  The strong spin-orbit coupling and the broken inversion symmetry in monolayer
transition metal dichalcogenides (TMDs) results in spin-valley coupled band
structures. Such a band structure leads to novel applications in the fields of
electronics and optoelectronics. Density functional theory calculations as well
as optical experiments have focused on spin-valley coupling in the valence
band. Here we present magnetotransport experiments on high-quality n-type
monolayer molybdenum disulphide (MoS$_{2}$) samples, displaying highly resolved
Shubnikov-de Haas oscillations at magnetic fields as low as $2~T$. We find the
effective mass $0.7~m_{e}$, about twice as large as theoretically predicted and
almost independent of magnetic field and carrier density. We further detect the
occupation of the second spin-orbit split band at an energy of about $15~meV$,
i.e. about a factor $5$ larger than predicted. In addition, we demonstrate an
intricate Landau level spectrum arising from a complex interplay between a
density-dependent Zeeman splitting and spin and valley-split Landau levels.
These observations, enabled by the high electronic quality of our samples,
testify to the importance of interaction effects in the conduction band of
monolayer MoS$_{2}$.
