Doubling the superconducting transition temperature of ultraclean wafer-scale aluminum nanofilms
Ching-Chen Yeh, Thi-Hien Do, Pin-Chi Liao, Chia-Hung Hsu, Yi-Hsin Tu, Hsin Lin, Tay‐Rong Chang, Siang-Chi Wang, Yu-Yao Gao, Yu-Hsun Wu, Chu-Chun Wu, Yu An Lai, Ivar Martin, Sheng‐Di Lin, C. Panagopoulos, Chi‐Te Liang
Abstract
We studied the role of reduced dimensionality and disorder in the superconducting properties of wafer-scale aluminum (Al) nanofilms. This new generation of ultrathin films were grown using molecular beam epitaxy and depict normal-state sheet resistance at least 20 times lower than the quantum resistance $h/(4{e}^{2})$. Defying general expectations, the superconducting transition temperature of our films increases with decreasing Al film thickness, reaching 2.4 K for a 3.5-nm-thick Al film grown on GaAs: twice that of bulk Al (1.2 K). Surface phonon softening is shown to impact superconductivity in pure ultrathin films, offering a route for materials engineering in two dimensions.