d-d Excitations in Cuprates and their Interplay with Superconductivity / Growth and Oxygenation of Infinite-Layer CaCuO2 and SrCuO2 Thin Films

20.10.2020 13:00 – 14:30

High-temperature superconductors (HTSC) earned the title of the most studied materials in the eld
of condensed matter, polarizing the research since their discovery in 1986. Cuprates have oered a
fertile ground to theoreticians as well as to experimentalists, since the development of new
spectroscopic techniques and at the same time of novel theoretical models and tools are clearly
triggered by the questions raised in these materials. Up to now, there are still open discussions and
disputes about many phenomena observed in cuprates phase diagram.
Bi2Sr2CaCu2O8+ (Bi2212)
is a prototypical cuprate superconductor where
the critical temperature attains 91 K at optimal
doping. This material has been widely investigated
by means of many spectroscopic techniques,
from optical spectroscopy to angle resolved photo
electron spectroscopy, from resonant inelastic x-ray
scattering (RIXS) to scanning tunneling microscopy.
Using RIXS, we developed a novel experimental
approach which allows to carefully track the
temperature dependence of the dierent collective
modes, while slowly ramping and crossing the superconducting critical temperature Tc. We were so
able to unveil which spectroscopic part of the spectrum is mostly reactive to the transition at Tc.
It turns out the region in 1-3 eV is that changing behaviour across Tc, which corresponds to the
localized d-d orbital excitons. More precisely, the temperature dependencies of the integrated counts
show a slope change for dierent doping level, once a constant drift spectrum is subtracted from the
experimental data. I will then discuss a simple model which leads to a possible short range
interaction between d-d excitons and the electrons carrying the superconducting current.

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The discovery of high-Tc superconductivity in the cuprates has led to intensive research aiming at
understanding these fascinating compounds. Here we focus on the so-called innite-layer (IL)
structure (ACuO2, A = Ca, Sr, Ba) [1] which was also studied in detail leading to a series of
interesting discoveries.
Among these
discoveries are the observation of superconductivity in articially
layered cuprates [2], interface superconductivity between
the IL CaCuO2 and SrTiO3 when deposited under highly-oxidising
growth conditions [3,4], and the complete rearrangement
of oxygen coordination driven by the polar nature of SrCuO2 [5].
The IL cuprates are therefore extremely sensitive to their
oxygen coordination environment and controlling this environment
is crucial to the reproducible control of their properties.
In this work, we study the oxygenation of IL lms grown by pulsed
laser deposition under various growth conditions. We study the
eect of the oxidation power during the growth using oxygen and
ozone as growth atmospheres. We show that the strain state is a crucial factor that aects the
oxygenation process, allowing precise control of the oxygen environment of the IL cuprates.

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Bâtiment: Ecole de Physique

Auditoire Stueckelberg

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