Magnetic field dependence of the critical current in stacked Josephson junctions. Evidence for fluxon modes in Bi2Sr2CaCu2O8+x mesas

Modulation of the critical current across layers, Ic(H), of stacked Josephson junctions (SJJs) as a function of an applied magnetic field parallel to the junction planes is studied theoretically and experimentally for different junction lengths and coupling parameters. It is shown that the Ic(H) patterns of long SJJs are very complicated without periodicity in H. This is due to interaction between junctions in the stack. This, in turn, gives rise to the existence of multiple quasi-equilibrium Josephson fluxon modes and submodes which are different with respect to the symmetry of the phase and the fluxon sequence in SJJs. The critical current of long SJJs is multiple valued and is governed by switching between energetically close fluxon modes/submodes. Due to this, the probability distribution of the critical current may become wide and may consist of multiple maxima each representing a particular mode/submode. Experimentally, multiple branched Ic(H) patterns and multiple maxima in the Ic probability distribution were observed for Bi2Sr2CaCu2O8+x intrinsic SJJs, which are in a good agreement with numerical simulations and support the idea of having different quasi-equilibrium fluxon modes/submodes in intrinsic SJJs.Comment: 5 pages, 5 figure

Temperature distribution in a large Bi2Sr2CaCu2O8+δ mesa

Joule heating in large Bi2Sr2CaCu2O8+delta mesas was numerically analyzed while taking into account typical thermal conductivities and their temperature dependences of all the materials involved in heat dissipation and removal. Such mesas are used in experiments on THz-range radiation. The analysis shows that the temperature increases with bias current and is distributed unevenly along the mesas. The temperature of the mesa's middle part can even exceed T-c at sufficiently high bias. The overall current-voltage characteristics are also calculated self-consistently, showing a negative differential conductance in a wide range of currents

Large Responsivity of Graphene Radiation Detectors With Thermoelectric Readout: Results of Simulations

Simple estimations show that the thermoelectric readout in graphene radiation detectors can be extremely effective even for graphene with modest charge-carrier mobility ∼ 1000 cm 2 /(Vs). The detector responsivity depends mostly on the residual charge-carrier density and split-gate spacing and can reach competitive values of ∼ 10 3 - 10 4 V/W at room temperature. The optimum characteristics depend on a trade-off between the responsivity and the total device resistance. Finding out the key parameters and their roles allows for simple detectors and their arrays, with high responsivity and sufficiently low resistance matching that of the radiation-receiving antenna structures

Nonequilibrium effects due to charge fluctuations in intrinsic Josephson systems

Nonequilibrium effects in layered superconductors forming a stack of intrinsic Josephson junctions are investigated. We discuss two basic nonequilibrium effects caused by charge fluctuations on the superconducting layers: a) the shift of the chemical potential of the condensate and b) charge imbalance of quasi-particles, and study their influence on IV-curves and the position of Shapiro steps.Comment: 17 pages, 2 figures, revised version slightly shortene

Pseudo-gap features of intrinsic tunneling in (HgBr_2)-Bi2212 single crystals

The c-axis tunneling properties of both pristine Bi2212 and its HgBr$_2$ intercalate have been measured in the temperature range 4.2 - 250 K. Lithographically patterned 7-10 unit-cell heigh mesa structures on the surfaces of these single crystals were investigated. Clear SIS-like tunneling curves for current applied in the $\it c$-axis direction have been observed. The dynamic conductance d$I/$d$V(V)$ shows both sharp peaks corresponding to a superconducting gap edge and a dip feature beyond the gap, followed by a wide maximum, which persists up to a room temperature. Shape of the temperature dependence of the {\it c}-axis resistance does not change after the intercalation suggesting that a coupling between $\rm CuO_2$-bilayers has little effect on the pseudogap.Comment: 6 pages, 5 figures; presented at the Second Int Conf. New3Sc-1999 (Las Vegas, NV

Measurements of weak localization of graphene in inhomogeneous magnetic fields

Weak localization in graphene is studied in inhomogeneous magnetic fields. To generate the inhomogeneous field, a thin film of type-II superconducting niobium is put in close proximity to graphene. A deviation from the ordinary quadratic weak localization behavior is observed at low fields. We attribute this to the inhomogeneous field caused by vortices in the superconductor. The deviation, which depends on the carrier concentration in graphene, can be tuned by the gate voltage. In addition, collective vortex motion, known as vortex avalanches, is observed through magnetoresistance measurements of graphene