Interacting composite fermions

Even though much of the dramatic physics of two-dimensional electrons in a high magnetic field is explicable in terms of weakly interacting composite fermions (CFs), the inter-CF interaction is responsible for many interesting, non-trivial phenomena. Here, we discuss four examples. (i) At small filling factors, a softening of the roton mode destroys the fractional Hall effect, giving way to the Wigner crystal. (ii) In higher Landau levels, the fractional Hall effect is destroyed due to a collapse of the energy of the neutral exciton. (iii) At ν=5/2, the Fermi sea of CFs is unstable to Cooper pairing of CFs, thereby opening up a gap and producing a fractional Hall effect. (iv) Prior to the transition into the Wigner crystal, the CF liquid exhibits the Bloch instability into a magnetically ordered, spontaneously broken symmetry phase.