  The valley degree of freedom in two-dimensional (2D) crystals recently
emerged as a novel information carrier in addition to spin and charge. The
intrinsic valley lifetime in 2D transition metal dichalcoginides (TMD) is
expected to be remarkably long due to the unique spin-valley locking behavior,
where the inter-valley scattering of electron requires simultaneously a large
momentum transfer to the opposite valley and a flip of the electron spin. The
experimentally observed valley lifetime in 2D TMDs, however, has been limited
to tens of nanoseconds so far. Here we report efficient generation of
microsecond-long lived valley polarization in WSe2/MoS2 heterostructures by
exploiting the ultrafast charge transfer processes in the heterostructure that
efficiently creates resident holes in the WSe2 layer. These valley-polarized
holes exhibit near unity valley polarization and ultralong valley lifetime: we
observe a valley-polarized hole population lifetime of over 1 us, and a valley
depolarization lifetime (i.e. inter-valley scattering lifetime) over 40 us at
10 Kelvin. The near-perfect generation of valley-polarized holes in TMD
heterostructures with ultralong valley lifetime, orders of magnitude longer
than previous results, opens up new opportunities for novel valleytronics and
spintronics applications.
