Many fundamental questions remain open about the mechanism of high temperature superconductivity (HTSC) in layered copper oxides. In low-temperature SCs Cooper pairing is mediated by phonons, but for HTSCs the pairing interaction is not yet confidently known. Intense paramagnon excitations have only been very recently identified in a large family of HTSCs and this finding enables quantitative tests of magnetic Cooper pairing models. Another challenge is to understand the competition and coexistence of HTS with other broken symmetry ground states. An increasing amount of experimental data obtained in recent years can be regarded as strong evidence for spontaneous symmetry breaking in the pseudogap phase of the cuprates and indicates that the charge density wave instability competes with HTS.
Spectroscopic studies of electrodynamics are emerging as the premier experimental tool of HTSC. The validity of pairing models based on recent experimental observations needs to be verified against optical data. We apply terahertz (THz) transmittance spectroscopy and wide-band (from far-IR to deep-UV) spectroscopic ellipsometry to study charge carrier dynamics in high-quality single crystals and epitaxial films of HTSC copper oxides. Our experimental approach is based on measurements of both the transmission\reflection coefficient and phase shift of the transmitted\reflected radiation. This approach allows measurements of the real and imaginary parts of the dielectric function with significantly higher accuracy and reproducibility than regular reflection spectroscopy. Well-characterized high-quality specimens of YBa2Cu3O7-δ and Bi2Sr2CaCu2O8+δ are afforded by recent advances in crystal preparation. Oxide Molecular Beam Epitaxy (MBE) provides crystalline films of La2−xSrxCuO4−δ grown in a layer-by-layer fashion and doped with atomically controlled distribution of cations.
Furthermore, THz polarization anisotropies, such as Faraday effects, linear and magneto-electric birefringence can be very sensitive tools for the detection of the broken symmetry states. In order to address this issue we have expanded our experimental capabilities with a new state-of-the-art high-resolution time-domain THz spectrometer (HASSP-THz) which utilizes two Ti:sapphire femtosecond lasers whose repetition rates of 1 GHz are linked with an offset of a few kHz. Its unique sensitivity and resolution will allow us to use an advanced optical layout with a combination of several rotating polarizers and to determine changes in the polarization of the transmitted light through the HTSC films to high precision.