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Energy sustainability is critical for social activities in the human world. The quaternary compound Cu2ZnSnSe4 (CZTSe), as a promising candidate for thin-film solar cell absorption with medium-level thermoelectric performance, is of interest for the purpose of utilizing solar energy. The defect chemistry and atomic ordering in this particular compound also triggers interests in understanding its crystallographic structure as well as defects. Hereby, high energy resolution X-ray absorption spectroscopy is employed to investigate the electronic and geometric structural complexity in pristine and cobalt-doped Cu2ZnSnSe4. The occupational atomic sites of Cu are found to be mixed with the Zn atoms, forming CuZn anti-defects, which serve as a knob to tune local electronic structures. With proper doping, the band structure can be manipulated to improve the optical and thermoelectric properties of the CZTSe compounds.

Investigation of ultrafast dynamic behaviors can provide novel insights about the coupling mechanisms among multiple degrees of freedom, such as lattice, magnetism and electronic structure, in condensed matters. Here we investigate both the ferromagnetic (FM) and antiferromagnetic (AFM) ultrafast dynamics of a strongly correlated oxide system, GdBaCo2O5.5 thin film by time-resolved x-ray magnetic circular dichroism in reflectivity (XMCDR) and resonant magnetic x-ray diffraction (RMXD). A photo-induced AFM-FM transition characterized by an increase of the transient XMCDR (sensitive to FM order) beyond the unpumped value and a decay of RMXD (sensitive to AFM order) was observed. The photon-energy dependence of the transient XMCDR and reflectivity could be interpreted as a concomitant photo-induced spin-state transition (SST). The AFM-FM transition and SST couple with each other in the time domain, resulting in unusual dynamic behaviors of the magnetism.

We investigate the ultrafast lattice dynamics in ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ via time-resolved x-ray diffraction measurements using an x-ray free-electron laser at SPring-8 Angstrom Compact free-electron LAser (SACLA). The profile of the Bragg peaks substantially changes after strong photoexcitation, where the $c$ axis length of the lattice initially contracts, followed by expansion. These ultrafast lattice contractions and expansions significantly depend on the pump fluence, and it is suggested that a strong normal stress is applied along the $c$ axis. To obtain a phenomenological understanding of the observed lattice dynamics, we performed calculations based on a two-temperature model and an ultrafast thermoelasticity model. Our results showed that a steep gradient of the electron temperature induced by photoexcitation generates a blast force, which acts as a strong normal stress. We propose that a temporal stress can be applied by a strong optical pump, and this scheme can be another route for the application of uniaxial stress to superconductors.

Using a time-resolved magneto-optical Kerr effect (TR-MOKE) microscope, we observed ultrafast demagnetization of inverse-spinel-type NiCo2O4 (NCO) epitaxial thin films of the inverse spinel type ferrimagnet NCO with perpendicular magnetic anisotropy. This microscope uses a pump-probe method, where the sample is pumped at 1030 nm, and magnetic domain images are acquired via MOKE microscopy at 515 nm (the second harmonic). We successfully observed the dynamics of the magnetic domain of the NCO thin film via laser irradiation, and obtained a demagnetization time constant of approximately 0.4 ps. This time constant was significantly smaller than the large time constants reported for other half-metallic oxides. These results, combined with the results of our x-ray photoemission spectroscopy study, indicate that this NCO thin film is a ferrimagnetic metal whose electronic structure deviates from the theoretically predicted half-metallic one., 5 pages, 5 figures

The discoveries of intrinsic ferromagnetism in atomically-thin van der Waals crystals have opened up a new research field enabling fundamental studies on magnetism at two-dimensional (2D) limit as well as development of magnetic van der Waals heterostructures. To date, a variety of 2D ferromagnetism has been explored mainly by mechanically exfoliating 'originally ferromagnetic (FM)' van der Waals crystals, while bottom-up approach by thin film growth technique has demonstrated emergent 2D ferromagnetism in a variety of 'originally non-FM' van der Waals materials. Here we demonstrate that V5Se8 epitaxial thin films grown by molecular-beam epitaxy (MBE) exhibit emergent 2D ferromagnetism with intrinsic spin polarization of the V 3d electrons despite that the bulk counterpart is 'originally antiferromagnetic (AFM)'. Moreover, thickness-dependence measurements reveal that this newly-developed 2D ferromagnet could be classified as an itinerant 2D Heisenberg ferromagnet with weak magnetic anisotropy, broadening a lineup of 2D magnets to those potentially beneficial for future spintronics applications.

The strain effect from a substrate is an important experimental route to control electronic and magnetic properties in transition-metal oxide (TMO) thin films. Using hard x-ray photoemission spectroscopy, we investigate the strain dependence of the valence states in LaNiO$_{3}$ thin films, strongly correlated perovskite TMO, grown on four substrates: LaAlO$_{3}$, (LaAlO$_{3}$)$_{0.3}$(SrAl$_{0.5}$Ta$_{0.5}$O$_{3}$)$_{0.7}$, SrTiO$_{3}$, and DyScO$_{3}$. A Madelung potential analysis of core-level spectra suggests that the point-charge description is valid for the La ions while it breaks down for Ni and O ions due to a strong covalent bonding between the two. A clear x-ray photon-energy dependence of the valence spectra is analyzed by the density functional theory, which points to a presence of the La 5$p$ state near the Fermi level., 6 pages, 4 figures, and 2 tables

We investigate near-Fermi-energy (${E}_{F}$) element-specific electronic and spin states of ferromagnetic van der Waals (vdW) metal ${\mathrm{Fe}}_{5}\mathrm{Ge}{\mathrm{Te}}_{2}$. The soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) measurement provides spectroscopic evidence of localized Fe $3d$ band. We also find prominent hybridization between the localized Fe $3d$ band and the delocalized Ge/Te $p$ bands. This picture is strongly supported from direct observation of the remarkable spin polarization of the ligand $p$ bands near ${E}_{F}$, using x-ray magnetic circular dichroism (XMCD) measurements. The strength of XMCD signal from ligand element Te shows the highest value, as far as we recognize, among literature reporting finite XMCD signal for nonmagnetic element in any systems. Combining SX-ARPES and elemental selective XMCD measurements, we collectively point to an important role of giant spin polarization of the delocalized ligand Te states for realizing itinerant long-range ferromagnetism in ${\mathrm{Fe}}_{5}\mathrm{Ge}{\mathrm{Te}}_{2}$. Our finding provides a fundamental elemental selective viewpoint for understanding mechanism of itinerant ferromagnetism in low-dimensional compounds, which also leads to insight for designing exotic magnetic states by interfacial band engineering in heterostructures.

Magnetocrystalline anisotropy, a key ingredient for establishing long-range order in a magnetic material down to the two-dimensional (2D) limit, is generally associated with spin-orbit interaction (SOI) involving a finite orbital angular momentum. Here we report strong out-of-plane magnetic anisotropy without orbital angular momentum, emerging at the interface between two different van der Waals (vdW) materials, an archetypal metallic vdW material NbSe2 possessing Zeeman-type SOI and an isotropic vdW ferromagnet V5Se8. We found that the Zeeman SOI in NbSe2 induces robust out-of-plane magnetic anisotropy in V5Se8 down to the 2D limit with a more than 2-fold enhancement of the transition temperature. We propose a simple model that takes into account the energy gain in NbSe2 in contact with a ferromagnet, which naturally explains our observations. Our results demonstrate a conceptually new magnetic proximity effect at the vdW interface, expanding the horizons of emergent phenomena achievable in vdW heterostructures.

This work explores the mechanism of interfacial coupling in LaMnO${}_{3}$/SrIrO${}_{3}$ superlattices by x-ray spectroscopies and first-principles calculations. The superlattice-period dependent properties of the Ir magnetic moments can be attributed to the realignment of electron spin during the formation of the interfacial molecular orbital.

SmO thin film is a new Kondo system showing a resistivity upturn around 10 K and was theoretically proposed to have a topologically nontrivial band structure. We perform hard x-ray and soft x-ray photoemission spectroscopy to elucidate the electronic structure of SmO. From the Sm $3d$ core-level spectra, we estimate the valence of Sm to be $\ensuremath{\sim}2.96$, proving that the Sm has a mixed valence. The valence-band photoemission spectra exhibit a clear Fermi edge originating from the Sm $5d$-derived band. The present finding is consistent with the theory suggesting a possible topological state in SmO and show that rare-earth monoxides or their heterostructures can be a new playground for the interplay of strong electron correlation and spin-orbit coupling.

Perovskite rhodates are characterized by intermediate strengths of both electronic correlation as well as spin-orbit coupling (SOC) and usually behave as moderately correlated metals. A recent publication [Phys. Rev. B 95, 245121 (2017)] on epitaxial ${\mathrm{SrRhO}}_{3}$ thin films reported a bad-metallic behavior and suggested the occurrence of antiferromagnetism below 100 K. We have further studied this ${\mathrm{SrRhO}}_{3}$ thin film by hard x-ray photoemission spectroscopy and found a very small density of states (DOS) at the Fermi level (${E}_{F}$), which is consistent with the previously reported bad-metallic behavior. However, the absence of DOS persists up to room temperature, which contradicts the explanation of antiferromagnetic transition at $\ensuremath{\sim}100$ K. We also employed electronic structure calculations within the framework of density functional theory and dynamical mean-field theory. In contrast to the photoemission results, our calculations indicate metallic behaviors of both bulk ${\mathrm{SrRhO}}_{3}$ and the ${\mathrm{SrRhO}}_{3}$ thin film, and a stronger correlation effect was observed in the thin film than that in the bulk. The calculated uniform magnetic susceptibility is substantially larger in the thin film than that in the bulk. We also investigated the role of SOC and found only a moderate modulation of the band structure. Hence SOC is not expected to significantly affect the electronic correlation in ${\mathrm{SrRhO}}_{3}$. Extrinsic effects of finite-thickness effects beyond our calculations and localization effects may play important roles to induce the negligible spectral weight at ${E}_{F}$.

Recently, hole-doped superconducting cuprates with the T'-structure La1.8-xEu0.2SrxCuO4 (LESCO) have attracted a lot of attention. We have performed x-ray photoemission and absorption spectroscopy measurements on as-grown and reduced T0-LESCO. Results show that electrons and holes were doped by reduction annealing and Sr substitution, respectively. However, it is shown that the system remains on the electron-doped side of the Mott insulator or that the charge-transfer gap is collapsed in the parent compound., Comment: 2 pages, 2 figures

We have synthesized polycrystalline samples of Eu2Pt6(Al1−xGax)15 (x = 0, 0.05, 0.1, 0.2, 0.3, and 1) and examined electrical resistivity, magnetic susceptibility, high-field magnetization, and X-r...

We examined the photo-induced dynamics of ferromagnetic Co/Pt thin films demonstrating perpendicular magnetic anisotropy with element specificity using resonant polar magneto-optical Kerr effect measurements at Pt~N${}_{6,7}$ and Co~M${}_{2,3}$ edges with an x-ray free electron laser. The obtained results showed a clear element dependence of photo-induced demagnetization time scales: $\tau_\textrm{demag.}^\textrm{Co}=80\pm60~\textrm{fs}$ and $\tau_\textrm{demag.}^\textrm{Pt}=640\pm140~\textrm{fs}$. This dependence is explained by the induced moment of the Pt atom by current flow from the Co layer through the interfaces. The observed magnetization dynamics of Co and Pt can be attributed to the characteristics of photo-induced Co/Pt thin film phenomena including all-optical switching.

Unraveling the origin of ultrafast demagnetization in multisublattice ferromagnetic materials requires femtosecond x-ray techniques to trace the magnetic moment dynamics on individual elements, but this could not yet be achieved in the hard x-ray regime. We demonstrate here the first ultrafast demagnetization dynamics in the ferromagnetic heavy 5d-transition metal Pt using circularly-polarized hard x-rays at an x-ray free electron laser (XFEL). The decay time of laser-induced demagnetization of L10-FePt is determined to be τ Pt = 0.61 ± 0.04 ps using time-resolved x-ray magnetic circular dichroism at the Pt L3 edge, whereas magneto-optical Kerr measurements indicate the decay time for the total magnetization as τ total < 0.1 ps . A transient magnetic state with a photo-modulated ratio of the 3d and 5d magnetic moments is demonstrated for pump–probe delays larger than 1 ps. We explain this distinct photo-modulated transient magnetic state by the induced-moment behavior of the Pt atom and the x-ray probing depth. Our findings pave the way for the future use of XFELs to disentangle atomic spin dynamics contributions.

In order to realize superconductivity in cuprates with the ${T}^{\ensuremath{'}}$-type structure, not only chemical substitution (Ce doping) but also postgrowth reduction annealing is necessary. In the case of thin films, however, well-designed reduction annealing alone without Ce doping can induce superconductivity in the ${T}^{\ensuremath{'}}$-type cuprates. In order to unveil the origin of superconductivity in the Ce-undoped ${T}^{\ensuremath{'}}$-type cuprates, we have performed bulk-sensitive hard x-ray photoemission and soft x-ray absorption spectroscopy on superconducting and nonsuperconducting ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4}$ ($x=0$, 0.15, and 0.19) thin films. By postgrowth annealing, core-level spectra exhibited dramatic changes, which we attributed to the enhancement of core-hole screening in the ${\mathrm{CuO}}_{2}$ plane and the shift of chemical potential along with changes in the band filling. The result suggests that the superconducting ${\mathrm{Nd}}_{2}{\mathrm{CuO}}_{4}$ film is doped with electrons despite the absence of the Ce substitution.

The electronic-structural modulations of ${\mathrm{Ir}}_{1\ensuremath{-}x}{\mathrm{Pt}}_{x}{\mathrm{Te}}_{2}$ ($0\ensuremath{\leqq}x\ensuremath{\leqq}0.12$) have been examined by resonant elastic x-ray scattering (REXS) and resonant inelastic x-ray scattering (RIXS) techniques at both the Ir and Te edges. Charge-density-wave-like superstructures with wave vectors of $\mathbf{Q}=(1/5\phantom{\rule{4pt}{0ex}}0\phantom{\rule{4pt}{0ex}}\ensuremath{-}1/5)$, $(1/8\phantom{\rule{4pt}{0ex}}0\phantom{\rule{4pt}{0ex}}\ensuremath{-}1/8)$, and $(1/6\phantom{\rule{4pt}{0ex}}0\phantom{\rule{4pt}{0ex}}\ensuremath{-}1/6)$ are observed on the same sample of ${\mathrm{IrTe}}_{2}$ at the lowest temperature, the patterns of which are controlled by the cooling speeds. In contrast, superstructures around $\mathbf{Q}=(1/5\phantom{\rule{4pt}{0ex}}0\phantom{\rule{4pt}{0ex}}\ensuremath{-}1/5)$ are observed for doped samples ($0.02\ensuremath{\leqq}x\ensuremath{\leqq}0.05$). The superstructure reflections persist to higher Pt substitution than previously assumed, demonstrating that a charge-density wave (CDW) can coexist with superconductivity. The analysis of the energy-dependent REXS and RIXS line shape reveals the importance of the Te $5p$ state rather than the Ir $5d$ states in the formation of the spatial modulation of these systems. The phase diagram reexamined in this work suggests that the CDW incommensurability may correlate with the emergence of superconducting states such as ${\mathrm{Cu}}_{x}{\mathrm{TiSe}}_{2}$ and ${\mathrm{Li}}_{x}{\mathrm{TaS}}_{2}$.

The spin states of ${\mathrm{Co}}^{3+}$ ions in perovskite-type ${\mathrm{LaCoO}}_{3}$, governed by the complex interplay between the electron-lattice interactions and the strong electron correlations, still remain controversial due to the lack of experimental techniques which can directly detect them. In this Letter, we revealed the tensile-strain dependence of spin states, i.e., the ratio of the high- and low-spin states, in epitaxial thin films and a bulk crystal of ${\mathrm{LaCoO}}_{3}$ via resonant inelastic soft x-ray scattering. A tensile strain as small as 1.0% was found to realize different spin states from that in the bulk.

We investigate the thickness effects on charge and magnetic orderings in Fe perovskite oxide ${\mathrm{La}}_{1/3}{\mathrm{Sr}}_{2/3}{\mathrm{FeO}}_{3}/{\mathrm{SrTiO}}_{3}$ thin films by hard x-ray and resonant soft x-ray scattering (RSXS) with changing thin-film thickness systematically. We found that the correlation lengths of the magnetic ordering along the in-plane and out-of-plane directions are comparable and proportional to the thickness, and show stronger thickness dependence than those of charge ordering. The magnetic ordered states disappear when the correlation length of magnetic ordering decreases to that of charge ordering through the intrinsic thickness effects. Surface-sensitive grazing-incident RSXS revealed that the orderings exist even in the surface region, which indicates that the observed orderings are not affected by surface effects like oxygen vacancies. Critical thickness is in 5--15 nm, which corresponds to 4--11 antiferromagnetic ordering period. This critical value seems to be common to other ferromagnetic oxide thin films.

Given the parallelism between the physical properties of Ce and Yb based magnets and heavy fermions due to the electron-hole symmetry, it has been rather odd that the transition temperature of the Yb based compounds is normally very small, as low as $\sim$ 1 K or even lower, whereas Ce counterparts may often have the transition temperature well exceeding 10 K. Here, we report our experimental discovery of the transition temperature reaching 20 K for the first time in a Yb based compound at ambient pressure. The Mn substitution at the Al site in an intermediate valence state of $\alpha$-YbAlB$_{4}$ not only induces antiferromagnetic transition at a record high temperature of 20 K but also transforms the heavy fermion liquid state in $\alpha$-YbAlB$_{4}$ into a highly resistive metallic state proximate to a Kondo insulator., Comment: 7 pages, 3 figures

The electronic structure of ${\mathrm{BaFe}}_{2}{X}_{3}$ ($X$ = S and Se) and ${\mathrm{CsFe}}_{2}{\mathrm{Se}}_{3}$ in which two-leg ladders are formed by the Fe sites are studied by means of x-ray absorption and resonant inelastic x-ray scattering spectroscopy. The x-ray absorption spectra at the Fe $L$ edges for ${\mathrm{BaFe}}_{2}{X}_{3}$ exhibit two components, indicating that itinerant and localized Fe $3d$ sites coexist. Substantial x-ray linear dichroism is observed in polarization dependent spectra, indicating the existence of orbital order or fluctuation in the Fe ladder even above the N\'eel temperature ${T}_{\mathrm{N}}$. Direct exchange interaction along the legs of the Fe ladder stabilizes the orbital and antiferromagnetic orders in ${\mathrm{BaFe}}_{2}{\mathrm{S}}_{3}$, while the ferromagnetic molecular orbitals are realized between the rungs in ${\mathrm{CsFe}}_{2}{\mathrm{Se}}_{3}$.

Resonant soft X-ray scattering is a new experimental technique to study charge-, orbital-, and spin- ordered states in transition-metal oxides. I present recent research using this technique, which includes future prospects.

Photoemission spectroscopy is a powerful experimental technique to study the electronic structures of solids, especially of transition-metal oxides. Recently, hard X-ray photoemission spectroscopy (HXPES) has emerged as a more relevant experimental technique to obtain clear information about bulk states. Here, we describe how HXPES can be conveniently applied to study the interesting subjects on oxide thin films such as the composition dependence and the film thickness dependence of the electronic structures and the interfacial electronic structure of multilayers.

We investigated the electronic structure of layered Mn oxide Bi3Mn4O12(NO3) with a Mn honeycomb lattice by x-ray absorption spectroscopy and model calculations. The valence of Mn was determined to be 4 + with a small charge-transfer energy of ∼ 1 eV. The values of (J1, J2, J3, Jc, Jc1, and Jc2) obtained by unrestricted Hartree–Fock calculations on Mn 3d–O 2p lattice models show that intra-layer second and third neighbor superexchange interactions J2 and J3 as well as inter-layer superexchange interactions Jc, Jc1, and Jc2 are enhanced due to Mn–O–O–Mn pathways, which are activated by the smallness of charge-transfer energy. The present analysis indicates that the ferromagnetic Jc1 and antiferromagnetic Jc2 are responsible to the antiferromagnetic inter-layer coupling and that the intra-layer exchange interactions with the ferromagnetic J2 and antiferromagnetic J3 have no frustration effect.

We have studied the chemical potential shift and changes in the electronic density of states near the Fermi level (E F) as functions of temperature and carrier concentration in Pr1−x Ca x MnO3 (PCMO) and Nd1−x Sr x MnO3 (NSMO) by measurements of photoemission spectra. Suppression of the chemical potential shift as a function of carrier concentration has been observed in PCMO and NSMO near and in the composition range where the CE-type antiferromagnetic charge-ordered (CO) phase appears at low temperatures. This result indicates that there is charge self-organization on a microscopic scale such as stripe formation in this composition range. In the ferromagnetic metallic phase of NSMO, we found a large temperature-dependent chemical potential shift at low temperatures and attributed this to double-exchange mechanism. Suppression of the temperature-dependent chemical potential shift near Curie temperature was observed, possibly associated with the formation of correlated polarons. In the valence band near the E F of PCMO, spectral weight was transferred with hole doping, leading to a finite intensity at E F even in the paramagnetic insulating phase above the CO phase for x≳0.3, and as the temperature was lowered, a clear gap opening was observed in the CO phase.

We have performed an in situ photoemission spectroscopy (PES) of well-ordered surfaces of high-quality epitaxial La1−x Srx MnO3(LSMO) thin films to investigate the temperature-dependent changes in the electronic structure of LSMO in detail. For ferromagnetic metal LSMO with x = 0.4 , the spectral intensity of the e g states near the Fermi level ( E F ) significantly increases with decreasing temperature. This result suggests that the stabilization of ferromagnetic ordering causes the enhancement of spectral weight near E F .

We studied the electronic and magnetic dynamics of ferromagnetic insulating BaFeO3 thin films by using pump-probe time-resolved resonant x-ray reflectivity at the Fe 2p edge. By changing the excitation density, we found two distinctly different types of demagnetization with a clear threshold behavior. We assigned the demagnetization change from slow (~ 150 ps) to fast (< 70 ps) to a transition into a metallic state induced by laser excitation. These results provide a novel approach for locally tuning magnetic dynamics. In analogy to heat assisted magnetic recording, metallization can locally tune the susceptibility for magnetic manipulation, allowing to spatially encode magnetic information., 5 pages, 4 figures

Novel interplay of spin-orbit coupling and electron correlations in complex Ir oxides recently emerged as a new paradigm for correlated electron physics. Because of a large spin-orbit coupling of ~0.5 eV, which is comparable to the transfer energy t and the crystal field splitting $\Delta$ and Coulomb U, a variety of ground states including magnetic insulator, band insulator, semimetal and metal, shows up in a narrow materials phase space. Utilizing such subtle competition of the ground states, we successfully tailor a spin-orbital magnetic insulator out of a semimetal SrIrO$_3$ by controlling dimensionality using superlattice of [(SrIrO$_3$)$_m$, SrTiO$_3$] and show that a magnetic ordering triggers the transition to magnetic insulator. Those results can be described well by a first-principles calculation. This study is an important step towards the design and the realization of topological phases in complex Ir oxides with very strong spin-orbit coupling.

Using resonant soft-x-ray scattering as a function of both temperature and magnetic field, we reveal a large number of almost degenerate magnetic orders in SrCo6O11. The Ising-like spins in this frustrated material in fact exhibit a so-called magnetic devil's staircase. It is demonstrated how a magnetic field induces transitions between different microscopic spin configurations, which is responsible for the magnetoresistance of SrCo6O11. This material therefore constitutes a unique combination of a magnetic devil's staircase and spin-valve effects, yielding a novel type of magnetoresistance system.

We investigated the electronic and magnetic properties of fully oxidized ${\mathrm{BaFeO}}_{3}$ thin films, which show ferromagnetic-insulating properties with cubic crystal structure, by hard x-ray photoemission spectroscopy (HAXPES), x-ray absorption spectroscopy (XAS), and soft x-ray magnetic circular dichroism (XMCD). We analyzed the results with configuration-interaction (CI) cluster-model calculations for ${\mathrm{Fe}}^{4+}$, which showed good agreement with the experimental results. We also studied ${\mathrm{SrFeO}}_{3}$ thin films, which have an ${\mathrm{Fe}}^{4+}$ ion helical magnetism in cubic crystal structure, but are metallic at all temperatures. We found that ${\mathrm{BaFeO}}_{3}$ thin films are insulating with large magnetization ( 1.$7{\ensuremath{\mu}}_{B}$/formula unit) under $\ensuremath{\sim}1$ T, using valence-band HAXPES and Fe $2p$ XMCD, which is consistent with the previously reported resistivity and magnetization measurements. Although Fe $2p$ core-level HAXPES and Fe $2p$ XAS spectra of ${\mathrm{BaFeO}}_{3}$ and ${\mathrm{SrFeO}}_{3}$ thin films are quite similar, we compared the insulating ${\mathrm{BaFeO}}_{3}$ to metallic ${\mathrm{SrFeO}}_{3}$ thin films with valence-band HAXPES. The CI cluster-model analysis indicates that the ground state of ${\mathrm{BaFeO}}_{3}$ is dominated by ${d}^{5}\underline{L}\phantom{\rule{0.222222em}{0ex}}(\underline{L}$: ligand hole) configuration due to the negative charge transfer energy, and that the band gap has significant O $2p$ character. We revealed that the differences of the electronic and magnetic properties between ${\mathrm{BaFeO}}_{3}$ and ${\mathrm{SrFeO}}_{3}$ arise from the differences in their lattice constants, through affecting the strength of hybridization and bandwidth.

Nowadays it has become feasible to perform angle-resolved photoemission spectroscopy (ARPES) measurements of transition-metal oxides with three-dimensional perovskite structures owing to the availability of high-quality single crystals of bulk and epitaxial thin films. In this article, we review recent experimental results and interpretation of ARPES data using empirical tight-binding band-structure calculations. Results are presented for SrVO$_3$ (SVO) bulk single crystals, and La$_{1-x}$Sr$_x$FeO$_3$ (LSFO) and La$_{1-x}$Sr$_x$MnO$_3$ (LSMO) thin films. In the case of SVO, from comparison of the experimental results with calculated surface electronic structure, we concluded that the obtained band dispersions reflect the bulk electronic structure. The experimental band structures of LSFO and LSMO were analyzed assuming the G-type antiferromagnetic state and the ferromagnetic state, respectively. We also demonstrated that the intrinsic uncertainty of the electron momentum perpendicular to the crystal surface is important for the interpretation of the ARPES results of three-dimensional materials., Comment: 25 pages, 12 figures

We have performed in situ photoemission and O 1s X-ray absorption studies of La 1−x Sr x FeO 3 ( 0 ≤ x ≤ 0.67 ) epitaxial thin films grown by laser molecular beam epitaxy. We found that hole doping induces spectral weight transfer from below the Fermi level to above it across the gap in a highly non-rigid-band-like manner. We also observed the temperature dependence on the spectra of the x = 0.67 sample, which shows a charge disproportionation at 190 K. With decreasing temperature, we observed gradual changes of the spectra with spectral weight transfer primarily within the e g band.

We investigate the electronic structure of a perovskite-type Pauli paramagnet ${\mathrm{SrMoO}}_{3}$ (${{t}_{2g}}^{2}$) thin film using hard x-ray photoemission spectroscopy and compare the results to realistic calculations that combine density functional theory within the local-density approximation (LDA) with dynamical mean-field theory (DMFT). Despite the clear signature of electron correlations in the electronic specific heat, the narrowing of the quasiparticle bands is not observed in the photoemission spectrum. This is explained in terms of the characteristic effect of Hund's rule coupling for partially filled ${t}_{2g}$ bands, which induces strong quasiparticle renormalization already for values of Hubbard interaction which are smaller than the bandwidth. This interpretation is supported by DMFT model calculations including Hund's rule coupling, which show a renormalization of low-energy quasiparticles without affecting the overall bandwidth. The photoemission spectra show additional spectral weight around $\ensuremath{-}2.5$ eV that is not present in the LDA+DMFT results, pointing to a source of correlations that is not present in our calculations that include only on-site interactions. We interpret this weight as a plasmon satellite, which is supported by the measured core-level spectra that all show satellites at this energy.

We studied the charge-orbital ordering in the superlattice of charge-ordered insulating Pr0.5Ca0.5MnO3 and ferromagnetic metallic La0.5Sr0.5MnO3 by resonant soft x-ray diffraction (RSXD) and hard x-ray photoemission spectroscopy (HXPES). A temperature-dependent incommensurability is found in the orbital ordering by RSXD. In addition, a large hysteresis is observed that is caused by phase competition between the insulating charge ordered and metallic ferromagnetic states. No magnetic phase transitions are observed in contrast to pure Pr0.5Ca0.5MnO3 thin films, confirming the unique character of the superlattice. Mn $2p$ HXPES spectra revealed a hysteresis in the metalicity, supporting the picture of phase competition. The deviation from the commensurate orbital order can be directly related to the decrease of ordered-layer thickness that leads to dimensional crossover from three-dimensional to two-dimensional orbital ordering., New Journal of Physics, 16, ISSN:1367-2630

Strongly correlated electron systems often exhibit very strong interactions between structural and electronic degrees of freedom that lead to complex and interesting phase diagrams1 2. For technological applications of these materials it is important to learn how to drive transitions from one phase to another. A key question here is the ultimate speed of such phase transitions and to understand how a phase transition evolves in the time domain3 4 5 6 7 8 9 10 11 12 13. Here we apply time resolved X ray diffraction to directly measure the changes in long range order during ultrafast melting of the charge and orbitally ordered phase in a perovskite manganite. We find that although the actual change in crystal symmetry associated with this transition occurs over different timescales characteristic of the many electronic and vibrational coordinates of the system the dynamics of the phase transformation can be well described using a single time dependent ‘order parameter’ that depends exclusively on the electronic excitation.

We report magnetotransport properties for epitaxial thin films of SrFeO${}_{3}$ and SrFe${}_{0.99}$Co${}_{0.01}$O${}_{3}$ with possible skyrmion-related spin textures. Resonant soft x-ray diffraction measurements revealed formation of helical spin structures for both samples ($Q//\ensuremath{\langle}111\ensuremath{\rangle}$). From magnetotransport measurements we found several distinct helimagnetic phases with multiple/single $Q$ vectors. A steep suppression of Hall resistivity is observed above the critical field to reach the high-field conical state, indicating the presence of skyrmionlike topological spin textures at lower fields responsible for the topological Hall effect.

We have performed an in situ photoemission study of Pr 1 - x Ca x MnO 3 thin films grown on LaAlO 3 (0 0 1) by laser molecular beam epitaxy. By utilizing Mn 2 p → 3 d resonant photoemission, we obtained the Mn 3d partial density of states. There was no density of states at the Fermi level, consistent with the insulating behavior of this material. Chemical potential pinning, which was observed in bulk samples, was not observed in the present study, consistent with the suppression of charge ordering under the compressive strain from the substrate.

We have performed an in situ angle-resolved photoemission spectroscopy (ARPES) on single-crystal surfaces of La 0.6 Sr 0.4 MnO 3 (LSMO) thin films to investigate the electronic structure near the Fermi level ( E F ). ARPES spectra near E F reveal the existence of a dispersive Mn-3d derived conduction band indicative of an electron pocket centered at the Γ point. Comparing the experimental band structure with the local density approximation + U band-structure calculation, the estimated size of the Fermi surface is consistent with the prediction of the calculation, while the measured bandwidth is significantly narrower than the calculation. The discrepancy between the two suggests that the renormalization effect is significant due to the strong electron correlation in LSMO.

We have investigated the electronic structure of a La 0.6 Sr 0.4 FeO 3 (LSFO)/La 0.6 Sr 0.4 MnO 3 (LSMO) heterointerface using Mn 2p-3d resonant photoemission spectroscopy and X-ray absorption spectroscopy. We have found that the spectral intensity of e g↑ states near the Fermi level is gradually reduced with increasing LSFO overlayer thickness. The spectral behavior is quite similar to that of hole-doped LSMO films, indicating hole-doping in LSMO layers. We have also found that Fe 2p X-ray absorption spectrum of a LSMO/LSFO/LSMO trilayer shows the spectral evidence for electron doping in the corresponding LSFO layer. These results indicate the occurrence of charge transfer at interfaces between LSMO and LSFO layers.

The fundamental electronic structure of the widely used battery material ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2}$ still remains a mystery. Soft x-ray absorption spectroscopy of ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2}$ reveals that holes with strong O $2p$ character play an essential role in the electronic conductivity of the ${\mathrm{Co}}^{3+}/{\mathrm{Co}}^{4+}$ mixed valence ${\mathrm{CoO}}_{2}$ layer. The oxygen holes are bound to the ${\mathrm{Co}}^{4+}$ sites and the Li-ion vacancy, suggesting that the Li-ion flow can be stabilized by oxygen hole back flow. Such an oxygen hole state of ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2}$ is unique among the various oxide-based battery materials and is one of the key ingredients to improving their electronic and Li-ion conductivities.

We report the detailed electronic structure of a hole-doped delafossite oxide CuCr${}_{1\ensuremath{-}x}$Mg${}_{x}$O${}_{2}$ ($0\ensuremath{\le}x\ensuremath{\le}0.03$) studied by photoemission spectroscopy (PES), soft x-ray absorption spectroscopy (XAS), and band-structure calculations within the local-density approximation $+U$ ($\mathrm{LDA}+U$) scheme. Cr/Cu 3$p$-$3d$ resonant PES reveals that the near-Fermi-level leading structure has primarily the Cr $3d$ character with a minor contribution from the Cu $3d$ through Cu $3d\ensuremath{-}$O $2p\ensuremath{-}$Cr $3d$ hybridization, having good agreement with the band-structure calculations. This indicates that a doped hole will have primarily the Cr $3d$ character. Cr $2p$ PES and $L$-edge XAS spectra exhibit typical Cr${}^{3+}$ features for all $x$, while the Cu $L$-edge XAS spectra exhibited a systematic change with $x$. This indicates now that the Cu valence is monovalent at $x=0$ and the doped hole should have Cu $3d$ character. Nevertheless, we surprisingly observed two types of charge-transfer satellites that should be attributed to Cu${}^{+}$ ($3{d}^{10}$) and Cu${}^{2+}$ ($3{d}^{9}$) like initial states in Cu $2p$-$3d$ resonant PES spectrum of at $x=0$, while Cu $2p$ PES spectra with no doubt shows the Cu${}^{+}$ character even for the lightly doped samples. We propose that these contradictory results can be understood by introducing not only the Cu $4s$ state, but also finite Cu $3d,4s\ensuremath{-}$Cr $3d$ charge transfer via O $2p$ states in the ground-state electronic configuration.

We have investigated the Mn3d spectral function in La0.6Sr0.4FeO3(LSFO)∕La0.6Sr0.4MnO3(LSMO) heterointerfaces as well as of La1−xSrxMnO3 films using Mn2p−3d resonant photoemission spectroscopy. The strong enhancement of the Mn3d spectra at the Mn2p−3d threshold enables us to extract the Mn3d spectra of LSMO layers in the vicinity of the interface with the LSFO overlayers. We have found that the spectral intensity of eg↑ states near the Fermi level is gradually reduced with increasing LSFO overlayer thickness and is finally saturated at 5–7ML. The close similarity in reduction of the intensity of the eg↑ states between the LSFO∕LSMO interface and hole-doped LSMO films indicates the occurrence of charge transfer at the interface between the LSMO and LSFO layers.

We have investigated the thickness-dependent transport properties of La1/3Sr2/3FeO3 thin films grown on SrTiO3 (001) and (111) substrates. At a thickness of ∼40 nm, both films show a clear transition in resistivity associated with the characteristic charge disproportionation at approximately 190 K. The transition temperature of the charge disproportionation is nearly unchanged with decreasing film thickness down to a certain thickness of ∼13 nm for both orientations, while the change in resistivity gradually decreases. Below this thickness, the transition becomes unclear, strongly suggesting the suppression of the charge disproportionation at the critical thickness of ∼13 nm. Furthermore, there is no significant difference in the thickness dependence of La1/3Sr2/3FeO3 thin films between the (001) and (111) orientations. The negligible crystallographic-orientation dependence may reflect the isotropic nature for the domain of charge disproportionation states in La1/3Sr2/3FeO3.

Single crystals of SrFe1-xCoxO3 solid solution with high valence ions (Fe4+/Co4+) were synthesized by combining floating-zone and high-pressure oxygen-annealing techniques. As the Co content (x) is increased, the ground state changes from the helimagnetic state (x = 0.2), with a high Curie temperature ranging between 245 and 337 K. We found that, within the helimagnetic state (x