Electronic properties of emergent topological defects in chiral pp-wave superconductivity

Chiral $p$-wave superconductors in applied magnetic field can exhibit more complex topological defects than just conventional superconducting vortices, due to the two-component order parameter (OP) and the broken time-reversal symmetry. We investigate the electronic properties of those exotic states, some of which contain clusters of one-component vortices in chiral components of the OP and/or exhibit skyrmionic character in the \textit{relative} OP space, all obtained as a self-consistent solution of the microscopic Bogoliubov-de Gennes equations. We reveal the link between the local density of states (LDOS) of the novel topological states and the behavior of the chiral domain wall between the OP components, enabling direct identification of those states in scanning tunneling microscopy. For example, a skyrmion always contains a closed chiral domain wall, which is found to be mapped exactly by zero-bias peaks in LDOS. Moreover, the LDOS exhibits electron-hole asymmetry, which is different from the LDOS of conventional vortex states with the same vorticity. Finally, we present the magnetic field and temperature dependence of the properties of a skyrmion, indicating that this topological defect can be surprisingly large in size, and can be pinned by an artificially indented non-superconducting closed path in the sample. These features are expected to facilitate the experimental observation of skyrmionic states, thereby enabling experimental verification of chirality in emerging superconducting materials

Threefold onset of vortex loops in superconductors with a magnetic core

A magnetic inclusion inside a superconductor gives rise to a fascinating complex of {\it vortex loops}. Our calculations, done in the framework of the Ginzburg-Landau theory, reveal that {\it loops always nucleate in triplets} around the magnetic core. In a mesoscopic superconducting sphere, the final superconducting state is characterized by those confined vortex loops and the ones that eventually spring to the surface of the sphere, evolving into {\it vortex pairs} piercing through the sample surface.Comment: 6 pages, 6 figures (low resolution), latex2

The Paramagnetic Meissner Effect (PME) in Metallic Superconductors

The experimental data in the literature concerning the Paramagnetic Meissner Effect (PME) or also called Wohlleben effect are reviewed with the emphasis on the PME exhibited by metallic, s-wave superconductors. The PME was observed in field-cool cooling (FC-C) and fieldcool warming (FC-W) m(T)-measurements on Al, Nb, Pb, Ta, in compounds such as, e.g., NbSe2, In-Sn, ZrB12, and others, and also in MgB2, the metallic superconductor with the highest transition temperature. Furthermore, samples with different shapes such as crystals, polycrystals, thin films, biand multilayers, nanocomposites, nanowires, mesoscopic objects, and porous materials exhibited the PME. The characteristic features of the PME, found mainly in Nb disks, such as the characteristic temperatures T1 and Tp and the apparative details of the various magnetic measurement techniques applied to observe the PME, are discussed. We also show that PME can be observed with the magnetic field applied parallel and perpendicular to the sample surface, that PME can be removed by abrading the sample surface, and that PME can be introduced or enhanced by irradiation processes. The PME can be observed as well in magnetization loops (MHLs, m(H)) in a narrow temperature window Tp < Tc, which enables the construction of a phase diagram for a superconducting sample exhibiting the PME. We found that the Nb disks still exhibit the PME after more than 20 years, and we present the efforts of magnetic imaging techniques (scanning SQUID microscopy, magneto-optics, diamond nitrogen-vacancy (NV)-center magnetometry, and low-energy muon spin spectroscopy, (LE-µSR)). Various attempts to explain PME behavior are discussed in detail. In particular, magnetic measurements of mesoscopic Al disks brought out important details employing the models of a giant vortex state and flux compression. Thus, we consider these approaches and demagnetization effects as the base to understand the formation of the paramagnetic signals in most of the materials investigated. New developments and novel directions for further experimental and theoretical analysis are also outlined

Magnetism in reduced dimensions

We propose a short overview of a few selected issues of magnetism in reduced dimensions, which are the most relevant to set the background for more specialized contributions to the present Special Issue. Magnetic anisotropy in reduced dimensions is discussed, on a theoretical basis, then with experimental reports and views from surface to single-atom anisotropy. Then conventional magnetization states are reviewed, including macrospins, single domains, multidomains, and domain walls in stripes. Dipolar coupling is examined for lateral interactions in arrays, and for interlayer interactions in films and dots. Finally thermally-assisted magnetization reversal and superparamagnetism are presented. For each topic we sought a balance between well established knowledge and recent developments.Comment: 13 pages. Part of a Special Issue of the C. R. Physique devoted to spinelectronics (2005

A numerical study of steady-state vortex configurations and vortex pinning in type-II superconductors

In part I, a numerical study of the mixed states in a mesoscopic type-II superconducting cylinder is described. Steady-state configurations and transient behavior of the magnetic vortices for various values of the applied magnetic field H are presented. Transitions between different multi-vortex states as H is changed is demonstrated by abrupt changes in vortex configurations and jumps in the B vs H plot. An efficient scheme to determine the equilibrium vortex configuration in a mesoscopic system at any given applied field, not limited to the symmetry of the system, is devised and demonstrated. In part II, a superconducting thin film is subject to a non-uniform magnetic field from a vertical magnetic dipole, consisting of two magnetic monopoles of opposite charges. For a film with constant thickness and with no pins, it has been found that the film carries two pairs of vortex-antivortex in the steady state in the imposed flux range of 2.15 < (Phi)+ < 2.90 (in units of flux quantum) and no vortex at all for (Phi)+ <= 2.15. Transitions from a superconducting state with 3 pairs of vortex-antivortex to one with 2 pairs, where a pair of vortex-antivortex annihilates, have been observed in the pseudo-time sequence. With a perturbation with antidots (holes), vortexantivortex pair has been created for lower magnetic fluxes down to (Phi)+ = 1.3. In the sample of size 16(Xi) x 16(Xi), the attraction force between the vortex and antivortex always dominates over the pinning force, so that they eventually come out of pins, move toward each other, and annihilate each other. The annihilation rate, measured with time taken for the annihilation, is reduced noticeably by the increase of the distance between pins, or the increase in the pin size. A simulation of the magnetic vortex pinning in the sample of size 32(Xi) x 32(Xi) suggests we are likely to achieve pinning of the vortex-antivortex pair with the sample size around this and vortex-antivortex separation of 22(Xi). Using this sample as a template, the maximum density of pinned vortices achievable is calculates to be about 7.6 x 10^14 vortices/m2 for (Xi) =~ 1.6A°

Vortex dynamics in nanostructured systems superconductor-superconductor and superconductor-metal of one and two bands

The overarching theme of the thesis are the emergent novel phenomena in two-component superconductors. My advisor in Colombia, Prof. Barba Ortega, has directed my research on two-component superconductors made of one same material, with properties spatially changed by either localized heating or nanostructuring. My advisor in Belgium, Prof. Milosevic, directed my research on samples made of two distinct superconducting materials, related to very recent experiments, with accompanying development of the theoretical model and the numerical implementation suited for high-performance computing. The principal objective of this thesis is to investigate the behavior of vortex matter under the effect of several configurations of pinning landscape with the inclusion of the enhanced surface, in several cases in this research, by using De Gennes’ boundary condition phenomenological parameter. Its effect is studied upon the vorticity, magnetic induction, Cooper pair density, magnetization and phase of the order parameter as functions of the external applied magnetic field.El tema general de la tesis son los nuevos fenómenos emergentes en superconductores de dos componentes. Mi asesor en Colombia, el Prof. Barba Ortega, ha dirigido mi investigación sobre superconductores de dos componentes hechos de un mismo material, con propiedades modificadas espacialmente por calentamiento localizado o nanoestructuración. Mi asesor en B´Bélgica, el Prof. Milosevic, dirigío mi investigación sobre muestras hechas de dos materiales superconductores distintos, relacionados con experimentos muy recientes, junto con el desarrollo del modelo teórico y la implementación numérica adecuada para la computación de alto rendimiento. El objetivo principal de esta tesis es investigar el comportamiento de los vórtices bajo el efecto de varias configuraciones de puntos de anclaje con la inclusión de una superficie mejorada, considerada en varios casos en esta investigación, usando la condición de frontera de De Gennes, parámetro fenomenológico b 0 . Se estudia su efecto sobre la vorticidad, la inducción magnética, la densidad de pares de Cooper, la magnetización y la fase del parámetro de orden como funciones del campo magnético externo aplicado. Se considera un caso particular de manipulación espacial de la distribución del condensado superconductor en películas delgadas mediante cambios locales de las propiedades térmicas, pero también, incluyendo las variaciones de anisotropía a través de variaciones de Tc y el calentamiento localizado del superconductor T. Este enfoque simple proporciona la alternativa perfecta para la modulación del colectivo de vórtices, emergiendo en los superconductores tipo II como una respuesta natural al campo magnético aplicado, que hasta ahora estaba controlada puramente a través de centros de anclaje estáticos nanofabricados, cuya la intensidad y distribución no se pueden cambiar una vez que se define su distribución. También se considera en esta investigación la modulación del condensado superconductor a nanoescala a través del control del espesor sobre la superficie, el cual produce una distribución del quantum de flujo magnético debido a la simetría seleccionada que permite imponer la creación de configuraciones de vórtice, pero también ajustar los parámetros críticos del superconductor que puede ser controlado. Nuestro resultado sugiere la posibilidad de modificar la superconductividad de una película delgada explotando efectos de tamaño cuántico dependientes del grosor. Dos manuscritos publicados en la Journal of low temperature physcis incluidos en este capítulo, en colaboración con el Profesor Edson Sardella (Universidade Estadual Paulista) y el Profesor José Barba, fueron citados más adelante en la revista Physical Review B por Baek, et al. [REF. [1]]. Finalmente, se presenta un estudio de la superconductividad de dos componentes, donde la muestra es una combinación de diferentes superconductores en una heteroestructura híbrida. Cubrimos una multitud de posibles híbridos superconductores de dos componentes, ya sea combinando dos materiales diferentes, o usando un mismo material con diferentes niveles de desorden y/o grosor diferente en dos regiones, donde esperamos que nuestros hallazgos mejoren la comprensión del procesos involucrados y ofrecer nuevas opciones para dispositivos cuánticos superconductores.Doctorad