arXiv subjects

Gabriela Pasquini

Publications and source records attributed to Gabriela Pasquini.

7 recordsLinked to original sources

arXiv2024

A Ginzburg-Landau approach to the vortex-domain wall interaction in superconductors with nematic order

In this work we study the interaction between vortices and nematic domain walls within the framework of a Ginzburg Landau approach. The free energy of the system is written in terms of a complex order parameter characteristic of $s$-wave superconductivity and a real (Ising type) order parameter associated to nematicity. The interaction between both order parameters is described by a biquadratic and a trilinear derivative term. To study the effects of these interactions we solve the time-dependent dissipative Ginzburg Landau equations using a highly performant pseudospectral method by which we calculate the trajectories of a vortex that, for different coupling parameters, is either attracted or repelled by a wall, as well as of the wall dynamics. We show that despite its simplicity, this theory displays many phenomena observed experimentally in Fe-based superconductors. In particular we find that the sign of the biquadratic term determines the attractive (pining) or repulsive (antipining) character of the interaction, as observed in FeSe and BaFeCoAs compounds respectively. The trilinear term is responsible for the elliptical shape of vortex cores as well as for the orientation of the axes of the ellipses and vortex trajectories with respect to the axes of the structural lattice. For the case of pining, we show that the vortex core is well described by a heart-shaped structure in agreement with STM experiments performed in FeSe.

cond-mat.supr-con↗

arXiv2023

Experimentation on stochastic trajectories: from Brownian motion to inertial confined dynamics

Statistical physics courses typically employ abstract language that describes objects too small to be seen, making the topic challenging for students to understand. In this work, we introduce a simple experiment that allows conceptualizing some of the underlying ideas of stochastic processes through direct experimentation. Students analyze stochastic trajectories of beads in a bouncing bed of smaller beads subjected to an external periodic drive. The analysis of the trajectories involves the application of a vast tool kit of statistical estimators that are useful in many fields of physics.

physics.ed-ph↗

arXiv2022

Vortices in a Ginzburg Landau Theory of Superconductors with Nematic Order

In this work we explore the interplay between superconductivity and nematicity in the framework of a Ginzburg Landau theory with a nematic order parameter coupled to the superconductor order parameter, often used in the description of superconductivity of Fe based materials. In particular, we focus on the study of the vortex-vortex interaction in order to determine the way nematicity affects its attractive or repulsive character. To do so, we use a dynamical method based on the solutions of the Time Dependent Ginzburg Landau equations in a bulk superconductor. An important contribution of our work is the implementation of a pseudo-spectral method to solve the dynamics, known to be highly efficient and of very high order in comparison to the usual finite differences/elements methods. The coupling between the superconductor and the (real) nematic order parameters is represented by two terms in the free energy: a biquadratic term and a coupling of the nematic order parameter to the covariant derivatives of the superconductor order parameter. Our results show that there is a competing effect: while the former independently of its competitive or cooperative character generates an attractive vortex-vortex interaction, the latter always generates a repulsive interaction.

cond-mat.supr-con↗

arXiv2019

Ac dynamic reorganization and non-equilibrium phase transition in driven vortex matter

Externally driven glassy systems may undergo non-equilibrium phase transitions (NEPTs). In particular, ac-driven systems display special features, like those observed in the vortex matter of $\mathrm{NbSe_2}$, where oscillatory drives reorganize the system into partially ordered vortex lattices. We provide experimental evidence for this dynamic reorganization and we show an unambiguous signature of its connection with an NEPT driven by ac forces. We perform a scaling analysis and we estimate critical exponents for this transition. Our results share similarities with some glass-to-viscous-liquid NEPTs and invite to search for similar physics in other elastic disordered media.

cond-mat.supr-con↗

arXiv2019

Thermal hysteresis of the Campbell response as a probe for bulk pinscape spectroscopy

In type-II superconductors, the macroscopic response of vortex matter to an external perturbation depends on the local interaction of flux lines with the pinning landscape (pinscape). The (Campbell) penetration depth $λ_{\mathrm{\scriptscriptstyle C}}$ of an $ac$ field perturbation is often associated with a phenomenological pinning curvature. However, this basic approach is unable to capture thermal hysteresis effects observed in a variety of superconductors. The recently developed framework of strong-pinning theory has established a quantitative relationship between the microscopic pinscape and macroscopic observables. Specifically, it identifies history-dependent vortex arrangements as the primary source for thermal hysteresis in the Campbell response. In this work, we show that this interpretation is well suited to capture the experimental results of the clean superconductor $\mathrm{Nb}\mathrm{Se}_{2}$; as observed through Campbell response (linear $ac$ susceptibility) and small-angle neutron scattering measurements. Furthermore, we exploit the hysteretic Campbell response upon thermal cycling to extract the temperature dependence of microscopic pinning parameters from bulk measurements; specifically the pinning force and pinning length. This spectroscopic tool may stimulate further pinscape characterization in other superconducting systems.

cond-mat.supr-con↗

arXiv2019

Transient magnetic domain wall AC dynamics by means of MOKE microscopy

The domain wall response under constant external magnetic fields reveals a complex behavior where sample disorder plays a key role. Furthermore, the response to alternating magnetic fields has only been explored in limited cases and analyzed in terms of the constant field solution. Here we unveil phenomena in the evolution of magnetic domain walls under the application of alternating magnetic fields within the creep regime, well beyond a small fuctuation limit of the domain wall position. Magnetic field pulses were applied in ultra-thin ferromagnetic films with perpendicular anisotropy, and the resulting domain wall evolution was characterized by polar magneto-optical Kerr effect microscopy. Whereas the DC characterization is well predicted by the elastic interface model, striking unexpected features are observed under the application of alternating square pulses: magneto-optical images show that after a transient number of cycles, domain walls evolve toward strongly distorted shapes concomitantly with a modification of domain area. The morphology of domain walls is characterized with a roughness exponent when possible and contrasted with alternative observables which result to be more suitable for the characterization of this transient evolution. The final stationary convergence as well as the underlying physics is discussed.

cond-mat.stat-mech↗

arXiv2003

Angular dependent vortex dynamics in superconductors with columnar defects

We explore in detail the angular dependent vortex dynamics in type II superconductors with aligned columnar defects introduced by irradiation with very energetic heavy-ions. We use dc magnetization measurements deep in the vortex solid phase, and ac susceptibility near the solid-liquid transition. We show that aligned columnar defects are an excellent tool to test models for vortex dynamics, particularly if they are tilted with respect to the crystallographic axes, so their effects can be easily distinguished from those arising from mass anisotropy, sample geometry, twin boundaries and intrinsic pinning. This allows us, for instance, to use the uniaxial pinning of the columnar defects as a probe to determine the orientation of the vortices inside a bulk material, which in general is different from the orientation of the applied fields. In some aspects we have found an excellent agreement with the theoretical expectations of the Bose-glass model. The field dependence of the lock-in angle follows remarkably well the 1/H prediction over the whole temperature range of our measurements. In turn, the temperature dependence of the lock-in angle gives strong support to the concept of an effective pinning energy dominated by the entropic smearing effect. On the other hand, both our ac and dc results show that columnar defects produce effective pinning over a wide angular range, and that correlated pinning dominates the scenario for all field orientations. One consequence of this is the existence of a rich variety of vortex staircases.

cond-mat.supr-con↗