Yi-Chun Hung

Yi-Chun Hung

A physics PhD student at NEU

Interaction-driven phases in flat bands

Flat bands, characterized by strongly suppressed kinetic energy, provide an ideal setting for studying interaction-driven quantum phases. In such systems, electron-electron interactions dominate over band dispersion, leading to a variety of unconventional correlated phenomena. Additionally, due to the suppress band dispersion, the quantum geometry of Bloch wave functions plays an essential role in determining collective behavior. Quantities such as the quantum metric and Berry curvature can influence superconductivity (Liang et al., 2017) and collective excitations (Ying and Li, 2025), providing new routes for understanding interaction-driven phases in flat-band systems.

My research explores how interactions and quantum geometry cooperate to generate emergent phenomena in flat-band and moiré systems. In pseudospin-1 Dirac flat bands, we showed that interactions drive a staggered virtual-loop-current (SVLC) order accompanied by a \(\sqrt{3}\times\sqrt{3}\) charge order by coupling the underlying pseudospin-1 Dirac cones at filling \(\nu=1/3\) [1]. The corresponding spontaneous time-reversal symmetry breaking can be probed via nonzero anomalous Hall conductivity and orbital magnetization. Furthermore, at \(\nu=2/3\), such an SVLC order coexists with a staggered loop-current order, which generates local magnetic moments that may be accessible via spin-neutron diffraction (Liege et al., 2024). Unlike pseudospin-1/2 Dirac flat bands such as those in twisted bilayer graphene, the onset of SVLC does not require a well-defined low-energy valleys. Instead, equivalence of flat-band wavefunctions near the pseudospin-1 Dirac cones plays a central role in coupling these Dirac cones.

We also showed that, in a flat band without spin-orbit coupling, interaction and Zeeman field drives the systems into an excitonica insulator phase [2]. The quantum metric of the flat band enters the corresponding superfluidity state through its phase stiffness (superfluid weight) and its superfluidensity fluctuation. We discover that the superfluid density fluctuation exhibits a negative kinetic coefficient due to the non-trivila quantum metric, which can be probed via momentum-resolved in-plane magnetization susceptibility. This work highlight how flat-band engineering, interactions, and quantum geometry can combine to produce novel collective quantum phenomena.

  • [1] Yi-Chun Hung and Arun Bansil. Staggered Virtual-Loop-Current Order in Pseudospin-1 Dirac Flat Bands. arXiv:2606.16155.
  • [2] Yi-Chun Hung, Xiaoting Zhou, and Arun Bansil. Quantum Geometry Driven Finite-Momentum Exciton Fluctuations in Flat-Band Systems. arXiv:2606.23598.