Research Interests

Topological Materials

Until recently it has been well understood that there are only two types of insulators. One kind is band insulator consistent with band theory of solid state physics, and the other is Mott insulator which is supposed to be metal based on the band theory, but it turns out to be insulating due to strong interaction between electrons. Very recently another “conceptually” distinct type of insulator was proposed named a topological insulator. A key character of this insulator is a conducting edge state while the bulk state has a band gap, and a main ingredient of being topological insulators is spin-orbit coupling. There are many materials suggested for candidates of topological insulators. ┬áThere have been recent attempts to search for topological characters in metallic systems as well. See our papers on arXiv. org.


Frustrated Magnetic Systems

Magnets are used in everyday life, but fundamental mechanisms of magnet (being magnetic) require understanding of quantum mechanics. In general materials with localized magnetic moment are supposed to be magnetically ordered below a certain transition temperature. However it has been found that many correlated systems with well-defined local moment do not show any signature of magnetic ordering down to lowest temperature (that can be achieved in laboratory). It turns out that underlying crystal structures where the local moments reside play a crucial role in determining magnetic ordering tendency. In some exotic materials called spin liquid, spins behave like fermi liquid while charge excitation is gapped, generating spin-charge separation of electrons (for more details; see our papers on

High Temperature Superconductors

High temperature superconductors are materials that superconduct at unusually high temperatures. Common ingredients of these materials are cooper-oxides (cuprates) and antiferromagnetic insulating phase nearby superconducting state. Since the discovery of superconducting state in the cuprates in 1986, the transition temperature has been raised up to above 150 Kelvin (under pressure). The cuprates have not only generated intensive research on possible realization of room temperature superconductors, but also provided a playground to study the rich physics of strongly correlated systems. Innovative ideas including spin liquid and competing order parameters are by-products of cuprates studies, which are now widely used in understanding properties of other correlated materials. See our papers in for details. In particular, ” An explanation for a universality of transition temperatures in families of copper oxide superconductors”, published in Nature 428, 53 (2004).

Electronic Nematic Liquid

Electrons in solids are either localized near host atoms or move freely to account for metallic behaviour. In correlated materials, electrons can self-assemble themselves in states that analogous to classical liquid crystal phases. The search for such phases in solid-state systems, in particular for the quantum version of an anisotropic liquid crystal, dubbed electronic nematic phase, has been of great interest. Such a phase spontaneously breaks the point-group symmetry of the underlying lattice thus characteristically modifying, e.g., transport properties. A variety of transition metal materials such as Cooper-, Ruthenium-based oxides and Iron Pnictides has been proposed to harbour an electronic nematic phase. We have extensively worked on interplay between orbitals and spins to understand microscopic mechanism of electronic nematic phases and associated phenomena. See our papers on (under publication).