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 very recently been 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 HTSC 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 HTSC. Furthermore, the effects of reduced dimensionality, disorder, underdoping/overdoping, and their interplay with the superconducting state remain of fundamental interest.

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 submillimeter (GHz) interferometry, terahertz (THz) time-domain 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 YBa₂Cu₃O_7-δ and Bi₂Sr₂CaCu₂O_8+δ are afforded by advances in large-crystal sample preparation. Oxide Molecular Beam Epitaxy (MBE) provides crystalline films of DyBa₂Cu₃O_7-δ, La_2-xSr_xCuO₄, and La_2-xCa_xCuO₄ grown in a layer-by-layer fashion and doped with an atomically controlled distribution of cations.

Effects arising from two-dimensional fluctuations of the superconducting state, such as the Berezinskii-Kosterlitz-Thouless vortex unbinding transition, manifest in the evolution of the optical conductivity and superconducting condensate density. These can therefore provide sensitive means to probe details of the superconducting state, critical behavior, and disorder in dimensionally constrained HTSCs [1]. Additionally, MBE growth of epitaxially stabilized films allows superconductivity to be accessed in the severely overdoped as well as the underdoped and optimally doped regimes of the copper oxides [2]. In order to take advantage of these capabilities, we will use our broadband (submillimeter to deep-UV) phase-sensitive spectroscopic techniques to carry out highly accurate and detailed studies of the superfluid density and optical conductivity in ultrathin DyBa₂Cu₃O_7-δ and La_2-xCa_xCuO₄ as a function of temperature, ­doping level, and magnetic field. These studies are expected to provide fresh insights into the behavior of 2D-SC and the severely overdoped regime in the copper oxide family of HTSCs.

[1] “Approaching Two-Dimensional Superconductivity in Ultrathin DyBa₂Cu₃O_7-δ”. R. D. Dawson, K. S. Rabinovich, D. Putzky, G. Christiani, G. Logvenov, B. Keimer, and A. V. Boris, PRL 125, 237001 (2020).

[2] “Optical conductivity and superconductivity in highly overdoped La_2-xCa_xCuO₄ thin films”. G. Kim, K. S. Rabinovich, A. V. Boris, A. N. Yaresko, Y. E. Suyolcu, Y.-M. Wu, P. A. van Aken, G. Christiani, G. Logvenov, and B. Keimer, PNAS 118, 30 (2021).