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Open Access Publications from the University of California

Scholarly Works (2 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2019)

Since the discovery of superconductivity, the potential applications of superconductors

has pushed the research of these materials into the forefront of physics. With the current rate

of technological developments, the discovery of room temperature superconductors seems only

steps away. The use of light, i.e. spectroscopy, has been one of the greatest tools used in modern

physics for furthering our understanding of material properties. Not only can we use light to

probe the underlying physics, but we can also use light to manipulate and control a materials

properties by design. In this thesis, I will present two different projects I have worked on involving

high-Tc superconductors and using light excitation to induce a metastable phase transition and

probe the materials nonlinear response. The first three chapters cover a brief background of

superconductors and recent advances in light-induced superconductivity, experimental techniques

built and developed by myself, and the method of analysis. The first research endeavour I report

on is photo-excitation of the highly charge-ordered cuprate La2-xBaxCuO4. After excitation we

are able to partially melt the competing charge-order and observe a long-lived metastable state

which shows signatures of superconducting behaviour above Tc. The second research project

explores the nonlinear c-axis response of La2-xSrxCuO4 using high field THz spectroscopy and

we observe third harmonic generation by driving the Josephson plasma resonance. A metamaterial

is also applied to the crystals surface and we observe pronounced nonlinear effects above and

below Tc not present for linear field strengths.

    Cover page: Ultrafast terahertz electrodynamics of cuprate superconductors
    • Article
    • Peer Reviewed
    UC San Diego Previously Published Works (2019)
    Quantum materials are amenable to nonequilibrium manipulation with light, enabling modification and control of macroscopic properties. Light-based augmentation of superconductivity is particularly intriguing. Copper-oxide superconductors exhibit complex interplay between spin order, charge order, and superconductivity, offering the prospect of enhanced coherence by altering the balance between competing orders. We utilize terahertz time-domain spectroscopy to monitor the c-axis Josephson plasma resonance (JPR) in La2-xBaxCuO4 (x = 0.115) as a direct probe of superconductivity dynamics following excitation with near-infrared pulses. Starting from the superconducting state, c-axis polarized excitation with a fluence of 100 μJ/cm2 results in an increase of the far-infrared spectral weight by more than an order of magnitude as evidenced by a blueshift of the JPR, interpreted as resulting from nonthermal collapse of the charge order. The photoinduced signal persists well beyond our measurement window of 300 ps and exhibits signatures of spatial inhomogeneity. The electrodynamic response of this metastable state is consistent with enhanced superconducting fluctuations. Our results reveal that La2-xBaxCuO4 is highly sensitive to nonequilibrium excitation over a wide fluence range, providing an unambiguous example of photoinduced modification of order-parameter competition.
      Cover page: Photoenhanced metastable c-axis electrodynamics in stripe-ordered cuprate La<sub>1.885</sub>Ba<sub>0.115</sub>CuO<sub>4</sub>.
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