États excités des isotopes de plomb de masse 202, 200 et 198

The low-lying excited levels of ^{198, 200, 202}Pb have been studied by observing γ- and β-rays and γ?γ coincidences in bismuth decay. Decay schemes have been proposed for these nuclei. Half-lives were determined for the 9^?, 7^?, 5^? and 4⁺ levels in ²⁰⁰Pb, and for the 5^? level in ¹⁹⁸Pb. Microscopic calculations have been made with Gaussian and surface delta interactions to account for level schemes. The results are discussed.     

Defects in Zn fired ZnTe : Detection of a double acceptor (SiTe ?)

We present a detailed account of electrical and optical measurements including resonantly excited luminescence and photoexcitation spectroscopy on the acceptor and donor defects associated to the annealing of ZnTe in molten Zinc. A new acceptor (X) is detected and tentatively attributed to a double neutral acceptor (Si_Te ?) according to Zeeman and stress experiments.     

Identification of Cu and Ag acceptors in CdTe

We report here on the identification of the two dominant acceptor levels in high purity p type CdTe, with Cu and Ag on Cd site. This identification is based on back doping experiments coupled with electrical measurements and photoluminescence studies. Cu and Ag can form easily complex centers when a supersaturation is achieved. The way of obtained good doping without complexation, is explained. The principal bound exciton lines are at 1,5896 eV (Cu) and at 1,5885 eV (Ag). The precise hole binding energies obtained from optical data are E_A (Cu) = 146 meV and E_A (Ag) = 108 meV.     

Critical point of the competition between different orbital- and Bi lone pair-orderings in the Bi-rich part of the BiCaMnO system

The present paper deals with the evidence of an unprecedented ordering in the Bi-rich part of the BiCaMnO diagram, where the spin glass state is observed at low temperature. The χ⁻¹(T) curves of three compounds, corresponding to different Ca/Bi + Ca ratio varying between 0.37 and 0.25, have been characterized and discussed in correlation with the structural transitions as a function of T. Three different behaviours have been observed for Bi_0.63Ca_0.37MnO₃, Bi_0.67Ca_0.33MnO₃ and Bi_0.71Ca_0.24MnO_3−δ. The anomalies in the χ⁻¹(T) curves of Bi_0.71Ca_0.24MnO_3−δ and Bi_0.63Ca_0.37MnO₃ appear to be associated to two different mechanisms, which are not observed in the intermediate Bi_0.67Ca_0.33MnO₃. For the Bi-rich compound, an unprecedented [111]_p-type ordering of the perovskite cell is reported, which is associated to a change of the slope in the magnetic curve. Bi_0.71Ca_0.24MnO_3−δ appears at the convergence point of a complex competition between the complementary lone pair ordering/orbital ordering of the ferromagnetic BiMnO₃ and the charge ordering/orbital ordering of the pseudo CE antiferro-magnetic Bi_0.63Ca_0.37MnO₃.     

Why pinning by surface irregularities can explain the peak effect in transport properties and neutron diffraction results in NbSe2 and Bi- 2212 crystals?

The existence of a peak effect in transport properties (a maximum of the critical current as function of magnetic field) is a well-known but still intriguing feature of Type II superconductors such as NbSe₂ and Bi-2212. Using a model of pinning by surface irregularities in anisotropic superconductors, we have developed a calculation of the critical current which allows estimating quantitatively the critical current in both the high critical current phase and the low critical current phase. The only adjustable parameter of this model is the angle of the vortices at the surface. The agreement between the measurements and the model is really very impressive. In this framework, the anomalous dynamical properties close to the peak effect is due to coexistence of two different vortex states with different critical currents. Recent neutron diffraction data in NbSe₂ crystals in the presence of transport current support this point of view.     

Two‐Orbital Three‐Electron Stabilizing Interaction for Direct Co2+?As3+ Bonds involving Square‐Planar CoO4 in BaCoAs2O5

The quest for new oxides with cations containing active lone‐pair electrons (E) covers a broad field of targeted specificities owing to asymmetric electronic distribution and their particular band structure. Herein, we show that the novel compound BaCoAs₂O₅, with lone‐pair As³⁺ ions, is built from rare square‐planar Co²⁺O₄ involved in direct bonding between As³⁺E and Co²⁺ d_z2 orbitals (Co As=2.51 Å). By means of DFT and Hückel calculations, we show that this σ‐type overlapping is stabilized by a two‐orbital three‐electron interaction allowed by the high‐spin character of the Co²⁺ ions. The negligible experimental spin‐orbit coupling is expected from the resulting molecular orbital scheme in O₃AsE–CoO₄ clusters.     

Nanosciences: Evolution or revolution?

In miniaturized objects fabricated by modern technology the smallest linear size may be of a few nanometers. In the field of microelectronics, the advantages of such a miniaturization are huge (increased complexity and reliability, reduced costs). The technology is now approaching the limits where further size reduction will be impossible, except for very novel techniques such as molecular electronics. Miniaturization research has also led to the discovery of nanometric objects such as carbon nanotubes, which turn out to be particularly appropriate for inventing new materials. Miniaturization techniques have been progressively applied in other fields, with the hope of obtaining improvements similar to those encountered in microelectronics. Examples are biochips, which concentrate on a few cm² the recognition of ADN sequences, or ‘lab-on-a-chip’ devices, each of which constitutes a whole laboratory of chemical analysis, or MEMs (Micro Electro Mechanical Systems). New therapies will use miniaturized objects with multiple functions: For instance a nanoparticle can both recognize the target organ thanks to an appropriate protein, and deliver the therapeutic molecule to this target. These results have only been possible through new observation instruments, able to observe and manipulate nano objects. Is the observed evolution really a revolution of science and techniques? This is a point discussed in the conclusion, which also deals with risks associated to nanotechnologies, while the need for a social regulation is stressed.     

Substrate and orientation influence on electrical properties of sputtered La-doped PZT thin films

Pb_1−2y/3La_yZr_xTi_1−xO₃ (PLZT) thin films have been prepared “in situ” by multi-target sputtering on Silicon, LaAlO₃ (LAO) and MgO substrates covered with a Pt bottom electrode. The purpose was to grow tetragonal PLZT films (Zr/Ti = 28/72 with different La contents) on these various substrates and to compare their electrical properties. To this aim, Pt was first deposited on the three different substrates to get (1 1 1)Pt/Si, (1 1 1)Pt/LAO and (2 0 0)Pt/MgO. Then PLZT was deposited in a same run on these three kinds of substrates and the influence of La contents and film orientation on electrical properties was investigated. The La content was varied from y = 0 to y = 32 in order to explore the phase transition between ferroelectric and paraelectric phases as a function of the substrate. For large amount of Lanthanum, relaxor behavior has been observed and studied.     

Measurement of vortex motion in a type-II superconductor: A novel use of the neutron spin-echo technique

We have used the neutron spin-echo technique to measure the small energy change of neutrons which are diffracted by a moving vortex lattice in a low-pinning Nb-Ta superconducting sample. A transport current was passed in the mixed state to cause flux line movement. In the case of uniform motion, the flux velocity v(L) was given as expected by the values of electric and magnetic fields, via E = -v(L)wedgeB. We show that with a nonuniformly moving vortex lattice, one can measure the dispersion of the velocities, opening up new possibilities for investigating moving vortex lines.     

Heat Conservation and Fluctuations Between Quantum Reservoirs in the Two-Time Measurement Picture

Journal of Statistical Physics - This work concerns the statistics of the Two-Time Measurement definition of heat variation in each reservoir of a thermodynamic quantum system. We study the...