Vortex Dynamics in Superconducting Thin Films under Microwave Fields

• Autores: Pedro de Jesús Cuadra Solis
• Directores de la Tesis: Antoni García Santiago (dir. tes.), Javier Tejada Palacios (dir. tes.)
• Lectura: En la Universitat de Barcelona ( España ) en 2015
• Idioma: inglés
• Tribunal Calificador de la Tesis: Xavier Obradors Berenguer (presid.), Sergio Valenzuela (secret.), Joan Manel Hernández Ferrás (voc.)
• Materias:
  • Física
    • Electromagnetismo
      • Interacción de ondas electromagnéticas con la materia
      • Radioondas y microondas
      • Superconductividad
    • Física del estado sólido
      • Propiedades magnéticas
• Enlaces
  • Tesis en acceso abierto en:  TDX  Dipòsit Digital de la UB 
• Resumen

  • In this thesis we have studied the vortex dynamics in type-2 superconducting thin films made of La1.82Sr0.18CuO4, patterned Pb and MgB2, by means of the combination of microwave, dc and ac measurements. In particular, we explore first the magnetic moment response in real time under continuous and pulsed microwave mode, and second commensurability effects both in presence of microwave fields under ac susceptibility mode and using microwave reflection spectrometry. To carry out these measurements, we have designed and developed a low-temperature microwave setup to be used inside of an rf-SQUID-based magnetometer. This layout has been used to perform measurements under different microwave modes, namely sweeping, pulsed and power reflected. A theoretical model has been proposed to obtain an estimation of the characteristic frequencies and the electric and magnetic fields of the resonant structure formed by a multi-turn coil with a sample loaded in its core. We presents the results on vortex dynamics of superconducting thin films made of La1.82Sr0.18CuO4, patterned Pb and MgB2 under continuous and pulsed microwave modes. In the first set of experiments, the temporal- and frequency-dependent magnetic moment has been investigated at different values of the temperature, dc magnetic fields, and nominal microwave power in the three samples. Irregularities forming ripples, small steps, jumps and negative peaks at discrete frequency values under different thermodynamics variables (temperature and dc magnetic field) and microwave nominal powers have registered. The presence of these frequency-triggered anomalies has been interpreted as nucleation of instabilities in the propagation of the flux front and explained by considering the energy fed at some specific resonant modes, the vortex dynamics in thin-film geometry, and the contribution from the microwave electric and magnetic fields generated in the coil. In the second set of experiments, the temporal evolution of the sample magnetization has been recorded when single microwave pulses are applied at fixed values of temperature, dc magnetic fields, pulse duration and nominal power in the La1.82Sr0.18CuO4, and MgB2 samples. The onset of a significant variation in the sample magnetization which exists below threshold values of temperature, dc magnetic field, and pulse duration is interpreted as an avalanche-like flux penetration. The microwave fields are suggested to contribute to the nucleation of the fast vortex diffusion. The additional microwave electric field is surmised to guarantees the occurrence vortex instabilities under adiabatic conditions via the enhancement of the flux flow resistivity. We expose results obtained for patterned Pb under ac susceptibility and microwave power reflection techniques. The in-phase and out-of-phase components of the ac susceptibility are measured under the combination of crossed dc and microwave magnetic fields at different fixed values of microwave power and frequency for temperature close to Tc. The results show that commensurability effects are strongly dependent on the values of the frequency and nominal power of the microwave radiation. The magnetic field dependence of the microwave backward reflection coefficient is studied in a wide range of temperature (from 6.80 K to 7.20 K) at different fixed microwave resonant modes and powers. Commensurability effects related to the matching of the vortex lattice parameter and the antidot array spacing at integer and fractional matching field values have been observed as peaks in the magnetic field dependence of the microwave backward reflection coefficient for different values of temperature, frequency and power. These peaks appear as a result of the contribution of the vortex dynamics to the reflected signal a the matching fields and their observation depends on the values of the microwave power supplied to the sample.