Monolayer Heterostructure*

We investigate low temperature photoluminescence of interlayer excitons in hexagonal boron nitride (hBN) encapsulated monolayer MoSe₂ and WSe₂ vertical heterostructure. Surprisingly, we find the interlayer exciton splits into four different resonances at 1305 meV, 1340 meV, 1360 meV, and 1387 meV. All four resonances exhibit long photoluminescence lifetimes of hundred nanoseconds, which verify their interlayer nature. Based on the temperature and excitation power dependence, we tentatively assign the lower energy resonances, 1305 meV and 1340 meV resonances to be indirect and direct interlayer exciton in the momentum space, respectively.