Quantifying Twist Angles in Cuprate Heterostructures Reveals Angle-Dependent Superconducting Properties

Artificially stacked, twisted van der Waals heterostructures represent a promising frontier in materials science, offering pathways to explore novel electronic states and emergent phenomena, but precise control and characterisation of the twist angle remains a significant challenge. Flavia Lo Sardo, from the Leibniz Institute for Solid State and Materials Research Dresden and Technische Universitaet Dresden, Marina Esposito of the University of Naples Federico II and the National Institute for Nuclear Physics, and Tommaso Confalone, also from IFW Dresden and Technische Universitaet Dresden, alongside Christophe Tremblay of Université de Sherbrooke, Valerii M. Vinokur from Terra Quantum AG, and Genda Gu, now demonstrate a fully non-invasive method for quantifying these critical twist angles in cuprate heterostructures. The team utilises high-resolution, polarization-resolved Raman spectroscopy to identify unique fingerprints of rotational misalignment between cuprate layers, specifically analysing the anisotropic vibrational modes of Bi2Sr2CaCu2O8+x. This approach overcomes the limitations of conventional characterisation techniques that can damage delicate twisted interfaces, offering a reliable and reproducible means to understand and control the angle-dependent properties of these advanced materials.

Artificially engineered twisted van der Waals (vdW) heterostructures are opening new avenues for exploring novel quantum phenomena and functionalities. These structures, created by stacking two-dimensional materials with a controlled twist angle, exhibit strongly correlated electronic behaviours and emergent properties not found in their individual constituents. Researchers now investigate these systems with unprecedented control, enabling possibilities for advanced electronic devices and fundamental physics research. Precise manipulation of interlayer coupling and electronic band structure through twist angle engineering allows for the creation of designer materials with tailored properties, offering a pathway towards realising novel quantum technologies. This approach represents a significant advancement in materials science and condensed matter physics, enabling exploration of previously inaccessible regimes of quantum behaviour.

Twisted 2D Superconductor Heterostructure Fabrication and Control

Research focuses on creating and controlling heterostructures by twisting two-dimensional materials, including graphene, hexagonal boron nitride, and superconducting materials, to tune their electronic properties. A key area of investigation involves integrating two-dimensional superconductors, specifically bismuth strontium calcium copper oxide (BSCCO), into practical electronic circuits. Scientists are developing heterostructures where layers can be dynamically rotated to control material properties, utilising advanced contact printing techniques for device fabrication. A significant focus lies on understanding high-temperature superconductivity in BSCCO, including controlling the arrangement of oxygen atoms within the material and investigating atomically thin BSCCO superconductors.

Researchers are exploring the role of oxygen interstitials and their ordering in influencing superconductivity, alongside charge density waves and lattice modulations. Characterization relies heavily on Raman spectroscopy to identify vibrational modes, study oxygen ordering, and analyse doping and temperature effects, while scanning tunneling microscopy images structural distortions at the atomic level. Theoretical studies, including ab initio calculations, help understand the electronic and vibrational properties of BSCCO and model oxygen ordering, aiming to develop flexible electronics and deepen understanding of high-temperature superconductivity.

Twist Angle Mapping Using Raman Spectroscopy

This work introduces a non-invasive method for determining the twist angle in artificially stacked BSCCO heterostructures using polarization-resolved Raman spectroscopy. Researchers successfully identified twist-dependent anisotropic vibrational Raman modes, focusing on the out-of-plane A1g vibrational mode of Bi/Sr at 116cm-1, to reveal clear fingerprints of rotational misalignment between cuprate layers. A customized high-resolution confocal Raman setup, equipped with polarization control and filtering, enabled reliable and reproducible measurements without compromising the material’s structural integrity. Detailed Raman spectra were acquired on BSCCO flakes ranging in thickness from 22nm to 370nm, revealing characteristic phonon features and confirming the presence of key lattice vibrations.

As flake thickness decreased, noticeable changes in the vibrational modes emerged, particularly below 60nm, demonstrating the ability to distinguish between parallel and cross-polarized spectra. Further analysis involved rotating the incident laser polarizer and analyzer, generating a color map of Raman mode intensity that highlighted the material’s anisotropic response and confirmed the sensitivity of the technique to rotational alignment. These results establish polarization-resolved Raman spectroscopy as a powerful, non-destructive method for twist-angle determination in BSCCO-based heterostructures, while also providing insight into subtle crystalline anisotropies.

Raman Spectroscopy Maps Layer Twist Angle

This work demonstrates a rapid, non-destructive, and reliable technique for determining the twist angle in artificially stacked BSCCO-based heterostructures. Researchers successfully employed Raman spectroscopy, leveraging the anisotropic angular dependence of Raman-active phonon modes, to accurately measure rotational misalignment between layers. Specifically, analysis of the out-of-plane vibrational mode at approximately 116cm-1 provided a robust and consistent indicator of twist angle, proving effective across varying flake thicknesses and even with material aging. The method was validated using simple twisted BSCCO flakes before being successfully applied to more complex heterostructures, and importantly, is not limited to materials composed of identical layers.

This versatility extends the technique to twisted heterostructures comprising dissimilar materials. The development of this Raman-based metrology provides a powerful tool for characterizing environmentally sensitive quantum materials and devices, enabling more controlled investigations of twist-dependent phenomena in low-dimensional superconductors and correlated systems. Future research directions include integrating this technique with cryogenic transport setups, which would allow for direct exploration of twist-angle-dependent superconducting, charge-density-wave, or pseudogap phenomena under controlled conditions, promising to deepen understanding of correlated physics and facilitate the engineering of novel quantum devices.