[Paper Review] Emergent Interfacial Superconductivity between Twisted Cuprate Superconductors

Sai Zhao, Nicola Poccia+12 more|arXiv (Cornell University)|Aug 30, 2021

Physics of Superconductivity and Magnetism16 references43 citations

TL;DR

The paper demonstrates atomically sharp twisted Bi2Sr2CaCu2O8+x interfaces that form Josephson junctions with twist-angle dependent coupling, revealing d-wave pairing characteristics and emergent co-tunneling superconductivity near 45 degrees, including fractional Shapiro steps.

ABSTRACT

Twisted interfaces between stacked van der Waals cuprate crystals enable tunable Josephson coupling between in-plane anisotropic superconducting order parameters. Employing a novel cryogenic assembly technique, we fabricate Josephson junctions with an atomically sharp twisted interface between Bi2Sr2CaCu2O8+x crystals. The Josephson critical current density sensitively depends on the twist angle, reaching the maximum value comparable to that of the intrinsic junctions at small twisting angles, and is suppressed by almost 2 orders of magnitude yet remains finite close to 45 degree twist angle. Through the observation of fractional Shapiro steps and the analysis of Fraunhofer patterns we show that the remaining superconducting coherence near 45 degree is due to the co-tunneling of Cooper pairs, a necessary ingredient for high-temperature topological superconductivity.

Motivation & Objective

• Investigate how twist between Bi2Sr2CaCu2O8+x layers controls interfacial Josephson coupling.
• Probe the pairing symmetry and current-phase relation across twisted cuprate interfaces.
• Demonstrate high-quality, cryogenic, solvent-free fabrication of twist Josephson junctions with preserved interfacial superconductivity.
• Explore signatures of co-tunneling and potential topological superconducting states near 45 degrees.

Proposed method

• Fabricate atomically sharp twisted BSCCO interfaces using a cryogenic, all-dry vdW transfer technique in argon.
• Measure four-terminal I-V characteristics and extract J_C and R_N to obtain I_C R_N versus angle.
• Analyze Fraunhofer patterns under in-plane magnetic field to infer effective thickness and 2nd-harmonic CPR signatures.
• Perform microwave-induced Shapiro step measurements to identify integer and half-integer steps and their relation to the current-phase relation.
• Compare angular dependence of I_C R_N to theoretical expectations for d-wave superconductors (cos(2θ) behavior).
• Use cross-sectional STEM/AFM to confirm interface quality and structural integrity.

Experimental results

Research questions

• RQ1 How does twist angle θ between BSCCO layers modulate the Josephson critical current density J_C across the interface?
• RQ2 Does the twisted cuprate interface display d-wave pairing signatures in the Josephson transport and Fraunhofer patterns?
• RQ3 Is there evidence for co-tunneling of Cooper pairs and an emergent order parameter near θ ≈ 45°?
• RQ4 What are the Shapiro step signatures and current-phase relation near the 45° twist?
• RQ5 Can the interface maintain high-Tc superconductivity and pristine interfacial structure under cryogenic, dry-assembly conditions?

Key findings

• J_C R_N follows an angular dependence ~ |cos(2θ̃)|, consistent with incoherent tunneling between d-wave superconductors.
• Near θ̃ ≈ π/4 (45°), J_C remains finite but strongly suppressed, with Fraunhofer patterns indicating increased effective thickness due to co-tunneling.
• Fraunhofer measurements near 45° show a shorter oscillation period corresponding to an enlarged effective thickness or higher-order current components.
• Half-integer Shapiro steps emerge for twist angles within about 1° of 45°, indicating a dominant second harmonic in the current-phase relation.
• Evidence of non-monotonic I_C R_N(T) with a maximum at a finite temperature T_M for angles near 45°, supporting a competition between nodal and antinodal contributions due to d-wave SOPs.
• Josephson coupling at θ = 0° achieves a high-quality junction with J_C ~ that of intrinsic BSCCO junctions, demonstrating interface quality comparable to intrinsic junctions.