Strongly nonequilibrium flux flow in the presence of perforating submicron holes

We report on the effects of perforating submicron holes on the vortex dynamics of amorphous Nb0.7Ge0.3 microbridges in the strongly nonequilibrium mixed state, when vortex properties change substantially. In contrast to the weak nonequilibrium - when the presence of holes may result in either an increase (close to Tc) or a decrease (well below Tc) of the dissipation, in the strong nonequilibrium an enhanced dissipation is observed irrespectively of the bath temperature. Close to Tc this enhancement is similar to that in the weak nonequilibrium, but corresponds to vortices shrunk due to the Larkin-Ovchinnikov mechanism. At low temperatures the enhancement is a consequence of a weakening of the flux pinning by the holes in a regime where electron heating dominates the superconducting properties.Comment: 6 pages, 5 figure

Symmetry Violation in a Superconducting Film with a Square Array of Ferromagnetic Dots

We study the equilibrium state of a superconducting film covered with a regular square array of perpendicularly magnetized magnetic dots. The dots induce vortices in the film directly under them and antivortices between the dots. We show that the symmetry of the dot array is spontaneously violated by the vortices. The positions of the vortices and the antivortices depend on the magnetization and the size of the dots.Comment: 6 pages, 3 figure

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

Band filling effects on coherence lengths and penetration depth in the two-orbital negative-U Hubbard model of superconductivity

The two-orbital superconducting state is modeled by on-site intra-orbital negative-U Hubbard correlations together with inter-orbital pair-transfer interactions. The critical temperature is mainly governed by intra-orbital attractive interactions and it can pass through an additional maximum as a function of band filling. For the certain number of electrons the clear interband proximity effect is observable in the superconducting state of the band with a smaller gap. The influence of band fillings and orbital site energies on the temperature dependencies of two-component superconductivity coherence lengths and magnetic field penetration depth is analyzed. The presence of proximity effect is probably reflected in the relative temperature behaviour of characteristic lengths.Comment: Physica C (2012) in pres

Driving force for commensurate vortex domain formation in periodic pinning arrays

Recent vortex images in periodic pinning arrays have revealed the formation of degenerate commensurate domains separated by domain walls near rational fractional filling. This phenomenon was entirely unanticipated since, in stark contrast to ferromagnetic materials, the energies and magnetisation of different domains are identical, and the driving force for domain formation and estimation of typical domain sizes has, until now, remained an unsolved problem. We use high-resolution scanning Hall probe microscopy to show that domain formation is driven by the efficient incorporation of mismatched excess vortices/vacancies at the corners of domain walls. Molecular dynamics simulations with a generic pinning potential reveal that domains are only formed when vortex–vortex interactions are long range. A semi-quantitative model of domain formation further discloses how domain sizes depend on both the pinning array period and effective penetration depth.<br/

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