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.

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

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