Magnetic Flux Penetration and Motion in Antiferromagnetic Superconductors

The paper reviews a concept of induced spin-flop domain inside vortices in an antiferromagnetic superconductor. Such phenomenon may occur when an external magnetic field is strong enough to flip over magnetic moments in the core of the vortex from their ground state configuration. The formation of the domain structure inside vortices modifies the surface energy barrier of the superconductor. During this process the entrance of the flux is stopped and a newly created state exhibits perfect shielding. Such behavior should be visible as a plateau on the dependence of flux density as a function of the external magnetic field. The end of the plateau determines the critical field, which has been called the second critical field for flux penetration. Moreover, it is predicted and described how this phenomenon modifies flux creep in layered superconductors. The various scenarios of changing the creep regime from thermal to quantum and vice versa at constant temperature are discussed.     

Flux Dynamics in Y358 and Gd358 Superconductors

We have studied the thermally activated flux creep in the newly fabricated superconductors Y₃Ba₅Cu₈O_18+δ and Gd₃Ba₅Cu₈O_18+δ with transition temperature above 100 K. The thermally activated flux creep model explains the electron share in the magnetoresistivity of the samples in the vicinity of transition temperature T _c . The activation energy U in a magnetic field below 15 kOe has been calculated by the corrected thermally activated flux creep model; and the pinning energy is exponentially related to the magnetic field.     

Relaxation Properties of the Trapped Flux of Bulk High-Temperature Superconductors at Different Magnetization Levels

The magnetic relaxation phenomenon is a crucial subject for the engineering applications of bulk high-temperature superconductors (HTS) in which the trapped field or levitation force behaves with a time-dependent decay due to the intrinsic flux creep inside the HTS materials. To fully exploit the high trapped field of bulk HTS, we have experimentally investigated the trapped flux relaxation properties, especially at different magnetization levels. With different excitation fields, the dependence between trapped field and relaxation rate was analyzed and compared in both field-cooling magnetization (FCM) and zero-field-cooling magnetization (ZFCM) conditions. In parallel, the relaxation rates for different trapped flux can be measured all at once during a single magnetization process. The relaxation rate is closely correlated to the trapped field and useful to reflect the effect of the following remagnetization processes. To suppress the relaxation, we further checked possible methods from the three aspects of material improvement, working temperature and flux annealing effect.     

Magnetic Susceptibility in Normal States of Quasi-One-Dimensional Superconductors

The longitudinal magnetoresistance of a two-dimensional superconductor, β’-Et₂Me₂P[Pd(dmit)₂]₂ (dmit=C₃S ₅ ²⁻ ) under hydrostatic pressure was measured at low temperatures with the field applied perpendicular to the conducting layers. At 0.58 GPa, the field-dependent isothermal interlayer resistanceR╧ (H) exhibited a peak below the superconducting transition temperature T _C . This peak effect can be explained by a model of resistively-shunted Josephson-Junctions. The peak is strongly suppressed at a higher pressure, 0.71 GPa.     

Spontaneous Generation of Magnetic Flux in Superconductors: Testing the Model

The Kibble-Zurek scenario of the generation of topological defects, applied to superconductors, predicts the spontaneous formation of magnetic flux during a rapid quench through T^c. The predicted amount of net flux scales as the 1/8 power of the cooling rate, and as the square root of the circumference of the sample. Our experimental data is broadly consistent with the first prediction, while the second one is yet to be tested. We describe a proposed experiment to test this prediction. In addition, we describe additional observations of spontaneous flux generated through a different mechanism. PACS numbers: 05.70.Ln, 74.40.+k, 11.27.+d.     

Local Magnetic Measurements of Trapped Flux Through a Permanent Current Path in Graphite

Temperature- and field-dependent measurements of the electrical resistance of different natural graphite samples suggest the existence of superconductivity at room temperature in some regions of the samples. To verify whether dissipationless electrical currents are responsible for the trapped magnetic flux inferred from electrical resistance measurements, we localized them using magnetic force microscopy on a natural graphite sample in remanent state after applying a magnetic field. The obtained evidence indicates that at room temperature a permanent current flows at the border of the trapped flux region. The current path vanishes at the same transition temperature \(T_c\approx 370\) K as the one obtained from electrical resistance measurements on the same sample. This sudden decrease in the phase is different from what is expected for a ferromagnetic material. Time-dependent measurements of the signal show the typical behavior of flux creep of a permanent current flowing in a superconductor. The overall results support the existence of room-temperature superconductivity at certain regions in the graphite structure and indicate that magnetic force microscopy is suitable to localize them. Magnetic coupling is excluded as origin of the observed phase signal.     

Trapped Flux Dependence of Bulk High-Temperature Superconductors Between 77 and 30 K under a Limited Excitation Field

The trapped-flux dependence of bulk high-temperature superconductors between two typical working temperatures of 77 and 30 K was investigated by a field-cooling magnetization (FCM) method under a limited excitation field of 5 T to approach actual application conditions. An exponential trapped-flux dependence between the two temperatures was obtained by fitting the experimental data. Its effectiveness was checked from the theoretical temperature relationship of the critical current density and trapped-field data in the literature. It was found that the achievable trapped flux at 30 K is closely dependent on the excitation field. The bulk samples with moderate performance around the trapped-field range of 1.0–1.5 T at 77 K can be efficiently activated to reach a trapped field over 4.6 T at 30 K. Bulk samples with better performance at 77 K will get saturation and cannot be fully excited by the 5 T excitation field. The presented results are useful for the appropriate estimation of trapped-flux performance from 77 to 30 K and may support some basic references for the applications of bulk HTS magnets.     

Effect of Magnetic Flux Distribution and Magnetic Powder Addition on the Magnetic Levitation Force of Sm123 Superconductors

Sm123 and Sm211 were prepared by melt-powder-melt-growth and solid-state-reaction techniques, respectively, to have the nominal composition of (Sm123)_0.75(Sm211)_0.25. After the preparation of this composition, the Fe-B magnetic powder (MP) was added to the composition for 0.000, 0.010, 0.015, 0.025, 0.050 wt.% to investigate the effect of MP addition on the levitation force density (LFD). Additionally, different permanent magnets (PM) were used as magnetic-field source to investigate the effect of magnetic-field gradient on the LFD of MP added superconductors. Our findings indicate that the MP addition acts as a flux pinning centre in the sample and enhances the LFD up to 0.025 wt.% adding amount, and the optimum flux gradient which produces a screening current in the sample occurs when the B/d (magnetic-field intensity/ diameter of PM) ratio is equal to 0.060. These results imply that the experimental data can be useful for fabricating process of superconducting samples with larger MLF values, and designing of superconducting magnets, flywheel energy storage and maglev systems.     

Electron-optical Observation of Magnetic Flux Quanta in Superconductors by Decoration and Vortex Microscopy

The stray field modulation of magnetic flux in superconductors can be converted into an optical or electron optical contrast by various decoration techniques, which are reviewed. Thin films decorated in this way are analysed in a transmission, scanning, or scanning transmission electron microscope and permit one to study the configuration of the magnetic flux relative to the crystalline structure. Results of electron interferometer experiments on the flux quantization in small superconducting hollow cylinders, on the motion of single flux quanta, and on their pinning forces at grain boundaries, will be surveyed.     

Vortex Dynamics at the Initial Stage of Resistive Transition in Superconductors with Fractal Cluster Structure

No Heading The effect of fractal normal-phase clusters on vortex dynamics in a percolative superconductor is considered. The superconductor contains percolative superconducting cluster carrying a transport current and clusters of a normal phase, acting as pinning centers. A prototype of such a structure is YBCO film, containing clusters of columnar defects, as well as the BSCCO/Ag sheathed tape, which is of practical interest for wire fabrication. Transition of the superconductor into a resistive state corresponds to the percolation transition from a pinned vortex state to a resistive state when the vortices are free to move. The dependencies of the free vortex density on the fractal dimension of the cluster boundary as well as the resistance on the transport current are obtained. It is revealed that a mixed state of the vortex glass type is realized in the superconducting system involved. The current-voltage characteristics of superconductors containing fractal clusters are obtained and their features are studied.PACS numbers: 74.81.–g, 74.25.Fy     

Direct Observation and Interpretation of Carrier Dynamics of Molecular Magnetic Superconductors

The temperature-dependent time-resolved reflectance spectra of the organic superconductors (OSs) κ- and λ-(BETS)₂MCl₄ (M=Fe, Ga) and κ-(ET)₂Cu[N(CN)₂]Br are reported. The π electrons of the organic molecules (BETS or ET) are responsible for the electrical conduction in these salts, while only the Fe-containing salts have localized spins on the d electrons in the Fe atoms (S=5/2) with π–d interaction. Analysis of the spectra provided the relaxation time τ of the carriers, as well as information concerning the number of carriers at the Fermi energy N(E _F). The carrier dynamics were classified in terms of high-T _C and low-T _C OSs. In these salts, the results indicate that (1) the electron-phonon interaction is important in the occurrence and stability of the superconducting phase, and (2) electron correlation can be related to the enhancement of T _C.     

Creep of Driven Flux Lines in Type-II Superconductors

The creep motions of flux lines in both Bragg glass (BrG) and vortex glass (VG) are studied. The power-law dependence of the creep activation barrier on the driving force is obtained. Two universal classes of creep motion are found with the exponent μ=0.5±0.02 for the BrG and μ=0.28±0.02 for the VG. The former is in good agreement with the prediction by the scaling theory and the functional-renormalization-group theory on creep, while the latter is a new estimate. The different dynamics of flux lines in the VG and in the BrG are investigated accordingly.     

Retardation of the Magnetic Relaxation in High-Temperature Superconductors Near a Ferromagnet

The creep of a magnetic flux trapped in a bulk high-temperature superconductor has been studied. It has been found that the magnetic relaxation is retarded when the superconductor is placed near a ferromagnet. The value of the retardation effect depends on the sequence of magnetization and the approach of the superconductor to a ferromagnet. The magnetic relaxation is fully suppressed when a superconducting sample first is magnetized and then is brought close to a ferromagnet. An interpretation of this effect has been discussed. Being magnetized, a ferromagnet produces its own magnetic field. While penetrating into a disk sample through its planes, the ferromagnet field induces screening currents, which circulate oppositely to the current that arises upon trapping of the magnetic flux. As a result, the stability of the magnetic structure is sharply improved since opposite driving forces can act on different sections of the vortices.     

Phase Fluctuations of s-Wave Superconductors on a Lattice

Based on an attractive U Hubbard model on a lattice with up to second neighbor hopping we derive an effective Hamiltonian for phase fluctuations. The superconducting gap is assumed to have s-wave symmetry. The effective Hamiltonian we finally arrive at is of the extended XY type. While it correctly reduces to a simple XY in the continuum limit, in the general case, it contains higher neighbor interaction in spin space. An important feature of our Hamiltonian is that it gives a much larger fluctuation region between the Berezinskii–Kosterlitz–Thouless transition temperature identified with T _c for superconducting and the mean field transition temperature identified with the pseudogap temperature.     

Static Magnetic Flux Dynamics in One-Dimensional (1D) Josephson Lattices

We investigated static magnetic flux dynamical properties of one-dimensional lattices of Josephson junctions. The discretized wave equations of the Josephson junction lattice were solved using a generalized relaxation iteration algorithm. Numerical simulations indicated that transitions between periodic state and chaotic state will occur as the physical parameters and geometric parameters such as external current y _n, magnetic field h ₀, h, and the length of Josephson junction _n and d _n , varied. A shot length of the Josephson junction favors stable periodic states.     

Computer Simulation of Flux Pinning in the High-Temperature Superconductors Bi-2212

The magnitude of critical current density J _c (or critical magnetic induction B _c), is closely related to the flux pinning property of the high-temperature superconductors (HTSC) materials. So, the investigation on flux pinning mechanism is an important research field in the high-temperature superconductors. There are different pinning centers by analyzing the flux pinning force of the HTSC. In this study, based on Sun and Zou et al.’s previous works, we studied the flux pinning mechanism in Bi-2212 single crystals with an improved scaling function of pinning density, which is simplified and possesses determined physical meaning. Then, a computer simulation was conducted to determine the category of flux pinning in Bi-2212 according to Sun’s experimental data. It is revealed that the simulation result is consistent with the experiment.     

The Effect of the Phase Separation in the Phase Diagram of Cuprate Superconductors

We show that the main features of the cuprates superconductors phase diagram can be derived considering the disorder as a key property of these materials. Our basic point is that the high pseudogap line is an onset of phase separation which generates compounds made up of regions with distinct doping levels. We calculate how this continuous temperature dependent phase separation process occurs in high critical temperature superconductors (HTSC) using the Cahn–Hilliard approach. Calculations with the BdG approach and an analysis of the local density of states (LDOS), yield good agreement with the measured values of T ^* and T _c . This work has been partially supported by CAPES, CNPq and CNPq-Faperj Pronex E-26/171.168/2003.     

Fluctuations of the Order Parameter's Phase in Layered Superconductors

The influence of the quantum fluctuations of the order parameter's phase on the critical temperature T _c is studied for a Josephson coupled layered superconductor. Two characteristic critical temperatures exist for a system, namely the superconducting critical temperature T ⁽²⁾ _c for a single layer estimated by the mean-field theory and the transition temperature for the outset or the superconducting phase coherence T* _c . The true critical temperature T _c is shown to vary inside the intervals T* _c ≤ T _c ≤ T ⁽²⁾ _c . For a strong quantum phase fluctuation limit, the superconducting layers become decoupled.