Optical Signatures Enable Probing of Buried TI-SC Interfaces and Quantum Geometry

The search for Majorana fermions, particles that are their own antiparticles, has intensified interest in proximity-induced superconductivity at the interface between topological insulators and conventional superconductors. Myungjun Kang, Yogeshwar Prasad, and Nikhil Danny Babu, all from the Department of Physics at Hanyang University, alongside colleagues from Yonsei University and Hanyang University including Jae Hoon Kim and Sangmo Cheon, have now explored a novel method for detecting these elusive states. Their theoretical work investigates the optical signature of these buried interfaces, offering a way to bypass the limitations of traditional surface-sensitive techniques. This research demonstrates that analysing the longitudinal optical response can directly reveal the superconducting gap at the interface, and crucially, link it to the underlying quantum geometry supporting Majorana edge modes. The findings suggest terahertz and infrared spectroscopy could become a vital, non-invasive tool for identifying and characterising materials hosting these potentially revolutionary particles.

Probing the interfacial superconductivity at buried interfaces presents a significant challenge when utilising conventional surface methods. This work presents a theoretical study of the longitudinal optical response of a TI-SC heterostructure, focusing on the complex interface sheet conductance as a direct and layer-selective probe of the interfacial superconducting gap. Within a minimal TI, SC model, the researchers demonstrate that proximity-induced superconductivity can be effectively investigated through this optical approach.

The approach involves modelling the optical response of the heterostructure, specifically examining how light interacts with the interface sheet conductance, allowing for a layer-selective analysis. By employing a minimal TI, SC model, the study aims to isolate and understand the fundamental physics governing the interfacial superconductivity. A specific contribution of this work is the demonstration of the complex interface sheet conductance as a quantifiable indicator of the interfacial superconducting gap, providing a pathway to experimentally probe this quantity using longitudinal optical spectroscopy. Furthermore, the model clarifies how the optical response is directly linked to the proximity-induced superconducting state at the interface, offering a new avenue for investigating and ultimately controlling topological superconductivity in these heterostructures.

Interface Conductance Probes Topological Superconductivity Emergence Researchers are

The study pioneers a theoretical approach to investigate proximity-induced superconductivity at interfaces between topological insulators and superconductors, crucial for realising topological superconductivity and potential Majorana boundary modes. Researchers addressed the challenge of probing buried interfaces, typically inaccessible with standard surface techniques, by focusing on the longitudinal optical response of the heterostructure. This work centres on the complex interface sheet conductance as a direct, layer-selective method for characterising the interfacial superconducting gap.

Scientists employed a minimal topological insulator-superconductor model, demonstrating that proximity-induced superconductivity generates a two-dimensional superconducting phase capable of supporting Majorana edge modes. Calculations of optical conductance were performed using a Bogoliubov-de Gennes slab model coupled with the Kubo formalism, a sophisticated technique for determining the linear response of a many-body system to external fields. A novel thickness-extrapolation protocol was introduced, enabling the isolation of the interface contribution to the overall optical signal and allowing for precise analysis of the buried interface. The resulting interface conductance exhibited a robust, thickness-independent coherence peak at an energy determined by the proximity-induced superconducting gap, clearly distinguishing it from the parent superconductor’s pair-breaking behaviour and the ungapped Dirac cone present on the top surface. Further analysis revealed that the low-frequency spectral weight of this interface resonance adheres to a quantum-metric sum rule, establishing a quantitative link between the optical response and the geometry of the proximitized interfacial state. This innovative connection highlights the importance of quantum geometry in understanding the emergent properties of these heterostructures, proposing terahertz and infrared spectroscopy of the interfacial sheet conductance as a non-invasive diagnostic tool for identifying Majorana-hosting topological insulator-superconductor interfaces.

Interface Superconductivity and Majorana Mode Potential

Scientists have demonstrated proximity-induced superconductivity at the interface of a topological insulator and a conventional superconductor, revealing a pathway to realizing Majorana boundary modes. The research team focused on the longitudinal optical response of this heterostructure, employing a novel method to directly probe the interfacial superconducting gap. Experiments utilising a Bogoliubov-de Gennes slab model and the Kubo formalism successfully isolated the interface contribution to the optical conductance, revealing a robust coherence peak at an energy determined by the proximity-induced gap.

This peak is clearly distinguishable from features originating in the bulk superconductor or the ungapped Dirac cone on the top surface, demonstrating the emergence of a two-dimensional superconducting phase supporting Majorana edge modes within the heterostructure. The team measured a thickness-independent coherence peak in the interface conductance, confirming its robustness and providing a direct signature of interfacial superconductivity. Further analysis revealed that the low-frequency spectral weight of this interface resonance adheres to a quantum-metric sum rule, quantitatively linking the optical response to the geometry of the proximitized interfacial state. This connection establishes a fundamental relationship between the observed optical properties and the underlying quantum characteristics of the superconducting interface. The study establishes terahertz/infrared spectroscopy of the interfacial sheet conductance as a non-invasive diagnostic tool for identifying Majorana-hosting topological insulator-superconductor interfaces, delivering a method for probing buried interfaces and overcoming limitations of conventional surface techniques.

Interface Superconductivity and Quantum Weight Measurement

This work demonstrates the realisation of a two-dimensional topological superconducting state at a buried interface within a topological insulator-superconductor heterostructure. Researchers have shown this state is spectroscopically accessible through measurement of the interface sheet conductance, offering a new method for its characterisation. By combining the Kubo formalism with a thickness scaling technique, they successfully isolated an optical response originating solely from the interface, identifying a resonance directly linked to the proximity-induced superconducting gap.

Furthermore, the interface conductance was found to be quantitatively governed by a quantum-metric sum rule, with the negative first moment of the optical response directly measuring the quantum weight of the gapped Dirac surface state. This establishes a conceptually new optical pathway for diagnosing emergent topological superconductivity and the physics underpinning Majorana modes in these heterostructures, complementing existing transport-based techniques. The authors acknowledge that their model employs a minimal description of the heterostructure, and future work should explore the impact of more complex band structures. Experimental verification of these findings is proposed through terahertz time-domain spectroscopy and infrared measurements on thin-film series, specifically looking for a thickness-independent conductance resonance at the scale of the induced interfacial gap. Additionally, the application of magnetic fields to tune time-reversal symmetry is suggested as a means to investigate Majorana hybridization and its controllability, potentially advancing the field of topological quantum information processing.