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SiIkeda, Hiroaki*; Suzuki, Michito; Arita, Ryotaro*; Takimoto, Tetsuya*; Shibauchi, Takasada*; Matsuda, Yuji*
Nature Physics, 8(7), p.528 - 533, 2012/07
Times Cited Count：132 Percentile：96.1(Physics, Multidisciplinary)The origin of the hidden-order phase transition of URuSi has been a long-standing mystery in condensed matter physics. We examine the complete set of multipole correlations allowed in URuSi based on a first-principles theoretical approach. The results uncover that the hidden-order parameter is a rank-5 multipole (dotriacontapole) order with nematic 
symmetry. This naturally provides comprehensive explanations of all key features in the hidden-order phase including anisotropic magnetic excitations, nearly degenerate antiferromagnetic-ordered state, and spontaneous rotational symmetry breaking.
Takimoto, Tetsuya; Hotta, Takashi; Ueda, Kazuo*
Physical Review B, 69(10), p.104504_1 - 104504_9, 2004/03
Times Cited Count：152 Percentile：96.83(Materials Science, Multidisciplinary)In order to discuss superconductivity in orbital degenerate systems, a microscopic Hamiltonian is introduced. Based on the degenerate model, a strong-coupling theory of superconductivity is developed within the fluctuation exchange (FLEX) approximation where spin and orbital fluctuations, spectra of electron, and superconducting gap function are self-consistently determined. Applying the FLEX approximation to the orbital degenerate model, it is shown that the 
-wave superconducting phase is induced by increasing the orbital splitting energy which leads to the development and suppression of the spin and orbital fluctuations, respectively. It is proposed that the orbital splitting energy is a controlling parameter changing from the paramagnetic to the antiferromagnetic phase with the -wave superconducting phase in between.
Takimoto, Tetsuya; Hotta, Takashi; Ueda, Kazuo*
Journal of Physics; Condensed Matter, 15(28), p.S2087 - S2093, 2003/07
Times Cited Count：14 Percentile：57.88(Physics, Condensed Matter)Rich physics is expected to appear due to the coupling between orbital and spin degrees of freedom in strongly correlated electron system with orbital degeneracy. Especially, it is considered that the superconducting transition in orbital degenerate system is one of the most interesting phenomena. For instance, triplet superconductivity occurs in SrRuO
where three cylindrical Fermi surfaces are observed. Also, heavy fermion superconductors are considered to have potentially orbital degree of freedom due to large degeneracy of 
-electron states. We discuss the effect of orbital fluctuations on superconductivity by using random phase approximation and fluctuation exchange (FLEX) approximation. It is also expected that the FLEX approximation is useful to study the correlation between spin and orbital fluctuations for the orbital degenerate system.
Takimoto, Tetsuya; Moriya, Toru*
Physical Review B, 66(13), p.134516_1 - 134516_7, 2002/10
Times Cited Count：28 Percentile：75.41(Materials Science, Multidisciplinary)Interplay between antiferromagnetism and superconductivity is studied by using the 3-dimensional nearly half-filled Hubbard model with anisotropic transfer matrices 
and 
. The phase diagrams are calculated for varying values of the ratio 
using the spin fluctuation theory within the fluctuation-exchange approximation. The antiferromagnetic phase around the half-filled electron density expands while the neighboring phase of the anisotropic 
-wave superconductivity shrinks with increasing 
. For moderate values of we find the second order transition, with lowering temperature, from the -wave superconducting phase to a phase where incommensurate SDW coexists with -wave superconductivity. Soft components of the incommensurate SDW spin fluctuation mode grow as the coexistent phase is approached.
Takimoto, Tetsuya; Hotta, Takashi; Maehira, Takahiro; Ueda, Kazuo*
Journal of Physics; Condensed Matter, 14(21), p.L369 - L375, 2002/06
Times Cited Count：55 Percentile：87.93(Physics, Condensed Matter)A microscopic Hamiltonian of the orbitally-degenerated Hubbard model reflecting correct symmetry of -orbitals is proposed to discuss superconductivity in -electron systems. In the degenerated region, cancellation among spin and orbital fluctuations basically destabilizes singlet superconductivity, while 
-wave superconductivity mediated by spin fluctuations emerges out of the suppression of orbital fluctuations due to the lift of orbital degeneracy. We argue that the present scenario can be applied to recently discovered Ce-based heavy-fermion superconductor.
(T=Rh, Ir, and Co); Band-calculation and tight-binding approachMaehira, Takahiro; Takimoto, Tetsuya; Hotta, Takashi; Ueda, Kazuo*; Higuchi, Masahiko*; Hasegawa, Akira*
Journal of the Physical Society of Japan, Vol.71, Supplement, p.285 - 287, 2002/00
Times Cited Count：8 Percentile：49.91(Physics, Multidisciplinary)Recently a new family of Ce-based heavy fermion superconductor 
(T=Rh, Ir, and Co) has been reported. The dHvA effect has been successfully observed in 
and 
.The angular dependence of dHvA branches is well explained by a quasi-two dimensional Fermi surface(F.S.). In order to clarify band structures of Ce-115 systems such as CeIrIn and CeCoGa, two-step strategy is here employed. In the first step, the F.S. are derived by using the RLAPW method. In the next step, the band structure near the Fermi level is reanalyzed by using a simple tight-binding model. Namely, hopping amplitudes of -electrons between neighboring Ce ions are parameterized to reproduce the F.S. obtained in the band-calculation. By further adding the Coulomb interaction terms into this tight-binding model, a simple many-body Hamiltonian for Ce-compounds can be constructed. This Hamiltonian opens a way to discuss magnetism and superconductivity in -electron systems from the microscopic viewpoint.
