Periodic Reporting for period 1 - TOP WSF (Tailoring topological properties in Weyl semimetal thin films)
Berichtszeitraum: 2021-05-01 bis 2023-04-30
In the quest for new quantum materials for electronics and energy, the investigation of topological states of matter has become a priority towards the realization of new quantum devices with unprecedented functionalities. Recently discovered Weyl semimetals attracted an incredible interest in the scientific community. Weyl semimetals are condensed matter systems in which relativistic Weyl fermions are found to exist in the form of quasiparticle excitations in correspondence of specific locations in the electronic band structure, called Weyl points. If a Weyl point is positioned close enough to the Fermi level, the transport properties of a Weyl semimetals can be dominated by the Weyl fermions instead of standard electrons, leading to exotic physical effects of both fundamental and technological interest. The uniqueness of Weyl semimetals, which make them so promising for applications, is their stability against external perturbations since the Weyl points are topologically protected and not protected by the system symmetries. This means that they could represent a robust platform for future quantum devices if we understand how to handle their properties.
In order to convert promising theoretical predictions into a concrete technological perspective, trigger and tune the exceptional properties associated to non-trivial topology at the micro- and nano-scale represents a fundamental step. In this regard, the establishment of methods to fabricate high quality thin films, exfoliated flakes and micro-structured bulk samples becomes a priority.
In this context, TOP-WSF aimed to explore the evolution of the transport properties of selected Weyl semimetals when their dimensions are reduced to micro and nano-structures. To reach the goal, three sub-objectives have been established:
• ralization of nanostructures in form of thin films of selected topological semimetals through the pulsed laser deposition (PLD) technique;
• fabrication of devices based on Weyl semimetals nanostructures for the characterization of electric and thermoelectric transport properties down to the micro and nano-scale by optical lithography, thermal scan lithography and focused ion beam (FIB) techniques;
• investigation of the transport properties of the selected Wel semimetals through the fabricated devices, by tuning different parameters: temperature, magnetic field, uniaxial strain, sample dimensionality and geometry.
In two years, TOP-WSF investigated several classes of Weyl semimetals, selected for different peculiar characteristics, each with a specific interest in the field of quantum technologies. The project contributed to the understanding of their electronic properties and the evaluation of their potential when the geometrical sizes are reduced to the micro- and nano-scale.
In order to convert promising theoretical predictions into a concrete technological perspective, trigger and tune the exceptional properties associated to non-trivial topology at the micro- and nano-scale represents a fundamental step. In this regard, the establishment of methods to fabricate high quality thin films, exfoliated flakes and micro-structured bulk samples becomes a priority.
In this context, TOP-WSF aimed to explore the evolution of the transport properties of selected Weyl semimetals when their dimensions are reduced to micro and nano-structures. To reach the goal, three sub-objectives have been established:
• ralization of nanostructures in form of thin films of selected topological semimetals through the pulsed laser deposition (PLD) technique;
• fabrication of devices based on Weyl semimetals nanostructures for the characterization of electric and thermoelectric transport properties down to the micro and nano-scale by optical lithography, thermal scan lithography and focused ion beam (FIB) techniques;
• investigation of the transport properties of the selected Wel semimetals through the fabricated devices, by tuning different parameters: temperature, magnetic field, uniaxial strain, sample dimensionality and geometry.
In two years, TOP-WSF investigated several classes of Weyl semimetals, selected for different peculiar characteristics, each with a specific interest in the field of quantum technologies. The project contributed to the understanding of their electronic properties and the evaluation of their potential when the geometrical sizes are reduced to the micro- and nano-scale.
The action has been organized into six work packages: WP1, WP2 and WP3 were related to the scientific work, WP4 to the PI training activity, WP5 to the dissemination and WP6 to the administrative part.
WP1 (months: 1 to 21): this WP was devoted to the realization of micro and nanostructures of selected Weyl semimetals. Two strategies have been followed:
• a bottom-up approach, consisting in the growth of thin films of selected Weyl semimetals by pulsed laser ablation (PLD);
• a top-down approach, consisting in the mechanical exfoliation of thin flakes from bulk single crystals.
WP2 (months: 2 to 22): in this WP we fabricated micro- and nano-structured devices to perform electric and thermoelectric transport property measurements on thin film and flakes obtained in WP1. To this aim we used optical and thermal scan lithography methods and focuse ion beam (FIB) techniques.
WP3 (months: 3 to 24): this WP was dedicated to the characterization of the micro and nanostructures realized in WP1 and WP2 through electric and thermoelectric transport properties as a function of different parameters including temperature, magnetic field, uniaxial strain, sample geometry.
WP4 (months 1 to 24): this WP included all the training activities performed by the PI, including: PLD techniques, x-ray diffraction, SEM and AFM microscopy, optical and thermal scan lithography and FIB methods.
WP5 (months 1 to 24): this WP was devoted to the dissemination activity. Some of the scientific results have been so far included in 2 publications as articles. The PI gave 3 seminars in international scientific institutes, participated to 2 conferences/workshops and he was member of the organization committee of 2 workshops. In addition, the PI participated to 2 outreach activities.
WP6:(months 1 to 24): this WP was dedicated to the management of the project.
Overview of the scientific results
-Thin films of Mn3Ge. TOP-WSF realized thin films of Mn3Ge, one of the most promising material in the field of antiferromagnetic spintronics. Although the optimization of Mn3Ge thin films will require more research time and energies, the action contributed to the identification of a protocol for their fabrication.
-Nanostructures of PtBi2: one of the most relevant result of the action regards the realization and characterization of the transport properties of flakes of PtBi2, one of the most intriguing materials among recently discovered topological non-trivial systems, hosting both Weyl semimetallic state and 2D superconductivity.
-Uniaxial strain on micrometric samples of W1-xMoxTe2: TOP-WSF demonstrated that the electronic properties of micrometric single crystals of type-II Weyl semimetals W1-xMoxTe2 are very susceptible to mechanical deformations of their crystal structure, showing the potential of this compounds in the field of straintronics.
-Weyl semimetal phase in MnBi4Te7 compounds: TOP-WSF experimentally identified Weyl semimetal state in the magnetic topological insulator MnBi4Te7 under the application of an external magnetic field. This is an important outcome, because it evidences that MnBi4Te7 hosts different non-trivial topological states, namely topological insulator and Weyl semimetal, in a single system.
-On-chip thermoelectric devices. As an important technical result, TOP-WSF realized different designs of on-chip devices for thermoelectric measurement, exploitable to different types of samples even outside the project.
WP1 (months: 1 to 21): this WP was devoted to the realization of micro and nanostructures of selected Weyl semimetals. Two strategies have been followed:
• a bottom-up approach, consisting in the growth of thin films of selected Weyl semimetals by pulsed laser ablation (PLD);
• a top-down approach, consisting in the mechanical exfoliation of thin flakes from bulk single crystals.
WP2 (months: 2 to 22): in this WP we fabricated micro- and nano-structured devices to perform electric and thermoelectric transport property measurements on thin film and flakes obtained in WP1. To this aim we used optical and thermal scan lithography methods and focuse ion beam (FIB) techniques.
WP3 (months: 3 to 24): this WP was dedicated to the characterization of the micro and nanostructures realized in WP1 and WP2 through electric and thermoelectric transport properties as a function of different parameters including temperature, magnetic field, uniaxial strain, sample geometry.
WP4 (months 1 to 24): this WP included all the training activities performed by the PI, including: PLD techniques, x-ray diffraction, SEM and AFM microscopy, optical and thermal scan lithography and FIB methods.
WP5 (months 1 to 24): this WP was devoted to the dissemination activity. Some of the scientific results have been so far included in 2 publications as articles. The PI gave 3 seminars in international scientific institutes, participated to 2 conferences/workshops and he was member of the organization committee of 2 workshops. In addition, the PI participated to 2 outreach activities.
WP6:(months 1 to 24): this WP was dedicated to the management of the project.
Overview of the scientific results
-Thin films of Mn3Ge. TOP-WSF realized thin films of Mn3Ge, one of the most promising material in the field of antiferromagnetic spintronics. Although the optimization of Mn3Ge thin films will require more research time and energies, the action contributed to the identification of a protocol for their fabrication.
-Nanostructures of PtBi2: one of the most relevant result of the action regards the realization and characterization of the transport properties of flakes of PtBi2, one of the most intriguing materials among recently discovered topological non-trivial systems, hosting both Weyl semimetallic state and 2D superconductivity.
-Uniaxial strain on micrometric samples of W1-xMoxTe2: TOP-WSF demonstrated that the electronic properties of micrometric single crystals of type-II Weyl semimetals W1-xMoxTe2 are very susceptible to mechanical deformations of their crystal structure, showing the potential of this compounds in the field of straintronics.
-Weyl semimetal phase in MnBi4Te7 compounds: TOP-WSF experimentally identified Weyl semimetal state in the magnetic topological insulator MnBi4Te7 under the application of an external magnetic field. This is an important outcome, because it evidences that MnBi4Te7 hosts different non-trivial topological states, namely topological insulator and Weyl semimetal, in a single system.
-On-chip thermoelectric devices. As an important technical result, TOP-WSF realized different designs of on-chip devices for thermoelectric measurement, exploitable to different types of samples even outside the project.
The scientific outcomes of the project consist of original sets of data, representing an advancement with respect to the state of the art on topological materials. Although the immediate results of TOP-WSF mainly constitute an advancement in fundamental condensed matter physics, in the mid-term the broad potential applications of topological materials make our study very interesting for various technological branches from electronics to spintronics, quantum computing and energy harvesting. Considering the non-technical the impact of the project, TOP-WSF contributed to enforce the international network of the beneficiary institution. In particular, the the action fostered the settlement of an official collaboration agreement between the University of Genova (the beneficiary) and the IFW Dresden institute, which was sealed with a joint workshop (Dresden 30.11.22 - 01.12.22). In addition TOP-WSF contributed to the education of early-career scientist, being the Pi co-supervisor of 4 students: 1 bachelor in Material Science, 2 masters in Physics and 1 Phd in Physics.