High-temperature-superconducting ramp-edge Josephson junctions operating at 77 K combine a SmBa2Cu3Oy (SmBCO) base electrode with a La-doped ErBa2Cu3Oy counter-electrode, with the Josephson coupling controlled by a nanometre-scale interfacial layer that forms spontaneously during fabrication. A 2016 electron-microscopy study of these junctions found a roughly 1.5-nm structurally disordered interfacial layer that is Sm-enriched and Cu-depleted, and proposed a four-step formation mechanism (preferential Cu sputtering during Ar-ion milling, thermally driven Sm-oxide segregation, a Cu-poor precursor layer, and trapping of excess ions during counter-electrode growth). However, its energy-dispersive X-ray and dark-field imaging data could not determine the layer's chemical state or phase identity, the mechanism was not tested by step-resolved experiments, and only single-chip SQUID numbers were reported. The open problem is to establish what the barrier is chemically and structurally, how and why the fabrication steps produce it, and how its structure and composition determine junction electrical metrics such as IcRnI_cR_n and magnetic modulation at 77 K. No subsequent study has determined the layer's phase; the only substantive successor study (2021, same group) examined deliberately deposited barrier materials and suggested, but did not determine, the phase formed there.

Background

Rare-earth barium cuprates of the YBa2Cu3Oy family (REBCO, where the rare-earth site can be occupied by Y, Er, Sm and others) are high-temperature superconductors with a superconducting transition above the boiling point of liquid nitrogen (77 K), which makes them attractive for superconducting electronic devices that do not require liquid-helium cooling. A Josephson junction — two superconductors separated by a thin non-superconducting barrier that supports Cooper-pair tunnelling — is the active element of a SQUID (superconducting quantum interference device), an extremely sensitive magnetometer used, for example, in nondestructive evaluation of steel structures. In the ramp-edge geometry, the base electrode is etched into a shallow ramp and the counter-electrode film is grown across it, so the two electrodes couple through the processed ramp surface. Because cuprate superconductors are layered ceramics whose superconductivity collapses when the crystal structure is disturbed, the useful barrier in these junctions is a nanometre-scale interfacial layer whose nature is set by the fabrication sequence itself rather than by a separately deposited, well-characterized tunnel insulator.

This question has a history. Transmission-electron-microscopy (TEM) studies of interface-modified high-Tc junctions showed early on that a structurally disordered, compositionally altered layer forms at the processed interface (Wen et al., 1999), and analysis of ramp-edge junctions with La-doped 123-type electrodes found rare-earth enrichment and Cu depletion in the interfacial layer and, for that system, proposed that the barrier consists of a thin layer of a cubic perovskite (Ba,La)(Y,Yb,Cu)O3 (Adachi et al., 2003) — the same enrichment tendency later seen in the junctions studied here, whose Sm-rich barrier has no determined phase. Depositing a Cu-poor precursor layer before growing the counter-electrode was found empirically to raise the product of critical current and normal-state resistance, IcRnI_cR_n, the key figure of merit for the voltage output of a SQUID (Wakana et al., 2007); the group's review of the device program (Adachi et al., 2012) summarized ramp-edge junction technology with the barrier microphysics still unsettled.

The source paper of the present problem (Adachi, Tsukamoto, Oshikubo and Tanabe, Physica C 530, 2016) examined ramp-edge junctions with a SmBa2Cu3Oy (SmBCO) base electrode and a La-doped ErBa2Cu3Oy counter-electrode of nominal composition La0.1Er0.95Ba1.95Cu3Oy, designed for 77-K SQUID operation. Cross-sectional imaging in the scanning-TEM mode called HAADF (high-angle annular dark-field, whose contrast scales with atomic number and resolves the atomic lattice) showed that the ordered triple-perovskite lattice of both electrodes is absent in a continuous interfacial layer about 1.5 nm thick. Spot-resolved energy-dispersive X-ray spectroscopy (STEM-EDX, which measures local elemental composition from characteristic X-rays excited by a roughly 0.2-nm electron probe) indicated that this layer is enriched in Sm and depleted of Cu relative to the adjacent electrodes (reported atomic ratio Ba:La:Sm:Er:Cu of about 23:1:25:10:41). The chip piece from which the TEM specimen was taken had been characterized as a dc SQUID at 77 K, with reported parameters 2Ic = 30 microampere, Rn/2 = 5.3 ohm (Rn the normal-state resistance) and IcRnI_cR_n of about 160 microvolt. The authors proposed a stepwise barrier-formation mechanism: Ar-ion milling (etching the ramp surface with an argon-ion beam) of the SmBCO ramp preferentially sputters Cu and leaves a damaged, Cu-depleted surface; subsequent heating drives surface reconstruction in which formation of Sm oxide (e.g. Sm2O3) is thermodynamically favoured over Ba and Cu oxides; the deliberately Cu-poor precursor layer maintains the Cu deficiency; and during counter-electrode growth and anneal the excess or displaced ions are trapped at the interface, producing the thin, disordered, Sm-rich and Cu-poor layer. The paper's data, however, are EDX composition ratios and dark-field contrast — they do not determine chemical state (oxidation state, bonding) or phase identity; the mechanism is inferred from the process sequence rather than measured; and the electrical data are a single-chip characterization, not a structure–property relationship.

Problem Statement

For ramp-edge Josephson junctions made of a SmBa2Cu3Oy (SmBCO) base electrode and a La0.1Er0.95Ba1.95Cu3Oy counter-electrode, designed as dc SQUIDs operating at 77 K, the problem is to establish:

(1) Chemical and phase identity of the barrier: determine the atomic-scale chemical state and phase identity of the roughly 1.5-nm disordered interfacial layer — for example, whether the Sm-rich region is predominantly an Sm-oxide phase such as Sm2O3, an amorphous Sm–Ba–Cu–O mixed oxide, or another phase — going beyond energy-dispersive X-ray (EDX) composition ratios and dark-field contrast.

(2) Formation mechanism: establish experimentally, by isolating each fabrication step, the causal role of (i) preferential sputtering of Cu during Ar-ion milling of the ramp, (ii) temperature-driven surface segregation and reconstruction of SmBCO during heating in oxygen, (iii) the efficacy and composition control provided by the Cu-poor precursor layer, and (iv) trapping of excess ions during counter-electrode deposition and anneal.

(3) Structure–property relationship: determine quantitatively how the barrier's structure and composition (the measured Sm enrichment and Cu depletion, and whatever phase is identified in point 1) set the Josephson electrical properties at 77 K — critical current, normal-state resistance, IcRnI_cR_n product, and magnetic modulation of SQUIDs built from these junctions.

These three questions are scientifically linked: without the barrier's chemical identity the formation mechanism cannot be pinned, and without both, the link to junction metrics cannot be made predictive. An answer that determines the barrier's identity, validates or refutes the formation mechanism with step-resolved measurements, and quantitatively accounts for the junction electrical metrics resolves the problem.

References

  1. S. Adachi, A. Tsukamoto, Y. Oshikubo, K. Tanabe, 'Barrier structure of ramp-edge-type Josephson junction made of La-doped ErBCO and SmBCO electrodes', Physica C: Superconductivity and its Applications 530 (2016) 79-81. DOI: 10.1016/j.physc.2016.02.007, https://doi.org/10.1016/j.physc.2016.02.007
  2. S. Adachi, A. Tsukamoto, Y. Oshikubo, T. Hato, K. Tanabe, 'Fabrication of HTS ramp-edge-type Josephson junctions and SQUIDs using various YBCO-derivatives as barrier materials', Japanese Journal of Applied Physics 60 (2021) 093001. DOI: 10.35848/1347-4065/ac1c8e, https://doi.org/10.35848/1347-4065/ac1c8e
  3. J. G. Wen, N. Koshizuka, S. Tanaka, T. Satoh, M. Hidaka, S. Tahara, 'Atomic structure and composition of the barrier in the modified interface high-Tc Josephson junction studied by transmission electron microscopy', Applied Physics Letters 75 (1999) 2470-2472. DOI: 10.1063/1.125051, https://doi.org/10.1063/1.125051
  4. S. Adachi, H. Wakana, Y. Wu, Y. Ishimaru, Y. Tarutani, K. Tanabe, 'Structure and formation mechanism of interface-modified layer in ramp-edge Josephson junctions with La-doped 123-type superconducting electrodes', IEEE Transactions on Applied Superconductivity 13 (2003) 877-880. DOI: 10.1109/TASC.2003.814075, https://doi.org/10.1109/TASC.2003.814075
  5. H. Wakana, S. Adachi, K. Nakayama, K. Tsubone, Y. Tarutani, K. Tanabe, 'Fabrication of ramp-edge junctions with high IcRn products by using Cu-poor precursor', IEEE Transactions on Applied Superconductivity 17 (2007) 233-236. DOI: 10.1109/TASC.2007.898694, https://doi.org/10.1109/TASC.2007.898694
  6. S. Adachi, A. Tsukamoto, T. Hato, J. Kawano, K. Tanabe, 'Recent developments of high-Tc electronic devices with multilayer structures and ramp-edge Josephson junctions', IEICE Transactions on Electronics E95-C (2012) 337-346. DOI: 10.1587/transele.e95.c.337, https://doi.org/10.1587/transele.e95.c.337
  7. R. Wang, X. Shi, B. Xiang, J. Lin, X. Cai, Z. Cao, X. Zhang, X. Lin, 'High-Temperature Superconducting SQUIDs: From Principles and Fabrication to Applications - A Comprehensive Review', Advanced Quantum Technologies 9 (2026) e00595. DOI: 10.1002/qute.202500595, https://doi.org/10.1002/qute.202500595
  8. A. Sugimoto, I. Iguchi, 'Observation of local magnetic distribution in ramp-edge Josephson junctions', Journal of Physics Communications 1 (2017) 025009. DOI: 10.1088/2399-6528/aa8aa1, https://doi.org/10.1088/2399-6528/aa8aa1