  When a topological insulator is made into a nanowire, the interplay between
topology and size quantization gives rise to peculiar one-dimensional states
whose energy dispersion can be manipulated by external fields. In the presence
of proximity-induced superconductivity, these 1D states offer a tunable
platform for Majorana zero modes. While the existence of such peculiar 1D
states has been experimentally confirmed, the realization of robust
proximity-induced superconductivity in topological-insulator nanowires remains
a challenge. Here, we report the realization of superconducting
topological-insulator nanowires based on (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ (BST)
thin films. When two rectangular pads of palladium are deposited on a BST thin
film with a separation of 100--200 nm, the BST beneath the pads is converted
into a superconductor, leaving a nanowire of BST in-between. We found that the
interface is epitaxial and has a high electronic transparency, leading to a
robust superconductivity induced in the BST nanowire. Due to its suitable
geometry for gate-tuning, this platform is promising for future studies of
Majorana zero modes.
