In the last 30 days

• Open/close Superconductivity: its role, its success and its setbacks in the Large Hadron Collider of CERN

  Lucio Rossi  2010 Supercond. Sci. Technol. 23 034001 Tag this article

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  The Large Hadron Collider (LHC), the particle accelerator at CERN, Geneva, is the largest and probably the most complex scientific instrument ever built. Superconductivity plays a key role because the accelerator is based on the reliable operation of almost 10 000 superconducting magnets cooled by 130 tonnes of helium at 1.9 and 4.2 K and containing a total stored magnetic energy of about 15 000 MJ (including detector magnets). The characteristics of the 1200 tonnes of high quality Nb–Ti cables have met the severe requests in terms of critical currents, magnetization and inter-strand resistance; the magnets are built with an unprecedented uniformity, about 0.01% of variation in field quality among the 1232 main dipoles, which are 15 m in length and 30 tonnes in weight. The results of this 20-year-long enterprise will be discussed together with problems faced during construction and commissioning and their remedies. Particular reference is made to the severe incident which occurred nine days after the spectacular start-up of the machine on 10 September 2008. The status of repair and the plan for the physics programme in 2010 are also presented.

• Open/close New Fe-based superconductors: properties relevant for applications

  M Putti et al  2010 Supercond. Sci. Technol. 23 034003 Tag this article

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  Less than two years after the discovery of high temperature superconductivity in oxypnictide LaFeAs(O, F) several families of superconductors based on Fe layers (1111, 122, 11, 111) are available. They share several characteristics with cuprate superconductors that compromise easy applications, such as the layered structure, the small coherence length and unconventional pairing. On the other hand, the Fe-based superconductors have metallic parent compounds and their electronic anisotropy is generally smaller and does not strongly depend on the level of doping, and the supposed order parameter symmetry is s-wave, thus in principle not so detrimental to current transmission across grain boundaries. From the application point of view, the main efforts are still devoted to investigate the superconducting properties, to distinguish intrinsic from extrinsic behaviors and to compare the different families in order to identify which one is the fittest for the quest for better and more practical superconductors. The 1111 family shows the highest T _c, huge but also the most anisotropic upper critical field and in-field, fan-shaped resistive transitions reminiscent of those of cuprates. On the other hand, the 122 family is much less anisotropic with sharper resistive transitions as in low temperature superconductors, but with about half the T _c of the 1111 compounds. An overview of the main superconducting properties relevant to applications will be presented. Upper critical field, electronic anisotropy parameter, and intragranular and intergranular critical current density will be discussed and compared, where possible, across the Fe-based superconductor families.

• Open/close Characterization of high-temperature superconductors by AC susceptibility measurements

  Fedor Gömöry  1997 Supercond. Sci. Technol. 10 523 Tag this article

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  A magnetic field harmonically varying in time (to probe the sample) and a lock-in technique (to register the sample response sensed by a pick-up coil) are widely used for characterizing superconductors. Measuring the temperature dependence of the complex AC susceptibility is the most common procedure of this type. This paper reviews these techniques, introducing in addition the complex AC susceptibility, the so-called `wide-band AC susceptibility'. The latter quantity refers to the magnetic flux and often offers an easier meeting between theory and experiment. Starting from models for linear flux diffusion, reversible screening, volume and surface flux pinning and the intermediate regime in a type II superconductor, the expressions for the complex AC susceptibility in different cases are presented and compared with those derived for the wide-band AC susceptibility. Derivation of the basic physical properties of high- superconducting materials from the AC data (resistivity, critical temperatures and fields, London and Campbell penetration depths, critical current density, granularity and content of superconducting phase, irreversibility line, pinning potential) is then thoroughly discussed.

• Open/close The effect of carbon and rare earth oxide co-doping on the structural and superconducting properties of MgB₂

  N Ojha et al  2010 Supercond. Sci. Technol. 23 045005 Tag this article

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  Carbon (C) and rare earth oxide (REO) co-doped bulk polycrystalline MgB ₂ samples with nominal compositions Mg _{1 − y}(REO) _y (B _0.95C _0.05) ₂ (where y = 0.00, 0.01, 0.03, 0.05 and REO = Eu ₂O ₃ or Pr ₆O ₁₁) have been synthesized via a solid state reaction route. The XRD results reveal the presence of impurity phases EuB ₆ in Eu ₂O ₃ and PrB ₄ and PrB ₆ in Pr ₆O ₁₁ co-doped samples along with the main hexagonal phase of MgB ₂ and a small amount of MgO. The values of upper critical field ( H _c2) and irreversibility field ( H _irr), except H _c2 of y = 0.05 for Eu ₂O ₃, have been found to increase at all temperatures (< T _c) with increasing doping concentration of REO. Improvements in the values of critical current density ( J _c) at 10 K for y = 0.01 of Eu ₂O ₃ and y = 0.01, 0.03 of Pr ₆O ₁₁ co-doped samples have been observed in high fields (>6.5 T) region. At 20 K enhancement in the high field (>6 T)  J _c values for y = 0.01, 0.03 of Eu ₂O ₃ and y = 0.01 of Pr ₆O ₁₁ co-doped samples are also reported in the present work. The correlations between the structural characteristics and the observed superconducting properties of the co-doped samples are described and discussed in this paper.

• Open/close Critical currents in magnetically pressed MgB₂ rods

  S P Anan'ev et al  2004 Supercond. Sci. Technol. 17 S274 Tag this article

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  Using a magnetic compression method a 2.3 mm diameter superconducting MgB ₂ rod was synthesized in a stainless steel tube. After the compression the MgB ₂ density inside the tube reached d = 2.45 g cm ⁻³, i.e. very near to the theoretical value 2.62 g cm ⁻³. At T = 4.2 K and B = 0.5 T the quench current reached 870 A. The critical current density versus temperature was determined from electric and magnetic measurements.

• Open/close Superconducting wind turbine generators

  A B Abrahamsen et al  2010 Supercond. Sci. Technol. 23 034019 Tag this article

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  We have examined the potential of 10 MW superconducting direct drive generators to enter the European offshore wind power market and estimated that the production of about 1200 superconducting turbines until 2030 would correspond to 10% of the EU offshore market. The expected properties of future offshore turbines of 8 and 10 MW have been determined from an up-scaling of an existing 5 MW turbine and the necessary properties of the superconducting drive train are discussed. We have found that the absence of the gear box is the main benefit and the reduced weight and size is secondary. However, the main challenge of the superconducting direct drive technology is to prove that the reliability is superior to the alternative drive trains based on gearboxes or permanent magnets. A strategy of successive testing of superconducting direct drive trains in real wind turbines of 10 kW, 100 kW, 1 MW and 10 MW is suggested to secure the accumulation of reliability experience. Finally, the quantities of high temperature superconducting tape needed for a 10 kW and an extreme high field 10 MW generator are found to be 7.5 km and 1500 km, respectively. A more realistic estimate is 200–300 km of tape per 10 MW generator and it is concluded that the present production capacity of coated conductors must be increased by a factor of 36 by 2020, resulting in a ten times lower price of the tape in order to reach a realistic price level for the superconducting drive train.

• Open/close Grain boundaries in the cuprate superconductors: tapes and tunneling spectroscopy

  P Chaudhari and Heejae Shim  2010 Supercond. Sci. Technol. 23 034002 Tag this article

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  Grain boundaries in the high temperature superconducting cuprates have played a central role in their development for practical applications and in the fundamental understanding of the nature of superconductivity in these materials. Tapes for energy use, SQUIDS, symmetry of the wavefunction, Qbits, applications related to the AC Josephson effect, and tunneling spectroscopy are some areas of current research. In this brief paper, the authors first summarize what we know about what limits the critical current densities of tapes and suggest a few experiments to further understand these limits to critical current densities and, secondly, discuss the use of grain boundary for carrying out tunneling spectroscopy in optimally doped La _1.84Sr _0.16CuO ₄ (LSCO). This includes new data and comparisons with theory and experiments. The background material and review were presented at the EUCAS 09 conference in Dresden as one of the plenary talks and are available from the authors.

• Open/close Superconductivity in a new iron pnictide oxide (Fe₂As₂)(Sr₄(Mg, Ti)₂O₆)

  Shinya Sato et al  2010 Supercond. Sci. Technol. 23 045001 Tag this article

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  We have discovered a new iron pnictide oxide superconductor (Fe ₂As ₂)(Sr ₄(Mg, Ti) ₂O ₆). This material is isostructural with (Fe ₂As ₂)(Sr ₄M ₂O ₆) (M = Sc, Cr, V), which were found in previous studies. The structure of this compound is tetragonal with a space group of P4/ nmm and consists of the anti-fluorite type FeAs layer and perovskite-type block layer. The lattice constants are a = 3.935 Å and c = 15.952 Å for (Fe ₂As ₂)(Sr ₄MgTiO ₆). Bulk superconductivity with a T _c(onset) of ~ 26 K was observed for a partially Co-substituted sample. Moreover, Co-free and Ti-rich samples exhibited higher T _c(onset)s above 35 K, which were further enhanced by applying high pressures up to ~ 43 K.

• Open/close High current, low cost YBCO conductors—what's next?

  J L MacManus-Driscoll et al  2010 Supercond. Sci. Technol. 23 034009 Tag this article

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  The Holy Grail for high temperature superconducting conductors is achieving high current material in a simple and cost-effective way. The current status is encouraging but even after more than twenty years of intense worldwide research, there are still many new avenues to be explored. Innovative functional oxide materials science is central to future progress. This paper discusses three key areas of our research focusing on new directions: highly tailored flux pinning using the new core pinning additives R ₃TaO ₇ and RBa ₂NbO ₆ for control of nanostructure formation; pinning using magnetic phase additives such as RFeO ₃ with the potential for a magnetic contribution to the flux pinning; and the use of liquid assisted growth enabling very high growth rates leading to thick films with no critical current degradation.

• Open/close Melt-textured growth of (LRE)–Ba–Cu–O by a cold-seeding method using SmBa₂Cu₃O_y thin film as a seed

  M Oda et al  2009 Supercond. Sci. Technol. 22 075012 Tag this article

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  A cold-seeding method for fabricating LRE-based (LRE: a light rare-earth element) bulk superconductors under a reduced oxygen atmosphere is reported. We used Sm123/MgO thin films as seed crystals, and succeeded in growing c-axis oriented single grains for both LRE =  Gd and LRE =  Sm. Although a slight contamination of Mg from the seed crystal was detected, high quality bulk superconductors with diameters of 30 mm were successfully fabricated under low oxygen partial pressure. In particular, the trapped field of the Sm-based bulk superconductor was 1.52 T at liquid nitrogen temperature.