Ramp-Edge Josephson Junctions Using Barriers of Various Resistivities

Masahiro HORIBE  Koh-ichi KAWAI  Akira FUJIMAKI  Hisao HAYAKAWA  

IEICE TRANSACTIONS on Electronics   Vol.E81-C   No.10   pp.1526-1531
Publication Date: 1998/10/25
Online ISSN: 
Print ISSN: 0916-8516
Type of Manuscript: INVITED PAPER (Special Issue on Low- and High-Temperature Superconductive Electron Devices and Their Applications)
Category: High-Tc Junction Technology
ramp-edge Josephson junction,  Ga doping,  Ca doping,  direct tunneling,  resonant tunneling,  

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We have studied the effect of Ga and Ca substitution in the PrBa2Cu3Oδ (PBCO) barrier on the parameters of high-temperature-superconductor ramp-edge Josephson junctions. Pr 1-XCa XBa2Cu3Oδ (X=0. 15, 0. 3) had reduced bulk barrier resistivity as small as 10 mΩcm which was close to the metal-insulator transition. Also, PrBa2Cu 3-ZGa ZOδ, written as GaZ-doped PBCO (Z=0. 15, 0. 3, 0. 6), had enhanced resistivity neater than 1 kΩcm at 4. 2 K. The transport mechanisms in these bulk barriers fitted well with the Mott variable hopping model. The critical current density Jc and normalized junction conductance (R nA)-1 decayed exponentially with almost the same decay length, as the barrier thickness increased. The decay length depended on the barrier material, and ranged from 1. 7 nm to 6. 5 nm for Jc, from 1. 9 nm to 7. 2 nm for (Rn A)-1. Because on these experimental results, we conclude that direct tunneling is the dominant transport mechanism for both quasi particles and paired particles in our junctions, while resonant tunneling should be considered as an additional transport mechanism of these two kinds of particles in the junctions with the PBCO-based barriers reported so far. It was also found that Ga doping raised the characteristic voltage Vc while Ca doping reduced it, though the Vc values obtained here were still small compared to the theoretically predicted values. The spacewise metal insulator transition at the interfaces caused by a high density of localized states in the barriers seemed to be responsible for the reduction in Vc. The best Vc value was 0. 32 mV at 77 K and 5. 2 mV at 4. 2 K using a Ga0. 6-PBCO barrier. These Vc values are suitable for electronics applications. Furthermore, superconducting-gap-like structures were observed in the junctions with highly resistive Ga-doped PBCO barriers.