Quantum Science on Strong Correlation


Basic research on Mottronics

Sub-theme Leader: Masashi Kawasaki

  • We call it Mottronics that the scientific principles for electronics application of metal-insulator transition (Mott transition) in correlated electron systems, where electronic phase is used to describe the state variable of switching devices. This research field was proposed by Prof. Y. Tokura, core researcher of FIRST program QS2C project, and named correlated-electronics or Mottronics, as associated with Sir N. F. Mott (Nobel Laureate in Physics 1977). One of the prominent features of correlated electron system appears upon the electronic phase transition where the ordered state of many electrons melt down into a disordered state in Mott transition. This phenomenon has already been considered in the industry as a possible candidate to realize a new type of nonvolatile resistance-change memory (ReRAM) application. This Mott transition has shown to occur on nano-scale, where a multiple domain state plays a crucial role in gigantic and ultra-fast change in, for example, its conductivity. In this FIRST program, we aim at the elucidation of basic principles that govern the applied physics of Mott transition through the following research topics studied for designed nano-structures.
  • ・Elucidation of the electro-magnetic ordered state and spin-orbital coupling originating from multiple degrees of freedom of correlated electrons in thin films, interfaces, and superlattices.
  • ・Mott transition of correlated electron junctions induced by photo-irradiation and its possible application for solar cells with multiple carrier generation.
  • ・Microscopic observation and artificial switching of nano-scale domain structures in phase-separated correlated electron system.
  • ・Theoretical elucidation of Mott transition dynamics induced by stimuli such as electric-field.
  • ・Realization of high critical temperature superconductivity induced by electric-field and ambipolar operation of superconducting transistors.

Emergent properties of strong correlation

Sub-theme Leader: Yoshinori Tokura

  • By exploiting the emergence arising from multiple degrees of freedom - charge, spin, orbital - of correlated electron, we aim at establishing the materials-design discipline to gigantically enhance the cross-correlation between electricity, magnetism, heat, and light actions. In particular, the cross-correlation between electricity and magnetism may lay the basis for the future spintronics functions, while the large thermoelectricity based on high electron entropy as well as the photo-induced Mott transition enabling the multiple-carrier generation may be guiding principles for future innovative energy function in a solid. Toward this, we focus the following research topics:
  • ・Quantum mutual and dynamic control of electric-polarization and magnetic structure.
  • ・Realization of gigantic thermoelectric and magneto-capacitance effects based on new principles, in particular exploiting the high-entropy feature of correlated electrons with multiple degrees of freedom
  • ・Clarification of ultrafast dynamics of electronic phase transitions induced by actions of electric field and photo-excitation
  • ・Determination of electronic structures in exotic superconductors and chiral surface states of topological insulators by the state-of-the-art photoemission spectroscopy

Principles of dissipationless electronics

Sub-theme Leader: Naoto Nagaosa

  • We will construct the principles of quantum state control based on disspationless topological currents (charge current, spin current, displacement current). Recently, there has been a very rapid progress in the understanding the fundamental properties of topological currents, which are driven by the interference phenomena of quantum mechanical electron waves, and the associated quantum transport. In addition, the discovery of topological insulators and quantum spin Hall systems has made it very promising that topological currents can be used in quantum circuits. If electron correlation effects are further taken into account, then we could expect astonishing functions, such as very robust quantum bit and gigantic magnetoelectric cross-correlations. We will particularly focus on the following themes.
  • ・proposal of principles for spin current design and discovery of unprecedented functions in magnetoelectric and magnetothermal correlations.
  • ・Complete analysis of quantum multiple orders with use of quantum beams (synchrotron X-ray, neutron).
  • ・Theoretical understanding of interface/surface/edge states in correlated materials with topological order (topological insulator), exploration of dissipationless currents and superconducting properties in such states, and construction/demonstration of the principles of electric-/magnetic-field control.