Enhancement of the critical field in nanoscale ultra-thin Pb film network

The ability for a superconductor to survive under high magnetic fields is crucial for its applications. We measured the MBE-grown Pb nano-mesh of 2nm thickness and found it maintains strong global phase rigidity, thus retaining the Tc close to that of a bulk crystal. The parallel critical field is significantly higher than the Clogston limit as a direct consequence of ultra-thin geometry and strong spin orbital coupling. In the perpendicular field, the orbital pair breaking is also quenched until the magnetic length becomes smaller than the lateral dimension of the nano-mesh. The inverse correlation of the lateral width and local HC⊥ points to a possibility to achieve much higher HC⊥. This work demonstrates that superconductivity pair breaking can be significantly suppressed by nanoscale engineering and opens new strategies to optimize superconducting quantum devices.
This work is in collaboration with Prof. Chih-Kang Shih (University of Texas), Prof. Allan MacDonald (University of Texas), Prof. Hua Chen (Colorado State University), Prof. Philip Adams (Louisiana State University).

Published online in Nam et al., Nature Communications 9, 5431 (2018) (21 Dec, 2018).

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Published online in 研之有物 and 泛科學 (5 Jun, 2017).

Superconducting Topological Surface States in PbTaSe2

The simplest approach to achieve topological superconductivity (TSC) is find a stoichiometric s-wave superconductor with topological surface states across Fermi level in the normal state. By using our SI-STM technique, we confirm the spectroscopic signature of the calculated topological band structures and superconducting properties in PbTaSe2. Fully gapped superconducting topological surface state is reported for the first time in a stoichiometric bulk material. Our work shows PbTaSe2 is a promising TSC candidate.
This work is in collaboration with National Taiwan University and National Tsing Hua University.

Published in Science Advances 2, e1600894 (23 Nov, 2016).

Highlighted by Institute of Physics, Academia Sinica : (English version), (Chinese version)

Anisotropic Energy Gaps of Iron-Based Superconductivity from Intraband Quasiparticle Interference in LiFeAs

We introduce intraband Bogoliubov QPI techniques for determination of superconducting gap structure, Δi (k) in superconducting LiFeAs. At T=1.2K, we can reach the necessary energy resolution of ~350μeV in order to resolve Δi (k). We identify three hole-like bands from our data. A comparison of our data to both ARPES and quantum oscillation measurements identifies these bands with those previously assigned as α1, α2 and γ by ARPES. Our results directly determine the anisotropy, magnitude and relative orientations of their Δi (k). The Δi (k) reported later in ARPES studies of LiFeAs appear in excellent agreement with our observations for the γ and α2 bands.
This work is in collaboration with Cornell University (USA) , Brookhaven National Lab(USA), University of St. Andrews (UK) and National Institute of Advanced Industrial Science and Technology(Japan)

Published in Science 336, 563 (4 May, 2012).