D. Koelle

Physics and technology of high temperature superconducting Josephson junctions

The controllable fabrication of reliable HTS Josephson junctions with sufficiently small spread of their characteristic parameters has not yet been achieved and prevents the successful use of HTS Josephson junctions in complex integrated circuits. The problems in HTS junction fabrication certainly are related to the specific properties of the cuprate superconductors, which make the fabrication of high quality interfaces in HTS junctions employing artificial barrier layers extremely difficult. Therefore, several types of HTS Josephson junctions make use of so-called intrinsic interfaces originating from grain boundaries or the intrinsic layer structure of the cuprates. Beyond the fabrication technology, the physics of HTS Josephson junctions is not well understood. In particular, the detailed mechanisms of charge transport in the various junctions types and the impact of an unconventional symmetry of the superconducting order parameter are unsettled issues. We summarize the key issues regarding the physics and technology of HTS Josephson junctions and discuss possible routes to a useful HTS junction technology.

dc SQUID magnetometers from single layers of YBa2Cu3O7−x

We have fabricated magnetometers patterned in a single layer of YBa2Cu3O7−x involving dc superconducting quantum interference devices (SQUIDs) with bicrystalline grain boundary junctions. The magnetometers consist of either a SQUID with a large area square washer or a single turn pickup loop coupled directly to the body of a small area SQUID. We found that the transfer function falls off with increasing SQUID inductance much more rapidly than predicted; implications for magnetometer performance are discussed. When operated at 77 K with a bias reversal technique a directly coupled magnetometer had a noise of (105±10) fT Hz−1/2 at 1 kHz, increasing to (145±10) fT Hz−1/2 at 1 Hz.

Hot spots and waves in Bi2Sr2CaCu2O8 intrinsic Josephson junction stacks: a study by low temperature scanning laser microscopy.

Recently, it has been shown that large stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2O8 emit synchronous THz radiation, the synchronization presumably triggered by a cavity resonance. To investigate this effect we use low temperature scanning laser microscopy to image electric field distributions. We verify the appearance of cavity modes at low bias and in the high input-power regime we find that standing-wave patterns are created through interactions with a hot spot, possibly pointing to a new mode of generating synchronized radiation in intrinsic Josephson junction stacks.

0-π josephson tunnel junctions with ferromagnetic barrier

We fabricated high quality Nb/Al2O3/Ni(0.6)Cu(0.4)/Nb superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions. Using a ferromagnetic layer with a steplike thickness, we obtain a 0-pi junction, with equal lengths and critical currents of 0 and pi parts. The ground state of our 330 microm (1.3lambda(J)) long junction corresponds to a spontaneous vortex of supercurrent pinned at the 0-pi step and carrying approximately 6.7% of the magnetic flux quantum Phi(0). The dependence of the critical current on the applied magnetic field shows a clear minimum in the vicinity of zero field.

Terahertz emission and detection both based on high-Tc superconductors: Towards an integrated receiver

We have combined a stand-alone Bi2Sr2CaCu2O8 intrinsic Josephson junction stack, emitting terahertz radiation, with a YBa2Cu3O7 grain boundary Josephson junction acting as detector. The detector is mounted on a lens, positioned 1.2 cm away from the emitter on a similar lens. With the emitter radiating at 0.5 THz, we observed up to 7 Shapiro steps on the current-voltage characteristic of the detector. The ac current induced in this junction was 0.9 mA, and the dissipated power was 1.8 μW. The setup, although far from being optimized, may be considered as a first step towards an integrated high-Tc receiver.

Dynamics of Semifluxons in Nb Long Josephson 0-pi Junctions

We propose, implement, and test experimentally long Josephson 0-pi junctions fabricated using conventional Nb-AlOx-Nb technology. We show that by using a pair of current injectors one can create an arbitrary discontinuity of the Josephson phase and, in particular, a pi discontinuity, just as in d-wave/s-wave or in d-wave/d-wave junctions, and study fractional Josephson vortices which spontaneously appear. Moreover, using such junctions, we can investigate the dynamics of the fractional vortices-a domain which is not yet available for natural 0-pi junctions due to their inherently high damping. We observe half-integer zero-field steps which appear on the current-voltage characteristics due to the hopping of semifluxons.

High quality ferromagnetic 0 and π Josephson tunnel junctions

The authors fabricated high quality Nb∕Al2O3∕Ni0.6Cu0.4∕Nb superconductor-insulatorferromagnet-superconductor Josephson tunnel junctions. Depending on the thickness of the ferromagnetic Ni0.6Cu0.4 layer and on the ambient temperature, the junctions were in the 0 or π ground state. All junctions have homogeneous interfaces showing almost perfect Fraunhofer patterns. The Al2O3 tunnel barrier allows one to achieve rather low damping, which is desired for many experiments especially in the quantum domain. The McCumber parameter βc increases exponentially with decreasing temperature and reaches βc≈700 at T=2.11K. The critical current density in the π state was up to 5A∕cm2 at T=2.11K, resulting in a Josephson penetration depth λJ as low as 160μm. Experimentally determined junction parameters are well described by theory taking into account spin-flip scattering in the Ni0.6Cu0.4 layer and different transparencies of the interfaces.

Semifluxons in long Josephson 0 − π -junctions

We investigate analytically long Josephson junctions with phase

Linewidth dependence of coherent terahertz emission from Bi2Sr2CaCu2O8 intrinsic Josephson junction stacks in the hot-spot regime

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Josephson vortex in a ratchet potential: Theory

-discontinuity points. Such junctions are usually fabricated as a ramp between a superconductor such as