In the last 30 days

• Open/close The effects of attrition and ball milling on the properties of magnesium diboride

  C E J Dancer et al 2010 Supercond. Sci. Technol. 23 065015 Tag this article

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  Commercially produced magnesium diboride powder was modified by attrition milling and ball milling in propan-2-ol for various durations. These powders were characterized by means of particle size distribution measurements using laser diffraction, impurity analysis using x-ray diffraction, energy dispersive spectroscopy and x-ray photoelectron spectroscopy, and scanning electron microscopy, and were then used to produce magnesium diboride samples through pressureless heat treatment at peak temperatures up to 1100 °C. X-ray diffraction, scanning electron microscopy, Vickers hardness measurements, and density measurements using the Archimedes method were used to characterize the properties of the samples, and a determination of their superconducting properties using the magnetization method was carried out. Magnesium diboride produced from powder milled under certain conditions had a higher J _{c, mag} than samples produced from as-purchased powder, but the relationship between the milling duration or energy and the superconducting performance is a complex one, affected by both the particle size and the impurity content of the starting powder.

• Open/close Superconducting nanowire single-photon detectors: physics and applications OPEN ACCESS

  Chandra M Natarajan et al 2012 Supercond. Sci. Technol. 25 063001 Tag this article

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  Single-photon detectors based on superconducting nanowires (SSPDs or SNSPDs) have rapidly emerged as a highly promising photon-counting technology for infrared wavelengths. These devices offer high efficiency, low dark counts and excellent timing resolution. In this review, we consider the basic SNSPD operating principle and models of device behaviour. We give an overview of the evolution of SNSPD device design and the improvements in performance which have been achieved. We also evaluate device limitations and noise mechanisms. We survey practical refrigeration technologies and optical coupling schemes for SNSPDs. Finally we summarize promising application areas, ranging from quantum cryptography to remote sensing. Our goal is to capture a detailed snapshot of an emerging superconducting detector technology on the threshold of maturity.

• Open/close Frequency-dependent dielectric characteristics of Tl–Ba–Ca–Cu–O bulk superconductor

  Ş Çavdar et al 2005 Supercond. Sci. Technol. 18 1204 Tag this article

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  A ceramic sample of a layered cuprate of Tl ₂Ba ₂Ca ₂Cu ₃O _x and Tl ₂Ba ₂Ca ₁Cu ₂O _x , so-called Tl-based 2223 and 2212 compounds, was investigated by the measurement of the dielectric constants (ε' and ε''), the dielectric loss (tanδ), and the conductivity (σ _ac) as a function of temperature (80–300 K) and frequency (100 Hz–10 MHz). A negative capacitance phenomenon has been observed and it is believed that the negative capacitance effect is due to the polarization effect. The negative dielectric constant has been calculated from the negative capacitance. The dielectric constant (ε') and the dielectric loss (tanδ) show strong dispersions at lower frequencies. The dielectric properties of the sample were found to have strong frequency and temperature dependence.

• Open/close Effect of high-energy ball milling time on superconducting properties of MgB₂ with low purity boron powder

  Chengduo Wang et al 2012 Supercond. Sci. Technol. 25 035018 Tag this article

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  MgB ₂ bulks and tapes were fabricated using ball-milled precursor powders. The selected B powders have the β-rhombohedral crystalline structure as a main phase, with a small amount of MgO impurity. Milling time was tested from 0.5 to 120 h to get an optimized value. It is found that ball milling can facilitate MgB ₂ phase formation, refine the MgB ₂ grains and increase the defect density. Moreover, the active area, B _c2 and flux pinning are improved, which is of benefit to the enhancement of J _c performance. The 80 h milled MgB ₂ bulk sample has a J _c value of 2.8 × 10 ⁴ A cm ⁻² at 5 K and 6 T, 41 times larger than that of the 0.5 h sample. At the same time, the transport J _c of the 80 h MgB ₂ tape reaches 2.3 × 10 ³ A cm ⁻² at 4.2 K and 9 T, 32 times larger than that of the 0.5 h one. Due to the increase of impurities and the release of lattice deformation and strain, the J _c was decreased for the sample milled up to 120 h.

• Open/close Superconducting boundary conditions for mesoscopic circular samples

  J Barba-Ortega et al 2011 Supercond. Sci. Technol. 24 015001 Tag this article

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  In the present paper we solve the time dependent Ginzburg–Landau (TDGL) equations by using the link variables technique for two shapes of circular geometry, a circular sector and a disc. The implemented algorithm is applied to a circular geometry surrounded by different kinds of material and immersed in an external magnetic field applied perpendicular to its plane. The properties of these materials are accounted for in the de Gennes boundary conditions with the de Gennes extrapolation length (the so called b parameter). We evaluate the magnetization, the superconducting electron density, the superconducting–normal magnetic field transition, and the applied magnetic field /b-parameter phase diagram. For the circular sector, our results point out that, under an appropriate choice of the b parameter, the third critical field H _c3 can be greatly increased. For the disc, we determine the b-limit for the occurrence of the Meissner, single and multi-vortex states in a type-II superconductor. In addition, we show that under an appropriate construction of the boundary, a type-II superconducting disc may behave like a type-I.

• Open/close Alcoholic beverages induce superconductivity in FeTe_{1 − x}S_x

  K Deguchi et al 2011 Supercond. Sci. Technol. 24 055008 Tag this article

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  We found that hot alcoholic beverages were effective in inducing superconductivity in FeTe _0.8S _0.2. Heating the FeTe _0.8S _0.2 compound in various alcoholic beverages enhances the superconducting properties compared to a pure water–ethanol mixture as a control. Heating with red wine for 24 h leads to the largest shielding volume fraction of 62.4% and the highest zero resistivity temperature of 7.8 K. Some components present in alcoholic beverages, other than water and ethanol, have the ability to induce superconductivity in the FeTe _0.8S _0.2 compound.

• Open/close Influence of Ni and Cu contamination on the superconducting properties of MgB₂ filaments

  A Jung et al 2010 Supercond. Sci. Technol. 23 095006 Tag this article

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  Technical MgB ₂ wires usually have a sheath composite consisting of different metals. For the inner sheath with direct contact to the superconducting filament, chemically inert Nb may be used as a reaction barrier and thermal stabilization is provided by a highly conductive metal like Cu. A mechanical reinforcement can be achieved by the addition of stainless steel. In order to illuminate the influence of defects in the reaction barrier, monofilament in situ wires with direct contact between the MgB ₂ filament and frequently applied reactive sheath metals like Cu, Ni or Monel are studied. Reactions of Mg and B with a Cu-containing sheath lead to Cu-based by-products penetrating the whole filament. Reactions with Ni-containing sheaths lead to Ni-based by-products which tend to remain at the filament–sheath interface. Cu and/or Ni contamination of the filament lowers the MgB ₂-forming temperature due to the eutectic reaction between Mg, Ni and Cu. Thus, for the samples heat-treated at low temperatures J _C and (partly) T _C are increased compared to stainless-steel-sheathed wires. At high heat treatment temperatures uncontaminated filaments lead to the highest J _C values. From the point of view of broken reaction barriers in real wires, the contamination of the filament with Cu and/or Ni does not necessarily constrain the superconductivity; it may even improve the properties of the wire, depending on the desired application.

• Open/close High-field properties of carbon-doped MgB₂ thin films by hybrid physical–chemical vapor deposition using different carbon sources

  Wenqing Dai et al 2011 Supercond. Sci. Technol. 24 125014 Tag this article

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  We have studied the high-field properties of carbon-doped MgB ₂ thin films prepared by hybrid physical–chemical vapor deposition (HPCVD). Carbon doping was accomplished by adding carbon-containing gas, such as bis(methylcyclopentadienyl)magnesium and trimethylboron, into the hydrogen carrier gas during the deposition. In both cases, T _c drops slowly and residual resistivity increases considerably with carbon doping. Both the a and c lattice constants increase with carbon content in the films, a behavior different from that of bulk carbon-doped MgB ₂ samples. The films heavily doped with trimethylboron show very high parallel H _c2 over 70 T at low temperatures and a large temperature derivative near T _c. These behaviors are found to depend on the unique microstructure of the films, which consists of MgB ₂ layers a few-nanometers thick separated by non-superconducting MgB ₂C ₂ layers. This leads to an increase in the parallel H _c2 by the geometrical effect, which is in addition to the significant enhancement of H _c2 due to changes in the scattering rates within and between the two bands present in films doped using both carbon sources. The high H _c2 and high-field J _c( H) values observed in this work are very promising for the application of MgB ₂ in high magnetic fields.

• Open/close An overview on iron based superconductors

  P M Aswathy et al 2010 Supercond. Sci. Technol. 23 073001 Tag this article

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  The discovery of superconductivity at relatively higher temperatures in a non-cuprate system, LnFeAsO _{1 − x}F _x (Ln = lanthanides) has created tremendous activity among the reseachers in this field. This review is an overview on the present status and the future scope for iron pnictides. The various structural categories of iron based superconductors, the structural aspects, different preparation techniques of the material and the necessity for its optimization are discussed. The highlighting features of iron pnictide, i.e. the very high upper critical field, moderate magneto-transport and thermal properties, are also included. The article gives a summary of the prevailing arguments of researchers to relate the material to cuprates and also the comparative features of classical and MgB ₂ superconductors. The existing challenges, such as optimizing synthesis methods for technological applications and clarifying the ambiguity in the superconducting mechanism and the flexibility of the material for any site substitution, will keep iron based superconductors on the frontiers of research for a long time, in parallel to HTS.

• Open/close Modeling of the electro-mechanical behavior of ITER Nb₃Sn cable in conduit conductors

  Marco Breschi et al 2012 Supercond. Sci. Technol. 25 054005 Tag this article

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  The coupling of electrical and mechanical modeling of superconducting cable in conduit conductors (CICCs) can be useful for the understanding of the complex phenomena occurring in cables based on strain-sensitive materials like Nb ₃Sn. The MULTIFIL model is a detailed mechanical model aimed at computing the strain distribution at the strand level in multistrand superconducting cables. MULTIFIL is a finite element code dealing with the contacts between beam assemblies that includes plasticity and it allows modeling both transverse and longitudinal loads applied to the cable. The electromagnetic part of the THELMA code is based on a distributed parameter electrical circuit model of CICCs and is aimed at computing the current distribution and electrical losses at an arbitrary cabling stage. In this work, the maps of strain have been computed with MULTIFIL for a petal of a relevant conductor for the central solenoid of the ITER magnet system. These strain maps were computed in three main cases, namely applying only a thermal compressive strain due to cable contraction in cool-down, considering thermal strain plus the strain due to Lorentz force in a virgin state, and thermal strain plus Lorentz force after a given number of electromagnetic cycles. These strain maps have been implemented in the THELMA code, in order to compute the E– T curves corresponding to the different cases, such deriving important parameters to be compared with experimental results ( T _cs, n value, effective strain). A methodology to account in the electrical model for the inhomogeneous strain on the strand cross section due to bending is described and the corresponding results are compared to the case of a homogeneous strain on the wire cross section. The model is able to explain the experimental difference between the physical strain that can be determined with T _c measurements and the effective strain that can be derived from the T _cs measurements.