Antiferromagnetic transition in botallackite Cu2Cl(OH)3

The natural cuprate botallackite, Cu₂Cl(OH)₃, is found to be a new antiferromagnet with Magnetic susceptibility properties under strong field show non-linear M-H properties indicating metamagnetism. The T_N and the super-exchange coupling are discussed and compared with its polymorph atacamite and other copper oxides on the basis of their structural parameters.     

Magnetic oscillations in the 2D network of compensated coupled orbits of the organic metal (BEDT-TTF) 8 Hg 4 Cl 12 (C 6 H 5 Br) 2

Interlayer magnetoresistance and magnetisation of the quasi-two dimensional organic metal (BEDT-TTF)₈Hg₄Cl₁₂(C₆H₅Br)₂ have been investigated in pulsed magnetic fields extending up to 60 T and 33 T, respectively. About fifteen fundamental frequencies, composed of linear combinations of only three basic frequencies, are observed in the oscillatory spectra of the magnetoresistance. The dependencies of the oscillation amplitude on the temperature and on the magnitude and orientation of the magnetic field are analyzed in the framework of the conventional two-dimensional Lifshitz-Kosevitch (LK) model. This model is implemented by damping factors which accounts for the magnetic breakthrough occurring between electron and hole orbits yielding conventional Shubnikov-de Haas closed orbits (model of Falicov and Stachowiak) and quantum interferometers. In particular, a quantum interferometer enclosing an area equal to the first Brillouin zone area is evidenced. The LK model consistently accounts for the temperature and magnetic field dependence of the oscillation amplitude of this interferometer. On the contrary, although this model formally accounts for almost all of the observed oscillatory components, it fails to give consistent quantitative data in most other cases. Received 4 September 2002 / Received in final form 14 November 2002 Published online 27 January 2003 RID="a" ID="a"e-mail: audouard@insa-tlse.fr RID="b" ID="b"UMR 5830: Unité Mixte de Recherche CNRS - Université Paul Sabatier - INSA de Toulouse RID="c" ID="c"UMS 5642: Unité Mixte de Service CNRS - Université Paul Sabatier - INSA de Toulouse     

CDW-induced negative thermal expansion in two-dimensional conductor η-Mo4O11

Two-dimensional oxide η-Mo₄O₁₁ exhibits almost isotropic thermal expansion in the normal phase, while anisotropic negative thermal expansion (NTE) in the charge density wave (CDW) phase below T_c1=105 K, where a remarkable anomalous expansion occurs along the a-axis. The incommensurate nesting vector along the b-axis is independent of temperature, q₁=(0.000(1) 0.2335(1) 0.0000(5)). We propose that the CDW-induced NTE occurs as a result of the structural relaxation of the MoO₆ octahedra layers along the stacking direction with the aid of the open spaces around the MoO₄ tetrahedra.     

Acoustic enhancement of diffusion in a porous material

Lattice Boltzmann simulations are used to model the enhancement of diffusion which results from Eckart (attenuation driven) acoustic streaming in model porous material. Comparisons are made to Fickian diffusion where no flow is present and the diffusion when a fluid jet is used, which represents a more conventional method of enhancement. We show that streaming can produce a higher diffusion rate for the same average flow velocity and propose that this is the result of the continuation within the material of the driving force that produces the acoustic streaming.     

Electrical,thermal and infrared studies on semicarbazide hydrochloride and semicarbazide hydrobromide

Semicarbazide hydrochloride (SEM.HCl) was reported to be a prototype of another possible family of ferroelectrics [Appl. Phys. Lett. 20, 49 (1972)]. The present dielectric, resistivity, thermal and infrared absorption studies carried out on SEM.HCl and SEM.HBr do not confirm this suggestion. However these studies reveal that some observable changes do occur in the thermal amplitude of the molecular cation with the lowering of temperature.     

Semiconductor interface studies using core and valence level photoemission

The application of core and valence level photoelectron spectroscopy to the study of semiconductor heterojunctions and metal-semiconductor interfaces (Schottky barriers) is outlined, with an emphasis on recent results and their explanation in terms of current theories. While the determination of transport barriers (valence band offsets and Schottky barriers) is stressed, the identification of chemical reactions at the interface is also discussed using several examples. Photoemission can precisely determine many important quantities in these junctions; also demonstrated, however, is the disturbing influence of the photoemission process itself through the creation of a surface photovoltage in metal-semiconductor interfaces, and its possible consequences for recent investigations of Schottky barrier heights in metal overlayers on low temperature substrates.     

Electronic structure and magnetic properties of Y4Co3

The electronic structure of Y₄Co₃ has been studied based on the density functional theory within the local-density approximation. The calculation indicates that Y₄Co₃ is very close to ferromagnetic instability. The Fermi surfaces are composed mainly of 3d electrons of Co and 4d electrons of Y.     

Detecting spatially localized excitons in InGaN quantum well structures with a micro-photoluminescence technique

Spatially localized excitons are observed in InGaN quantum well structures at 4 K by using a micro-photoluminescence (PL) technique. By combining PL and nano-lithographic techniques, we are able to detect PL signals with a 0.2 μm spatial resolution. A sharp PL line (linewidth of <0.4 meV) is clearly obtained, which originates from a single localized exciton induced by a quantum dot like a local potential minimum position. Sharp PL spectra detected in three QWs with different indium compositions confirm that there are exciton localization effects in quantum wells in the blue-green (about 2.60 eV, 477 nm) to purple (about 3.05 eV, 406 nm) regions.