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FeSb2: a riddle, inside an insulator, wrapped in a metal Electric and magnetic properties of the unconventional insulator iron diantimonide 10.17863/CAM.91726 Eaton, Alexander 0000000166690807 Sebastian, Suchitra The distinction between metals and insulators forms a fundamental tenet of condensed matter physics. Over the past decade or so, the explosion of interest in topological insulators – materials that host a metallic surface state atop their insulating bulks – has demonstrated how the contrasting properties of insulators and metals can be synergistically combined. In contrast, recent observations of Fermi surface signatures – conventionally presumed to be characteristic of a (metallic) Fermi liquid – from within a new class of unconventional insulators has profoundly called into question our conventional understanding of this fundamental tenet. This dissertation focusses on experimental studies of the strongly correlated insulator iron diantimonide (FeSb2). It is found that FeSb2 exhibits anomalous behaviour in both its magnetic and electrical properties. Under the application of high magnetic fields, quantum oscillations in the magnetisation are observed, with an inverse-field periodicity indicating the presence of Landau quantisation. Remarkably, these are shown to emanate from the insulating bulk of the material – in sharp contrast to our conventional interpretation of Landau’s Fermi liquid model. These findings are discussed in the context of previously discovered unconventional insulators. Notably, unlike other unconventional insulators reported to date, the unconventional quantum oscillatory profile in FeSb2 is found to be closely related to changes in the bulk magnetisation of the material. At low fields, a marked enhancement in the growth of the magnetic torque signal is accompanied by a likewise enhancement in both the quantum oscillatory amplitude and frequency. Then, at the highest fields, a sharp metamagnetic transition abruptly quenches the insulating quantum oscillations – in marked contrast to numerous tunnelling and magnetic-breakdown posited models of unconventional insulating behaviour. These findings point strongly to a magnetic origin being responsible for the manifestation of this paradoxical phenomenon. Additionally, a suite of electrical transport measurements is presented, demonstrating the presence of surface-dominated conduction at low temperatures, corroborating previous reports from angle resolved photoemission spectroscopy studies. Notable comparisons with the putative topological Kondo insulator samarium hexaboride (SmB6) are discussed, including signatures of non-Ohmic electrical transport. Evidence indicative of metallic surface states on SmB6 has been reported from multiple sources. Several of these measurements are repeated here on FeSb2, and excellent correspondence between the features of both materials is observed. Therefore, the mechanism driving surface-dominated conduction in FeSb2 is very likely to be the same as that in SmB6. University of Cambridge 2022-09-24 Thesis Doctoral Doctor of Philosophy (PhD) eng EPSRC UK St Catharine's College Cambridge https://www.repository.cam.ac.uk/handle/1810/344301 https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/4e738320-0eef-4481-85d1-60d260d57d10/download 817c1efee53f22dced16df6f6a355f94 https://creativecommons.org/licenses/by/4.0/ quantum oscillations topological insulator quantum matter unconventional insulator