Quantum Surface and Interface Research Unit

Principal Investigator

PI Name Shuaihua Ji
Title Unit Leader
Brief Resume
2008Ph.D., Institute of Physics, Chinese Academy of Sciences, Beijing, China.
2008Postdoctoral Fellow, IBM T. J. Watson Research Center, NY, USA
2011Postdoctoral Fellow, Columbia University, NY, USA
2012Assistant Professor, Department of Physics, Tsinghua University
2014Unit Leader, Quantum Surface and Interface Research Unit, Cross-Divisional Materials Research Program, RIKEN Center for Emergent Matter Science (-present)
2017Associate Professor, Department of Physics, Tsinghua University (-present)


Our research unit focuses on the emergent phenomena on the surface/interface of the newly discovered topological related quantum materials, such as topological insulators and topological crystalline insulators. We are interested in the fundamental properties of those symmetry protected surface states. In particular, we are interested in topological surface states with the gap opening induced by the time reversal symmetry breaking, or gauge symmetry breaking, or interaction of two surface states through tunneling effect, which could lead to the exotic phenomena, such as surface quantum hall effect, topological magnetoelectric effect and topological superconducting.

Research Fields

Physics, Material Science


Scanning tunneling microscopy
Low dimensional superconductors
Topological quantum matters
Thin films and interfaces
Surface and heterogeneous interfaces


Molecular beam epitaxy growth of superconducting LiFeAs film on SrTiO3(001) substrate

The stoichiometric “111” iron-based superconductor, LiFeAs, has attacted great research interest in recent years. For the first time, we have successfully grown a LiFeAs thin film by molecular beam epitaxy on a SrTiO3(001) substrate, and studied the interfacial growth behavior by reflection high-energy electron diffraction and low-temperature scanning tunneling microscope. The effects of substrate temperature and Li/Fe flux ratio were investigated. A uniform LiFeAs film as thin as 3 quintuple layers (QL) is formed. A superconducting gap appears in LiFeAs films thicker than 4QL at 4.7K. When the film is thicker than 13QL, the superconducting gap determined by the distance between the coherence peaks is about 7meV, close to the value of bulk material. The ex situ transport measurement of a thick LiFeAs film shows a sharp superconducting transition around 16K. The upper critical field, Hc2(0)=13.0T, is estimated from the temperature-dependent magnetoresistance. The precise thickness and quality control of the LiFeAs film paves the road for growing similar ultrathin iron arsenide films.

MBE growth of LiFeAs film. (a) The tetragonal structure of LiFeAs. (b) RHEED pattern of the (001) surface of the SrTiO3 substrate, which shows the 2×2 reconstruction. The arrows indicate the 1×1 stripes. (c), (d): RHEED patterns of a LiFeAs thin film on the SrTiO3 substrate after 8 (c) and 60 (d) minutes deposition. The weak spots marked by the arrows show the formation of a small amount of Li3As clusters on the surface during growth. (e), (f): STM topographic images of SrTiO3(001) (e) and 25QL LiFeAs film (f). Image size: 500 nm × 250 nm; sample bias: 1.0V; tunneling current: 100 pA. (g) Atomically resolved STM topography image on the surface of a 25QL LiFeAs film (10 nm×10nm, 8.0mV, 2.0nA). (h) The evolution of the in-plane lattice constant ain-plane determined by RHEED as a function of the film thickness. (i) ThedI/dV spectrum on the surface of a 13QL LiFeAs film.


Shuaihua Ji

Unit Leader shuaihua.ji[at]riken.jp