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1D-electrons coupled to dissipation: a novel approach for understanding and engineering superconducting materials and devices

Periodic Reporting for period 4 - 1D-Engine (1D-electrons coupled to dissipation: a novel approach for understanding and engineering superconducting materials and devices)

Período documentado: 2022-06-01 hasta 2023-11-30

Superconductivity is a remarkable property of certain materials which manifests itself by the transport of electrical current without any losses, as well as perfect diamagnetism, whereby the material attempts to expel any magnetic fields that try to penetrate it. So far, at atmospheric pressure, any material with these properties can only enter such a superconducting state when it is cooled to very low temperatures – the best materials require about -135° Celsius, and many far below even that. The temperature where superconductivity sets in is called the materials critical temperature. Superconductors that can operate up to or above the temperature at which nitrogen liquefies (a comparatively cheap cooling agent), at about -195° Celsius, are designated “high-temperature” superconductors – this is strictly meant in contrast to the earliest discovered superconductors that required liquefied helium (a rare, expensive cooling agent), requiring operating temperatures of -269° Celsius or below.

Even with these temperature-limitations, and the attendant need for powerful cooling, superconductors already offer unique advantages for a range of existing and potential near-term applications, from medical imaging, where they enable todays magnetic resonance imaging (MRI) machines, to telecommunication relays and generators/motors with very high power-to-weight ratios, e.g. for wind turbines or load-carrying electric vehicles. On the side of fundamental physics, superconductors have been an extremely important area within which physicists have already radically expanded human knowledge and insight into systems of many interacting electrons, which in their many different incarnations represent the most important frontier of condensed matter physics today and in the foreseeable future. The theories explaining the properties of the first generation of superconductors have been a breakthrough in our understanding of many-electron systems. However, even today, close to four decades after the discovery of high-temperature superconductors, there is no comparable theory available to explain their properties or to predict new ones.

However, without such a theory, there can be no program to deliberately engineer superconductors that work at even higher temperatures, in order to further reduce cooling requirements and thus vastly increase their beneficial use in areas where they would be currently impractical. Another limitation of all existing superconductors is that any wires made out of them loose their superconducting properties when they become too small. This represents a severe impediment to the development of integrated high-performance superconducting electronics that has the prospect of enormously increased computing and signal processing speeds compared to todays systems based on conventional semiconductors, while simultaneously greatly reducing power consumption.

This project aims at providing a novel and fundamental theoretical framework that would show how both these goals, being able to deliberately design superconductors with higher critical temperatures, while still functioning at nanoscale sizes, could be achieved. It will do so by leveraging the unparalleled theoretical insight of condensed matter physics into systems of one-dimensional (1D) interacting electrons, compared to higher-dimensional ones (the existing high-temperature superconductors are based on stacks of weakly coupled two-dimensional crystal lattices which are much harder to treat theoretically, one of the main reasons their properties cannot be deliberately engineered). We are developing the basic theory that would allow to reliably predict, and thus purposefully design, the superconducting properties of bulk materials made from such 1D building blocks, and that further hold the prospect of exhibiting high critical temperatures that would be amenable to systematic improvements. Our approach further aims at theory that would allow to engineer superconducting nanowires, a basic building block of highly integrated superconducting electronics.

This framework is further targeted at explaining the basic mechanisms behind a large group of already existing quasi-1D superconductors, which are made up out of systems of weakly coupled 1D electrons, such as the Bechgaard and Fabre salts and chromium pnictide. While these compounds do not show high critical temperatures, their fundamental mechanism for superconductivity is widely assumed to be of the same general type as that of the existing high-temperature superconductors, i.e. based on electron pairing due to repulsive interactions. This mechanism has so far defied a universally accepted theoretical description in any material, which is why this project is targeting our new theory framework to resolve it in the quasi-1D superconductors.
After 30 months, the state of the project can be summarized thusly:

- We have developed the first stage of the new theory framework that allows to predict the properties of a bulk system made up out of 1D electrons. This framework applies in the regime in which pairs of electrons do not break up when they move from one 1D system to an adjacent one. But even in this, an effectively weak-coupling regime, we see the clear potential for such bulk systems to become superconducting at surprisingly high temperatures, and we show how their transition temperatures can be systematically tuned in that direction. We are further developing a proposal to test this new framework using a so-called analogue quantum simulator (a type of quantum computer), i.e. high-precision experiments with ultracold clouds of atomic gas confined in standing laser-fields.

- We have entered the development of the second stage of this theory framework, where the coupling between 1D systems of electrons in the bulk system is strong enough to break up pairs of electrons as they move through the system. This second and final of the two stages is necessary and interesting because the existing superconductors made out of weakly coupled 1D systems of electrons are in this regime, and explaining their properties, which this project has as one of its targets, requires being able to address this regime. Further it is quite possible that any new bulk materials this project might propose for chemical synthesis could end up to be in this regime as well.

- We have developed the basics of a powerful framework that will allow this project to study how contact with a three-dimensional piece of metal might be able to stabilize superconductivity in a 1D nanowire that could not be superconducting on its own due to its effectively one-dimensional nature.
This project targets progress beyond the state of the art on several fronts:

- Development of a new theoretical numerical framework for high-temperature superconductivity for bulk systems comprised of weakly coupled 1D electrons. Due to this framework the properties of these bulk systems would be amenable to systematic engineering and improvement through the tuning of the systems microscopic parameters, which would allow to propose new high-temperature superconducting materials for chemical synthesis based on reliable, fundamental numerical theory. The first of two stages of this framework has already been developed, and will soon be proposed for a first experimental validation using an analog quantum computer.

- Development of a new theoretical framework for how currently infeasible superconducting nanowires for use in e.g. superconducting electronics could be achieved using contact with an external metallic bulk. The basic approach for this framework has been devised now.

- Development of numerical theory capable of explaining the superconductivity in a range of existing materials comprised of weakly coupled systems of 1D electrons, such as the Bechgaard and Fabre salts or chromium pnictide, at a fundamental level.