SPring-8, the large synchrotron radiation facility

Language: English

Title: Electron-phonon coupling in high temperature superconducting oxychlorides, a light element model for cuprates

Abstract:
The copper oxychloride cuprate Ca₂CuO₂Cl₂ (CCOC) system, with vacancy or Na doping on the Ca site, is unique among the high temperature superconducting cuprates (HTSCs) since it: lacks high Z atoms; has a simple I4/mmm 1-layer structure, typical of 214 (LSCO) cuprates, but which is stable at all doping and temperatures; and has a strong 2D character due to the replacement of apical oxygen with chlorine ^[1]. It also shows a remarkable phase diagram, with a superconducting Tc growing to the optimal doping without any minimum around 1/8 doping, despite the observation of charge modulations by by near-field spectro-microscopy ^[2]. Due to the reduced number of electrons, advanced calculations that incorporate correlation effects, such as quantum Monte Carlo ^[3], are easier, but relatively little is known about CCOC from an experimental point of view. We are now filling this gap by a comprehensive experimental study covering the whole phase diagram, in particular of the (para)magnon ^[4] and phonon dispersion ^[5]. In this talk, we will focus on the phonons modes, particularly of the stretching of the Cu-O bond, as the possible responsible of the very large isotope effect in underdoped cuprate ^[6]. We will show how this can be linked to anomalies in the dispersion of Cu-O bond stretching mode, and most probably to the kink affecting ARPES bands at the same energy ^[7], and how RIXS unveil a strong electron-phonon coupling with that mode ^[8]. We will show how simple DFT fail to describe the mode dispersion ^[5], and henceforth its coupling with charge carriers. Finally, we will show that the enhancement of the electron-phonon coupling around 1/8 doping is caused by charge order proximity, which is particularly interesting in the oxychlorides, and the needs for a measurement of the bare phonon dispersion in that compound, that will take place in a forthcoming experiment at BL35XU(SPring-8).

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