[论文解读] Spontaneous momentum polarization and diodicity in Bernal bilayer graphene

Jiangxiazi Lin, Yibang Wang+5 more|arXiv (Cornell University)|Feb 8, 2023

Graphene research and applications被引用 9

一句话总结

该论文通过二次谐波角分辨非线性输运观测伯纳尔双层石墨烯中的自发动量极化，揭示与谷/同位自旋序相关的动量极化态级联。并确立 ARNTM 作为多层石墨烯自发对称性破缺的敏感探针。

ABSTRACT

The low-temperature phase diagram of multilayer graphene heterostructures is largely defined by the exchange-driven instability that lifts the four-fold isospin degeneracy. Such instability gives rise to the quarter- and half-metal phases, which are key to our understanding of other emergent phenomena. Recent theoretical works shed light on a new type of Coulomb-driven instability. It is proposed that the exchange interaction between trigonal-warping-induced Fermi pockets could induce charge carriers to condense into one of the Fermi pockets, giving rise to a net polarization in the momentum space. Here, we report the observation of spontaneous momentum polarization in Bernal bilayer graphene using angle-resolved nonlinear transport measurement at the second-harmonic frequency. With excellent angular precision, we show that the polar axis of the momentum polarization is tunable with varying carrier density, electric field, and magnetic field. The dominating influence of the momentum-space instability reveals a natural connection between broken symmetries, and the isospin degeneracy lifting in the half- and quarter-metal phases.

研究动机与目标

• 理解在多层石墨烯中通过交换驱动的不稳定性提升同位自旋简并性的动机与认识。
• Demonstrate spontaneous momentum polarization 作为 BLG 的库仑驱动不稳定性。
• Map how carrier density, displacement field, and magnetic field tune momentum-space polarization.
• Link momentum-space order to known isospin-ordered (half- and quarter-metal) phases.

提出的方法

• Use angle-resolved nonlinear transport measurements at the second-harmonic frequency to detect symmetry breaking.
• Fit the angular dependence V2ω(φ) to a combination of one-fold and three-fold symmetric components: |V2ω(φ)=V1 cos(φ−β1)+V3 cos(3(φ−β3)).
• Analyze n-D (carrier density vs displacement field) maps to track polar axis orientation β1 and nonlinear amplitude V1.
• Employ an out-of-plane and in-plane magnetic field to study its effect on momentum-space instability and Fermi-pocket occupation.
• Interpret ARNTM data to infer Fermi pocket occupation and valley polarization across transitions I–IV in the n-D map.
• Correlate nonlinear transport signals with magneto-oscillations and Landau level filling to elucidate Fermi-surface topology.

实验结果

研究问题

• RQ1 Can spontaneous momentum polarization be directly observed in Bernal bilayer graphene?
• RQ2 How does the polar axis orientation β1 and the nonlinear response V1 evolve with carrier density and displacement field?
• RQ3 What is the relationship between momentum polarization, valley isospin order, and known half-/quarter-metal phases?
• RQ4 How do magnetic fields (in-plane and out-of-plane) tune the momentum-space instability and Fermi-pocket occupation?
• RQ5 Can ARNTM distinguish momentum polarization from other sources of second-harmonic responses (Berry curvature dipole, skew scattering, nematicity)?

主要发现

• Spontaneous momentum polarization is observed, with the polar axis aligned along occupied Fermi pockets in valley K′ across most of the n-D map.
• The nonlinear response shows one-fold symmetry dominated by V1, with β1 rotating in response to transitions between regimes A, B, A′, B′, and C, forming a fan-like phase diagram.
• Evidence of a momentum-unpolarized (MUP) phase appears as narrow density ranges between transitions, consistent with sequential pocket occupation across valleys.
• Applying B fields induces rotations of β1 and reveals a cascade of momentum-polarized states within isospin-degenerate Landau levels, indicating spontaneous rotational symmetry breaking driven by momentum-space instability.
• Momentum polarization competes with spin polarization; large in-plane B suppresses PVP/PMP phases, while small B preserves them, suggesting a competition between spin and orbital channels.
• ARNTM is established as a sensitive tool to resolve spontaneously broken symmetries and to connect momentum-space instability with isospin degeneracy lifting and reduced rotational symmetry.