Growth and Characterization of Superconducting Bulk Crystal [(SnSe)$_{1+\delta}$]$_m$(NbSe$_2$) Misfit Layer Compounds

Abstract

[(SnSe)$_{1+\delta}$]$_m$(NbSe$_2$) ($m$ = 1-6, 8, and 12) highly orientated crystals 1-2 mm in size and well-defined c-planes were successfully grown using CsCl/KCl flux, including the first growth of crystals with $m = 12$. The stacked layers along the $c$ axis in the obtained crystals were directly observed by transmission electron microscopy as m alternating layers of SnSe and single layers of NbSe$_2$. The superconducting transition temperature of the obtained [(SnSe)$_{1+\delta}$]$_m$(NbSe$_2$) crystals decreased with an increase in the number of SnSe layers per unit cell. As the superconducting anisotropy parameters increase, a significant increase is observed between $m = 4$ and 5. This indicates that the superconducting dimensionality becomes more two-dimensional with an increasing $m$.