arXiv subjects

Bokai Liang

Publications and source records attributed to Bokai Liang.

4 recordsLinked to original sources

arXiv2026

Displacement-Field-Driven Semimetal-Superconductor Transition in Magic-Angle Twisted Trilayer Graphene

Magic-angle twisted trilayer graphene(MATTG) hosts versatile displacement-field-tuned correlated phenomena. MATTG consists of a dispersive Dirac cone which hybridizes with the flat band from a twisted bilayer graphene (TBG) sector. The hybridization strength increases with the displacement field $D$ and naively one may expect D-driven heavy fermion physics. However, the TBG Hubbard bands have a momentum-selective Mott gap, which is small at the $\Gamma$ point due to the band topology, and a rigid local moment description as in the familiar Kondo lattice model is invalid. Here we show that the dominant effect of the displacement field is to induce an energy shift of the Dirac cone and self-doping into the TBG sector. We illustrate this picture in a concrete calculation using a slave-particle theory at the filling $\nu=\pm 2$. We find that increasing $D$ drives a transition from a semimetal into a superconducting state. We also discuss the enhancement of the superconductivity by $D$ near $\nu=\pm2$ and the particle-hole asymmetry of the phase diagram. Our results provide a unified picture for electric-field-tunable superconductivity, Mottness, and heavy-fermion-like behavior in MATTG.

cond-mat.str-el↗

arXiv2025

Topological Flat Minibands and Fractional Chern Insulators in Rashba Systems with Tunable Superlattice Potentials

Fractional Chern insulators are interaction-driven topological states that realize the lattice analogue of the fractional quantum Hall effect without external magnetic fields. Here we propose a programmable platform for engineering topological flat bands by combining a Zeeman field with a tunable electrostatic superlattice potential in a Rashba spin-orbit-coupled thin film. The Zeeman field generates a nearly flat segment of the Rashba band, which is isolated by the superlattice potential into a flat band with Chern number $\mathcal{C}=1$. Using many-body exact diagonalization, we show that this band supports fractional Chern insulators at filling factors $n = 1/3$ and $2/3$, both of which remain robust over broad parameter ranges. We further identify realistic material candidates and the corresponding device conditions that enable experimental realization. These results establish a versatile and experimentally accessible platform for engineering topological flat bands and exploring correlated topological phases.

cond-mat.mes-hall↗

arXiv2024

Resolving and routing the magnetic polymorphs in 2D layered antiferromagnet

Polymorphism, commonly denoting the variety of molecular or crystal structures, is a vital element in many natural science disciplines. In van der Waals layered antiferromagnets, a new type of magnetic polymorphism is allowed by having multiple layer-selective magnetic structures with the same total magnetization. However, resolving and manipulating such magnetic polymorphs remain a great challenge. Here we use the phase-resolved magnetic second-harmonic generation microscopy to elucidate such magnetic polymorphism in the 2D semiconducting layered antiferromagnet chromium sulfur bromide (CrSBr), and demonstrate how the magnetic polymorphs can be deterministically switched in an unprecedented layer-selective manner. With the nonlinear magneto-optical technique unveiling the magnetic symmetry information through the amplitude and phase of light, we could unambiguously resolve the polymorphic spin-flip transitions in CrSBr bilayers and tetralayers. Remarkably, the deterministic routing of polymorphic transitions originates from the breaking of energy degeneracy via a magnetic layer-sharing effect: the spin-flip transitions in a tetralayer are governed by the laterally extended bilayer, which acts as a control bit. We envision such controllable magnetic polymorphism to be ubiquitous for van der Waals layered antiferromagnets, and could lead to conceptually new design and construction of spintronic and opto-spintronic devices for probabilistic computation and neuromorphic engineering.

cond-mat.mes-hall↗

arXiv2022

Giant efficiency of long-range orbital torque in Co/Nb bilayers

We report unambiguously experimental evidence of a strong orbital current in Nb films with weak spin-orbit coupling via the spin-torque ferromagnetic resonance (ST-FMR) spectrum for Fe/Nb and Co/Nb bilayers. The sign change of the damping-like torque in Co/Nb demonstrates a large spin-orbit correlation and thus great efficiency of orbital torque in Co/Nb. By studying the efficiency as a function of the thickness of Nb sublayer, we reveal a long orbital diffusion length (~3.1 nm) of Nb. Further planar Hall resistance (PHE) measurements at positive and negative applying current confirm the nonlocal orbital transport in ferromagnetic-metal/Nb heterostructures.

physics.app-ph↗