Induced Superconductivity and Engineered Josephson Tunneling Devices in Epitaxial (111)-Oriented Gold/Vanadium Heterostructures

New heterostructure for topological superconductor: superconductivity in (111)-oriented gold

Research work led by Professor Jagadeesh Moodera from Department of Physics, Massachusetts Institute of Technology in Cambridge and Professor Peng Wei at University of California, Riverside have now shown a unique experimental approach to create topological superconductors by inducing superconductivity into epitaxial metallic thin film hosting surface states with strong spin-orbit coupling. The research team demonstrated high quality ultrathin ( ∼4nm) (111) –oriented gold (Au) films grown on s-wave superconductor-vanadium (V) that exhibits clear superconductivity transition at T_C∼3.9K. The new findings appeared in the journal, Nanoletters.

“It has been shown theoretically that epitaxial noble metal thin-films with well-defined crystalline surface such as Ag (III) and Au (III) and so forth, are ideal materials for robust p+ip semiconductors due to giant Rashba spin-orbit coupling.”, mentioned by Prof. Moodera. “The surface quasiparticles gain spin-orbit coupling owing to the nature of broken inversion symmetry and short Fermi wavelength in metallic systems”, he added.

Dr. Wei mentioned: “A way of creating hybridized quasiparticle with combined interactions of spin-orbit coupling, Zeeman splitting and s-wave superconductivity has a challenge in terms of epitaxial growth of ultrathin noble metal films with well-ordered crystalline surface on other superconductors or ferromagnetic insulators.” He further explained: “Due to large surface energy of silver and gold, island type growth mode forms which leads to the prevention of continuous and smooth films formation in thickness down to a few nanometers.”

The team solved this challenge by innovatively using a multistep grown/annealing sequences and a seed layer from the bottom superconducting material vanadium. Although a non-epitaxial gold layer in proximity to a superconductor has been shown before, no one was able to induce superconductivity to epitaxial well (111)-oriented ultra-thin gold layer.

“The 1nm seed layer of vanadium significantly improved the heterostructure quality.”, mentioned by Dr. Wei. The growth of the seed layer was initially diffusive at room temperature, which then becomes crystallized upon the proper in-situ annealing under ultra-high vacuum environment. Sharp diffraction streaks confirmed the crystallization of the vanadium seed layer. Following this high quality baseline, epitaxial 20 nm superconducting vanadium layer can be grown following quasi 2D growth mode. Following that however, as soon as ultra-thin gold (within only 4 Å) was evaporated, the growth immediately demonstrated 2D growth mode suggesting the formation of smooth continuous layer. Such growth mode extended all the way to 4 nm thick gold and produce epitaxial layer confirmed by reflection high-energy electron diffraction (RHEED). RHEED diffraction along different surface crystalline directions proved the (111) terminated gold surface. The extracted lattice constant of gold surface (4.07±0.25Å) matches very well with that in bulk gold. This high quality heterostructure also demonstrated Laue interference fringes in x-ray diffraction measurements, which yielded roughness of gold surface to be ∼0.89±0.03nm and roughness of vanadium/gold interface to be ∼0.43±0.01nm.

“The Cooper pair tunneling in the engineered Josephson junctions of 4 nm (111)-Au clearly confirmed the induced superconductivity.”, noted by Prof. Moodera. The tunneling measurement was carried at 1 K. The associated Andreev reflections and subharmonic gap structures (SGS) was tuned systematically as a function of the junction resistance, which proved the induced superconductivity in ultra-thin epitaxial gold. The surface states of (111)-Au are thus expected to acquire superconductivity. “Our results paved the way for engineering thin film heterostructures for p-wave superconductivity and the corresponding nanodevices exploiting Majorana Fermions for quantum computing.”, as mentioned by Prof. Moodera.

This study opens the door for further research investigating unknown superconductivity in hybrid systems in presence of various method interactions which may lead to new observation of new phenomena in Physics.

Acknowledgement: This research has been supported by John Templeton Foundation along with National Science Foundation and Office of Naval Research.