Unveiling the nature of superconductivity in moiré quantum matter

Project Information

mKoire

Grant agreement ID: 101087014

DOI

10.3030/101087014

EC signature date 6 June 2023

Start date 1 October 2023

End date 30 September 2028

Funded under

• European Research Council (ERC)

Total cost

€ 2 802 250,00

EU contribution

€ 2 802 250,00

Coordinated by

FUNDACION DONOSTIA INTERNATIONAL PHYSICS CENTER
Spain

Periodic Reporting for period 1 - mKoire (Unveiling the nature of superconductivity in moiré quantum matter)

Reporting period: 2023-10-01 to 2026-03-31

Summary of the context and overall objectives of the project

Graphene moiré materials constitute a new paradigm in condensed matter physics for the study of superconductivity and other collective electronic phases given their chemical and structural simplicity as well as large tunability as compared to other many-body systems. However, a comprehensive picture of the fundamental properties of superconductivity in graphene moiré materials is still missing due to the limited information attained so far at the microscopic level. Therefore, the need for local probe techniques that enable the exploration of superconductivity in moiré matter with outstanding energy and spatial resolution is pressing, and constitutes the main motivation of this project.

The ultimate objective of this project is to unveil the microscopic mechanisms driving superconductivity in graphene moiré materials. To this end, I propose the local characterization of the superconductivity in various twisted graphene systems with unprecedented resolution (few µeV) by means of STM/STS measurements at milikelvin temperatures. Here I aim to unveil (i) the symmetry of the order parameter(s) in unambiguous connection with the valley, orbital and spin degrees of freedom, (ii) the origin(s) of the attractive interactions governing Cooper pairing, and (iii) the nature of the neighboring correlated phases as well as their interplay with the superconducting state.

The intellectual merit of this project lies in the fact that will provide ground-breaking insights to long-standing questions in condensed matter physics, initially posed in complex unconventional superconductors, here instead tackled using simple, highly tunable moiré materials. Furthermore, this project has the potential to unveil forms of superconductivity never seen in nature before by using state-of-the-art microscopy technology that combines atomic-scale spatial resolution with the finest energy resolution.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

To date, we have established the full experimental platform required to realize the project’s objectives. We designed and fabricated dedicated NaviGate microstructured chips to host twisted micron-scale graphene heterostructures. These chips integrate a global backgate for electrostatic gating and biasing, together with micron-scale navigation markers. This architecture enables rapid and reliable localization of the twisted devices with the STM tip in the tunneling regime, the application of gate and bias voltages, and the systematic calibration of tip apexes for STM/STS measurements. In parallel, we set up a dedicated nanofabrication laboratory for twisted graphene heterostructures, including the design of the lab space and the purchase and installation of key equipment. The lab was fully installed and became operational in summer 2024. In addition, we designed the ultra-quiet STM room in the new DIPC experimental building to host the mK-STM, imposing stringent requirements on acoustic noise (< 35 dB), electromagnetic fields (< 20 nT) and mechanical vibrations (< 500 nm/s), thus ensuring noise-free, high-resolution STM/STS measurements beyond current standards.

On the instrumentation side, we completed the custom design, installation and setup of a mK-UHV-STM system. The system operates at base temperatures down to 19 mK in ultra-high vacuum conditions, and is optimized for atomic-resolution imaging and spectroscopy of micron-sized twisted graphene moiré devices under electrostatic gating and vector magnetic fields. Particular effort has been devoted to minimizing RF noise to reach an energy resolution of 15 µeV, which is essential for the project’s scientific goals. In parallel, we devoted substantial effort to the optimized fabrication of our own twisted graphene devices (bilayer, trilayer and tetralayer). Over more than one year, we systematically compared fabrication recipes, substrates and chemical treatments, and evaluated device quality and cleanliness using optical microscopy, AFM, Raman spectroscopy and room-temperature STM. As a result, we now routinely produce high-quality twisted graphene heterostructures integrated onto gateable NaviGate chips, providing a robust and reproducible device base for the forthcoming mK-STM studies

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The integration of twisted graphene heterostructures into NaviGate chips, a microfabricated back-gated platform with a binary-encoded navigation pattern for STM access to twisted micron-scale heterostructures, represents a clear advance beyond the state-of-the-art. To the best of our knowledge, no previous platform combines deterministic STM access, electrostatic gating and encoded spatial coordinates specifically optimized for micron-sized twisted devices. The NaviGate architecture will remain a central workhorse for the remainder of the project and is readily adaptable to other families of 2D and layered materials. Importantly, its layout has been conceived to be fully compatible with both our sub-Kelvin (300 mK) STM and the mK-STM, allowing the same device platform to be interrogated across our complete low-temperature STM infrastructure.

In addition, the combination of an ultra-quiet STM room with the custom mK-UHV-STM (19 mK, vector magnetic fields, RF-noise-optimized for ~15 µeV resolution) also pushes well beyond standard low-temperature STM capabilities and positions the laboratory at a genuinely world-leading level for spectroscopic studies of correlated moiré phases. While the development of advanced infrastructure and device platforms was planned, the specific NaviGate concept (with binary navigation) and the level of performance achieved in terms of stability, energy resolution and device yield have exceeded the original expectations and can be considered partially unplanned outcomes of the project.