The Synthesis, Tunable Properties and Emergent Superconductivity in the New Misfit Layered Compounds (EuS)_1+δ(NbSe₂)_n (n = 2,3)

Heterostructured materials typically consist of two component materials layered on each other and bonded via “intermolecular” van der Waals (VdW) forces. Initially proposed as semiconductor materials due to their potential for advancing manufacture as well as enabling new technologies, substantial recent interest has focused on the use of 2D transition metal dichalcogenides (TMDs), as building blocks to provide novel electronic and magnetic properties.<br/>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. Normally, the layering of these structures would not be expected to form spontaneously as the degree of lattice mismatch is very high. However, due to the incommensurate structure (the ratio of the two structures is not an integer), it is possible with the misfit compounds. Misfit layered compounds can possess entirely different properties compared to their component materials which offers potential for the discovery of useful physical properties and phenomena. It is also possible to combine materials that typically contain mutually exclusive phenomena in a single material, such as ferromagnetism and superconductivity.<br/>Here we report implementation of this strategy through combining ferromagnetic EuS and superconductive TMD NbSe₂. The new 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 misfit is neither ferromagnetic nor superconducting down to T = 0.4 K. We find evidence of a field-driven transition to a ferromagnetic state due to reorientation of ferromagnetic layers. On the other hand, n = 3 is a superconductor with a transition temperature at T_c ≈ 2.8 K. In both compounds the magnetic moment observed in the paramagnetic regime show that the magnetism is almost entirely caused by Eu²⁺ ions in the EuS layer. Transport properties, on the other hand, are expected to come primarily from TMD layer. The appearance of superconductivity in n = 3 compound can be attributed to the addition of a third NbSe₂ layer. As it is positioned between two other NbSe₂layers, there is no direct contact with EuS layer. Lattice strain and charge transfer are therefore minimized allowing for the emergence of superconductivity.