After a short introduction to the physics of two-leg ladder copper oxygen compounds, we present results of our combined experimental and theoretical study of the electronic gap features and the phonon anomalies occurring in the infrared spectra of the ladder compound Sr14−xCaxCu24O41 (SCCO).

First, we present a phenomenological model that describes the anomalous doping- and temperature-dependence of some of the phonon features in the a-axis response (field along the rungs of the ladders) of SCCO. It assumes that the phonons are coupled to charge oscillations within the ladders. Their changes with decreasing temperature reveal the formation of a crystal (density wave) of hole pairs that are oriented along the rungs.

Second, we confirm that an insulator-like pseudogap develops in the a-axis conductivity of SCCO which closely resembles the well-known one occurring in the c-axis conductivity (field perpendicular to the CuO2 planes) of the underdoped high-Tc superconductor YBa2Cu3O6+x (YBCO). We offer some insights into the origin of this enigmatic pseudogap based on results of our numerical study of t-J ladders.

Third, we find that the c-axis conductivity (field along the legs of the ladders) of SCCO is strikingly similar to the in-plane one (field parallel to the CuO2 planes) of YBCO. Notably, in both cases a dip feature develops in the normal state spectra that is connected with a spectral weight shift towards low frequencies and can be possibly associated with precursor superconducting pairing correlations that are lacking macroscopic phase coherence. This SCCO-YBCO analogy indicates that collective degrees of freedom contribute to the low-energy response of underdoped high Tc cuprates and it even suggests that the charges in the CuO2 planes tend to segregate forming quasi-one-dimensional structures similar to the two-leg ladders, as predicted for the stripe-scenario or certain intertwinned states.

P. Adamus, B. Xu, P. Marsik, A. Dubroka, P. Barabasová, H. Růžičková, P. Puphal, E. Pomjakushina, J. L. Tallon, Y. L. Mathis, D. Munzar, and C. Bernhard, Reports on Progress in Physics 86, 044502 (2023).