There is still plenty room on the surface: two case studies

by Prof. Xi Chen (陈曦) (Tsinghua University)

Friday 16 Jan 2026, 10:30 → 11:30 Asia/Shanghai

Tsung-Dao Lee Institute/N4F-N400 - meeting room (Tsung-Dao Lee Institute)

Host: Prof. Haibiao Zhou  

Venue: TDLI Meeting Room N400

Tencent Meeting link:  https://meeting.tencent.com/dm/YkB97vKHleNl

Meeting ID: 663333283, no password

 

Abstract:

We will discuss two experiments demonstrating the strategies to reveal new physics in two-dimensional systems. (1) The interplay between disorder and superconductivity is a subtle and fascinating phenomenon in quantum many body physics. In the weak disorder regime, the conventional superconductors are insensitive to nonmagnetic impurities, known as the Anderson’s theorem. Destruction of superconductivity and even superconductor-insulator transitions occur in the regime of strong disorder. Hence disorder-enhanced superconductivity is rare and has only been observed in some alloys or granular states. Because of the entanglement of various effects, the mechanism of enhancement is still under debate. Here we report well-controlled disorder effect in the recently discovered monolayer NbSe2 superconductor. The superconducting transition temperatures of NbSe2 monolayers are substantially increased by disorder. Realistic theoretical modeling shows that the unusual enhancement arises from the multifractality of electron wave functions. This work provides the first experimental evidence of the multifractal superconducting state. (2) In monolayer FeSe, we report the direct observation of BEC-BCS crossover. The scanning tunneling spectra demonstrate the evolution from the BCS to the BEC regions in real space. In the BCS region, the superconducting gap closes and re-opens under magnetic field, which reveals a quantum phase transition from the superconducting state to the pseudogap state. The spectra are nearly independent of temperature in the BEC region. In the BEC-BCS crossover region, a pseudogap above superconducting transition temperature is observed.

 

Biography:

Xi Chen received B.S. (1993) and M.S. (1996) degrees from Tsinghua University. He carried out graduate education and research in physics at Cornell University under the guidance of Professor Wilson Ho. After completion of his Ph.D. work in 2004, he spent more than one year at University of Californian, Irvine as a postdoctoral associate. He became an assistant professor of physics at Tsinghua University in 2006 and was promoted to a professor of physics in 2010. He was elected a fellow of APS in 2019. His current research focuses on the design, growth and probing of quantum matter with unconventional electronic properties. The studies span a wide range of effects that can emerge from electron correlation and spin-orbit coupling.