Interplay between magnetism and superconductivity in 2D van der Waals heterostructures

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The interplay between superconducting states and states with broken time-reversal symmetry allows for unconventional superconductivity. We propose to explore the nature of proximity induced superconductivity in various 2D van der Waals heterostructures formed from combinations of superconducting and non-superconducting van der Waals systems. Of particular interest are novel forms of Cooper pairing, especially triplet pairing, engendered by proximity coupling of a conventional s-wave superconductor with magnetic systems or, more generally, systems with broken space and time reversal symmetries. We propose to prepare both vertical and lateral Josephson junctions formed from van der Waals heterostructures. Recently we have demonstrated that such systems can exhibit novel properties including a Josephson Diode Effect in which the critical supercurrent density for current flowing in opposite directions can vary by up to 80%. We have demonstrated such effects in Josephson Junctions (JJs) where the barrier is formed from the van der Waals semimetals NiTe2, PtTe2 and WTe2 using superconducting electrodes formed from both the 2D Ising superconductor 2H-NbSe2 as well as conventional niobium. The detailed origin of the diode effect remains to be determined. We plan to address this by exploring the relationship of the diode effect to the detailed electronic and magnetic structures of heterostructures formed using a wide variety of 2D materials. We also propose to explore proximity induced superconductivity in heterostructures formed using 2D magnetic systems that exhibit topological non-collinear spin textures (e.g. skyrmions) in combination with 2D superconducting materials. The emergent superconducting states in these systems will be explored using both magneto-transport techniques as well as superconducting tunneling microscopy (STM) and superconducting tunneling spectroscopy (STS) to explore their spatial extent. An important goal is to both understand and control the contribution of triplet pairing to the proximity induced superconducting states. Van der Waals heterostructures will be formed using both molecular beam epitaxy and mechanical exfoliation techniques. We have demonstrated the MBE growth of high-quality van der Waals films displaying several ferroic properties as thin as a single atomic layer and also as lateral monolayer heterostructures. Our proposal directly addresses the research area on “collective and correlated phenomena (including spin, magnetic and superconducting proximity effects, Mott insulators) and novel topological states emerging in 2D vdW [hetero]structures” in the SP 2244-2DMP. Strong collaborations with several groups within the SPP in both theory and experiment are envisaged.