La2-xSrxCuO4 thin films and nanostructures to study local ordering phenomena in a striped superconductor

Examensarbete för masterexamen
Master's Thesis
Övrigt, MSc
Biagi, Marco
Since their discovery in 1986, cuprates high critical temperature superconductors (HTS) represent one of the most fascinating class of materials in condensed matter physics. Understanding the underlying mechanism behind high-Tc superconductivity is a real challenge, which could results in the possibility to design in the future a room-temperature superconductor, a technological holy grail allowing an energy-efficiency revolution, and the large-scale realization of applications such as magnetically levitated trains and quantum computers. The strong electron-electron correlations in HTS lead to the formation of exotic charge and spin orders such as charge density waves (CDW) and spin density waves (SDW), that are respectively charge and spin density periodic spatial modulations. In La-based cuprates, as La2-xSrxCuO4 (LSCO), CDW and SDW are characterized, in a well-defined portion of the phase diagram, by a well-defined relation of periodicity, forming the so-called stripe order. The understanding of these local orders is crucial, since they have been recently reported to be responsible for the superconducting and normal state of HTS. Their nature can be effectively investigated in thin films, where the strain induced by the substrate, and the confinement of the HTS at the nanoscale, have been proven to be two powerful knobs to manipulate these orders and understand their mutual interaction. The fabrication of HTS nanostructures is a very challenging task, and up to now relevant results were obtained mainly for YBa2Cu3O7-δ. We optimized the growth of 20 nm thick optimally doped LSCO thin films on LaSrAlO4 (001) substrates by Pulsed Laser Deposition. The films are smooth, as confirmed by atomic force microscopy and reflection high-energy electron diffraction, and highly crystalline, as confirmed by X-ray diffraction. Our best films show a Tc ∼ 39 K, comparable to the bulk value. Finally, we realize LSCO nanowires down to 50 nm width. We measure their Jc values and study the Jc(T). To prove the high degree of homogeneity of our nanowires, we compare the value of J0 c , obtained by fitting the Jc(T) with the Bardeen expression, to the Ginzburg-Landau theoretical limit for the depairing current Jv, due to vortex motion. Our results pave the way for the study of LSCO ground state at the nanoscale.
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