Vacuum cleaving of superconducting niobium tips to optimize noise filtering and with adjustable gap size for scanning tunneling microscopy

TL;DR

This paper presents a simple vacuum cleaving method to produce superconducting niobium tips for STM, enabling optimized noise filtering and tunable gap sizes for improved surface science measurements.

Contribution

It introduces a straightforward vacuum cleaving technique for Nb tips, demonstrating their use in noise characterization and tunable superconducting gaps for STM applications.

Findings

Vacuum cleaving produces fully gapped Nb tips in one step.

Nb tips can be used to measure the STM base temperature.

Ar$^{+}$ sputtering allows tuning of the superconducting gap.

Abstract

Superconducting (SC) tips for scanning tunneling microscopy (STM) can enhance a wide range of surface science studies because they offer exquisite energy resolution, allow the study of Josephson tunneling, or provide spatial contrast based on the local interaction of the SC tip with the sample. The appeal of a SC tip is also practical. An SC gap can be used to characterize and optimize the noise of a low-temperature apparatus. Unlike typical samples, SC tips can be made with less ordered materials, such as from SC polycrystalline wires or by coating a normal metal tip with a superconductor. Those recipes either require additional laboratory infrastructure or are carried out in ambient conditions, leaving an oxidized tip behind. Here, we revisit the vacuum cleaving of an Nb wire to prepare fully gapped tips in an accessible one-step procedure. To show their utility, we measure the SC gap of Nb on Au(111) to determine the base temperature of our microscope and to optimize its RF filtering. The deliberate coating of the Nb tip with Au fully suppresses the SC gap and we show how sputtering with Ar$^{+}$ ions can be used to gradually recover the gap, promising tunability for tailored SC gaps sizes.