No abstract

We have imaged the current noise with atomic resolution in a Josephson scanning tunneling microscope with a Pb-Pb junction. By measuring the current noise as a function of applied bias, we reveal the change from single electron tunneling above the superconducting gap energy to double electron charge transfer below the gap energy when Andreev processes become dominant. Our spatially resolved noise maps show that this doubling occurs homogeneously on the surface, also on impurity locations, demonstrating that indeed the charge pairing is not influenced by disruptions in the superconductor smaller than the superconducting coherence length.

We report on the fabrication and performance of a new kind of tip for scanning tunneling microscopy. By fully incorporating a metallic tip on a silicon chip using modern micromachining and nanofabrication techniques, we realize so-called smart tips and show the possibility of device-based STM tips. Contrary to conventional etched metal wire tips, these can be integrated into lithographically defined electrical or photonic circuits, as well as mechanical systems. We experimentally demonstrate that the performance of the smart tips is on par with conventional ones, both in stability and resolution. In situ tip preparation methods are possible and we verify that they can resolve the herringbone reconstruction and Friedel oscillations on Au (111) surfaces. In addition, these devices can be made to accommodate two isolated tips with sub-50 nm apex-to-apex distance to measure electron correlations at the nanoscale using a new type of double-tip experiment described in this letter.Scanning tunneling microscopy (STM) is one of the leading tools for probing electronic and topographic information at the atomic scale [1]. Since its inception a few decades ago, data quality has dramatically improved by focusing on mechanical stability, tip preparation and lower temperatures. New possibilities have emerged and greatly extended the range of STM, including quasi-particle interference studies with density of states mapping [2, 3], spin-polarized STM [4], and ultralow temperature operation [5].Here, we introduce a platform for bringing new, devicebased functionality to STM, in order to utilize decades of progress in device engineering for the field of scanning probe. We replace the conventional electrochemically etched, pointy metal wire with an integrated metal tip on a silicon chip. This new platform, which we call smart tip, allows in principle to directly add additional capabilities to a STM tip, including novel spin-sensitivity, local heating, local magnetic fields, local gating, highfrequency compatible coplanar waveguides, resonators, qubits, and double tips. However, it is a priori unclear, whether such a nanofabricated tip will function for STM measurements, as several challenges arise: the stability needs to be below the picometer scale, stringent requirements exist on the shape and sharpness of the freestanding tip, and contamination from fabrication residues need to be absent. In this letter, we demonstrate the feasibility of the novel smart tip platform. We first discuss our newly developed fabrication procedure and then experimentally show the functionality of these tips in standard STM measurements. Finally, we present a theory of how such nanofabricated double tips will allow to measure electron correlations, if the tip-to-tip distance is on the * s.groeblacher@tudelft.nl † allan@physics.leidenuniv.nl