Quantum oscillations and topological magnetotransport in micron-scale Hall bars of the chiral semimetal CoSi
Topological quantum materials are a recently-discovered class of material systems exhibiting exotic physical phenomena, such as quantum oscillations, giant thermoelectric, photogalvanic and magnetoresistance effects –. In the growing landscape of topological quantum materials, comprising for instance topological insulators, Dirac and Weyl semimetals, the chiral semimetal CoSi has attracted a deep interest in the scientific community for it is predicted to behave as a nearly-ideal topological conductor . Recent experimental studies have already demonstrated the occurrence of unconventional magnetotransport, photo-galvanic properties and quantum oscillations in CoSi bulk single crystals –. To date, the quest for exploiting the topological quantum properties of CoSi in micro- and nanostructured devices is still open. In our work, we have fabricated micron-scale, multiterminal Hall bar devices by carrying out focused ion beam (FIB) milling on a bulk single crystal of CoSi and characterized their magnetotransport properties. Initially, FIB-cut lamellas were analyzed by means of scanning transmission electron microscopy in order to identify the crystallographic orientation of the original CoSi single crystal. Afterwards, FIB milling was used to fabricate multiterminal Hall bar devices with a typical size of about 25 μm x 4 μm x 1.5 μm. The magnetotransport properties of the CoSi devices, including transverse, longitudinal magnetoresistance and Hall effect, were probed as a function of temperature (1.7 – 300 K) and applied magnetic field (up to ± 9 T) by making use of a physical properties measurement system and a lock-in amplifier. We observe that when a current of about 50 μA is applied along the  crystallographic direction, the CoSi Hall bar devices present a metallic behavior with a residual resistivity ratio of about 6. Furthermore, the transverse magnetoresistance is positive and reaches a maximum value of about 25% at 1.7 K. Interestingly, when the magnetic field is applied parallel to the flowing current, the response of the longitudinal magnetoresistance changes from positive to negative regime upon decreasing temperature. The observation of a negative longitudinal magnetoresistance may be related to the occurrence of the chiral anomaly in CoSi, i.e., the breaking of conservation of chirality in topological Weyl semimetals when current and magnetic field are parallel to each other. This observation is corroborated by the presence of quantum oscillations both in the transverse and longitudinal magnetoresistance signals in proximity of 2 K. The results of the Hall effect data indicate that the main charge carriers contributing to the transport properties are holes with a charge carrier concentration nh ≈ 2.2 · 1020 h+/cm3 and a mobility μh ≈ 1400 cm2 V / s at 2 K. In conclusion, our results provide new perspectives on the still largely-unexplored magnetotransport properties of the chiral topological semimetal CoSi upon decreasing the characteristic length scale from bulk to the micron-scale. The authors acknowledge funding from the European Union’s Horizon2020 research and innovation program under Grant Agreement ID 829044 "SCHINES" and ID 898113 “InNaTo”. We thank the Cleanroom Operations Team of the Binnig and Rohrer Nanotechnology Center (BRNC) for their help and support.  P. Narang, C. A. C. Garcia, C. Felser, Nat. Mater. 20, 293–300, 2021.  J. Gooth, G. Schierning, C. Felser, K. Nielsch, MRS Bull. 43, 187–192, 2018.  N. Kumar, S. N. Guin, K. Manna, C. Shekhar, C. Felser, Chemical Reviews. 121, 2780–2815, 2020.  D. S. Sanchez et al., Nature 567, 500–505, 2019.  X. Xu et al., Phys. Rev. B 100, 045104, 2019.  D. S. Wu et al., Chinese Phys. Lett. 36, 077102, 2019.  Z. Ni et al., Nat. Commun. 12, 1–8, 2021.