title: Sensing the quantum limit in scanning tunnelling spectroscopy.
abstract: The tunnelling current in scanning tunnelling spectroscopy (STS) is typically and often implicitly modelled by a continuous and homogeneous charge flow. If the charging energy of a single-charge quantum sufficiently exceeds the thermal energy, however, the granularity of the current becomes non-negligible. In this quantum limit, the capacitance of the tunnel junction mediates an interaction of the tunnelling electrons with the surrounding electromagnetic environment and becomes a source of noise itself, which cannot be neglected in STS. Using a scanning tunnelling microscope operating at 15 mK, we show that we operate in this quantum limit, which determines the ultimate energy resolution in STS. The P(E)-theory describes the probability for a tunnelling electron to exchange energy with the environment and can be regarded as the energy resolution function. We experimentally demonstrate this effect with a superconducting aluminium tip and a superconducting aluminium sample, where it is most pronounced.
authors.affiliation: Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, 70569 Stuttgart, Germany.
authors.forename: Christian R
authors.initials: CR
authors.lastname: Ast
authors.affiliation: Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, 70569 Stuttgart, Germany.
authors.forename: Berthold
authors.initials: B
authors.lastname: Jäck
authors.affiliation: Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, 70569 Stuttgart, Germany.
authors.forename: Jacob
authors.initials: J
authors.lastname: Senkpiel
authors.affiliation: Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, 70569 Stuttgart, Germany.
authors.forename: Matthias
authors.initials: M
authors.lastname: Eltschka
authors.affiliation: Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, 70569 Stuttgart, Germany.
authors.forename: Markus
authors.initials: M
authors.lastname: Etzkorn
authors.affiliation: Institut für Komplexe Quantensysteme and IQST, Universität Ulm, Albert-Einstein-Allee 11, 89069 Ulm, Germany.
authors.forename: Joachim
authors.initials: J
authors.lastname: Ankerhold
authors.affiliation: Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, 70569 Stuttgart, Germany.|Institut de Physique, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland.
authors.forename: Klaus
authors.initials: K
authors.lastname: Kern