Novel quantum phenomena and excitation modes in type-I superconductors and magnetic vortices

Abstract

The aim of this thesis was to study quantum phenomena and excitation modes in type-I superconductors and magnetic vortices.

The intermediate state in type-I superconductors is characterized by the gradual penetration of magnetic flux and the coexistence of normal and superconducting domains. This thermodynamic phase shows a magnetic irreversibility of topological origin, even in the case of defect-free samples. This irreversibility has been explored in disk-shaped samples made of lead (the prototype of a type-I superconductor) using a magnetic field applied perpendicularly to the disk plane, by means of the measurement of hysteresis cycles at different temperatures, zero-field-cooled and field-cooled magnetization curves at different magnetic fields and magnetic relaxations along the descending branch of the hysteresis cycles. Non-thermal magnetic relaxations have been observed in these samples at low temperatures, which have been attributed to the tunnel effect of normal-superconductor interfaces through pinning energy barriers. A quantum model based on the Caldeira-Leggett theory for dissipative systems have been developed to explain these experimental observations. The interface is described as a 2D elastic manifold that is pinned by a planar defect. The pinning barrier can be controlled by a supercurrent that exerts a force on the interface.

The vortex state turns out to be the ground state of magnetic disks for a wide variety of thicknesses and radii. It is characterized by the curling of the magnetization in the plane of the disk, leaving virtually no magnetic ‘charges’. The very weak uncompensated magnetic moment of the disk sticks out of a small area confined to the vortex core. The low-frequency dynamics of the vortex state is characterized by the spiral-like precessional motion of the vortex core as a whole (gyrotropic mode), which can be induced by the application of an in-plane magnetic field. The presence of structural defects in these magnetic disks affects the dynamics of the vortex state, which is indicative of the elastic nature of the vortex core along the axial direction of the disk. It has been studied whether the gyrotropic mode allows spatial dispersion similar to spin waves of a finite wavelength in ferromagnets. The excitation spectrum splits into two branches, one related to the gyrotropic mode with a gap given by the gyrofrequency of the disk and the other related to the existence of an effective mass associated to the vortex core. The magnetic irreversibility of the vortex state has been also explored by means of an experimental protocol analogous to that used in the case of type-I superconductors. Non-thermal magnetic relaxations have been observed again at low temperatures, which is attributed to the tunnel effect of a segment of the vortex core line through pinning barriers. A quantum model based on the Caldeira-Leggett theory for dissipative systems have been developed to explain these experimental observations. The interface is described as a 1D elastic manifold that is pinned by a linear defect.

To conclude, the effect of the vortex state on the supercurrent of a Josephson junction has been studied in the case where the non-superconducting layer consists of a magnetic disk with the vortex as the ground state. It has been concluded that the variation of the Josephson current with tiny displacements of the vortex core can be detected experimentally.

El objetivo de esta tesis ha sido estudiar fenómenos cuánticos y modos de excitación en superconductores tipo-I y vórtices magnéticos.

La irreversibilidad magnética en muestras de plomo con forma de disco en el estado intermedio ha sido explorada mediante medidas de ciclos de histéresis a diferentes temperaturas, medidas de las curvas de magnetización zero-field-cooled y field-cooled a diferentes campos y relajaciones magnéticas a lo largo de la rama descendiente de los ciclos de histéresis. Se han observado relajaciones magnéticas independientes de la temperatura en estas muestras, las cuales se atribuyen al efecto túnel de las interficies normal-superconductor a través de barreras de anclaje. Un modelo de efecto túnel basado en la teoría de Caldeira-Leggett para sistemas disipativos se ha construido para explicar estas observaciones experimentales, donde la interfície se trata como una variedad 2D elástica que se ancla a defectos planares. La barrera de anclaje se puede controlar mediante la inyección de supercorriente en el sistema.

El núcleo del estado vórtice muestra una naturaleza elástica a lo largo de la dirección axial de los discos magnéticos que lo presentan como estado fundamental. Se ha estudiado bajo qué condiciones el modo girótropo es compatible con una dispersión espacial semejante a las ondas de espín de longitud de onda finita presentes en un ferromagneto. El espectro de excitaciones axiales presenta dos ramas bien definidas, una asociada al modo girótropo y la otra originada por la existencia de una masa efectiva asociada al núcleo. También se ha explorado la irreversibilidad magnética del estado vórtice mediante un protocolo análogo al de los superconductores tipo-I. De nuevo se ha observado un comportamiento no térmico a bajas temperaturas en las relajaciones magnéticas, el cual es atribuido al efecto túnel de un segmento del núcleo vorticial a través de las barreras de anclaje. Un modelo de efecto túnel basado en la teoría de Caldeira-Leggett para sistemas disipativos se ha construido para explicar estas observaciones experimentales, donde el núcleo vorticial se trata como una variedad 1D elástica anclada a un defecto lineal.

Por último, se ha estudiado cuál sería el efecto del estado vórtice sobre la supercorriente de una unión Josephson si como capa no superconductora se escogiera un disco magnético con este estado fundamental. Se ha concluido que la variación de la corriente Josephson con desplazamientos pequeños del núcleo vorticial es detectable experimentalmente.