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Unconventional Superconductivity at Microkelvin Temperatures

Periodic Reporting for period 1 - UCSMT (Unconventional Superconductivity at Microkelvin Temperatures)

Reporting period: 2019-09-01 to 2021-08-31

The “Unconventional Superconductivity at Microkelvin Temperatures” (UCSMT) project aims at developing novel measurement techniques at ultra-low temperatures and studying superconducting properties of elemental Bismuth and quantum critical YbRh2Si2 single crystals, two unique superconductors. Bismuth is exceptional element with unique electronic properties such as a very small density of states, and exhibits strong quantum effects that have helped in uncovering many interesting phenomena in condensed matter research such as the Seebeck, Nernst, Shubnikov–de Haas, and de Haas–van Alphen (dHvA) effects right from early 1900s. The superconductivity in bulk Bismuth crystals was only recently observed at extremely low temperatures below 530 micro-kelvin using magnetisation measurements making it one of the lowest carrier density superconductor with a upper critical field of 5.2 micro-tesla. The UCSMT project aims at performing first transport measurements on bismuth crystals at micro-kelvin temperatures using current sensing noise thermometry (CSNT) techniques to detect the resistive transition into the superconducting state. The YbRh2Si2, on the other hand is a prototype quantum critical, heavy fermion metal which exhibits anti-ferromagnetic (AFM) order below 70 mK. A small magnetic field of 60 mT, when applied in the basal plane of the tetragonal structure, can suppress the AFM order and drive YbRh2Si2 towards a quantum critical point. Magnetic measurements on high quality single crystal samples at the lowest fields find evidence for superconductivity. Calorimetric measurements suggest the emergence of nuclear spin order around 2 mK. This is suggested to weaken the electronic AFM order so that superconductivity of probably unconventional nature emerges at a QCP shifted to zero field. Analysis of the upper critical field seems to suggest heavy fermion superconductivity. The nuclear Kondo-effect has also been suggested as the possible source of the heavy fermion character of YbRh2Si2. The UCSMT project aims at developing a novel, state-of-the-art magnetically shielded, sensitive magnetization measurement setup to measure the lower critical field and it’s anisotropy in YbRh2Si2, to further enhance the understanding on the superconducting ground.

The development of new measurement techniques in the microkelvin temperature regime is likely to produce new sensors, sensitive thermometers and software which can be used in other fields of cryogenic industry and electronics. Identification of a high-quality single crystal material that can host topological superconductivity is an important step towards topologically protected quantum computing that are expected to play very important role in solving challenging problems in economics, fundamental science and health sectors.

The overall objectives of the UCSMT project are to develop the understanding of superconductivity in Bismuth and YbRh2Si2, and to make significant advances in our ability to study quantum materials into the ultra-low temperature regime, through the development of novel measurement techniques suitable for sub-milli-kelvin temperatures. A parallel goal of this MSCA Individual Fellowship is to foster the development of the individual researcher as well as enable the exchange of knowledge and expertise with the host institute.
The Management, Training and Transfer of Knowledge objectives of the UCSMT project was successfully implemented by means of regular scientific meetings, various training and workshop activities offered by the host institution relevant to the fellows development as an independent researcher. These included (but not limited to) workshops on writing successful research grant proposals, protection of intellectual properties and fair research practices. The transfer of the knowledge was ensured with working with the researchers in the host institute and regular engagements throughout the project. The measurement of critical fields of YbRh2Si2 needed the development of highly magnetically shielded environment and sensitive magnetometry techniques. At the start of the UCSMT project, computer simulations were performed using COMSOL Multiphysics software to study the shielding performance of the high permeability materials. Based on the simulation results, a magnetic shielding cell is designed consisting of two layers of high permeability material Cryophy sandwiched between two niobium tubes which act as superconducting shields. The shielding factor of this setup was estimated to be greater than 10 million at 4.2K (1e7) using COMSOL Multiphysics this was successfully validated by the experimental tests. The magnetization measurement is performed using a first order gradiometer coil made of superconducting NbTi wire and the magnetization setup was calibrated a Zinc (Zn) sample, a known type-I superconductor. The calibration tests confirmed high sensitivity of the magnetization setup and showed that the magnetization measurements can be carried out in remarkably low magnetic fields of 10-20 nano-tesla using this setup. Unfortunately, a lot of experimental time was lost due to COVID-19 restrictions and the measurements on the YbRh2Si2 single crystals are currently in progress. A similar magnetically shielded setup is also developed for the transport measurements on the Bismuth crystals. The transport measurements are performed using current sensing noise thermometry techniques harnessing unprecedented sensitivity of dc-SQUIDS (Superconducting quantum interference devices). The first transport measurements suggest a need to improve the thermalization techniques for Bismuth crystals and a revised measurement is being setup. As there are new results from the UCSMT project coming towards the end of the fellowship duration, the scientific findings and results would be presented in upcoming international conferences. The work on development and setting up of magnetically shielded cells and their testing is being written and will be submitted to a peer reviewed journal for publication.
The magnetization measurement setup developed during the fellowship is an state-of-the-art technique. It is tested to be capable of measuring magnetization of small samples at the lowest applied field of 10-20 nano-tesla with excellent signal to noise ratio down to 400 micro-kelvin. This provides a great platform for measurement of small magnetization signatures to the users and members of the European micro-kelvin platform as well as guest experiments and is a great addition to the measurement capabilities of the ultra-low temperature group at the host institution. The test results of the measurement setup will be published in an open access journal and is likely to attract new collaborations.
Magnetisation measurement setup installed on the nuclear stage of the dilution fridge.
Magnetic shielding and field noise inside the magnetisation cell at 4.2K