V. V. Ryazanov

Coupling of two superconductors through a ferromagnet: evidence for a pi junction

We report measurements of the temperature dependence of the critical current, I(c), in Josephson junctions consisting of conventional superconducting banks of Nb and a weakly ferromagnetic interlayer of a CuxNi1-x alloy, with x around 0.5. With decreasing temperature I(c) generally increases, but for specific thicknesses of the ferromagnetic interlayer, a maximum is found followed by a strong decrease down to zero, after which I(c) rises again. Such a sharp cusp can be explained only by assuming that the junction changes from a 0-phase state at high temperatures to a pi phase state at low temperatures.

Superconductor-ferromagnet-superconductor π-junctions

Superconductor—ferromagnet—superconductor (SFS) π- junctions1 were investigated experimentally. The transitions to π-state and back to the conventional 0-state were found to result from temperature decrease or ferromagnetic interlayer thickness increase. Thickness dependence of the critical current was studied and nonmonotonic behavior was observed. The SF-proximity effect was also examined on SF-bilayers. It is shown that, due to the spatial oscillations of the induced superconducting order parameter in the ferromagnet, the critical temperature of the SF-bilayer has a minimum when the thickness of the ferromagnetic layer is close to a quarter of the oscillation period. PACS numbers: 74.50.+r, 85.25.Cp.

Proximity effect and spontaneous vortex phase in planar SF structures

The proximity effect in SF structures is examined. It is shown that, due to the oscillations of the induced superconducting order parameter in a ferromagnet, the critical temperature of an SF bilayer becomes minimal when the thickness of the ferromagnetic layer is close to a quarter of the period of spatial oscillations. It is found that the spontaneous vortex state arising in the superconductor due to the proximity of the magnetic domain structure of a ferromagnet brings about noticeable magnetoresistive effects.

Josephson magnetic rotary valve

We propose a control element for a Josephson spin valve. It is a complex Josephson device containing ferromagnetic (F) layer in the weak-link area consisting of two regions, representing 0 and π Josephson junctions, respectively. The valves state is defined by mutual orientations of the F-layer magnetization vector and boundary line between 0 and π sections of the device. We consider possible implementation of the control element by introduction of a thin normal metal layer in a part of the device area. By means of theoretical simulations, we study properties of the valves structure as well as its operation, revealing such advantages as simplicity of control, high characteristic frequency, and good legibility of the basic states.

Magnetic properties of two-dimensional Josephson networks: Self-organized criticality in magnetic flux dynamics

The field dependence of the magnetic moment of square (100×100) Josephson networks was examined with the use of a SQUID magnetometer. The field dependence of the magnetic moment was found to be regular with features corresponding to integer and half-integer numbers of flux quanta per cell. At temperatures below 5.8 K, jumps in the magnetization curves associated with the entry and exit of avalanches of tens and hundreds of fluxons were observed. It was shown that the probability distribution of these processes corresponded to the theory of self-organized criticality. An avalanche character of flux motion was observed at temperatures at which the size of the fluxons did not exceed the size of the cell, that is, when a discrete vortex structure occurred.

Fluctuation conductivity in superconducting MgB2

According to the crystal structure of MgB2 and band structure calculations, quasi-two-dimensional (2D) boron planes are responsible for the superconductivity. We report on critical-field and resistance measurements of 5.6-μm-thick MgB2 films grown on a sapphire single-crystal substrate. Resistivity measurements yield a temperature dependence of the fluctuation conductivity above the critical temperature, which agrees with the Aslamazov-Larkin and Maki-Thompson theory of fluctuations in layered superconductors, indicating a quasi-two-dimensional nucleation of superconductivity in MgB2.

Magnetic properties of two-dimensional SNS-type Josephson junction arrays

The field dependence of the magnetic moment of square (100×100) SNS-type Josephson junction arrays was studied by a SQUID magnetometer. A considerable difference in the behavior of this dependence was revealed for the cases of the entry of magnetic flux into the array and its exit from it. The branches of the curve obtained with increasing field strength had the form of regular periodic dependences with peaks corresponding to integer and half-integer numbers of flux quanta per cell. The curves obtained with decreasing field strength had no noticeable features and, in particular, no periodic structure. Magnetic flux avalanches were not observed in the SNS-type arrays, although the critical parameter of the system was sufficiently great (LIC/Φ0≫1) to satisfy the necessary condition of the self-organized criticality. The quasi-hydrodynamic flux motion in the array was explained by the considerable viscosity characterizing the vortex motion through the Josephson junctions.

Seebeck effect of weak links in a high-Tc superconductor

The authors have observed experimentally the analog of the superfluid helium fountain effect in a polycrystalline YBaCuO film. In a superconductor a temperature gradient induces the counterflow of superconducting electrons and quasiparticles. The supercurrent creates a phase difference at the intra- and intergrain weak links. In the presence of a temperature gradient parallel to an electric current they have detected an asymmetry of the current-voltage characteristic. Reversing the direction of the temperature gradient also reverses the sign of the current asymmetry. They estimate the thermoelectric coefficient for the quasiparticles in the YBaCuO sample.

Observation of Magneto-Thermoelectric Effects in High-Tc Superconducting Tl2Ba2CaCu2Ox Single Crystals

Anomalous peaks on the temperature dependences of the magnetic moment and the transverse Nernst voltage were observed in Tl 2 Bd 2 CaCu 2 O x single crystals, when the perpendicularly directed temperature gradient and transport current were applied. The results obtained are interpreted within the context of fluctuation vortex dynamics near T c .

Reentrant superconducting behavior of the Josephson SFS junction. Evidence for the π-phase state

Critical supercurrents, Ic in Nb_Cu1−xNix_Nb Josephson SFS junctions with F-layers prepared from ferromagnetic Cu1−xNix alloys have been studied. For value x=0.52 and particular F-layer thickness we have observed Ic(T) oscillations with Ic vanishing for some values of T. We associate this reentrant superconducting behavior with a crossover of the SFS junction from ‘0’- to ‘π’-state that is related to temperature dependence of spatial oscillation period of induced superconducting order parameter in the weak ferromagnet. We argue this is the first experimental evidence of the π-behavior of a Josephson junction, that is the special feature of superconducting pair flow through a ferromagnet predicted for SFS junctions by Bulaevskii, Buzdin et al [1].