Doctor of Philosophy, The Ohio State University, 2023, Physics

Nominally metallic systems can be rendered insulating by electronic interactions, disorder, or both, leading to a myriad of interesting many-body phases. In this thesis, we present electronic transport data on a variety of such insulator materials, each with their own unique emergent phenomena. We start with few-layer graphene (FLG), the multilayer counterpart to monolayer graphene, and show that electronic interactions can lead to the development of an electronic energy gap in the band structure near charge neutrality. Previously, this has been associated with spontaneous inversion symmetry breaking, but has only been observed in suspended devices of the highest quality. Here, we show that similar physics can be observed in hexagonal boron nitride-encapsulated devices, alleviating the requirement for suspension. Moreover, in very thick FLG samples, typically thick enough to be considered as three-dimensional graphite, we show the existence of fractional quantum Hall states that are extended through the bulk of the material. Next, we turn to Pt-doped TiSe2, where the interplay between a charge density wave state and a newly discovered quasi one-dimensional insulating state gives rise to ultra slow time-scale physics, along with a strong resistance anisotropy. Finally, transport data as well as angle-resolved photoemission spectroscopy data on Se-doped Ge2Sb2Te5 devices are shown. Here, a disorder-induced metal-to-insulator transition exhibits unique properties, which we attribute to the onset of strong electronic interactions.

Committee: Stuart Raby (Committee Member); Nandini Trivedi (Committee Member); Roland Kawakami (Committee Member); Marc Bockrath (Advisor) Subjects: Physics

Master of Sciences, Case Western Reserve University, 2020, Physics

In this thesis we explore mechanical analogs of electronic topological insulators. We develop continuum models for the mechanical instability and spontaneous symmetry breaking for monolayer antimonene and bilayer graphene. We find that walls form between domains corresponding to different symmetry breaking minima. These domain walls are solitons in our model. Perturbations about the symmetry breaking equilibria propagate as waves with a gapped dispersion in the bulk but there is a gapless mode with linear dispersion that propagates along the domain wall in a manner reminiscent of the electronic edge modes of a topological insulator. We establish that monolayer antimonene is a mechanical topological insulator by demonstrating a mapping between our continuum model and an underlying Dirac equation of the symmetry class BDI which is known to be a topological insulator in one dimension and a weak topological insulator in two dimensions. Following a similar argument we expect that bilayer graphene as well is a weak topological insulator in two dimensions. We surmise that the effects studied here (namely low scale symmetry breaking, strain solitons and gapless edge modes) are not limited to antimonene and bilayer graphene but are common features of two dimensional materials.

Committee: Harsh Mathur (Advisor) Subjects: Condensed Matter Physics; Physics

Doctor of Philosophy, The Ohio State University, 2019, Physics

In crystalline materials, the symmetry of the crystal lattice imposes strict conditions on the observable properties of the material. These symmetry restricted conditions can be, in turn, probed by light via the electromagnetic interaction. Studying the electromagnetic excitations in solids can reveal many fundamental properties of these systems. A quick introduction and guide to symmetry in solids will be given, with an emphasis on how it can be used to interpret spectroscopic measurements. The measurement techniques used will also be described. Time domain Terahertz spectroscopy (TDTS) is the main technique used in this dissertation. Important experimental considerations pertaining to the construction of the THz spectrometer will be given. In the multiferroic Sr_2 FeSi_2O_7, we found multiple excitations in the few meV energy scale (THz), in the material's paramagnetic phase. Measurements with varying temperature and magnetic field revealed that these excitations are both electric and magnetic dipole active. By considering the ground state of the Fe 2+ magnetic ion in Sr 2 FeSi 2 O 7 , we concluded that our observation is coming from the spin-orbital coupled states of the ion. This realization demonstrated that spin-orbit coupling plays a crucial role in these exotic materials. Interestingly, these spin-orbital THz excitations persist into the magnetically ordered phase. The single-ion picture of the paramagnetic phase needs to be expanded theoretically to explain our observations. CaFe_2O_4 orders antiferromagnetically below ~ 200 K. Two co-existing magnetic structures (A and B phase) have been measured previously by neutron diffraction. The anti-phase boundaries between these two phases have been proposed to be the cause of the quantized magnetic excitations (magnons) measured by an inelastic neutron scattering study. We measured two antiferromagnetic resonances (magnons) with TDTS. Our observation can be explained by the orthorhombic crystal anisotropy of CaF (open full item for complete abstract)

Committee: Rolando Valdes Aguilar (Advisor); P. Chris Hammel (Committee Member); Nandini Trivedi (Committee Member); Douglass Schumacher (Committee Member) Subjects: Physics

Doctor of Philosophy (PhD), Ohio University, 2015, Physics and Astronomy (Arts and Sciences)

In this dissertation we study transport properties of graphene within the low energy Dirac approximation. We utilize partial wave scattering methods and relate the scattering matrix elements to physical observables such as the elastic time, transport time, and skewness of scattering. We suggest that experimentally measurable quantities, such as the transport to elastic time ratio, indicate the presence of perturbations that lead to the reduction of symmetries of graphene, as well as spin-orbit interactions. This result relies on the fact that perturbations that leave graphene symmetries untouched, such as potential scatterers, display a constant ratio of transport to elastic times at low energies, making this ratio robust to random scatterer size and strength disorder. We also show that this ratio is not robust to either symmetry breaking perturbations or spin-orbit interactions, as these interactions lead to the ratio deviating from its ideal value. Even though both kinds of perturbations, symmetry breaking and spin-orbit interactions, lead to changes in the ratio, we show that the qualitatively different dependence on energy for each of these perturbations allows the experimental identification and quantification of both effects simultaneously. We have also shown, in relation to the spin Hall effect detection in graphene, that even though the local enhancement of spin-orbit interactions leads to the appearance of a spin Hall effect signal robust to potential and size disorder, the breaking of effective time reversal symmetry through local perturbations leads to the appearance of a valley Hall effect through skew scattering. This valley skew processes contribute to the non-local resistance that helps quantify the Hall effect. Similarly, we show that multiple potential scatterers with space dependence that breaks parity in graphene, also lead to the appearance of a valley Hall effect due to the separation of electrons from different valleys in space through skew (open full item for complete abstract)

Committee: Ulloa Sergio E. PhD (Advisor) Subjects: Condensed Matter Physics; Physics; Solid State Physics

Doctor of Philosophy, The Ohio State University, 2003, Physics

Although the Standard Model of fundamental particles and their interactions has enjoyed much success over the past quarter-century, a portion of the theory has eluded experimental verification. Mass is a manifest quality of the constituents of our universe; it remains to be understood however in the context of the Standard Model why the fundamental particles have the masses they possess. The imposition of mass is a consequence of the breaking of the symmetry that unifies the weak and electromagnetic forces; electroweak symmetry breaking is accomplished in the Standard Model via the Higgs mechanism. This elegant portion of the theory not only provides the dynamics for the symmetry breaking, but also predicts a physically observable scalar particle, the Higgs boson, which is yet to be discovered. A new search for the Higgs boson has been performed in the proton – antiproton collisions at center-of-mass energy of 1.8 TeV provided by the Tevatron accelerator at Fermi National Accelerator Laboratory. In this analysis, a neural network was utilized to aid in the rejection of collision events that share the equivalent signature as Higgs events but are produced via other, less interesting production mechanisms. The neural network was implemented as part of an advanced event selection that in simulation studies was shown to provide a 34% increase in signal sensitivity over conventional methods. When the technique is applied to the data collected by the CDF collaboration during the Tevatron's Run 1 (1992—1995), an excess of events is identified above the background expectation. The limit on the Higgs production cross section is calculated for six Higgs mass hypotheses in the range 100 GeV/c2 < MH < 150 GeV/c2. The WH production cross section upper limit was determined to be 18—22 pb in the range MH < 130 GeV/c2 at 95% confidence; the limit in the range MH > 130 GeV/c2 is considerably larger. The measured limit is a factor of two larger than the limit from a priori studies. Th (open full item for complete abstract)

Committee: Brian Winer (Advisor); Richard Hughes (Other); Eric Braaten (Other); Linn Van Woerkem (Other) Subjects: