The recent discovery of the field-free Josephson diode effect in the Nb3Br8 based devices, and the unconventional superconducting behaviour in 4Hb-TaS2 [2,3] has motivated the exploration into the nature of their correlated ground state. The fingerprints of such strong electronic correlations are embedded in the electronic structure of these systems in the form renormalised energy dispersions and spectral gaps. Experimentally, identifying unique signatures of electronic correlations is challenging because both band and Mott insulators exhibit similar response in optical and transport measurements. Nevertheless, angle-resolved photoemission spectroscopy (ARPES) allows us to conveniently map the entire Fermi surface of the solid, offering direct access to the spectral function in the momentum and energy space.
In the first part of the talk, I will focus on our recent work where we describe an approach to identify the Mott insulating state of Nb3Br8 simply by observing the periodicity of the spectral weight corresponding to the band maximum along the out-of-plane momentum direction [4]. Subsequently, I will talk about 4Hb-TaS2, discussed as a candidate chiral superconductor. In this material a key question is the filling of "flat band" that emerges from the CDW reconstruction within the T layers, with numerous theories building from the existence of such correlated electrons, which might even be Mott localised near half-filling. Illustrating the importance of spatially resolved ARPES, I will show unique signatures of interlayer charge transfer in 4Hb-TaS2 that drives the flat band away from half-filling on the T termination, while fully emptying the flat band in the bulk. Our results leaving no room for electron localization [5]. In the context of superconductivity, our measurements discount all the explanations based on electronic correlations in the T layers, and in fact point towards Josephson-like coupling between separated H layers.
1. H Wu, et al., Nature 604, 653–656 (2022)
2. A. Almoalem, et al., Nat. Commun. 15 4623 (2024)
3. Y. Wang, et al., Nature, 606 896 (2022)
4. M.Date, et al, Nature Communications 16, 4037 (2025)
5. M. Date, et al., arXiv: 2508.16411 [cond-mat.supr-con] (2025)