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HomeScienceDynamics of High-Energy Quasiparticles in Superconducting Nanowire Leading to 'Hotspot Formation'

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


Experimental setup. a, A nanowire in the center of a target of lines (blue squares) with four microconnectors (at the edges). b, Schematic diagram of a cross-sectional view of a nanowire. c, The nanowires are probed by an STM tip with a bias potential of V.B And the current tunnel iR while iwires-Fifthwires The property is monitored. credit: nature physics (2023). DOI: 10.1038/s41567-023-01999-4

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.

Dynamics of high-energy quasiparticles

Critical current as a function of bias voltage and injected power. Model N03. a, Critical current as a function of bias voltage VB For different tunnel streams iR At T = 250 mK. The dashed line indicates the critical current value Ic= 96.3 μA when quasiparticles are not injected. b, the same data as a function of IRFifthB. The inset shows a zoom of the data in the gray rectangle. credit: nature physics (2023). DOI: 10.1038/s41567-023-01999-4

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.

Dynamics of high-energy quasiparticles

Critical current scanning microscope. a, Format of nanowires. b, Critical current (dots) as a function of STM tip position x along the nanowire and averaged along y for different tunneling conditions at T = 180 mK and critical current (dashed lines) obtained from numerical solutions of equation (1) for Σ = 6 x 109 W k−5M-3. Without the injection current, ic = 18.5 μA. Model N06. credit: nature physics (2023). DOI: 10.1038/s41567-023-01999-4

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.

Dynamics of high-energy quasiparticles

Effect of force injected by quasiparticles on electronic temperature. a, Normalized critical current measured without quasiparticle injection Ic(T/Tc) I0c, where I0c is the critical current value at T = 0 K as a function of low temperature. b, Low electronic temperature as a function of the quasiparticle force injected by the STM tip. For the sake of completeness, in this set of experiments we altered and measured Tb with the help of a heater and thermometer attached to the sample holder. The horizontal colored lines on the left indicate the corresponding bath temperature dropB = TB / h. The solid lines correspond to theoretical predictions of equation (1) with Σ = 6 × 1096 x 109 0.8 x 109 and 8 x 109 W.K–5M-32for sample N03, N06, N08 and N07, respectively. credit: nature physics (2023). DOI: 10.1038/s41567-023-01999-4

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.

Dynamics of high-energy quasiparticles

Relaxation dynamics of injected quasiparticles. Critical current icIn a certain strength iRFifthBIt decreases further when the quasi-particle injection rate is low. Solid lines are scalar fits with τrel = 40 ps. The dashed line is the constant critical current I stat c, and the inset is a low power zoom. credit: nature physics (2023). DOI: 10.1038/s41567-023-01999-4


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.

more information:
T. Jalabert et al., Thermal conversion and dynamics of high-energy quasiparticles in superconducting nanowires, nature physics (2023). DOI: 10.1038/s41567-023-01999-4

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the quote: Dynamics of ‘hot spot forming’ high-energy quasiparticles in superconducting nanowires (2023, April 25) Retrieved April 25, 2023 from https://phys.org/news/2023-04-dynamics-hotspot-high -energy-quasiparticles -superconductivity. html

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