Probing Quantum Anomalous Hall States in Twisted Bilayer WSe2 via Attractive Polaron Spectroscopy

Abstract

Moiré superlattices in semiconductors exhibit a rich variety of interaction-induced topological states, including quantum anomalous Hall (QAH) effects. A recent study hinted that twisted WSe2 homobilayer (tWSe2) could host a QAH state but lacked direct evidence of ferromagnetism, a key hallmark of this phase. Here, we report the first direct evidence of QAH states in tWSe2 with spontaneous ferromagnetism. Specifically, we employ polarization-resolved attractive polaron spectroscopy on a dual-gated, 2 degree tWSe2 and observe direct signatures of spontaneous time-reversal symmetry breaking at hole filling ν= 1. Together with a Chern number measurement via Streda formula analysis, we identify this magnetized state as a topological state, characterized by C = 1. Furthermore, we demonstrate that these topological and magnetic properties are tunable via a finite displacement field, between a QAH ferromagnetic state and an antiferromagnetic state. Our findings position tWSe2 as a highly versatile, stable, and optically addressable platform for investigating topological order and strong correlations in two-dimensional landscapes.

Figures (4)

• Figure 1: a, Illustration of the moiré pattern in a R-stacked twisted $\text{WSe}_2$ homobilayer. Orange and red circles denote the high-symmetry atomic registries MX and XM, respectively (see inset). b, Schematic of an attractive polaron (AP) formed by an exciton in one valley (here at $-K$) and a hole in the opposite valley ($+K$). The exciton consists of an electron in the lowest conduction moiré band (dashed) and a hole in the topmost valence moiré band ($V_1$). A second valence moiré band, $V_2$, is separated from $V_1$. c, Reflection contrast (RC) spectrum, defined as $\frac{R-R_0}{R_0}$, at temperature 4K. The hole-filling fraction $\nu$ is tuned by gates (see Methods for calibration). The horizontal dashed lines (and arrows) indicate the integrated energy range of the attractive polaron. d, Derivative of RC with respect to $\nu$. e, Doping dependence of the integrated AP intensity ($R_{\mathrm{AP}}$).
• Figure 2: RC as a function of energy and hole filling fraction $\nu$ under a,$\sigma^-$ and b,$\sigma^+$ excitation, respectively. c, Spectrally resolved reflection magnetic circular dichroism (RMCD) at various $\nu$. d, Polarization-resolved reflection spectrum at $\nu = 1$. e, Magnetic hysteresis loop measured at $\nu = 1$; green and orange indicate the scan directions with respect to the magnetic field ($\pm\Delta B$). f, Schematic of the many-body bands at $\nu = 1$, incorporating electrostatic interaction at the mean-field level wu2019topological. v$_1$ and v$_2$ denote the first and the second moiré valence bands, respectively. This diagram is specific to $\nu=1$ and does not represent the band structure at other fillings.
• Figure 3: a, Integrated AP intensity as a function of magnetic field $\mu_0H$ and hole filling $\nu$. b, Extracted maxima of the integrated AP intensity at each $\mu_0H$ near $\nu \simeq 1, 3$, indicated by blue, and orange points, respectively. Error bars represent 3% deviation from the centroid for $\nu=1$ and 5% deviation for $\nu=3$ (see Methods). Red and dark green lines near $\nu = 1$ and $\nu = 3$ indicate a linear fit to the data near $\nu=1$ and $\nu=3$, from which the slope indicates Chern number $C=1$ for $\nu = 1$, and $C=0$ for $\nu = 3$.
• Figure 4: a, RMCD as a function of the displacement field $D$ and hole filling fraction $\nu$ at magnetic field $B=0$ and temperature $T_{\rm{lattice}}$=66 mK. b, (Red, left y-axis) Reflection magnetic circular dichroism (RMCD) as a function of the displacement field $\frac{D}{\epsilon_0}$ at $B=0$. (Blue, right y-axis) derivative of RMCD with respect to magnetic field $B$ at $B=0$ measured as a function of $\frac{D}{\epsilon_0}$. c, Magnetic susceptibility as a function of temperature for two displacement field values. The dashed line is the Curie-Weiss fit. All the small displacement field fittings are done with data points above the Curie temperature of $\sim$1.2 K, see Supplemental Information Fig. 3, Fig. 9. d, Curie-Weiss temperature $T_{CW}$ as a function of $\frac{D}{\epsilon_0}$. Linear fitting error bars are also shown. The inset schematics indicate the (bottom) QAH FM phase and the (top) AFM phase, respectively.