Unexpectedly Large Electron Correlation Measured in Auger Spectra of Ferromagnetic Iron Thin Films: Orbital-Selected Coulomb and Exchange Contributions

A set of electron-correlation energies as large as 10 eV have been measured for a magnetic 2 ML Fe film deposited on Ag(001). By exploiting the spin selectivity in angle-resolved Auger-photoelectron coincidence spectroscopy and the Cini-Sawatzky theory, the core-valence-valence Auger spectrum of a spin-polarized system have been resolved: correlation energies have been determined for each individual combination of the two holes created in the four subbands involved in the decay: majority and minority spin, as well as e_{g} and t_{2g}. The energy difference between final states with parallel and antiparallel spin of the two emitted electrons is ascribed to the spin-flip energy for the final ion state, thus disentangling the contributions of Coulomb and exchange interactions.

Microscopic origin of electron accumulation in In2O3

Angle-resolved photoemission spectroscopy reveals the presence of a two-dimensional electron gas at the surface of In(2)O(3)(111). Quantized subband states arise within a confining potential well associated with surface electron accumulation. Coupled Poisson-Schrödinger calculations suggest that downward band bending for the conduction band must be much bigger than band bending in the valence band. Surface oxygen vacancies acting as doubly ionized shallow donors are shown to provide the free electrons within this accumulation layer. Identification of the origin of electron accumulation in transparent conducting oxides has significant implications in the realization of devices based on these compounds.

Spin-dependent on-site electron correlations and localization in itinerant ferromagnets

Spin selectivity in angle-resolved Auger photoelectron coincidence spectroscopy (AR-APECS) is used to probe electron correlation in ferromagnetic thin films. In particular, exploiting the AR-APECS capability to discriminate Auger electron emission events characterized by valence hole pairs created either in the high or in the low total spin state, a strong correlation effect in the Fe M(2,3)VV Auger line shape (measured in coincidence with the Fe 3p photoelectrons) of Fe/Cu(001) thin films is detected and ascribed to interactions within the majority spin subband. Such an assignment follows from a close comparison of the experimental AR-APECS line shapes with the predictions of a model based on spin polarized density functional theory and the Cini-Sawatzky approach.

Insight on hole-hole interaction and magnetic order from dichroic auger-photoelectron coincidence spectra

The absence of sharp structures in the Auger line shapes of partially filled bands has severely limited the use of electron spectroscopy in magnetic crystals and other correlated materials. By a novel interplay of experimental and theoretical techniques we achieve a combined understanding of the photoelectron, Auger, and Auger-photoelectron coincidence spectra (APECS) of the antiferromagnetic CoO. A recently discovered dichroic effect in angle resolved (DEAR) APECS reveals a complex pattern in the Auger line shape, which is here explained in detail, labeling the final states by their total spin. Since the dichroic effect exists in the antiferromagnetic state but vanishes at the Néel temperature, the DEAR-APECS technique detects the phase transition from its local effects, thus providing a unique tool to observe and understand magnetic correlations where the usual methods are not applicable.

The attenuation length of low energy electrons in Yb

Photoelectron emission spectra in a photon energy range between 7.5 and 21 eV are measured for in situ grown polycrystalline Yb films. By comparing bulk and surface core level shifted 4f components we give an estimation of the effective attenuation length (EAL) for low energy (6-20 eV) electrons in Yb, establishing a moderate increase of the EAL upon electron energy decrease. The experimental EAL data are found to be a factor of four smaller than those predicted from the so-called 'universal curve'.

Coherent peaks and minimal probing depth in photoemission spectroscopy of Mott-Hubbard systems

We have measured hard x-ray photoemission spectra of pure vanadium sesquioxide (V(2)O(3)) across its metal-insulator transition. We show that, in the metallic phase, a clear correlation exists between the shakedown satellites observed in the vanadium 2p and 3p core-level spectra and the coherent peak measured at the Fermi level. Comparing experimental results and dynamical mean-field theory calculations, we estimate the Hubbard energy U in V(2)O(3) (4.20+/-0.05 eV). From our bulk-sensitive photoemission spectra we infer the existence of a critical probing depth for investigating electronic properties in strongly correlated solids.

Tuning the magnetic coupling across ultrathin antiferromagnetic films by controlling atomic-scale roughness

Characterization and control of the interface structure and morphology at the atomic level is an important issue in understanding the magnetic interaction between an antiferromagnetic material and an adjacent ferromagnet in detail, because the atomic spins in an antiferromagnet change direction on the length scale of nearest atomic distances. Despite its technological importance for the development of advanced magnetic data-storage devices and extensive studies, the details of the magnetic interface coupling between antiferromagnets and ferromagnets have remained concealed. Here we present the results of magneto-optical Kerr-effect measurements and layer-resolved spectro-microscopic magnetic domain imaging of single-crystalline ferromagnet-antiferromagnet- ferromagnet trilayers. Atomic-level control of the interface morphology is achieved by systematically varying the thicknesses of the bottom ferromagnetic and the antiferromagnetic layer. We find that the magnetic coupling across the interface is mediated by step edges of single-atom height, whereas atomically flat areas do not contribute.