arXiv subjects

Manabu Takahashi

Publications and source records attributed to Manabu Takahashi.

At least 19 recordsLinked to original sources

arXiv2019

Magnetic circular dichroism in hard x-ray Raman scattering as a probe of local spin polarization

We argue that the magnetic circular dichroism (MCD) of the hard x-ray Raman scattering (XRS) could be used as an element selective probe of local spin polarization. The magnitude of the XRS-MCD signal is directly proportional to the local spin polarization when the angle between the incident wavevector and the magnetization vector is $135^{\circ}$ or $-45^{\circ}$. By comparing the experimental observation and the configuration interaction calculation at the $L_{2,3}$ and $M_{2,3}$ edges of ferromagnetic iron, we suggest that the integrated MCD signal in terms of the transferred energy could be used to estimate the local spin moment even in the case where the application of the spin sum-rule in X-ray absorption is questionable. We also point out that XRS-MCD signal could be observed at the $M_{1}$ edge with a magnitude comparable to that at the $M_{2,3}$ edge, although the spin-orbit coupling is absent in the core orbital. By combining the XRS-MCD at various edges, spin polarization distribution depending on the orbital magnetic quantum number would be determined.

cond-mat.mtrl-sci↗

arXiv2012

Resonant x-ray scattering from chiral materials, $α$-quartz and $α$-berlinite

We study the resonant x-ray scattering at Si and Al K-edges from chiral materials, $α$-quartz and $α$-berlinite. We derive the general form of the scattering matrix for the dipole transition by summing up the local scattering matrices which satisfy the symmetry requirement. The oscillation term is obtained in the spectral intensity as a function of azimuthal angle with an expression of possible phase shift. We evaluate the parameters undetermined by the symmetry argument alone on the basis of underlying electronic structures given by the bond-orbital model. The spectra are calculated on forbidden spots $(001)$, $(00\bar{1})$, $(002)$, and $(00\bar{2})$ in circular polarizations without adjustable parameter, reproducing well the experimental curves depending on polarization, chirality, and scattering vector. Some discrepancies remain in the phase shift in $α$-quartz.

cond-mat.mtrl-sci↗

arXiv2012

Ab initio study of 2p-core level x-ray photoemission spectra in ferromagnetic transition metals

We study the 2p-core level x-ray photoemission spectra in ferromagnetic transition metals, Fe, Co, and Ni using a recently developed ab initio method.The excited final states are set up by distributing electrons on the one-electron states calculated under the fully screened potential in the presence of the core hole. We evaluate the overlap between these excited states and the ground state by using one-electron wave functions, and obtain the spectral curves as a function of binding energy. The calculated spectra reproduce well the observed spectra displaying interesting dependence on the element and on the spin of the removed core electron. The origin of the spectral shapes is elucidated in terms of the one-electron states screening the core hole. The magnetic splitting of the threshold energy is also estimated by using the coherent potential approximation within the fully screened potential approximation. It decreases more rapidly than the local spin moment with moving from Fe to Ni. It is estimated to be almost zero for Ni despite the definite local moment about 0.6μ_B, in agreement with the experiment.

cond-mat.mtrl-sci↗

arXiv2010

An ab initio study of 3s core-level x-ray photoemission spectra in transition metals

We calculate the $3s$- and $4s$-core-level x-ray photoemission spectroscopy (XPS) spectra in the ferromagnetic and nonmagnetic transition metals by developing an \emph{ab initio} method. We obtain the spectra exhibiting the characteristic shapes as a function of binding energy in good agreement with experimental observations. The spectral shapes are strikingly different between the majority spin channel and the minority spin channel for ferromagnetic metals Ni, Co, and Fe, that is, large intensities appear in the higher binding energy side of the main peak (satellite) in the majority spin channel. Such satellite or shoulder intensities are also obtained for nonmagnetic metals V and Ru. These behaviors are elucidated in terms of the change of the one-electron states induced by the core-hole potential.

cond-mat.mtrl-sci↗

arXiv2008

Charge Excitations in Cuprate and Nickelate in Resonant Inelastic X-Ray Scattering

We analyze the resonant inelastic x-ray scattering (RIXS) spectra at the Cu and Ni K edges in La2CuO4 and La2NiO4, respectively. We make use of the Keldysh-Green-function formalism, in which the RIXS intensity is described by a product of incident-photon-dependent factor and density-density correlation function in the 3d states. The former factor is calculated using the $4p$ density of states given by an ab initio band structure calculation and the latter using the wavefunctions given by a Hartree-Fock calculation of a multi-orbital tight-binding model. The initial state is described within the Hartree-Fock approximation and the electron correlations on charge excitations are treated within the random phase approximation. The calculated RIXS spectra well reproduce several characteristic features in the experiments. Although several groups have interpreted the RIXS peaks as bound excitons, our calculation indicates that they should be interpreted as band-to-band excitations augmented by electron correlations. The difference in RIXS spectra between La2CuO4 and La2NiO4 is explained from this point of view.

cond-mat.str-el↗

arXiv2008

An ab-initio calculation of the core-level x-ray photoemission spectra -Fe 3s- and 1s-core levels-

We develop a method of the ab-initio calculation for the core-level x-ray photoemission spectroscopy (XPS). By calculating one-electron states in the presence of core-hole potential, excited states are constructed by distributing electrons on these one-electron states. The overlap integrals between the excited states and the ground state are evaluated by using the wavefunctions with and without the core-hole potential, and finally the XPS spectra are obtained. Implementing the procedure, we obtain the spin-resolved 3s-core XPS spectra in ferromagnetic iron without any adjustable parameters, in good agreement with the experiment. The spectral shapes are quite different for different spin channels. We explain the behavior in terms of the difference in the one-electron states screening the core-hole potential.

cond-mat.mtrl-sci↗

arXiv2008

Theoretical Analysis of Resonant Inelastic X-Ray Scattering Spectra in LaMnO3

We analyze the resonant inelastic x-ray scattering (RIXS) spectra at the K edge of Mn in the antiferromagnetic insulating manganite LaMnO3. We make use of the Keldysh-type Green-function formalism, in which the RIXS intensity is described by a product of an incident-photon-dependent factor and a density-density correlation function in the 3d states. We calculate the former factor using the 4p density of states given by an ab initio band structure calculation and the latter using a multi-orbital tight-binding model. The ground state of the model Hamiltonian is evaluated within the Hartree-Fock approximation. Correlation effects are treated within the random phase approximation (RPA). We obtain the RIXS intensity in a wide range of energy-loss 2-15 eV. The spectral shape is strongly modified by the RPA correlation, showing good agreement with the experiments. The incident-photon-energy dependence also agrees well with the experiments. The present mechanism that the RIXS spectra arise from band-to-band transitions to screen the core-hole potential is quite different from the orbiton picture previously proposed, enabling a comprehensive understanding of the RIXS spectra.

cond-mat.str-el↗

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.

cond-mat.str-el↗

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.

cond-mat.str-el↗

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.

cond-mat.str-el↗

arXiv2005

4p states and X-Ray Spectroscopy

The 4p states in transition metals and their compounds usually play minor roles on their physical quantities. Recent development of resonant x-ray scattering (RXS) at the K-edge of transition metals, however, casts light on the 4p states, because the signals on orbital and magnetic superlattice spots are brought about by the modulation in the 4p states. The 4p states are extending in solids and thereby sensitive to electronic states at neighboring sites. This characteristic determines the mechanism of RXS that the intensity on the orbital superlattice spots are mainly generated by the lattice distortion and those on magnetic superlattice spots by the coupling of the 4p states with the orbital polarization in the 3d states at neighboring sites. Taking up typical examples for orbital and magnetic RXS, we demonstrate these mechanisms on the basis of the band structure calculation. Finally, we study the MCD spectra at the K-edge, demonstrating that the same mechanism as the magnetic RXS is working.

cond-mat.mtrl-sci↗

arXiv2004

Resonant magnetic X-ray scattering spectra in SDW Cr -- ab initio study -----

Using ab-initio band structure calculation based on the local density approximation, Cr K-edge resonant X-ray magnetic scattering spectra are analyzed in the spin density wave (SDW) state of chromium. We perform band structure calculation, assuming an ideal bcc lattice structure with the lattice constant observed at the spin-flip temperature T_{SF} and a commensurate SDW state with the propagation vector close to the observed value at T_{SF}. Taking account of the spin-orbit interaction, we obtain the orbital moment on each Cr site induced in proportion to the local spin moment, which is quite small, at most a tenth of those in nickel or iron. In spite of the tiny 3d orbital moment, the orbital polarization is found to have large fluctuations as a function of energy. We obtain the scattering intensity at the Cr K-edge on the SDW magnetic Bragg spot, which shows resonant enhancement in good agreement with the experiment. The 3d orbital polarization is found to be highly correlated with the intensity of the resonant main peak, indicating that the 4p orbital polarization is mainly induced by the 3d orbital polarization through the p-d hybridization.

cond-mat.mtrl-sci↗

arXiv2003

An ab-initio calculation of magnetic resonant x-ray scattering spectra in NiO

We investigate the magnetic resonant x-ray scattering spectra around the $K$ edge of Ni in antiferromagnetic NiO using an \textit{ab-initio} band-structure calculation based on the density-functional theory. By taking account of orbital polarization through the spin-orbit interaction, we reproduce well the spectra obtained experimentally, thus demonstrating the usefulness of the \textit{ab-initio} calculation. It is shown that the main-edge peak, which mainly comes from the dipolar ($1s \to 4p$) transition, is a direct reflection of the orbital polarization of the $4p$ states. It is clarified that the $4p$ orbital polarization is mainly induced from the spin polarization on the $4p$ states by the spin-orbit interaction. The 3d orbital polarization at neighboring Ni sites gives rise to only a minor contribution to the $4p$ orbital polarization through a $p$-$d$ mixing. It is also shown that the pre-edge peak, which mainly originates from the quadrupolar ($1s \to 3d$) transition, is a direct reflection of the orbital polarization of the unoccupied 3d states. It shows a Fano-type antiresonant dip due to interference with the nonresonant contribution, in agreement with the experimental result.

cond-mat.str-el↗

arXiv2003

X-Ray Magnetic Circular Dichroism at the K edge of Mn3GaC

We theoretically investigate the origin of the x-ray magnetic circular dichroism (XMCD) spectra at the K edges of Mn and Ga in the ferromagnetic phase of Mn3GaC on the basis of an ab initio calculation. Taking account of the spin-orbit interaction in the LDA scheme, we obtain the XMCD spectra in excellent agreement with the recent experiment. We have analyzed the origin of each structure, and thus elucidated the mechanism of inducing the orbital polarization in the p symmetric states. We also discuss a simple sum rule connecting the XMCD spectra with the orbital moment in the p symmetric states.

cond-mat.mtrl-sci↗

arXiv2001

Effect of Crystal Structure to Resonant X-Ray Scattering on YTiO3

We investigate the mechanism of the resonant X-ray scattering on the Ti $K$ edge region of YTiO$_3$ using the band structure calculation combined with the local density approximation. A large intensity is obtained for the orbital superlattice spots. The calculated spectra consist of several peaks as a function of photon energy in agreement with the recent experiments. Against a naive interpretation that directly relates the intensity to the orbitally polarized $3d$ states, the obtained large intensity arises from the distorted crystal structure, i.e., the tilt of the TiO$_6$ octahedra and the Jahn-Teller distortion, which considerably modifies the $4p$ states in the intermediate states of the dipolar process. This casts doubt on a prevailing assertion that the resonant x-ray scattering is a direct observation of the orbital order.

cond-mat.str-el↗

arXiv2001

Mechanism of resonant x-ray magnetic scattering in NiO

We study the resonant x-ray magnetic scattering (RXMS) around the K edge of Ni in the antiferromagnet NiO, by treating the 4p states of Ni as a band and the 3d states as localized states. We propose a mechanism that the 4p states are coupled to the magnetic order through the intra-atomic Coulomb interaction between the 4p and the 3d states and through the p-d mixing to the 3d states of neighboring Ni atoms. These couplings induce the orbital moment in the 4p band, and thereby give rise to the RXMS intensity at the K edge in the dipolar process. It is found that the spin-orbit interaction in the 4p band has negligibly small contribution to the RXMS intensity. The present model reproduces well the experimental spectra. We also discuss the azimuthal angle dependence of the intensity.

cond-mat.str-el↗

arXiv2000

Resonant X-Ray Magnetic Scattering from CoO

We analyze the recent experiment [W. Neubeck {\em et al.}, Phys. Rev. B \vol(60,1999,R9912)] for the resonant x-ray magnetic scattering (RXMS) around the K edge of Co in the antiferromagnet CoO. We propose a mechanism of the RXMS to make the $4p$ states couple to the magnetic order: the intraatomic exchange interaction between the $4p$ and the $3d$ states and the $p$-$d$ mixing to the $3d$ states of neighboring Co atoms. These couplings induce the orbital moment in the $4p$ states and make the scattering tensor antisymmetric. Using a cluster model, we demonstrate that this modification gives rise to a large RXMS intensity in the dipole process, in good agreement with the experiment. We also find that the pre-edge peak is generated by the transition to the $3d$ states in the quadrupole process, with negligible contribution of the dipole process. We also discuss the azimuthal angle dependence of the intensity.

cond-mat.str-el↗

arXiv2000

Pre-K-Edge Structure on Anomalous X-Ray Scattering in LaMnO3

We study the pre-K-edge structure of the resonant X-ray scattering for forbidden reflections (anomalous scattering) in LaMnO3, using the band calculation based on the local density approximation. We find a two-peak structure with an intensity approximately 1/100 of that of the main peak. This originates from a mixing of 4p states of Mn to 3d states of neighboring Mn sites. The effect is enhanced by an interference with the tail of the main peak. The effect of the quadrupole transition is found to be one order of magnitude smaller than that of the dipole transition, modifying slightly the azimuthal-angle dependence.

cond-mat.str-el↗