Abstract
One of the most widely discussed features of the cuprate high Tc superconductors is the presence of a pseudogap in the normal state. Recent transport and specific heat measurements have revealed an abrupt transition at the pseudogap critical point, denoted p*, characterized by a drop in carrier density and a strong mass enhancement. In order to give more details about this transition at p*, we performed low-temperature infrared spectroscopy in the normal state of cuprate superconductors La2-xSrxCuO4 (LSCO) and La1.8-xEu0.2SrxCuO4 (Eu-LSCO) for doping contents across the pseudogap critical point p* (from p = 0.12 to 0.24). Through the complex optical conductivity we can extract the spectral weight, K*, of the narrow Drude peak due the coherent motion of the quasi-particles, and the spectral weight enclosed inside the mid-infrared (MIR) band, KMIR, caused by coupling of the quasi-particles to collective excitations of the many-body system. K* is smaller than a third of the value predicted by band calculations, and KMIR forms a dome as a function of doping. We observe a smooth doping dependence of K* through p*, and demonstrate that this is consistent with the observed doping dependence of the carrier density and the mass enhancement. We argue that the superconducting dome is the result of the confluence of two opposite trends, namely the increase of the density of the quasi-particles and the decrease of their coupling to the collective excitations as a function of doping.
Highlights
The rich phase diagram of hole doped cuprates is a major challenge in condensed matter physics
When the carrier density exceeds the value where the highest Tc is observed, a critical point p∗ is reached where the pseudogap vanishes. At this critical doping p∗ two important signatures have been observed: firstly the carrier density n observed by Hall effect [3,4,5] and thermal transport [6] jumps from p to 1 + p indicating a major Fermi-surface reconstruction, secondly the density of states and associated effective mass m∗ observed by NMR [7] and specific heat [8,9] is strongly enhanced at p∗ and exhibits a logarithmic divergence in tem
If we take into account coupling to phonons or collective excitations of the electron many-body system, part of the Drude spectral weight is transferred to a mid-infrared band (MIR band) at higher energy, carrying a spectral weight KMIR
Summary
The rich phase diagram of hole doped cuprates is a major challenge in condensed matter physics. The strongly overdoped side of the phase diagram has remained unexplored and the high-Tc of the Hg1201 and Bi2212 families has limited the accessible temperature range where the normal state properties could be analyzed. To elucidate this puzzle, we measured infrared optics and extracted the complex optical conductivity σ in the normal state of two cuprates, LSCO and Eu-LSCO for four dopings across the pseudogap critical point p∗: 0.12, 0.15, 0.21, and 0.24 (a total of six samples). A polarizer was used for selecting the optical response parallel to the CuO2 planes
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