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We review recent measurements of upper (Hc2) and lower (Hc1) critical fields in clean single crystals of MgB2, and their anisotropies between the two principal crystallographic directions. Such crystals are far into the "clean limit" of Type II superconductivity, and indeed for fields applied in the c-direction, the Ginzburg-Landau parameter k is only about 3, just large enough for Type II behaviour. Because m0Hc2 is so low, about 3 T for fields in the c-direction, MgB2 has to be modified for it to become useful for high-current applications. It should be possible to increase Hc2 by the introduction of strong electron scattering (but because of the electronic structure and the double gap that results, the scatterers will have to be chosen carefully). In addition, pinning defects on a scale of a few nm will have to be engineered in order to enhance the critical current density at high fields., Comment: BOROMAG Conference Invited paper. To appear in Supercond. Sci. Tech

We discuss pinning properties of MgB2 thin films grown by pulsed-laser deposition (PLD) and by electron-beam (EB) evaporation. Two mechanisms are identified that contribute most effectively to the pinning of vortices in randomly oriented films. The EB process produces low defected crystallites with small grain size providing enhanced pinning at grain boundaries without degradation of Tc. The PLD process produces films with structural disorder on a scale less that the coherence length that further improves pinning, but also depresses Tc.

Measurements of the magnetic field distribution (magnetic images) above flat superconducting samples provide information on the pattern of current flow within the material. While several deconvolution procedures to calculate the current flow have been proposed, we show here that they do not always give accurate results in situations where the current flow extends to regions that are outside of the magnetic image window, as with transport currents in a conductor. We describe two new procedures which overcome the problem. The most accurate (and most direct) approach is to incorporate magnetic images at different heights above the sample. The second method relates to the more conventional case using only one field image at a finite height above a thin sample (but of finite thickness). The results compare favourably to existing approaches.

Second generation Y1Ba2Cu3O7-δ (YBCO) coated conductors promise greater performance than first generation powder-in-tube technology. Along with the development of these materials, advances in characterization techniques have enabled detailed studies of current flow, grain boundaries and homogeneity. We have developed a scanning probe system which can be configured to map either the magnetic field or electric potential at the sample surface (with a local Hall sensor or voltage probe respectively). While the magnetic image can be used to determine the current flow pattern, the potential map allows direct measurement of the local dissipation within the samples.

Measurements of the temperature dependence of the surface resistance at 3 GHz of 100 micron size grains of MgB_2 separated powder are presented and discussed. The microwave surface resistance data is compared to experimental results of Nb, Bi_2Sr_2CaCu_2O_{8+\delta} (BSCCO) and theoretical predictions of s-wave weak coupling electron-phonon theory (BCS)., Comment: 4 pages, 2 figures; accepted in SUST

Power applications of high temperature superconductors (HTS) are emerging rapidly towards the market place. While first generation Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10/ (BSCCO) powder in tube (PIT) conductors are already used in the industrial sector (magnet inserts, transformers and power transmission cables), second generation Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) coated conductors are being developed which promise even greater performance, particularly for applications which require high magnetic fields. Along with the development of these materials, advances in characterization techniques have enabled detailed studies of the associated loss mechanisms. In particular, magnetic imaging techniques (both scanning Hall probe and magneto optics (MO)) have been able to establish the pattern of current flow within the samples, allowing study of grain boundary efficiency, current homogeneity and filament interconnection in multifilamentary PIT tapes. However, as the technology moves towards high field applications, high-field magnetic imaging is required. Presently MO is inherently limited to low fields (

A new approach to the fabrication process of bulk HTSC materials is proposed, using the initial mixture of components (YBa 2 Cu 3 O 7-x and non-superconducting inclusions) in the form of nanocrystalline powders. The preparation of ultra-fine powders is by means of the pulse-plasma channel method, which allows metallic, oxide and alloy materials etc (including HTSC) to be dispersed into spherical particles of 20-200 nm size. Bulk specimens of YBa 2 Cu 3 O 7-x (d ∼ 250 nm) + 2% ZrO 2 (d ∼ 70 nm) were produced by pulling the initial mixed powders through a gradient furnace. The results of structural, magnetic, and magneto-optical measurements are reported for the powders and the bulk samples. The critical current density, j c , of the bulk composites range from 10 4 to 2 x 10 4 A cm -2 at 77 K in zero magnetic field. The magnetic measurements and magneto-optical observations reveal that j c is independent of direction in the composite and corresponds to the transport current through the whole sample. The mean distance between pinning sites coincides with the mean grain size of YBa 2 Cu 3 O 7-x , which confirms homogeneous distribution of the inclusion particles.

The new capabilities of AC calorimetry, when working at frequencies above the classical limit, were shown. The idea of the method was to use the information about the phases and the amplitudes of the temperature oscillations on both sides of a plate-like sample for simultaneous determination of the sample’s complex heat capacity Cs(ω) and thermal conductivity κs(ω), when the oscillating heat flow was supplied to one face of the sample. The mathematical algorithm for simultaneous determination of real and imaginary parts of the sample’s heat capacity and thermal conductivity was developed. The advanced AC technique was applied for polymers. As a first step, the approximation of the frequency independent thermal conductivity was used to determine the complex heat capacity Cs(ω) of polymers in the melting region. It was found that the processes of reversing (more definitely heat-reversing) melting and completely irreversing melting can be distinguished by the complex heat capacity measurements. Thus, the different physical processes at melting can be distinguished by the complex heat capacity measurements.

A solution is obtained for the Bean critical-state model in an inclined magnetic field which leads to localization of the irreversible magnetic moment induced by the shielding currents. An experimental investigation of YBa2Cu3Oy single crystals and epitaxial films, and also plates of classical superconductors using vector magnetometry yields results fully consistent with the model calculations. The established geometric effects have an important influence on measurements of the angular dependence of the critical current and its anisotropy in a high-temperature superconductor.

The method of Temperature Waves Transmission Spectroscopy is developed on the basis of the classical AC calorimetry technique. The idea of the method is to use the information about the phase and amplitude of the temperature wave transmitted through a plate-like sample for simultaneous determination of the sample's heat capacity and thermal conductivity. For the correct application of the method in an experimental set up, it was necessary to study the problem of plane temperature wave transmission through a plate-like multilayered system. This problem was solved analytically for a wide range of experimental conditions. It was shown that the AC calorimeter can be used at relatively high frequencies, when the temperature oscillations in the sample are not quasi-static. Therefore, the width of the appropriate frequency range of classical AC calorimetry can be enlarged. This is of great importance for the applicability of classical AC calorimetry for low heat-conductive materials, such as polymers. It is shown that the dynamic heat capacity and thermal conductivity of polymers can be measured in real time and in a broad frequency region. The method was applied for polystyrene near glass transition and for 4.4′-n-octyloxycyanobiphenyl (8OCB) liquid crystal in melting region, smectic–nematic and nematic–isotropic transitions. The possibility of dynamic heat capacity measurements is demonstrated.

We report on vector magnetization studies of a La{sub 1.9}Sr{sub 0.1}CuO{sub 4} single crystal, cut in the form of a disk with the anisotropy (c) axis parallel to the disk plane. We observe a sharp threshold in the angular dependences of the isothermal remanent magnetization. When the magnetic field is applied within a threshold angle to the ab plane, the remanent moment is strictly parallel to ab, and also the c-axis component of magnetization is reversible. The threshold angle scales with temperature in the same manner as the lower critical field parallel to the c axis, H{sub c1}{sup c}. These observations give direct support to the presence of a vortex lock-in transition to the CuO planes in our crystal. {copyright} {ital 1997} {ital The American Physical Society}

We solve the Bean critical-state model for the screening current distribution in an infinite superconducting slab in a tilted magnetic field {bold H}, and calculate from it the magnitude and direction of the induced magnetic moment {bold m}. As the tilt angle increases, {bold m} remains directed close to the plate normal {bold n} until {bold H} is almost perpendicular to {bold n}, then {bold m} rotates very rapidly. We consider several generalizations of the model: finite length, a more realistic current-voltage characteristic, and critical-current anisotropy, but find that they have only a minor quantitative effect on the results derived for the simplest case. Also, we prove that the closure currents always contribute half the moment. Vector magnetic moment measurements of YBa{sub 2}Cu{sub 3}O{sub y} single crystals and an epitaxial film, and also on samples of conventional superconductors, confirm the model calculations. These geometrical effects are important for the analysis of the angular behavior of the critical currents and their anisotropy in HTS materials; also, they assist the observation of vortex locking to twin boundaries in HTS crystals, but tend to obscure vortex locking to CuO planes. {copyright} {ital 1997} {ital The American Physical Society}

The magnetic behavior of the non-superconducting Sr overdoped La 2− x Sr x CuO 4 x > 0.25 system was studied by Mossbauer spectroscopy (MS) and ESR studies on 0.3% 57 Fe doped materials. Both experimental techniques indicate clearly that in the metallic state, the CuO 2 planes are paramagnetic. The MS spectra are a superposition of two sub-spectra corresponding to inequivalent Fe sites. At 4.1 K, the complicated spectra are interpreted as due to local paramagnetic fluctuations of dilutte e, with slow relaxation rates. The ESR spectra are composed of two lines. The centroid of the most intense resonance line is independent of temperature and corresponds to well isolated paramagnetic Fe ions. Thus competition between SC and AFM exists only in the region of low x .

Magnetic hysteresis loops of LaSrCuO superconducting single crystals were studied by means of a vibrating sample magnetometer at T = 4.2 K. It was directly confirmed that two types of secondary maxima are present in the magnetization curves. By means of vector magnetization measurements we proved that low-field secondary maxima in tilted magnetic field are caused by the flux line lattice rotation in the process of the sample magnetization. The high-field magnetization maximum in tilted field, corresponding to an anomalous increase of the critical current (‘fishtail’) is strongly correlated with the alignment of the magnetization vector parallel to the c axis. It is discussed that fishtails originate due to stretched pinning sites in an anisotropic superconductor.

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.

We have performed a comparative study of two thin films of magnesium diboride (MgB2) grown by different techniques. The critical current density at different temperatures and magnetic fields was evaluated from magnetisation curves, the structure of superconducting order parameter was obtained from point-contact spectroscopy, and the scattering rates were evaluated by fitting the temperature dependent normal-state resistivity to the two-band model. The films have similar critical temperatures close to 39 K, but the upper critical fields were different by a factor of 2 (5.2T and 2.5 T at 20 K). We have found that the film with higher Hc2 also had stronger scattering in the sigma band and smaller value of the superconducting gap in this band. As the scattering in sigma band is primarily due to the defects in boron plane, our results are consistent with the assumption that disordering the boron planes leads to enhanced Hc2 and better pinning properties in magnetic field., Paper presented at EUCAS'03

We present magnetisation data on crystalline and polycrystalline MgB2 before and after irradiation with ~1MeV protons. In the virgin crystal the critical current density is below our noise floor of 103A/cm2 over most of the field range. However, after irradiation a peak occurs in the current density as a function of applied magnetic field as the upper critical field Hc2 is approached. After subsequent ageing over a time period of three months, the peak effect is greatly enhanced, exhibiting much stronger pinning over a wide field range, and the upper critical field is approximately doubled, accompanied by a 2K reduction in transition temperature. Similar studies were made on polycrystalline fragments, where irradiation leads to an increased irreversibility field (Jc is enhanced at high fields but decreased at low fields) and a suppression in transition temperature. However, after two years of ageing both parameters returned towards those of the virgin sample., Comment: 18 pages 4 figs

The experimental study of La/sub 2-x/Sr/sub x/CuO/sub 4-/spl delta// superconducting single crystals is reported. The magnetization measurements were carried out at T=4.2 K in magnetic fields tilted by an arbitrary angle /spl theta/ to the crystal c axis. The irreversibility field H/sub irr/, defined by extrapolation of pinning force curves, shows a strong dependence on /spl theta/. At 30/spl deg/ >

Bulk samples of MgB2 were prepared with 5, 10, and 15% wt.% Y2O3 nanoparticles added using a simple solid-state reaction route. Transmission electron microscopy (TEM) showed a fine nanostructure consisting of ~3-5 nm YB4 nanoparticles embedded within MgB2 grains of ~400 nm size. Compared to an undoped control sample, an improvement in the in-field critical current density Jc was observed, most notably for 10% doping. At 4.2K, the lower bound Jc value was ~2x105A.cm-2 at 2T. At 20K, the corresponding value was ~8x104A.cm-2. Irreversibility fields were ~11.5 T at 4.2K and 5.5T at 20K, compared to ~4T and ~8T, respectively, for high-pressure synthesized bulk samples.

Using a transverse vibrating sample magnetometer, we have made vector magnetisation measurements on a disk-shaped single crystal La1.9Sr0.1CuO4. Reversible behaviour of the c-axis component of magnetisation, and the angular dependences of the remanent moment are evidence of the vortex lock-in state. The lock-in angle dependence on temperature and applied magnetic field are in agreement with theory.

Using VSM with two perpendicular sense coils, we studied the vector magnetic moment of a high purity YBa 2 Cu 3 O y single crystal near the ab-plane. The results support the vortex lock-in to the CuO planes.

We present magnetic flux creep measurements on a twinned YBCO crystal taken using a Vibrating Sample Magnetometer, in which the sample orientation may be varied with respect to the applied field. The measured flux creep rate is sometimes of negative sign, opposite to that which is usually observed. The data are discussed in terms of the inferred electrodynamics of the sample.

Using VSM with two perpendicular pick-up coil sets we have studied the vector magnetic moment of YBa 2 Cu 3 O y single crystals with columnar defects. Completely different behavior is found for components of the magnetic moment parallel and perpendicular to the columns. Analysis shows different regions dominated by locking, trapping and matching of vortices.

Using a transverse field vibrating sample magnetometer in which the sample may be rotated with respect to the applied field, we have made magnetisation measurements of twinned YBCO crystals over a wide range of angle. We examine the influence of the vortex to twin boundary interaction on the vortex dynamics.

Superconducting single crystals of La 2- x Sr x CuO 4-δ ( x = 0.15 and 0.20) were studied by means of the vector magnetization technique. The effective mass anisotropy factors are estimated to be Γ = 100 and Γ = 2, respectively, for the two samples. A procedure is proposed to obtain the thermodynamic orientation of the flux line lattice from the irreversible magnetization. Direct evidence is reported of vortex penetration along the easy plane. The data agree with numerical results for a three-dimensional anisotropic superconductor.

The dependence of the remanent moment ( M r ) anisotropy on the structure and shape of superconductor and on the magnetizing procedure was studied. It was found that the main contribution to the anisotropy is due to the surface screening currents. The effective demagnetization factors for decoupled ceramic samples are calculated. The influence of inhomogenious grain magnetization and global bulk currents on the shape of the angular dependences of M r curves is investigated.