Subcategory: Physics (not Nanoscience)
Pavel Shibayev - Rutgers University
Co-Author(s): Maryam Salehi, Jisoo Moon, and Seongshik Oh, Rutgers University, Piscataway, NJ
In the past few years, there has been an increased interest within a subset of the experimental condensed matter physics community in understanding the superconducting behavior of iron selenide (FeSe). While bulk FeSe is known to be superconducting at 8K , Tc substantially increases in FeSe films. Past efforts of others aimed at growing thin films of FeSe have yielded some success in reaching and replicating Tc of approximately 40K [2,3], followed by reports of 65K . However, last year one group  reported superconductivity above 100K for a similar structure, which was admittedly impossible to replicate by anyone to date. Given the resulting inconsistency in consensus among groups working on this problem, we set a goal of growing FeSe via a modified recipe by optimizing both the growth temperature and the compound to use as protection layer. In our quest to high temperature superconductivity in thin iron selenide, we concentrate on keeping track of each compound’s structural evolution with temperature via RHEED, an aspect almost always overlooked in papers describing FeSe growth, thus presenting a unique perspective to tackling this multifaceted challenge. In particular, our group has grown single-unit-cell (1 UC) and three-unit-cell (3 UC) FeSe on insulating SrTiO3 (STO) substrates using a state-of-the-art molecular beam epitaxy (MBE) system in our lab. Transport measurements are done via a cryogenic system, allowing cooling to below 6K. At present, this is an ongoing study with an expected time horizon of several months. After numerous tests, our methods involve cleaning the STO substrate in the presence of oxygen at temperature of 800C and setting the growth temperature to 600C. Iron is deposited in the presence of selenium based on QCM flux measured in advance. The protection layer (capping) to prevent oxidation upon sample take-out at present is indium, but will be replaced by another element as a result of further recipe optimization. In addition, 1 UC FeSe/STO heterostructures with FeTe protection layers will be grown to enable comparison of transport and scanning tunneling spectra (STS) data to both data involving our own capping and that obtained by others  for the sake of completeness of this comprehensive study.
References:  Hsu, F. C. et al. Superconductivity in the PbO-type structure α-FeSe. Proc. Natl. Acad. Sci. USA 105, 14262-14264 (2008).
 Wang, Q.-Y. et al. Interface-induced high-temperature superconductivity in single unit-cell FeSe films on SrTiO3, Chin. Phys. Lett. 29, 037402 (2012).
 Sun, Y. et al. High temperature superconducting FeSe films on SrTiO3 substrates, Scientific Reports 4, 6040 (2014)
 Tan, S. et al. Interface-induced superconductivity and strain-dependent spin density waves in FeSe/SrTiO3 thin films, Nat. Mater. 12, 634 (2013).  J.-F. Ge et al, Superconductivity above 100K in single-layer FeSe films on doped SrTiO3, Nat. Mater. 14, 285 (2015).
Funder Acknowledgement(s): This work is supported by NSF Emerging Frontiers in Research and Innovation (EFRI) Scholars program (1542798), the Gordon and Betty Moore Foundation’s EPiQS Initiative (GBMF4418), and Fellowship sponsored by the U.S. Department of Education for Graduate Assistance in Areas of National Need.
Faculty Advisor: Seongshik Oh, firstname.lastname@example.org
Role: I am the lead person in this project, and as such my roles included, but were not limited to, devising and modifying the growth recipe (upon consultation with the PI, Professor Oh, and my colleague, Maryam Salehi), operated our MBE system to grow each compound, performed transport measurements, and am writing the paper.