arXiv subjects

Takuji Nomura

Publications and source records attributed to Takuji Nomura.

At least 19 recordsLinked to original sources

arXiv2025

Unconventional $s$-Wave Pairing with Point-Node-Like Gap Structure in UTe$_2$

We explore the pairing state and gap structure of UTe$_2$ using a six-orbital model which we call the $f$-$d$-$p$ model. Our model accurately reproduces the quasi-two-dimensional Fermi surfaces consistent with recent de Haas-van Alphen oscillation measurements and the $(0, \pm π, 0)$ antiferromagnetic spin fluctuations observed by neutron scattering. We incorporate on-site Coulomb repulsion for $f$ electrons and solve the linearized Eliashberg equation within the third-order perturbation theory to investigate the superconducting symmetry in UTe$_2$. The most likely state is found to be an $s$-wave state with a highly anisotropic superconducting gap structure that exhibits a point-node-like behavior of the specific heat at low temperatures.

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arXiv2025

Theoretical Study of Impurity Effects on Superconductivity in UTe2

This study investigates the impurity effects on UTe2 within the self-consistent Born approximation using the six-orbital f-d-p model which contains two uranium and tellurium atoms in the minimum unit cell. We analyze the dependence of superconducting transition temperature (Tc) on impurity concentration for various pairing symmetries proposed by experiments and theories. It clarifies that the decrease of Tc significantly depends on which atom sites the impurities reside. Particulalry, the analysis shows that the impurity at U-site has dominant effect on the change of Tc. Then, either the singlet state in the case of magnetic impurities or the triplet states in both non-magnetic and magnetic impurities are consistent with experiments. Thus, this indicates that elucidating the magnetic properties of impurities (i.e. magnetic or non-magnetic) is crucial for identifying the pairing symmetry of UTe2.

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arXiv2024

Spin-lattice relaxation for point-node-like s-wave superconductivity in f-electron systems

In this study, we examined the temperature dependence of the spin-lattice relaxation using an f-d-p model, which is an effective model of UTe2. Solving the linearized Eliashberg equation in the f-d-p model based on third-order perturbation theory, we obtain a point-node-like s-wave pairing state. Our result shows that the Hebel-Slichter peak in the point-node-like s-wave pairing state is smaller than that in the isotropic s-wave pairing state. However, the Hebel-Slichter peak remains robust even in the point-node-like s-wave pairing state, and the point-node-like s-wave state is inconsistent with the results of nuclear magnetic resonance measurements.

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arXiv2020

Weak-coupling Mean-field Theory of Magnetic Properties of NiGa$_2$S$_4$

We report a mean-field theoretical study of a triangular lattice magnet NiGa$_2$S$_4$. Specifically, spiral mean-field theory is applied to a 17-band $d$-$p$ model constructed from the maximally localized Wannier functions. Our itinerant-model approach shows that the most stable spiral magnetic state has an ordering vector near ${\bf Q}=(0.15,0.15,0)$, consistent with neutron scattering experiments, when we assume the Ni-site Coulomb interaction is not so strong ($U \approx 2$ eV). To map onto a classical Heisenberg spin model, we estimate spin exchange interactions from the mean-field results, and find that the nearest-neighbor exchange is ferromagnetic and the largest (larger than the third nearest-neighbor exchange, in contrast to early studies). We also calculate the dynamical spin correlation function $S({\bf q}, ω)$, using the same model within the random-phase approximation (RPA). Calculated $S({\bf q}, ω)$ has a spectral structure quite different from that of conventional spin-wave excitations.

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arXiv2019

Detecting Crystallographic Lattice Chirality using Resonant Inelastic X-ray Scattering

The control and detection of crystallographic chirality is an important and challenging scientific problem. Chirality has wide ranging implications from medical physics to cosmology including an intimate but subtle connection in magnetic systems, for example Mn$_{1-x}$Fe$_{x}$Si. X-ray diffraction techniques with resonant or polarized variations of the experimental setup are currently utilized to characterize lattice chirality. We demonstrate using theoretical calculations the feasibility of indirect $K$ -edge bimagnon resonant inelastic X-ray scattering (RIXS) spectrum as a viable experimental technique to distinguish crystallographic handedness. We apply spin wave theory to the recently discovered $\sqrt {5}\times\sqrt {5}$ vacancy ordered chalcogenide Rb$_{0.89}$Fe$_{1.58}$Se$_{2}$ for realistic X-ray experimental set up parameters (incoming energy, polarization, and Bragg angle) to show that the computed RIXS spectrum is sensitive to the underlying handedness (right or left) of the lattice. A Flack parameter definition that incorporates the right- and left- chiral lattice RIXS response is introduced. It is shown that the RIXS response of the multiband magnon system RbFeSe arises both from inter- and intra- band scattering processes. The extinction or survival of these RIXS peaks are sensitive to the underlying chiral lattice orientation. This in turn allows for the identification of the two chiral lattice orientations.

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arXiv2017

Two-magnon excitations in resonant inelastic x-ray scattering studied by spin-density-wave formalism

We study two-magnon excitations in resonant inelastic x-ray scattering (RIXS) at the transition-metal $K$-edge. Instead of working with effective Heisenberg spin models, we work with a Hubbard-type model ($d$-$p$ model) for a typical insulating cuprate La$_2$CuO$_4$. For the antiferromagnetic ground state within the spin-density-wave (SDW) mean-field formalism, we calculate the dynamical correlation function within the random-phase approximation (RPA), and then obtain two-magnon excitation spectra by calculating the convolution of it. Coupling between the $K$-shell hole and the magnons in the intermediate state is calculated by means of diagrammatic perturbation expansion in the Coulomb interaction. Calculated momentum dependence of RIXS spectra agrees well with that of experiments. A notable difference from previous calculations based on the Heisenberg spin models is that RIXS spectra have a large two-magnon weight near the zone center, which may be confirmed by further careful high-resolution experiments.

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arXiv2015

Theoretical Study of $L$-edge Resonant Inelastic X-ray Scattering in La$_2$CuO$_4$ on the Basis of Detailed Electronic Band Structure

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. It is demonstrated that the single-magnon excitation intensity is maximized when the polarization directions of incoming and outgoing photons are perpendicular to each other.

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arXiv2014

Microscopic Analysis of Resonant Inelastic X-Ray Scattering in Orbital-Ordered KCuF3

We analyze resonant inelastic x-ray scattering (RIXS) at the Cu K edge in a typical orbital-ordered compound KCuF3 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-dγorbitals, 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.

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arXiv2012

Analysis of Resonant Inelastic X-Ray Scattering in Stripe-Ordered Nickelate

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_xNiO_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 Ni3dγand O2p_{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 Q, being consistent with a recent experimental result. The origin of this anomalous momentum dependence of RIXS spectra is discussed microscopically.

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arXiv2011

Evaluation of Spin-Triplet Superconductivity in Sr2RuO4

This review presents a summary and evaluations of the superconducting properties of the layered ruthenate Sr2RuO4 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 Sr2RuO4. After an introduction to superconductors with possible odd-parity pairing, accumulated evidence for the pairing symmetry of Sr2RuO4 is examined. Then, significant recent progress on the theoretical approaches to the superconducting pairing by Coulomb repulsion is reviewed. A section is devoted to some experimental properties of Sr2RuO4 that seem to defy simple explanations in terms of currently available spin-triplet scenario. The next section deals with some new developments using eutectic boundaries and micro-crystals, which reveals novel superconducting phenomena related to chiral edge states, odd-frequency pairing states, and half-fluxoid states. Some of these properties are intimately connected with the properties as a topological superconductor. The article concludes with a summary of knowledge emerged from the study of Sr2RuO4 that are now more widely applied to understand the physics of other unconventional superconductors, as well as with a brief discussion of relatively unexplored but promising areas of ongoing and future studies of Sr2RuO4.

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arXiv2009

Perturbation Theory of High-Tc Superconductivity in Iron Pnictides

The high-transition-temperature (high-Tc) superconductivity discovered recently in iron pnictides is analyzed within a perturbation theory. Specifically, the probable pairing symmetry, the doping dependence of the transition temperature and the pairing mechanism are studied by solving the Eliashberg equation for multi-band (2- and 5-band) Hubbard models with realistic electronic structures. The effective pairing interaction is expanded perturbatively in the on-site Coulomb integrals up to third order. Our perturbative weak-coupling approach shows that sufficiently large eigenvalues of the Eliashberg equation are obtained to explain the actual high transition temperatures by taking realistic on-site Coulomb integrals in the 5-band model. Thus, unconventional (non-phonon-mediated) superconductivity is highly likely to be realized. The superconducting order parameter does not change its sign on the Fermi surfaces, but it does change between the electron and hole Fermi surfaces. Consequently, the probable pairing symmetry is always "a nodeless extended s-wave symmetry (more specifically, an s_{+-}-wave symmetry)" over the whole parameter region that we investigated. It is suggested that the 2-band model is insufficient to explain the high values of Tc.

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arXiv2008

Possibility of Unconventional Pairing Due to Coulomb Interaction in Fe-Based Pnictide Superconductors: Perturbative Analysis of Multi-Band Hubbard Models

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 Tc 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 Tc.

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arXiv2008

Analysis of Incident-Photon-Energy and Polarization Dependent Resonant Inelastic X-Ray Scattering from La$_{2}$CuO$_{4}$

We present a detailed analysis of the incident-photon-energy and polarization dependences of the resonant inelastic x-ray scattering (RIXS) spectra at the Cu $K$ edge in La$_{2}$CuO$_{4}$. Our analysis is based on the formula developed by Nomura and Igarashi, which describes the spectra by a product of an incident-photon-dependent factor and a density-density correlation function for 3d states. We calculate the former factor using the $4p$ density of states from an ab initio band structure calculation and the latter using a multiorbital tight-binding model within the Hartree-Fock approximation and the random phase approximation. We obtain spectra with rich structures in the energy-loss range 2-5 eV, which vary with varying momentum and incident-photon energy, in semi-quantitative agreement with recent experiments. We clarify the origin of such changes as a combined effect of the incident-photon-dependent factor and the density-density correlation function.

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arXiv2007

Theory of resonant inelastic x-ray scattering at the K edge in La$_2$CuO$_4$ - Multiple scattering effects -

We develop a theory of resonant inelastic x-ray scattering (RIXS) at the $K$ edge in La$_2$CuO$_4$ on the basis of the Keldysh Green's function formalism. In our previous analysis (Phys. Rev. B 71, 035110 (2005)), the scattering by the core-hole potential was treated within the Born approximation, and a crude-model density of states was used for the $4p$ band. We improve the analysis by taking account of the multiple scattering in Cu3d-O$2p$ bands and by using a realistic $4p$ DOS obtained from a band calculation. The multiple scattering effect is evaluated with the use of the time representation developed by Nozières and De Dominicis. It is found that the multiple scattering effect makes the $K$-edge peak in the absorption coefficient shift to the lower energy region as a function of photon energy, that is, the photon energy required to excite the $1s$ electron to the $K$-edge peak reduces. It is also found that the multiple-scattering effect does not change the two-peak structure in the RIXS spectra but modifies slightly the shape as a function of energy loss. These findings suggests that the multiple scattering effect could mainly be included into a renormalization of the core-level energy and partly justify the Born approximation, leading to a future application to the RIXS in three-dimensional systems.

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arXiv2006

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

We analyze the resonant inelastic x-ray scattering (RIXS) spectra at the Ni $K$ edge in an antiferromagnetic insulator NiO by applying the theory developed by the present authors. It is based on the Keldysh Green's function formalism, and treats the core-hole potential in the intermediate state within the Born approximation. We calculate the single-particle energy bands within the Hartree-Fock approximation on the basis of the multi-orbital tight-binding model. Using these energy bands together with the $4p$ density of states from an ab initio band structure calculation, we calculate the RIXS intensities as a function of energy loss. By taking account of electron correlation within the random phase approximation (RPA), we obtain quantitative agreement with the experimental RIXS spectra, which consist of prominent two peaks around 5 eV and 8 eV, and the former shows considerable dispersion while the latter shows no dispersion. We interpret the peaks as a result of a band-to-band transition augmented by the RPA correlation.

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arXiv2005

Theory of Transport Properties in the p-wave Superconducting State of Sr2RuO4 - A Microscopic Determination of the Gap Structure -

We provide a detailed quantitative analysis of transport properties in the p-wave superconducting state of Sr2RuO4. Specifically, we calculate ultrasound attenuation rate and electronic thermal conductivity within the mean field approximation. The impurity scattering of the quasi-particles are treated within the self-consistent T-matrix approximation, and assumed to be in the unitarity limit. 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 Sr2RuO4. On the basis of the microscopic theory, we can naturally expect nodal structures along the c-axis on the cylindrical Fermi surfaces, even if we assume the chiral pairing state (i.e., Δ(k) \sim k_x \pm {\rm i} k_y). Consequently, we obtain the temperature dependence of the transport coefficients in agreement with the experimental results. We can clarify that actually the thermal excitations on the passively superconducting bands contribute significantly to the thermal conductivity in a wide temperature range, in contrast to the case of other physical quantities.

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arXiv2005

Analysis of resonant inelastic x-ray scattering in La2CuO4

We provide a semiquantitative explanation of a recent experiment on the resonant inelastic x-ray scattering (RIXS) in insulating cuprate La2CuO4 (Y.J. Kim et al., Phys. Rev. Lett. 89, 177003 (2002).). We show theoretically that there are three characteristic peaks in RIXS spectra, two of which are attributed to the charge transfer excitation and are reasonably assigned to those observed experimentally. The lowest energy peak has a relatively large dispersion ($\sim 0.8$ eV) and is suppressed near the zone corner (π, π), in agreement with the experiment. We stress that electron correlation is an essential factor for explaining the overall energy-momentum dependence of the RIXS spectra consistently.

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arXiv2004

Theoretical Study on Resonant Inelastic X-Ray Scattering in Quasi-One-Dimensional Cuprates

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 Cu1s-4p 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.

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