Achieving room-temperature ferromagnetism (RTFM) in diluted magnetic semiconductors (DMSs) has been a long-standing challenge, with doping transition metals (TM) into oxide semiconductors being one of the most common approaches. However, the underlying physical mechanisms remain poorly understood, particularly for Co-doped ZnO (Co:ZnO) films, which exhibit high Curie temperatures (Tc) above 300 K. A promising mechanism proposed for high-Tc ferromagnetism is the donor impurity band exchange model, in which donor electrons mediate the coupling between TM spins. Despite its theoretical significance, the nature of the donor band electrons has yet to be experimentally identified. In this work, we use polarization-dependent, bulk-sensitive hard x-ray photoemission spectroscopy (HAXPES) to investigate Co-doped ZnO epitaxial films. Our results reveal the presence of a weak electron donor band, crossing the Fermi level, and from a polarization dependence analysis, it is unambiguously identify it as having "s-character." This finding offers new insight into the ferromagnetic mechanism in Co-doped ZnO, where Zn1+4s1 states mediate the ferromagnetism, contributing to metallic-like transport and Co2+ spin ordering. These results not only elucidate the complementary role of dopant-host electronic states but also open avenues for designing novel room-temperature magnetic semiconductors, particularly in the context of 2D DMSs.

It is well-known that in elemental metals, the onsite Coulomb energy of transition-metal (TM) d-electrons, Udd, is significantly smaller than Uff of f-electron rare-earth (RE) metals. Consequently, Udd is often neglected in RE-TM intermetallic alloys. In spite of the low value of Udd compared to Uff, we quantify and clarify the important role of Udd in partially filled d-bands of RE-TM alloys. We investigate the electronic structure of a typical RE-TM ferrimagnetic series Gd6(Mn1-xMx)23 (M = Fe, Co; x = 0.0, 0.3), which shows promising magnetocaloric properties. Resonant photoemission and constant initial state spectroscopy is used to identify the Mn 3d, Fe 3d, and Co 3d partial density of states (PDOS) in the valence band. The photon energy-dependent spectral evolution allows us to separate out the lower Hubbard band and the two-hole correlation satellites in the Mn, Fe, and Co 3d PDOS. Using the Cini-Sawatzky method, we determine an average Udd = 2.1 +/- 0.4 eV, 2.2 +/- 0.4 eV, and 2.9 +/- 0.4 eV for the Mn 3d, Fe 3d, and Co 3d states, respectively. The relatively larger Udd for Co compared to Fe 3d states results in lower DOS for the coherent feature at the Fermi level (EF) and higher DOS in the lower Hubbard band away from EF in Gd6(Mn0.7Co0.3)23 compared to Gd6(Mn0.7Fe0.3)23. To understand the role of Coulomb correlations on the electronic structure and magnetic properties, ab initio electronic structure calculations using density functional theory with onsite Coulomb correlations (DFT+U) were carried out for the parent Gd6Mn23. The results show that the calculated Mn magnetic moments are consistent with experiments Mn = 0.75 eV, corresponding to Udd = 1.65 eV and Jdd = 0.9 eV. Further, using the calculated Gd and Mn PDOS and known photoionization cross-sections, the simulated Gd6Mn23 spectrum is fairly consistent with the experimental valence band spectrum. The results indicate the crucial role of d-d correlations in the presence of large f-f correlations for tuning the electronic structure and magnetic properties of RE-TM intermetallics.

The filled skutterudite PrRu4P12 shows a metal-insulator (MI) transition at 62.3 K which is yet to be understood. It is believed that the Pr 4 f electrons play an integral role in the MI transition however the order parameter associated with the Pr 4 f shells remains elusive. In this study, we investigate PrRu4P12 using temperature dependent resonant soft x-ray diffraction in combination with x-ray absorption spectroscopy at the Pr M-4,M-5 edges. A resonant enhancement of the (100) reflection at the Pr M-4,M-5 edges signaling the order parameter of the Pr 4 f shells was observed below T-MI, with a steady increase with decreasing temperature. The experimental spectra and subsequent analysis rule out the existence of magnetic, orbital and significant charge order, as well as any Pr lattice displacement. The ordered state below T-MI is likely the periodic electronic energy-level modulation of Pr 4 f shells. Our results indicate that a synergistic coupling of Pr 4 f states and lattice distortion plays an important role in the metal-insulator transition associated with charge density wave state formation.

AbstractMany large unit-cell rare-earth transition metal ternary alloys of the type Ra(M1−xM’x)b exhibit non-monotonic ferrimagnetic Curie temperatures (TC) coupled to monotonic composition-controlled magnetization. Its origin remains an important long-standing puzzle in the absence of studies probing their temperature-dependent element-specific magnetism. Here, in order to resolve this issue and identify design principles for new R-M-M’ permanent magnets, we carry out x-ray magnetic circular dichroism (XMCD) for the series Gd6(Mn1−xFex)23, x = 0.0 − 0.75. The results show that the net Mn-moment reduces and switches from parallel to antiparallel for x ≥ 0.2, while the Fe-moment is always antiparallel to the Gd-moment. Kouvel-Fisher analyses of XMCD data reveals distinct sublattice TC’s and 3D Heisenberg criticality. Band structure calculations show magnetic moments and density of states consistent with experiments. The magnetic phase diagram shows three regions characterized by (i) Mn-sublattice bulk-TC > Gd-sublattice TC, (ii) a reduced common-TC for all sublattices, and (iii) Fe-sublattice bulk-TC > Gd-sublattice TC. The Mn-moment switching and gradual increase of Fe-moment combine to cause non-monotonic TC’s with monotonic magnetization. The study indicates the importance of element-specific TC’s for tuning magnetic properties.

We study the high Curie temperature ferromagnet (${\mathrm{Cr}}_{0.35}{\mathrm{Sb}}_{0.65}{)}_{2}\mathrm{Te}{}_{3}$ (${T}_{C}=192\phantom{\rule{0.16em}{0ex}}\mathrm{K}$), using $T$-dependent x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and angle-resolved photoemission spectroscopy (ARPES). The $T$-dependent (25--220 K) XAS-XMCD evolution of $\mathrm{Cr}\phantom{\rule{0.16em}{0ex}}3d$ and $\mathrm{Te}\phantom{\rule{0.16em}{0ex}}5p$ unoccupied site- and orbital-projected states shows a systematic modification, which we interpret as due to spin-splitting below $T{}_{C}$. The $T$-dependent XMCD intensity and leading-edge spin-sensitive shifts ${\ensuremath{\gamma}}_{\text{expt}}(T)$ follow bulk magnetization. ARPES measurements with $h\ensuremath{\nu}=78\phantom{\rule{4pt}{0ex}}\mathrm{eV}$ show a metallic state with Sb $5p$ band dispersions at and near Fermi level (${E}_{F}$), consistent with bulk band-structure calculations for ${({\mathrm{Cr}}_{0.33}{\mathrm{Sb}}_{0.67})}_{2}\mathrm{Te}{}_{3}$. However, surface-sensitive ARPES with $h\ensuremath{\nu}=8.4\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ above and below $T{}_{C}$ show linear band dispersions just below ${E}_{F}$, suggesting a remnant of Dirac-type dispersions. Assuming the linear dispersion survives above ${E}_{F}$, it implies a topologically trivial ferromagnet as the estimated Dirac point energy lies above the largest ${\ensuremath{\gamma}}_{\text{expt}}\phantom{\rule{4pt}{0ex}}(T=25\phantom{\rule{0.16em}{0ex}}\mathrm{K})$. The $\mathrm{Cr}\phantom{\rule{0.16em}{0ex}}3d$ XAS-XMCD spectra can be simulated by charge transfer multiplet cluster model calculations with an exchange field ${H}_{\text{ex}}$ which quantitatively reproduces the experimental XMCD. At $T=25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, the required exchange field ${H}_{\text{ex}}$ of $\ensuremath{\sim}48\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ corresponds to a Zeeman energy $\ensuremath{\zeta}=2.8\phantom{\rule{0.16em}{0ex}}\mathrm{meV}\phantom{\rule{0.16em}{0ex}}<\phantom{\rule{4pt}{0ex}}{T}_{C}=192\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ (= 16.5 meV) $\ensuremath{\ll}{\ensuremath{\gamma}}_{\text{expt}}\ensuremath{\sim}140\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$. The results indicate the role of $\mathrm{Cr}\phantom{\rule{0.16em}{0ex}}3d$ exchange interactions in causing spin-sensitive shifts in $\mathrm{Cr}\phantom{\rule{0.16em}{0ex}}3d$ states, and inducing comparable spin-sensitive shifts via hybridization in $\mathrm{Te}\phantom{\rule{0.16em}{0ex}}5p$ states of ${({\mathrm{Cr}}_{0.35}{\mathrm{Sb}}_{0.65})}_{2}\mathrm{Te}{}_{3}$.

We study the local electronic structure of an equimolar-multimetal solid solution of high-entropy metal disulfide (Fe, Co, Ni, Cu)S2 and its parent compounds MS2 (M = Fe, Co, Ni, and Cu) using x-ray absorption spectroscopy (XAS). The Fe, Co, and Ni L2,3-edge absorption spectra indicate a divalent metal state both in (Fe, Co, Ni, Cu)S2 and its parent compounds, except for the Cu L2,3-edge absorption spectra. The Cu L2,3-edge spectra of CuS2 and (Fe, Co, Ni, Cu)S2 show satellites, which rule out the divalent Cu but can be analyzed as a combination of monovalent and trivalent copper states. The L2,3-edge XAS spectral analysis with charge-transfer multiplet cluster model calculations was carried out for (Fe, Co, Ni, Cu)S2 and its parent compounds. The estimated electronic parameters indicate a negative charge-transfer energy for the parent compounds and highentropy compound (with the Ni L edge in the high-entropy compound being an exception), which corresponds to a p-p type lowest energy excitation in the extended Zaanen-Sawatzky-Allen phase diagram. The analysis shows that the charge-transfer energy A decreases and the on-site Coulomb energy Udd increases systematically from Fe to Cu. The results suggest that in the high-entropy compound compared to the parent compounds, the hybridization strengths are weaker for Fe 3d-S 3p and Co 3d-S 3p and stronger for Ni 3d-S 3p and Cu 3d-S 3p bonds. This behavior is consistent with the longer Fe-S and Co-S bond distances and shorter Ni-S and Cu-S bond distances in the high-entropy compound compared to the parent compounds. The results indicate modifications in the structural lattice parameters of the high-entropy compound are reflected in the electronic structure and provides evidence for the so-called cocktail effect in the high-entropy compound.

It is well-known that in elemental metals, the onsite Coulomb energy of transition-metal (TM) $d$-electrons, ${U}_{dd}$, is significantly smaller than ${U}_{ff}$ of $f$-electron rare-earth (RE) metals. Consequently, ${U}_{dd}$ is often neglected in RE-TM intermetallic alloys. In spite of the low value of ${U}_{dd}$ compared to ${U}_{ff}$, we quantify and clarify the important role of ${U}_{dd}$ in partially filled $d$-bands of RE-TM alloys. We investigate the electronic structure of a typical RE-TM ferrimagnetic series ${\mathrm{Gd}}_{6}{({\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{M}}_{x})}_{23}$ (M = Fe, Co; $x=0.0,0.3$), which shows promising magnetocaloric properties. Resonant photoemission and constant initial state spectroscopy is used to identify the Mn $3d$, Fe $3d$, and Co $3d$ partial density of states (PDOS) in the valence band. The photon energy-dependent spectral evolution allows us to separate out the lower Hubbard band and the two-hole correlation satellites in the Mn, Fe, and Co $3d$ PDOS. Using the Cini-Sawatzky method, we determine an average ${U}_{dd}=2.1\ifmmode\pm\else\textpm\fi{}0.4\phantom{\rule{0.16em}{0ex}}\mathrm{eV}, 2.2\ifmmode\pm\else\textpm\fi{}0.4\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, and $2.9\ifmmode\pm\else\textpm\fi{}0.4\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ for the Mn $3d$, Fe $3d$, and Co $3d$ states, respectively. The relatively larger ${U}_{dd}$ for Co compared to Fe $3d$ states results in lower DOS for the coherent feature at the Fermi level (${E}_{F}$) and higher DOS in the lower Hubbard band away from ${E}_{F}$ in ${\mathrm{Gd}}_{6}{({\mathrm{Mn}}_{0.7}{\mathrm{Co}}_{0.3})}_{23}$ compared to ${\mathrm{Gd}}_{6}{({\mathrm{Mn}}_{0.7}{\mathrm{Fe}}_{0.3})}_{23}$. To understand the role of Coulomb correlations on the electronic structure and magnetic properties, ab initio electronic structure calculations using density functional theory with onsite Coulomb correlations ($\mathrm{DFT}+U$) were carried out for the parent ${\mathrm{Gd}}_{6}{\mathrm{Mn}}_{23}$. The results show that the calculated Mn magnetic moments are consistent with experiments when ${U}_{\text{Mn}}^{\text{DFT}}=0.75\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, corresponding to ${U}_{dd}=1.65\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ and ${J}_{dd}=0.9\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. Further, using the calculated Gd and Mn PDOS and known photoionization cross-sections, the simulated ${\mathrm{Gd}}_{6}{\mathrm{Mn}}_{23}$ spectrum is fairly consistent with the experimental valence band spectrum. The results indicate the crucial role of d-d correlations in the presence of large f-f correlations for tuning the electronic structure and magnetic properties of RE-TM intermetallics.

We study the high Curie temperature ferromagnet (Cr0.35Sb0.65)2Te 3 (TC=192 K ), using T -dependent x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and angle-resolved photoemission spectroscopy (ARPES). The T -dependent (25-220 K) XAS-XMCD evolution of Cr 3 d and Te 5 p unoccupied site- and orbital-projected states shows a systematic modification, which we interpret as due to spin-splitting below T C . The T -dependent XMCD intensity and leading-edge spin-sensitive shifts gammaexpt(T ) follow bulk magnetization. ARPES measurements with h nu =78 eV show a metallic state with Sb 5 p band dispersions at and near Fermi level (EF), consistent with bulk band-structure calculations for (Cr0.33Sb0.67)2Te 3 . However, surface-sensitive ARPES with h nu =8.4 eV above and below T C show linear band dispersions just below EF, suggesting a remnant of Dirac-type dispersions. Assuming the linear dispersion survives above EF, it implies a topologically trivial ferromagnet as the estimated Dirac point energy lies above the largest gammaexpt(T =25 K ) . The Cr 3 d XAS-XMCD spectra can be simulated by charge transfer multiplet cluster model calculations with an exchange field Hex which quantitatively reproduces the experimental XMCD. At T =25 K , the required exchange field Hex of similar to 48 T corresponds to a Zeeman energy zeta =2.8 meV C=192 K (= 16.5 meV) <exptsimilar to 140 meV . The results indicate the role of Cr 3 d exchange interactions in causing spin-sensitive shifts in Cr 3 d states, and inducing comparable spin-sensitive shifts via hybridization in Te 5 p states of (Cr0.35Sb0.65)2Te 3 .

Recent studies have identified novel magnetic properties involving pure spin-transport, spin-orbit torque, spin- texture, and spin-dynamics in rare earth (R)-transition metal (M) ferrimagnetic alloys. These properties make R-M ferrimagnets very suitable for spintronic applications. For a deeper understanding of their electronic and magnetic properties, it is necessary to clarify the role of element-specific on-site Coulomb correlations as well as ligand fi eld effects which determine magnetic moments. In this work, we present a brief review of recent progress in element-specific electron spectroscopy studies of R-M ferrimagnets. We focus on how bulk sensitive hard x-ray photoemission spectroscopy (HAXPES), soft x-ray resonant photoemission spectroscopy (RESPES), x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) can be used to study element specific electronic structure of R-M ferrimagnetic alloys. We discuss examples of applying these techniques to characterize the electronic structure of R-M alloys, including RM (R = Gd; M = Fe, Co, Ni), RM2 (= Dy, Ho; M = Co), R2Fe14B alloys (R = Nd, Y), and Gd6(Mn1-xMx)23 (M = Fe, Co). For the Gd6(Mn1-xMx)23 (M = Fe, Co) series, we discuss the relevance of quantifying electronic parameters using RESPES, and comparison of HAXPES valence band with density of states from electronic structure calculations. Simplified ligand fi eld cluster model calculations were carried out for metal L-edge XAS-XMCD spectra of Gd6(Mn1-xMx)23 (M = Fe, Co; x = 0.0, 0.3). The results show the important role of ligand fi eld calculations in combination with synchrotron based element-specific electron spectroscopy for understanding electronic structure of ferrimagnets, and can help in designing improved ferrimagnets for applications.

The one-band and three-band Hubbard models which describe the electronic structure of cuprates indicate very different values of effective electronic parameters, such as the on-site Coulomb energy and the hybridization strength. In contrast, a comparison of electronic parameters of several cuprates with corresponding values from spectroscopy and scattering experiments indicates similar values in the three-band model and cluster model calculations used to simulate experimental results. The Heisenberg exchange coupling $J$ obtained by a downfolding method in terms of the three band parameters is used to carry out an optimization analysis consistent with $J$ from neutron scattering experiments for a series of cuprates. In addition, the effective one-band parameters $\tilde{U}$ and $\tilde{t}$ are described using the three band parameters, thus revealing the hidden equivalence of the one-band and three-band models. The ground-state singlet weights obtained from an exact diagonalization elucidates the role of Zhang-Rice singlets in the equivalence. The results provide a consistent method to connect electronic parameters obtained from spectroscopy and the three-band model with values of $J$ obtained from scattering experiments, band dispersion measurements and the effective one-band Hubbard model.

We study the electronic structure of electron-doped Pr_1.3-xLa_0.7Ce_xCuO_4 (PLCCO ; T_c = 27 K, x = 0.1) and hole-doped Bi_2Sr_2CaCu_2O_8+δ (Bi2212 ; T_c = 90 K) cuprate superconductors using x-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (Res-PES). From Res-PES across the O K-edge and Cu L-edge, we identify the O 2p and Cu 3d partial density of states (PDOS) and their correlation satellites which originate in two-hole Auger final states. Using the Cini-Sawatzky method, analysis of the experimental O 2p PDOS shows an oxygen on-site Coulomb energy for PLCCO to be U_p = 3.3±0.5 eV and for Bi2212, U_p = 5.6±0.5 eV, while the copper on-site Coulomb correlation energy, U_d = 6.5±0.5 eV for Bi2212. The expression for the Heisenberg exchange interaction J in terms of the electronic parameters U_d, U_p, charge-transfer energy Δ and Cu-O hopping t_pd obtained from a simple Cu_2O cluster model is used to carry out an optimization analysis consistent with J known from scattering experiments. The analysis also provides the effective one band on-site Coulomb correlation energy Ũ and the effective hopping t̃. PLCCO and Bi2212 are shown to exhibit very similar values of Ũ/t̃ ∼9-10, confirming the strongly correlated nature of the singlet ground state in the effective one-band model for both the materials.

We study the electronic structure of electron-doped Pr$_{1.3-x}$La$_{0.7}$Ce$_{x}$CuO$_{4}$ (PLCCO ; $T_{c}$ = 27 K, x = 0.1) and hole-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (Bi2212 ; $T_{c}$ = 90 K) cuprate superconductors using x-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (Res-PES). From Res-PES across the O K-edge and Cu L-edge, we identify the O 2p and Cu 3d partial density of states (PDOS) and their correlation satellites which originate in two-hole Auger final states. Using the Cini-Sawatzky method, analysis of the experimental O 2p PDOS shows an oxygen on-site Coulomb energy for PLCCO to be $U_{p}$ = 3.3$\pm$0.5 eV and for Bi2212, $U_{p}$ = 5.6$\pm$0.5 eV, while the copper on-site Coulomb correlation energy, $U_{d}$ = 6.5$\pm$0.5 eV for Bi2212. The expression for the Heisenberg exchange interaction $J$ in terms of the electronic parameters $U_{d}$, $U_{p}$, charge-transfer energy $Δ$ and Cu-O hopping $t_{pd}$ obtained from a simple Cu$_2$O cluster model is used to carry out an optimization analysis consistent with $J$ known from scattering experiments. The analysis also provides the effective one band on-site Coulomb correlation energy $\tilde{U}$ and the effective hopping $\tilde{t}$. PLCCO and Bi2212 are shown to exhibit very similar values of $\tilde{U}$/$\tilde{t}$ $\sim$9-10, confirming the strongly correlated nature of the singlet ground state in the effective one-band model for both the materials.

We study the electronic structure of electron-doped Pr$_{1.3-x}$La$_{0.7}$Ce$_{x}$CuO$_{4}$ (PLCCO ; $T_{c}$ = 27 K, x = 0.1) and hole-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212 ; $T_{c}$ = 90 K) cuprate superconductors using x-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (Res-PES). From Res-PES across the O K-edge and Cu L-edge, we identify the O 2p and Cu 3d partial density of states (PDOS) and their correlation satellites which originate in two-hole Auger final states. Using the Cini-Sawatzky method, analysis of the experimental O 2p PDOS shows an oxygen on-site Coulomb energy for PLCCO to be $U_{p}$ = 3.3$\pm$0.5 eV and for Bi2212, $U_{p}$ = 5.6$\pm$0.5 eV, while the copper on-site Coulomb correlation energy, $U_{d}$ = 6.5$\pm$0.5 eV for Bi2212. The expression for the Heisenberg exchange interaction $J$ in terms of the electronic parameters $U_{d}$, $U_{p}$, charge-transfer energy $\Delta$ and Cu-O hopping $t_{pd}$ obtained from a simple Cu$_2$O cluster model is used to carry out an optimization analysis consistent with $J$ known from scattering experiments. The analysis also provides the effective one band on-site Coulomb correlation energy $\tilde{U}$ and the effective hopping $\tilde{t}$. PLCCO and Bi2212 are shown to exhibit very similar values of $\tilde{U}$/$\tilde{t}$ $\sim$9-10, confirming the strongly correlated nature of the singlet ground state in the effective one-band model for both the materials.

We study the electronic structure of electron-doped Pr1.3-xLa0.7CexCuO4 (PLCCO; Tc = 27 K, x = 0.1) and hole-doped Bi2Sr2CaCu2O8+s (Bi2212; Tc = 90 K) cuprate superconductors using x-ray absorption spectroscopy and resonant photoemission spectroscopy (Res-PES). From Res-PES across the O K-edge and Cu L-edge, we identify the O 2p and Cu 3d partial density of states (PDOS) and their correlation satellites, which originate in two-hole Auger final states. Using the Cini-Sawatzky method, analysis of the experimental O 2p PDOS shows an oxygen on-site Coulomb energy for PLCCO to be Up = 3.3 +/- 0.5 eV, and for Bi2212, Up = 5.6 +/- 0.5 eV, while the copper on-site Coulomb correlation energy is Ud = 6.5 +/- 0.5 eV for Bi2212. The expression for the Heisenberg exchange interaction J in terms of the electronic parameters Ud, Up, charge-transfer energy A, and Cu-O hopping tpd obtained from a simple Cu2O cluster model is used to carry out an optimization analysis consistent with J known from scattering experiments. The analysis also provides the effective one-band on-site Coulomb correlation energy U similar to and the effective hopping t similar to.PLCCO and Bi2212 are shown to exhibit very similar values of U similar to/t similar to similar to 9-10, confirming the strongly correlated nature of the singlet ground state in the effective one-band model for both materials.

We present a resonant inelastic x-ray scattering (RIXS) study across the temperature (T ) driven gamma -alpha transition in Ce0.93Sc0.07. RIXS measurements across the Ce M5 edge unambiguously identify the f 1 -> f0 and f 1 -> f2 charge excitations, which provide a quantification of the Ce on-site Coulomb repulsion energy, Uf f . Calculation with a simplified single-impurity Anderson model combined with full multiplet theory reproduces the charge excitations and establishes that the very different Kondo temperatures of the gamma to alpha phase are reflected in RIXS spectra. A systematic T-dependent hysteresis is observed for the f0 final state spectral intensity upon cycling across the gamma -alpha transition. In addition, a fluorescencelike structure also follows the same hysteretic behavior and shows it is directly connected to the weight of the f0 configuration in the ground state. The results indicate that the Ce M-edge RIXS is a reliable quantitative probe of the electronic structure of strongly correlated Ce-based Kondo systems and is sensitive to the emergent Kondo energy scale.

The possibility that valency changes due to the Kondo effect induce a charge-density-wave (CDW) transition and lead to zero-thermal-expansion by compensating the accompanying structural changes is appealing from both a fundamental and applied physics perspective. Theoretical studies have predicted CDW-order caused by the Kondo effect, whereby a material would exhibit a temperature-dependent dual Kondo effect comprising of two sublattices with different single-ion Kondo temperatures, but its experimental realization remains elusive. Here, we show direct evidence of a dual Kondo effect providing the electronic energy gain for a CDW accompanied by zero-thermal-expansion, in a strongly correlated f -electron material. YbPd undergoes a cubic to tetragonal transition with an incommensurate-CDW below T 1 = 130 K, which becomes commensurate below T 2 = 105 K. Bulk-sensitive spectroscopy reveals temperature-independent ytterbium single-site mixed-valence above T 1 , and a clear temperature-dependent mixed-valence charge-disproportionation of two crystallographic ytterbium sites in the CDW phases. Simplified single-impurity Anderson model calculations prove existence of a dual Kondo mixed-valency coupled to the CDW changes associated with the two ytterbium sites, and quantify site-dependent single-ion Kondo temperatures. The dual Kondo temperatures track the evolution of lattice parameters, resulting in a cell-volume compensated Kondo-CDW phase. The results provide a route to develop room temperature intermetallic zero-thermal-expansion materials.

We study the evolution of the electronic structure of the intermetallic series Gd-6(Mn1-xFex)(23), x = 0.0-0.75, which shows nonmonotonic ferrimagnetic ordering temperatures T-C but with a systematic reduction of the total bulk magnetization upon increasing Fe content, x. We have carried out hard x-ray photoemission spectroscopy to elucidate the relation between electronic structure and properties of the series. The Gd 3d and Gd 4d core-level spectra indicate trivalent Gd3+ multiplets in the intermediate-coupling scheme with features due to L-S and j-J coupling. The Fe 2p core levels show asymmetric single peak metal-like spectra, while the Mn 2p core levels show asymmetric doublet peaks. The relative intensities of the Mn 2p doublets as a function of x indicate occupancy changes of distinct crystallographic sites associated with Mn up-spin and down-spin states. The valence band spectra identify the Gd 4 f states at high binding energies (similar to 7.4 eV). The Mn 3d states occur at the Fermi level and as a broad feature between 2 and 5 eV binding energy in Gd6Mn23. Upon substitution, the Fe 3d states show up as small shifts to higher binding energies compared to Mn 3d states. The Fe 3s and Mn 3s spectra show exchange split peaks, allowing an estimate of the Mn and Fe magnetic moments using a Van Vleck analysis, which also provides a quantification of occupancy changes with x. The overall results are consistent with the bulk net magnetization, indicating that Mn up-spin sites become Fe down-spin sites on substitution, while the nonmonotonic T-C originates in a change from Mn sublattice to Fe sublattice derived ordering.

Resonant inelastic x-ray scattering (RIXS) studies across the Ce M-5 edge have been carried out to investigate the electronic structure of the ferromagnetic CeAgSb2 Kondo system. The RIXS spectra exhibit energy loss features corresponding to final states usually observed by combining photoemission and inverse photoemission spectroscopy. At low energy loss, a clear signature of the spectral features corresponding to the spin-orbit interaction is also observed. Polarization dependence provides evidence for the S-1(0) symmetry singlet ground state by a total suppression of the f(0) final state. A simplified single-impurity Anderson model combined with full multiplet theory allows an accurate description of the spin-charge excitations. The RIXS data also reveal a strong temperature T dependence of the fluorescencelike structure. We conjecture that this behavior reflects the T dependence of the Kondo resonance.

We present X-ray spectroscopic evidence for the evolution of valence-specific spin states and tetragonal distortions in single-layer cobaltates. Measurements of Co $L_3$-edge resonant inelastic X-ray scattering reveal the $t_{2g}$ electronic structure of Co for hole-doped La$_{2-x}$Sr$_x$CoO$_4$ ($x$ = 0.5, 0.7 and 0.8). As the Sr-doping $x$ increases, the tetragonal splitting of the $t_{2g}$ states of high-spin Co$^{2+}$ decreases, whereas that of low-spin Co$^{3+}$ increases and the fraction of high-spin Co$^{3+}$ increases. The results enable us to clarify the origin of the change of magnetic anisotropy and in-plane resistivity in a mixed-valence cobaltate caused by the interplay of spin-orbit coupling and tetragonal distortion.

Combination of low-dimensionality and electron correlation is vital for exotic quantum phenomena such as the Mott-insulating phase and high-temperature superconductivity. Transition-metal dichalcogenide (TMD) 1T-TaS2 has evoked great interest owing to its unique nonmagnetic Mott-insulator nature coupled with a charge-density-wave (CDW). To functionalize such a complex phase, it is essential to enhance the CDW-Mott transition temperature TCDW-Mott, whereas this was difficult for bulk TMDs with TCDW-Mott < 200 K. Here we report a strong-coupling 2D CDW-Mott phase with a transition temperature onset of ~530 K in monolayer 1T-TaSe2. Furthermore, the electron correlation derived lower Hubbard band survives under external perturbations such as carrier doping and photoexcitation, in contrast to the bulk counterpart. The enhanced Mott-Hubbard and CDW gaps for monolayer TaSe2 compared to NbSe2, originating in the lattice distortion assisted by strengthened correlations and disappearance of interlayer hopping, suggest stabilization of a likely nonmagnetic CDW-Mott insulator phase well above the room temperature. The present result lays the foundation for realizing monolayer CDW-Mott insulator based devices operating at room temperature.