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Strongly Correlated Materials Group

The major focus of our group research involves the discovery and investigation of novel cooperative phenomena such as anisotropic superconductivity, unusually large thermoelectrics, and multiferroics in transition-metal oxides. A large fraction of the group's effort is devoted to the discovery of new materials that show unique physical properties caused by strong electronic correlation and/or cross-correlation relating to charge, spin and orbital degrees of freedom. High-pressure technique is a useful method to achieve the purpose.

Objective

The major subjects are
  • Search for unconventional superconductivity,
  • Search for exotic magnetic materials and its physical properties,
  • Search for unusual thermoelectric materials,
  • Study of electron transport and magnetodielectric effect for functional ceramics, and
  • Crystal growth of transition-metal oxides.



Facilities

Belt-type high-pressure apparatus


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Selected Recent Results

Jeff = 1/2 spin-orbit Mott state in the novel layered iridate Ba2IrO4

Spin-orbit (SO) Mott insulating state is a novel quantum phase induced by electronic correlation associated with large SO coupling in 5d electron systems such as iridates. In the novel layered perovskite Br2IrO4, spins are strongly coupled with orbitals to form a “pseudo-spin” (Kramers doublet) state with an effective total angular momentum Jeff = | – L + S|. The Jeff = 1/2 pseudo spins form an antiferromagnetic long-range order on the IrO2 2-D square-lattice plane. The in-plane superexchange interaction is very large and comparable to those in high-TC cuprates. It is theoretically predicted that electron doping may induce a quite unconventional superconducting state characterized by a d-wave “pseudo-spin singlet” Cooper pair formed by the Jeff = 1/2 Kramers doublet, which contains inter-orbital as well as both singlet and triplet components of t2g electrons. Search for the novel superconductivity in iridates is a challenging subject which may create a new research field in condensed matter physics.


"Fig. 1	Crystal structure of Ba2irO4, antiferromagnetic long-range order of pseudo-spins in the IrO2 plane, and resonant X-ray scattering study for the Kramers doublet state" Image

Fig. 1 Crystal structure of Ba2irO4, antiferromagnetic long-range order of pseudo-spins in the IrO2 plane, and resonant X-ray scattering study for the Kramers doublet state




Superconductivity in the novel noncentrosymmetric silicide SrAuSi3

It has been theoretically predicted that noncentrosymmetric (= no spatial inversion symmetry) systems can give an unconventional superconducting state, where the wave function of the Cooper pair is composed of both the spin-singlet and -triplet characters with an admixture of even and odd parity. Such an unusual state can be realized by large antisymmetric spin-orbit (SO) coupling in 4d or 5d electron systems.
Recently, we successfully synthesized a novel strontium-gold silicide SrAuSi3 using a high-pressure technique. SrAuSi3 crystalizes in a BaNiSn3-type structure with a space group of I4mm, which has no inversion symmetry in real space. Since gold (Au) is the heaviest transition-metal element, it is potentially able to yield large SO coupling through orbital hybridization in the compound. We found that SrAuSi3 shows a bulk superconductivity at the critical temperature (TC) of 1.6 K. We have studied the superconducting properties and its electronic state in detail from a viewpoint of the noncentrosymmetric superconductivity.


"Fig. 2	Crystal structure, STEM lattice image, and superconducting properties (zero resistivity) of SrAuSi3" Image

Fig. 2 Crystal structure, STEM lattice image, and superconducting properties (zero resistivity) of SrAuSi3




Novel multiferroic material RMnO3 (R = Ho, Er, Tm, Yb, Lu)

Multiferroics are new functional materials with magnetic-field induced ferroelectricity or electric-field induced magnetization. We found new multiferroic compounds RMnO3 (R = Ho, Er, Tm, Yb, Lu) that show E-type antiferromagnetic order and extremely large polarization. This discovery has attracted a great deal of attention because the origin of multiferroicity is different from the conventional multiferroics with spiral magnetic order.


National Institute for Materials Science (NIMS)
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