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Doping ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ with Ti stabilizes the corundum paramagnetic metallic phase and restricts the transition to the monoclinic antiferromagnetic insulating phase at lower temperatures. In this study, we investigated the dependence of phase stability on Ti concentration for ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ nanocrystals as small as 20 nm. Ti doping of several percent induces a high-temperature insulating phase preserving the magnetic transition without the crystal symmetry change. The emergence of the insulating phases, which are peculiar to this crystal size, can be attributed to the enhancement of the on-site Coulomb interaction $U$ and increased mixing between ${a}_{1g}$ and ${e}_{g}^{\ensuremath{\pi}}$ states accompanied by characteristic lattice parameter changes in the high Ti doping region.

Nanocrystalline Ni can adopt a hexagonal close-packed (hcp) structure,which does not appear in a bulk form under any conditions.Structural studies found its $>$ 20% expansion of volume per atom compared with face-centered cubic (fcc) Ni.Hard x-ray photoemission spectroscopy of the hcp phase revealed a $>$ 0.14 valence electron transfer from 3$d$ to 4$s$ and 4$p$ orbitals and a 0.25 eV reduction in band energy per electron compared with the fcc phase.Its nonmagnetic nature was attributed to a low density of states at the Fermi level.Thermodynamic stability of the hcp phase can be explained by the band energy gain and the electrostatic potential energy loss as related to the large lattice expansion., 6 pages, 5 figures

A two-dimensional electron gas interacting with an external periodic potential attracts attention as a designable artificial material to explore topological phases. Here, to introduce a periodic potential into a Shockley state, superstructures of CO molecules have been fabricated on a Cu(111) surface by atom manipulation with a low-temperature scanning tunneling microscope. Local electronic states have been investigated in relation to specific locations on the CO triangular lattice. All tunneling spectra of the lattice exhibit a reduction at the bottom energy of the surface-state band, which reflects absorption of surface-state electrons into the bulk. For an ($8\ifmmode\times\else\texttimes\fi{}8$) CO structure, spectra measured at positions corresponding to two equivalent triangular sublattices of artificial graphene have different features near the Fermi level. This sublattice site dependence is not observed for a ($6\ifmmode\times\else\texttimes\fi{}6$) structure. First-principles calculations for a ($4\ifmmode\times\else\texttimes\fi{}4$) structure have reproduced the local density of states that depend on the sublattice sites. These results can be understood that coupling between the surface state and the bulk is strengthened via scattering by the adsorbates, and that the external periodic potential is perturbed by the second layer of the Cu(111) substrate. The periodicity of the external potential appears to be the key parameter which dominates the equivalence of pseudospin states in the artificial graphene.

In this study of fabricated one-dimensional (1D) nanostructures of Fe adatoms on vicinal Au(111) surfaces, the growth mechanism and electronic structures are investigated by scanning tunneling microscopy (STM) and by angle-resolved photoemission spectroscopy (ARPES). STM observations reveal that dosed Fe atoms are trapped at the lower corners of the steps. They create nucleation centers near the intersections between steps and discommensuration lines, and grow into evenly spaced Fe fragments located at face-centered-cubic (fcc) stacking regions of the substrate. The connection of these fragments aligned along the steps results in the formation of Fe monatomic rows. As the Fe coverage increases, the Fe growth proceeds predominantly at the fcc stacking regions, and forms quasi-1D nanostructures with undulating edges. At an Fe coverage of ∼0.6 ML, the fast-growing parts connect with the adjacent Fe structures and a two-dimensional network structure is built up. ARPES measurements reveal that the decoration o...

Magnetic behavior of Fe nanostructures on a vicinal Au(1 1 1) surface have been investigated by X-ray magnetic circular dichroism (XMCD) measurements. It is found that one-dimensional (1D) Fe chains of 2–3 atomic rows exhibit a ferromagnetic long-range order, and have a larger orbital moment in comparison with stripes and two-dimensional (2D) nanostructures. From the temperature-dependent XMCD measurements, the 1D chains exhibit two phase transitions, which are qualitatively ascribed to a ferromagnetic-to-superparamagnetic and a superparamagnetic-to-paramagnetic phase transions. These results suggest that the magnetism of the 1D chains behaves superparamagnetically.

The electronic structures of nanostructured transition metals (Fe, Co, Ni) grown by step decoration of the Au(788) surface are investigated by normal emission angle-resolved photoemission spectroscopy (ARPES). As the coverage of the transition metals is increased, the intensity of Au 5d valence bands decreases but the peak positions do not change, suggesting that the mixing between Au 5d and 3d of transition metals is negligibly weak. The coverage dependences of ARPES spectra near the Fermi level (E F ) for Fe, Co and Ni are discussed and compared with the photoemission results on bulk materials. In the higher coverage regions, we identify the observed 3d peaks near E F as ferromagnetic exchange splitting bands with similar exchange energy to the bulk. In contrast, at lower coverages, the exchange splitting vanishes for Fe and Co and becomes small for Ni. The origins of this behaviour are attributed to atomic volume, structural dimensionality and finite temperature effects.

The electronic structure of nanostructured Co adatoms grown by step decoration of a vicinal Au(1 1 1) surface has been investigated by angle-resolved photoemission spectroscopy (ARPES). With increased Co coverage, the intensity of Au 5d valence bands decreases but their peak positions do not change, suggesting that the mixing between Au 5d and Co 3d bands is negligibly weak. The coverage dependence of ARPES spectra near the Fermi level for Co was compared with bulk photoemission results. At the coverage of 1.28 ML Co, the observed Co 3d bands were identified as the exchange splittings between 3d bands with the similar energy value to the bulk.

The electronic and magnetic nature of Fe monatomic chains and two-dimensional (2D) adlayers grown by step decoration of a vicinal Au (7 8 8) surface has been investigated by angle-resolved photoemission spectroscopy (ARPES). By comparing the coverage dependence of ARPES spectra with bulk photoemission data and band structure calculations, we found that the Fe chains have a low ferromagnetic spin state or a nonmagnetic state, and that the 2D Fe adlayers have a high ferromagnetic spin state similar to the bulk Fe. We have succeeded in mapping out the band dispersion of the Fe chains and the 2D adlayers. The band dispersion of the monatomic chains derived from the ARPES measurement show a strong anisotropic feature reflecting a one-dimensional (1D) character. On the other hand, the ARPES spectra of the 2D adlayers show a finite energy dispersion even in the direction perpendicular to the steps, implying the break down of the 1D character. These results suggest that the electronic structures and the magnetism strongly depend on the dimensionality of the Fe adatom structures.

We have investigated the growth of Fe on Au(4 5 5) and Au(7 8 8) surfaces by scanning tunneling microscopy and examined the electronic structures by angle-resolved photoemission spectroscopy. We found that deposited Fe atoms adsorb at the lower corners of the steps, and form one-dimensional monatomic Fe chains. In addition, Fe deposition has significant influence on the surface potential of the regularly stepped surface. After decorating the step edges with monatomic Fe rows, photoemission spectra measured in the direction perpendicular to the steps show parabolic dispersion and a downward energy shift of the bottom of the surface state band. This result suggests that Fe adatoms at step edges act as attractive scatterers and reduce the step potential barrier height.

Reactivity of water molecules on atomically flat SrTiO 3 (0 0 1) surfaces was investigated with a wide range of the water partial pressure from 10 −4 to 10 3 Pa, by means of scanning probe microscopy (SPM) at room temperature. The reactivity on SrTiO 3 (0 0 1) was observed by monitoring the microscopic friction contrast reflecting SrO and TiO 2 terraces, since the contrast is enhanced by a preferential water reaction on the SrO terraces. The remarkable enhancement of the water reactivity was observed at the partial pressure of 10 3 Pa. A simple estimation revealed that, at this pressure, a condensed water layer is formed on the surface, which was confirmed by the temperature dependence measurements. We conclude that the water reaction on SrTiO 3 (0 0 1) is promoted by the condensation, and propose possible promotion mechanisms relating to the condensed water properties.

Local variation of frictional properties associated with adatoms of Sr and Cu with SrTiO3(100) surfaces was investigated by friction force microscopy. On the surfaces partially covered with Sr, strong friction was observed on Sr adlayers. In contrast, the areas covered with Cu showed weaker friction than the substrates. We found that a thick Cu layer became easily worn by scanning, so the stepped surface of the substrate appeared in the scanned area. Furthermore, inversion of the contrast in friction between the SrO and TiO2 terraces was observed in the same region during measurement. The origins of this friction behavior will be discussed in terms of the interaction between adsorbates and the substrate.

The perovskite manganite BiMnO 3 thin film was synthesized by a pulsed laser deposition method on a SrTiO 3 single crystal substrate, and its magnetic properties were investigated. The Curie temperature ( T C ) was about 105 K, consistent with the previous report for BiMnO 3 bulk, prepared under high pressure. However, the saturation magnetic moment was 2.8 μ B , which is smaller than that of the bulk (3.6 μ B ). Effect of Sr substitution for the Bi site was also investigated.

Adsorbed states of π-bonded ethylene at 250 K and ethynyl at 300 K on a Pd(110) surface have been studied by scanning tunneling microscopy (STM). Adsorbed π-bonded ethylene occupies an on-top site of the Pd atom. On the other hand, ethynyl exhibits a pair of depressed spots at both sides, located in the neighbor Pd rows. with a protrusive part between them. At low coverage, π-bonded ethylene molecules form a one-dimensional (3 × 1) structure, which reflects the anisotropic intermolecular interactions, while ethynyl species do not form an ordered structure.

We study the magnetic structure of perpendicularly magnetized Fe nanowires grown on Au(788) by means of x-ray magnetic circular dichroism. The average size of ferromagnetic domains in the nanowires exponentially decreases with temperature, leading to a steeper decay in magnetization compared to nanoislands of Fe. The temperature dependence of magnetization and domain structures can be well described in terms of local atomic geometry, the number of atomic chains in nanowires, exchange coupling between adjacent spins, and magnetic anisotropy. The nonlinear coverage dependence of the activation energy for the creation of magnetic domains can be explained by overall shape of nanowires, which also implies that the magnetic switching processes are strongly shape dependent at the nanoscale.

In this study of fabricated one-dimensional (1D) monatomic Fe wires on a Au(788) substrate, electronic and magnetic features are investigated by angle-resolved photoemission spectroscopy (ARPES) and x-ray magnetic circular dichroism (XMCD). The ARPES spectra show little dispersion both in the parallel and perpendicular directions to the wires revealing a strongly localized nature of the $\mathrm{Fe}\phantom{\rule{0.2em}{0ex}}3d$ electrons. Finite intensity of the XMCD signals has angular dependence reflecting an anisotropic feature of the localized magnetic moments with an easy axis perpendicular to the substrate. Temperature and field dependence of the XMCD signals shows superparamagnetic behavior well described by the 1D Ising model exhibiting large hysteresis below the blocking temperature, which is ruled by the anisotropy energy. Increase of the average size of the spin blocks with reduced temperature enhances the dramatic change of the magnetic properties of the 1D spin system from soft to hard within a narrow temperature range.

A first-order phase transition in a bulk material is generally considered to arise at extended defects such as grain boundaries or dislocations, where the energetic barrier between the two phases is reduced. Downsizing a crystal to the nanoscale can exclude the number of defects, leading to enhanced kinetic stabilization of the metastable phase. Here, the disappearance of the first-order metal–insulator transition in defect-free V2O3 nanocrystals and the revival of the transition by introducing a certain Cr or Ti impurity content are investigated. The hysteresis width of the transition corresponding to the barrier height decreases with the impurity content. It is proposed that homogeneous impurity doping is a universal method that can control the occurrence of a first-order phase transition in nanoscale materials.

We fabricated monatomic Fe wires on vicinal Au(111) surfaces and found that decoration of step edges with Fe adatoms has a significant influence on the behavior of surface state electrons confined between regularly arranged steps. On a surface with Fe monatomic rows, angle-resolved photoemission spectra measured in the direction perpendicular to the steps shows parabolic dispersion, in contrast to one-dimensional quantum-well levels observed on a clean surface. Simple analysis using a one-dimensional Kronig-Penney model reveals potential barrier reduction from 20 to 4.6 eV A, suggesting an attractive nature of the Fe adatoms as scatterers.

We have obtained monodisperse (V1−xCrx)2O3 nanocrystals with crystal sizes of 21.0 ± 4.1 nm using organic-phase synthesis. The (V1−xCrx)2O3 nanocrystals clearly show the transition from a corundum structured paramagnetic metal to a monoclinic structured antiferromagnetic insulator in contrast to non-doped V2O3 nanocrystals, in which the disappearance of the metal-insulator transition has been observed. We have found that Cr doping works effectively in narrowing the a1g band, which tends to be broadened by nanocrystallization. This result suggests that chemical doping is useful for control of material phase transitions at the nanoscale.

The vicinal (001) surface of a Nb-doped SrTiO3 single crystal has been investigated by scanning tunneling microscopy and low energy electron diffraction. The stepped surface prepared by annealing in ultrahigh vacuum at 250 °C exhibits a complex atomic structure composed of four types of reconstructions, which shows short-range variation within nanoscale regions. SrO layers show a c(6×2) structure being stable up to 1000 °C, while √13×√13-R33.7°, c(√13×√13)-R33.7°, and c(√2×√18)-R45° structures are formed on TiO2 layers, which disappear at 450–750 °C followed by the formation of 2×2 and √5×√5-R26.6° structures. These results indicate instability of the reconstructions on the TiO2 terminated surface due to the variation in Sr adatom density caused by multikinetic processes, in contrast to the thermodynamically stable SrO terminated surface.