The First Approximation-Free Theoretical Study of the Cause
for High Critical Temperature in FeSe Films on SrTiO3
In 2012 , monolayer FeSe films grown on SrTiO3 (STO) substrate were discovered to have high superconducting critical temperatures, Tc. They are almost an order of magnitude higher than those of the bulk FeSe and are highest among all known Fe-based superconductors. In a recent work , a research team made up of Beijing and Berkeley scientists carried out the first approximation-free theoretical study to identify the cause of such high Tc in this system.
Fig.1: Schematic drawing of an atomically thin FeSe film grown on SrTiO3 substrate.
In the superconducting state, electrons bind together to form pairs. They are called "Cooper pairs". The binding energy of Cooper pairs is a measure of the robustness of the superconducting state; a larger binding energy implies a higher superconducting transition temperature. In Ref. , Li et al. performed the first numerically-exact sign-problem-free quantum Monte Carlo simulations to iron-based superconductors. By examining a number of electron-electron and electron-phonon interactions that are potentially responsible for causing the formation of Cooper pairs, they gave a tentative answer to the following question: why is Tc is so high in monolayer FeSe films grown on SrTiO3?
In particular, the size of the binding energy and the symmetry of pair wavefunctions were able to be determined through quantum Monte-Carlo simulation (Fig. 2). The Cooper pair wavefunction is very similar to the molecular wavefunction for, say, H2. It is the quantum mechanical wavefunction of a Cooper pair as a function of the relative coordinates between the two electrons. The symmetry of the pair wavefunction refers to its phase behavior upon rotating the relative coordinate. With the calculated binding energy and symmetry of the pair wavefunction, Li et al. made comparisons to experiments and suggested the most likely interaction that triggers superconductivity when there is no SrTiO3 substrate.
Fig.2: Cooper pair wavefunctions with different symmetry.
In the presence of the SrTiO3 substrate, the pair binding energy was found to be enhanced by the interaction between the FeSe electron and SrTiO3 phonons and this confirms that such interactions indeed substantially raises Tc. Based on these results, a "phase diagram" like the one in Fig.3 has been constructed. Here, the difference between the red solid circles and open circles reflect the Tc enhancement by the substrate phonons.
Fig.3: The phase diagram deduced in Ref. .
In addition to its importance to the understanding of high Tc in FeSe/SrTiO3, these results also point out two separate but cooperative mechanisms driving high temperature superconductivity. Hopefully it will also provide hints as to where we might find other higher temperature superconducting materials.
This research was funded by the National Science Foundation of China (Grant No. 11474175 and 11374018) and the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, grant DE-AC02-05CH11231.
 QY Wang, Z Li, WH Zhang, ZC Zhang, JS Zhang, W Li, H Ding, YB Ou, P Deng, K Chang, J Wen Jing, CL Song, K He, JF Jia, SH Ji, YY Wang, LL Wang, X Chen, XC Ma and QK Xue, Chin. Phys. Lett. 29, 037402 (2012).
 Z-X Li, F Wang, H Yao and D-H Lee, Sci. Bull. 61, 925 (2016).
Wenjuan Zou received her PhD in theoretical physics from Nanjing Normal University. Since 2007, she has worked as a COE researcher in Shizuoka University, Japan. She is presently an editor of Science Bulletin, which is a semi-monthly international journal publishing high-caliber peer-reviewed research on a broad range of natural sciences and high-tech fields on the basis of the originality, scientific significance and overall general interest of the research.