[en] We have used a constituent quarks model to describe the nucleon structure function, F₂(x, Q²), for a wide range of x = [10^-6, 1] and Q² = [0.5, 5000]GeV². We have found that although F₂ rises as x decreases, but there exists some x₀ ≤ 10^-4 - 10^-5, below which the rise of F₂ subsides drastically and hence, exhibits an almost flat behavior, compatible with the latest results from HERA, at least for low Q²

[en] Temperature dependence of the resistivity and the Hall effect has been measured from room temperature down to 4.2K for polycrystalline URu₂Si₂ samples. The resistivity has a small peak at T/sub o/ = 19.5K. Observation of a similar peak at 17.5K in case of single crystals¹ has been interpreted to be caused by an antiferromagnetic phase transition. Their Hall effect data shows a change of sign from negative to positive at T/sub o/ = 20K as the sample temperature is decreased. This change in sign is a signature of an antiferromagnetic phase transition. The temperature dependence of the Hall effect and the resistivity have very similar shape as also observed for other heavy fermions. Details of temperature dependences of the resistivity and the Hall constant will be presented

[en] The influence of Cr substitution by various metals (M=V, Mn, Fe, Cu and Al) on the magnetic state of the itinerant intermetallics La(Cr,M)Sb₃ was studied by magnetization and magnetic susceptibility measurements up to 55 kG at 5 K and from 4.2 to 400 K, in a magnetic field of 1000 G, respectively. It was found that the Curie temperature (T_C) and magnetization (M) of these compounds depend nonlinearly on the concentration, remaining in the vicinity of the values of T_C and M measured for LaCrSb₃. Curie temperatures and magnetization values at 55 kG are suppressed by Mn, Fe, V, and Cu, and have a slight maximum at low Al concentration (about 5%). [copyright] 2001 American Institute of Physics

[en] We report Mn K-edge x-ray absorption near edge structure (XANES) calculations of the La_1-xCa_xMnO₃ manganites considering three pair correlations, according to the three fundamental degrees of freedom governing their unusual electronic properties, namely, the electronic structure of the unoccupied states probed by the Mn K-edge profile versus (i) local atomic distortions; (ii) local magnetic ordering; and (iii) the charge-transfer nature of the Mn-O bonds. The calculations are accompanied by Mn K-edge XANES measurements in the temperature range 30--300 K. The main features of the absorption edge can be qualitatively reproduced in terms of single-electron multiple-scattering calculations for an 87-atom cluster. Lattice polaronic distortions in La_0.7Ca_0.3MnO₃ are simulated assuming a strongly distorted orthorhombic structure above and an almost undistorted rhombohedral structure below T_c. The results roughly reproduce the energy ''shift'' across T_c observed experimentally. Mn K-edge spin-polarized XANES spectra of the x=0, 0.3, and 1 samples are presented. An energy splitting between the majority- and minority-spin spectra of 0.5--1.1 eV contributes to the total XANES broadening below the Neel (Curie) temperature. A small feature B₃ standing approximately 6 eV above the main absorption peak is beyond the scope of single-electron calculations; it is assigned to a shake-up transition. To illustrate, the calculated Mn K edge is obtained as the convolution product of the single-electron XANES and the spectrum of many-body excitations in the Mn-O electronic states upon the sudden switching on of the Mn 1s core hole. We investigate the charge-transfer (CT) versus. Mott-Hubbard-type ground state using the U_d, Δ, and the T_dp parameters determined by previous Mn 2p x-ray photoemission spectroscopy measurements and find that LaMnO₃ should be viewed as a CT-type insulator with a substantial O 2p component in the ground state. In light of these results the controversial issue of Mn valence states in the manganites is critically reexamined. We argue that the disproportionation may be understood as a mixture of the CT many-body electronic configurations coupled with spin and lattice degrees of freedom

[en] Thin polycrystalline diamond films have been deposited using both conventional chemical vapour deposition (CVD) and time-modulated CVD (TMCVD) processes. Films deposited using conventional CVD methods were found to be rough and exhibited columnar growth characteristics. The average crystallite size and the surface roughness of the as-grown films increased with film thickness. It was found that with increasing methane concentration the overall nucleation density increased. However, increasing the methane concentration deteriorated the quality of the films, in terms of diamond carbon phase purity, as evident from the Raman spectra. The TMCVD process involved pulsing methane gas initially at higher concentration, into the CVD reactor during the nucleation stage of the diamond CVD process followed by the extended growth duration at a lower methane concentration. The TMCVD process produced films that displayed smoother surfaces. In addition, the films were grown at higher growth rates compared to the conventional CVD films. A preliminary model explaining the mechanism of the TMCVD process is presented

[en] Magnetic irreversibility in the form of thermal hysteresis of the magnetic susceptibility χ(T) between zero-field-cooled and field-cooled states has been observed in bulk SDW Cr alloys containing Fe and Co. The observed irreversibility effects are discussed from the point of view of frustration of Cr moments in the vicinity of 3d impurities similar to that which may occur at the interface in FM/AFM layered systems

No abstract available

[en] The ‘axial thickening’, a morphological character of certain Eocene orthophragminids refers to the thickening of the equatorial and/or lateral layers in axial directions forming radial external structures, namely the ribs. The occurrence of ribs is considered to be a diagnostic specific character of the orthophragminids in the periMediterranean region and Europe (western Tethys). In the Bartonian orthophragminids of the Indian subcontinent we observe a new type of axial thickening that we name ‘bulges’. The bulges are semi-rounded to rounded, localized structures on the test surface formed solely by the greater development of lateral chamberlets. These specimens, with trybliolepidine to umbilicolepidine type embryon configuration, co-occur with ribbed discocyclinids having completely different internal features. We also record another type of axial thickening of the lateral layers in notably small omphaloid tests that are characterized by a thick umbo, deeply depressed at the center such that the periphery of it forms a thick circular structure. The reported omphaloid specimens possess a small, semiisolepidine to nephrolepidine type embryon configuration and characteristic early chambers, not comparable to any known species in the Tethys. These specimens occasionally may have incomplete and/or irregular septula, reminiscent of certain Caribbean orthophragminids. The specimens with bulges, identified in two geographically distant regions of the Indian subcontinent, possibly represent endemic foraminiferal fauna confined to the IndoPakistan region. We introduce two new species, Discocyclina kutchensis sp. nov. (characterized by bulges) and ‘D’. sulaimanensis sp. nov. (characterized by circular structures in omphaloid tests). The status of both taxa in the Tethys is discussed. (Author)

[en] The authors have measured magnetic susceptibility, thermal expansion, electrical resistivity, and magnetoresistance of the intermetallic compound YMn₂. YMn₂ orders antiferromagnetically below T_N ≅ 110 K. There is a large thermal hysteresis such that the ordering temperature is T_N = 86 K on cooling and T_N = 116 K on warming. This is a first-order phase transition from the paramagnetic to antiferromagnetic phase. Their electrical resistivity and magnetoresistance measurements in the ordered state show a localized magnetic moment for Mn atoms in YMn₂ below T_N. The Mn moments collapse at T_N which is reflected in the sharp decrease in the thermal expansivity at T_N. Above T_N the Mn moment gradually recovers with increasing temperature which is reflected as an increase in χ with increasing temperature in the paramagnetic state. This behavior at T > T_N could be described by weak itinerant electron magnetism

[en] A spin glass (SG) phase was observed in the spin-density wave (SDW) alloys Cr_1-xCo_x at high Co concentrations (x≥13%). After zero-field cooling, the temperature dependence of the magnetization M(T) of Cr_0.87Co_0.13 and Cr_0.85Co_0.15, measured in an applied field H=100Oe with a SQUID magnetometer, exhibits a low T maximum, characteristic of a SG. Cooling in the measuring field, however, gives quite a different behavior. The field dependence of the magnetization M(H) is nonlinear, with a pronounced hysteresis. A strong time relaxation M(t) is also an indication of a SG phase. SG in Cr_1-xCo_x is formed at high impurity concentrations, possibly due to clustering of the magnetic impurities. [copyright] 2001 American Institute of Physics