Husimi function and phase-space analysis of bilayer quantum Hall systems at $ν=2/λ$
We propose localization measures in phase space of the ground state of bilayer quantum Hall (BLQH) systems at fractional filling factors $ν=2/λ$, to characterize the three quantum phases (shortly denoted by spin, canted and ppin) for arbitrary $U(4)$-isospin $λ$. We use a coherent state (Bargmann) representation of quantum states, as holomorphic functions in the 8-dimensional Grassmannian phase-space $\mathbb{G}^4_{2}=U(4)/[U(2)\times U(2)]$ (a higher-dimensional generalization of the Haldane's 2-dimensional sphere $\mathbb{S}^2=U(2)/[U(1)\times U(1)]$). We quantify the localization (inverse volume) of the ground state wave function in phase-space throughout the phase diagram (i.e., as a function of Zeeman, tunneling, layer distance, etc, control parameters) with the Husimi function second moment, a kind of inverse participation ratio that behaves as an order parameter. Then we visualize the different ground state structure in phase space of the three quantum phases, the canted phase displaying a much higher delocalization (a Schrödinger cat structure) than the spin and ppin phases, where the ground state is highly coherent. We find a good agreement between analytic (variational) and numeric diagonalization results.