Periodic Reporting for period 1 - InNaTo (Investigation of Nanoscale properties of Topological Weyl semimetals)
Periodo di rendicontazione: 2020-07-01 al 2022-06-30
Topological materials, comprising for instance topological insulators, Dirac and Weyl semimetals, are a recently-discovered class of materials exhibiting exotic electronic band structures that give rise to several unconventional physical phenomena, such giant magnetoresistance, photogalvanic and thermoelectric effects, the violation of classical laws of physics (e.g. the Wiedemann-Franz law and the conservation of chirality) and quantum phenomena. Thanks to their outstanding physical properties, topological materials have the potential to be implemented in novel applications and functionalities in the areas of valleytronics, quantum computing, sensing and catalysis. To date, most of the research activities on topological materials have been primarily devoted to the investigation of bulk single crystals. However, in order to implement topological materials in novel technologies it is of primary importance to understand how and to which extent their properties can be manipulated at the nanoscale. The research activities during the project InNaTo were focused on the topological material system CoSi, a high-order Weyl semimetal, which was recently found to exhibit longest surface Fermi arcs in case of CoSi bulk single crystals. At low dimensionality, owing to the increased surface-to-volume ratio, the topological surface states of CoSi may give rise to unprecedented transport properties. The goal of InNaTo was to investigate the properties of CoSi at low dimensions following two main approaches: CoSi nanoscale thin films prepared by molecular beam epitaxy (MBE) and CoSi micro-scale Hall bars fabricated by focused ion beam (FIB) milling. The experimental results on the magnetotransport properties of CoSi micro-scale Hall bars provide a valuable reference for a comparison with bulk CoSi single crystals and CoSi thin films. In particular, it was found that CoSi micro-scale Hall bars prepared by FIB display a negative longitudinal magnetoresistance, which is possibly manifested as a consequence of the phenomenon of chiral anomaly. Concerning CoSi thin films prepared by MBE method, several unexpected anomalies have been observed in the magnetotransport properties, which are correlated with the strong influence of structural and chemical disorder. Furthermore, an unusual resistivity scaling behavior has been observed in amorphous CoSi thin films, which, contrary to conventional metals, display a decrease in resistivity upon reducing the film thickness down to 2 nm. In conclusion, this action opens the path to explore the complexity and the challenges involved in the prospective exploitation of the topological chiral semimetal CoSi in micro- and nanoscale thin films and devices.
CoSi thin films were grown by MBE method onto MgO single-crystal substrate varying different parameters, such as film thickness (2 – 80 nm), growth temperature (30 – 450 ˚C) and stoichiometry (0.4 < x < 0.6 in Co1-xSix). The main objective was to examine the correlation between microstructure, chemical composition and magnetotransport properties and to look for possible signatures of topological properties at the nanoscale. Relevant magnetotransport parameters, such as longitudinal resistivity, magnetoresistance ratio, charge carrier density and mobility, have been extracted for amorphous and textured Co1-xSix thin films under different conditions of temperatures and applied magnetic field. It has been found that the magnetotransport properties (e.g. magnetoresistance and Hall effect) have a strong dependence on both microstructure and chemical composition. A thickness dependence survey revealed an unusual decrease of resistivity upon decreasing the thickness of amorphous CoSi films from 80 to about 2 nm. This is a quite interesting aspect, given the fact that conventional metals show exactly the opposite trend, i.e. an increase in resistivity upon decreasing film thickness. Our investigation on Co1-xSix thin films poses awareness on challenges and opportunities for the potential implementation of this topological material into technological devices.
As a parallel project, micro-scale CoSi Hall bars were prepared via FIB milling of CoSi bulk single crystals prepared by our collaborators in Max Planck Institute Dresden. The purpose of FIB-cut CoSi samples was twofold: (i) to study the properties of crystalline CoSi in an intermediate, micro-scale range between macro- and nanoscales and (ii) to serve as a reference for the MBE-grown CoSi thin films. The high crystalline quality of the original CoSi single crystals was to a great extent preserved also in the FIB-cut CoSi Hall bars. In general, the good crystallinity of FIB-cut CoSi devices allowed to obtain larger values of Hall mobilities and magnetoresistance as compared to the values of CoSi films grown by MBE method.
The experimental results of InNaTo have been presented at three international conferences. Dissemination was also carried out by participation to outreach events (such as 24 Hours of Science (2021) organized by IBM) and by realization of a dedicated website on the project InNaTo (https://www.zurich.ibm.com/st/nanoscale/InNaTo.html(si apre in una nuova finestra)). Manuscripts reporting details on the experimental results are in preparation.
As a parallel project, micro-scale CoSi Hall bars were prepared via FIB milling of CoSi bulk single crystals prepared by our collaborators in Max Planck Institute Dresden. The purpose of FIB-cut CoSi samples was twofold: (i) to study the properties of crystalline CoSi in an intermediate, micro-scale range between macro- and nanoscales and (ii) to serve as a reference for the MBE-grown CoSi thin films. The high crystalline quality of the original CoSi single crystals was to a great extent preserved also in the FIB-cut CoSi Hall bars. In general, the good crystallinity of FIB-cut CoSi devices allowed to obtain larger values of Hall mobilities and magnetoresistance as compared to the values of CoSi films grown by MBE method.
The experimental results of InNaTo have been presented at three international conferences. Dissemination was also carried out by participation to outreach events (such as 24 Hours of Science (2021) organized by IBM) and by realization of a dedicated website on the project InNaTo (https://www.zurich.ibm.com/st/nanoscale/InNaTo.html(si apre in una nuova finestra)). Manuscripts reporting details on the experimental results are in preparation.
Topological materials, to date mostly studied in the form of bulk single crystals, may have a tremendous impact in the area of microelectronics, valleytronics, as well as quantum and neuromorphic computing. InNaTo contributed to the understanding of the low-dimensional properties of such new class of topological materials. In particular we carried out comprehensive evaluation of the correlation between microstructure, chemical composition and magnetotransport properties of nanoscale CoSi thin films. Besides, the unusual scaling behavior observed in amorphous CoSi thin films upon scaling down the film thickness open the path for the exploration of topological amorphous materials as potential replacements of conventional metals in back end of line interconnects. Projects like InNaTo offer the opportunity to explore new materials and technologies for potential applications in the sectors of semiconductor industry and microelectronics.