The study of phonon modes of Cu 1− xTl xBa 2Ca 3Cu 4O 12− y superconductor thin films by FTIR absorption spectroscopy

Abstract

The superconducting properties of Cu 1− x Tl x Ba 2Ca 3Cu 4O 12− y thin films prepared by amorphous phase epitaxial (APE) method have been studied, by resistivity measurements, critical current density measurements, infrared spectroscopy and X-ray diffraction. The resistivity measurements of all the samples have shown metallic variation down to onset of superconductivity and T c in the range of 95–112 K. The critical current densities of samples were also measured and it was observed that the samples with higher thallium contents in the unit cell have low critical current density, J c (∼10 3 A/cm 2) while lower thallium contents in the material give, higher critical current density J c (∼10 6 A/cm 2). The XRD measurements showed the material to be single phase and oriented along c-axis. The φ-scan of (1 0 3) reflection of Cu 1− x Tl x -1234 material showed the crystal to be a-axis oriented. The main emphasis of this research work is on the study of phonon modes of vibration of different atoms in the unit cell of Cu 1− x Tl x Ba 2Ca 3Cu 4O 12− y superconductor. The effects of post-annealing, of the samples in air atmosphere, on the phonon modes absorption is also studied. Three major phonon modes around 450–475, 900 and 1200 cm −1 in Cu 1− x Tl x -1234 have been observed. These modes are assigned to the vibrations of apical oxygen, O 3 and C–O respectively. The apical oxygen mode at 464 cm −1 is hardened to 474 cm −1 when the preparation temperature is increased from 905 to 920 °C while these modes substantially reduce in intensity after the post-annealing in air at 650 °C. The 945 cm −1 O 3 mode is softened to 880 °C when the synthesis temperature is above 910 °C. The softening of O 3 mode increases the critical temperature of the material, which also showed that the presence of O 3 atoms provide maximum stability to T1 atoms of the unit cell. The post-annealing of the materials in air atmosphere at 650 °C substantially reduces the intensity of 465–475 and 850 cm −1 mode. The reduced intensity of these modes shifts the material towards highly resistive regime and superconductivity is destroyed.