Hidden magnetic transitions in thermoelectric layered cobaltite, [Ca2_2CoO3_3]0.62_{0.62}[CoO2_2]

A positive muon spin rotation and relaxation ($\mu^+$SR) experiment on [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$], ({\sl i.e.}, Ca$_3$Co$_4$O$_9$, a layered thermoelectric cobaltite) indicates the existence of two magnetic transitions at $\sim$ 100 K and 400 - 600 K; the former is a transition from a paramagnetic state to an incommensurate ({\sf IC}) spin density wave ({\sf SDW}) state. The anisotropic behavior of zero-field $\mu^+$SR spectra at 5 K suggests that the {\sf IC-SDW} propagates in the $a$-$b$ plane, with oscillating moments directed along the c-axis; also the {\sf IC-SDW} is found to exist not in the [Ca$_2$CoO$_3$] subsystem but in the [CoO$_2$] subsystem. In addition, it is found that the long-range {\sf IC-SDW} order completes below $\sim$ 30 K, whereas the short-range order appears below 100 K. The latter transition is interpreted as a gradual change in the spin state of Co ions %% at temperatures above 400 K. These two magnetic transitions detected by $\mu^+$SR are found to correlate closely with the transport properties of [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$].Comment: 7 pages, 8 figures. to be appeared in Phys. Rev.

An Analysis of Spectroscopic Results for the NO/Ni Adsorption System

A simple model of open-shell adsorbates on narrow-band transition metals has been applied to the special case of NO adsorbed on Ni. Properties of both the ground state and some excited states relevant to ultraviolet photoemission (UPS) are considered in some detail. Assuming the surface bond to be dominated by the NO-2π* orbitals, it is found that: (a) the ground state configuration corresponds to a spin-paired solution, (b) no multiplet structure is predicted for UPS from inner levels (4σ, 1π, 5σ) and (c) UPS from 2π electron states should give a peak at about 2 eV below the Fermi level. These conclusions which are somewhat unexpected from the properties of the NO molecule in the gas phase (i.e. paramagnetic) agree with experimental observations

Semiconductor-metal transition of the single-domain K/Si(100)-(2×1) interface by Fermi-surface determination

The semiconductor-metal electronic transition of the K/Si(100)-(2×1) interface is studied by exploring the Fermi surface with photoemission spectroscopy. Once metallized at a critical coverage the surface remains metallic up to saturation. The experimentally determined Fermi surface consists of hole pockets centered around the Γ + ̄ points of the surface Brillouin zone. These results are fairly well reproduced by calculations based on a 2D Mott-Hubbard model. The metallization process is related to the overlap of Si-confined electron clouds surrounding the K atoms rather than to changes in the surface atomic structure. © 1998 The American Physical SocietyThis work was partially funded by the Spanish agency DGCYT under Grant No. PB94-0022-c02-01 and PB92- 0030 .Peer Reviewe

Long-lived core-hole excited states and high-energy thresholds in stimulated desorption processes: Cl/Si(100)-(2 × 1)

The valence-electron response to an adsorbate core-hole is studied by means of the configuration-lattice approach in order to include correlation effects as accurately as possible. For the specific case of Cl adsorption on the reconstructed Si(100)-(2 × 1) surface, we find that removal of an electron from the Cl 3s orbital leads to long-lived excited states at 22 and 41 eV, in excellent agreement with the high-energy thresholds found in the stimulated desorption of Cl ions. The first excitation corresponds to a neutral valence-electron distribution (Cl+ desorption) while the second one, which leads to Cl2+, requires transfer of a valence electron from the adsorbate to the semiconductor accompanied by a strong shake-up of electron-hole pairs. These results are compared with those of a previous calculation where the core-hole was not included. © 1998 Elsevier Science B.V.This work has been partially supported by the Spanish Ministry of Education and Science through D G I C Y T Project N. PB95-0069.Peer Reviewe

Electron-Hole Shake-Up in Adsorption Systems: Halogens on Semiconductor Surfaces

In order to account for strong correlation effects, we have carried out a many body calculation of the electronic structure of the Cl/Si(100)-(2 × 1) system. The one-electron Green function is found directly from its Lehmann representation in terms of N - 1 and N + 1 electron states. These are obtained approximately by means of an iteration scheme (the lattice-configuration approach) whereby states with an increasing number of electron-hole pairs are successively included. This approach gives in a natural way the configurational composition of all the spectral features, which allows following in detail the character of possible relaxation processes. The resulting density of states is in quantitative agreement with the photoemission spectrum and describes qualitatively the desorption data of positive and negative Cl ions. It is found that most of the above spectral features are dominated by a strong shake-up of electron-hole pairs which largely invalidates a one-electron picture. This is ultimately traced to the (probably nonlinear) substrate response to the strong disruption produced by the Cl atoms (of large electronegativity).Peer Reviewe