ON INFLUENCE OF MAGNETIC STRUCTURE ON THE ELECTRIC CHARGE TRANSPORT IN SAMARIUM AND THULIUM THIN FILMS

The temperature dependence of the electrical resistivity of samarium and thulium thin films has been studied in the temperature range from 4.2 K to 300 K. The influence of magnetic structure on the resistivity is clearly seen on the obtained resistivity vs. tem- perature dependences of Sm films prepared by evaporation in high vacuum as well as that of Tm films prepared in UHV. The obtained values of Néel temperatures are lower than those of bulk samples and decrease with decreasing film thickness. Residual resistance ratio exhibits similar behaviour as Néel temperature

Stabilization of Pancake Bonding in (TCNQ)₂.⁻ Dimers in the Radical‐Anionic Salt (N−CH₃−2‐NH₂−5Cl−Py)(TCNQ)(CH₃CN) Solvate and Antiferromagnetism Induction

We report a new antiferromagnetic radical‐anion salt (RAS) formed from 7,7,8,8‐tetracyanquinonedimethane (TCNQ) anion and 2‐amino‐5‐chloro‐pyridine cation with the composition of (N−CH3−2‐NH2−5Cl−Py)(TCNQ)(CH3CN). The crystallographic data indicates the formation of (TCNQ)2.− radical‐anion π‐dimers in the synthesized RAS. Unrestricted density functional theory calculations show that the formed π‐dimers characterize with strong π‐stacking “pancake” interactions, resulting in high electronic coupling, enabling efficient charge transfer properties, but π‐dimers cannot be stable in the isolated conditions as a result of strong Coulomb repulsions. In a crystal, where (TCNQ)2.− π‐dimers bound in the endless chainlets via supramolecular bonds with (N−CH3−2‐NH2−5‐Cl−Py)+ cations, the repulsion forces are screened, allowing for specific parallel π‐stacking interactions and stable radical‐anion dimers formation. Measurements of magnetic susceptibility and magnetization confirm antiferromagnetic properties of RAS, what is in line with the higher stability of ground singlet state of the radical‐anion pair, calculated by means of the DFT. Therefore, the reported radical‐anion (N−CH3−2‐NH2−5Cl−Py)(TCNQ)(CH3CN) solvate has promising applications in novel magnetics with supramolecular structures

Regularisation of non-local actions in two-dimensional field theories

Taking the induced action for gauge fields coupled to affine currents as an example, we show how loop calculations in non-local two-dimensional field theories can be regulated. Our regularisation method for one loop is based on the method of Pauli and Villars. We use it to calculate the renormalisation factors for the corresponding effective actions, clearing up some discrepancies in the literature. In particular, it will be shown explicitly that vector gauge transformation invariance and Haar invariance of functional integral measures impose different requirements, but they are related by a counterterm (which is local in terms of group variables). For higher loops, we use the method of covariant derivatives combined with Pauli-Villars to argue that the one loop result remains unaltered.Comment: 29 pp, LATEX, LBL-33777, UCB-PTH-93/08, KUL-TF-93/2

Annealing-Dependent Morphotropic Phase Boundary in the BiMg0.5Ti0.5O3–BiZn0.5Ti0.5O3 Perovskite System

The annealing behavior of (1-x)BiMg0.5Ti0.5O3–xBiZn0.5Ti0.5O3 [(1-x)BMT–xBZT] perovskite solid solutions synthesized under high pressure was studied in situ via X-ray diffraction and piezoresponse force microscopy. The as prepared ceramics show a morphotropic phase boundary (MPB) between the non-polar orthorhombic and ferroelectric tetragonal states at 75 mol. % BZT. It is shown that annealing above 573 K results in irreversible changes in the phase diagram. Namely, for compositions with 0.2 < x < 0.6, the initial orthorhombic phase transforms into a ferroelectric rhombohedral phase. The new MPB between the rhombohedral and tetragonal phases lies at a lower BZT content of 60 mol. %. The phase diagram of the BMT–BZT annealed ceramics is formally analogous to that of the commercial piezoelectric material lead zirconate titanate. This makes the BMT–BZT system promising for the development of environmentally friendly piezoelectric ceramicspublishe