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 Landé 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.
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We are thankful for discussions with S. Ilani, D. Orgad, A. Yacoby, P. Jarillo-Herrero, E. Rossi and E. Andrei. M. Aprili, C. H. L. Quay, and M. Kuzmenovic assisted with high magnetic field measurements. Device fabrication and characterization were carried out at the Harvey M. Krueger Family Center for Nanoscience and Nanotechnology. Part of this work was performed at the LNCMI, a member of the European Magnetic Field Laboratory. Work was supported by ERC-2014-STG Grant No. 637298 and ISF Quantum Initiative grant No. 994/19. A.Z. and T. D. Are supported by an Azrieli Fellowship. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan and and the CREST (JPMJCR15F3), JST.
© 2020, The Author(s).