Magnetic Materials Laboratory

Outline

We are exploring a wide variety of electronic phases formed by strongly interacting electrons (strongly correlated electrons) and the physics behind them. Our playgrounds are in transition metal oxides and rare earth intermetallics where d-electrons and f-electrons dominate the electronic and magnetic properties.

Currently ongoing projects include elucidating the mechanism of Fe-based superconductors, exploration of new superconductors and thermoelectrics, the physics of quantum spin liquids, novel electronic states produced by strong spin-orbit coupling, and the development of a single spin detection probe.

Recent Research Topic

Revolutional functions of matter by complex electrons

Fig. 1 Atomic-resolution topographic image of Fe(Se,Te) single crystal observed by STM/STS

Fig. 2 Magnetic-field induced change in the Fourier-transformed quasi-particle interference pattern suggesting the s±-wave superconductivity

Fig. 3 The crystal structure of Ir2O4 and an epitaxial growth chamber used for synthesis

Our projects include mechanisms of high-Tc superconductivity, spin-orbit coupling physics, development of phase change functions including in ice packs, and the exploration of novel oxides using high-pressure synthesis and thin-film growth. We abstract here two topics representing those activities, (i) unconventional s-wave superconducting gap identified in an iron-based superconductor, and (ii) novel electronic phases induced by spin-orbit coupling discovered in complex Ir oxides.

(i) Unconventional s-wave superconducting gap

The new iron-based superconductors have been attracting much attention because of their extraordinary high transition temperature. Their Fermi surface consists of electron and hole pockets. A pairing mechanism based on spin fluctuations, caused by nesting between these disconnected Fermi pockets, has been widely discussed. If this unconventional mechanism is the case, the sign of the superconducting gap should be reversed between the hole and the electron pockets, while the gap on each pocket should be isotropic. Experimental verification of this so-called s±wave symmetry has been long desired. In order to tackle this issue, we have performed Fourier analyses of quasi-particle interference (QPI) patterns using spectroscopic-imaging STM. The phase of the superconducting gap is highlighted in the field dependence of QPI amplitudes through the coherence factor. The tunneling spectrum of Fe(Se,Te) in the superconducting state showed the absence low-energy excitations around the Fermi energy, indicating that the “amplitude” of the superconducting gap is isotropic. In Fourier-transformed QPI patterns, the intensity of scattering between electron pockets is enhanced by a magnetic field, while that of scattering between electron and hole pockets is suppressed. These clearly indicate the “phase” change of superconducting gap between the Fermi pockets and provide the very first experimental evidence for the s±wave superconductivity [T. Hanaguri, et al. Science 2010, 328, 474]. It is highly likely that superconductivity in iron-based superconductors is driven by spin fluctuations.

(ii) Novel electronic phases induced by spin-orbit coupling

Spin-orbit coupling has emerged as one of the most important keywords in solid state physics today and is indeed a source of recently discovered novel phenomena such as spin Hall effect and topological insulator. By a resonant x-ray diffraction performed at SPring-8, we discovered that very strong spin-orbit coupling gives rise to a new class of Mott insulating state in a layered iridium oxide Sr2IrO4 [B.J. Kim, et al. Science 2009, 323, 1329]. If such a unique state is coupled with geometrically frustrated lattice, an even more exotic ground state such as a correlated topological insulator may be anticipated: even a room-temperature quantum spin Hall effect is predicted there. Therefore we searched for novel iridium oxides with a unique topological character by epitaxial thin-film growth and successfully synthesized a new compound Ir2O4. Ir2O4 crystallizes in a spinel structure without cations in the tetrahedral site. This Ir spinel is a narrow gap insulator, in remarkable contrast to the metallic ground state of rutile-type IrO2. Since Ir ions form a pyrochlore sublattice with strong geometrical frustration, Ir2O4 is unique in terms of quantum magnetism. We have also succeeded in fabricating other iridium oxides such as Na2IrO3 with a honeycomb lattice in the form of thin films. Based on these results, we are initiating a project to examine the quantum spin Hall effect.