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Séminaire - Julien Varignon

Lundi 23 Janvier 2022 à 14h00, Julien Varignon (Laboratoire de Cristallographie et Sciences des Matériaux CRISMAT ENSICAEN) donnera un séminaire intitulé : "Charge ordering as the driving mechanism for superconductivity in bismuth and nickel oxides"

Along with the famous cuprates, ABiO3 (A=Ba, Sr) bismuth oxide perovskites and nickel oxides, either as an infinite layer RNiO2 or Reduced Ruddlesden-Popper phase Rn+1 Nin O2(n+1) (R=La, Pr or Nd), belong to the few oxide systems exhibiting superconductivity once appropriately doped. They are thus alternative platforms for understanding the formation of bound electrons at the core of superconductivity. However, these two systems look rather different in their pristine form : (i) Ni+ cations exhibit a magnetic moment while Bi 4+ cations does not, (ii) nickelates are prone to correlation effects while bismuthates do not, (iii) RNiO2 compounds are metallic while bismuthates are insulating and (iv) nickelates present a Mott-like regime while ABiO3 materials possess a charge-transfer like behavior.

Although different at first glance, we reveal on the basis of Density Functional Theory (DFT) calculations, involving all relevant degrees of freedom and an exchange-correlation functional sufficiently amending self-interaction errors, that bismuthates and nickelates share much more in common than expected. In particular, both material exhibit disproportionation effects of Bi and Ni cations, at 0 and half hole doping, respectively. It results in a charge ordering and an insulating ground state. It is further accompanied by a bond disproportionation producing compressed and extended O complexes around Bi and Ni cations. Using the charge ordered states as a starting point, doping is revealed to be a knob driving the material in a metallic regime at the vicinity of the charge ordered phase. Within these metallic states, the vibration of the bond disproportionation effect is sufficient to explain the formation of Cooper pairs and to reproduce the evolution of the critical temperature versus doping content observed experimentally for the two compounds.

The results suggest that undoped materials are not necessarily a proper starting point for understanding superconductivity and the search for instabilities in the doped phase diagram thus appears crucial. Finally, strong electronic correlations appearing in the nickelates likely have a marginal effect on the superconducting mechanism, although local Ni spin formation remains an essential degree of freedom for describing superconductivity in the simulation.


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