Soutenance de thèse : Tianyou Yi



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Modeling of dynamical vortex states in charge density waves

Tianyou Yi, LPTMS

Electronic Crystals is a common form of organization in conducting solids. They take forms of Wigner crystals at hetero-junctions and nano-wires, charge density waves (CDWs) in chain compounds, spin density waves in organic conductors, stripes in doped oxides and high-Tc superconductors. In the CDW ground state, the elementary units can be readjusted by absorbing or rejecting pairs of electrons. Such a phase-slip process should go via topologically nontrivial configurations: solitons and dislocations – the CDW vortices. An experimental access to those states came from studies of nano-fabricated mesa-junctions, from the STM visualizations and from the X-ray micro-diffraction.

 Following these requests, we performed a program to model the stationary states and their transient dynamic for the CDW in restricted geometries under the applied field or the passing current. A particular care had to be taken to derive a gauge invariant and current conserving scheme for the interacting condensed and normal charge densities. The model takes into account multiple fields in mutual nonlinear interactions: the amplitude and the phase of the CDW complex order parameter, distributions of the electric field, the density and the current of normal carriers. We have found that vortices are formed stepwise in the junction when the voltage across, or the current through, exceed a threshold. The vortex core concentrates the voltage drop, working as a self-tuned microscopic tunnelling junction. The studied reconstruction in junctions of the CDW is a convenient playground for modern efforts of field-effect transformations in strongly correlated materials with spontaneous symmetry breakings

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