M. Jirsa

Bio: M. Jirsa is an academic researcher from Czechoslovak Academy of Sciences. The author has contributed to research in topics: Lorentz force & Relaxation (NMR). The author has an hindex of 2, co-authored 2 publications receiving 72 citations.

Correlation between magnetic hysteresis and magnetic relaxation in YBaCuO single crystals

Abstract: The dependence of the magnetic momentm obtained from the hysteresis loops on the speed of the magnetic field sweep $$\dot H$$ =dH ext/dt is explained on the basis of Anderson's interpretation of the magnetic flux creep. In addition, a phenomenological model is suggested which predicts a linear dependence ofm on ln ‖ $$\dot H$$ ‖ with the slope ∂m/∂ ln ‖ $$\dot H$$ ‖, numerically equal to the relaxation rate ∂m/∂ ln(t) from the usual magnetic relaxation. Such linear relations betweenm and ln ‖ $$\dot H$$ ‖ were observed experimentally in single crystals of YBaCuO. Preliminary experiments on the complementary time dependent relaxation ofm after a simulated step change ofH ext gave mostly relaxation rates close to the predicted values. The model here presented also enables one to compare the critical state in the superconductor at a field sweep rate $$\dot H$$ with the critical state at some timet eff after a step change ofH ext. The values of $$\dot H$$ analyzed in our experiments actually correspond to the critical state at timest eff between0.04 and4 sec after an imaginary large step change ofH ext.

Magneto-optical investigations of superconductors

Abstract: Magneto-optical investigations of the flux distributions in high-Tc superconductors are reviewed. The various techniques (which are all based on the Faraday effect) are compared with each other concerning resolution and working range of temperature and external magnetic field. A short description of the historical development of the magneto-optical methods is given and the existing equipment is classified. Various aspects of flux visualization are presented acid special observations made by means of magneto-optical techniques are shown. The capability of the magneto-optical techniques to observe dynamic processes allows observation of flux motion due to thermal activation or even quantum creep, and under the influence of transport currents. It is shown that magneto-optical techniques are a unique tool to show that flux patterns are extremely sensitive to the presence of defects.

Vortex dynamics in superconducting MgB2 and prospects for applications.

Abstract: The recently discovered1 superconductor magnesium diboride, MgB2, has a transition temperature, Tc, approaching 40 K, placing it intermediate between the families of low- and high-temperature superconductors. In practical applications, superconductors are permeated by quantized vortices of magnetic flux. When a supercurrent flows, there is dissipation of energy unless these vortices are ‘pinned’ in some way, and so inhibited from moving under the influence of the Lorentz force. Such vortex motion ultimately determines the critical current density, Jc, which the superconductor can support. Vortex behaviour has proved to be more complicated in high-temperature superconductors than in low-temperature superconductors and, although this has stimulated extensive theoretical and experimental research2, it has also impeded applications. Here we describe the vortex behaviour in MgB2, as reflected in Jc and in the vortex creep rate, S, the latter being a measure of how fast the ‘persistent’ supercurrents decay. Our results show that naturally occurring grain boundaries are highly transparent to supercurrents, a desirable property which contrasts with the behaviour of the high-temperature superconductors. On the other hand, we observe a steep, practically deleterious decline in Jc with increasing magnetic field, which is likely to reflect the high degree of crystalline perfection in our samples, and hence a low vortex pinning energy.

An overview on iron based superconductors

Abstract: The discovery of superconductivity at relatively higher temperatures in a non-cuprate system, LnFeAsO1 − xFx (Ln = lanthanides) has created tremendous activity among the reseachers in this field. This review is an overview on the present status and the future scope for iron pnictides. The various structural categories of iron based superconductors, the structural aspects, different preparation techniques of the material and the necessity for its optimization are discussed. The highlighting features of iron pnictide, i.e. the very high upper critical field, moderate magneto-transport and thermal properties, are also included. The article gives a summary of the prevailing arguments of researchers to relate the material to cuprates and also the comparative features of classical and MgB2 superconductors. The existing challenges, such as optimizing synthesis methods for technological applications and clarifying the ambiguity in the superconducting mechanism and the flexibility of the material for any site substitution, will keep iron based superconductors on the frontiers of research for a long time, in parallel to HTS.

Magnetization and relaxation curves of fast relaxing high-Tc superconductors

Abstract: The magnetization curves of rapidly relaxing type-II superconductors are calculated by means of Monte Carlo simulations and numerical solutions of the partial differential equation for thermally activated flux-creep. Several models for the current dependence of the activation energy ( U ( j )= U c (1- j / j c ), U ( j )=( U c / μ )(( j c / j ) μ -1) and U ( j )= U c ( j c / j )) are considered. The calcu lated curves reproduce all the features exhibited by experimental magnetization curves, even when the critical current is assumed to be field-independent. This remarkable result shows explicitly that strong flux relaxation effects can lead to spurious field-dependent critical currents. The characteristic features of the magnetization curves are related to the relaxation behaviour of the corresponding flux density profiles. The dependence of hysteresis loops on the magnetic field sweep rate is investigated in detail and is shown to contain basically the same information as the time dependence of the magnetization during relaxation.

Open questions in the magnetic behaviour of high-temperature superconductors

Abstract: A principally experimental review of vortex behaviour in high-temperature superconductors is presented. The reader is first introduced to the basic concepts needed to understand the magnetic properties of type II superconductors. The concepts of vortex melting, the vortex glass, vortex creep, etc are also discussed briefly. The bulk part of the review relates the theoretical predictions proposed for the vortex system in high temperature superconductors to experimental findings. The review ends with an attempt to direct the reader to those areas which still require further clarification.