arXiv subjects

Shin-ichi Fujimori

Publications and source records attributed to Shin-ichi Fujimori.

At least 19 recordsLinked to original sources

arXiv2024

Correlated Ligand Electrons in the Transition-Metal Oxide SrRuO$_3$

In transition-metal compounds, the transition-metal d electrons play an important role in their physical properties; however, the effects of the electron correlation between the ligand p electrons have not been clear yet. In this Letter, the Ru 4d and O 2p partial density of states (PDOS) in transition-metal oxide SrRuO$_3$ involving Weyl fermions are investigated by resonant photoemission spectroscopy. The observations demonstrate that the O 2p PDOS is distorted from that predicted by first-principles calculations than the Ru 4d PDOS. The results indicate that the electron correlation in the ligand orbitals will be important to understand the electronic structure of the p-d hybridized state in strongly correlated electron systems, even with topological states.

cond-mat.str-el↗

arXiv2023

Impact of the Ce $4f$ states in the electronic structure of the intermediate-valence superconductor CeIr$_3$

The electronic structure of the $f$-based superconductor $\mathrm{CeIr_3}$ was studied by photoelectron spectroscopy. The energy distribution of the $\mathrm{Ce}~4f$ states were revealed by the $\mathrm{Ce}~3d-4f$ resonant photoelectron spectroscopy. The $\mathrm{Ce}~4f$ states were mostly distributed in the vicinity of the Fermi energy, suggesting the itinerant character of the $\mathrm{Ce}~4f$ states. The contribution of the $\mathrm{Ce}~4f$ states to the density of states (DOS) at the Fermi energy was estimated to be nearly half of that of the $\mathrm{Ir}~5d$ states, implying that the $\mathrm{Ce}~4f$ states have a considerable contribution to the DOS at the Fermi energy. The $\mathrm{Ce}~3d$ core-level and $\mathrm{Ce}~3d$ X-ray absorption spectra were analyzed based on a single-impurity Anderson model. The number of the $\mathrm{Ce}~4f$ states in the ground state was estimated to be $0.8-0.9$, which is much larger than the values obtained in the previous studies (i.e., $0-0.4$).

cond-mat.str-el↗

arXiv2023

Allotropic transition of Dirac semimetal α-Sn to superconductor β-Sn induced by irradiation of focused ion beam

Diamond-type structure allotrope α-Sn is attracting much attention as a topological Dirac semimetal (TDS). In this study, we demonstrate that α-Sn undergoes a phase transition to another allotrope β-Sn with superconductivity at low temperature by irradiating with a focused Ga ion beam (FIB). To clarify the transition mechanism, we performed X-ray photoemission spectroscopy (XPS) measurements on an α-Sn thin film irradiated with FIB and an as-grown α-Sn thin film. The XPS results suggest that the local annealing, which is one of the side effects of FIB, causes the transformation from α-Sn into β-Sn. Furthermore, the difference in the chemical states between α-Sn and β-Sn can be quantitatively explained by the crystal structures rather than the degree of metallicity reflecting the conductivity. These results propose a new way of fabricating TDS/superconductor in-plane heterostructures based on α-Sn and β-Sn.

cond-mat.mtrl-sci↗

arXiv2022

Electronic Structure of ThPd$_2$Al$_3$: an impact of the U $5f$ states in the electronic structure of UPd$_2$Al$_3$

The electronic structure of ThPd$_2$Al$_3$, which is isostructural to the heavy fermion superconductor UPd$_2$Al$_3$, was investigated by photoelectron spectroscopy. The band structure and Fermi surfaces of ThPd$_2$Al$_3$ were obtained by angle-resolved photoelectron spectroscopy (ARPES), and the results were well-explained by the band-structure calculation based on the local density approximation. The comparison between the ARPES spectra and the band-structure calculation suggests that the Fermi surface of ThPd$_2$Al$_3$ mainly consists of the Al $3p$ and Th $6d$ states with a minor contribution from the Pd $4d$ states. The comparison of the band structures between ThPd$_2$Al$_3$ and UPd$_2$Al$_3$ argues that the U $5f$ states form Fermi surfaces in UPd$_2$Al$_3$ through hybridization with the Al $3p$ state in the Al layer, suggesting that the Fermi surface of UPd$_2$Al$_3$ has a strong three-dimensional nature.

cond-mat.str-el↗

arXiv2021

Development of magnetism in Fe-doped magnetic semiconductors: Resonant photoemission and x-ray magnetic circular dichroism studies of (Ga,Fe)As

Fe-doped III-V ferromagnetic semiconductors (FMSs) such as (In,Fe)As, (Ga,Fe)Sb, (In,Fe)Sb, and (Al,Fe)Sb are promising materials for spintronic device applications because of the availability of both n- and p-type materials and the high Curie temperatures. On the other hand, (Ga,Fe)As, which has the same zinc-blende crystal structure as the Fe-doped III-V FMSs, shows paramagnetism. The origin of the different magnetic properties in the Fe-doped III-V semiconductors remains to be elucidated. To address this issue, we use resonant photoemission spectroscopy (RPES) and x-ray magnetic circular dichroism (XMCD) to investigate the electronic and magnetic properties of the Fe ions in a paramagnetic (Ga$_{0.95}$,Fe$_{0.05}$)As thin film. The observed Fe 2$p$-3$d$ RPES spectra show that the Fe 3$d$ states are similar to those of ferromagnetic (Ga,Fe)Sb. The estimated Fermi level is located in the middle of the band gap in (Ga,Fe)As. The Fe $L_{2,3}$ XMCD spectra of (Ga$_{0.95}$,Fe$_{0.05}$)As show pre-edge structures, which are not observed in the Fe-doped FMSs, indicating that the minority-spin ($\downarrow$) $e_\downarrow$ states are vacant in (Ga$_{0.95}$,Fe$_{0.05}$)As. The XMCD results suggest that the carrier-induced ferromagnetic interaction in (Ga$_{0.95}$,Fe$_{0.05}$)As is short-ranged and weaker than that in the Fe-doped FMSs. The experimental findings suggest that the electron occupancy of the $e_\downarrow$ states contributes to the appearance of ferromagnetism in the Fe-doped III-V semiconductors, for p-type as well as n-type compounds.

cond-mat.mtrl-sci↗

arXiv2021

Single-domain perpendicular magnetization induced by the coherent O 2p-Ru 4d hybridized state in an ultra-high-quality SrRuO3 film

We investigated the Ru 4d and O 2p electronic structure and magnetic properties of an ultra-high-quality SrRuO3 film on SrTiO3 grown by machine-learning-assisted molecular beam epitaxy. The high itinerancy and long quantum lifetimes of the quasiparticles in the Ru 4d t2g-O 2p hybridized valence band are confirmed by observing the prominent well-screened peak in the Ru 3d core-level photoemission spectrum, the coherent peak near the Fermi energy in the valence band spectrum, and quantum oscillations in the resistivity. The element-specific magnetic properties and the hybridization between the Ru 4d and O 2p orbitals were characterized by Ru M2,3-edge and O K-edge soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements. The ultra-high-quality SrRuO3 film with the residual resistivity ratio of 86 shows the large orbital magnetic moment of oxygen ions induced by the strong orbital hybridization of the O 2p states with the spin-polarized Ru 4d t2g states. The film also shows single-domain perpendicular magnetization with an almost ideal remanent magnetization ratio of 0.97. These results provide detailed insights into the relevance between orbital hybridization and the perpendicular magnetic anisotropy in SrRuO3/SrTiO3 systems.

cond-mat.mtrl-sci↗

arXiv2021

Electronic structure of URu$_2$Si$_2$ in paramagnetic phase: Three-dimensional angle resolved photoelectron spectroscopy study

The three-dimensional (3D) electronic structure of the hidden order compound URu$_2$Si$_2$ in a paramagnetic phase was revealed using a 3D angle-resolved photoelectron spectroscopy where the electronic structure of the entire Brillouin zone is obtained by scanning both incident photon energy and detection angles of photoelectrons. The quasi-particle bands with enhanced contribution from the $\mathrm{U}~5f$ state were observed near $E_\mathrm{F}$, formed by the hybridization with the $\mathrm{Ru}~4d$ states. The energy dispersion of the quasi-particle band is significantly depend on $k_z$, indicating that they inherently have a 3D nature. The band-structure calculation qualitatively explain the characteristic features of the band structure and Fermi surface although the electron correlation effect strongly renormalizes the quasi-particle bands. The 3D and strongly-correlated nature of the quasi-particle bands in URu$_2$Si$_2$ is an essential ingredient for modeling its hidden-order transition.

cond-mat.str-el↗

arXiv2021

UTe$_2$: a nearly insulating half-filled $j=\frac{5}{2}$ $5f^3$ heavy fermion metal

Correlated band theory implemented as a combination of density functional theory with exact diagonalization [DFT+U(ED)] of the Anderson impurity term with Coulomb repulsion $U$ in the open 14-orbital $5f$ shell is applied to UTe$_2$. The small gap for $U$=0, evidence of the half-filled $j=\frac{5}{2}$ subshell of $5f^3$ uranium, is converted for $U$=3 eV to a flat band semimetal with small heavy-carrier Fermi surfaces that will make properties sensitive to pressure, magnetic field, and off-stoichiometry, as observed experimentally. The predicted Kondo temperature around 100 K matches the experimental values from resistivity. The electric field gradients for the two Te sites are calculated by DFT+U(ED) to differ by a factor of seven, indicating a strong site distinction, while the anisotropy factor $η=0.18$ is similar for all three sites. The calculated uranium moment $ ^{1/2}$ of 3.5$μ_B$ is roughly consistent with the published experimental Curie-Weiss values of 2.8$μ_B$ and 3.3$μ_B$ (which are field-direction dependent), and the calculated separate spin and orbital moments are remarkably similar to Hund's rule values for an $f^3$ ion. The $U$=3 eV spectral density is compared with angle-integrated and angle-resolved photoemission spectra, with agreement that there is strong $5f$ character at, and for several hundred meV below, the Fermi energy. Our results support the picture that the underlying ground state of UTe$_2$ is that of a half-filled $j=\frac{5}{2}$ subshell with two half-filled $m_j=\pm\frac{1}{2}$ orbitals forming a narrow gap by hybridization, then driven to a conducting state by configuration mixing (spin-charge fluctuations). UTe$_2$ displays similarities to UPt$_3$ with its $5f$ dominated Fermi surfaces rather than a strongly localized Kondo lattice system.

cond-mat.str-el↗

arXiv2020

Core-Level Photoelectron Spectroscopy Study of UTe$_2$

The valence state of UTe$_2$ was studied by core-level photoelectron spectroscopy. The main peak position of the U $4f$ core-level spectrum of UTe$_2$ coincides with that of UB$_2$, which is an itinerant compound with a nearly $5f^3$ configuration. However, the main peak of UTe$_2$ is broader than that of UB$_2$, and satellite structures are observed in the higher binding energy side of the main peak, which are characteristics of mixed-valence uranium compounds. These results suggest that the U 5$f$ state in UTe$_2$ is in a mixed valence state with a dominant contribution from the itinerant $5f^3$ configuration.

cond-mat.str-el↗

arXiv2020

Alternation of Magnetic Anisotropy Accompanied by Metal-Insulator Transition in Strained Ultrathin Manganite Heterostructures

Fundamental understanding of interfacial magnetic properties in ferromagnetic heterostructures is essential to utilize ferromagnetic materials for spintronic device applications. In this paper, we investigate the interfacial magnetic and electronic structures of epitaxial single-crystalline LaAlO$_3$ (LAO)/La$_{0.6}$Sr$_{0.4}$MnO$_3$ (LSMO)/Nb:SrTiO$_3$ (Nb:STO) heterostructures with varying LSMO-layer thickness, in which the magnetic anisotropy strongly changes depending on the LSMO thickness due to the delicate balance between the strains originating from both the Nb:STO and LAO layers, using x-ray magnetic circular dichroism (XMCD) and photoemission spectroscopy (PES). We successfully detect the clear change of the magnetic behavior of the Mn ions concomitant with the thickness-dependent metal-insulator transition (MIT). Our results suggest that double-exchange interaction induces the ferromagnetism in the metallic LSMO film under tensile strain caused by the SrTiO$_3$ substrate, while superexchange interaction determines the magnetic behavior in the insulating LSMO film under compressive strain originating from the top LAO layer. Based on those findings, the formation of a magnetic dead layer near the LAO/LSMO interface is attributed to competition between the superexchange interaction via Mn 3$d_{3z^2-r^2}$ orbitals under compressive strain and the double-exchange interaction via the 3$d_{x^2-y^2}$ orbitals.

cond-mat.mtrl-sci↗

arXiv2020

Soft x-ray irradiation induced metallization of layered TiNCl

We have performed soft x-ray spectroscopy in order to study the photoirradiation time dependence of the valence band structure and chemical states of layered transition metal nitride chloride TiNCl. Under the soft x-ray irradiation, the intensities of the states near the Fermi level (EF) and the Ti3+ component increased, while the Cl 2p intensity decreased. Ti 2p-3d resonance photoemission spectroscopy confirmed a distinctive Fermi edge with Ti 3d character. These results indicate the photo-induced metallization originates from deintercalation due to Cl desorption, and thus provide a new carrier doping method that controls the conducting properties of TiNCl.

cond-mat.mtrl-sci↗

arXiv2020

Evolution of the Fe-3$d$ impurity band state as the origin of high Curie temperature in p-type ferromagnetic semiconductor (Ga,Fe)Sb

(Ga$_{1-x}$,Fe$_x$)Sb is one of the promising ferromagnetic semiconductors for spintronic device applications because its Curie temperature ($T_{\rm C}$) is above 300 K when the Fe concentration $x$ is equal to or higher than ~0.20. However, the origin of the high $T_{\rm C}$ in (Ga,Fe)Sb remains to be elucidated. To address this issue, we use resonant photoemission spectroscopy (RPES) and first-principles calculations to investigate the $x$ dependence of the Fe 3$d$ states in (Ga$_{1-x}$,Fe$_x$)Sb ($x$ = 0.05, 0.15, and 0.25) thin films. The observed Fe 2$p$-3$d$ RPES spectra reveal that the Fe-3$d$ impurity band (IB) crossing the Fermi level becomes broader with increasing $x$, which is qualitatively consistent with the picture of double-exchange interaction. Comparison between the obtained Fe-3$d$ partial density of states and the first-principles calculations suggests that the Fe-3$d$ IB originates from the minority-spin ($\downarrow$) $e$ states. The results indicate that enhancement of the interaction between $e_\downarrow$ electrons with increasing $x$ is the origin of the high $T_{\rm C}$ in (Ga,Fe)Sb.

cond-mat.mtrl-sci↗

arXiv2019

Electronic Structure of UTe$_2$ Studied by Photoelectron Spectroscopy

The electronic structure of the unconventional superconductor UTe$_2$ was studied by resonant photoelectron spectroscopy (RPES) and angle-resolved photoelectron spectroscopy (ARPES) with soft X-ray synchrotron radiation. The partial $\mathrm{U}~5f$ density of states of UTe$_2$ were imaged by the $\mathrm{U}~4d$--$5f$ RPES and it was found that the $\mathrm{U}~5f$ state has an itinerant character, but there exists an incoherent peak due to the strong electron correlation effects. Furthermore, an anomalous admixture of the $\mathrm{U}~5f$ states into the $\mathrm{Te}~5p$ bands was observed at a higher binding energy, which cannot be explained by band structure calculations. On the other hand, the band structure of UTe$_2$ was obtained by ARPES and its overall band structure were mostly explained by band structure calculations. These results suggest that the $\mathrm{U}~5f$ states of UTe$_2$ have itinerant but strongly-correlated nature with enhanced hybridization with the $\mathrm{Te}~5p$ states.

cond-mat.str-el↗

arXiv2019

Manifestation of electron correlation effect in $\mathrm{U}~5f$ states of uranium compounds revealed by $\mathrm{U}~4d-5f$ resonant photoemission spectroscopy

We have elucidated the nature of the electron correlation effect in uranium compounds by imaging the partial $\mathrm{U}~5f$ density of states (pDOS) of typical itinerant, localized, and heavy fermion uranium compounds by using the $\mathrm{U}~4d-5f$ resonant photoemission spectroscopy. Obtained $\mathrm{U}~5f$ pDOS exhibit a systematic trend depending on the physical properties of compounds. The coherent peak at the Fermi level can be described by the band-structure calculation, but an incoherent peak emerges on the higher binding energy side ($\lesssim 1~\mathrm{eV}$) in the \Uf pDOS of localized and heavy fermion compounds. As the $\mathrm{U}~5f$ state is more localized, the intensity of the incoherent peak is enhanced and its energy position is shifted to higher binding energy. These behaviors are consistent with the prediction of the Mott metal-insulator transition, suggesting that the Hubbard-$U$ type mechanism takes an essential role in the $5f$ electronic structure of actinide materials.

cond-mat.str-el↗

arXiv2018

Electronic structure of the novel high-$T_{\rm C}$ ferromagnetic semiconductor (Ga,Fe)Sb: x-ray magnetic circular dichroism and resonance photoemission spectroscopy studies

The electronic structure and the magnetism of the novel ferromagnetic semiconductor (Ga,Fe)Sb, whose Curie temperature $T_{\rm C}$ can exceed room temperature, were investigated by means of x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and resonance photoemission spectroscopy (RPES). The line-shape analyses of the XAS and XMCD spectra suggest that the ferromagnetism is of intrinsic origin. The orbital magnetic moments deduced using XMCD sum rules were found to be large, indicating that there is a considerable amount of 3$d^{6}$ contribution to the ground state of Fe. From RPES, we observed a strong dispersive Auger peak and non-dispersive resonantly enhanced peaks in the valence-band spectra. The latter is a fingerprint of the correlated nature of Fe 3$d$ electrons, whereas the former indicates their itinerant nature. It was also found that the Fe 3$d$ states have finite contribution to the DOS at the Fermi energy. These states presumably consisting of majority-spin $p$-$d$ hybridized states or minority-spin $e$ states would be responsible for the ferromagnetic order in this material.

cond-mat.str-el↗

arXiv2017

Electronic structures of UX$_3$ (X=Al, Ga, and In) studied by photoelectron spectroscopy

The electronic structures of UX$_3$ (X=Al, Ga, and In) were studied by photoelectron spectroscopy to understand the relationship between their electronic structures and magnetic properties. The band structures and Fermi surfaces of UAl$_3$ and UGa$_3$ were revealed experimentally by angle-resolved photoelectron spectroscopy (ARPES), and they were compared with the result of band-structure calculations. The topologies of the Fermi surfaces and the band structures of UAl$_3$ and UGa$_3$ were explained reasonably well by the calculation, although bands near the Fermi level ($E_\mathrm{F}$) were renormalized owing to the finite electron correlation effect. The topologies of the Fermi surfaces of UAl$_3$ and UGa$_3$ are very similar to each other, except for some minor differences. Such minor differences in their Fermi surface or electron correlation effect might take an essential role in their different magnetic properties. No significant changes were observed between the ARPES spectra of UGa$_3$ in the paramagnetic and antiferromagnetic phases, suggesting that UGa$_3$ is an itinerant weak antiferromagnet. The effect of chemical pressure on the electronic structures of UX$_3$ compounds was also studied by utilizing the smaller lattice constants of UAl$_3$ and UGa$_3$ than that of UIn$_3$. The valence band spectrum of UIn$_3$ is accompanied by a satellite-like structure on the high-binding-energy side. The core-level spectrum of UIn$_3$ is also qualitatively different from those of UAl$_3$ and UGa$_3$. These findings suggest that the U~$5f$ states in UIn$_3$ are more localized than those in UAl$_3$ and UGa$_3$.

cond-mat.str-el↗

arXiv2017

Electronic structure of ThRu2Si2 studied by angle-resolved photoelectron spectroscopy: Elucidating the contribution of U 5f states in URu2Si2

The electronic structure of ThRu2Si2 was studied by angle-resolved photoelectron spectroscopy (ARPES) with incident photon energies of hn=655-745 eV. Detailed band structure and the three-dimensional shapes of Fermi surfaces were derived experimentally, and their characteristic features were mostly explained by means of band structure calculations based on the density functional theory. Comparison of the experimental ARPES spectra of ThRu2Si2 with those of URu2Si2 shows that they have considerably different spectral profiles particularly in the energy range of 1 eV from the Fermi level, suggesting that U 5f states are substantially hybridized in these bands. The relationship between the ARPES spectra of URu2Si2 and ThRu2Si2 is very different from the one between the ARPES spectra of CeRu2Si2 and LaRu2Si2, where the intrinsic difference in their spectra is limited only in the very vicinity of the Fermi energy. The present result suggests that the U 5f electrons in URu2Si2 have strong hybridization with ligand states and have an essentially itinerant character.

cond-mat.str-el↗

arXiv2016

Correlation effect in Sr$_{1-x}$La$_x$RuO$_3$ studied by soft x-ray photoemission spectroscopy

To clarify how the electronic state of Sr1-xLaxRuO3 evolves with La doping, we conducted photoemission (PES) experiments using soft x-rays. The spectral shape of the Ru 4d derived peak near the Fermi level changes significantly with increasing x. This variation indicates that a spectral weight transfer from the coherent to incoherent component occurs due to an enhancement of the electron correlation effect. Resonant PES experiments at the La 3d_{5/2} edge have confirmed that there is no significant contribution of the La 5d state in the energy range where the spectral weight transfer is observed. Using the dependence of the photoelectron mean free path on the photon energy, we subtracted the surface components from the PES spectra and confirmed that the enhancement of the electron correlation effect with La doping is an intrinsic bulk phenomenon. On the other hand, a large portion of the coherent component remains at the Fermi level up to x = 0.5, reflecting that the Ru 4d state still has itinerant characteristics. Moreover, we found that the PES spectra hardly depend on the temperature and do not exhibit a discernible change with magnetic ordering, suggesting that the temperature variation of the exchange splitting does not follow the prediction of the Stoner theory. The presently obtained experimental results indicate that the electron correlation effect plays an important role in Sr1-xLaxRuO3 and that the Ru 4d electrons possess both local and itinerant characteristics.

cond-mat.str-el↗