Phonon anomalies with doping in superconducting oxychlorides Ca2-xCuO2Cl2

Abstract

We measure the dispersion of the Cu-O bond-stretching phonon mode in the high-temperature superconducting parent compound Ca2CuO2Cl2. Our density functional theory calculations predict a cosine-shaped bending of the dispersion along both the (ξ00) and (ξξ0) directions, while comparison with previous results on Ca1.84CuO2Cl2 show it only along (ξ00), suggesting an anisotropic effect which is not reproduced in calculation at optimal doping. Comparison with isostructural La2−xSrxCuO4 suggests that these calculations reproduce well the overdoped regime, however they overestimate the doping effect on the Cu-O bond-stretching mode at optimal doping. PACS numbers: 74.72.Gh, 63.20.D-, 63.20.kd, 78.70.Ck ∗ [email protected] 1 ar X iv :1 90 4. 08 25 8v 1 [ co nd -m at .s tr -e l] 1 7 A pr 2 01 9 The role of electron-phonon coupling in high-temperature superconducting (HTS) cuprates has been debated since their discovery [1]. Although the general belief is that coupling with phonons is not the main mechanism driving Cooper pair formation in HTS cuprates [2], their role is still not completely understood. For example, the electron-phonon coupling exhibits anomalous doping dependence with a very large oxygen isotope effect close to 1/8 doping [3, 4]. A decade ago, the debate around the role of electron-phonon coupling was revived by the observation of a strong kink in the electronic band dispersion measured by angle-resolved photoemission spectroscopy (ARPES) [5] which was thought to originate from phonon interactions. The Cu-O bond stretching phonon [6–12], which softens with doping, is the most likely candidate for this interaction [13]. Subsequent density functional theory (DFT) calculations [14–16], could explain rather well the phonon softening despite small electron-phonon coupling, but they could not explain the large ARPES kink. However, it was suggested that large couplings may still exist due to many-body effects in the presence of strong electron-electron correlations [17] which are not captured by these DFT calculations. In this Letter, we present inelastic x-ray scattering (IXS) measurements of the parent compound Ca2CuO2Cl2 [20, 21]. We demonstrate that doping induces a softening of the CuO bond-stretching phonon by comparing with previous reports [22] on the vacancy-doped compound Ca1.84CuO2Cl2 [23], which is near optimal doping. This result is consistent with the above cited reports of doping-induced softening in other HTS cuprates. The softening however is anisotropic which disagrees with our DFT calculations. We show by comparison with La2−xSrxCuO4 [24], since Ca2−xCuO2Cl2 cannot be overdoped, that DFT calculations actually reproduce the strongly overdoped region in HTS cuprates. The failure of the DFT calculations to reproduce this important phonon mode near optimal doping naturally explains its inability to reproduce the observed large ARPES kink. In the future our results on Ca2CuO2Cl2, coupled with previous reports on Ca1.84CuO2Cl2 [22], may help bridge this gap between theory and experiment in the HTS cuprates. The Ca2CuO2Cl2 system is ideally suited to advanced many-body calculations trying to capture the predicted larger electron-phonon coupling due to electronic correlations because of its light elements and simple structure [25, 26]. Single crystals of Ca2CuO2Cl2 were grown by the flux method as described in Ref. 19. The phonons were measured using inelastic x-ray scattering (IXS) at the BL35XU beamline of