Novel forms of magnetism in 2D transition metal dichalcogenides

Project Information

MAGTMD

Grant agreement ID: 101033538

DOI

10.3030/101033538

Project closed

EC signature date 23 March 2021

Start date 1 April 2021

End date 31 March 2023

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 160 932,48

EU contribution

€ 160 932,48

Coordinated by

FUNDACION DONOSTIA INTERNATIONAL PHYSICS CENTER
Spain

Periodic Reporting for period 1 - MAGTMD (Novel forms of magnetism in 2D transition metal dichalcogenides)

Reporting period: 2021-04-01 to 2023-03-31

Summary of the context and overall objectives of the project

In the last decade, there has been immense interest in exploring two dimensional (2D) materials such as graphene as it has been a model system to explore physics confined in 2D and can used for a multitude of device applications. In this same category of van der Waals layered materials, transition metal dichalcogenides (or TMDs), have emerged as an ideal playground for the investigation of exotic electronic phases due to greatly enhanced many-body interactions attributed to its large spin-orbit coupling and absence of inversion symmetry in the single layer limit. Many of these TMDs have the potential to host magnetic order which often coexist with other electronic states such as superconductivity and charge density wave order. With reduced dimensionality, much of the physics related to strong correlations can be observed and studied in more detail.
Investigating such 2D materials requires spectroscopic tools or technique with high sensitivity as well as pristine and defect-free crystals, or in some cases, having precisely controlled defect density. While these 2D materials can be exfoliated by a simple method using scotch tape, atomic level investigations demand crystals of high purity that can be measured in a clean environment.
The ultimate goal of MAGTMD project is to demonstrate that the enhanced electronic correlations in TMD lead to different expressions of magnetism in the 2D limit. The principal tool of investigation is scanning tunneling microscopy at ultra-low temperatures which can provide atomic level insights on the electronic density of states of the material spatially mapped onto its 2D lattice. With an external magnetic field applied to the sample surface, the impact of magnetic order in the ground state electronic properties can be studied. For this purpose, high crystallinity 2D materials will be prepared by molecular beam epitaxy in an ultrahigh vacuum environment. A key component of MAGTMD is the modification of ground state properties by modulating the charge carrier density at the fermi level to access rich electronic phases in proximity to a quantum critical point.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The results in this project are based on ultra-low temperature (0.4 K and 4.2 K) scanning tunneling microscopy and spectroscopy (STM/STS) for single layer TMD that are grown on bilayer graphene substrate, using molecular beam epitaxy.
In the initial stage, the researcher was involved in studying substrate effects on the superconducting properties of niobium diselenide. The results pointed towards weak interaction on graphene (and on boron nitride), which provided evidence that graphene (as a substrate) was a good candidate for studying intrinsic properties of TMDs. Further STS measurements at 340 millikelvin in Niobium diselenide showed resonances outside the superconducting gap in which the origin of these peaks has been found to arise from competing pairing instabilities based on spin fluctuations.
For a better understanding of magnetic ground state in two dimensions, the researcher has investigated bilayer heterostructure of tantalum diselenide which is known to exhibit Kondo type behavior. Through high resolution STS measurements at 340 millikelvin, it was found that the typical Kondo peak, here, was comprised of two symmetric peaks split at the fermi level. With the help of magnetic field dependent STS measurements, it was concluded that the ground state lies deep in the magnetic side of the Doniach phase diagram.
In the final part, the intrinsic properties of pure niobium diselenide and tantalum diselenide were modified by means of chemical substitution with a heterovalent impurity (an another transition metal atom). Aliovalent alloy of Mo doped Niobium diselenide, of various doping levels, has allowed to investigate the atomic scale evolution of the electronic ground state across three different phase transitions, that is, superconductor-metal, CDW and metal-semiconductor. Following this innovative approach, the researcher was able to induce superconductivity in tantalum diselenide (doped with tungsten) due to an increase of the density of states (DOS), as the Fermi surface approaches a van Hove singularity.
All results of this project have been published in reputed peer-review journals with green and gold access options. These findings have also been communicated to an international audience through conferences in a timely fashion.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The researcher has developed an innovative approach to modify the electronic properties of single layer TMD crystals by means of chemical substitution. This alloying method can be used for a wide range of combinations thus providing a tuning tool for material science researchers.
For back-gated devices, the researcher has optimized the growth of TMD on substrates prepared in different chemical environments, in this case monolayer graphene transferred onto a silicon oxide substrate and grown by chemical vapor deposition. This allows researchers to collaborate more efficiently across different disciplines.

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