Low-temperature structural transition in FeCr_2S_4

Transmission electron microscopy studies of [110] and [111] oriented FeCr_2S_4 single crystals at different temperatures reveal a structural transition at low temperatures indicating a cubic-to-triclinic symmetry reduction within crystallographic domains. The overall crystal symmetry was found to be reduced from Fd3m to F-43m. The triclinic distortions were suggested to result from the combined actions of tetragonal distortions due to the Jahn-Teller active Fe^2+ ions and trigonal distortions due to a displacement of the Cr^3+ ions in the direction.Comment: 4 pages, 8 figure

Phonon anomalies and charge dynamics in Fe_{1-x}Cu_{x}Cr_{2}S_{4} single crystals

A detailed investigation of phonon excitations and charge carrier dynamics in single crystals of Fe_{1-x}Cu_{x}Cr_{2}S_{4} (x = 0, 0.2, 0.4, 0.5) has been performed by using infrared spectroscopy. In FeCr_{2}S_{4} the phonon eigenmodes are strongly affected by the onset of magnetic order. Despite enhanced screening effects, a continuous evolution of the phonon excitations can be observed in the doped compounds with x = 0.2 (metallic) and x = 0.4, 0.5 (bad metals), but the effect of magnetic ordering on the phonons is strongly reduced compared to x = 0. The Drude-like charge-carrier contribution to the optical conductivity in the doped samples indicates that the colossal magneto-resistance effect results from the suppression of spin-disorder scattering.Comment: 8 pages, 6 figure

Dynamics of the superconducting condensate in the presence of a magnetic field. Channelling of vortices in superconducting strips at high currents

On the basis of the time-dependent Ginzburg-Landau equation we studied the dynamics of the superconducting condensate in a wide two-dimensional sample in the presence of a perpendicular magnetic field and applied current. We could identify two critical currents: the current at which the pure superconducting state becomes unstable ($J_{c2}$ \cite{self1}) and the current at which the system transits from the resistive state to the superconducting state ($J_{c1}<J_{c2}$). The current $J_{c2}$ decreases monotonically with external magnetic field, while $J_{c1}$ exhibits a maximum at $H^*$. For sufficient large magnetic fields the hysteresis disappears and $J_{c1}=J_{c2}=J_c$. In this high magnetic field region and for currents close to $J_c$ the voltage appears as a result of the motion of separate vortices. With increasing current the moving vortices form 'channels' with suppressed order parameter along which the vortices can move very fast. This leads to a sharp increase of the voltage. These 'channels' resemble in some respect the phase slip lines which occur at zero magnetic field.Comment: 5 pages, 4 figures, Proceedings of Third European Conference on Vortex Matter in Superconductor

Negative magnetoresistance and phase slip process in superconducting nanowires

We argue that the negative magnetoresistance of superconducting nanowires, which was observed in recent experiments, can be explained by the influence of the external magnetic field on the critical current of the phase slip process. We show that the suppression of the order parameter in the bulk superconductors made by an external magnetic field can lead to an enhancement of both the first $I_{c1}$ and the second $I_{c2}$ critical currents of the phase slip process in nanowires. Another mechanism of an enhancement of $I_{c1}$ can come from decreasing the decay length of the charge imbalance $\lambda_Q$ at weak magnetic fields because $I_{c1}$ is inversely proportional to $\lambda_Q$. The enhancement of the first critical current leads to a larger intrinsic dissipation of the phase slip process. It suppresses the rate of both the thermo-activated and/or quantum fluctuated phase slips and results in decreasing the fluctuated resistance.Comment: 7 pages, 4 figure

Multiple phase slips phenomena in mesoscopic superconducting rings

We investigate the behavior of a mesoscopic one-dimensional ring in an external magnetic field by simulating the time dependent Ginzburg-Landau equations with periodic boundary conditions. We analyze the stability and the different possible evolutions for the phase slip phenomena starting from a metastable state. We find a stability condition relating the winding number of the initial solution and the number of flux quanta penetrating the ring. The analysis of multiple phase slips solutions is based on analytical results and simulations. The role of the ratio of two characteristic times u is studied for the case of a multiple phase slips transition. We found out that if u>>1, consecutive multiple phase slips will be more favorable than simultaneous ones. If u>1 is often a necessary condition to reach the ground state. The influence of the Langevin noise on the kinetics of the phase transition is discussed.Comment: 8 pages, 6 figure

Evidence for two-dimensional nucleation of superconductivity in MgB2_{2}

According to the crystal structure of MgB$_{2}$ and band structure calculations quasi-two-dimensional (2D) boron planes are responsible for the superconductivity. We report on critical fields and resistance measurements of 30 nm thick MgB$_{2}$ films grown on MgO single crystalline substrate. A linear temperature dependence of the parallel and perpendicular upper critical fields indicate a 3D-like penetration of magnetic field into the sample. Resistivity measurements, in contrast, yield a temperature dependence of fluctuation conductivity above T$_{c}$ which agrees with the Aslamazov-Larkin theory of fluctuations in 2D superconductors. We consider this finding as an experimental evidence of two-dimensional nucleation of superconductivity in MgB$_{2}$.Comment: 5 RevTex pages, 3 PostScript Figures ZIPed in archive Sidoren.zip. Submitted to EuroPhys. Lett. December 3, 200

Spin-lattice coupling in the ferrimagnetic semiconductor FeCr2S4 probed by surface acoustic waves

Using surface acoustic waves, the elastomagnetic coupling could be studied in thin single crystalline plates of the ferrimagnetic semiconductor FeCr2S4 by measuring the attenuation and the frequency tracking in the temperature range 4.2 K to 200 K. The data clearly display the anomalies found in low-field magnetization measurements.Comment: 15 pages, 3 figures. To appear in J. Appl. Phys., 99 (2006

Experimental and theoretical analysis of the upper critical field in FSF trilayers

The upper critical magnetic field H_{c2} in thin-film FSF trilayer spin-valve cores is studied experimentally and theoretically in geometries perpendicular and parallel to the heterostructure surface. The series of samples with variable thicknesses of the bottom and of the top Cu_{41}Ni_{59} F-layers are prepared in a single run, utilizing a wedge deposition technique. The critical field H_{c2} is measured in the temperature range $0.4-8$ K and for magnetic fields up to 9 Tesla. A transition from oscillatory to reentrant behavior of the superconducting transition temperature versus F-layers thickness, induced by an external magnetic field, has been observed for the first time. In order to properly interpret the experimental data, we develop a quasiclassical theory, enabling one to evaluate the temperature dependence of the critical field and the superconducting transition temperature for an arbitrary set of the system parameters. A fairly good agreement between our experimental data and theoretical predictions is demonstrated for all samples, using a single set of fit parameters. This confirms adequacy of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) physics in determining the unusual superconducting properties of the studied Cu_{41}Ni_{59}/Nb/Cu_{41}Ni_{59} spin-valve core trilayers.Comment: 16 pages, 7 figures; published versio

Memory Effect and Triplet Pairing Generation in the Superconducting Exchange Biased Co/CoOx/Cu41Ni59/Nb/Cu41Ni59 Layered Heterostructure

We fabricated a nanolayered hybrid superconductor-ferromagnet spin-valve structure, the resistive state of which depends on the preceding magnetic field polarity. The effect is based on a strong exchange bias (about -2 kOe) on a diluted ferromagnetic copper-nickel alloy and generation of a long range odd in frequency triplet pairing component. The difference of high and low resistance states at zero magnetic field is 90% of the normal state resistance for a transport current of 250 {\mu}A and still around 42% for 10 {\mu}A. Both logic states of the structure do not require biasing fields or currents in the idle mode.Comment: 9 pages, 4 figures, Accepted to Applied Physics Letter

Negative magnetoresistance of ultra-narrow superconducting nanowires in the resistive state

We present a phenomenological model qualitatively explaining negative magnetoresistance in quasi-one-dimensional superconducting channels in the resistive state. The model is based on the assumption that fluctuations of the order parameter (phase slips) are responsible for the finite effective resistance of a narrow superconducting wire sufficiently close to the critical temperature. Each fluctuation is accompanied by an instant formation of a quasi-normal region of the order of the non-equilibrium quasiparticle relaxation length 'pinned' to the core of the phase slip. The effective time-averaged voltage measured in experiment is a sum of two terms. First one is the conventional contribution linked to the rate of the fluctuations via the Josephson relation. Second term is the Ohmic contribution of this quasi-normal region. Depending on material properties of the wire, there might be a range of magnetic fields where the first term is not much affected, while the second term is effectively suppressed contributing to the experimentally observed negative magnetoresistance.Comment: 10 pages including 2 figure