A nanoscale gigahertz source realized with Josephson scanning tunneling microscopy

Abstract

Using the AC Josephson effect in the superconductor-vacuum-superconductor tunnel junction of a scanning tunneling microscope (STM), we demonstrate the generation of GHz radiation. With the macroscopic STM tip acting as a λ/4-monopole antenna, we first show that the atomic scale Josephson junction in the STM is sensitive to its frequency-dependent environmental impedance in the GHz regime. Further, enhancing Cooper pair tunneling via excitations of the tip eigenmodes, we are able to generate high-frequency radiation. We find that for vanadium junctions, the enhanced photon emission can be tuned from about 25 GHz to 200 GHz and that large photon flux in excess of 1020 cm−2 s−1 is reached in the tunnel junction. These findings demonstrate that the atomic scale Josephson junction in an STM can be employed as a full spectroscopic tool for GHz frequencies on the atomic scale.

Quantum Brownian Motion at Strong Dissipation Probed by Superconducting Tunnel Junctions

“…This is illustrated by the frequency-dependent part of the simulated system impedance in Fig. 2(c), where the tip resonance modes appear as sharp peaks [18]. In the low frequency limit ν → 0, the environmental impedance Z(0) is, therefore, dominated by the coupling of the tunneling Cooper pairs to the electromagnetic vacuum in the gap between tip and sample (tunnel barrier) with the STM being operated in ultra-high vacuum conditions.…”

“…Owing to the large output impedance of our constant current source, R B = 1.33 GΩ, the circuit features a horizontal load-line. Concerning the electromagnetic environment of the tunnel junction, we obtain a virtual DC impedance for frequencies up to the low GHz regime by choosing an STM tip of adequate length, moving the tip resonance modes in the environmental impedance Z(ν) to higher frequencies [18]. This is illustrated by the frequency-dependent part of the simulated system impedance in Fig.…”

“…For decreasing bias currents starting from I B > I S , the current is fixed to the in-gap current before switching back into the phase-diffusion branch. At our experimental conditions, the in-gap current originates from quasiparticle excitations due to life-time effects of Cooper pairs in the STM tip and tunneling of individual Cooper pairs via interaction with resonance modes in the environment [18,21,22].…”

Visualising the Charge and Cooper-Pair Density Waves in Cuprates

“…Another facet of the ultra-small size of SJTM junctions is that they can have a junction capacitance, C J , as small as ∼1fF and a shunt resistance provided by the SIS single-particle tunnelling channel of ∼ 0.1 − 100MΩ [139][140][141][142][143][144][145]. On the face of it the small capacitance would favour over-damping while the large resistance favours underdamping.…”

“…Firstly, it is known that at magnetic impurities in s-wave superconductors the superconducting order parameter, Ψ, is suppressed whereas the single-particle gap, ∆, is unperturbed [134][135][136][137]. Similarly, In the face of these adversities a handful of groups have initiated efforts to develop SJTM as a direct probe of the superconducting order parameter [139][140][141][142][143][144][145]. In our group we have focussed on an implementation of SJTM using high temperature d -wave superconducting tips to search for a pair density wave in cuprate superconductors [132].…”