Superconducting Properties of the 3-K Phase of Sr2RuO4 near Hc2

The superconducting transition temperature, T _c , of the impurity-free, intrinsic Sr₂RuO₄ is as high as 1.50 K. However, we recently showed that T _c is remarkably increased up to 3 K in the Sr₂RuO₄–Ru eutectic system, in which plate-like microdomains of Ru metal are embedded in the primary-phase Sr₂RuO₄. The phase diagram of the anisotropic upper critical field of the 3-K phase indicates that H _c2 for the field parallel to the RuO₂ plane is strongly suppressed at low temperatures. We argue that the reorientation of the Cooper-pair spin direction near the Sr₂RuO₄–Ru interface may be responsible for this suppression. In addition, we observed unusual hysteresis in the out-of-plane resistivity, ρ _c , at low temperatures and near H _c2, only when the field was applied parallel to the RuO₂ plane.     

Resistive upper critical field of single crystals of Tl2Ba2CuO6

We have measured the resistive upper critical field of overdoped single crystals of Tl₂Ba₂CuO₆ from the zero-field transition temperatureT _c (approximately 20 K) to temperatures as low as 12 mK, corresponding to less than 0.001T _c. In sharp contrast to the predictions of standard theories of superconductivity, the critical field is found to rise steeply with positive curvature as the temperature is reduced, and no sign of saturation is observed down to the lowest temperatures reached.     

Comparison of 2-D Quantum and Thermal Critical Fluctuations of Underdoped Bi2Sr2CaCu2O8+δ with Ultrathin YBa2Cu3O7−δ

This paper addresses two issues. Underdoped Bi-2212 has been shown to exhibit two-dimensional quantum fluctuations, with T _c vs. N _s (0) scaling qualitatively similar to ultrathin YBCO films that are 2-D by construction. The present paper shows that the quantitative difference in scaling is resolved by growing Bi-2212 films that are much smoother than previously used. Also, the absence of 2-D thermal critical behavior from the superfluid density of Bi-2212 films that exhibit 2-D quantum scaling is not likely due to an inhomogeneous T _c smearing out the 2-D thermal transition. Finally, we emphasize that there is as yet no consensus on the physics that drives the T-dependence of N _s at low to intermediate temperatures.     

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.     

Implications of T c-Variation in UBe13 for a Possible Fulde–Ferrell–Larkin–Ovchinnikov Phase

We measure the heat capacity of a UBe₁₃ sample with an unusually low T _c for a polycrystal. We find an upturn in the upper critical field H _c2(T) below about T _c/2, much as for higher-T _c samples. Comparing the critical fields in our sample and in samples with higher T _c's shows that the low temperature limit of H _c2 is proportional to T _c(H = 0), as expected if the upturn comes from an FFLO phase and strong coupling.     

Study of anisotropic magnetoresistance in the thermodynamic fluctuation regime of single crystals of Bi2Sr2CaCu2O8+x

Measurements of the in-plane and out-of-plane magnetoresistance in Bi₂Sr₂CaCu₂O_8+x single crystals with a critical temperature T_c=93 K are reported for temperatures between T_c and 200 K and in magnetic fields up to 13 T. Both the transverse and the longitudinal in-plane magnetoresistance are positive and originate entirely from the suppression of superconducting fluctuations by the magnetic field. The longitudinal out-of-plane magnetoresistance is negative at all temperatures and magnetic fields. As possible origins two recent models are discussed, the reduction of the quasiparticle density of states due to condensation into superconducting fluctuations and thermally activated transport over an interplane pseudogap.     

Effect of Nano-AlN Addition on the Critical Current Density of MgB2 Superconductors

A series of MgB₂ polycrystalline pellets with and without aluminum nitride (nano)addition have been synthesized by a solid-state reaction. No detectable shift in the XRD peak position of MgB₂ has been observed with AlN nanoparticles addition indicating no substitution for Mg or B. Surface morphology displays randomly oriented well-defined grains. The presence of AlN nanoparticles between the grain boundaries and also on the grain surface is observed in AlN-added MgB₂. Addition of AlN nanoparticles decreases the superconducting transition temperatures from 38.5 to 37 K. Resistivity data confirm an improved connectivity of the grains in MgB₂ pellets for low-level addition of AlN nanoparticles. Magnetization data measured at various temperatures between ±6 T are used to estimate the critical current density (J _c ) of undoped and AlN nanoparticles doped MgB₂ pellets. J _c enhances significantly up to 4 T at 20 K for MgB₂ containing 0.5 wt.% AlN. Our analysis confirms excellent correspondence of the measured field dependence of critical current density (J _c ) in terms of collective pinning model. A new universal scaling behavior is established between the reduced critical current density, J _n (=J _c /J _c (0)) and the reduced field h _n (=H/H ₀) for both undoped and AlN-doped MgB₂ pellets at different temperatures, where J _c (0) and H ₀ are the best fit parameters obtained from the collective pinning model. Our analysis further confirms the dominance of δT _c -type pinning for pure MgB₂, and for n-AlN-added MgB₂ pellets at low temperatures the major contribution comes from δl-type pinning.     

Optically induced changes and long-term relaxations of resistivity and critical temperature in He irradiated YBa2Cu3Ox

Thin films of YBa₂Cu₃O_x with superconducting critical temperatures (T_c) ranging from 58 K to 72 K were prepared by pulsed-laser deposition and subsequent irradiation with 75 keV He⁺ ions. Optical excitation of the films with a He-Ne laser resulted in a small resistivity reduction, when the experiment was carried out a short period of time after the ion irradiation. The value of T_c was essentially unchanged by the optical treatment. Further light excitation experiments did not show a significant effect on the resistivity of the samples. We conclude that ion-irradiated metallic YBa₂Cu₃O_x does not exhibit the persistent photoconductivity observed in oxygen-depleted samples with comparable critical temperatures. On the other hand we have observed a long-term relaxation of T_c towards larger values that takes place even when the samples are kept in the dark. Our results indicate that T_c may increase after ion irradiation to an asmptotic value about 20% over the initial value on a typical time scale of years. This observation may be important for possible applications of light-ion irradiated YBa₂Cu₃O_x.     

Superconducting Properties of the 3-K Phase of Sr2RuO4 near Hc2

The superconducting transition temperature, T _c , of the impurity-free, intrinsic Sr₂RuO₄ is as high as 1.50 K. However, we recently showed that T _c is remarkably increased up to 3 K in the Sr₂RuO₄–Ru eutectic system, in which plate-like microdomains of Ru metal are embedded in the primary-phase Sr₂RuO₄. The phase diagram of the anisotropic upper critical field of the 3-K phase indicates that H _c2 for the field parallel to the RuO₂ plane is strongly suppressed at low temperatures. We argue that the reorientation of the Cooper-pair spin direction near the Sr₂RuO₄–Ru interface may be responsible for this suppression. In addition, we observed unusual hysteresis in the out-of-plane resistivity, _c , at low temperatures and near H _c2, only when the field was applied parallel to the RuO₂ plane.     

Intrinsic critical current of Ag-clad (Bi,Pb)2Sr2Ca2Cu3O z tapes

True zero-field critical current densityJ _c of a well-characterized BPSCCO/Ag tape has been determined by means of high-resolution ac susceptibility in the temperature range 77–110 K. The resultant values (30,000 A/cm² at 77 K) agree well with the transportJ _c of the same tape. Because of a very thin BPSCCO, the coreJ _c determined from the imaginary part of the ac susceptibility is nearly the same as the zero field one. AllJ _c 's follow the same (1-T/T _c )ⁿ withn=1.45 dependence.J _c shows an approximateH ^?0.5 field dependence over the explored temperature range. Accordingly, the variations ofJ _c withT andH seem to be determined by the flux creep.     

Scaling Behavior in Aluminum Doped MgB2 Superconductor

Magnetization measurements have been performed on Mg_1?x Al _x B₂ (0≤x≤0.1) at various temperatures below T _c and in fields up to 9 T. The reversible magnetization has been used to calculate the thermodynamic critical field (H _c ) which is then used to obtain a new universal behavior for both the critical current density (J _c ) and the pinning force (P _f ). This approach is based on vortex core energy ( ${\sim}H^{2}_{c}$ ) and clearly exhibits the deviation from single vortex pinning with increasing temperature and higher concentration of Al-doping.     

Superconducting Properties and Microstructure of Nanocrystalline MgB2 Bulk Prepared by Sintering of Ball-Milled Powders

Ultrafine nanostructured MgB₂ bulks with an average grain size less than 10 nm have been fabricated by high-energy ball milling and subsequent high pressure sintering. Microstructural evolution in MgB₂ subjected to high-energy ball milling has been investigated by means of X-ray diffraction (XRD). The finer grain size of MgB₂ powders of about 7 nm has been estimated from Rietveld refinement analysis of XRD data, which is confirmed by a transmission electron microscope (TEM) observation. There is almost no grain growth in the subsequent sintering at low temperature of 600?°C under pressure of 3?C5 GPa for 10?C30 min. The nanocrystalline MgB₂ bulks exhibit the lower onset critical transition temperatures (T _{c onset}) of 32?C33?K. The relative wider width of the magnetic hysteresis loops at high external magnetic field and the higher critical current density (J _c ) are obtained in nanocrystalline bulks. J _c is as high as 10⁵?A/cm² in 8?T at 10?K and 2.7×10³?A/cm² in 4?T at 20?K.     

Study of Flux-Creep in Bulk Melt-Textured YBa2Cu3Ox in the Regime of Individual Pinning

Magnetic relaxation, with magnetic fields applied parallel to the c axis, was measured in a high-quality melt-textured YBa₂Cu₃O_x bar. Zero-field-cooled magnetization-versus-time data were obtained within a regime of field and temperatures dominated by single vortex pinning. The activation pinning energy U = U _M was calculated from experimental data and compared to U = U ₀ ln(J _c /J), where U, is a constant, J _c is the critical current density, and J the current density. This expression for U obtains within the individual pinning regime. A logarithmic dependence of U _M on J was found, and the data indicated that a scaling function of U _M , g(T/T _c ) = (1?T/T _c )^1/2, was appropriate for describing the single-vortex pinning regime.     

Investigation of the Upper Critical Magnetic Field and Activation Energy in MgB2 Thin Film

MgB₂ film with a thickness of about 600?nm was deposited on the MgO (100) single crystal substrate using a ??two-step?? synthesis technique. First, deposition of boron thin film was carried out by rf magnetron sputtering on MgO substrates and followed by a post deposition annealing at 850?°C in magnesium vapor. The upper critical field H _c2 has been estimated from temperature dependences of resistivity curves in both directions of the magnetic fields perpendicular and parallel to the c-axis. Resistivity measurements of the film were performed using a standard four-probe method under different magnetic fields up to 70?kOe in zero fields cooling regime. The upper critical magnetic field H _c2(0) at T=0?K for 90?% of R _n was calculated by the extrapolation H _c2(T) to the temperature T=0?K. The results showed that H _c2??ab(0) and H _c2??c(0) was found to be around 22?T and 18?T, respectively. Using extracted data, the zero-temperature coherence lengths and field anisotropy ratio were calculated. In order to determine the activation energy of thermally activated flux flow of the film, Arrhenius law was taken into account.     

Scaling Behavior and Estimation of Activation Energy of Polycrystalline RuSr2GdCu2O8 Superconductor from AC Susceptibility Measurements

In this study, the measured curves of AC susceptibility (ACS) components, χ′(T) and χ″(T), of polycrystalline RuSr₂GdCu₂O₈ (Ru-1212) superconductor were scaled onto a single curve using the peak temperature of its imaginary part (T _p) as the scaling parameter for various AC field amplitudes from 0.5 to 24 G. The dependence of the AC magnetic field amplitude on T _p is scaled as: H _ac ～ (1?T _p/T _c)^2.25. Similarly, the current density J _c, extracted from the AC field amplitude is also scaled as: J _c ～ (1 ? T _p/T _c)^2.25. The dependencies of T _p on frequency and AC field amplitude are also investigated and the time parameter t ₀ of the order of 10^?8 s is estimated from the dependence of T _p on frequency. The dependencies of activation energy on temperature, T, and the field amplitude, H _ac, are obtained from the Arrhenius-like semilog plot of frequency (ν) and T _p. Such dependencies on temperature and field amplitude can be described by a scaling law of the form: U(H _ac,T) = U ₀[1 ? T/T _p]H _ac ^?0.17.     

Transport Properties of the Iron-based Superconductor SrFe2(As,P)2 in High Magnetic Fields

We have investigated the resistive upper critical field (μ ₀ H _c2) of the iron-based superconductor SrFe₂(As_1?x P _x )₂ (x=0.35, T _c～29 K) in pulsed high magnetic fields of up to 52 T. For H∥ab, $\mu_{0}H_{\mathrm{c}2}^{ab}(T)$ exhibits the Werthamer-Helfand-Hohenberg (WHH) like behavior. While, for H∥c, $\mu_{0}H_{\mathrm {c}2}^{c}(T)$ is almost linear against T and exhibits a slightly upward curvature near T _c. The anisotropy of μ ₀ H _c2 decreases from 2.5 near T _c to ～1.2 at T=0 K monotonically. Similar isotropic behavior of μ ₀ H _c2 at low temperatures has also been observed in the Fe_1+δ (Te,Se). We demonstrate the results of the two-band analysis for μ ₀ H _c2(T) on the present sample and discuss the anisotropy of μ ₀ H _c2 by comparing with those in some kinds of iron-based superconductors.     

MgB2 Thin Films on Metal Substrates for Superconducting RF Cavity Applications

Magnesium diboride is a promising material for superconducting RF (SRF) cavity applications. Compared to the currently used superconductor for SRF cavities Nb, MgB₂ has the potential to achieve lower RF loss and higher acceleration field due to its higher critical temperature and thermodynamic critical magnetic field. Since the RF field only penetrates a few penetration depths into a superconductor, a superconducting coating of several hundred nanometers on a metal cavity is sufficient for superb SRF cavity performances. In this work, we report the properties of MgB₂ thin films deposited by the hybrid physical–chemical vapor deposition (HPCVD) technique on different metal substrates including Nb, Mo, Ta, and stainless steel. All the films were polycrystalline, as indicated by X-ray diffractometry and scanning electron microscopy, and showed T _c ～39 K, determined by resistance versus temperature, magnetic susceptibility, and dielectric resonator measurements. MgB₂ films deposited on Nb substrates polished to various degrees of smoothness exhibit similar T _c . The result is a promising step in the investigation of using MgB₂ as an alternative to Nb for SRF cavities.     

Measurements ofH c2(T) in Bi-Sr-Cu-O

H _c2(T) has been measured for thin BSCO films at temperatures down to 65 mK and pulsed fields up to 35 T.H _c2(T) diverged anomalously as the temperature decreased. At the lowest temperature, it was five times that expected for a conventional superconductor.     

Magnetic Characterization of MgB2 Bulk Superconductor for Magnetic Field Mitigation Solutions

Magnetic shielding properties of MgB₂ bulk samples synthesized by the SPS (Spark?CPlasma?CSintering) technique were characterized in low applied magnetic fields at temperatures ranging from 20 to 37 K. The used growth technique allows one to produce this compound in different shapes and sizes required for shielding applications. In this framework, shielding magnetic-induction field profiles generated by MgB₂-based shield components, shaped as planar thick disks, were measured by means of a suitable Hall probe in-plane array. The magnetic field distribution at different vertical distances above the sample was also obtained by a micrometric motion of the probe ensemble. Magnetic field profiles were then analyzed in the framework of the critical state model and the critical current density, J _c , was evaluated. The J _c magnitude indicates that the material under test is a good candidate for passive magnetic shield manufacturing up to temperatures close to the transition one.