Dynamics of High-Energy Quasiparticles in Superconducting Nanowire Leading to ‘Hotspot Formation’

Active quasiparticles possess a range of quantum properties that act in a particle-like manner in superconducting nanostructures, and can undergo relaxation by including many cascade interactions between electrons, phonons, and cooper pairs. These interactions are important for the performance of devices such as qubits or photon detectors, yet they still need to be well understood across the spectrum. semiparticle Organized experiments. Typically, these experiments involve rigid tunnel junctions with a fixed tunnel barrier.

In a new report in nature physicsT. Jalabert and a team of researchers in France used A A scanning tunneling microscope To independently adjust the energy and rate of quasiparticle injection via bias voltage and tunneling current. High-energy quasiparticles relied on the injected force and injection rate to produce a low critical current on the nanowires. The results highlight a thermal mechanism underlying the critical current reduction to provide insights into the rapid dynamics of the generated hotspot.

Superconducting hardware performance

Superconducting devices are often limited or governed by quasiparticle dynamics, where superconducting quasiparticles are not useful for devices such as Superconducting mini coolers And superconducting qubits. However, knowledge of the exact mechanism underlying quasiparticle dynamics is important in order to improve device performance and provide a prerequisite for the operation of photon detectors. Despite extensive research efforts, physicists still understand the processes at stake during energy relaxation of quasiparticles in current-carrying superconductors.

In a recent proposal, experimental physicists have developed an all-metal Josephson field effect transistor, which relied on regulating its critical current after applying a gate voltage. This has generated a lot of controversy since it was proposed Heating effect after injection of high-energy quasiparticles. Previous experiences too It works routinely The method, however, prevented separation of the effects of current and voltage, which Jalbert and colleagues overcome by using a scanning tunneling microscope (STM) to inject quasiparticles locally into a superconducting nanowire and simultaneously measure its critical current.

Experimental physics with scanning tunneling microscopy (STM)

Locating and contacting an individual nanostructure using a scanning tunneling microscope is challenging due to the intrinsic incompatibility of the microscope to isolate a nanostructure. Physicists and materials scientists had previously focused on fusion Atomic force microscopy and STM, although both methods are technically tedious. In this study, Jalbert and the team used STM to locate and quantify the nanodevice and studied six superconducting nanowires with different nominal total thicknesses of niobium/gold.

They measured the critical current after injecting the quasiparticles into the middle of the nanowire, and noted how the critical current depends on the injected energy. In a thermodynamic framework, each injected quasiparticle into the setup relaxes its energy phonons, resulting in the breaking of hundreds of Cooper pairs to generate many unbalanced quasiparticles that formed the so-called hot spot. This low conversion sequence happened in a very short time frame in picosecond order.

hotspot dynamics

Using tunneling microscopy, Jalabert and colleagues map the current as a function of tip position for stationary tunneling conditions. As the position of the tip moved away from the leads or as the injected force increased, the critical current decreased further. The physicists determined the local electronic temperature from the measured critical current and noted a remarkable agreement between the one-dimensional heat model. All experimental data were supportive of the hot quasiparticle-induced thermal reduction of the critical current. They showed how the increase in quasiparticles in the hot spot reduced the density of Cooper pairs available to carry the superfluid current, which agreed that with previous studies.

Hotspot dynamics depended on the balance between quasiparticles proliferating in the down-conversion chain and their escape through diffusion. In the models described here, only the increasing number of out-of-equilibrium quasiparticles matters. The hotspot formation time was 40 picoseconds, which matches the time required for quasiparticles to diffuse across the width of the nanowire. The team plans to conduct further studies to solve the dual kinetic equations for the interacting quasiparticles and phonons. outside the current scope of work.

prospects

In this way, T. Jalabert and colleagues have formed a powerful new method to study local quasiparticle dynamics in superconducting nanostructures to tune the tunneling rate and quasiparticle energy. The physicists used the experimental setup to show how the critical current for nanowires can be significantly reduced by injecting a quasiparticle injection current of several magnitudes.

They attributed the result to the phenomenon of convective heating of the quasiparticles. The results have an immediate impact on the function of superconducting nanodevices such as field effect transistor And photon detectorswith the added ability to design superconducting quantum circuits with improved quasiparticle effects in the future.

© 2023 Science X Network