Quantum-dot assisted spectroscopy of degeneracy-lifted Landau levels in graphene.

Energy spectroscopy of strongly interacting phases requires probes which minimize screening while retaining spectral resolution and local sensitivity. Here, we demonstrate that such probes can be realized using atomic sized quantum dots bound to defects in hexagonal Boron Nitride tunnel barriers, placed at nanometric distance from graphene. With dot energies capacitively tuned by a planar graphite electrode, dot-assisted tunneling becomes highly sensitive to the graphene excitation spectrum. The spectra track the onset of degeneracy lifting with magnetic field at the ground state, and at unoccupied excited states, revealing symmetry-broken gaps which develop steeply with magnetic field - corresponding to Lande g factors as high as 160. Measured up to B = 33 T, spectra exhibit a primary energy split between spin-polarized excited states, and a secondary spin-dependent valley-split. Our results show that defect dots probe the spectra while minimizing local screening, and are thus exceptionally sensitive to interacting states. Here, the authors develop a spectroscopic technique whereby individual defects in an ultrathin hBN dielectric, placed in proximity to graphene, act as quantum dots. Dot-assisted tunneling is highly sensitive to the nearby graphene excitation spectrum, and allows probing of energy splitting in the excited Landau levels.