Heusler-type ternary and pseudobinary compounds Y2ZX ( Y and Z denote transition metal atoms and X is a metalloid) in the L21 or DO3 structure, which exhibit remarkable “site-selectivity” properties, have received renewed interest owing to their ferromagnetic shape memory, half-metallic electronic structure and thermoelectric properties etc. Among them, Fe3-xVxSi (0 ≤ x ≤ 1) alloys show some remarkable electronic and magnetic property. It is known that magnetic structure of Fe3-xVxSi changes from ferromagnetic to antiferromagnetic ones with increasing the content of nonferromagnetic V. It is also reported that Fe3-xVxSi compounds show anomalous resistivity properties 1) and negative Seebeck coefficient (thermoelectric power) at larger x 2). In order to clarify the origin of these novel properties of Fe3-xVxSi alloys in terms of the electronic structures, the valence-band as well as core-level photoelectron spectroscopy has been performed at BL-23SU of SPring-8 with soft X-ray and at BL-29XU of SPring-8 with hard X-ray. In this study it is to be clarified that the main structures of 1s, 2p core level spectra of Si, Fe and V shift to the higher binding energy with the addition of the V concentration due to the chemical potential shift; as for the spectra of valence band, with the increase of V concentration, the main structures shift to higher binding energy, which should be the evidences for the movement of Fermi level to higher energy; the high resolution X-ray photoemission spectra also show that the intensity around Fermi level decreases with adding V, and a new sharp structure formed for x = 1, which indicate the formation of pseudo-gap at large x. The comparisons of experimental valence band spectra with those of the total DOS calculated with KKR-CPA method 1) are also done in this study. *This work was done under the approval of the SPring-8 Proposal Assessing Committee (Proposal No. 2005B3811）and was also supported by the Ministry of Education, Culture, Sports, Sciences and Technology of Japan. 1) Y. Nishino, et al, Phys. Rev. B 48, (1993)13607. 2) O. Nashima, et al, J. Alloys Compd., 383, (2004)298. 3) A. Bansil, et al, Phys. Rev. B 60, (1999)13396.

from x = 0 to 1 is consistent with the corresponding theoretical value for the FeII and Si 1s core levels, whereas for the FeI and V core levels the computed shifts are generally larger than the experimental values. We ascribe these discrepancies to the effects of the core-hole screening in the final state which are not accounted for in the computations. In a rigid-band model the chemical potential and the core-level binding energies are expected to decrease with V doping as electrons are depleted from the Fermi energy. The observed increase in the binding energy of core levels thus supports a picture of the electronic structure where V doping induces a “pseudogap” or a region of reduced density of states in the vicinity of the Fermi energy.

The electronic structures of the Heusler type compounds Fe$_{3-x}V$_x$Si in the concentration range between x = 0 and x = 1 have been probed by photoemission spectroscopy (PES). The observed shift of Si 2p core- level and the main valence band structres indicate a chemical potential shift to higher energy with increasing x. It is also clarified that the density of state at Fermi edge is owing to the collaboration of V 3d and Fe 3d derived states. Besides the decrease of the spectral intensity near Fermi edge with increasing x suggests the formation of pseudo gap at large x.

Core-level photoemission spectra of Fe3-xVxSi alloys with inequivalent Fe-I and Fe-II sites are investigated via hard x-ray photoemission spectroscopy over the entire doping range x=0-1. All the measured 1s core-level peaks are found to shift to higher binding energy with increasing V concentration. First-principles, all electron charge- and spin-self-consistent electronic structure computations within the framework of the local-spin-density approximation are used to interpret the experimental results. The measured size of energy shift in going from x=0 to 1 is consistent with the corresponding theoretical value for the Fe-II and Si 1s core levels, whereas for the Fe-I and V core levels the computed shifts are generally larger than the experimental values. We ascribe these discrepancies to the effects of the core-hole screening in the final state which are not accounted for in the computations. In a rigid-band model the chemical potential and the core-level binding energies are expected to decrease with V doping as electrons are depleted from the Fermi energy. The observed increase in the binding energy of core levels thus supports a picture of the electronic structure where V doping induces a "pseudogap" or a region of reduced density of states in the vicinity of the Fermi energy.

The interaction of cobalt atoms with silicon (1 1 1) surface has been investigated by means of scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Besides the Co silicide islands, we have successfully distinguished two inequivalent Co-induced 13×13 reconstructions on Si(1 1 1) surface. Our high-resolution STM images provide some structural properties of the two different 13×13 derived phases. Both of the two phases seem to form islands with single domain. The new findings will help us to understand the early stage of Co silicide formations.

Scanning tunneling microscopy and scanning tunneling spectroscopy (STS) have been examined for an Al nanocluster periodic array grown on $p$-type silicon substrate. Local density of states at Al and Si sites within the nanocluster has been extracted from site-resolved STS spectra by taking into account tip-induced band bending (TIBB) effect. Besides, it has been clarified that the surface-potential-energy shift caused by TIBB effect directly influences the tunneling current spectra. Consequently, a good correspondence has been found between the experimental spectra and the theoretical local density of states except for the energy gap. The energy gap has been experimentally determined as 1.7 eV, which is much larger than the predicted value. This discrepancy could be ascribed to the local-density approximation rather than the TIBB effect.

We have observed an intermediate surface structure between two different surface phases of Al/Si (111) using the scanning tunneling microscope. The intermediate structure between the Al nanocluster and the gamma phase shows a large cluster with the size of a half-unit cell. The structural model of the gamma-phase Al/Si (111) has been reasonably proposed through the intermediate Al cluster.

We have deduced a local magnetic susceptibility from the temperature dependence of Mn L-23 soft x-ray magnetic circular dichroism spectra of c(2x2) CuMn/Cu(001) two-dimensional ordered surface alloy. The higher temperature magnetic susceptibility above similar to 40 K obeys the Curie-Weiss law, suggesting that the surface alloy is in a paramagnetic phase. The deduced spin magnetic moment from the temperature dependence of the local magnetic susceptibility is found to be highly suppressed from the predicted value.

We have done scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS) as well as current imaging tunneling spectroscopy (CITS) of an Al nanocluster periodic array of a submonolayer-Al/Si(111) system. To understand the formation mechanism of this system in detail, a phase diagram of the Al/Si(111) has been carefully produced. Among several structural phases, the nanocluster phase appears for the deposition thickness of 0.24-0.5 ML at a substrate temperature of 550 degrees C. It has been confirmed that these growth conditions should be carefully chosen to produce the well-defined nanocluster. The site-resolved STS spectra of Al nanocluster at 78 K show an insulating energy gap of 3.3 eV. In the CITS images, we have found different features of space-resolved tunneling current for the filled and empty states, which markedly depend on atomic sites and also on inequivalent half-unit cells.

The electronic structures of the Hensler type compounds Fe3-xVxSi in the concentration range between x = 0 and x = 1 have been probed by photoemission spectroscopy (PES). The observed shift of Si 2p core-level and the main valence band structres indicates a chemical potential shift to higher energy with increasing x. It is also clarified that the density of state at Fermi edge is owing to the collaboration of V 3d and Fe 3d derived states. Besides the decrease of the spectral intensity near Fermi edge with increasing x suggests the formation of pseudo gap at large x. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We have applied scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS) as well as current imaging tunneling spectroscopy (CITS) [1] to an Al nanocluster periodic array of a submonolayer-Al/Si(111) system. These clusters become visible with an Al deposition thickness of 0.24–0.5 ML at a substrate temperature of 550 °C. The site-resolved STS spectra of the Al nanocluster at 78 K show an insulating energy gap of 3.4 eV. In the CITS images, we have found different features of space-resolved tunneling current for the filled and empty states, which markedly depend on atomic sites and also on inequivalent half unit cells.

We have tried to clarify the magnetic property of a c(2×2) CuMn two-dimensional ordered surface alloy, which has been probed by Mn L23 X-ray magnetic circular dichroism (XMCD) spectra using a helicity modulation technique for the incident circularly polarized synchrotron radiation at BL 25 SU of SPring-8. In this work, a local magnetic susceptibility has been firstly applied to this surface alloy system, which has been deduced from the temperature dependence of the XMCD spectra. It is found that the local magnetic susceptibility above 40 K obeys the Curie-Weiss law. The evaluated positive value of the Weiss temperature Θ = 20 K clearly suggests the ferromagnetic interaction between the Mn atoms, although the expected Curie temperature is much reduced compared to the predicted value.

X-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) have been used to study self-assembled Co nanoclusters on Si(111)-7 x 7 substrate. The multiplet fine structures of the XAS spectrum indicate that there is a contribution of Co3+ atoms in the cluster due to the Co 3d-Al 3p hybridization at the Co-Al interface. The very small and positive L-2 peak compared to the negative peak at the L-3 edge appearing in the XMCD spectrum indicates a localized atomic-like character of the 3d electrons. Estimation based on the sum rules gives a very large value of m(L)/(m(S) + 7m(T)) = 0.37 for the Co nanoclusters.

Co and Mn 3d electronic states of Co2MnGe have been studied by 2p core absorption (XAS) and X-ray magnetic circular dichroism (XMCD). The evaluated Co and Mn 3d spin magnetic moments are consistent with the value by the band structure calculation. We have found that the orbital magnetic moment of the Co 3d and even the Mn 3d states are recognizable, which suggests that a spin–orbit coupling should be reconsidered in the energy band structure in order to verify a half-metallic nature of this alloy.

We have observed the site-resolved (Co, Mn and Ge) magnetic moments of Co2MnGe with the use of 2p core absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). The evaluated Co and Mn 3d spin magnetic moments are consistent with the values given by the neutron scattering experiment. We have found that the orbital magnetic moment is large for the Co 3d states and is recognizable even for the Mn 3d state, suggesting that a spin-orbit coupling should be reconsidered in the energy band structure in order to verify a half-metallic nature of this alloy. Moreover, we have revealed that the magnetic moment is induced on the Ge site in Co2MnGe.

We have performed x-ray magnetic circular dichroism experiments to study the cancellation of spin and orbital magnetic moments in $(\mathrm{Sm},\mathrm{Gd}){\mathrm{Al}}_{2}$, a ferromagnet without net magnetization at a certain compensation temperature, ${T}_{\mathit{comp}}$. We verified the existence of long-range order for both spin and orbital magnetic moments at ${T}_{\mathit{comp}}$. The spin and orbital magnetic moments of the Sm ion are found always antiparallel coupled and the magnitude of its orbital magnetic moment is always larger than that of spin one, so the cancellation of magnetic moments cannot be achieved by only Sm $4f$ electrons. We show that the addition of spin magnetic moments of Gd ions and conduction electrons, which are ferromagnetically coupled with the spin magnetic moment of Sm ions, cancels out the surplus orbital magnetic moments in Sm ions completely and results in the zero magnetization at ${T}_{\mathit{comp}}$. All our experimental results can be reproduced well by atomic multiplet calculations.

Mn nanoclusters have been prepared on a Si(1 1 1)-7×7 surface by substrate-induced spontaneous clustering due to delicate control of growth kinetics. An ex situ soft X-ray absorption spectroscopy (XAS) using synchrotron radiation has been used to clarify electronic structures of the nanoclusters. The small value of XAS L3–L2 branching ratio and spectrum line-shape for the Mn clusters show that the Mn3+ and Mn4+ electronic configuration should be taken into account.