Research highlights in 2012

I have worked on the fields of the quantum Hall effect, high-Tc superconductivity, first-principles determination of crystal and electronic structures of condensed matter. Currently, my main interests are many-body and topological effects in electron systems, i.e., superconductivity, magnetism and topological phenomena, for which we envisage a materials design for correlated electron systems and novel non-equilibrium phenomena should be realised. Studies in recent years include:



Research interests

Superconductivity

A recent highlight of my studies on superconductivity has culminated in one of the first theories on the iron-based superconductor discovered a few years ago in Japan. There, I have applied my ideas and expertise on an intimate relation between the electronic structure and the way in which the electron-mechanism superconductivity appears. Our conclusion is the iron-based should have an ingenious ``s+-" pairing arising from disconnected Fermi surface originating from multiple orbitals.

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An entirely new class of organic superconductors has been discovered, also in Japan, in aromatic compounds such as picene. I have also obtained their electronic structures for the first time.

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Now, given a variety of classes of superconductors, we can re-visit the high-Tc cuprate, which still boasts of the highest Tc. We have actually clarified that the cuprate, usually regarded as a one-band system, is in fact a two-orbital system. This not only enables us to understand a strong material dependence and pressure-dependence of Tc, but also gives a clue how to enhance Tc.

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In an entirely novel avenue, I am now devoted to superconductivity induced in non-equilibrium, which has emerged from merging my interests in superconductivity and nonequilibrium physics. Namely, if we drive a repulsively correlated electron systems out of equilibrium with intense ac fields, the states are converted into Floquet states, and, coupled with an inverted (negative-T) population, a dynamical conversion of the particle-particle interaction from repulsion to attraction is predicted to be realised.

I have also looked at collective modes in multi-band superconductors

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Ferromagnetism

I have studied possibilities for metallic ferromagnetism in light-element materials with the flat-band mechanism. This includes my 1993 Phys Rev Lett, which proposes metallic ferromagnetism in what may now be called graphene with an antidot array, decades before the advent of graphene physics. I have also extended my studies to ferromagnetism in cold atoms and spin Hall effect in the iron-based superconductors.

Topological systems: Quantum Hall systems and graphene

I have worked on both integer and fractional quantum Hall effects (QHE). My most recent work on the fractional QHE is a theoretical contribution to the fractional quantum Hall effect in oxides discovered by experimental colleagues in our university. Ever after graphene physics was kicked off, I am studying graphene QHE, for which topological properties (such as a generalised chiral symmetry proposed by us) are of my interests. I have also introduced a concept of optical (THz) Hall effect in graphene, which is now stimulating experimental people.

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We have also proposed what we call ``photovoltaic Hall effect in graphene", which is predicted to occur when graphene is illuminated by a circularly-polarised light. This is actually a control of a topological property by non-equilibrium, another product from merging different interests of mine. The model has later been identified to be equivalent to Haldane's model for a QHE in zero magnetic field.

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Non-equilibrium and nonlinear phenomena in correlated electron systems

I have already described some applications of non-equilibrium physics in various topics. All these started from our study on non-linear transport in the dielectrically broken Mott insulators, which has a curious analogy with the QED vacuum decay intensively studied back in the 1950's by Schwinger and others.

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Selected Papers

  • Kazuhiko Kuroki, Seiichiro Onari, Ryotaro Arita, Hidetomo Usui, Yukio Tanaka, Hiroshi Kontani and Hideo Aoki: Unconventional superconductivity originating from disconnected Fermi surfaces in LaO1-xFxFeAs,

    Phys. Rev. Lett. 101, 087004 (2008).

    This paper (cited more than 660 times so far) is one of the first theoretical papers on the iron-based superconductor, with a new theoretical concept of s+- pairing, which is now being experimentally confirmed with phase-sensitive measurements.

  • Kazuhiko Kuroki, Hidetomo Usui, Seiichiro Onari, Ryotaro Arita and Hideo Aoki: Pnictogen height as a possible switch between high-Tc nodeless and low-Tc nodal pairings in the iron based superconductors,

    Phys. Rev. B 79, 224511 (2009) (Editors' Suggestion; Featured in Physics).

    This is also on the iron-based superconductor, cited more than 250 times, and is significant as a paper which clarified the material dependence in the multi-orbital systems.

  • Hirofumi Sakakibara, Hidetomo Usui, Kazuhiko Kuroki, Ryotaro Arita and Hideo Aoki: Two orbital model explains why the single-layer Hg cuprate have higher superconducting transition temperature than the La cuprate,

    Phys. Rev. Lett. 105, 057003 (2010).

    This explains a material-dependence of Tc on the cuprates, and opens a way for examining the system in terms of a two-orbtial physics, with a possible hint for raising Tc.

  • Naoto Tsuji, Takashi Oka, Philipp Werner and Hideo Aoki: Changing the interaction of lattice fermions dynamically from repulsive to attractive in ac fields,

    Phys. Rev. Lett. 106, 236401 (2011). (Editors' Suggestion; Viewpoint)

    In the cold atom physics you can readily change the interaction from repulsive to attractive with the Feshbach resonance, but that seems simply inconceivable for electron systems. Still, this paper proposed a way to realise that, by putting the system out of equilibrium. James Freericks, in his Viewpoint article, described the work as ``Changing repulsion into attraction with the quantum Hippy Hippy Shake".

  • Takahiro Morimoto, Yasuhiro Hatsugai and Hideo Aoki: Optical Hall conductivity in ordinary and graphene QHE systems,

    Phys. Rev. Lett. 103, 116803 (2009).

    This PRL introduced a concept of plateau structures in optical Hall conductivity in the quantum Hall regime for the two-dimensional electron gas and for graphene. The former has subsequently observed experimentally in THz regime, by a colleague of mine, Ryo Shimano, which produced another PRL. Most recently Shimano, jointly with myself, is now submitting a paper on an observation of the effect in graphene.