Current transport in polycrystalline magnesium diboride is highly non-uniform (percolative) due to the presence of secondary phases and also due to the intrinsic anisotropy of the material. The influence of secondary phases on the transport properties of MgB2 was investigated. Bulk samples were prepared from a mixture of MgB2 and MgO powders by the ex-situ technique in order to vary the MgO content systematically. The samples were characterized by resistive and magnetization measurements. The reduced MgB2 fraction is modeled by a reduced effective cross section (connectivity), which was assessed directly by the experiments. The presence of MgO also increases the percolation threshold, which reduces the zero resistivity (or irreversibility) field.
We have investigated the microstructure, normal-state electrical connectivity, and critical current density of ex-situ MgB2 polycrystalline bulks prepared by systematically varying the sintering conditions under low pressure. Samples heated at a high temperature of ~900°C for a long period showed an increased packing factor, a larger intergrain contact area, and a significantly enhanced electrical connectivity, all of which indicate solid-state self-sintering of MgB2. Sintered ex-situ MgB2 bulks from a laboratory-made ball-milled powder exhibited a greatly enhanced connectivity of 28%, which i…
Using a recently developed scaling procedure that allows to establish the temperature dependence of the normalized upper critical field from measurements of the equilibrium isothermal magnetization, we analyze experimental data obtained on polycrystalline and ceramic samples. We show that the scaling procedure works quite well in all cases that we have considered. This provides a very strong evidence that the temperature dependencies of the normalized upper critical field are independent of the orientation of an applied magnetic field with respect to the crystallographic axes of these material…
The intrinsic properties of MgB$_2$ form the basis for all applications of this superconductor. We wish to emphasize that the application range of polycrystalline MgB$_2$ is limited by the upper critical field H$_{c2}$ and its anisotropy. In wires or tapes, the MgB$_2$ grains are randomly oriented or only slightly textured and the anisotropy of the upper critical field leads to different transport properties in different grains, if a magnetic field is applied and the current transport becomes percolative. The irreversibility line is caused by the disappearance of a continuous superconducting c…
The current transport in polycrystalline MgB$_2$ is strongly influenced by the intrinsic anisotropy of this superconductor. Untextured bulks and wires are macroscopically isotropic, but the grains retain their anisotropic properties and the field dependence of the critical currents is much stronger than in isotropic superconductors. Weakly or partially textured tapes are macroscopically anisotropic, but the anisotropy of the zero resistivity (or irreversibility) field is smaller than the intrinsic upper critical field anisotropy, $γ$. The \Jc-anisotropy is field and temperature dependent and c…
We investigate the in-field critical current density Jc(B) of SNS Josephson Junctions (JJs) and polycrystalline superconductors with grain boundaries modelled as Josephson type planar defects, both analytically and through computational time-dependent Ginzburg-Landau (TDGL) simulations in 2D and 3D. For very narrow SNS JJs, we derive analytic expressions for Jc(B) that to our knowledge are the first high field solutions for Jc(B) for JJs across the entire field range up to the effective upper critical field. They generalise the well-known (low-field) exponential junction thickness dependence f…
A series of polycrystalline SmO1-xFxFeAs bulks (x=0.15, 0.2, 0.3 and 0.4) were prepared by the conventional solid state reaction. Resistivity, susceptibility, magnetic hysteresis, critical current density and microstructure of these samples have been investigated. It is found that critical transition temperature Tc increases steadily with increasing fluorine content, with the highest onset Tc=53 K at x=0.4. On the other hand, the superconductivity seems correlated with lattice constants; that is, Tc rises with the shrinkage of a-axis while resistivity increases with the enlargement of c-axis. …
A series of polycrystalline SmFeAs1-xOx bulks was prepared to systematically investigate the influence of sample density on flux pinning properties. Different sample densities were achieved by controlling the pelletizing pressure. The superconducting volume fraction, the critical current densities Jcm and the flux pinning force densities Fp were estimated from the magnetization measurements. Experimental results manifest that: (1) the superconducting volume fraction decreases with the decreasing of sample density. (2) The Jcm values have the similar trend except for the sample with very high d…
Flux pinning scaling laws were developed to explain the observed increase in the critical current density Jc caused by increased density of grain boundaries in polycrystalline low temperature superconductors (LTS) such as Nb3Sn. For four decades they have provided the framework for the successful development of the LTS materials that carry high Jc in high magnetic fields. However, the discovery of the weak-link problem suggested that transmission of supercurrent flow through the grain boundaries limited Jc in high temperature superconductors (HTS) such as YBa2Cu3O7-δ (YBCO). Here we provide ev…
We report enhancement in the magnetic critical current density of indium added polycrystalline SmFeAsO1-xFx. The value of magnetic Jc is around 25 kA/cm2 at 4.2 K under self-magnetic field. Polycrystalline SmFeAsO1-xFx is mainly composed of the superconducting grains and a little of amorphous FeAs compounds. These areas randomly co-exist and amorphous areas are located between superconducting grains. Therefore, the superconducting current is prevented by the amorphous areas. In this study, it is found that indium addition to polycrystalline SmFeAsO1-xFx removes these amorphous areas and induce…
We report the results from resistivity and magnetic measurements on polycrystalline Ce oxypnictide (CeFeAsO1-xFx) samples where x spans from 0.13 to 0.25. We find that the orbital limiting field is as high as 150 T and it systematically decreases with increasing doping. The Maki parameter is greater than one across the phase diagram and the large Maki parameter suggests that orbital and Pauli limiting effects contribute to the upper critical field. The broadening of the superconducting transition in the resistivity data was interpreted using the thermally activated flux flow (TAFF) model where…
A novel method to prepare bulk Fe(Se0.5Te0.5) samples is presented, based on a melting process and a subsequent annealing treatment. With respect to the standard sintering technique, it produces much more homogeneous and denser samples, characterized by large and well interconnected grains. The resulting samples exhibit optimal critical temperature values, sharp resistive and magnetic transitions, large magnetic hysteresis loops and high upper critical fields are observed. Interestingly, the global critical current density is much enhanced as compared to the values reported in literature for b…
The influence of anisotropy on the transport current in MgB$_2$ polycrystalline bulk samples and wires is discussed. A model for the critical current density is proposed, which is based on anisotropic London theory, grain boundary pinning and percolation theory. The calculated currents agree convincingly with experimental data and the fit parameters, especially the anisotropy, obtained from percolation theory agree with experiment or theoretical predictions.
Polycrystalline NdFeAsO0.88F0.12 superconductors prepared by high pressure (HP) and ambient pressure (AP) method were comparatively studied by magnetization and transport measurements. Upper critical field (Hc2), irreversibility field (Hirr) and the anisotropy parameter were estimated from resistance transition curves. The broadening of transition width was observed, and ascribed to both Hc2 anisotropy and superconductivity inhomogeneity of samples. Magnetic hysteresis loops (MHLs) in low fields were measured to detect the trace of weak-link behavior. The reclosed hysteresis loops in low field…
Polycrystalline Sr0.6K0.4Fe2As2 samples with various Pb additions (0-20 wt%) were prepared using a one-step solid state reaction. X-ray diffraction analysis shows no evidence for chemical reaction between the Pb and the FeAs-based superconductor. However, the presence of the Pb can affect the microstructure and superconducting properties of the final products. The critical transition temperature Tc indicates no degradation up to 20 wt% Pb addition, and dramatic improvements of magnetic Jc and irreversibility field Hirr were observed for appropriate Pb concentration. Transport critical current …
The influence of Ca substitution on the superconducting properties of polycrystalline R1-xCaxBa2Cu3Oz (R = Eu, Gd, Er; x=0; 0.2; 0.25 and 0.3) samples has been studied by X-ray powder diffraction, ac susceptibility and dc magnetization measurements. The superconducting parameters such as critical temperature, inter- and intra-granular critical current and flux pinning are found to be strongly dependent both on Ca content and type of R element. The best combination of these parameters is found for the system Gd1-xCaxBa2Cu3Oz forming R1+yBa2-yCu3Oz clusters. The level of overdoping and the type …
The magnetic properties of LiFeAs, as single crystalline and polycrystalline samples, were investigated. The lower critical field deduced from the vortex penetration of two single crystals appears to be almost isotropic with a temperature dependence closer to that of two-gap superconductors. The parameters extracted from the reversible magnetizations of sintered polycrystalline samples are in good agreement with those from the single crystal data.
Continuous Nb wires, 7-15 nm in diameter, have been fabricated by sputter-coating single fluorinated carbon nanotubes. Transmission electron microscopy revealed that the wires are polycrystalline, having grain sizes of about 5 nm. The critical current of wires thicker than ~12 nm is very high (10^7 A/cm^2) and comparable to the expected depairing current. The resistance versus temperature curves measured down to 0.3 K are well described by the Langer-Ambegaokar-McCumber-Halperin (LAMH) theory of thermally activated phase slips. Quantum phase slips are suppressed.
A method is proposed for estimating the length scale of currents circulating in superconductors. The estimated circulation radius is used to determine the critical current density on the basis of magnetic measurements. The obtained formulas are applicable to samples with negligibly small demagnetizing factors and to polycrystalline superconductors. The proposed method has been verified using experimental magnetization loops measured for polycrystalline YBa$_2$Cu$_3$O$_{7-d}$ and Bi$_{1.8}$Pb$_{0.3}$Sr$_{1.9}$Ca$_2$Cu$_3$O$_x$ superconductors.
The AC susceptibility at zero DC magnetic field of a polycrystalline sample of LaFeAsO_{0.94}F_{0.06} (T_c = 24 K) has been investigated as a function of the temperature, the amplitude of the AC magnetic field (in the range Hac = 0.003 - 4 Oe) and the frequency (in the range f = 10 kHz - 100 kHz). The temperature dependence of the AC susceptibility exhibits the typical two-step transition arising from the combined response of superconduncting grains and intergranular weak-coupled medium. The intergranular part of the susceptibility strongly depends on both the amplitude and the frequency of th…
The effect of Ag addition (0-20 wt%) on polycrystalline Sr0.6K0.4Fe2As2 superconductor has been investigated. It is found that the critical transition temperature Tc was not depressed, and the irreversibility field Hirr and hysteresis magnetization were significantly enhanced upon Ag addition. Characterization study reveals that larger grains are observed in the Ag-added samples. Moreover, the formation of glassy phase as well as amorphous layer, which are present in almost all the grain edges and boundaries in pure samples, are suppressed by Ag addition. The improvement of superconducting pro…
Here we describe the results of an atomic resolution study of oxygen incorporation into bulk MgB2. We find that ~20-100 nm sized precipitates are formed by ordered substitution of oxygen atoms onto boron lattice sites, while the basic bulk MgB2 crystal structure and orientation is preserved. The periodicity of the oxygen ordering is dictated by the oxygen concentration in the precipitates and primarily occurs in the (010) plane. The presence of these precipitates correlates well with an improved critical current density and superconducting transition behavior, implying that they act as pinning…
We have prepared high-quality polycrystalline FeTe0.5Se0.5 at temperature as low as 550°C. The transport critical current density evaluated by the current-voltage characteristics is over 700 A/cm2 at 4.2 K under zero field, which is several times larger than FeTe0.5Se0.5 superconducting wires. The critical current density estimated from magneto-optical images of flux penetration is also similar to this value. The upper critical field of the polycrystalline FeTe0.5Se0.5 at T = 0 K estimated by Werthamer-Helfand-Hohenberg theory is 585 kOe, which is comparable to that of single crystals. This st…
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, ac…
The distribution of magnetic field in single-phase polycrystalline bulk MgB2 has been measured using a Magneto-Optical (MO) technique for an external magnetic field applied perpendicular to the sample surface. The MO studies indicate that an inter-granular current network is readily established in this material and the current is not limited by weak-linked grain boundaries. The grain boundaries are observed to resist preferential magnetic field penetration, with the inter-grain mechanism dominating the current flow in the sample at temperatures up to 30K. The results provide clear evidence tha…
The critical current density (Jc) through a superconductor in high magnetic fields is controlled by the inclusions and microstructure of the material that hold fluxons stationary to keep the resistance zero and is described using Ginzburg-Landau (G-L) theory. Although the functional form of Jc for superconducting-normal-superconducting (SNS) Josephson-Junctions (J-Js) is known in the low field limit, includes the local properties of the junctions and has been confirmed experimentally in many systems, there are no general solutions for Jc of J-Js in high fields. Scaling laws describe the functi…
I-V characteristics of polycrystalline Y(1-x)CaxBa2Cu3O(7-δ) samples (x=0.025 and 0.20) have been measured at different temperatures and magnetic fields in the range 0.1 T-6.9 T. The scaling behavior has been established for both samples at all magnetic fields. The dynamic exponent z displays some morphology dependence with higher value for small grain size sample Y0.8Ca0.2Ba2Cu3O(7-δ). The static exponent ν has been determined from ρ vs. T dependence at given magnetic field. The critical exponents are field independent with one only exception (ν - for Y0.975Ca0.025Ba2Cu3O(7-δ) sample). This i…
The dependence of Tc for MgB2 on purely hydrostatic or nearly hydrostatic pressure has been determined to 23 GPa for single-crystalline and to 32 GPa for polycrystalline samples, and found to be in good agreement. Tc decreases from 39 K at ambient pressure to 15 K at 32 GPa with an initial slope dTc/dP = -1.11(2) K/GPa. Evidence is presented that the differing values of dTc/dP reported in the literature result primarily from shear-stress effects in nonhydrostatic pressure media and not differences in the samples. Although comparison of these results with theory supports phonon-mediated superco…
In this work we present the study of the elastic properties of polycrystalline samples of superconducting YBa_2Cu_3O_7 prepared by the sol-gel method. The quality of all samples was checked by x-ray diffraction and scanning electronic microscopy while their physical properties were verified by transport and magnetic measurements. The elastic study was performed using the standard pulse-echo technique through measuring the phase velocity and the attenuation of ultrasonic waves (in the range of a few MHz) as a function of temperature. We have focused this study on the low temperatures interval (…
We study the effect of 100 MeV Silicon and 200 MeV Gold ion irradiation on the inter and intra grain properties of superconducting thin films of Magnesium Diboride. Substantial decrease in inter-grain connectivity is observed, depending on irradiation dose and type of ions used. We establish that modification of sigma band scattering mechanism, and consequently the upper critical field and anisotropy, depends on the size and directional properties of the extrinsic defects. Post heavy ion irradiation, the upper critical field shows enhancement at a defect density that is five orders of magnitud…
The influence of two important features of magnesium diboride on the macroscopic transport properties of polycrystalline MgB2 is discussed in the framework of a percolation model. While two band superconductivity does not have significant consequences in the field and temperature range of possible power applications, the opposite is true for the anisotropy of the upper critical field. The field dependence of the critical current densities strongly increases and the macroscopic supercurrents disappear well below the apparent upper critical field. The common scaling laws for the field dependence…
Bulk superconductor samples of YBa2Cu3-xZnxO7-δ with x = 0, 0.01, 0.03 are synthesized by solid-state reaction route. The structural characterisation of all samples has been carried out by x-ray-diffraction (XRD) and transmission electron microscopy (TEM) techniques. The x-ray diffraction patterns indicate that the gross structure/phase of YBa2Cu3-xZnxO7-δ do not change with the substitution of Zn up to x=0.03. In TEM investigations of Zn-doped Y-based cuprates a number of ZnO nano-flower and nano-rod of Y-211 phase are found dispersed in regular YBa2Cu3-xZnxO7 matrix. These dispersed nano-flo…
We studied the influence of the disorder introduced in polycrystalline MgB2 samples by neutron irradiation. To circumvent self shielding effects due to the strong interaction between thermal neutrons and 10B we employed isotopically enriched 11B which contains 40 times less 10B than natural B. The comparison of electrical and structural properties of different series of samples irradiated in different neutron sources, also using Cd shields, allowed us to conclude that, despite the low 10B content, the main damage mechanisms are caused by thermal neutrons, whereas fast neutrons play a minor rol…
In this letter, we present the superconducting property characterization of a phase pure reasonably good quality YBa2Cu3O7-δ sample. Studied compound is crystallized in orthorhombic Pmmm space group with lattice parameters a, b, and c are 3.829(2) Å, 3.887(1) Å and 11.666(3) Å respectively. Bulk superconductivity is observed below 90K as evidenced by resistivity and dc/ac magnetization measurements. The resistivity under magnetic field (ρTH) measurements showed clearly both the intra-grain and inter-grain transitions, which are supplemented by detailed (varying frequency and amplitude) ac susc…
Early studies have found quasi-reversible magnetization curves in polycrystalline bulk rare-earth iron oxypnictides that suggest either wide-spread obstacles to intergranular current or very weak vortex pinning. In the present study of polycrystalline samarium and neodymium rare-earth iron oxypnictide samples made by high pressure synthesis, the hysteretic magnetization is significantly enhanced. Magneto optical imaging and study of the field dependence of the remanent magnetization as a function of particle size both show that global currents over the whole sample do exist but that the interg…
The real $R_s(T)$ and imaginary $X_s(T)$ parts of the surface impedance $Z_s(T)=R_s(T)+iX_s(T)$ in polycrystalline MgB$_2$ samples of different density with the critical temperature $T_c\approx 38$ K are measured at the frequency of 9.4 GHz and in the temperature range $5\le T<200$ K. The normal skin-effect condition $R_s(T)=X_s(T)$ at $T\ge T_c$ holds only for the samples of the highest density with roughness sizes not more than 0.1 $μ$m. For such samples extrapolation $T\to 0$ of the linear at $T<T_c/2$ temperature dependences $λ_L(T)=X_s(T)/ωμ_0$ and $R_s(T)$ results in values of the London…
The upper critical field and flux pinning in MgB2 single crystals were investigated. The implications of these properties for technical applications are discussed and compared with transport properties of polycrystalline bulk samples and wires. In these untextured materials current percolation is important, especially at high magnetic fields. It is shown that the anisotropy of the upper critical field influences the "irreversibility line" and that the application range of MgB2 is limited by the smallest upper critical field (i.e., for the field direction perpendicular to the boron planes). Dis…
Here we describe the results of an atomic resolution study of the structure and composition of both the interior of the grains, and the grain boundaries in polycrystalline MgB2. We find that there is no oxygen within the bulk of the grains but significant oxygen enrichment at the grain boundaries. The majority of grain boundaries contain BOx phases smaller than the coherence length, while others contain larger areas of MgO sandwiched between BOx layers. Such results naturally explain the differences in connectivity between the grains observed by other techniques.
Carbon-doped magnesium diboride was fabricated from pre-reacted pure MgB2 by mechanical alloying. The sample set had excellent critical current densities Jc(8T,4.2K) ranging from 15-60 kA/cm2, depending on composition. Magneto-optical imaging detected regions up to 0.5 mm in size which were nearly 100% dense with Jc 2-6 times that of the matrix. Evaluation of resistivity curves using the Rowell method predicts that only 10-50% of the cross sectional area carries the normal state measurement current, suggesting that considerable increases in Jc in these ~80% dense MgB2 samples would be possible…
A method to measure the electrical connectivity between square superconducting plates joined by weak link interfaces is presented. It is based on observation of lines where the flow of critical current abruptly changes direction due to the presence of weak links, and the confinement created by the shape of the sample. The method is demonstrated using magneto-optical imaging (MOI) of systems consisting of up to 2 X 2 plates joined to form a larger square. Common features are found in the current flow patterns, which allow to measure the electrical connectivity between the plates by observing an…
The anisotropy of the critical current density (Jc) and its influence on measurement of irreversibility field (Birr) has been investigated for high quality, in-situ MgB2 strands. Comparison of transport and magnetization measurements has revealed the onset of a regime where large differences exist between transport and magnetically measured values of the critical current density and Birr. These effects, initially unexpected due to the lack of crystalline texture in these in-situ processed strands, appear to be due to a fibrous microstructure, connected with the details of the wire fabrication …
We report the synthesis and measurements of magnetic, transport, and thermal properties of polycrystalline Nb0.18Re0.82, which has a superconducting transition at Tc ~ 8.8 K. The non-centrosymmetric alpha-Mn structure of the compound is confirmed by X-ray diffraction. Using the measured values for the lower critical field Hc1, upper critical field Hc2, and the specific heat C, we estimate the thermodynamic critical field Hc(0), coherence length ξ(0), penetration depth λ(0), and the Ginzburg-Landau parameter κ(0). The specific heat jump at Tc, ΔC/γTc = 1.86, suggests that Nb0.18Re0.82 is modera…
Recent studies suggests that in disordered ultrathin films superconducting (SC) state may be intrinsically inhomogeneous. Here we investigate the nature of SC state in ultrathin Nb films, of thickness $d$ ranging from 1.2 nm to 20 nm, which undergo a transition from amorphous to polycrystalline structure at the thickness $d \simeq 3.3$ nm. We show that the properties of SC state are very different in polycrystalline and amorphous films. The upper critical field ($H_{c2}$) is orbitally limited in the first case, and paramagnetically limited in the latter. The magnetic field induced superconduct…
Pressure is well known to significantly raise the superconducting transition temperature, Tc, in both iron pnictides and cuprate based superconductors. Little work has been done, however, on how pressure can affect the flux pinning and critical current density in the Fe-based superconductors. Here, we propose to use hydrostatic pressure to significantly enhance flux pinning and Tc in polycrystalline pnictide bulks. We have chosen Sr4V2O6Fe2As2 polycrystalline samples as a case study. We demonstrate that the hydrostatic pressure up to 1.2 GPa can not only significantly increase Tc from 15 K (un…
We report a direct current transport study of the local intergrain connections in a polycrystalline SmFeAsO0.85 (Sm1111) bulk, for which we earlier estimated significant intergranular critical current density Jc. Our combined low temperature laser scanning microscopy (LTLSM) and scanning electron microscopy observations revealed only few grain-to-grain transport current paths, most of which switched off when a magnetic field was applied. These regions typically occur where current crosses Fe-As, which is a normal-metal wetting-phase that surrounds Sm1111 grains, producing a dense array of supe…
Temperature (T) dependence of 59Co-NMR Knight shifts K of Na0.3CoO2.1.3H2O has been studied, where samples of randomly oriented powder and aligned crystals have been used for the applied magnetic fields H // ab plane and H //c-axis, respectively. For both directions of H, the shift K decreases below the superconducting transition temperature Tc(H) with decreasing T, indicating that the superconducting electron pairs are in the singlet state. The upper critical fields Hc2(T) determined from the K(H)-T curves are found to be consistent with the values reported by the resistivity measurements for…
The noncentrosymmetric superconductor (NCS) AuBe is investigated using a variety of thermodynamic and resistive probes in magnetic fields of up to 65~T and temperatures down to 0.3~K. Despite the polycrystalline nature of the samples, the observation of a complex series of de Haas-van Alphen (dHvA) oscillations has allowed the calculated bandstructure for AuBe to be validated. This permits a variety of BCS parameters describing the superconductivity to be estimated, despite the complexity of the measured Fermi surface. In addition, AuBe displays a nonstandard field dependence of the phase of d…
The surface resistance (Rs) and the real part (sigma_1) of the microwave complex conductivity of a ~380 nm-thick polycrystalline MgB2 film with the critical temperature (Tc) of 39.3 K were investigated at ~8.5 GHz as a function of temperature. Two coherence peaks were observed in the sigma_1 versus temperature curve at temperatures of ~0.5 Tc and ~0.9 Tc, respectively, providing a direct evidence for the two-gap nature of MgB2. The film appeared to have a pi-band gap energy of 1.8 meV. For the MgB2 film ion-milled down to the thickness of ~320 nm, two coherence peaks were still observed with t…
Nearly Isotropic Quantum-Critical Transport in Single-Crystal CeNiC2
Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；School of Physical Sciences, University of Chinese Academy of Sciences（中国科学院大学）；Institute for Solid State Physics, The University of Tokyo（东京大学固体科学研究所）；Graduate School of Science and Technology, Yamaguchi University（山口大学理工学研究科）；Faculty of Science and Engineering, Tokyo City University（东京都立大学理工学部）
For many applications of polycrystalline high-Tc superconductors the small critical currents of the grain boundaries pose a severe problem. To solve this problem we derive novel designs for the microstructure of coated conductors.
We investigated the magnetization and crystal structure of FeSe polycrystalline samples with various grain sizes. For the samples with large grains, a large magnetic critical current density Jc was observed. For the samples with small grains, superconductivity signals were not observed; instead, magnetization hysteresis, which is not a result of superconductivity, was observed. In the X-ray diffraction pattern for the samples with small grains, broad additional peaks were observed, corresponding to the formation of the non-superconducting (monoclinic) Fe-Se phase at the FeSe grain surface. The…
The superconducting and critical current properties of thin films of NiBi3 formed on the surface of carbon microfibers and sapphire substrates are reported. The NiBi3 coated carbon microfibers were prepared by reacting 7 μm diameter Ni-coated (~ 80 nm) carbon fibers with Bi vapor, and thin films on sapphire were formed by exposing electron-beam deposited Ni films (~ 40 - 120 nm) to Bi vapor. The microfibers and films show Tc = 4.3 K and 4.4 K,respectively, which were slightly higher than that reported for bulk polycrystalline NiBi3. The critical current density (Jc) was measured below the tran…
We present a detailed magneto-optical investigation of the magnetic flux penetration in polycrystalline MgB2 slabs made by direct reaction of B and Mg. Our results unambiguously indicate a uniform, Bean critical state magnetization behavior with almost no electromagnetic granularity. From the measured magnetic flux profiles we were able to extract the temperature dependence of the critical current density Jc(T). The Jc(T) value reaches 1.8x10^5 A/cm2 at 10K and 0.12T, in good agreement with global magnetization measurements.
We report the magnetotransport properties of thin polycrystalline films of the recently discovered non-oxide perovskite superconductor MgCNi3. CNi3 precursor films were deposited onto sapphire substrates and subsequently exposed to Mg vapor at 700 C. We report transition temperatures (Tc) and critical field values (Hc2) of MgCNi3 films ranging in thickness from 7.5 nm to 100 nm. Films thicker than ~40 nm have a Tc ~ 8 K, and an upper critical field Hc2 ~ 14 T, which are both comparable to that of polycrystalline powders. Hall measurements in the normal state give a carrier density, n =-4.2 x 1…
We report significant enhancement in superconducting properties of yttrium substituted Ce1-xYxOFFeAs superconductors. The polycrystalline samples were prepared by two step solid state reaction technique. X-ray diffraction confirmed tetragonal ZrCuSiAs structure with decrease in both a and c lattice parameters on increasing yttrium substitution (with fixed F content). With smaller ion Y in place of Ce, the transition temperature increased by 6 K. Yttrium doping also lead to higher critical fields as well as stronger inter and intra-granular current density. The magnetization critical current de…
We have synthesized the polycrystalline samples of Eu1-xKxFe2As2 (x = 0-1) and carried out systematic characterization using x-ray diffraction, ac & dc magnetic susceptibility, and electrical resistivity measurements. We have seen a clear signature of the coexistence of superconducting transition (Tc = 5.5 K) with SDW ordering in our under doped sample viz. x = 0.15. The spin density wave transition observed in EuFe2As2 get completely suppressed at x = 0.3 and superconductivity arises below 20 K. Superconducting transition temperature Tc increases with increase in K content and a maximum Tc = …
