Competing pinning mechanisms in Bi2Sr2CaCu2Oy single crystals by magnetic and defect structural studies

Quantized dislocations

A dislocation, just like a phonon, is a type of atomic lattice displacement but subject to an extra topological constraint. However, unlike the phonon which has been quantized for decades, the dislocation has long remained classical. This article is a comprehensive review of the recent progress on quantized dislocations, aka the 'dislon' theory. Since the dislon utilizes quantum field theory to solve materials defects problems, we adopt a pedagogical approach to facilitate understanding for both materials science and condensed matter communities. After introducing a few preliminary concepts of dislocations, we focus on the necessity and pathways of dislocation's quantization in great detail, followed by the interaction mechanism between the dislon and materials electronic and phononic degrees of freedom. We emphasize the formality, the new phenomena, and the predictive power. Imagine the leap from classical lattice wave to quantized phonon; the dislon theory may open up vast opportunities to compute dislocated materials at a full quantum many-body level.

Comparative study of vortex dynamics in CaKFe4As4 and Ba0.6K0.4Fe2As2 single crystals

We investigate the vortex dynamics in two typical hole doped iron based superconductors CaKFe₄As₄ (CaK1144) and Ba_0.6K_0.4Fe₂As₂ (BaK122) with similar superconducting transition temperatures. It is found that the magnetization hysteresis loop exhibits a clear second peak effect in BaK122 in wide temperature region while it is absent in CaK1144. However, a second peak effect of critical current density versus temperature is observed in CaK1144, which is however absent in BaK122. The different behaviors of second peak effect in BaK122 and CaK1144 may suggest distinct origins of vortex pinning in different systems. Magnetization and its relaxation have also been measured by using dynamical and conventional relaxation methods for both systems. Analysis and comparison of the two distinct systems show that the vortex pinning is stronger and the critical current density is higher in BaK122 system. It is found that the Maley's method can be used and thus the activation energy can be determined in BaK122 by using the time dependent magnetization in wide temperature region, but this is not applicable in CaK1144 system. Finally we present the different regimes with distinct vortex dynamics in the field-temperature diagram for the two systems.

Second magnetization peak effect, vortex dynamics, and flux pinning in 112-type superconductor Ca0.8La0.2Fe(1-x)CoxAs2

Investigation of vortex pinning and its relaxation is of great importance for both basic physics and technological applications in the field of superconductivity. We report a great improvement of superconducting properties in the recently discovered 112-type superconductors (Ca, La)FeAs2 through Co co-doping. High critical current density Js(5 K) > 2(*)10(6) A/cm(2) is obtained and pronounced second peak effect is observed in magnetization hysteresis loops. Both the dynamic and static relaxation studies result in comparable and sizable relaxation rates S or Q, indicating a fast vortex creep. The second magnetization peak (SMP) is found to be strongly associated with a crossover from elastic to plastic vortex creep. Above the crossover, plastic vortex creep governs the vortex dynamics in a wide range of temperatures and fields. A good scaling behavior of the normalized pinning force density fp by formula fp = h(p)(1-h)(q) ((p) = 1.44, q = 1.66, h = 0.44) is revealed, which demonstrates an important contribution from core normal point-like pinning sites. To better understand the SMP phenomenon, we discuss the related physical scenario as well as the affecting factors in the SMP occurrence.

Peak effect in a driven lattice gas model

We study the peak effect (PE), i.e., a sharp peak observed in the critical current as a function of the particle density, discovered in transport properties of a driven lattice gas model. We show that the PE corresponds to a first-order phase transition found in the undriven system at equilibrium, which in turn gives rise to an "anomalous" second peak in magnetic hysteresis loops. We also explain the "history" dependent phenomena observed in the PE region by investigating the system characteristic time scales, which diverge at low T and have a broad maximum as a function of the external field around the PE. The model we consider can be related to a coarse grained description of vortex lines in superconductors and we discuss the relations of the PE described here and the one experimentally observed in these systems.

Static and dynamic coupling transitions of vortex lattices in disordered anisotropic superconductors

We use three-dimensional molecular dynamics simulations of magnetically interacting pancake vortices to study vortex matter in disordered, highly anisotropic materials such as BSCCO. We observe a sharp 3D-2D transition from vortex lines to decoupled pancakes as a function of relative interlayer coupling strength, with an accompanying large increase in the critical current reminiscent of a second peak effect. We find that decoupled pancakes, when driven, simultaneously recouple and order into a crystalline-like state at high drives. We construct a dynamic phase diagram and show that the dynamic recoupling transition is associated with a double peak in dV/dI.

High Critical-Current Density in the Heavily Pb-Doped Bi2Sr2CaCu2O8+delta Superconductor: Generation of Efficient Pinning Centers

Critical-current density (Jc) is a parameter of primary importance for potential applications of high-temperature copper oxide superconductors. It is limited principally by the breakdown of zero-resistive current due to thermally activated flux flow at high temperatures and high magnetic fields. One promising method to overcome this limitation is to introduce efficient pinning centers into crystals that can suppress the flux flow. A marked increase in Jc was observed in Bi2Sr2CaCu2O8+delta (Bi-2212) single crystals doped with a large amount of Pb. By electron microscopy, characteristic microstructures were revealed that probably underlie the observed enhancement in Jc: thin (10 to 50 nanometers), platelike domains having a modulation-free structure appeared with spacings of 50 to 100 nanometers along the b axis.