Bi-2212 Intrinsic Josephson Junctions Research Articles

Periodic oscillations of Josephson-vortex flow resistance in oxygen-deficientYBa2Cu3Ox

We measured the Josephson vortex flow resistance as a function of magnetic field applied parallel to the ab-planes using annealed Y1Ba2Cu3Ox intrinsic Josephson junctions having high anisotropy (around 40) by oxygen content reduction. Periodic oscillations were observed in magnetic fields above 45-58 kOe, corresponding to dense-dilute boundary for Josephson vortex lattice. The observed period of oscillations, agrees well with the increase of one fluxon per two junctions ($H_{p}$\textit{=$\Phi $}$_{0}$\textit{/2Ls}), may correspond to formation of a triangular lattice of Josephson vortices as has been reported by Ooi et al. for highly anisotropic (larger than 200) Bi-2212 intrinsic Josephson junctions.

Preparation conditions and characterization of self-planarized Bi2Sr2CaCu2Ox stacks

We investigated the preparation conditions and device characterization of stacked Bi2Sr2CaCu2Ox (Bi-2212) intrinsic Josephson junctions. The stacks were fabricated by self-planarizing process. In this process the Bi-2212 crystal around the stack was modified to insulator by soaking it into the solution of dilute (⩽0.2%) hydrochloric acid. The Auger electron spectroscopy (AES) depth profile for the acid-treated layer indicated the reduction of O, Ca and Sr atoms in the crystal. In atomic force microscopy (AFM) images, no step was observed at the edge of the junction window for the sample soaked it into the dilute (⩽0.2%) hydrochloric acid for 5s. We also analyzed the temperature dependences of the critical and return currents of the stacks.

Temperature dependence of the return current in Bi2Sr2CaCu2Ox stacks fabricated by self-planarizing process

We investigated the temperature variation of the return currents of stacked Bi2Sr2CaCu2Ox (Bi-2212) intrinsic Josephson junctions fabricated using self-planarizing process. In this process, the Bi-2212 crystal around the junction window was changed to an insulator by soaking it into the solution of dilute hydrochloric acid. The current-voltage characteristics of the stacks with areas as large as 30×40μm2 exhibited large hystereses and multiple branches at 4.2K, which can be explained by a series connection of highly capacitive Josephson junctions. At higher temperatures than 40K, we found that the return current Ir of the intrinsic junction depended strongly on the temperature, T. We analyzed the Ir-T characteristics using simple resistively and capacitively shunted junction model in view of the temperature dependence of the c-axis resistance of the stack.

Possible formation of rectangular Josephson-vortex lattice in narrow Bi-2212 intrinsic Josephson junctions by the enhanced edge effect

Josephson vortex lattice in narrow Bi-2212 intrinsic Josephson junctions have been studied. Formation of rectangular Josephson vortex lattice is expected in narrow width due to the edge effect. Current–voltage ( I – V ) characteristics were measured as a function of the magnetic field applied parallel to their a – b planes. Maximum zero bias current ( I C,branch ) and flux-flow voltage were measured. We observed that the I C,branch monotonically decreased with magnetic field up to 0.6 T. Above 0.6 T, I C,branch showed an enhancement, accompanied by an enhancement of flux-flow voltage thus flux-flow speed, up to 1.0 T. This result indicates a change in the configuration of vortex lattices from triangular to other structure (possibly rectangular) by the edge effect in which configuration both the flux-flow voltage and I C,branch are enhanced due to an increase in the maximum velocity of the vortex lattice.

Probing the intrinsic Josephson coupling potential inBi2Sr2CaCu2O8+δsuperconductors by thermal activation

We study thermal fluctuation phenomena in small Bi-2212 intrinsic Josephson junctions. Being able to measure switching currents of a {\it single} intrinsic junction, we observe that it's statistics can be very well described by thermal activation from a periodic Josephson potential with the sinusoidal current-phase relation. This is a direct evidence for the dc-intrinsic Josephson effect and the first unambiguous confirmation of the tunnelling nature of interlayer transport in strongly anisotropic high temperature superconductors. Furthermore, the fluctuation-free critical current, extracted from the analysis of switching current statistics, exhibits a temperature dependence typical for superconductor- insulator- superconductor tunnel junctions.

Microwave-Induced Steps Observed in Bi-2212 Intrinsic Josephson Junctions

Microwave-induced steps observed in the I-V characteristics of Bi-2212 intrinsic Josephson junctions are investigated and the mechanism producing the zero-crossing steps is discussed. It is found that (1) a junction area smaller than 40 /spl mu/m/sup 2/ shows uniform in-plane distribution of the c-axis dc current and (2) the zero-crossing steps are observed in the uniform distribution. The observation of steps in samples of different sizes under the uniform current condition indicates that the observed steps are produced by the inverse ac Josephson effect.

Shapiro step responses in the flux-flow state of bi-2212 intrinsic Josephson junctions with cooperation of pancake vortices

Flux-flow properties of one-dimensionally long Bi-2212 intrinsic Josephson junction stacks are studied. Quasioptically irradiated millimeter waves drives Josephson vortices and reveals current steps in equal voltage intervals corresponding to the Josephson relation. Differential conductance properties and the power dependence of the current step height are also measured. The experimental results indicate that the responses are consistent with the Shapiro step. Periodic potentials produced by pancake vortices are considered to be effective to realize the in-phase collective motion of the Josephson vortices.

Tunneling Characteristics for Bi2212 Intrinsic Josephson Junctions With Little Influence of Self-Heating Using Short Pulse Technique on 60 ns Time Scale

A small mesa structure (10 /spl mu/m /spl times/ 10 /spl mu/m, 15 nm thick) of intrinsic Josephson junctions in Bi/sub 2/Sr/sub 2/CaCu/sub 2/o/sub 8+/spl delta// is employed to observe the tunneling characteristics and the superconducting gap magnitude by short pulse technique (short pulse interlayer tunneling spectroscopy). In the measurement on 60 ns time scale, the effect of self-heating is almost negligible in the sample, whose resistivity is as small as 10 /spl Omega/cm. In slightly overdoped region, the maximum Josephson current density increases from 0.85 to 2.1 kA/cm/sup 2/ with increasing doping level. The superconducting gap magnitude 2/spl Delta/ decreases from 62 to 37 meV with increasing doping level. The value for 2/spl Delta//k/sub B/T/sub c/ decreases from 8.3 to 5.2 with increasing doping level. These values in slightly overdoped region are smaller than those in optimum and underdoped region.

Simple growth technique of Bi 2Sr 2CaCu 2O 8+ δ single crystals and transport characteristics of intrinsic Josephson junctions

High-quality Bi 2Sr 2CaCu 2O 8+ δ (Bi-2212) single crystals were grown by a self-flux method without mixing and grinding of the reagent of starting powders, to simplify the growth process of the crystals. During the crystal growth process, a pressure (<0.4 N/cm 2) was applied using a brick placed on the cap on an alumina crucible. We obtained high-quality single crystals with zero-resistance transition temperature T c,zero≈90 K in the best sample. The Bi-2212 intrinsic Josephson junctions (IJJs) were fabricated using standard photolithographic and Ar-ion milling techniques. Multi-branch structures were clearly observed in the I– V curves of the Bi-2212 IJJs. The critical current of the junction was sensitive to external magnetic-fields parallel to the a– b plane at temperature up to 89 K.

Ion implantation effects on tunneling properties of Bi/sub 2/Sr/sub 2/Ca/sub 1/Cu/sub 2/O/sub 8+y/ intrinsic Josephson junctions

We propose a feasible method to modify the tunneling properties of Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+y/ (Bi-2212) intrinsic Josephson junctions(IJJ's) using silicon ion implantation. The implantation is performed on 150 nm-height mesas at an acceleration voltage of 80 keV with doses ranging from 1/spl times/10/sup 13/ to 5/spl times/10/sup 15/ ions/cm/sup 2/. The critical current of IJJ's rapidly decreases with increasing doses, while the critical temperature hardly changes. The small amount of Si impurities affect on the interlayer coupling but not the gap of the CuO/sub 2/ bilayers. The RF response of a Si-implanted IJJ is demonstrated and reveals clear Shapiro steps as the plasma frequency decreases.

パルス管冷凍機と可変磁場永久磁石によるＢｉ‐２２１２固有ジョセフソン接合の磁束フロー特性

A cryogen-free measurement system has been developed to examine the flux-flow phenomena in Bi2Sr2CaCu2O8+y (Bi-2212) intrinsic Josephson junctions. The system consists of a pulse-tube cryocooler and a variable-field permanent magnet. Flux-flow measurements were carried out at 44 K at which the Bi-2212 intrinsic Josephson junction exhibited 90% of the critical current at 4.2 K. Field direction could be set parallel to the superconducting CuO2 layers of the intrinsic junctions accurately, free from pancake vortices. The current-voltage characteristics measured under magnetic fields up to 1 T revealed a broad velocity-matching step corresponding to the strongly coupled Josephson junction stack.

Submicron-scale high- Tc superconducting Bi-2212 stack fabrication for single-Cooper-pair tunneling

We report the characteristics of Bi-2212 intrinsic Josephson junctions (IJJ) showing single-Cooper-pair tunneling effect with a decrease of their in-plane area, S, smaller than a micron scale. The junctions show the typical slope of critical current and current peak-like structure up to 37 K.

Relationship between the Out-Of-Plane Resistance and the Subgap Resistance of Intrinsic Josephson Junctions inBi2Sr2CaCu2O8+δ

We have experimentally demonstrated that the c-axis magnetoresistance peak effect is determined by the subgap resistance of intrinsic Josephson junctions in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} (Bi2212). Given a Bi2212 single crystal, the effect may be predicted from the c-axis current-voltage (I-V) characteristics in {ital zero} magnetic field. At high magnetic field H, the I-V characteristics preserve their overall nonlinear shape at the transition temperature T{sub c}(H){le}T{sub c}(0) suggesting a smooth change of the vortex system with temperature. Observation of the gap feature below T{sub c}(0) at all {mu}{sub 0}H{le}7 T means that the upper critical field is not attained at these fields. {copyright} {ital 1997} {ital The American Physical Society}

Josephson vortex motion in stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2O8+x

Abstract We have investigated the motion of Josephson vortices in stacks of intrinsic Josephson junctions in Bi 2 Sr 2 CaCu 2 O 8+ x (BSCCO). The stacks were realized as mesa structures on top of BSCCO single crystals. In external magnetic fields up to 5 T, oriented parallel to the layers, the current–voltage ( I – V ) characteristics exhibit a field dependent branch. By microwave emission measurements we show that this branch is due to a collective motion of Josephson vortices. For magnetic fields larger than 17 kOe we find a pronounced flux-flow step in the I – V characteristic that corresponds to a Josephson frequency of 1.5 THz. In this regime, we observe an intensive non–Josephson emission signal. We propose that this signal is due to Cherenkov radiation emitted from Josephson vortices moving faster than the lowest mode velocity of the stack. Numerical simulations using the coupled sine-Gordon model demonstrate downconversion of Cherenkov radiation due to the mixing of different modes.