Quantum Lifetime Spectroscopy and Magnetotunneling in Double Bilayer Graphene Heterostructures*

Energy resolved lifetime measurements provide unique insight into the fundamental relaxation mechanisms of quantum states in a material. We describe a tunneling spectroscopy technique in a double bilayer graphene heterostructure where momentum-conserving tunneling between different energy bands serves as an energy filter for the tunneling carriers, and allows a measurement of the quasi-particle state broadening at well-defined energies and temperatures.  The data reveal the quasi-particle state broadening increases linearly with the state energy separation from the Fermi level, and is relatively insensitive to temperature. These observations suggest the quasi-particle state broadening at the Fermi level is disorder limited, and at excited states is dominated by carrier-carrier interactions or phonon emission. We use magnetotunneling spectroscopy to confirm the high degree rotational alignment of the two bilayer graphenes and an absence of momentum randomizing processes. Using a combination of measurements and calculations, we determine the interlayer separation in the double bilayer graphene heterostructure.