Magnetoresistance and surface acoustic waves measurements of quantum materials

Abstract

This thesis consists of three separate parts. The first part of the thesis focuses on the quantum oscillation study of the quasiparticle properties of thin-film \sro grown on a (LaAlO$_3$)$_{0.29}$ (SrAl$_{1/2}$Ta$_{1/2}$O$_3$)$_{0.71}$ substrate. The second part concerns the magnetotransport study of the Fermi surface of the high-\Tc cuprate, \NdLSCO. The third part of the thesis focuses on the development of contactless, finite-wavelength quantum transport in high-quality 2D heterostructures using surface acoustic waves (SAWs). In the first part of the thesis, first, we show that in clean materials where the mean free path of the electron is long compared to the cyclotron radius, the Shubikov-de Hass effect can be observed in these materials, and the quasiparticle properties, including the Fermi surface volumes, cyclotron effective masses, and quantum lifetimes, can be accurately determined from the quantum oscillation analysis. An extensive quantum oscillation analysis of thin-film \sro grown on an (LaAlO$_3$)$_{0.29}$-(SrAl$_{1/2}$Ta$_{1/2}$O$_3$)$_{0.71}$ substrate is presented. The transport lifetime is calculated by solving the semiclassical Boltzmann transport equation. We find that the transport lifetime is longer than the quantum lifetime, indicating that extended defects are the dominant source of quasiparticle scattering, confirmed by cross-sectional scanning transmission electron microscopy. In the second part of the thesis, we show that with angle-dependent magnetoresistance (ADMR) measurement, the Fermi surface and the quasiparticle lifetime can also be accurately determined, in materials where the mean free path is short, and quantum oscillation cannot be observed with an achievable magnetic field. We perform a thorough angle-dependent study of the high-\Tc cuprate, \NdLSCO at two dopings, one above the critical doping and one below. Simulations of the ADMR are performed by numerically solving the Boltzmann transport equation. We find that the ADMR data can be described by a Fermi surface geometry consistent with angle-resolved photoemission data and a highly anisotropic scattering rate outside the pseudogap phase. In the pseudogap phase, we find that the ADMR is qualitatively different. We tested several scenarios, including quasiparticle scattering rate change and Fermi surface reconstruction. We find that the data is best described by a Fermi surface consisting of small, nodal hole pockets. In the third part of the thesis, we focus on the development of SAW resonant cavities on LiNbO$_3$ substrates for contactless conductivity measurements in the quantum transport regime of 2D heterostructures. There are two major challenges, finding a suitable substrate, compatible with high-mobility device fabrication and electrostatic gating and increasing signal size. In this thesis, we try to address both of them. To address the first challenge, we analyze the basic property of SAWs and their interaction with a 2D conducting sample placed near the substrate surface and pick the piezoelectric material \lno as substrate material. To address the second challenge, we compare the two ways of performing SAW measurement, using SAW delay lines and SAW resonators, and show that the resonant cavity geometry increases signal-to-noise by two orders of magnitude over the traditional delay-line geometry. Finally, we demonstrate that the substrate is compatible with high-mobility device fabrication and electrostatic gating, and the quantum transport regime is achieved with a detailed analysis of the quantum oscillations in the SAW cavity frequency in the quantum Hall regime of graphene.