FORUM DQMP - Unravelling the Phases of Cuprate Superconductivity by Heavy Overdoping & Effect of Flexoelectricity on the Nano-mechanical Properties of Ferroelectrics

30.04.2019 13:00 – 14:30

Unravelling the Phases of Cuprate Superconductivity by Heavy Overdoping
Tim Gadzic (group of Prof. Renner)

Understanding of high temperature superconductivity has challenged physicists for decades. As with other compounds such as transition metal oxides, a core part of the endeavour is understanding the interplay between the strong electronic interactions and superconductivity.
In cuprates, the doping is usually controlled by oxygen intercalation. Bi2Sr2CaCuO8+ has an extremely high critical temperature (94K) at optimal doping (p=0.16), and therefore provides a perfect playground for investigation of these phases. A complication arises from the similarity of standing wave signatures due to charge density modulations (CDM) and Bogoliubov quasiparticle interference (BQPI) hinting on an inextricable link between the two very different phenomena. As doping is increased, the CDM have been shown to disappear simultaneously with a Fermi Surface (FS) reconstruction at p=0.19. Another asset of the overdoped (OD) side of the phase diagram is that transport measurements show it to approach a simpler, Fermiliquid behaviour. We use scanning tunneling spectroscopy on the heavily OD side of the phase diagram to investigate the potentially clearer picture of the pairing. We discover trademarks of reconstructed FS, a remnant presence of CDM on the OD side and new ordered states that are yet to be explained in their entirety.


Effect of Flexoelectricity on the Nano-mechanical Properties of Ferroelectrics
Kumara Cordero Edwards (group of Prof. Paruch)

Hitherto, it has been believed that the mechanical properties are invariant with respect to space inversion, that is to say that measuring them on one side or on the opposite side of a crystal should not change their value, even when the material in question is crystallographically asymmetric, such as ferroelectrics.
However, this situation can change in the presence of strain gradients, because deforming a ferroelectric material in an inhomogeneous way, will give two sources of polarization: the piezoelectric one due to strain, and the flexoelectric one due strain gradients. These two polarizations can be parallel or antiparallel depending on the ferroelectric polarity, which in turn will result in two different electrostatic energy costs of the deformation. As consequence, the mechanical response of ferroelectrics depends not just on the orientation but also on the sign of their polarization.
Our work demonstrates experimentally that, in the presence of strain gradients, mechanical inversion symmetry breaks down: the mechanical response of ferroelectrics depends not just on the orientation but also on the sign of their polarization. This result represents a paradigm shift in the physics of solid state mechanics and fracture physics, and opens up new and interesting functional concepts such as mechanical reading of polarization


Forum Committee: C. Lichtensteiger, N. Ubrig, A. Tamai (22.04.2019)

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Bâtiment: Ecole de Physique

Auditoire Stückelberg

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