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Physical review. B./Physical review. B

Josephson effect through YBa2Cu3O7−δ /Au-encapsulated nanogaps

Reza Baghdadi, Simon Abay, Dmitry S. Golubev, Thilo Bauch, Floriana Lombardi

Published May 17, 2017• 10 citations
Publisher

Abstract

We successfully fabricated and studied planar superconductor-normal-superconductor (SNS) Josephson junctions made of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) high-${\mathrm{T}}_{C}$ superconductor (S) and Au normal metal (N). The crucial step in fabrication was done by using a Pt/Au bilayer encapsulation of the patterned YBCO nanogap. This procedure preserves the YBCO nanogap from the degradation due to the contact with resists, water, and chemicals involved in further processing for hybrid devices. The junctions exhibit Fraunhofer-like modulation patterns and Shapiro steps when they are irradiated by a rf field. Furthermore, we analyzed the ${I}_{C}(T)$ characteristics of our hybrid junctions employing a ${\mathrm{SINI}}^{\ensuremath{'}}\mathrm{S}$ (superconductor--insulator--normal conductor--insulator--superconductor) model.

DOI
10.1103/physrevb.95.174510

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Planar S/N/S Josephson junctions were successfully fabricated by multi-layered thin films of Bi-based high-Tc oxide superconductors. Two Bi2Sr2Ca1Cu2O(x) films and Bi2Sr2Cu1O(y) film were successively layered as superconducting electrodes and normal conducting buffer layer, respectively. These films were epitaxially deposited on (100) MgO substrate by in situ process of RF magnetron sputtering. Microjunction areas were defined by photolithography and Ar ion milling. These S/N/S type junctions clearly exhibited the AC Josephson effect under the irradiation of radio frequency waves of 12 GHz. More than 20th Shapiro steps were observed and those step heights were oscillated according to the irradiation power. These characteristics were stable and reproducible for different configuration of function area.
We have fabricated step edge superconductor-normal metal-superconductor microbridges using YBa2Cu3O7−x(YBCO) and noble metals with critical current-normal resistance (IcRN) products as high as 10 mV and normal resistances up to 38 Ω. Our fabrication process achieves high values of the IcRN product by exploiting the anisotropy in the properties of epitaxial YBCO films, allowing contact only between normal metal and superconductor through the crystalline axes which support the largest Josephson coupling. This results in a dramatic increase in the normal resistance of a junction without decreasing its critical current. We discuss the role of the superconductor-normal metal boundary resistance on the junction electrical properties. We have coupled submillimeter wave rf currents quasioptically into junctions integrated at the feeds of noble metal planar log periodic antennas and have induced up to 7 Shapiro steps in the current-voltage characteristics with a 760 GHz beam from a far infrared laser.
Superconducting electrodes are an integral part of hybrid Josephson junctions used in many applications including quantum technologies. We report on the fabrication and characterization of superconducting hybrid Au/YBa2Cu3O7-x (YBCO) electrodes on vicinal substrates. In these structures, superconducting CuO2-planes face the gold film, resulting in a higher value and smaller variation of the induced energy gap compared to the conventional Au/YBCO electrodes based on films with the c-axis normal to the substrate surface. Using scanning tunneling microscopy, we observe an energy gap of about 10-17 meV at the surface of the 15- nm-thick gold layer deposited in situ atop the YBCO film. To study the origin of this gap, we fabricate nanoconstrictions from the Au/YBCO heterostructure and measure their electrical transport characteristics. The conductance of the nanoconstrictions shows a series of dips due to multiple Andreev reflections in YBCO and gold providing clear evidence of the superconducting nature of the gap in gold. We consider the Au/YBCO electrodes to be a versatile platform for hybrid Josephson devices with a high operating temperature.
The integration of ferroelectric barriers into high-temperature Josephson junctions offers a pathway to tunable superconducting quantum devices. Here, we demonstrate robust Josephson coupling in YBa2Cu3O7-x (YBCO)/Bi0.5Na0.5TiO3 (BNT)/YBCO trilayer junctions incorporating a 40 nm ferroelectric BNT barrier. Epitaxial trilayers were fabricated by pulsed laser deposition on SrTiO3 substrates and investigated under broadband terahertz (THz) irradiation (0.5-2.5 THz). At 1.5 THz, close to the Josephson plasma resonance, the BNT polarization increased from 33.4 to 46.4 uC/cm2 at 30 K, enhancing superconducting transport. The junctions exhibited a well-defined zero-voltage supercurrent branch, with the critical current Ic(T) remaining nearly constant up to 60 K and reaching 468.2 uA under 1.5 THz excitation, evidencing strong phase coherence. Scanning tunneling spectroscopy revealed an enhanced superconducting gap in YBCO/BNT/YBCO (Delta = 1.3 meV) compared with pure YBCO (Delta = 1.05 meV), while optical conductivity measurements showed a reduction in conductivity and gap with increasing temperature, consistent with BCS theory under THz excitation. Atomic force microscopy and X-ray reflectivity confirmed uniform morphology and sharp interfaces, excluding defect-mediated transport. Magnetic field modulation of Ic(B) exhibited a canonical Fraunhofer interference pattern, and resistance mapping revealed alternating lobes of high and low dissipation, indicating coherent Josephson tunneling. These results establish that Josephson coupling is intrinsic to the YBCO/BNT/YBCO junctions, enabled by the dipolar character and dynamic THz response of the BNT barrier. This study identifies BNT as a viable, tunable barrier material for next-generation high-Tc superconducting quantum devices.
Abstract Superconducting electrodes are an integral part of hybrid Josephson junctions used in many applications including quantum technologies. We report on the fabrication and characterization of superconducting hybrid Au/YBa2Cu3O7−x (YBCO) electrodes on vicinal substrates. In these structures, superconducting CuO2-planes face the gold film, resulting in a higher value and smaller variation of the induced energy gap compared to conventional Au/YBCO electrodes based on films with the c-axis normal to the substrate surface. Using scanning tunneling microscopy, we observe an energy gap of about 10–17 meV at the surface of the 15- nm-thick gold layer deposited in situ atop the YBCO film. To study the origin of this gap, we fabricate nanoconstrictions from the Au/YBCO heterostructures and measure their electrical transport characteristics. The conductance of the nanoconstrictions shows a series of dips due to multiple Andreev reflections in YBCO and gold, providing clear evidence of the superconducting nature of the gap in gold. We consider the Au/YBCO electrodes to be a versatile platform for hybrid Josephson devices with a high operating temperature.
A new fabrication method for superconductor-normal metal-superconductor junctions employing focused ion beam patterning of a superconductor-normal metal bilayer has been used to make series junction arrays. We investigated their current-voltage (I-V) characteristic and examined their response to microwaves at 4.2 K. Multiple integer Shapiro steps were observed in the I-V response of an array to microwaves. In addition, we found that the properties of each junction in a double-junction series array did not change in the junction spacing range studied (0.2-8 µm). Modelling the I-V characteristic of large arrays showed the junction variation of the critical current was 20%.