Discovery of “Orbital Switching” Phenomenon

Expectations for Understanding and Control of Ground States in Frustrated Magnetic Systems

2012.06.15
(2012.07.19 Update)

National Institute for Materials Science
The University of Tokyo Institute for Solid State Physics

A joint research team of the NIMS Strongly Correlated Materials Group, Superconducting Properties Unit and The University of Tokyo Institute for Solid State Physics discovered a new physical phenomenon, “orbital switching,” which appears accompanying a structural phase transition in a frustrated magnet.

Abstract

  1. A joint research team consisting of Dr. Hiroyuki Yoshida, a Postdoctoral Research Fellow, and Dr. Masaaki Isobe, Group Leader of the Strongly Correlated Materials Group, Superconducting Properties Unit (Unit Director: Shinya Uji) of the National Institute for Materials Science (President: Sukekatsu Ushioda), and Assistant Professor Junichi Yamaura, Assistant Professor Yoshihiko Okamoto, Dr. Gøran Nilsen, a Postdoctoral Research Fellow, and Professor Zenji Hiroi of The University of Tokyo Institute for Solid State Physics (Director: Yasuhiro Iye) discovered a new physical phenomenon called orbital switching, which appears accompanying a structural phase transition in a frustrated magnet.
  2. Solids are made up of a lattice in which atomic nuclei are arranged periodically and regularly, and a large number of electrons. In strongly correlated electron systems such as the transition-metal oxides, etc., there are many cases in which electrons strongly couple with the lattice system, and various phase transitions (orderings) occur depending on the selectivity of the degrees of freedom of the three attributes of electrons, namely, charge, spin, and orbitals.
  3. For example, in NaNiO2, the structural phase transition occurs between the low-temperature phase of the monoclinic system and the high-temperature phase of the trigonal system at around 480 K. At this time, on the lower temperature side than the phase transition temperature (low-temperature phase), the lattice is distorted by the cooperative Jahn-Teller effect, and unpaired electrons occupy designated orbitals. Because those orbitals are regularly arranged spatially, orbital ordering occurs. On the higher temperature side than the phase transition temperature (high-temperature phase), the orbital ordering is destroyed and electrons randomly occupy degenerate orbitals. In other words, orbital ordering can be defined as a type of order-disorder transition.
  4. In contrast to the phenomena described above, in the newly-discovered phenomenon of orbital switching observed in this research, a phase transition occurs from a certain ordered state (orbital occupation state) to a different ordered state. This is qualitatively different from the orbital ordering known to date. The research team discovered this phenomenon of orbital switching in volborthite, Cu3V2O7(OH)2·2H2O, which is a kind of copper mineral. Volborthite is known as a frustrated magnetic system having a quasi-kagomé lattice. Although it had not been possible to produce single crystals of this substance until now, in this research, single crystals of volborthite were fabricated for the first time by applying ingenuity to the hydrothermal synthesis process. Furthermore, the team found that orbital switching is only observed in single crystal specimens.
  5. The team also discovered that the phenomenon of orbital switching affects the magnetic ground state of the frustrated magnetic system. Orbital switching induces a magnetic phase transition at a cryogenic temperature through unbalancing of the magnetic interactions. Theoretically, the ground state of a spin-1/2 quasi-kagomé frustrated magnet is predicted to be a strong quantum fluctuation state called a “spin liquid.” However, the existence of that state had not been conclusively confirmed experimentally. Because the present research demonstrated that it is possible to change the magnetic ground state by orbital switching, it may be possible to realize the spin liquid state in the future by skillfully controlling orbital switching.
  6. These research results were published in the English scientific journal “Nature Communications” (electronic edition): http://www.nature.com/ncomms/journal/v3/n5/full/ncomms1875.html