Paired states in the even integer quantum Hall effect

A type of quantum Hall state requiring nonperturbative Landau-level mixing is argued to arise at low electron density. In these states, up- and down-spin electrons pair to form spinless bosons that condense into a bosonic quantum Hall state. We show that a BCS instability arising in two-component Hall metals stabilizes this paired state at ν=2. We discuss various experimental implications including a new global phase diagram for the integral quantum Hall effect with spin.