Skyrmionic Transport and First Order Phase Transitions in Twisted Bilayer Graphene Quantum Hall Ferromagnet

TL;DR

The work investigates multicomponent quantum Hall physics in large-angle twisted bilayer graphene (TBLG), where eightfold degenerate Landau levels arise from spin, valley, and layer pseudospins and strong Coulomb interactions drive symmetry-breaking ground states. Using tilted-field and displacement-field dependent transport in high-mobility single- and dual-gated devices, they reveal skyrmion-textured charged excitations and a field-induced insulating transition to an intervalley-coherent (IVC) Kekulé-ordered state at $\nu_{tot}=0$. Finite displacement field $D$ drives charge imbalance and multidomain nucleation, producing pronounced hysteresis in exchange-dominated transport and signaling first-order phase transitions between QH ferromagnetic ground states; dual-gated devices show layer-coherent ground states at $D=0$ with Ising-like pseudospin order giving way to domain-wall skyrmions under finite $D$. Tilt-field activation-gap analysis yields spin-skyrmion textures with effective slopes $g_{\parallel}$ exceeding the bare $g_0$, supporting a transport picture controlled by skyrmionic excitations in the low-energy LLs, paralleling graphene but enriched by layer pseudospin physics.

Abstract

Large-angle twisted bilayer graphene (TBLG) realizes a multicomponent quantum Hall (QH) platform of spin, valley and layer pseudospins with strong Coulomb interaction-driven symmetry broken phases. Here, we investigate the low energy Landau-level spectrum of layer-decoupled TBLG and identify skyrmion-textured charged excitations and a field-induced insulating transition to an intervalley coherent state at zero-filling factor. Symmetric potential difference perpendicular to TBLG demonstrated layer coherent population of ground states with uniform energy barriers, while the charge imbalance in the layers at finite displacement field led to multidomain nucleation and a pronounced hysteresis in the exchange-dominated transport regime suggesting first order phase transitions between different QH ferromagnetic ground states.

Figures (4)

• Figure 1: (a) Schematic illustration of the bottom-gated h-BN encapsulated TBLG in Hall bar geometry.(b) Landau fan diagram showing $R_{xx}$ as a function of $B$ and $n_{\text{tot}}$ at 1.6 K. Different $\nu_{\text{tot}}$ values are shown in white dashed line.(c) $\Delta R_{xx}$ vs. $1/B$ for different $n_{\text{tot}}$ ($\times 10^{12} \, \text{cm}^{-2}$) values at 1.6 K (left) and corresponding FFT for each $n_{\text{tot}}$ (right).(d) The upper layer ($n_U$), lower layer ($n_L$), and total ($n_{\text{tot}}$) carrier densities as function of gate voltage ($V_{\mathrm{bg}}$). The dashed lines are the theoretical fits of graphene double layer model to experimental $|n|$ values estimated from FFT in (c).
• Figure 2: (a) Evolution of $R_{xx}$ vs. $\nu_{\text{tot}}$ for e-h sweep at different B at 1.6 K with arrows indicating broken symmetry states. (b) $R_0$ (i.e., $R_{xx}$ at $\nu_{\text{tot}} = 0$) vs. $(1-h)^{-1/2}$ fitted using the KT equation (dashed line). Insets: Activation gap at $\nu_{\text{tot}} = 0$, $\Delta^0$ as a function of $B$ and schematic of IVC Kekulé order phase. (c) $R_{xx}$ vs. $\nu_{\text{tot}}$ for various tilt angles $\theta$ at $B = 11~\text{T}$ as shown in left , center and right plots. Left inset: schematic of the tilted-field geometry. Righ inset: $R_{0}$ vs. $B_{p}$ at different $\theta$. (d)(i)-(iii) Activation gaps $\Delta^\nu$ vs. $B$ for $\nu_{\text{tot}} = 1, 2, 3,$ at fixed $B_{p} = 6~\text{T}$. The slope of the linear fit for each filling yields $g_{\parallel}$ at the corresponding $\nu_{\text{tot}}$.
• Figure 3: (a)-(b) Dual-sweep measurements of $R_{xx}$ and $\sigma_{xy}$ vs. $\nu_{\text{tot}}$ at $B = 9 \,\text{T}$ and $T = 1.6 \,\text{K}$. Black and red arrows indicate the direction of the voltage sweep. The insets in (a) show ground and excited states of the $\nu_{\text{tot}} = 0$ IVC phase. (c) and (d) Dual-sweep measurements of $R_{xx}$ and $\sigma_{xy}$ as a function of total filling factor $\nu_{\text{tot}}$ for D1 under three conditions: (i) $B=3$ T, $T=1.6$ K (ii) $B=9$ T, $T=7$ K and (iii) $B=9$ T, $T=15$ K.
• Figure 4: (a) Dual-sweep $R_{xx}$ vs. $\nu_{\text{tot}}$ at several $D/\varepsilon_0$ (in unit of mV/nm) for D2 at $B=9$ T and $T=1.6$ K indicating broken symmetry states and hysteresis.(b) Contour plot of $R_{xx}$ vs. $D/\varepsilon_0$ and $\nu_{\text{tot}}$ for D2 at $B=9$ T and $T=1.6$ K. The LL filling contributions from the upper and lower layers $(\nu_{U} + \nu_{L})$ are illustrated.