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Journal Articles

Exact diagonalization study on nonmagnetic impurity effects in high-$$T_{c}$$ superconductors

Tsutsui, Kenji; Toyama, Atsushi*; Toyama, Takami*; Maekawa, Sadamichi*

Journal of Physics and Chemistry of Solids, 69(12), p.3365 - 3368, 2008/12

 Times Cited Count:0 Percentile:0(Chemistry, Multidisciplinary)

Substitution of impurities Zn and Ni for Cu in high-$$T_{c}$$ superconductors has attracted much attention. The recent neutron scattering experiment show that Ni doping traps holes in the vicinity of Ni sites. We theoretically examine the electronic state with impurity based on the $$d-p$$ model where impurity Zn and Ni 3$$d$$ orbitals are explicitly treated. We carry out the numerically exact diagonalization method on small cluster. We find the region where a hole is trapped in the Ni site so that the effective hole concentration decreases and the ground state has strong antiferromagnetic correlations, as indicated from the experimental study.

Journal Articles

Momentum-resolved charge excitations in high-$$T$$$$_{rm c}$$ cuprates studied by resonant inelastic X-ray scattering

Ishii, Kenji; Hoesch, M.*; Inami, Toshiya; Kuzushita, Kaori*; Owada, Kenji; Tsubota, Masami; Murakami, Yoichi; Mizuki, Junichiro; Endo, Yasuo; Tsutsui, Kenji*; et al.

Journal of Physics and Chemistry of Solids, 69(12), p.3118 - 3124, 2008/12

 Times Cited Count:3 Percentile:19.29(Chemistry, Multidisciplinary)

Journal Articles

On-site pairing interaction and quantum coherence in strongly correlated systems

Yamada, Susumu; Machida, Masahiko; Kano, Takuma; Imamura, Toshiyuki*; Koyama, Tomio*

Journal of Physics and Chemistry of Solids, 69(12), p.3395 - 3397, 2008/12

 Times Cited Count:0 Percentile:0(Chemistry, Multidisciplinary)

We numerically investigate a dependence of pairing interaction strength on quantum coherence and related tunneling dynamics in order to clarify whether the strong pairing interaction suppresses quantum coherent characters. The simulation is performed by using the attractive Hubbard model with double well potential. The simulation result reveals that the tunneling dynamics between the separated wells is dramatically suppressed with increasing the pairing interaction. This fact implies that the pair coherence is easily lost under randomness when the pairing interaction becomes very strong. We claim that such an issue is related to the loss of the pair coherence in under-doped High-$$T$$$$_{rm c}$$ superconductors. In addition, we note that the tunneling dynamics can be directly confirmed by using the crossover from BCS to BEC in atomic Fermi gases.

Journal Articles

DMRG studies for 1-D random Hubbard chain close to the half-filling

Okumura, Masahiko; Yamada, Susumu; Machida, Masahiko

Journal of Physics and Chemistry of Solids, 69(12), p.3324 - 3326, 2008/12

 Times Cited Count:0 Percentile:0(Chemistry, Multidisciplinary)

We numerically study the repulsive Hubbard model with random potential by using the density-matrix renormalization group method. When a few holes are doped to the half-filling, i.e., in slightly doped Mott insulator, we clearly find that "Mott plateaus" and "hole localized valleys" well-separately form in strong on-site repulsive $$U/t$$ and strong randomness. These nano-scale structures are non-trivial ones created due to strong correlation incorporating randomness. We suggest that these structures qualitatively capture phenomena seen in heavily under-doped High-$$T$$$$_{rm c}$$ superconductors, while cold atomic Fermi gases loaded on an optical lattice can experimentally confirm the structures under a well controllable manner.

Journal Articles

Ginzburg-Landau simulation for a vortex around a columnar defect in a superconducting film

Nakai, Noriyuki; Hayashi, Nobuhiko; Machida, Masahiko

Journal of Physics and Chemistry of Solids, 69(12), p.3301 - 3303, 2008/12

 Times Cited Count:1 Percentile:7.97(Chemistry, Multidisciplinary)

By means of numerical simulations based on Ginzburg-Landau theory, we study the vortex depinning from a columnar defect in a superconducting film. We evaluate the limiting thickness of the film, below which the depinning does not occur even under an application of the magnetic field perpendicular to the columnar defect. The limiting thickness is a measure of the pinning strength of the columnar defect. The dependence of this limiting thickness on the magnitude of the applied field is obtained for two types of columnar defect.

Journal Articles

Josephson effect between conventional and non-centrosymmetric superconductors

Hayashi, Nobuhiko; Iniotakis, C.*; Machida, Masahiko; Sigrist, M.*

Journal of Physics and Chemistry of Solids, 69(12), p.3225 - 3227, 2008/12

 Times Cited Count:1 Percentile:7.97(Chemistry, Multidisciplinary)

We analyze the Josephson effect between a conventional superconductor (SC) and a non-centrosymmetric SC with the purpose to identify qualitative features originating form the pairing symmetry of the latter. As a concrete example, we consider the non-centrosymmetric SC CePt$$_3$$Si where Rashba spin-orbit coupling plays a crucial role and affects the Josephson pair tunneling. We find that the Josephson coupling is composed of two parts, a spin-singlet-like component and a spin-triplet-like one. The triplet-like component can lead to a Josephson coupling that is shifted by $$ pi $$ relative to the singlet-like coupling. This has important implications on the interference effects and may explain some recent experimental results for the Al/CePt$$_3$$Si junction.

Journal Articles

Ni-impurity effect in high-$$T_c$$ cuprates studied by neutron scattering and XAFS spectroscopy

Hiraka, Haruhiro*; Wakimoto, Shuichi; Matsuda, Masaaki; Matsumura, Daiju; Nishihata, Yasuo; Mizuki, Junichiro; Yamada, Kazuyoshi*

Journal of Physics and Chemistry of Solids, 69(12), p.3136 - 3138, 2008/12

 Times Cited Count:2 Percentile:13.91(Chemistry, Multidisciplinary)

Neutron scattering experiments using La$$_{2-x}$$Sr$$_{x}$$Cu$$_{1-y}$$Ni$$_{y}$$O$$_{4}$$ clarified that the parallel spin-density modulations (SDMs) in the superconducting phase are susceptible to Ni, in the same way as the diagonal SDMs in the insulating spin-glass phase. Ni substitution reduces the mobile hole concentration from $$x$$ down to $$x$$ - $$y$$. Polarized XAFS measurements using Ni $$K$$-edge probe two types of Ni valence states; Ni$$^{2+}$$ and Ni$$^{2+alpha}$$. It indicates that a strong hole localization occurs around Ni, resulting in an effective spin-$$frac{1}{2}$$ value at Ni sites. Therefore, a charge impurity nature of Ni is most likely realized when $$x$$ $$ge$$ $$y$$.

Oral presentation

Magnetic excitations in lightly-doped La$$_{2-x}$$Sr$$_x$$CuO$$_4$$

Matsuda, Masaaki; Wakimoto, Shuichi; Fujita, Masaki*; Yamada, Kazuyoshi*; Tranquada, J. M.*

no journal, , 

no abstracts in English

Oral presentation

Theory of magnetic properties in the spin-triplet superconducting state of Sr$$_2$$RuO$$_4$$

Nomura, Takuji; Hirashima, Dai*; Ikeda, Hiroaki*

no journal, , 

We calculate the spin susceptibility $$chi({bf q}, omega)$$ and the NMR relaxation rate $$1/T_1$$ in the spin-triplet superconducting state of Sr$$_2$$RuO$$_4$$. We start with a three-band Hubbard model possessing a realistic electronic structure. The calculated NMR relaxation rate shows Korringa relation above $$T_c$$ and $$T^3$$ behavior from just below $$T_c$$, without a Hebel-Slichter peak. This temperature dependence is consistent with experiments, but the calculated anisotropy of $$1/T_1$$ ($$[1/T_1]_c/[1/T_1]_{ab}$$) is too small to explain the large experimental value, i.e., about two. We will discuss the anisotropy and the momentum and frequency dependences of $$chi({bf q}, omega)$$. We will investigate also possible spin-wave modes for the same superconducting order parameter in the three-band model.

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