory. This finding is expected to lead to the development of magnetic materials with new physical properties, as well as materials in which magnetism and conductivity are intricately intertwined.

This work was published in Physical Review Letters on December 23, 2011.

^*Currently at the Comprehensive Research Organization for Science and Society of Japan.

Background

Since hollandite-type oxides have a nano-sized tunnel structure, they have recently attracted attention as functional materials. Moreover, 1) they contain several ions with different valences, 2) their crystal structures are low-dimensional, and 3) they have conductivity and magnetic frustration;^*7 therefore, they have the ability to exhibit a variety of physical properties and thus have been at the focus of materials research.

Ueda and colleagues have discovered that K₂Cr₈O₁₆ has a transition from a metal to an insulator, while maintaining ferromagnetism; however, the underlying mechanism remained unknown. In a strongly-correlated electron system, such as the hollandite-type oxides, ferromagnetism and metallic nature are closely related with each other. Furthermore, ferromagnetic insulators are extremely rare in nature. Elucidation of the underlying mechanism, thus, has been much anticipated.

Methods and Results

In this study Nakao and her research team examine the precise crystal structure of the ferromagnetic insulating phase of hollandite-type oxide K₂Cr₈O₁₆, which exhibits the transition from ferromagnetic metal to insulator, using synchrotron radiation at the BL-8 beamline of the KEK’s Photon Factory. The study revealed structural changes in four Cr-O-Cr chains that make up a one-dimensional tunnel in the low-temperature insulating phase. Experimentally obtained Cr-O chains that have different bond distances are arranged alternately in one-dimension columns (bond alternation), and Cr ions form tetramers (Figure). In addition, Konishi, Ohta, Khomskii and colleagues made a band calculation using crystal structural analysis results, and showed that K₂Cr₈O₁₆ has a Peierls instability.^*5 In the ferromagnetic insulating phase, corresponding to Cr⁺³(3d³):Cr⁺⁴(3d²) = 1:3, one extra electron per four tetramerized Cr ions is shared to induce the Peierls transition, leading to insulator behavior.

In the normal Peierls mechanism,^*5 a crystal lattice is distorted and a bond alternation is induced, while spin-up and spin-down electrons make a pair to form a singlet state. In the present case, however, it is a ferromagnetic state in which all electron spins align (full polarization) undergoes the Peierls transition, while maintaining spin polarization, indicating that it is a special case of the Peierls transition where the spin freedom of electrons plays no role. This finding is expected to lead to the development of materials with novel physical properties in which magnetism and conductivity are intricately intertwined with each other.

Publication
“Peierls Mechanism of the Metal-Insulator Transition in Ferromagnetic Hollandite K₂Cr₈O₁₆” was published in Physical Review Letters on December 23, 2011.

Reference Figure