STUDYING SUPERCONDUCTING FLUCTUATIONS AND COLLECTIVE MODES IN OPTIMALLY DOPED FeSe1-xTex AN UNCONVENTIONAL SUPERCONDUCTOR IN THE BCS-BEC CROSSOVER REGIME

Open Access
Author:
Vitalone, Rocco Alexander
Area of Honors:
Physics
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Prof. Richard Wallace Robinett, Honors Advisor
  • Prof. Jie Shan, Thesis Supervisor
Keywords:
  • High temperature superconductivity
  • Terahertz
  • Spectroscopy
  • Pump-Probe Spectroscopy
  • Unique Superconductors
Abstract:
The BCS-BEC crossover regime has historically been studied in the realm of ultra-cold fermionic gas systems. However, this regime is also particularly interesting for superconductivity. The highest temperature superconductors are theorized to exist in this regime, where the pair coherence length is on the order of the average distance between particles. Therefore, studying this regime can lead to a fundamental understanding of the physical properties that facilitate high temperature superconductivity and thus allow for the advancement of superconductor technology. One clear indicator of this crossover region is the presence of high temperature superconducting fluctuations – the formation of Cooper pairs before the onset of long range coherence. For this reason, in this experiment we utilize THz spectroscopy, which is resonant with the gap energy, to experimentally detect the superconducting fluctuations in FeSe0.8Te0.2. This optimally doped 50nm thin film sample on a CaF¬2 substrate likely inhabits this BCS-BEC crossover regime based on its Fermi temperature of 100 K and a critical temperature of 20 K. In our first THz TDS of this sample, we characterized the critical temperature to be 20 K and one of the superconducting gaps of the sample to be slightly less than 0.5 THz. Then, we detected the Higgs Mode oscillations in the sample using THz Pump – THz Probe spectroscopy, confirming the validity of using such a measurement for future studies of the sample. Finally, using TPTP, we detected high temperature signals in both peak dynamics and optical conductivity spectrums exceeding 40 K. These, we argue, may in fact be high temperature superconducting fluctuations, indicating BCS-BEC crossover. However, they may also indicate either a structural phase transition or the onset of competing orbital order, and therefore future studies of the sample are required. If we can show that they are indeed high temperature superconducting fluctuations, we argue that this would be the route to higher temperature superconductors.