Showing papers by "Paul Seidel published in 2014"

Transition from Coulomb blockade to resonant transmission regime in superconducting tunnel junctions with W-doped Si barriers

Abstract: We have fabricated trilayered sandwiches consisting of superconducting electrodes made of MoRe alloy with a critical temperature of about 10 K and a silicon interlayer of thicknesses up to 20 nm doped by tungsten with atomic concentrations up to 10 at%. For concentrations below 5 at%, measurements of transport characteristics have revealed the presence of charging energy in ultra-small dopant granules (the Coulomb-blockade effect) without any supercurrent through the junction. Unexpectedly, a persistent current at zero voltage bias has exposed itself at higher W concentrations and even for the thickest W:Si layers. Microwave-radiation experiments have proven that in this case we are dealing with a Josephson current. The observation is explained as the fingerprint of 'open' channels in the charge transmission due to resonance-percolating trajectories inside the strongly inhomogeneous silicon interlayer with metallic nanoclusters. We have calculated the ratio of super- and excess currents using a universal distribution function for randomly arranged localized states and found good agreement with our experimental data without any adjustable parameters. The novel functionalities due to the disorder in doped semiconducting films make it possible to fabricate trilayered junctions with enhanced conductance properties and, at the same time, with well separated metallic electrodes.

Investigation of TiO x barriers for their use in hybrid Josephson and tunneling junctions based on pnictide thin films

Abstract: We tested oxidized titanium layers as barriers for hybrid Josephson junctions with high IcRn-products and for the preparation of junctions for tunneling spectroscopy. For that we firstly prepared junctions with conventional superconductor electrodes, such as lead and niobium, respectively. By tuning the barrier thickness, we were able to change the junction's behavior from a Josephson junction to tunnel-like behavior applicable for quasi-particle spectroscopy. Subsequently, we transferred the technology to junctions using Co-doped BaFe2As2 thin films prepared by pulsed laser deposition as base electrode and evaporated Pb as counter electrode. For barriers with a thickness of 1.5 nm, we observe clear Josephson effects with IcRn≈90 μV at 4.2 K. These junctions behave SNS'-like (SNS: superconductor-normal conductor-superconductor) and are dominated by Andreev reflection transport mechanism. For junctions with barrier thickness of 2.0 nm and higher, no Josephson but SIS'- (SIS: superconductor-insulator-superc...

The influence of external separate heating on the synchronization of Josephson junctions

Abstract: Recent experiments on synchronized radiation from intrinsic Josephson junctions in high-temperature superconductors showed that the difference of temperatures of the junctions can influence synchronization. We investigated numerically the mechainsm of this influence on the example of two overdamped Josephson junctions loaded by a shunt consisting of the resistance and the inductance. We showed that strong synchronization of radiation was obtained by means of external separate heating/cooling of junctions. Due to the change of critical currents in the process of heating or cooling, the current–voltage (I–V) characteristics of the junctions cross each other and junctions are synchronized by the ac current in the load in the vicinity of this crossing. The maximal tolerant spread of critical currents for this mechanism of synchronization is 15–20% that is twice larger than the maximal tolerant spread for this system without the separate heating. I–V characteristics of two separately heated junctions. Phase locking in this system is impossible without heating for such large spreads of critical currents.

Preparation of hybrid Josephson junctions on Co-doped Ba-122 single crystals

Abstract: In this paper we present a method for processing a hybrid Josephson junction on Co-doped (Ba-122) single crystals with a thin film Pb-counter electrode and a barrier layer of . This includes the leveling and polishing of the crystals and structuring them with thin film techniques such as photo lithography, sputtering and ion beam etching. The junctions show hysteretical resistively and capacitively shunted junction-like I?V characteristics with an -product of about 800 .

Preparation of hybrid Josephson junctions on Co-doped Ba-122 single crystals

Abstract: In this paper we present a method for processing a hybrid Josephson junction on Co-doped BaFe2As2(Ba-122) single crystals with a thin film Pb-counter electrode and a barrier layer of TiOx. This includes the leveling and polishing of the crystals and structuring them with thin film techniques such as photo lithography, sputtering and ion beam etching (IBE). The junctions show hysteretical resistively and capacitively shunted junction (RCSJ)-like I-V characteristics with an IcRn-product of about 800uV.

Phase dynamics of two parallel stacks of coupled Josephson junctions

Abstract: Two parallel stacks of coupled Josephson junctions (JJs) are investigated to clarify the physics of transitions between the rotating and oscillating states and their effect on the IV-characteristics of the system. The detailed study of phase dynamics and bias dependence of the superconducting and diffusion currents allows one to explain all features of simulated IV-characteristics and demonstrate the correspondence in their behavior. The coupling between JJ in the stacks leads to the branching of IV-characteristics and a decrease in the hysteretic region. The crucial role of the diffusion current in the formation of the IV-characteristic of the parallel stacks of coupled JJs is demonstrated. We discuss the effect of symmetry in a number of junctions in the stacks and show a decrease of the branching in the symmetrical stacks. The observed effects might be useful for development of superconducting electronic devices based on intrinsic JJs.

Bicrystalline Grain Boundary Junctions of Co-doped and P-doped Ba-122 Thin Films

Abstract: We prepared GB junctions of Ba(Fe0.9Co0.1)2As2 thin films on bicrystalline [00 l]-tilt SrTiO3 substrates. The junctions show clear Josephson effects. Electrical characterization shows asymmetric I-V characteristics which can be described within the resistively shunted junction (RSJ) model. A large excess current is observed. Their formal ICRN product is 20.2 μV at 4.2 K, which is decreased to 6.5 μV when taking Iex into account. Fabrication methods to increase this value are discussed. Additionally, measurements on GB junctions of BaFe2(As0.66P0.34)2 thin films on LSAT bicrystalline substrates are shown. Their symmetric RSJ/flux flow-behavior exhibits a formal ICRN product of 45 μV, whereas the excess corrected value is ll μV.

Influence of the spreading resistance on the conductance spectrum of planar hybrid thin film SNS' junctions based on iron pnictides

Abstract: To investigate the superconducting properties of iron pnictides we prepared planar hybrid SNS' junctions in thin film technology with a pnictide base electrode, a gold barrier layer and a lead counter electrode. Our design allows characterization of the electrodes and the junction independently in a 4-probe method. We show how both electrodes influence the measured spectra due to their spreading resistance. While the Pb electrode has a constant resistance above its Tc, the contribution of the pnictide electrode is clearly current-dependent and thus it needs a more advanced method to be corrected. We present an empirical method, which is simple to apply and allows to deal with the spreading resistance in our junctions to recalculate the actual conductance and voltage of one junction at given temperature.

Coherent emission of intrinsic Josephson junctions

Abstract: The model of synchronization of intrinsic Josephson junctions in a mesa structure of a high-temperature superconductor is developed. According to our model, the mesa structure does not consist of continuous layers but is divided into small stacks of layers. These stacks form the continuous environment which is considered as a transmission line. We showed that the conventional s-wave dynamic model for the Josephson junction and the d-wave dynamic model give nearly the same particularities in IV-characteristics. We showed also that the model describes numerically the dependence of the frequency of the coherent emission on the width of samples obtained in experiments.

The Resonant Interaction of Intrinsic Josephson Junctions With Standing Waves

Abstract: IV-characteristics of shunted microbridges made of Tl2Ba2CaCu2O8 (TBCCO) were measured. The TBCCO films were grown epitaxially on 20° misaligned LaAlO3 substrates and then covered by the gold shunting layer. Each of the microbridges consisted of about 360 shunted intrinsic Josephson junctions. Self-induced resonant particularities with a main period of about 17 mV were found on IV-curves. We showed that these particularities did not depend on the characteristics of the individual junctions and that they were connected with the geometrical dimensions of the system. The origin of particularities was the interaction of Josephson generation with the standing wave appeared in the system. We modeled this interaction considering radiation of junctions into the transmission line and found that the model described the main resonant structure of IV-characteristics. Strong synchronization of junctions appeared in the vicinity of the first particularity.

Increasing the sensor performance using Au modified high temperature superconducting YBa2Cu3O7-delta thin films

Abstract: We prepared planar, galvanically coupled gradiometers whereby the antenna structures of some of them were modified by incorporating Au nanoparticles. Gradiometers with gold modified antennas were compared with conventional ones to investigate the influence of gold induced pinning on the performance of superconducting sensor devices. We found that a local inclusion of gold nanoparticles offers the possibility of increasing the pinning of flux lines in the antenna regions, thus significantly reducing flux noise, especially in the low-frequency range. Since also the properties of grain boundary Josephson Junctions are strongly affected by Au particles, SQUID and antenna regions in gradiometric sensor devices can be separately optimized.

Shapiro Step on the rc-Branch in the IV-characteristic of a Josephson Junction with an LC-shunting

Abstract: The effect of external radiation on the phase dynamics of Josephson junctions shunted by an LC circuit is examined. It is shown that additional resonant circuit (rc) branches appear in the current voltage characteristics of the junctions when the Josephson frequency is equal to the frequency of the formed resonance circuit. We show that the amplitude dependence of the Shapiro step width crucially changes when the Shapiro step is on the rc-branch in comparison to the case of Josephson junction without shunt. The experimental implementation of these effects might give very important advantages for existing methods and technologies.