All ETDs from UAB

Advisory Committee Chair

Ilias E Perakis

Advisory Committee Members

David J Hilton

Kannatassen K Appavoo

Stephen A McGill

Eugenia Kharlampieva

Document Type

Dissertation

Date of Award

2020

Degree Name by School

Doctor of Philosophy (PhD) College of Arts and Sciences

Abstract

This thesis will study the photoexcitation dynamics observed in optimally doped La2-xSrxCuO2 (LSCO) thin films while under magnetic field. The high temperature superconducting cuprates display strong spin-fluctuations and appearances of mixed phase regimes that are heavily dependent on their doping levels. This thesis focuses on the optimal Sr doping level x = 0.15. The experimental technique uses femtosecond pulses to probe the normal metallic and superconducting phases in this material while applying external electric and magnetic perturbations. The broadband (0.1 - 10 THz) optical pump-terahertz probe spectrometer that is intended for studying this material was designed at the University of Alabama at Birmingham (UAB) to operate under extreme high magnetic field conditions of the 25 T Split Florida-Helix magnet system at the National High Magnetic Field Laboratory (NHMFL). We demonstrated the capabilities of this instrument by measuring the excitation dynamics in bulk MoS2 under applied external magnetic field. In these measurements, there were observational changes to the ground state electronic band structure from the applied field that were attributed to the Auger scattering process and twisted domain characteristics of the bulk material. The observations acquired on short time scales at high magnetic fields showed the efficacy of using this instrument for studying the effects of dimensionality on electronic properties. Measurements for the LSCO thin films involving a lower magnetic field (< 7T) were performed at the Center for Integrated Nanotechnologies (CINT) at both Sandia and Los Alamos National Labs (SNL and LANL, respectively). We observe the suppression of the superconducting condensate below the critical temperature under applied external magnetic fields. We also observe a strong conductive response under applied fields that includes an enhancement of real conductivity up to 5 T, following the superconductor to metal transition. At even higher fields, the imaginary conductivity shows a negative resistive response, suggesting the enhancement of a magnetic driven current. We also performed optical pump-terahertz probe measurements involving the application of a photoexcitation pulse under magnetic field. The results suggest a strong interplay between the magnetic and optical effects on breaking Cooper pairs, changing the dynamic quasiparticle recovery to equilibrium.

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