Anderson Impurity Model Calculations Research Articles

Dynamical Screening of Local Spin Moments at Metal–Molecule Interfaces

Transition-metal phthalocyanine molecules have attracted considerable interest in the context of spintronics device development due to their amenability to diverse bonding regimes and their intrinsic magnetism. The latter is highly influenced by the quantum fluctuations that arise at the inevitable metal-molecule interface in a device architecture. In this study, we have systematically investigated the dynamical screening effects in phthalocyanine molecules hosting a series of transition-metal ions (Ti, V, Cr, Mn, Fe, Co, and Ni) in contact with the Cu(111) surface. Using comprehensive density functional theory plus Anderson's Impurity Model calculations, we show that the orbital-dependent hybridization and electron correlation together result in strong charge and spin fluctuations. While the instantaneous spin moments of the transition-metal ions are near atomic-like, we find that screening gives rise to considerable lowering or even quenching of these. Our results highlight the importance of quantum fluctuations in metal-contacted molecular devices, which may influence the results obtained from theoretical or experimental probes, depending on their possibly material-dependent characteristic sampling time-scales.

Spin-orbit coupling and Kondo resonance in the Co adatom on the Cu(100) surface: DFT plus exact diagonalization study

We report density functional theory plus exact diagonalization of the multiorbital Anderson impurity model calculations for the Co adatom on the top of a Cu(001) surface. For the Co atom $d$-shell occupation ${n}_{d}\ensuremath{\approx}8$, a singlet many-body ground state and Kondo resonance are found, when the spin-orbit coupling is included in the calculations. The differential conductance is evaluated in good agreement with the scanning tunneling microscopy measurements. The results illustrate the essential role which the spin-orbit coupling is playing in the formation of a Kondo singlet for the multiorbital impurity in low dimensions.

Valence fluctuations and Kondo resonance in Co adatom on Cu2N/Cu(100) surface: DFT + ED study

We report density functional theory plus exact diagonalization of the multi-orbital Anderson impurity model calculations including the spin–orbit coupling for the Co adatom on the top of Cu2N/Cu(001) surface. It is found that experimentally observed zero-bias peak in differential conductance can occur for quasi-degenerate many-body solution representing a mixed valence state of the Co adatom with the non-integer d manifold occupation of 7.4. Estimated 5 meV inelastic step energy is in a quantitative agreement with the experimental scanning tunnelling spectroscopy value of 6 meV. We find the non-zero spin and orbital magnetic moments in the applied magnetic field, in a reasonable agreement with the x-ray magnetic circular dichroism data. The presence of large orbital magnetic moment was not considered in previous spin-only theoretical interpretations, and can unravel complexity of the physics behind the experimental spectra for Co and other adatoms on Cu2N/Cu(001) surface.

Unusual coexistence of negative and positive charge transfer in mixed-valenceNaxCa1−xCr2O4

We have investigated the electronic structure of Na$_x$Ca$_{1-x}$Cr$_2$O$_4$ using x-ray absorption spectroscopy together with Anderson impurity model calculations with full multiplets. We show Na$_x$Ca$_{1-x}$Cr$_2$O$_4$ taking a novel mixed-valence electronic state in which the positive charge-transfer (CT) and the negative self-doped states coexist. While CaCr$_2$O$_4$ (one end member) exhibits a typical CT nature with strong covalent character, Na substitution causes a self-doped state with an oxygen hole. In NaCr$_2$O$_4$ (the other end member), positive CT and negative self-doped states coexist with equal weight. This unusual electronic state is in sharp contrast to the conventional mixed-valence description, in which the ground state can be described by the mixture of Cr$^{3+}$ ($3d^3$) and Cr$^{4+}$ ($3d^2$).

Adjacent Fe-Vacancy Interactions as the Origin of Room Temperature Ferromagnetism in(In1−xFex)2O3

Dilute magnetic semiconductors (DMSs) show great promise for applications in spin-based electronics, but in most cases continue to elude explanations of their magnetic behavior. Here, we combine quantitative x-ray spectroscopy and Anderson impurity model calculations to study ferromagnetic Fe-substituted In2O3 films, and we identify a subset of Fe atoms adjacent to oxygen vacancies in the crystal lattice which are responsible for the observed room temperature ferromagnetism. Using resonant inelastic x-ray scattering, we map out the near gap electronic structure and provide further support for this conclusion. Serving as a concrete verification of recent theoretical results and indirect experimental evidence, these results solidify the role of impurity-vacancy coupling in oxide-based DMSs.

Electronic band gap reduction and intense luminescence in Co and Mn ion-implanted SiO2

Cobalt and manganese ions are implanted into SiO2 over a wide range of concentrations. For low concentrations, the Co atoms occupy interstitial locations, coordinated with oxygen, while metallic Co clusters form at higher implantation concentrations. For all concentrations studied here, Mn ions remain in interstitial locations and do not cluster. Using resonant x-ray emission spectroscopy and Anderson impurity model calculations, we determine the strength of the covalent interaction between the interstitial ions and the SiO2 valence band, finding it comparable to Mn and Co monoxides. Further, we find an increasing reduction in the SiO2 electronic band gap for increasing implantation concentration, due primarily to the introduction of Mn- and Co-derived conduction band states. We also observe a strong increase in a band of x-ray stimulated luminescence at 2.75 eV after implantation, attributed to oxygen deficient centers formed during implantation.

Electronic Structure Changes across the Metamagnetic Transition in FeRh via Hard X-Ray Photoemission

Stoichiometric FeRh undergoes a temperature-induced antiferromagnetic (AFM) to ferromagnetic (FM) transition at ~350 K. In this Letter, changes in the electronic structure accompanying this transition are investigated in epitaxial FeRh thin films via bulk-sensitive valence-band and core-level hard x-ray photoelectron spectroscopy with a photon energy of 5.95 keV. Clear differences between the AFM and FM states are observed across the entire valence-band spectrum and these are well reproduced using density-functional theory. Changes in the 2p core levels of Fe are also observed and interpreted using Anderson impurity model calculations. These results indicate that significant electronic structure changes over the entire valence-band region are involved in this AFM-FM transition.

Valence fluctuations in thin films and theαandδphases of Pu metal determined by4fcore-level photoemission calculations

The average number of $5f$ electrons making up the valence state in plutonium metal together with the electronic fluctuations on each metal site has been a recent subject of debate. For the $\ensuremath{\delta}$ phase of Pu, where compared to the $\ensuremath{\alpha}$ phase increased localization (more atomiclike character) leads to decreased overlap and volume increase, an $f$ count close to either 5 or 6 has been proposed depending on the type of electronic structure calculation. In order to resolve the controversy, we analyze the $\text{Pu}\text{ }4f$ photoemission spectrum, which displays well screened and poorly screened peaks that can be used as a measure for the degree of localization. A simple analytical two-level model already shows on general grounds that the $f$ count for Pu must be between 5 and 5.5. Furthermore, we present detailed Anderson impurity model calculations including the full multiplet structure for $\text{Pu}\text{ }4f$ photoemission, which are compared to previous experimental results obtained from 1 to 9 monolayers thin films of Pu on Mg and from Pu metal in the $\ensuremath{\alpha}$ and $\ensuremath{\delta}$ phases. The trend in the satellite to main peak intensity ratio as a function of the Pu layer thickness gives a clear indication that Pu metal has an $5{f}^{5}$ like ground state. For the Pu allotropes and thicker films an $f$ count of 5.22 is obtained with a Coulomb interaction $U=4\text{ }\text{eV}$. The $5f$ fluctuations in Pu metal are very prominent and strongly material dependent. The calculations give a ground state with 9.6% ${f}^{4}$, 58.8% ${f}^{5}$, and 31.6% ${f}^{6}$ for the $\ensuremath{\alpha}$ phase and 5.7% ${f}^{4}$, 66.4% ${f}^{5}$, and 27.8% ${f}^{6}$ for the $\ensuremath{\delta}$ phase while for the thin films the amount of ${f}^{5}$ and the localization strongly increase with reduced thickness. The obtained findings are in agreement with recent electronic structure calculations for $\ensuremath{\delta}$ Pu using local-density approximation with dynamical mean-field theory and with the branching-ratio analysis of the $\text{Pu}\text{ }{N}_{4,5}$ edge in electron-energy-loss spectroscopy.

Understanding the2pcore-level spectra of manganese: Photoelectron spectroscopy experiments and Anderson impurity model calculations

Using high-resolution core-level photoelectron spectroscopy and modified Anderson impurity model calculations, we study the $\mathrm{Mn}\phantom{\rule{0.2em}{0ex}}2p$ spectrum of manganese metal and resolve the current debate about its spectral shape. An unusual satellite feature, $1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ from the main peak, is observed in the $\mathrm{Mn}\phantom{\rule{0.2em}{0ex}}2{p}_{3∕2}$ spectrum of a thick Mn layer grown on Al. It originates from intra-atomic multiplet effect related to Mn atoms with large local moment. The satellite decreases in intensity for thin Mn layers and for Al deposition on bulklike Mn because of enhanced $\mathrm{Mn}\phantom{\rule{0.2em}{0ex}}3d$ hybridization with Al $s,p$ bandlike states. The reason for the absence of a charge-transfer satellite is discussed.

Intra- versus inter-site electronic excitations in NdNiO 3 by resonant inelastic ultra-soft X-ray scattering at Ni 3p edge

Resonant ultra-soft X-ray scattering measurements performed on NiO, as a model system, and NdNiO 3 reveal the presence of pronounced inelastic structures within a few eV below the elastic peak in both cases. A comparison of experimental data with the results of Anderson impurity model calculations indicates that for both oxides the inelastic part of recorded spectra mainly originates from the intra-atomic d–d excitations. Remarkably, no distinct signal from inter-site d–d transitions can be identified for both metallic and insulating NdNiO 3 at small energy-losses. The lack of significant spectral contribution from the latter transitions in this compound indicates that states on the top (bottom) of the valence (conduction) band have predominantly O 2p character and the NdNiO 3 insulator is rather of a p–p type.

Resonant inelastic X-ray scattering as a probe of optical scale excitations in strongly electron-correlated systems: quasi-localized view

Abstract An application of resonant inelastic X-ray scattering technique for studying optical scale excitations in electron-correlated materials is discussed. Examples are given including data obtained for 3d transition metal, lanthanide, and actinide systems. In some cases, the data are compared with the results of crystal-field multiplet and Anderson impurity model calculations. Advantages of this technique are pointed out, such as an ability to probe an extended multiplet structure of the ground state configuration, which is not fully accessible by other spectroscopies, an extreme sensitivity of spectral profiles to the chemical state of the element in question and to the crystal-field strength, and a great potential in probing the ground state character (for example, ground state J-mixing in rare-earths) due to the technique’s elemental selectivity and strict selection rules. Issues are addressed, such as a possible deviation from the linear dispersion of inelastic scattering structures, corresponding to charge-transfer excitations, with varying excitation energies and an estimation of values for model parameters, involved in the description of charge-transfer processes.

Temperature dependence of the Kondo resonance in YbAl3.

We report the observation of a strong temperature dependence of the Kondo resonance in the valence-band photoemission spectrum and of the 3d\ensuremath{\rightarrow}4f white line in the x-ray absorption spectrum of ${\mathrm{YbAl}}_{3}$. The asymmetric Kondo resonance has a binding energy of \ensuremath{\sim}35 meV with a \ensuremath{\sim}50 meV width. The temperature dependence of the Yb 4f spectral features and valence are well reproduced by Anderson impurity model calculations with a Kondo temperature of ${\mathit{T}}_{\mathit{K}}$\ensuremath{\approxeq}400 K, demonstrating that both spectroscopic and near ground state properties can be described within a unified theoretical framework.

Soft-x-ray emission spectroscopy study of La2CuO4 and three high-Tc superconductors.

High-resolution x-ray emission spectroscopy measurements have been performed on sintered samples of ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$, ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$, ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (\ensuremath{\delta}0.1), and compressed polycrystalline ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$. The Cu L\ensuremath{\alpha} and O K\ensuremath{\alpha} emission bands, which reflect the local partial Cu 3d and O 2p states, respectively, are compared to photoelectron spectra and ground-state local-density-approximation (LDA) densities-of-states (DOS) calculations. Our results show that solid-state effects are dominant. In particular, the differences in shape of the O K\ensuremath{\alpha} emission bands follow the trends predicted by LDA, although it must be stressed that they mainly arise from O 2p states at oxygen sites not directly involved in the Cu-O planes. The Cu-side 3d states, on the other hand, vary little from one cuprate to the next and are more strongly localized than expected from LDA predictions. The Anderson-impurity model calculations of Kotani and co-workers are discussed, and it is concluded that they provide a satisfactory explanation of the Cu L\ensuremath{\alpha} emission data, which reflect strong on-site Coulomb interaction. We conclude that the apparently good agreement between LDA-calculated DOS and low-energy photoemission experiments recently reported results from the high sensitivity of the latter to O 2p states. Discrepancies are to be expected in excitation spectra that involve mainly 3d electrons.