Beyond Mean-Field Microscopic Theoretical Framework in Many-Body Systems

by Prof. Fei Yang (杨飞) (HKUST)

Tuesday 22 Sept 2026, 14:00 → 15:00 Asia/Shanghai

Tsung-Dao Lee Institute/N6F-N600 - Lecture Room (Tsung-Dao Lee Institute)

Host: Prof. Tao Dong  

Venue: TDLI Meeting Room N600

Tencent Meeting link: https://meeting.tencent.com/dm/12EW8miKQBxq

Meeting ID: 261744988, no password

 

Abstract:

Understanding how collective fluctuations reshape ordered states beyond mean-field theory is a central challenge in modern many-body physics, especially in low-dimensional and strongly interacting systems. In this talk, we present a unified microscopic framework for phase transitions, crossovers, and decoherence phenomena driven by collective bosonic modes in superconductors, Josephson systems, and charge-density-wave materials.

For two-dimensional superconductors, we develop a self-consistent theory incorporating quasiparticles, phase fluctuations, disorder, and BKT physics. We show how long-range Coulomb interactions modify phase modes, overcoming the conventional Mermin–Wagner instability while retaining strong fluctuation effects. The theory captures gate-tunable superconductivity in atomically thin systems and the disorder-driven evolution of superconducting transitions and pseudogap regimes.

We further apply this framework to low-dimensional Josephson systems, demonstrating that thermal phase fluctuations can suppress interlayer coherence, quench the Josephson diode effect, and drive phase-incoherent states even when the superconducting gap remains finite.

Finally, we develop a microscopic theory of lattice-driven charge-density waves, showing that thermally enhanced phason fluctuations lead to phason softening, hidden depinning crossovers, amplitudon damping, and ultimately first-order transitions.

Overall, these results establish a unified beyond-mean-field perspective on how collective bosonic fluctuations govern ordering, decoherence, and hidden crossovers across a broad class of quantum materials.

 

Biography:

Dr. Fei Yang is a theoretical condensed matter physicist specializing in quantum materials and many-body physics. He received his Ph.D. in Physics from the University of Science and Technology of China (USTC) in 2021. He has carried out postdoctoral research at USTC, The Pennsylvania State University, and The Hong Kong University of Science and Technology (HKUST), where he investigated emergent phenomena in quantum materials, including unconventional superconductivity, charge density waves, ferroelectricity, ultrafast quantum dynamics, and correlated electronic phases. His research focuses on developing microscopic theories to understand the collective behaviors and novel quantum states arising from strong interactions in quantum materials.