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Journal Articles

Weak-coupling mean-field theory of magnetic properties of NiGa$$_{2}$$S$$_{4}$$

Nomura, Takuji*; Yamamoto, Yuji*; Yoshii, Kenji

Journal of the Physical Society of Japan, 89(2), p.024704_1 - 024704_10, 2020/02

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

We report a mean-field theoretical study of a triangular magnet NiGa$$_{2}$$S$$_{4}$$. Using the localized Wannier functions obtained from a band calculation, the most stable magnetic state was found to be spiral with the incommensurate structure, consistent with the result from neutron diffraction. The Ni-site Coulomb interaction was not strong (U about 2 eV). The spin value at the Ni site was also not so large about 1.1 mb. These results mean that the hybridization between Ni and S may play an important role. The dynamical spin correlation function was found to in accord with the results from neutron scattering.

Journal Articles

Resonant inelastic X-ray scattering study of entangled spin-orbital excitations in superconducting PrFeAsO$$_{0.7}$$

Nomura, Takuji*; Harada, Yoshihisa*; Niwa, Hideharu*; Ishii, Kenji*; Ishikado, Motoyuki*; Shamoto, Shinichi; Jarrige, I.*

Physical Review B, 94(3), p.035134_1 - 035134_9, 2016/07

 Times Cited Count:9 Percentile:49.94(Materials Science, Multidisciplinary)

Low-energy electron excitation spectra were measured on a single crystal of a typical iron-based superconductor PrFeAsO$$_{0.7}$$ using resonant inelastic X-ray scattering (RIXS) at the Fe-$$L_{3}$$ edge.

Journal Articles

Theoretical study of $$L$$-edge resonant inelastic X-ray Scattering in La$$_2$$CuO$$_4$$ on the basis of detailed electronic band structure

Nomura, Takuji

Journal of the Physical Society of Japan, 84(9), p.094704_1 - 094704_13, 2015/09

 Times Cited Count:3 Percentile:32.69(Physics, Multidisciplinary)

We study theoretically resonant inelastic X-ray scattering (RIXS) at the Cu $$L_3$$-edge in a typical parent compound of high-$$T_c$$ cuprate superconductors La$$_2$$CuO$$_4$$ on the basis of a detailed electronic band structure. We construct a realistic and precise tight-binding model by employing the maximally-localized Wannier functions derived from a first-principles electronic structure calculation, and then take account of the Coulomb repulsion between $$d$$ electrons at each Cu site. The antiferromagnetic ground state is described within the Hartree-Fock approximation, and take account of electron correlations in the intermediate states of RIXS within the random-phase approximation (RPA). Calculated RIXS spectra agree well with the experimentally observed features including low-energy magnon excitation, $$d$$-$$d$$ excitations, and charge-transfer excitations, over a wide excitation-energy range. In particular, we stress the importance of photon polarization dependence: the intensity of magnon excitation and the spectral structure of $$d$$-$$d$$ excitations depend significantly not only on the polarization direction of incident incoming photons but also that of outgoing photons.

Journal Articles

Resonant inelastic X-ray scattering study of electronic excitations in insulating K$$_{0.83}$$Fe$$_{1.53}$$Se$$_2$$

Gretarsson, H.*; Nomura, Takuji; Jarrige, I.*; Lupascu, A.*; Upton, M. H.*; Kim, J.*; Casa, D.*; Gog, T.*; Yuan, R. H.*; Chen, Z. G.*; et al.

Physical Review B, 91(24), p.245118_1 - 245118_8, 2015/06

 Times Cited Count:6 Percentile:33.5(Materials Science, Multidisciplinary)

Journal Articles

Microscopic analysis of resonant inelastic X-ray scattering in orbital-ordered KCuF$$_3$$

Nomura, Takuji

Journal of the Physical Society of Japan, 83(6), p.064707_1 - 064707_11, 2014/06

 Times Cited Count:5 Percentile:44.38(Physics, Multidisciplinary)

We analyze resonant inelastic X-ray scattering (RIXS) at the Cu $$K$$ edge in a typical orbital-ordered compound KCuF$$_3$$ on the basis of a microscopic theory. Spectral shape and its dependence on polarization direction and momentum transfer of photons are explained consistently with experimental data within our microscopic calculation. According to our microscopic orbital-resolving analysis, high-energy spectral weights (above 5 eV) originate from charge-transfer excitations related to the Cu-$$dgamma$$ orbitals, while the low-energy weights (below 2 eV) originate from the $$d$$-$$d$$ orbital excitations among the five Cu-$$d$$ orbitals. We assign specifically the RIXS weights to microscopic orbital-excitation processes, beyond the previous phenomenological assignment based on symmetry properties.

Journal Articles

Resonant inelastic X-ray scattering study of charge excitations in superconducting and nonsuperconducting PrFeAsO$$_{1-y}$$

Jarrige, I.*; Nomura, Takuji; Ishii, Kenji; Gretarsson, H.*; Kim, Y.-J.*; Kim, J.*; Upton, M.*; Casa, D.*; Gog, T.*; Ishikado, Motoyuki*; et al.

Physical Review B, 86(11), p.115104_1 - 115104_4, 2012/09

 Times Cited Count:8 Percentile:38.69(Materials Science, Multidisciplinary)

We report the first observation by momentum-resolved resonant inelastic X-ray scattering of charge excitations in an iron-based superconductor and its parent compound, PrFeAsO$$_{0.7}$$ and PrFeAsO respectively, with two main results. First, using calculations based on a 16 band $$dp$$ model, we show that the energy of the lowest-lying excitations, identified as $$dd$$ interband transitions of dominant $$xz$$, $$yz$$ orbital character, exhibits a dramatic dependence on electron correlation. This enables us to estimate the Coulomb repulsion $$U$$ and Hund's coupling $$J$$, and to highlight the role played by $$J$$ in these peculiar orbital-dependent electron correlation effects. Second, we show that short-range antiferromagnetic correlations, which are a prerequisite to the occurrence of these excitations at the $$Gamma$$ point, are still present in the superconducting state.

Journal Articles

Analysis of resonant inelastic X-ray scattering in stripe-ordered nickelate

Nomura, Takuji; Kaneshita, Eiji*

Journal of the Physical Society of Japan, 81(2), p.024707_1 - 024707_11, 2012/02

 Times Cited Count:2 Percentile:21.1(Physics, Multidisciplinary)

We analyze theoretically the resonant inelastic X-ray scattering (RIXS) at the Ni $$K$$ edge in the stripe-ordered state of La$$_{2-x}$$Sr$$_x$$NiO$$_4$$ at $$x$$ = 1/3. In the calculation of RIXS spectra, the stripe-ordered ground state is described within the Hartree-Fock approximation by using a realistic tight-binding model for Ni 3$$dgamma$$ and O 2$$p_{x,y}$$ orbitals, and the electron correlations in the electronic excitation processes are taken into account within the random-phase approximation. The calculated RIXS spectrum shows a tail toward the low-energy region when the momentum transfer of photons equals the stripe vector $${bf Q}$$, being consistent with a recent experimental result. The origin of this anomalous momentum dependence of RIXS spectra is discussed microscopically.

Journal Articles

Evaluation of spin-triplet superconductivity in Sr$$_2$$RuO$$_4$$

Maeno, Yoshiteru*; Kittaka, Shunichiro*; Nomura, Takuji; Yonezawa, Shingo*; Ishida, Kenji*

Journal of the Physical Society of Japan, 81(1), p.011009_1 - 011009_29, 2012/01

 Times Cited Count:370 Percentile:99.49(Physics, Multidisciplinary)

This review presents a summary and evaluations of the superconducting properties of the layered ruthenate Sr$$_2$$RuO$$_4$$as they are known in the autumn of 2011. This paper appends the main progress that has been made since the preceding review by Mackenzie and Maeno was published in 2003. Here, special focus is placed on the critical evaluation of the spin-triplet, odd-parity pairing scenario applied to Sr$$_2$$RuO$$_4$$.

Journal Articles

Perturbation theory of iron-pnictide superconductivity

Nomura, Takuji

Physica C, 470(Suppl.1), p.S365 - S367, 2010/12

 Times Cited Count:0 Percentile:0(Physics, Applied)

The high-$$T_{rm c}$$ superconductivity in iron pnictides is studied within a perturbation theory. We employ a five-band Hubbard model taking account of five Fe3$$d$$-like local orbitals. The Eliashberg equation is solved by calculating the pairing interaction within the third-order perturbation expansion in the Coulomb interaction at iron sites. The superconducting order parameter changes sign between the disconnected Fermi surfaces, and the superconducting gap is fully opened without any nodes on the Fermi surface. The most favorable pairing symmetry is an $$s_pm$$-wave. It is suggested that the superconductivity disappears in high doping regions where the electron pockets or the hole pockets disappear.

Journal Articles

Perturbation theory of high-$$T_{rm c}$$ superconductivity in iron pnictides

Nomura, Takuji

Journal of the Physical Society of Japan, 78(3), p.034716_1 - 034716_12, 2009/03

 Times Cited Count:27 Percentile:78.3(Physics, Multidisciplinary)

High-$$T_{rm c}$$ superconductivity discovered recently in iron pnictides is analyzed within a perturbation theory. Specifically, the probable pairing symmetry, the doping dependence of $$T_{rm c}$$ and a pairing mechanism etc. are studied by solving the Eliashberg equation for multi-band (2- and 5-band) Hubbard models. The effective pairing interaction is expanded perturbatively in the on-site Coulomb integrals up to third order. Our weak-coupling approach suggests: sufficiently large eigenvalues of the Eliashberg equation are obtained to explain the actual high $$T_{rm c}$$ for realistic on-site Coulomb integrals in the 5-band model. Thus unconventional (non-phonon-mediated) superconductivity is much likely to be realized. The superconducting order parameter does not change its sign on the Fermi surfaces, but change between the electron and hole Fermi surfaces. Consequently the probable pairing symmetry is always "nodeless extended $$s$$-wave (more specifically, $$s_pm$$-wave)". It is suggested that the 2-band model is insufficient to explain actual high $$T_{rm c}$$'s.

Journal Articles

Erratum; "Roles of electron correlations in the spin-triplet superconductivity of Sr$$_2$$RuO$$_4$$" [J. Phys. Soc. Jpn. 71 (2002) 1993]

Nomura, Takuji; Yamada, Kosaku*

Journal of the Physical Society of Japan, 78(1), P. 018001_1, 2009/01

The minus signs at the heads of the third and fifth lines in the right-hand side of eq. (37) should be plus signs.

Journal Articles

Possibility of unconventional pairing due to Coulomb interaction in Fe-based pnictide superconductors; Perturbative analysis of multi-band Hubbard models

Nomura, Takuji

Journal of the Physical Society of Japan, 77(Suppl.C), p.123 - 124, 2008/11

 Times Cited Count:14 Percentile:64.59(Physics, Multidisciplinary)

Possibility of unconventional pairing due to Coulomb interaction in iron-pnictide superconductors is studied by applying a perturbative approach to realistic 2- and 5-band Hubbard models. The linearized Eliashberg equation is solved by expanding the effective pairing interaction perturbatively up to third order in the on-site Coulomb integrals. The numerical results for the 5-band model suggest that the eigenvalues of the Eliashberg equation are sufficiently large to explain the actual high $$T_{rm c}$$ for realistic values of Coulomb interaction and the most probable pairing state is spin-singlet $$s$$-wave without any nodes just on the Fermi surfaces, although the superconducting order parameter changes its sign between the small Fermi pockets. On the other hand the 2-band model is quite insufficient to explain the actual high $$T_{rm c}$$.

Journal Articles

Possible collective spin excitation in the spin-triplet superconducting state of Sr$$_2$$RuO$$_4$$; Multi-band theory

Nomura, Takuji; Hirashima, Dai*; Yamada, Kosaku*

Journal of the Physical Society of Japan, 77(2), p.024701_1 - 024701_10, 2008/02

 Times Cited Count:6 Percentile:44.81(Physics, Multidisciplinary)

We investigate possible collective spin excitation by calculating the dynamical spin susceptibility in the spin-triplet superconducting state of Sr$$_2$$RuO$$_4$$. The effect of the fluctuating triplet-pairing condensate on the spin susceptibility is taken into account. The triplet pairing symmetry is assumed to be the most promising form: ${bf $d$}$(${bf $k$}$) $$sim$$ ($$k_x$$ + $rm i$k_y)hat{z}$$. For small (maybe realistic) anisotropy of pairing interaction in spin space, we expect a possible low-energy spin-wave mode as a peak structure in the imaginary part of the dynamical spin susceptibility, isolated from the spin excitation continuum. The present work is a natural extension of previous single-band calculations to a more realistic three-band calculation.

Journal Articles

Analysis of resonant inelastic X-ray scattering at the $$K$$ edge in NiO

Takahashi, Manabu*; Igarashi, Junichi*; Nomura, Takuji

Physical Review B, 75(23), p.235113_1 - 235113_9, 2007/06

 Times Cited Count:18 Percentile:62.74(Materials Science, Multidisciplinary)

Resonant inelastic X-ray scattering at the $$K$$ edge in NiO is analyzed theoretically. The antiferromagnetic groud state is described by the Hartree-Fock approximation, the core-hole potential is treated within the Born approximation, and the electron correlation effect is considered within the random-phase approximation. Consequently, the observed spectral shape and its dependence on the momentum transfer of scattered photons are explained quantitatively.

Journal Articles

Theory of resonant inelastic X-ray scattering in transition-metal compounds

Nomura, Takuji; Igarashi, Junichi*; Takahashi, Manabu*

Hoshako, 20(3), p.171 - 179, 2007/05

Recently, electronic excitation spectra have been observed in strongly correlated electron systems such as transition-metal compounds, by utilizing the highly brilliant synchrotron radiation. Among them, momentum dependent excitation spectra are observed in resonant inelastic X-ray scattering (RIXS) experiments at the transition-metal absorption $$K$$-edge in the hard X-ray regime. The microscopic theory of RIXS constructed recently by the present authors is explained. An essential point is that RIXS spectrum is closely related to the dynamical density correlation function of correlated transition-metal $$d$$ electrons. Spectral shapes and momentum dependence observed in several transition-metaloxides are semiquantitatively explained.

Journal Articles

Theory of transport properties in the $$p$$-wave superconducting state of Sr$$_2$$RuO$$_4$$; A Microscopic determination of the gap structure

Nomura, Takuji

Journal of the Physical Society of Japan, 74(6), p.1818 - 1829, 2005/06

 Times Cited Count:24 Percentile:73.44(Physics, Multidisciplinary)

We provide a detailed quantitative analysis of transport properties in the $$p$$-wave superconducting state of Sr$$_2$$RuO$$_4$$. Specifically, we calculate ultrasound attenuation rate and electronic thermal conductivity within the mean field approximation. The momentum dependence of the gap function is determined by solving the Eliashberg equation for a three-band Hubbard model with the realistic electronic structure of Sr$$_2$$RuO$$_4$$. Consequently, we obtain the temperature dependence of the transport coefficients in agreement with the experimental results.

Journal Articles

Analysis of resonant inelastic X-ray scattering in La$$_2$$CuO$$_4$$

Nomura, Takuji; Igarashi, Junichi*

Physical Review B, 71(3), p.035110_1 - 035110_5, 2005/01

 Times Cited Count:45 Percentile:83.01(Materials Science, Multidisciplinary)

The experimental result of resonant inelastic X-ray scattering in La$$_2$$CuO$$_4$$ is theoretically analyzed. The ground state of the system is described within the Hartree-Fock approximation, and the electronic excitation processes are taken account of within the random phase approximation. In the experiment, the spectral shape shows a strong dependence on the transferred momentum of the photon. We have succeeded in reproducing theoretically this dependence quantitatively.

Journal Articles

Progress in the theory of anisotropic superconductivity on the basis of the fermi liquid theory

Nomura, Takuji; Yamada, Kosaku*

Nihon Butsuri Gakkai-Shi, 59(12), p.893 - 897, 2004/12

A number of superconductors exhibiting anisotropic pairing have been discovered so far. In anisotropic superconductors, the attractive force originates from the $$l>0$$ component of the pair-scattering amplitude. We provide a unified view on the microscopic mechanism of anisotropic superconductivity on the basis of the Fermi liquid theory.

Journal Articles

Theoretical study on resonant inelastic X-ray scattering in quasi-one-dimensional cuprates

Nomura, Takuji; Igarashi, Junichi*

Journal of the Physical Society of Japan, 73(7), p.1677 - 1680, 2004/07

 Times Cited Count:26 Percentile:74.84(Physics, Multidisciplinary)

We study theoretically the resonant inelastic X-ray scattering in quasi-one-dimensional insulating copper oxides, where the incident photon energy is tuned to the Cu1$$s$$-4$$p$$ absorption energy. Our attention is focused particularly on the strong momentum-transfer dependence of the spectral shape observed in recent experiments. We describe the antiferromagnetic ground state within the Hartree-Fock theory, and consider charge excitations from the ground state within the random phase approximation. By taking account of the electron correlation effects perturbatively, we obtain detailed momentum-transfer dependence of the spectra in a semiquantitative agreement with the experiments.

Journal Articles

Theory of superconductivity in strongly correlated electron systems

Yanase, Yoichi*; Jujo, Takanobu*; Nomura, Takuji; Ikeda, Hiroaki*; Hotta, Takashi; Yamada, Kosaku*

Physics Reports; A Review Section of Physics Letters, 387(1-4), 149 Pages, 2003/11

In this article we review essential natures of superconductivity in strongly correlated electron systems from a universal point of view. Here we consider materials such as high-$$T_{rm c}$$ cuprates, BEDT-TTF organic superconductors, ruthenate Sr$$_2$$RuO$$_4$$, and heavy fermion superconductors. After the review of experimental results, we explain the formalism to discuss superconducting properties of strongly correlated electron systems based on the Dyson-Gor'kov equations. Then, we introduce theoretical retults on cuprates, organics, ruthenate, and heavy fermion superconductors in this order.

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