Two-dimensional weak localization effects in high temperature superconductor Nd2−x CexCuO4−δ

A systematic study of the resistivity and Hall effect in single-crystal films Nd_2−x Ce_xCuO_4−δ (0.12⩽x⩽0.20) is presented, with special emphasis on the low-temperature dependence of the normal state conductance. Two-dimensional weak localization effects are found both in a normally conducting underdoped sample (x=0.12) and in situ superconducting optimally doped (x=0.15) or overdoped (x=0.18) samples in a high magnetic field B>B _c2. The phase coherence time τ _ϒ (5.4 · 10⁻¹¹ s at 2 K) and the effective thickness of a CuO₂ conducting layer d (≃1.5Å) have been estimated by fitting 2D weak localization theory expressions to magnetoresistivity data for magnetic fields perpendicular to the ab plane and in plane. Estimates of the parameter d ensure strong carrier confinement and justify a model consisting of almost decoupled 2D metallic sheets for the Nd_2−x Ce_xCuO_4−δ single crystal. Zh. éksp. Teor. Fiz. 116, 1723–1734 (November 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Borylation of calix[4]resorcinols under ultrasonic irradiation

The use of ultrasonic irradiation in the reactions of trialkyl borates with octahydroxytetraoctyl[l ₄] metacyclophane allowed us obtaining tetraalkoxyboratocavitands with higher yields and under milder conditions. We first obtained 1,3,2-dioxaborolane and 1,3,2-dioxaborinane derivatives of tetraoctyl[l ₄]metacyclophane using ultrasonic irradiation. Boratocavitands were converted into the corresponding amine complexes with isobutylamine.     

Effect of disorder on the transport properties of the high-T c superconductor Nd2− x CexCuO4+δ

The temperature dependences ρ_ab(T) of Nd_2?x Ce_xCuO_4+δ single crystals with 0≤x≤0.20 are studied and analyzed on the basis of the concepts in the theory of disordered 2D systems. The results are compared with the data obtained for other copper-oxide HTSC. It is found that a transition to the superconducting state in the optimal doping region 0.14≤x≤0.18 occurs only in crystals with a fairly small degree of disorder (k _Fl≥2, where l is the mean free path). This transition is compatible with the weak 2D-localization mode as long as the localization radius is longer than the characteristic size of a Cooper pair. The superconducting transition temperature in the optimal doping region increases monotonically with the parameter k _Fl characterizing the degree of disorder in the crystal. The degradation of superconducting properties upon a further increase in the doping level (x>0.18) is apparently associated with a transition from 2D to 3D conductivity in the single crystal.     

Thiophosphorylation of calix[4]resorcinarenes with 2,4-diaryl-1,3-dithia-2λ<Superscript>5</Superscript>,4λ<Superscript>5</Superscript>-diphosphetane 2,4-disulfides

Reactions of 2,4-diaryl-1,3-dithia-2λ⁵,4λ⁵-diphosphetane 2,4-disulfides with tetraheptylcalix[4]-resorcinarene and octa-O-(2-hydroxyethyl)tetraoctylcalix[4]resorcinarene gave the corresponding octakis(aryldithiophosphonic acids).     

Anisotropic low-temperature in-plane magnetoresistance in electron doped Nd<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Ce<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CuO<Subscript>4+δ</Subscript>

Nominally electron doped antiferromagnetic tetragonal nonsuperconducting Nd_2?xCe _x CuO_4+δ(x=0.12) has been shown to manifest strong angular dependence of the in-plane magnetoresistance on the orientation of the external magnetic field within the ab plane in many aspects similar to that observed in hole doped YBa₂Cu₃O_7?δ and La_2?xSr_xCuO₄. Specific fourfold angular magnetoresistance anisotropy amounting to several percents was observed in oxygen annealed films at low temperatures and in an external magnetic field up to 5.5 T. The strong temperature dependence and fourfold symmetry observed in our sample points to a specific role of rare-earth (Nd) ions in magnetoresistance anisotropy. At low temperature T = 1.4 K, we observed the unusual transformation of magnetoresistance response with increasing the external magnetic field, which seems to be a manifestation of a combined effect of a crossover between first and second order spin-flop transitions and a field-dependent rare-earth contribution to quasiparticle magnetotransport.     

Disperse aqueous systems based on (S)-lysine in a wide range of concentrations and physiologically important temperatures

Russian Chemical Bulletin - It was shown for the first time using a complex of physicochemical methods (dynamic and electrophoretic light scattering, nanoparticle tracking analysis, transmission...     

Aqueous Systems Based on Metaphos in Low Concentration: The Relationship between Self-Assembly,Physico-Chemical,and Biological Properties

It has been shown for the first time that aqueous solutions of metaphos pesticide (1×10^–4–1×10^–17 mol/L) are disperse systems, the disperse phase undergoing rearrangement upon dilution. The relationship between nonmonotonic concentration dependences of the nanoassociate size, physico-chemical properties of the systems, and their action on higher plants and hydrocoles has been found.     

Density of states in the impurity d band and inelastic electron scattering by a system of mixed-valence iron ions in HgSe: Fe crystals

The resistivity and the Hall coefficient of HgSe: Fe crystals with various iron content are experimentally studied in the temperature range 1.3≤T≤300 K and in magnetic fields up to 60 kOe. The temperature dependences of the density and mobility of conduction electrons in these crystals are determined. The influence of spatial charge ordering in the system of mixed-valence iron ions on impurity states in HgSe: Fe crystals is considered. The density of states in the impurity d band is theoretically analyzed, and inelastic electron scattering in which bi-and trivalent iron ions are recharged is discussed. It is shown that the experimentally detected features in the dependence of the density and mobility of electrons on temperature and iron impurity content can be explained by the influence of Coulomb correlations in the mixed-valence iron ion system on the structure of the impurity d band.

     

Influence of magnetic field on the superconducting transition in a Nd1.82Ce0.18CuO4−δ film

The temperature and magnetic-field dependences of the resistivity ρ and Hall effect R(j‖ab, B‖c) in a Nd_1.82Ce_0.18CuO_4−δ single crystal film (T _c =6 K) is investigated at temperatures 1.4≤T≤20 K and magnetic fields 0≤B≤5.5 T. At the lowest temperature T=1.4 K the resistive state (exhibiting resistivity and the Hall effect) arises in a magnetic field B=0.5 T. A transition to the normal state is completed at B _c 2≃3 T, where the Hall coefficient becomes nearly constant. The negative magnetoresistance due to the weak-localization effect in the normal state is observed for B>3 T. The nonmonotonic behavior and the inversion of the sign of R(B) in the mixed state are explained in a reasonable way by the flux-flow model with the effect of pinning taken into account. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 6, 407–411 (25 September 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.