J. Albino Aguiar

Enhanced stability of vortex-antivortex states in two-component mesoscopic superconductors

R. Geurts,1 M. V. Milošević,1,2 J. Albino Aguiar,3 and F. M. Peeters1,* 1Departement Fysica, Universiteit Antwerpen, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium 2Departamento de Fı́sica, Universidade Federal do Ceará, 60455-900 Fortaleza, Ceará, Brazil 3Departamento de Fı́sica, Universidade Federal de Pernambuco, Cidade Universitária, 50670-901 Recife-PE, Brazil (Received 6 September 2012; revised manuscript received 7 November 2012; published 3 January 2013)

EVALUATION OF Sr2YSbO6 AS A NEW SUBSTRATE FOR YBa2Cu3O7-δ SUPERCONDUCTOR THIN FILMS

A complex perovskite oxide Sr2YSbO6 has been synthesized by solid-state reaction. X-ray diffraction (XRD) pattern reveals that Sr2YSbO6 has an ordered complex cubic structure characteristic of A2BB...

Ferromagnetism in Mn half-doped LaCrO3 perovskite

In this work, we present results on the structural, electrical, and magnetic properties of pure and half-doped LaCrO3 compounds. It is observed that including Mn the lattice structure changes from orthorhombic to rhombohedral. The electrical transport in LaCrO3 can be described by a band gap model, whereas in La2CrMnO6, a charge localization occurs and the electrical transport is described by a variable range hopping model. The magnetic measurements show that LaCrO3 is a G-type antiferromagnetic with a Neel temperature at 290 K, and the Mn doping favors the ferromagnetic order and reduces the transition temperature.

Distinct magnetic signatures of fractional vortex configurations in multiband superconductors

Vortices carrying fractions of a flux quantum are predicted to exist in multiband superconductors, where vortex core can split between multiple band-specific components of the superconducting condensate. Using the two-component Ginzburg-Landau model, we examine such vortex configurations in a two-band superconducting slab in parallel magnetic field. The fractional vortices appear due to the band-selective vortex penetration caused by different thresholds for vortex entry within each band-condensate, and stabilize near the edges of the sample. We show that the resulting fractional vortex configurations leave distinct fingerprints in the static measurements of the magnetization, as well as in ac dynamic measurements of the magnetic susceptibility, both of which can be readily used for the detection of these fascinating vortex states in several existing multiband superconductors.

NUCLEATION OF SUPERCONDUCTIVITY IN A THIN DISK WITH A RING-LIKE DEFECT

We report on the influence of a circular defect on the vortex configuration in a mesoscopic superconducting sample. Effects associated with the pinning force of the circular defect on the configuration and on the vortex entry fields are studied for a very thin disk. We calculate the magnetization loop, vorticity, free energy and superconducting electrons for the disk in presence of an external magnetic field applied perpendicular to the disk plane. The magnetization curves are hysteretic, with paramagnetic response in part of the downward branch, also, in this part we found a vortex–anti-vortex state.

Vortex configurations and metastability in mesoscopic superconductors

Abstract The vortex dynamics in mesoscopic superconducting cylinders with rectangular cross section under an axially applied magnetic field is investigated in the multivortex London regime. The rectangles considered range from a square up to an infinite slab. The flux distribution and total flux carried by a vortex placed in an arbitrary position of the sample is calculated analytically by assuming Clems solution for the vortex core. The Bean–Livingston energy barrier is also analytically calculated in this framework. A Langevin algorithm simulates the flux penetration and dynamical evolution of the vortices as the external field is slowly cycled. The simulated magnetization process is governed by metastable states. The magnetization curves are hysteretic, with paramagnetic response in part of the downward branch, and present a series of peaks corresponding to the entry or expulsion of a single vortex. For elongated rectangles, the vortices arrange themselves into parallel vortex chains and an additional modulation of the magnetization, corresponding to creation or destruction of a vortex chain, comes out.

NORMAL STATE MAGNETISM OF Zn-DOPED AND OXYGEN DEFICIENT CaLaBaCu3O7 SUPERCONDUCTOR

Both on-site Zn substitution and oxygen deficiencies in the CaLaBaCu3O7 system decrease the superconducting transition temperature Tc of the pristine sample. We observed that the normal state magnetism, measured in a field of 5 KOe, shows a Curie-Weiss behavior, in terms of a localized magnetic moment, presumably on Cu sites, in both cases. The pristine system, i.e. without Zn substitution or oxygen deficiencies, shows a weakly temperature dependent small magnetic susceptibility, in the normal state, i.e. above Tc. It has been argued that the normal state magnetism of HTSC systems plays a vital role in determining the Tc of these materials.

Atomically flat superconducting nanofilms: multiband properties and mean-field theory

Recent progress in materials synthesis enabled fabrication of superconducting atomically flat single-crystalline metallic nanofilms with thicknesses down to a few monolayers. Interest in such nano-thin systems is attracted by the dimensional 3D–2D crossover in their coherent properties which occurs with decreasing the film thickness. The first fundamental aspect of this crossover is dictated by the Mermin–Wagner–Hohenberg theorem and concerns frustration of the long-range order due to superconductive fluctuations and the possibility to track its impact with an unprecedented level of control. The second important aspect is related to the Fabri–Perot modes of the electronic motion strongly bound in the direction perpendicular to the nanofilm. The formation of such modes results in a pronounced multiband structure that changes with the nanofilm thickness and affects both the mean-field behavior and superconductive fluctuations. Though the subject is very rich in physics, it is scarcely investigated to date. The main obstacle is that there are no manageable models to study a complex magnetic response in this case. Full microscopic consideration is rather time consuming, if practicable at all, while the standard Ginzburg–Landau theory is not applicable. In the present work we review the main achievements in the subject to date, and construct and justify an efficient multiband mean-field formalism which allows for numerical and even analytical treatment of nano-thin superconductors in applied magnetic fields.

Superconductivity and magnetism in the KxMoO2−δ

Coexistence between superconductivity and magnetism is reported for the KxMoO2−δ samples. Photoemission experiments show that the presence of Mo3+ ions is responsible for the weak ferromagnetic ordering observed in the KxMoO2−δ samples. Magnetic ordering temperature and superconducting critical temperature (TC) ratio range from 7 to 18 in this compound. These are the highest ratios reported so far for a magnetic superconductor. TC decreases with increasing potassium composition (x). For the first time, TC near 10 K is reported in the K-Mo-O system.

Irreversible matching effects in homogeneous and layered superconducting films

Abstract Numerical simulation is used to study the vortex dynamics in thin superconducting homogeneous films and multilayers. The films are assumed to have thickness less than or equal to the penetration depth and subjected to a parallel magnetic field. The structure of the multilayers is treated as a periodic pinning potential. We use Langevin dynamics and London theory to calculate the vortex configurations when the external field is increased from zero to a maximum value and turned back to zero. The results show that the vortex lattice consists of parallel linear chains of vortices and goes through transitions from n to n +1 chains in particular values of the increasing field. The strong surface effects yield an irreversible magnetization curve even for the homogeneous films. Moreover, the pinning structure of the multilayers presents only small influence on the vortex lattice.