The multi-layered physics of layered superconductors

Muon spin rotation experiments provide unique microscopic insight into the superconductivity and magnetism of transition metal dichalcogenides — and reveal complex and unconventional patterns, hinting towards a common mechanism for and electronic origin of ‘unconventional’ superconductivity.

The family of transition metal dichalcogenides (TMDs) has been studied for decades, but over the past few years, the interest in these materials has experienced a marked upturn. That renewed interest stems primarily from the fact that TMDs consist of layers with strong in-plane bonding and weak out-of-plane interactions. This means that two-dimensional layers can be exfoliated from a crystal, similarly as graphene can be peeled off graphite. But also for bulk TMD materials, the layered nature of the systems gives rise to intriguing physical, chemical, and mechanical properties, including topological physics with Dirac-type dispersion, exotic optical and transport behaviour originating from valley splitting, and beyond.

Unique insight into the physics of TMDs has come recently through a series of muon spin rotation (μSR) experiments performed in the Laboratory for Muon Spin Spectroscopy (LMU) at PSI. In these experiments, the muons serve as tiny magnetometers that reveal how deeply an external magnetic field penetrates the material, and how that depth changes with varying parameters such as temperature and hydrostatic pressure. For superconductors, the superfluid density can be inferred from these data, providing microscopic information about this phase. As they report now in a paper just published in Science Advances [1], a team led by PSI Tenure-Track scientist Zurab Guguchia — working in close collaboration with Fabian von Rohr, an SNSF Ambizione fellow and group leader at the University of Zurich, and with other colleagues from the University of Zurich, Columbia University and Princeton University — has used the unique capabilities at LMU to study TMD superconductors under hydrostatic pressure. Their results establish an unconventional relationship between critical temperature and superfluid density across different TMD systems. Intriguingly, a similar relation had been found before for other classes of layered superconductors, namely cuprates and iron pnictides, yet with a different proportionality constant.

Deep insight into unconventional superconductors

Crystal structure of 2H-NbSe2.
(Adapted from ref. [1]))

Guguchia and his co-workers studied three different TMD superconductors: 2H-NbSe2 and 4H-NbSe2 (which differ by the stacking sequence of the layers), and the orthorhombic Td phase of MoTe2. The measurements revealed strong pressure effects on the superfluid density and its unconventional scaling with the critical temperature. They also found that the values of the superconducting gaps are insensitive to the suppression of the charge density wave (CDW) ordered state, indicating that CDW pairing has only a minimal effect on the superconductivity in 2H-NbSe2. Moreover, earlier work on MoTe2 [2] showed the energy-gap structure is consistent with the topologically non-trivial s+/- pairing, classifying MoTe2 as the potentially first known example of a time-reversal invariant topological (Weyl) superconductor. Topological superconductivity in the Weyl semimetal MoTe2 and the similarity between TMDs and other exotic superconductors is an important finding that contributes to overall understanding of unconventional superconductors.

Temperature dependence of the muon spin depolarization rate σSC(T) measured in 4H-NbSe2 at ambient pressure and in 2H-NbSe2 at various hydrostatic pressures in an applied magnetic field of 70 mT. The superfluid density can be deduced from the depolarization rate.
(Adapted from ref. [1]))

Furthermore, extended μSR studies of the semiconducting 2H phase of MoTe2 and MoSe2 revealed the presence of static magnetic order in these systems [3]. This feature has been confirmed in subsequent studies based on Scanning Tunnelling Microscopy (STM) and measuring magnetic susceptibility. The discovery of magnetism in these systems is highly significant for those working on TMDs and two-dimensional layered conducting systems: The discovery suggests a central role of magnetic interactions in electronic structures of TMDs, and extend general commonalities of various unconventional superconductors to this important family of two-dimensional conductors.

As these two-dimensional materials can be easily layered by van der Waals heteroepitaxy, combining unconventional and topological superconducting TMDs and magnetic TMDs should enable the creation of unique new device concepts in the future. Moreover, there are close connections to a parallel development where superconductivity and possibly magnetism have been discovered in twisted sheet of bilayer graphene by a group at the Massachusetts Institute of Technology (MIT). 

Unique results thanks to interdisciplinary collaboration

Compiling this body of results has only been possible due to interdisciplinary collaboration. On one hand, the synthesis of phase-pure samples — especially of 4H-NbSe2 — is challenging. Also the different polymorphs of MoTe2 produced for these studies are the best ones available today. On the other hand, the measurements were only possible thanks to experimental capabilities that are worldwide unique to the high pressure muon facility at PSI.