Evidence for unconventional superconductivity in twisted bilayer graphene

Abstract: The emergence of superconductivity with doping from correlated insulators in magic-angle twisted bilayer graphene (MATBG) has raised the intriguing possibility that its pairing mechanism is distinct from that of conventional superconductors, as described by the Bardeen-Cooper-Schrieffer (BCS) theory. While there is now ample evidence for strong electronic correlations in MATBG, recent studies have claimed that unlike correlated insulators, superconductivity persists even when these interactions are partially screened. This suggests that the pairing in MATBG might be conventional in nature, a consequence of the large density of states of its nearly flat bands, perhaps phonon-mediated as in BCS superconductors. Here we combine tunneling and Andreev reflection spectroscopy with the scanning tunneling microscope (STM) to observe several key experimental signatures for unconventional superconductivity in MATBG. We show that the tunneling spectra below the transition temperature $T_c$ are inconsistent with that of a conventional s-wave superconductor, but rather resemble that of a nodal superconductor with an anisotropic pairing mechanism. We observe a large discrepancy between the tunneling energy gap $\Delta_T$, which far exceeds the mean-field BCS ratio (with $2\Delta_T/k_BT_c \sim 25$) and the energy gap $\Delta_{AR}$ extracted from Andreev reflection spectroscopy ($2\Delta_{AR}/k_BT_c \sim 6$). The tunneling gap persists even when superconductivity is suppressed, indicating its emergence from a pseudogap phase, with a suppressed density of states at the Fermi level. Moreover, the pseudogap state and superconductivity are both absent when MATBG is aligned with the hexagonal boron nitride (hBN) underneath. These findings and other observations reported here provide a preponderance of evidence for a non-BCS mechanism for superconductivity in MATBG.

• The paper demonstrates experimental evidence for non-BCS superconductivity in MATBG, highlighted by a high 2ΔT/k_BT_c ratio of approximately 25.
• It employs density-tuned scanning tunneling and point-contact spectroscopy under dilution-refrigerator conditions to map carrier-dependent superconducting gaps.
• Results reveal that substrate alignment with hBN suppresses superconductivity, underscoring the role of structural symmetry in stabilizing the superconducting phase.

Unconventional Superconductivity in Twisted Bilayer Graphene

This paper presents experimental evidence for unconventional superconductivity in magic-angle twisted bilayer graphene (MATBG). The study employs advanced techniques such as tunneling and Andreev reflection spectroscopy combined with scanning tunneling microscopy (STM) to explore the superconducting properties of MATBG.

Experimental Methodology

The authors conducted a series of experiments using a homebuilt dilution-refrigerator STM to perform density-tuned scanning tunneling and point-contact spectroscopy (DT-STS and DT-PCS). These techniques allowed for precise control and measurement of carrier densities in MATBG. The experiments focused on the electronic behavior below the superconducting transition temperature $T_c$ and across various carrier densities to capture the nuances of superconductivity in this material.

Unconventional Superconductivity Observed

The analysis of the tunneling spectra reveals deviations from conventional s-wave superconductivity, typically described by Bardeen-Cooper-Schrieffer (BCS) theory. Instead, the spectra resemble those of a nodal superconductor with an anisotropic pairing mechanism. Indicative features such as large tunneling gaps ($\Delta_T$) persisting even when superconductivity is suppressed, and a high 2$\Delta_T$/$k_BT_c$ ratio of approximately 25, point towards non-BCS characteristics. Intriguingly, this tunneling gap persists when MATBG is aligned with hexagonal boron nitride (hBN), highlighting the essential role of substrate alignment in the emergence of superconductivity.

Key Findings and Implications

The research identifies two distinct energy scales: one from tunneling ($\Delta_T$) and another from Andreev reflection ($\Delta_{AR}$). These scales further confirm the unconventional nature of superconductivity in MATBG, with $\Delta_T$ significantly larger than $\Delta_{AR}$. Notably, the pseudogap phase, a precursor to superconductivity, emerges above $T_c$ and remains unaffected by moderate magnetic fields, suggesting phase coherence without full superconductivity at these elevated conditions.

The study also explores the influence of substrate alignment. In MATBG aligned with hBN, neither superconductivity nor the pseudogap phase is observed, indicating that the structural characteristics and possibly the C2T-symmetry of unaligned MATBG are crucial for stabilizing these phases.

Future Directions

The results underscore the complexity underlying the superconducting mechanisms in MATBG and point to the need for theoretical models beyond the conventional BCS framework. Future work could focus on understanding the precise nature of the anisotropic pairing mechanism and the role of potential bosonic modes in mediating superconductivity. Additionally, exploring other materials with similar moiré patterns or alignments could reveal more about the interplay between structural characteristics and electronic properties.

Conclusion

This paper contributes significant experimental evidence challenging the conventional understanding of superconductivity. By offering insights into the unique properties of MATBG, it opens new avenues for researching unconventional superconductivity and understanding the emergent phenomena in two-dimensional materials.