Transition Metal Chalcogenides under Extreme Pressures

The aim of this research project is to study the high-pressure phase diagram of transition metal chalcogenides (TMCs). We will search for new high-pressure structures, determine their superconducting properties, and investigate lattice instabilities. With respect to conventional superconductivity (SC), a highly topical class of compounds being intensely investigated at the moment are van der Waals (vdW) crystals, i.e., stacked materials whose sublayers are loosely held together by weak vdW forces. Prototypical examples for this class are the transition metal dichalcogenides (TMC2), and of particular interest with respect to SC are its metallic members, as the superconducting state coexists or appears in very close vicinity to a charge density wave (CDW) phase. (The electron density in a CDW forms a standing wave pattern and is accompanied by a periodic lattice distortion.) These two phases have originally be thought of as being mutually exclusive, since they both depend on the same interaction between electrons close to the Fermi energy and lattice vibrations. The coexistence of these two phases in TMC 2 offers the exceptional opportunity to study the mutual influence of SC and CDW order. In the proposed project we will employ evolutionary search methods to explore the high-pressure phase diagram of TMCs, with particular focus on identifying novel superconducting structures. We also envisage to use state-of-the-art methods to investigate the properties of the superconducting state within the CDW phase in several TMC2 completely from first-principles, and shed light on the effects of pressure on the electronic and vibrational structure, electron-phonon coupling, SC and charge-order. this project will contribute to obtain a more complete picture of the high-pressure phase diagram of TMC and similar materials, providing a solid foundation for future high-pressure research. The results of this project will also help to improve the understanding of the interactions between SC and charge order in TMC, and clarify the effects of high-pressure on atomic bonding, and the mechanisms behind SC and CDW order in these materials. In addition,

With respect to conventional superconductivity (SC), a highly topical class of compounds being intensely investigated at the moment are van der Waals (vdW) crystals, i.e., stacked materials whose sublayers are loosely held together by weak vdW forces. Prototypical examples for this class are the transition metal dichalcogenides (TMC2), and of particular interest with respect to SC are its metallic members, as the superconducting state coexists or appears in very close vicinity to a charge density wave (CDW) phase. While considerable attention has been devoted to studying the stable phases of transition metal chalcogenides under ambient and low pressures, high-pressure environments remain relatively unexplored. Our project aimed to address this gap by investigating the high-pressure phase diagrams of this material class, with a focus on identifying novel phases exhibiting superconductivity and/or CDW ordering. To achieve this, we employed a comprehensive approach combining fully ab initio and state-of-the-art methods. This allowed us to search for new high-pressure structures, analyze electronic and vibrational properties, assess electron-phonon coupling, evaluate superconducting properties, and explore instabilities towards CDW ordering. Our investigation centered on four systems: Nb-S, Nb-Se, Mo-S, and Mo-Se, with the goal of understanding how the electronic configurations of elements influence structure stability under high pressures. Our results, consistent with existing literature for known phases, revealed several new materials and unveiled intricate phase diagrams across pressure ranges. Notably, stable phases predominantly comprise various stackings and orientations of building blocks. Low-pressure phases are characterized by trigonal prismatic and octahedral geometries, found in many transition metal dichalcogenides, while high-pressure phases favor a cubic CsCl-like geometry. The prevalence of certain structures is largely determined by the transition metal, with Nb systems exhibiting more stable ambient-pressure phases and all binary phases displaying metallic properties. Mo systems, on the other hand, primarily features the semiconducting transition metal dichalcogenide phase at ambient pressure. Noteworthy among metallic phases were Nb2S and NbSe3, which show pronounced CDW instabilities while supporting conventional superconductivity, representing rare examples of coexisting CDW order and superconductivity at high pressures. Our findings provide valuable insights into this well-studied system, prompting a reevaluation of similar systems with the potential to uncover novel phases. As we continue to advance scientific knowledge, our work contributes to a deeper understanding of the properties and behaviors of vdW crystals under varying conditions.