Simulation of the interband s–d and intraband s–s electron–phonon contributions to the temperature dependence of the electrical resistivity in Fe/Cr multilayers

High-resolution electrical resistivity (rho, d rho/dT) measurements were performed in three series of [Fe-30 Cr-Angstrom(t) (Angstrom)] multilayers in the temperature range 15-300 K, with an applied magnetic saturation field (7.5 kOe). The samples were deposited by molecular beam epitaxy on MgO substrates and by sputtering on MgO and Si substrates. For T 150 K the resistivity attains the classical regime with rho proportional to T. To simulate the observed rho(i)(T) we have used a model that takes into account intraband s-s and interband s-d electron-phonon scattering, written as rho(sd) = A X f(1)(T) and rho(ss) = B X f(2)(T), where f(1) and f(2) are functions only of the temperature, A and B are sample-dependent constants and rho(i) = rho(sd) + rho(ss). The model predicts that rho(i) proportional to T-3 at low temperatures and rho(i) proportional to T at high temperatures as observed in our multilayers. The experimental curves of rho(i) and d rho/dT are well reproduced in the whole temperature range (15-300 K) and from the fits to these curves A and B are determined for each sample. By plotting B vs A we find that each point from all the multilayers falls in a straight line indicating that B is proportional to A. The simulated resistivity thus predicts that rho(i) = beta f(T) where f(T) = alpha(1) x f(1)(T) + alpha(2) x f(2)(T) is a function only of the temperature, as observed experimentally

Spin state crossover, vibrational, computational and structural studies of new Fe isopropyl-4H-tetrazole derivatives

Mononuclear complexes of formula [Fe(iptz)6]X2 {X =BF4 – (1), ClO4 – (2); iptz = 1-isopropyl-1H-tetrazole} have been synthesized and characterized spectroscopically and structurally. These materials display a gradual and incomplete spin conversion around 95 K on cooling, as well as a reversible thermochromic effect. This magnetic behavior was satisfactorily simulated using the Sorai and Seki domain model (ΔH = 5.9 kJ mol–1, ΔS = 57 Jmol–1 K–1 for 1, and ΔH = 5.4 kJ mol–1, ΔS = 63 J mol–1 K–1 for 2). Although spin crossover was observed earlier on a powdered sample of 1, we present the Xray crystal structure and magnetic properties of the crystalline compound for the first time. The crystal structures of four new iron complexes are also provided: two polymorphs of [Fe(iptz)4(NCS)2] (3), and compounds [Fe(iptz)4Cl2] (4) and [Fe(iptz)2(H2O)2(CH3SO3)2] (5). Compound 3, which is the first FeII mononuclear thiocyanato complex with coordinated 1-R-1H-tetrazole ligands, is a high-spin complex. DFT computations enabled the estimation of the spin state energies for the trans and cis isomers of [Fe(iptz)4(NCS)2] as well as the [Fe(iptz)6]2+cation. The stereoisomerism in 3 related to the mutual orientation of the ligand rings was also studied, along with π-acceptor properties of the iptz ligand, which are modified by the orientation of the coordinated ligand. The dependence between the Fe–N(CS) distance and the electronic energy of the tilt of the coordinated thiocyanato anion for trans-[FeL2(NCS)2] and trans-[FeL4(NCS)2] complexes is discussed, based on the comparison of X-ray data and DFT modelling for selected complexes

Ajustement des propriétés supraconductrices des nanoconstrictions de niobium par électro-annealing

In this work we report the possibility to create weak links in nanostructured Nb thin films using electro-annealing. By using electron-assisted atom diffusion in Nb nanconstrictions, we were able to locally change the material properties (superconducting critical temperature and normal resistance). When the critical temperature of the nanoconstriction is sufficiently decreased, the field dependence of the critical current exhibits a Fraunhofer-like pattern, confirming the formation of the weak link. Scanning electron microscopy images clearly reveals the structural change underwent by the constriction resulting from the effect of the high current density. Numerical simulations within the Ginzburg-Landau formalism are also presented. These findings represent the first evidence of weak link formation by electro-annealing and provide an easy method for the fabrication of Josephson junctions with tunable resistance and critical temperature in Nb thin films