Mechanical measurements of two-dimensional flux lattices: Observation of two-stage melting

Using a high-Q silicon mechanical oscillator we have studied flux lattice melting in two-dimensional amorphous composite In/${\mathrm{InO}}_{\mathrm{x}}$ superconducting films. In addition to the previously observed solid and liquid phases, the flux lattice exhibits a third stable phase, intermediate between the solid and liquid, which exists at high sheet resistances, large magnetic fields, and low temperatures. For a range of sheet resistances, this phase coexists at a tricritical point with the liquid and solid phases. In the limit of high sheet resistance, only this new phase is stable, preempting the solid phase. We argue that this intermediate phase is unlikely to be an hexatic phase, and is more likely to be an amorphous solid phase.