Heterostructured materials typically consist of two component materials layered on each other and bonded via “intermolecular” van der Waals (VdW) forces. Substantial recent interest has focused on the use of 2D transition metal dichalcogenides (TMDs), as building blocks to provide novel electronic and magnetic properties.
An emerging class of materials that offers significant potential for the discovery of useful physical properties and phenomena are class of chemically bonded heterostructured materials, misfit layered compounds. The “misfit” portion of the misfit compound arises from the incompatibility in crystal structures of the component materials, most commonly a pairing of [001]-oriented cubic (NaCl type) and either hexagonal or trigonal structures. The pontaneous formation of these structure is stabilized due to the incommensurate nature, meaning the two components have a non-integer lattice ratio. Misfit layered compounds can possess entirely different properties compared to their component materials. It is also possible to combine materials that typically contain mutually exclusive phenomena in a single material, such as ferromagnetism and superconductivity.
Here we report implementation of this strategy through combining ferromagnetic EuS and superconductive TMD NbSe₂. The new mixed-anion misfit layered compounds (EuS)_1+δ(NbSe₂)_n (δ ≈ 0.13; n = 2,3) have been successfully synthesized by solid state reaction (SSR) and chemical vapor transport (CVT). High resolution transmission electron microscopy (HRTEM), powder and single crystal X-ray diffraction confirm the misfit structure with c = 18.2 Å and 24.6 Å for n = 2 and 3, respectively. Magnetization, electrical resistivity, heat capacity, and thermal transport measurements show that the n = 2 material is a doped semiconductor with a low thermal conductivity of ~1 W/m-K and an antiferromagnetic ordering transition at T_N = 4.7 K. In contrast to the parent materials, the n = 2 misfit is neither ferromagnetic nor superconducting down to T = 0.4 K. On the other hand, n = 3 is a superconductor with a transition temperature at T_c ≈ 2.8 K.
The appearance of superconductivity in n = 3 compound can be attributed to the intercalation of a third NbSe₂ layer. As the distance between EuS layers increases, antiferromagnetic order is suppressed. Simultaneously, the middle NbSe₂ layer is no longer in direct contact with EuS, allowing superconductivity to emerge, with a T_c approaching single-layer NbSe₂ limit.

Daniel Foley

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