Infrared properties of high-temperature superconductors with single and triple copper oxygen planes

Abstract

<p>This thesis includes work on two high temperature superconducting systems (HTSC), namely, La2-x Sr x CuO4 and HgBa2 Ca2 Cu 3 O8+δ . The CuO2 planes contained within the crystal structure of these materials, as well as in all the other HTSC, hold the key to superconductivity. From one point of view the two systems described in this thesis represent two extremes within a wide variety of HTSC. La2-x Srx CuO 4 is the classic representative of a material with a single CuO 2 plane per unit cell and has one of the lowest critical temperatures. On the other hand, the crystallographic structure of HgBa2 Ca 2 Cu3 O8+δ is based on three CuO2 planes. Despite the fact that both of these systems were discovered quite some time ago relatively little work has been done on them. Infrared spectroscopy has established itself as a very powerful technique for studying the properties of superconductors. One out of the vast majority of excitations that infrared spectroscopy can probe is the excitation responsible for the pairing mechanism in superconductivity. Furthermore, the energy range covered is compatible with the predicted value of the superconducting energy gap. This work is focused on, but not confined to, the infrared study of the normal properties of both La2-x Sr x CuO4 and HgBa2 Ca2 Cu 3 O8+δ . The reflectivity of La2- x Srx CuO4 single crystals has been measured in the frequency range 30-9,000 cm-1 (0.004-1 eV) in directions parallel and perpendicular to the CuO2 planes (ab -plane), whereas HgBa2 Ca 2 Cu3 O8+δ single crystals were studied only in the direction of the ab -plane. Both the doping and the temperature dependence were studied. For the first time, the high-temperature optical spectra were obtained. This allowed us to investigate the charge dynamics of the anomalous normal state up to 400 K. We observe a gap-like depression in the effective scattering rate 1/τ(ω, T ) below a temperature T* in both systems. This characteristic behavior of the frequency and temperature dependent scattering rates suggests the existence of a pseudogap state in both the under- and overdoped regimes of the single-layer HTSC system La2-x Sr x CuO4 for temperatures exceeding 300 K as well as in the underdoped regime of HgBa2 Ca2 Cu3 O 8+δ . The signature of the pseudogap state in the direction perpendicular to the CuO2 planes is also discussed.</p>