94 research outputs found
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
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
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
Intrinsic tunneling spectra of Bi_2(Sr_{2-x}La_x)CuO_6
We have measured intrinsic-tunneling spectra of a single CuO-layer La-doped
Bi_2Sr_{2-x}La_xCuO_{6+\delta} (Bi2201-La_x). Despite a difference of a factor
of three in the optimal superconducting critical temperatures for
Bi2201-La_{0.4} and Bi2212 (32 and 95 K, respectively) and different spectral
energy scales, we find that the pseudogap vanishes at a similar characteristic
temperature T*\approx 230-300K for both compounds. We find also that in
Bi2201-La_x, PG humps are seen as sharp peaks and, in fact, even dominate the
intrinsic spectra.Comment: Submitted to Phys. Rev. Let
Collective resonance modes of Josephson vortices in sandwiched stack of BiSrCaCuO intrinsic Josephson junctions
We observed splitting of the low-bias vortex-flow branch in a
dense-Josephson-vortex state into multiple sub-branches in current-voltage
characteristics of intrinsic Josephson junctions (IJJs) of
BiSrCaCuO single crystals in the long-junction limit.
Each sub-branch corresponds to a plasma mode in serially coupled Josephson
junctions. Splitting into low-bias linear sub-branches with a spread in the
slopes and the inter-sub-branch mode-switching character are in good
quantitative agreement with the prediction of the weak but finite
inter-junction capacitive-coupling model incorporated with the inductive
coupling. This suggests the importance of the role of the capacitive coupling
in accurately describing the vortex dynamics in serially stacked IJJs.Comment: 4 pages, 3 figures, 1 tabl
Large-area uniform graphene-like thin films grown by chemical vapor deposition directly on silicon nitride
Large-area uniform carbon films with graphene-like properties are synthesized by chemical vapor deposition directly on Si3N4/Si at 1000 degrees C without metal catalysts. The as deposited films are atomically thin and wrinkle- and pinhole-free. The film thickness can be controlled by modifying the growth conditions. Raman spectroscopy confirms the sp(2) graphitic structures. The films show ohmic behavior with a sheet resistance of similar to 2.3-10.5 k Omega/square at room temperature. An electric field effect of similar to 2-10% (V-G=-20 V) is observed. The growth is explained by the self-assembly of carbon clusters from hydrocarbon pyrolysis. The scalable and transfer-free technique favors the application of graphene as transparent electrodes
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