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Terahertz time domain spectroscopy of GdBCO superconducting thin films

  • Ji, Gangseon (Department of Physics, Chungbuk National University) ;
  • Park, Woongkyu (Department of Physics and Astronomy and Center for Atom Scale Electromagnetism, Seoul National University) ;
  • Lee, Hyoung-Taek (Department of Physics, Chungbuk National University) ;
  • Song, Chang-Yun (Department of Physics, Chungbuk National University) ;
  • Seo, Choongwon (Department of Physics, Chungbuk National University) ;
  • Park, Minjo (Department of Physics, Chungbuk National University) ;
  • Kang, Byeongwon (Department of Physics, Chungbuk National University) ;
  • Kim, Kyungwan (Department of Physics, Chungbuk National University) ;
  • Kim, Dai-Sik (Department of Physics and Astronomy and Center for Atom Scale Electromagnetism, Seoul National University) ;
  • Park, Hyeong-Ryeol (Department of Physics, Chungbuk National University)
  • Received : 2019.03.05
  • Accepted : 2019.03.27
  • Published : 2019.03.31

Abstract

We present terahertz optical properties of $GdBa_2Cu_3O_{7-x}$ (GdBCO) superconducting thin films. GdBCO films with a thickness of about 105 nm were grown on a $LaAlO_3$ (LAO) single crystal substrate using a conventional pulsed laser deposition (PLD) technique. Using an Ar ion milling system, the thickness of the GdBCO film was reduced to 58 nm, and its surface was also smoothened. Terahertz (THz) transmission spectra through two different GdBCO films are measured over the range between 0.2 and 1.5 THz using THz time domain spectroscopy. Interestingly, the THz transmission of the thinner GdBCO film has been increased to six times larger than that of the thicker one, while the thinner film is still maintaining its superconducting property at below 90 K.

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Fig. 1. Schematic diagram of a transmission-type THz time domain spectroscopy based on a GaAs photoconductive antenna and a 1 mm-thick ZnTe (110) crystal as a detector with a low temperature cryostat. QWP: Quarter Wave-Plate. BS: Beam Splitter.

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Fig. 3. Scanning electron microscope (SEM) images of the cross-section of the two GdBCO films with the thicknesses of (a) 105 nm and (b) 58 nm.

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Fig. 5. Temperature-dependent transmittance curves of the two GdBCO films with the thicknesses of 105 nm (blue triangle) and 58 nm (red circle) at 0.7 THz.

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Fig. 2. (a) Electro-optic sampling signal in time-domain, through the bare aperture (black dashed), LAO (red solid), and STO (blue dashed dot). (b) Transmitted electric field amplitude spectra of LAO and STO.

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Fig. 4. (a) THz time-domain signals through the 58 nm-thick GdBCO film depending on the temperature in the range of 22 K and 200 K. (b) Fourier-transformed THz transmittance spectra in the frequency range between 0.2 and 1.5 THz.

Acknowledgement

Supported by : National Research Foundation of Korea (NRF), Seoul National University

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