Fundamental research into novel electronic systems—superconductors, spintronics and hybrid materials—underpins and guides our industrial research programmes.
High temperature superconductors
Superconductors are the only materials that carry electric current without energy loss. In superconductors the electrical resistance drops abruptly to zero when the material is cooled below its critical temperature. This means when an electric current circulates in a superconducting wire it doesn't dissipate energy as heat, and once started will flow continuously. Zero resistance allows superconductors to carry extremely large electric currents, resulting in strong magnetic fields. The unique physical properties of superconductors make them extremely useful for a range of practical applications, including powerful MRI scanners, frictionless levitating trains, and lightweight generators for electric aircraft.
Conventional superconductors need to be cooled to below −243 °C to achieve superconductivity. For most applications this is done using liquid helium as a coolant, a resource that is becoming increasingly scarce and expensive. Fortunately, some copper-oxide based ceramic materials have critical temperatures above −196 °C. These materials are known as high-temperature superconductors (HTS) and have several advantages over conventional low-temperature superconductors (LTS), not least that they can often be cooled using liquid nitrogen or low maintenance cryocoolers. RRI has multiple current magnet system projects that employ modern cryogenic refrigerators and high temperature superconducting wire.
A complete theoretical explanation of HTS remains elusive, and is an ongoing area of research. At RRI we employ a mix of measurements and computational modelling to investigate HTS materials and test proposed theories. Our areas of interest include ion-size and pressure effects, the role of Fermi-surface reconstruction, and modelling critical currents and flux pumping effects. It is hoped that by pursuing a fundamental understanding of high-temperature superconductors we can inform the development of new materials, with new applications.
Solving the mystery of the pseudogap in HTS
Superconductivity in cuprate HTS overlaps an even more mysterious electronic correlation called the pseudogap. Key questions concern its origin and whether it is essential in any way to high-temperature superconductivity. To answer these questions we test models of the electronic structure in the pseudogap state by calculating the expected thermodynamic, spectroscopic, and transport properties then comparing those calculations with measured data.
Of particular interest is a surface of constant energy in the electronic structure, called the Fermi surface, which dictates the electronic properties of a material. Recently we showed that a collapse in charge carrier density on entering the pseudogap state is precisely described by an abrupt transformation in the size and shape of the Fermi surface, from a large cylinder into several small pockets [J.G. Storey, EPL 113, 27003 (2016)]. Research is ongoing into the cause of this transformation.
Another area of interest concerns the onset of the superconductivity in HTS. The superconducting state is accompanied by an energy gap in the electronic structure, known as the superconducting gap. In a conventional LTS the superconducting gap closes as the material is warmed through the superconducting transition temperature Tc. However, in HTS, our studies suggest that the superconducting transition in HTS is governed by a strong increase in pair-breaking scattering near the Tc, rather than the closure of the superconducting gap. Strongly temperature dependent scattering is expected to occur when superconductivity is coupled to spin fluctuations, hinting that this might be the cause of superconductivity in these materials [J.G. Storey, New Journal of Physics 19, 073026 (2017)].
For more information on this research, contact James Storey:
New iron-based superconductors (FeSCs) can potentially be commercially viable for high-performing superconducting wire applications because they have very high irreversibility fields, which suggests high critical currents. We have already shown that electrical current-carrying density (Jc) above 0.1 MA/cm2 are possible, but strategies are now required to further increase the critical current.
Several magnetic, structural, and charge-ordered states compete with or complement superconductivity in the FeSCs, but they are yet to be exploited to enhance Jc. Determining the fundamental property that will significantly enhance Jc in these superconductors will not only aid their application but has the potential to radically shift current thinking on how Jc can be enhanced in other superconductors. By utilising the intrinsic properties of superconductors, as well as by engineering extrinsic properties, we plan to increase Jc of new iron-based superconductors by an order of magnitude.
We are also looking at doping methods that employ organic molecules. By studying layered inorganic/organic superconductors where spin correlations can be eliminated, we are informing the debate on the role of spin correlations in HTS. Our previous work has shown that the properties of such hybrid systems can be reliably controlled by changing the nature of the organic molecules, and we are now searching for novel high-Tc superconductors that lack spin correlations.
The combination of this and our interest in Jc enhancement has prompted us to examine whether a new ionic liquid gating architecture can be designed to enable a real-time monitoring of doping effect on thin-layered materials. This is fundamentally interesting and would potentially lead to a controllable doping mechanism that can be built into a superconducting device.
For more information on this research, contact Shen Chong:
Ionic liquid gating
In the study of condensed matter, thin films behave often differently from their bulk counterparts. Their chemical composition can also be controlled to a much higher degree, almost atom-by-atom. The ionic liquid gating technique allows us to change the electric carrier concertation in a thin film to an unprecedented degree. This allows us to reach states that are inaccessible to conventional chemical doping. We use this technique to advance our understanding of a range of condensed matter phenomena such as superconductivity and magnetism.
The technological potential of magnetic skyrmions in thin film systems has driven research into the electronic detection of these topological magnetic solitons. Our recent work has focused on thin films of Heusler alloys, a class of exceptionably tunable materials with properties pertinent to spintronics. Many are ferromagnetic with high Curie temperatures and highly spin polarized band structures, making them attractive for spintronic device applications. In 2017, we discovered skyrmions in a member of this family of materials, Mn2CoAl.
Skyrmions are the smallest possible perturbations of a magnetic film and are great candidates to be used in future generations of computer systems as they allow to transfer information at extremely low energy costs. Being able to generate skyrmions in these materials expands device possibilities substantially, particularly with regards to racetrack memory and its variants. We observed these skyrmions from 3K to up to room temperature by measuring the anomalous and the topological Hall effect.
In my ongoing research, I also use the electric double layer gating of superconducting films with the aim to push them towards their predicted higher temperatures. Maybe this will allow us to finally reach the holy grail of the field: superconductivity at room temperature.
For more information on this research, contact Guy Dubuis:
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The field of spin electronics—or 'spintronics'—is one of the most active and exciting areas of condensed matter physics today. Spintronics involves manipulating spin to perform logic operations in devices and exploits a fundamental property of the electron, called ‘spin ‘. Spin is the intrinsic angular momentum of an electron, and makes it behave like a tiny magnet. Depending on the orientation of the angular momentum an electron can be spin up or spin down. Traditional electronics uses charge to represent information as zeros and ones, and the same binary data can be represented in spintronics by the two spin states. Taking advantage of the electron’s spin can lead to devices with higher performance and lower power use than is possible with conventional charge-based electronics, which are rapidly approaching their physical limits.
There is growing interest in magnetic materials as candidates for use with spintronic devices. There are lots of concepts for spintronic devices that will replace the aging designs and materials of conventional electronics, and some, like magnetic RAM (MRAM), are already being sold by companies like Everspin. Nevertheless, magnetic materials with ideal properties for spintronic devices are needed to bring these concepts to reality. Spintronic devices that used materials that were both magnetic and semiconducting would enable the control of both electron spin and electron charge. This additional degree of freedom, which can be used for information storage and processing, would make spintronic devices faster and more energy efficient than existing devices. However, despite over two decades of research into adding magnetic elements into semiconductors, materials suitable for room temperature spintronic devices remain elusive.
Magnetic Heusler alloy thin films for spintronics
Our current spintronics research is focused on magnetic Heusler alloys such as Co2(Mn,Fe)Si, Co2MnGa, and Mn2CoAl. These have an extraordinary range of electronic band structures, including ferromagnetic metallic, half-metallic (fully spin-polarised), and spin gapless types, all of which will enable next-generation spintronic devices. The fingerprints of these band structures can be seen in the exotic magnetic and magnetotransport properties of thin films of the Heuslers, which is the subject of our current research.
Our current research focii with magnetic Heusler thin films include:
- spin texturing and skyrmions
- perpendicular magnetic anisotropy
- magnetic proximity effect
- anomalous Hall and anomalous Nernst effects
- spin waves/magnonics.
Simon and Bob
A new approach is to investigate materials that are intrinsically magnetic and semiconducting, especially at room temperature. Such materials are rare, however, recent theoretical calculations indicate that a Mn rich composition derived from the half-metal Heusler alloy Co2MnAl should have the required ideal properties. Co2MnAl is a magnetic metal, and the other end member Mn2CoAl is a spin-gapless semiconductor, that is, magnetic with a semiconducting energy gap of zero. An intermediate composition is predicted to have the required magnetic and semiconducting properties; a semiconductor-like band gap for one spin orientation and a metal-like band for the other spin, that is, electron conduction with a single spin polarisation or 100 percent spin polarisation.