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Contour plots of symmetrized energy distribution curves (EDCs) from ARPES data for hole and electron bands of Sr2VO3FeAs. A clear gap—an indicator of superconductivity—persists above Tc for the hole band, but not the electron band.
The iron-based superconductor (FeSC) emerged as a novel platform for unconventional superconductivity in 2008. It shares several characteristics with the original unconventional superconductors, the cuprate superconductors (CuSC). Notably, FeSC exhibits antiferromagnetism in its mother compounds, with superconductivity arising upon the introduction of additional charge carriers. In contrast to cuprates, the multi-orbital nature of FeSC offers a unique opportunity to explore the role of orbital degrees of freedom in superconductivity. These orbital degrees of freedom play a crucial role in the transition from the magnetically ordered phase to superconductivity. Our focus is on identifying the signature of orbital degrees of freedom in the electronic structure of FeSC and understanding their potential role in unconventional superconductivity.
Transition metal dichalcogenides - charge density wave
Transition metal dichalcogenides (TMDs) denoted as MX2 (with M = Nb, Ta, Va, Ti, etc., and X = S, Se, Te) are a notable system where charge density waves (CDW) manifest. Similar to cuprate and iron-based superconductors, superconductivity arises when external factors like pressure or charge carrier doping disrupt the CDW phase, implying a close link between CDW (or its fluctuations) and superconductivity. Despite extensive research on these materials, the precise mechanism driving CDW formation remains elusive, and the study of the connection between CDW and superconductivity is still in its early stages. In this context, our aim is to unveil the origin of CDWs and explore potential connections to superconductivity.
Topological insulating phases and topological superconductivity
ARPES measurements show a strain-induced transition from a weak TI to a strong TI in the quasi-1D superconductor TaSe3.
Topological insulators (TIs) are unique materials that, while insulating in their bulk, can conduct electrical current on their surfaces via spin-polarized channels. These surface channels, known as topological surface states (TSS), are remarkably robust to disorder.
Materials are classified based on their topological invariant, derived from the electronic structure of the bulk. When band inversion occurs due to strong spin-orbit coupling, the topological invariant changes, potentially resulting in a TI. The TSS that emerge between such induced bulk band gaps can be directly observed using angle-resolved photoemission spectroscopy (ARPES). TIs fall into two categories: Weak TIs, which exhibit TSS only on their sides, and Strong TIs, which display TSS on all surfaces.
Topological superconductors (TSCs) are characterized by a bulk superconducting gap and surface-conducting Majorana quasiparticles. These materials have garnered significant attention due to the presence of Majorana zero modes in their vortex cores or edges, which hold great promise for fault-tolerant quantum computing. In particular, for one-dimensional TSCs, there is potential for a quantum qubit system where multiple TSCs are interconnected to manipulate the mode. Consequently, numerous TSC candidates have been proposed and studied. Some unconventional superconductors are predicted to become TSCs, although they are extremely rare and challenging to implement.
2D-TMD heterostructures - application
The field of two-dimensional (2D) materials has recently seen significant growth due to their diverse physical properties, emerging phenomena, and potential applications. Transition metal dichalcogenides (TMDs) represent a category of 2D materials, similar to graphite, characterized by van der Waals bonding, making their dimensionality easily adjustable. TMDs, in particular, exhibit exceptional potential compared to other 2D materials due to the wide range of compounds possible from various combinations of transition metals (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, ...) and chalcogens (S, Se, Te). Building upon this diversity, a novel material concept arises, involving the design of new materials based on 2D monolayers of different TMDs, known as 2D TMD heterostructures or van der Waals heterostructures. Our objective is to investigate this innovative material platform by examining the variations in the electronic structure of potential 2D-TMD heterostructures.
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