Order formation and superfluidity of excitons in type-II semiconductor quantum wells

The condensed state and superfluidity of excitons in type-II semiconductor quantum wells (QW's) are investigated theoretically. Since the excitons in type-II QW's have translational motion along the layer, the assembly of the m is regarded as an interacting dilute quasi-two-dimensional Bose gas. This system is advantageous for our purpose because those excitons have a long lifetime of the order of 10(-6) s, and their transport mechanism can be dir ectly studied in experiments by observing electric current since the excito ns consist of spatially separated electron-hole pairs. Using the exciton wa ve functions obtained by the variational method, the exciton-exciton intera ction is calculated and found to be repulsive when the thickness of the QW is thinner than a critical value. To illustrate the situation, we carry out the numerical computation adopting a model system with material constants appropriate to GaAs/AlAs type-II QW's. The basic equation for the phase of the condensate wave function is derived when the exciton system is irradiat ed by a weak laser light at zero temperature. Solving the equation in the p resence of the external current J(ex), we study the stationary spatial patt ern of the phase of the condensate wave function. It is shown that there ap pears a vortex lattice with a net supercurrent when J(ex) is larger than a critical value; the period of the lattice is determined as a function of J( ex). We calculate the magnetic field induced by the current in the vortex l attice, and discuss a possibility of an experimental observation of the cri tical current. Such a direct observation of the exciton transport will prov ide unambiguous experimental evidence for the superfluidity of excitons. [S 0163-1829(99)15931-9].