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Linking atomic-scale properties of 2D correlated materials with their mesoscopic transport and mechanical response

Periodic Reporting for period 2 - LINKSPM (Linking atomic-scale properties of 2D correlated materials with their mesoscopic transport and mechanical response)

Reporting period: 2020-04-01 to 2021-09-30

Two-dimensional (2D) materials’ properties are highly susceptible to external perturbations. This presents tremendous new opportunities for manipulating the behavior of novel 2D layered materials and, ultimately, achieving unprecedented control over their performance when integrated into highly specific functional devices. However, strategies that enable such control are sorely lacking to date and remain an outstanding challenge for the materials science community. Progress here requires of a comprehensive microscopic picture of the fundamental properties of 2D materials in clear connection to their macroscopic behavior, a knowledge that is still missing due to the lack of experimental techniques that simultaneously probe multiple length regimes.

In LINKSPM, we aim at achieving control over the electronic ground state of 2D materials via external stimuli such as strain and electromagnetic fields. To this purpose, we focus on strongly correlated 2D materials as they exhibit rich electronic phase diagrams with diverse collective electronic phases such as superconductivity and charge density wave order. Therefore, the project’s first goal will be to unveil their phase diagram of the key 2D materials in the monolayer limit. Subsequently, we will investigate the interplay between these external stimuli and microscopic electronic phases, and to unambiguously correlate them with mesoscopic electrical transport and mechanical response.

This project comprises three research thrusts: (i) Development of new instrumentation that provides a direct way to correlate atomic-scale and mesoscopic properties of materials, and to establish links between (ii) the electrical conductivity and (iii) the mechanical response of 2D correlated materials with their atomic-scale structure and stimulus-dependent electronic phase diagram.
The project has already made significant progress in the detection and further electronic and magnetic characterization of collective ground states in selected strongly correlated 2D materials. So far, we have explored monolayers of several transition metal dichalcogenides (TMD) such as NbSe2, VSe2, self-intercalated TMDs, and 2D TMD alloys, all of them showing electronic phases of different nature. First, we have achieved the MBE growth of all these 2D systems in our laboratory with outstanding crystal quality beyond the state of the art. The most remarkable result to date is the detection of collective modes in the family of 2D TMD superconductors, which emerge from the mixing of singlet-triplet pairing channels. This result unveils the relevant role of magnetism in TMD superconductors and opens the door to the study of unconventional Cooper pairing mechanisms in strictly 2D superconductors. In parallel to the microscopic characterization, we have also carried out electron transport measurements in order to correlate their atomic-scale properties with the mesoscopic electronic response. Finally, we have also made key progress in integrating the correlated 2D materials in back-gated devices to enable their electrostatic doping and, therefore, a precise control of the DOS.
In light of our first results regarding the character of the superconducting state in TMD monolayers, we have an unprecedented opportunity to explore the most fundamental aspects of unconventional superconductivity (UcS) in simple and accessible 2D superconductors. Therefore, we plan to put significant research efforts here to confirm the unconventional character of superconductivity in 2D transition metal dichalcogenide (TMD) superconductors, and to gain knowledge about the Cooper pair formation and its relation to magnetic and other collective electronic phases of their phase diagram. Furthermore, our findings open new avenues for the exploration of UcS in exotic electronic environments via the combination of monolayers of TMDs with, for example, magnetic substrates to lead to topological superconductivity and Majorana fermionics. Aside from this major research opportunity, we also plan to investigate further the plausible existence of exotic forms of electronic order in other types of 2D TMD alloys, which are not present in regular TMD materials.