Single electron effects and tunneling spectroscopy with a cryogenic scanning tunneling microscope.

Abstract

We study experimentally tunnel junction systems that involve a very small conductive element (typical length scale of 100A). In these so-called mesoscopic tunnel junctions, the capacitive charging energy is larger than the thermal energy at the experimental temperatures ($\sim$4.2K). Experimental problems realizing current-driven single junction charging effects motivate the consideration of two serially connected junctions driven by a voltage source. A brief theoretical treatment based on the semi-classical model for this system is presented, including details for an analytic solution for the calculation of current-voltage (I-V) characteristics. Basic physical assumptions used in the theoretical account are stated. A historical perspective on tunneling spectroscopy and single electron charging effects is taken for the introduction of these subjects. We describe an experiment where the STM probes an indium droplet (diameter $\sim$100A) that sits on an oxidized aluminum substrate. This system forms two mesoscopic tunnel junctions connected in series. Data in the form of a Coulomb staircase is of a quality that allows a detailed comparison to a theory based on the semi-classical model of mesoscopic tunnel junctions. The data shows that the typical capacitance associated with the droplets is 10$\sp{-18}$F. We conclude that the double junction system views the external circuit as an ideal voltage source (the effective source can respond during the time of tunneling), and that the electronic states of the droplet can be considered continuous. We examine single junctions formed with the STM and find that surface oxide effects of both the tip and sample can mask spectroscopic information of the underlying metals, such as a superconducting density of states. We present strong evidence that in some cases capacitive charging effects taking place within the oxides are responsible for the masking effect. The observation of Coulomb staircases in the I-V characteristics at some sites on bulk etched tungsten provide this evidence. These measurements lead to a much different interpretation of Coulomb blockade-like I-V characteristics measured with a cryogenic STM. Possible implications for other tunnel junction systems are discussed. Finally, we present a tunneling spectroscopy study of superconducting thin film $YBa\sb2Cu\sb3O\sb7$ $(T\sb c \sim 85\rm K)$. Common in the data are Coulomb staircases, indicating charging effects taking place in the granular films. We fit an example of such an I-V characteristic. Our results provide a better understanding of what role charging effects play in the properties of high-$T\sb c$ superconducting films. The most common type of I-V characteristic observed has a region of high conductance about zero bias. We present plausible explanations for this phenomena. We also present and discuss data showing negative differential resistance and BCS gap-like structure.