Halperin states of particles and holes in ideal time reversal invariant pairs of Chern bands and the fractional quantum spin Hall effect in moiré <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>MoTe</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>
Inti Sodemann
Abstract
An experiment in moir\'e ${\mathrm{MoTe}}_{2}$ bilayers reported the observation of a topologically ordered state with zero Hall conductivity and half of the edge conductance of a standard time-reversal invariant quantum spin Hall insulator [K. Kang et al., Nature (London) 628, 522 (2024)]. This state is believed to emerge at total filling one of a pair of bands with Chern numbers $C=\ifmmode\pm\else\textpm\fi{}1$ related by time-reversal symmetry. By viewing these bands as a pair of Landau levels with opposite magnetic fields, and starting from a parent magnet with one filled band, we demonstrate that a class of Halperin states constructed by adding particles to the empty Chern band and holes to the occupied Chern band have all the properties observed in ${\mathrm{MoTe}}_{2}$. Remarkably, these states break time-reversal symmetry but have exactly zero Hall conductivity and helical edge conductance of ${e}^{2}/2h$. These states also feature a spinless composite fermion with the same charge as the electron but split equally between both valleys. In a standard Halperin 331 state, this particle would be a neutral Bogoliubov composite fermion. However, in our context this composite fermion is charged but remains itinerant because it is split into the two valleys that effectively experience opposite magnetic fields. The existence of such charged itinerant particles is a key difference between Landau levels with opposite magnetic fields and standard multicomponent Landau levels, where all the itinerant particles are charge neutral, such as the magnetoroton of the Laughlin state or the Bogoliubov composite fermion of the Moore-Read state. When the electron density changes away from the ideal filling and these itinerant charged particles are added to the parent state, the disorder potential is less efficient at localizing them as compared to standard Landau levels. This can explain why the state reported in K. Kang et al. [Nature (London) 628, 522 (2024)] did not display a robust Hall plateau upon changing the electron density.