Enhanced superconductivity in Al nanofilms grown by molecular beam epitaxy

Chi-Te Liang^1,2*, Sheng-Di Lin³, Christos Panagopoulos⁴

¹Department of Physics, National Taiwan University, Taipei, Taiwan
²CQSE, National Taiwan University, Taipei, Taiwan
³Institute of Electronics, National Yang Ming Chiao Tung University, Hsinchu, Taiwan
⁴Division of Physics and Applied Physics, Nanyang Technological University, Singapore

* Presenter:Chi-Te Liang, email:ctliang@phys.ntu.edu.tw

Superconducting properties of thin films can be vastly different from those of bulk materials. Seminal work has shown the critical temperature T_c of elemental superconductors decreases with decreasing film thickness when the normal-state sheet resistance is lower than the quantum resistance h/(4e²). Sporadic examples on disordered films, however, hinted an enhancement in T_c although, structural and strain characterization was not possible since samples were prepared on a cold substrate in situ. To clarify the role of reduced dimensionality and disorder on the superconducting properties of thin films we employed MBE to grow high-quality aluminium (Al) nanofilms with normal-state sheet resistance at least 20 times lower than the quantum resistance h/(4e²) and investigated their electronic and structural properties ex situ. Defying general expectations, T_c increases with decreasing Al film thickness, reaching 2.4 K for 3.5-nm-thick Al film grown on GaAs: twice that of bulk Al (1.2 K). DFT calculations indicate surface phonon softening impacts superconductivity in pure ultra-thin films, offering a new route for materials engineering in two dimensions. This work was done in collaboration with Ching-Chen Yeh, Thi-Hien Do, Pin-Chi Liao, Chia-Hung Hsu, Yi-Hsin Tu, Hsin Lin, T.-R. Chang, Siang-Chi Wang, Yu-Yao Gao, Yu An Lai, Yu-Hsun Wu, Chu-Chun Wu, and Ivar Martin.

Keywords: Aluminium, Superconductivity, MBE, Nanofilms, Substrate effects