Local magnetometry using miniature Hall-probe array was used to study vortex distribution in superconducting single crystal of Cu_{x}TiSe_{2}, with x=0.064 and T_{c}=3.2 K. We show that vortices after penetration into the sample move towards the center, resulting into a dome-shape field profile. Such a profile is a signature of relatively low pinning. We show that these measured profiles are consistent with a model proposed for the samples in the absence of bulk pinning. Modifications necessary to obtain quantitative agreement between the model and the data are presented.
In the paper there is given an analysis of the critical current problems in irradiated multilayered high temperature oxide superconductors. Critical current for the direction of current flow inside the layers is investigated basing on analysis of the capturing interaction of pancake vortices with nanodefects, created by fast neutrons irradiation. Analysis of the pinning potential barrier formation is presented. From performed calculations of current-voltage characteristics fitted to measurements the inherent pinning centers concentration and their average dimensions were estimated. The case of perpendicular to layers current has been regarded too, taking into account the intrinsic Josephson junctions formation. The influence of nanodefects concentration on the Josephson penetration depth has been considered, whose length determines the Swihart velocity and current-voltage characteristics of junction.
Electron-phonon coupling is one of the most common interaction in superconducting materials ranging from Nb₃Sn, MgB₂, iron-based superconductors and high temperature cuprates such as YBa₂Cu₃O_{7-δ} and HgBa₂Ca₂Cu₃O_{8+δ}. However an importance of the electron-phonon coupling constant, λ_{ep}, should not be underestimated for characterisation of the electronic properties of superconducting materials, but it is important that the enhanced flux pinning mechanism can be hold responsible for the applicability of the A15 superconducting materials in emerging hydrogen cryomagnetic technology where temperature of the liquid H₂ can be as low as 14 K.
In the paper we report experimental results of AC magnetization losses in a multi-filamentary BSCCO-2223 superconducting composite tape due to coaxial AC and DC magnetic fields subjected perpendicularly to the plane of the tape. Such superposition of magnetic fields usually leads to some reduction of magnetization losses. A distinguished minimum in the AC loss is observed, at a certain DC bias magnetic field and at a certain fixed magnetic field amplitude. It is shown that the minimum of the AC losses very strongly depends on magnetic history of the investigated tape, which is directly related to the magnetic flux trapping within tape's superconducting filaments region. Measurements were carried out with a sinusoidally varying magnetic fields at amplitudes up to 100 mT and a superimposed DC magnetic field up to 50 mT, at frequency range of 21-113 Hz.
The penetration depths of bulk Tl_2Ba_2Ca_2Cu_3O_y and Tl_{0.58}Pb_{0.4}Sr_{1.6}Ba_{0.4}Ca_2Cu_3O_y superconductors with the critical temperatures 112 K and 114 K, respectively, were determined from the AC susceptibility measurements. When the samples are in the Meissner state, the dispersive components of AC susceptibility as well as their temperature dependences reflect the changes of the penetration depths at various temperatures. In these bulk ceramic superconductors the penetration depths are of the order of few μm and they are comparable to the grains sizes in the ceramics.
A mixed state in dc-biased thin films of II-type superconductors realizes the Abrikosov magnetic vortices/antivortices, which are the result of the current-self magnetic field penetration into the film at temperatures lower than its critical temperature T_{c}. A nucleation of vortices/antivortices at the superconducting film's edges, their motion perpendicular to the direction of biasing current, and the annihilation in the film's center originates from a current dissipation in the superconductor and expresses itself in experiments as a dc voltage. This work reports on the results of simulation of current density in a 50 μm wide, 100 μm long, and 0.3 μm thick YBa_2Cu_3O_{7 - x} microbridges containing Π-shaped 5 μm wide single channel of easy vortex motion fabricated by means of laser-writing technique. Analyzing a two-dimensional-net of resistors and assuming that, due to the Meissner-Ochsenfeld effect, the magnetic flux penetration into superconducting film is nonlinear, we demonstrate that presence of a Π-shaped channel causes a non-homogeneous distribution of current in the microbridge.
A current-self-induced magnetic field H_{j}, such that H_{c1} < H_{j} < H_{c2} at T < T_{c}, penetrates a thin-film, type-II superconductor forming the Abrikosov magnetic vortex-antivortex pairs in the film's areas of weakest superconductivity. Our atomic force microscopy and scanning tunneling microscopy images confirm that in 50 μm wide, 100 μm long and 0.3 μm thick YBa_2Cu_3O_{7 - x} superconducting devices magnetic flux penetrates first into a 5 μm wide, Π-shaped and partially deoxygenated (x ≈ 0.2) channel for easy vortex motion. When the Lorentz force overcomes pinning force in the channel, the flux starts to move and its drift dissipates energy inducing dc voltage. This work reports on the density of coherently moving vortices along the channel vs. temperature in range from 0.93T_{c} to 0.97T_{c}. Our simulations show that the vortex density vs. temperature dependence extracted from I-V measurements of our devices follows the temperature dependence of magnetic field penetration depth and the coherence length of the superconductor.
A challenging aspect of the usage of patterned nanostructures relates to the development of superconducting devices operating with the Abrikosov vortices in some pinning potential. To provide such a potential we have used thin epitaxial films of Nb with washboard-like nanostructures in the form of grooves or Co stripes. The nanostructures were prepared by focused ion beam milling or focused electron beam induced deposition, respectively. The results of transport measurements affirm the existence of two fluxonic effects, the guided vortex motion and the vortex ratchet effect, both invoked by the nanostructuring. In particular, the effects represent the basis for the development of advanced fluxonic devices using a directional or orientational control of the net vortex motion in Nb films nanostructured by focused particle beam techniques.
New results of the investigations critical current problems in the high temperature oxide superconductors are presented, basing on the flux pinning analysis. Model of the pinning interaction of the nanosized centers with pancake vortices is proposed, while comparison of the received results concerning the dependence of the potential barrier versus transport current with other pinning forces approaches is presented. Special attention is devoted to the investigations of the influence the initial position of the vortex captured at pinning center on the current-voltage characteristics and critical current.
The magnetic field penetration depth into YBa₂Cu₃Oₓ film on polycrystalline Ag substrate with the critical temperature of 90.4 K was determined from the AC susceptibility measurements. The 95 μm thick YBCO film was deposited directly on Ag substrate by the sedimentation process. When the sample is in the Meissner state, the dispersive component of the AC susceptibility as well as its temperature dependence reflects the changes of the penetration depth with the temperature. The penetration depth of this film is found to be 5.4 μm.
The temperature widths of the resistive transition as well as the thermal fluctuations of bulk (Bi_{0.6}Pb_{0.4})_2Sr_2Ca_2Cu_{3}O_{x} superconductor with T_{c,50%} = 107.1 K were studied. The applied magnetic field widens the resistive transition according to the following formula: ΔT = CH^{m} + ΔT_{0}. The value of exponent m = 0.44 suggests that there is strong pining of vortices, especially at lower temperatures. The critical exponents λ of the conductivity were calculated using the following equation: Δσ = Kε^{-λ}, with the λ_1 = 1.75 close to the zero critical temperature and λ_2 = 3.77 at higher temperatures. These values were discussed within the available theoretical models.
In this work we numerically modelled a periodic magnetic flux pattern which qualitatively reproduces the so-called sand avalanches scenario in type-II superconductors. To model these sand-pile patterns we consider a perturbation on the critical current which, as a first approximation, follows a periodic function which depends on the position.
Using the magnetoresistance measurements we study the phase transition line and the activation energy for vortex pinning in superconductor/ferromagnet bilayer, built of a ferromagnetic Co/Pd multilayer with perpendicular magnetic anisotropy, and a niobium film, with insulating layer in-between to eliminate proximity effect. The domain width is reversibly pre-defined using the angle-dependent demagnetization. We find that the enhancement of the activation energy for vortex pinning by magnetic domains is rather modest, by a factor of about 2.1. We attribute this to large domain width, and large dispersion of the domain width in this bilayer.
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