General survey of polarization observables in deuteron electrodisintegration

Polarization observables in inclusive and exclusive electrodisintegration of the deuteron using a polarized beam and an oriented target are systematically surveyed using the standard nonrelativistic framework of nuclear theory but with leading-order relativistic contributions included. The structure functions and the asymmetries corresponding to the various nucleon polarization components are studied in a variety of kinematic regions with respect to their sensitivity to realistic NN-potential models, to subnuclear degrees of freedom in terms of meson exchange currents, isobar configurations and to relativistic effects in different kinematical regions, serving as a benchmark for a test of present standard nuclear theory with effective degrees of freedom.     

Final-state interaction in semi-inclusive DIS off nuclei

The Final-State Interaction (FSI) in Deep-Inelastic Scattering (DIS) of leptons off a nucleus A, due to the propagation of the struck nucleon debris and its hadronization in the nuclear environment is considered. The effective cross-section of such a partonic system with the nucleons of the medium and its time dependence are estimated, for different values of the Bjorken scaling variable, on the basis of a model which takes into account both the production of hadrons due to the breaking of the color string, which is formed after a quark is knocked out off a bound nucleon, as well as the production of hadrons originating from gluon radiation. It is shown that the interaction, the evolution and the hadronization of the partonic system in the nuclear environment can be thoroughly investigated by a new type of semi-inclusive process, denoted A(e, e'(A - 1))X, in which the scattered lepton is detected in coincidence with a heavy nuclear fragment, namely a nucleus (A - 1) in low energy and momentum states. As a matter of fact, if the FSI is disregarded, the momentum distribution of (A - 1) is directly related to the momentum distribution of the nucleon before γ^* absorption, i.e. the same quantity which appears in the conventional A(e, e'N)X process, where N denotes a nucleon. The rescattering of the struck nucleon debris with the medium damps and distorts the momentum distributions of (A - 1) in a way which is very sensitive to the details of the effective cross-section of the debris with the nucleons of the medium. The total cross-section of the process A(e, e'(A - 1))X on ⁴He, ¹⁶O, and ⁴⁰Ca, related to the probability that after a target nucleon experiences a DIS process, the recoiling (A - 1) nucleus remains intact in spite of the strong FSI, is evaluated, and the distorted momentum distribution of the recoiling (A - 1) system is obtained. It is shown that both quantities are very sensitive to the details of the early stage of hadronization of the nucleon debris in the nuclear medium. Received: 28 June 2002 / Accepted: 15 January 2003 / Published online: 5 May 2003     

Structural, electronic and elastic properties of M2SC (M = Ti, Zr, Hf) compounds

Using ab initio calculations, we have studied the structural, electronic and elastic properties of M₂SC, with M = Ti, Zr and Hf. Geometrical optimization of the unit cell are in good agreement with the available experimental data. The band structures show that all three materials are conducting. The analysis of the site and momentum projected densities shows that the bonding is achieved through a hybridization of M-atom d states with S and C-atom p states. The Md-Sp bonds are lower in energy and are stiffer than Md-Cp bonds. The elastic constants are calculated using the static finite strain technique. We derived the bulk and shear moduli, Young's moduli and Poisson's ratio for ideal polycrystalline M₂SC aggregates. We estimated the Debye temperature of M₂SC from the average sound velocity. This is a quantitative theoretical prediction of the elastic properties of Ti₂SC, Zr₂SC, and Hf₂SC compounds, and it still awaits experimental confirmation.     

Fluorescence resonance energy transfer between perylene and riboflavin in micellar solution and analytical application on determination of vitamin B2

Fluorescence resonance energy transfer (FRET) between perylene and riboflavin is studied in micellar solution of sodium dodecyl sulfate. The fluorescence of perylene is quenched by riboflavin and quenching is in accordance with Stern-Volmer relation. The efficiency of energy transfer is found to depend on the concentration of riboflavin. The value of critical energy transfer distance (R₀) calculated by using Foster relation is 32.13 Å, and as it is less than 50 Å, it indicates efficient energy transfer in the present system. The analytical relation was established between extent of sensitization and concentration of riboflavin, which helped to estimate vitamin B₂ directly from pharmaceutical tablets.     

Magnetization and neutron scattering studies of the pressure effect on the magnetic transition in Er Y Co

Neutron powder diffraction was employed to study the pressure effect on the magnetic transition in the pseudobinary Laves-phase compound Er_0.57Y_0.43Co₂ and to determine the magnetic moments of the Er- and Co-subsystems. Our studies reveal that the onset of long-range magnetic order for both the localized 4 f (Er) and itinerant 3 d (Co) electron moments appears at about the same temperature at ambient pressure. The pressure effect on T_c is found to be negative and equal for both sublattices, namely T _c / p ∼ - 0.4 K/kbar. The values of the magnetic moments of the Er and the Co ions are found = 5.40±0.15μ _B /atom, = 0.50±0.07μ _B /atom and 5.35±0.15μ _B /atom, 0.37±0.09μ _B /atom, for p = 0 and 6 kbar, respectively. Our experimental results give evidence for short-range magnetic order formation at temperatures already above T_c and for a coexistence short- and long-range order below T_c down to 4 K. Received 20 December 2001 / Received in final form 12 June 2002 Published online 31 October 2002 RID="a" ID="a"e-mail: andrew.podlesnyak@psi.ch     

Extraction of G<Subscript>M</Subscript><Superscript>n</Superscript> from inclusive electron scattering on D, <Superscript>4</Superscript>He

We use the relation between Structure Functions (SFs) of nuclei A and nucleons N in order to fomulate a criterion which isolates the QE part out of the total inclusive cross-section. From data points around the QEP we extract the reduced neutron magnetic form factor 〈α _n = G _M ⁿ/μ _n G _d〉. The latter shows an unexpected decrease up to Q ² = 10GeV^2, the largest measured.     

Ground state structures in the polonium based II-VI compounds

We have performed ab-initio self-consistent calculations using the full-potential linear augmented plane-wave method to investigate the structural and the electronic properties of the less known II-VI compounds: ZnPo, CdPo, and HgPo. Total energy calculations of the cubic zinc-blende, wurtzite, rocksalt, cesuim chloride, orthorhombic Cmcm, and tetragonal PbO phases are investigated. Ground state parameters are computed, and compared with available theoretical and experimental works. The zinc-blende structure is found to be the ground state phase of ZnPo and CdPo, while HgPo prefers the tetragonal PbO structure. The calculated band structure of II-Po shows features that differ considerably from those of typical II-VI semiconductors. In particular we found an inverted band gap, reflecting a semi-metallic character for these compounds.     

Dependence of T<Subscript>c</Subscript> on hydrostatic pressure in a 123 superconductor

Hydrostatic pressure studies on the tetragonal 123 superconductor (La_1-xCa_x) (Ba_1.75-xLa_0.25+x)Cu₃O_y for x =0.1 and variable oxygen content y show that increases rapidly under pressure (+5 K/GPa) for underdoped, optimally doped and overdoped samples. This points to a common cause for the pressure-induced changes in the superconductivity at all levels of doping, with negligible effects from charge transfer. Weak relaxation behavior in is observed only for the most underdoped sample. Received 26 August 1999     

Colossal pressure-induced negative resistance change in La2/3Ca1/3MnO3 thin films

2/3 Ca_1/3MnO₃ thin films as a function of temperature from 4 to 300 K are studied. The application of external pressure increases the temperature of the metal–insulator transition (T_MI). For a film showing T_MI at about 177 K, a colossal change in resisitivity (R(0)-R(p))/R(p) qualitatively comparable to the magnetoresistance (R(0)-R(B))/R(B) around the transition temperature, is observed. However, this change for the film with high T_MI (267 K) is smaller by a factor of about 100. The increase of T_MI with pressure is intimately associated with the pressure-induced contraction and alignment of Mn-O-Mn bonds and the possible enhancement of the double-exchange interaction with pressure. Received: 11 September 1998/Accepted: 12 September 1998     

Scaling analysis of high temperature superconductors under pressure

The scaling relation of single parameter scaling hypothesis is applied to the study of the scaling behavior of high temperature superconductors under pressure. The data of resistance and specific heat coefficient under various pressures are scaled onto a universal curve according to this scaling relation. The scaling parameters are pressure dependent while temperature independent. It is found that the controlling parameter Bi equals to the relative critical temperature tcP, which indicates that the superconducting energy gap at the zero temperature 2Δs0 is the controlling parameter in this scaling.