Research

Research interests:

I am interested in correlation, topology, and geometric aspects of theoretical condensed matter physics. My research interests cover various topics of condensed matter physics: unconventional superconductivity and magnetism, topological superconductors and Majorana fermions, heterostructures and Josephson junctions, topological insulators and semimetals, 2D and moir\'e materials, density-wave physics, nonlinear optics, strongly correlated physics, as well as applying the concepts of quantum information in condensed matter physics.

Research highlights:

Quasi-one-dimensional Supersolids in Luther-Emery Liquids

The supersolid is a long-sought phase in condensed matter physics, characterized by the coexistence of density wave and superfluid orders. This phase is counterintuitive, as different symmetry-breaking orders typically compete with one another. A deeper understanding of how such a state forms in condensed matter systems remains an open question, especially in quasi-one-dimensional correlated systems. In this work, we investigate the emergence of supersolids in Luttinger-Emery liquids using a variational method. As the system consists of coupled Luttinger-Emery liquid chains, we refer to this phase as a quasi-one-dimensional supersolid. Notably, we demonstrate that the quasi-one-dimensional supersolid phase is energetically favorable in chains with finite size or short-range order. Furthermore, we investigate the collective dynamics of these coexisting charge density waves and superconducting states, identifying a quasi-Goldstone mode. Our theory provides valuable insights into both the ground state and the dynamic properties of supersolids in strongly correlated systems.

Read more: To appear on PNAS (2025)

Dynamical and Geometric effects in density wave materials

Recently, I studied the dynamics and geometric effects in materials that support charge or spin density wave orders. In [PRL24'], we systematically explore the sliding dynamics of Skyrmion and Helix in chiral magnets in the limit of large current density. Moreover, we also investigate the nonreciprocal nonlinear responses in moving charge density waves [npj quantum materials, 24']. The geometric effects induced by the interference of triple-Q charge or spin density waves are also recently highlighted in [PRB 24'].

Orbital Fulde-Ferrell Pairing State in Moiré Ising Superconductors

Because of the large Ising SOC, the depairing effect caused by the Zeeman field is negligible and the in-plane upper critical field (Bc2) is determined by the orbital effects. This allows us to study the effect of large orbital fields. Interestingly, when the applied in-plane field is larger than the conventional orbital Bc2, a finite-momentum pairing phase would appear which we call the orbital Fulde-Ferrell (FF) state. This orbital field-driven FF state is different from the conventional FF state driven by Zeeman effects in Rashba superconductors. Remarkably, we predict that the FF pairing would result in a giant superconducting diode effect under electric gating when layer asymmetry is induced. An upturn of the Bc2 as the temperature is lowered, coupled with the giant superconducting diode effect, would allow the detection of the orbital FF state.

Valley-polarized state mediated unconventional Josephson junctions in superconducting TBG

Recently, we first helped Dmitri E Efetov's group at LMU to explain the highly unconventional Fraunhofer patterns seen in the MATBG Josephson junction (as schematically plotted in (a)) [Nat. Commun. 23']. Interestingly, we further found that the valley-polarized state connecting two superconducting regions of MATBG would give rise to a long-sought-after purely electric-controlled ϕ0-junction in which the two superconductors acquire a finite phase difference in the ground state [PRR 23']. Furthermore, based on this theoretical framework, we obtained the Josephson diode effect in a valley-polarized Josephson junction, which was also observed in the experiment [PRL 23'].

Intervalley-coherent state induced Topological Josephson Junctions in Bernal Bilayer Graphene

Recent experiments on Bernal bilayer graphene (BLG) deposited on monolayer WSe2 revealed robust, ultra-clean superconductivity coexisting with sizable induced spin-orbit coupling. Here we propose BLG/WSe2 as a platform to engineer gate-defined planar topological Josephson junctions, where the normal and superconducting regions descend from a common material. More precisely, we show that if superconductivity in BLG/WSe2 is gapped and emerges from a parent state with inter-valley coherence, then Majorana zero modes can form in the barrier region upon applying weak in-plane magnetic fields. Our results spotlight a potential pathway for `internally engineered' topological superconductivity that minimizes detrimental disorder and orbital-magnetic-field effects.

Spin-orbit-parity Coupled superconductivity

For clean superconductors, the enhancement of the upper critical field (Bc2) beyond the Pauli limit is conventionally realized in noncentrosymmetric superconductors. However, the gated-induced superconducting 1T' WTe2 (normal state is a quantum spin Hall insulator) is centrosymmetric and displays the enhancement of Bc2 in the experiment. Motivated by this experiment, we find a new type of superconductivity arising from the spin-orbit-parity coupling (SOPC) in centrosymmetric superconductors, where the SOPC pins the electron spins and renormalizes the effect of external Zeeman fields. As a result, the Bc2 is enhanced. We dubbed such superconductors as SOPC superconductors. This finding explains the enhancement of in-plane upper critical field Bc2 in topological monolayer WTe2. This work identified a new property of centrosymmetric superconductivity with topological band inversions and was published on PRL 20'. Shortly after our theoretical prediction, the SOPC superconductivity was demonstrated in the superconducting centrosymmetric atomically thin 2M-WS2, a material that has been predicted to exhibit topological band inversions. We provided the theoretical support for this work and the results are published Nat. Phys. 23'. The observation of SOPC superconductivity was recently highlighted in phys.org.

Kramers nodal line metal

Recently, it was pointed out that all chiral crystals with spin-orbit coupling (SOC) can be Kramers Weyl semimetals (KWSs) which possess Weyl points pinned at time-reversal invariant momenta. In this work, we show that all achiral non-centrosymmetric materials with SOC can be a new class of topological materials, which we term Kramers nodal line metals (KNLMs). In KNLMs, there are doubly degenerate lines, which we call Kramers nodal lines (KNLs), connecting time-reversal invariant momenta. The KNLs create two types of Fermi surfaces, namely, the spindle torus type and the octdong type. Interestingly, all the electrons on octdong Fermi surfaces are described by two-dimensional massless Dirac Hamiltonians. These materials support quantized optical conductance in thin films. We further show that KNLMs can be regarded as parent states of KWSs. Therefore, we conclude that all non-centrosymmetric metals with SOC are topological, as they can be either KWSs or KNLMs. Based on our work, some real materials of KNLMs have recently been studied in the experiments and the KNL superconductors have been explored in the experiments.