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During the last years we have witnessed colossal advancements in all-electrical doping control on
cuprates. In the vast majority of cases, the tuning of charge carrier density has been achieved via electric field effect by means of either a ferroelectric polarization or by using a dielectric or electrolyte gating. Unfortunately, these approaches are constrained to rather thin superconducting layers and require large electric fields in order to ensure sizable carrier modulations [1, 2]. In this master thesis, we focus on the investigation of oxygen doping in an extended region through current-stimulated oxygen migration in Y B a 2 Cu 3 O 7−δ superconducting bridges. The underlying methodology is rather simple and avoiding sophisticated overlay nanofabrication process steps and complex electronics. A patterned 
multiterminal transport bridge configuration allows us to electrically assess the directional counterflow of oxygen atoms and vacancies. Importantly, the emerging propagating front of current-dependent doping δ isprobed in situ by polarized optical microscopy and scanning electron microscopy. The resulting imaging techniques, together with photo-induced conductivity and Raman scattering investigations reveal an inhomogeneous oxygen vacancy distribution with a controllable propagation speed permitting us to estimate the oxygen diffusivity. These findings provide direct evidence that the microscopic mechanism at play in electrical doping in cuprates involves diffusion of oxygen atoms with the applied current. The resulting fine control of the oxygen content in complex oxides paves the way towards a systematic study of complex phase diagrams and the design of electrically addressable devices.

superconductivity --- ybco --- high temperature superconductors --- electromigration --- photoconductivity --- cuprate superconductivity --- Physique, chimie, mathématiques & sciences de la terre > Physique --- Physique, chimie, mathématiques & sciences de la terre > Chimie

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Since the discovery of quantum entanglement, characterization and detection of multipartite entanglement has remained an open question. In this work, we present how one can map the separability problem onto the truncated moment problem in probability theory. It leads to a necessary and sufficient condition for the separability of arbitrary quantum systems with arbitrary symmetries between the subparts. A semidefinite algorithm is presented, whose outcome provides a certificate of separability, or entanglement.

Entanglement --- Semidefinite optimization --- Truncated moment problem --- Convex optimization --- Separability problem --- Certificate of entanglement --- Certificate of separability --- Intrication --- Optimisation semidefinite --- Problème des moments tronqué --- Optimisation convexe --- Problème de la séparabilité --- Certificat d'intrication --- Certificat de séparabilité --- Physique, chimie, mathématiques & sciences de la terre > Physique