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Pengzi Liu1 2 Piranavan Kumaravadivel1 2 3 Yujun Xie1 2 Eric Miller4 Yuta Ebine4 Sungwoo Sohn1 2 Judy Cha1 2 5

1, Yale University, New Haven, Connecticut, United States
2, Yale West Campus, West Haven, Connecticut, United States
3, University of Manchester, Manchester, , United Kingdom
4, Hitachi High Technologies America, Inc., Clarksburg, Maryland, United States
5, Canadian Institute for Advanced Research Azrieli Global Scholar, Toronto, Ontario, Canada

Because the topological protection of conducting surface states stems from the crystal symmetry for topological crystalline insulators (TCIs), crystal defects on the surface of TCI SnTe can strongly influence its topological and transport properties. As a derivative, In-doped SnTe was recently rediscovered as a candidate for time-reversal-invariant topological superconductors (TSCs) [1]. However, the high concentration of Sn vacancies inhibits surface states to be easily revealed [2], and the inhomogeneous distribution of In dopants in conjunction with catalyst impurities induced by chemical vapor deposition (CVD) may result in mixed behaviors of superconductivity [3]. Therefore, to manipulate topological and superconducting properties of In-doped SnTe, it is important to investigate how point defects, dislocations, surface topography and transport properties correlate with each other. We propose that the crystalline quality can be improved by suppressing the formation of surface defects on SnTe and In-doped SnTe micro/nanocrystals synthesized by CVD [4].

Our work focuses on the study of morphology-dependent superconductivity of In-doped SnTe nanostructures [3]. Superconducting transitions are obtained in transport measurements of In-doped SnTe nanostructures synthesized by CVD using gold nanoparticles as a catalyst. For nanoplates, complete superconducting transitions are observed at ~2.0 K, whereas lower-dimensional nanoribbons and nanowires present gradual and possible multiple transitions at lower temperatures of ~1.4-1.75 K. Saturating resistances in some nanowires also indicate incomplete superconducting transitions. We propose that inhomogeneity, such as surface In-doping inhomogeneity and gold impurities, is exaggerated in the transport measurements of smaller nanostructures, leading to variations of superconducting behaviors between different morphologies.

Moreover, to investigate surface defects that may degrade the topological characteristics of In-doped SnTe, we characterize surface pits, open cores and steps on {100} and {111} surfaces of SnTe micro/nanocrystals with and without In doping [4]. We show that enlarging, deepening, and faceting of the surface pits are enabled by sublimation of open cores and anisotropic movement of steps during natural cooling after the CVD growth. Distinct morphologies of surface pits arise from the crystal symmetry, surface termination, presence of In dopants, and dislocations of SnTe. Fast cooling suppresses the formation of surface pits, although growth conditions need to be optimized to decrease the density of dislocations that are responsible for the surface defects of SnTe micro/nanocrystals.

References:

[1] Sasaki,S., and Yoichi Ando. Crystal Growth & Design 15 (2015): 2748-2752.
[2] Novak, M., et al.. Physical Review B 88 (2013) 140502.
[3] Kumaravadivel P., et al.. APL Materials 5 (2017) 076110.
[4] Liu, P. et al.. Journal of Physics and Chemistry of Solids, DOI: 10.1016/j.jpcs.2017.12.016 (2017).

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