UW-Madison MRSEC

Novel Two-gap Superconductivity In MgB₂ and Its Implications for Physics and Applications

Senior participants

From the University of Wisconsin-Madison

C. B. Eom, (ASC, MS&E, Physics), A. Gurevich (ASC), E. Hellstrom (ASC, MS&E), D. C. Larbalestier (ASC, MS&E, Physics), A. Polyanskii (ASC), M. Rzchowski (ASC, Physics), P. M. Voyles (MS&E), together with 3 students, two sabbatical visitors (Prof S. Patnaik of the J. Nehru University, Delhi India and Dr A. Matsumoto of NIMS Japan)

External Collaborators

P. Canfield and D. Finnemore (Ames Lab), I. Aranson and V. Vinokur (ANL), N. Newman, J. Rowell (Arizona State U), L. Cooley (BNL), B. Moeckly (Conductus/STI), A. Siri and Carlo Ferdeghini (U. of Genoa, Italy), V. Beilin (Hebrew U, Jerusalem Israel), R. Vaglio (INFN-Naples, Italy), J. Redwing, XX. Xi (Penn State U), and S Dou (U. of Wollongong, Australia).

Summary of Recent Work

The IRG2 thrust on MgB₂ was initiated in early 2001 just after its discovery and conducted in parallel with our grain boundary studies in the cuprate high-T_c superconductors. MgB₂ became our principal focus after discussion of the balance between the two types of study at the 2002 MRSEC Advisory Committee visit.  The unique two-gap structure of MgB₂, along with the different anisotropy of the related electronic bands, produces a novel superconductor important to basic research, as well as to technological applications. Original work within the IRG has shown that the two gaps afford a direct tunability of the superconducting properties not possible in other materials, and that the different gap structures provide a mechanism for selective charge injection. In particular, a combination of theoretical and experimental effort within IRG2 has demonstrated the route to extremely high critical fields by selective modification of scattering in the two bands. By extensive collaboration, both within the US and internationally, we have been able to make major progress on that front.  Our three Nature papers on MgB₂ have received almost 600 citations already.

Measurements of the Superconducting Gap

During 2004-2005 Mark Rzchowski and his graduate student Tom Heitmann made extensive STM studies of the superconducting gap.  They were able to make superconducting tips using the clean and dense filaments developed in Paul Canfield's group at Ames, tunneling into a variety of bulk and thin-film MgB₂ samples. They measured the 3-dimensional p-band gap and the larger 2-dimensional s-band gap in both tip and sample, but particularly exciting was the observation that the orthogonal orbital symmetry of the bands strongly modulated the tunneling depending on the relative crystallographic orientation. In orientations such that tunneling between p-bands in the tip and sample is suppressed by these matrix-element effects, no tunneling current is observed below a voltage equal to the sum of the s- and p-band superconducting gaps. This results in a junction with a much larger effective gap. These results have generated a variety of novel ideas involving preferential injection into just one of the bands, and proposals to implement these ideas in multilayer MgB₂ structures involving normal metal or ferromagnetic overlayers made by Chang-Beom Eom's group as smooth epitaxial growth of this compound improves.  In collaboration with Xiaoxing Xi at Penn State, they are also measuring the spatial dependence of the superconducting gap in carbon-doped MgB₂ epitaxial films. Preliminary results suggest that the carbon doping is not completely uniform in the films, but there is as yet no clear correlation with grain structure.

Thin Films

Research efforts around the world have demonstrated that controlled thin-film growth of MgB₂ is a challenging task. Chang-Beom Eom and his graduate students Jonathan Giencke have developed Variable Environmental Physical Vapor Deposition (VEPVD) which allows the growth of MgB₂ thin film heterostrcutures in situ. This technique combines sequential B sputtering in a differentially-pumped chamber and a reaction in a high-pressure in-situ Mg chamber in which independent control of the temperature of the sample heater and reactor.  Atomic layer controlled artificial heterostructures can be produced by depositing alternating layers of B and C-doped B.  The growth of heterostructures containing anisotropic MgB₂ layers plays a major role in both understanding the 2-gap physics of MgB₂ and unlocking its full potential. We have built a prototype reactor and installed in our existing MgB₂ sputtering chamber. MgB₂ thin films grown in tis system at 405 C shows a full transition at 22 K. We are presently implementing multiple sputter sources to permit both co-sputtering and sequential deposition of B and other alloying elements so as to manipulate the scattering.  

It showed that C-doping of MgB₂ permits H_c2(0)^{^} to rise to 40T while H_c2(0)^|| becomes so high that their measurement is currently beyond experimental capability at the National High Magnetic Field Laboratory and is extrapolated to be ~70T.  Ongoing collaborations of the PSU group with Gurevich on explaining the shape of H_c2(T) and extracting the ratio of p to s scattering rates from experimental data has continued

The upper critical field

The upper critical field (H_c2) behavior of MgB₂ continues to be a strong focus of Gurevich, Hellstrom and Larbalestier, both in thin film and bulk, theoretically and experimentally.  Our broad study of H_c2 carried out in collaboration with the groups of Xiaoxing Xi and Joan Redwing at Penn State, Nate Newman and John Rowell at Arizona State, Brian Moeckly at Conductus/STI, Carlo Ferdeghini at the University of Genoa, Italy and Ruggiero Vaglio at INFN in Naples, Italy was published during this period.  It showed that C-doping of MgB₂ permits H_c2(0)^{^} to rise to 40T while H_c2(0)^|| becomes so high that their measurement is currently beyond experimental capability at the National High Magnetic Field Laboratory and is extrapolated to be ~70T.  Ongoing collaborations of the PSU group with Gurevich on explaining the shape of H_c2(T) and extracting the ratio of p to s scattering rates from experimental data has continued, as also with Eom’s group to use four-circle x-ray diffraction system with a high resolution four-bounce monochromator and 2-D area detector to reveal structural properties of both pure MgB₂ and C-doped MgB₂ thin films.  A present powerful marker for enhanced H_c2 is an expanded c-axis lattice parameter which may be the source of the extra p scattering seen in the highest H_c2 samples.  Hellstrom and Larbalestier’s student Senkowicz has shown that C can be milled into MgB₂ and in so doing has raised H_c2(0) to ~33T.  However, like all bulk-form alloying results reported so far, this value is only about half that of the best thin films.  Two undergraduates are working with Senkowicz on various schemes for high-energy milling of MgB₂ to try to build in disorder that might enhance H_c2. 

MgB₂ – High Field Conductor of the Future

The significant potential of MgB₂ continues to attract wide interest.  It is clearly and widely appreciated that the exceptionally high H_c2 values (T_c 35-40K, H_c2^{^}(0) 40T) first reported by us show that MgB₂ is capable of replacing Nb₃Sn (T_c 18K, H_c2(0) 30T) as the present high-field superconductor of choice, since MgB₂ exceeds the critical field envelope of Nb₃Sn at all fields and temperatures and has low raw material costs.  Expanded research on superconducting materials is emphasized by the draft report of the COHMAG (Committee on High Magnetic Fields) panel of the National Research Council Board on Physics and Astronomy, a study funded by NSF to explore the future uses, needs and applications of high magnetic fields.  Applications for high field conductors of MgB₂ come both from sources wanting cheaper superconductor – for example the magnets for the International Tokamak Experimental Reactor (ITER) will use about $600 million worth of Nb₃Sn – and higher field conductors, for example the NMR community where there is widespread agreement that magnets beyond 1 GHz will not be possible with Nb₃Sn technology. General Electric has just started an internally funded program on MgB₂ for MRI use and this has stimulated Larbalestier’s group to prepare some simple multifilamentary wires suitable for magnet construction.  Amplifying this effort is the sabbatical of Akiyoshi Matsumoto, a young scientist from the National Institute of Materials Science (NIMS) in Tsukuba, Japan.  He is spending the year in Larbalestier’s and Hellstrom’s groups, trying to work out the influence of SiC alloying of MgB₂.  As noted originally by another of our collaborators, Shi Dou of the University of Wollongong Australia, SiC enhances both H_c2 and the current density of bulk form MgB₂.  In collaboration with Paul Voyles, we are hoping to understand the way that C enters the lattice of MgB₂, either when directly alloyed as by Senkowicz or by reaction with SiC.

IRG2 May 2004- April 30, 2005 Publications

Some publications may be available for download at:

http://www.asc.wisc.edu/publications/ascsearchnew.htm

Primary support by MRSEC

V. Braccini, A. Gurevich, J. E. Giencke, M. C. Jewell, C. B. Eom, D. C. Larbalestier, and et al, "High-field Superconductivity in Alloyed MgB₂ Thin Films," Physical Review B, 71: 012504 (4 pages), 2005.

S. X. Dou, V. Braccini, S. Soltanian, R. Klie, Y. Zhu, S. Li, X. L. Wang, and D. C. Larbalestier, "Nanoscale-SiC Doping for Enhancing J_c and H_c2 in the Superconducting MgB₂," Journal of Applied Physics,  96: 7549-7555. 2004.

T.W. Heitmann, M.S. Rzchowski, D.K. Finnemore, P.C. Canfield, "Effects of orbital symmetry on MgB₂ quasiparticle tunneling", submitted to Phys. Rev. Lett.

S. Patnaik, A. Gurevich, S. D. Bu, J. Choi, C. B. Eom, and D. C. Larbalestier, "Thermally-activated Current Transport in MgB₂ Films," Physical Review B, 70: 064503, 2004.

A. Polyanskii, V. Beilin, I. Felner, M. I. Tsindlekht, E. Yashchin, E. Dul'kin, E. Galystan, M. Roth, B. Senkowicz, and E. E. Hellstrom, "Magneto-optical and Electromagnetic Studies of Core Connectivity and Weak-link Behavior in Cu/MgB₂ and Ni/MgB₂ Wires and Tapes," Superconductor Science and Technology, 17: 363-370, 2004.

A. V. Pogrebnyakov, J. M. Redwing, J. E. Giencke, C. B. Eom, V. Vaithyanathan, D. G. Schlom, A. Soukiassian, S. B. Mi, C. L. Jia, J. Chen, Y. F. Hu, Y. Cui, Q. Li, and X. X. Xi, "Carbon-Doped MgB₂ Thin Films by Hybrid Physical-Chemical Vapor Deposition," accepted for IEEE Trans. On Appl. Superconductivity 15: 2005.

A. V. Pogrebnyakov, X. X. Xi, J. M. Redwing, V. Vaithyanathan, D. G. Schlom, A. Soukiassian, S. B. Mi, C. L. Jia, J. E. Giencke, C. B. Eom, J. Chen, Y. F. Hu, Y. Cui, and Q. Li, "Properties of MgB₂ Thin Films with Carbon Doping," Applied Physics Letters, 85: 2017-2019, 2004.

B. J. Senkowicz, J. E. Giencke, S. Patnaik, C. B. Eom, E. E. Hellstrom, and D. C. Larbalestier, "Improved H_c2 in Bulk-Form Magnesium Diboride by Mechanical Alloying with Carbon," submitted to Applied Physics Letters, 2004.

X. Song, V. Braccini, and D. C. Larbalestier, "Inter- and Intra-granular Nanostructure and Possible Spinodal Decomposition in Low Resistivity Bulk MgB₂ with Varying Critical Fields," Journal of Materials Research, 19(8): 2245-2255, 2004.

Secondary support by MRSEC

D. Abraimov, D. M. Feldmann, A. A. Polyanskii, A. Gurevich, G. Daniels, D. C. Larbalestier, A. P. Zhuravel, and A. V. Ustinov, "Scanning Laser Imaging of Dissipation in YBa₂Cu₃O_x Coated Conductors," Applied Physics Letters, 85(13): 2568-2570, 2004.

I. S. Aranson, A. Gurevich, M. S. Welling, R. J. Wijngaarden, V. K. Vlasko-Vlasov, V. M. Vinokur, and U. Welp, "Dendritic Flux Avalanches and Nonlocal Electrodynamics in Thin Superconducting Films," Physical Review Letters, 94: 037002 (4 pages), 2005.

D. V. Abraimov, D. M. Feldmann, A. A. Polyanskii, A. Gurevich, S. Liao, G. Daniels, D. C. Larbalestier, A. P. Zhuravel, and A. V. Ustinov, "Imaging Local Dissipation and Magnetic Field in YBCO Films with Artificial Defects," accepted for IEEE Trans. On Appl. Superconductivity 15: 2005.

A. Gurevich, "Critical Currents In Superconductors," accepted for publication in "Encylopedia of Condensed Matter Physics," Amsterdam, the Netherlands: Elsevier, 2004.

R. M. Scanlan, A. P. Malozemoff, and D. C. Larbalestier, "Superconducting Materials for Large Scale Applications," Proceedings of the IEEE, 92(10): 1639-1654, 2004.

M. Yethiraj, D. K. Christen, A. A. Gapud, D. McK. Paul, S. Crowe, C. D. Dewhurst, R. Cubitt, L. Porcar, and A. Gurevich,  “Temperature and Field Dependence of the Flux-Line Lattice Symmetry in V₃Si”, Physical Review Letters, (2004, submitted)

Facilities support by MRSEC

P. Bauer, L. Bellantoni, T. Berenc, M. C. Jewell, D. Larbalestier, P. J. Lee, A. Gurevich, A. Polyanskii, A. Squitieri, et al, "SRF Cavity and Materials R&D at Fermilab," in proceedings of LINAC 2004, Lubeck, Germany, August 16, 2004, pp. 229-231.

J. H. Choi, "Initial Stage Growth of Heteroepitaxial Oxide Thin Films on Oxide and Metal Substrates," Ph.D. Thesis, Materials Science and Engineering, University of Wisconsin - Madison, June, 2004.

L. D. Cooley, C. M. Fischer, P. J. Lee, and D. C. Larbalestier, "Simulations of the Effects of Tin Composition Gradients on the Superconducting Properties of Nb₃Sn Conductors," Journal of Applied Physics, 96(4): 2122-2130, 2004.

D. M. Feldmann, "Resolution of Two-Dimensional Currents in Superconductors from a Two-Dimensional Magnetic Field Measurement by the Method of Regularization," Physical Review B, 69: 144515 (1-14), 2004.

A. Godeke, M. C. Jewell, C. M. Fischer, A. A. Squitieri, P. J. Lee, and D. C. Larbalestier, "The Upper Critical Field of Filamentary Nb₃Sn Conductors," to appear in Journal of Applied Physics, 2005.

E. Gregory, B. A. Zeitlin, M. Tomsic, T. Pyon, M. D. Sumption, E. W. Collings, E. Barzi, D. R. Dietderich, R. M. Scanlan, A. A. Polyanskii, and P. J. Lee, "Attempts to Reduce AC Losses in High Current Density Internal-Tin Nb₃Sn," Advances in Cryogenic Engineering, 50B: 789-796, 2004.

A. Gurevich, "Thermal RF Breakdown of Superconducting Cavities," accepted for publication in proceedings of RF Superconductivity Workshop, Batavia, IL, 2004.

M. C. Jewell, A. Godeke, P. J. Lee, and D. C. Larbalestier, "The Upper Critical Field of Stoichiometric and Off-Stoichiometric Bulk, Binary Nb₃Sn," Advances in Cryogenic Engineering, 50B: 474-481, 2004.

J. Jiang, X. Y. Cai, Y. Yuan, A. A. Polyanskii, E. E. Hellstrom, D. C. Larbalestier, V. A. Maroni, T. G. Holesinger, and Y. Huang, "Long Term Anneal Study and Composition Variation for Reducing Residual Bi₂Sr₂CaCu₂O_x (2212) in (Bi,Pb)₂Sr₂Ca₂Cu₃O_x (2223) Wires," accepted for IEEE Trans. On Appl. Superconductivity 15: 2005.

P. J. Lee, L. Motowidlo, M. Rudziak, and T. Wong, "Development of Point-Array APC Nanostructures in Nb-Ti Based Superconducting Strand,"Advances in Cryogenic Engineering, 50B: 314-321, 2004.

P. J. Lee and D. C. Larbalestier, "Microstructural and Microchemical Homogeneity for High Critical Current Density in Nb₃Sn," submitted for publication in proceedings of 1st International Workshop on Progress in Nb based Superconductors, Tsukuba, Japan, February 2, 2004.

P. J. Lee and D. C. Larbalestier, "Microstructure, Microchemistry and the Development of Very High Nb₃Sn Layer Critical Current Density," accepted for IEEE Trans. On Appl. Superconductivity 15: 2005.

F. Lu and E. E. Hellstrom, "Deposition of Gd₂O₃ Buffer Layers on Ni-based Substrates by Aerosol Spray Pyrolysis," accepted for IEEE Trans. On Appl. Superconductivity 15: 2005.

O. Miura, D. Ito, P. J. Lee, and D. C. Larbalestier, "Flux Pinning Properties in Nb-Ti Composites Having Nb and Ti Mixed Artificial Pins," Advances in Cryogenic Engineering, 50B: 307-313, 2004.

L. R. Motowidlo, M. K. Rudziak, T. Wong, L. D. Cooley, and P. J. Lee, "Comparison of the Properties and Microstructure of Niobium-47 Titanium Superconductors with Magnetic and Non-magnetic Island Pinning Centers," Advances in Cryogenic Engineering, 50B: 322-329, 2004.

A. Polyanskii, P. Bauer, P. J. Lee, C. Boffo, L. Bellantoni, H. Edwards, A. Gurevich, M. Jewell, D. C. Larbalestier, G. K. Perkins, and A. Squitieri, "An Investigation of the Properties of BCP Niobium for Superconducting RF Cavities," submitted for publication in proceedings of Pushing the Limits of RF Superconductivity, Argonne National Laboratory, Argonne, IL, September 22, 2004.

B. J. Senkowicz, M. Takayasu, P. J. Lee, J. V. Minervini, and D. C. Larbalestier, "Effects of Bending on Cracking and Critical Current of Nb₃Sn ITER Wires," accepted for IEEE Trans. On Appl. Superconductivity 15: 2005.

K. Venkataraman, E. E. Hellstrom, and M. Paranthaman, "Growth of Lanthanum Manganate Buffer Layers for Coated Conductors via a Metal-Organic Decomposition Process," accepted for IEEE Trans. On Appl. Superconductivity 15: 2005.

Y. Yuan, "Advancing Materials Processing, Eliminating Current Limiting Mechanism, and Enhancing Critical Current Density of Ag-Sheathed (Bi,Pb)₂Sr₂Ca₂Cu₃O_x Composite Conductors," Ph.D. Thesis, Materials Science and Engineering, University of Wisconsin - Madison, June, 2004.

Y. Yuan, X. Y. Cai, J. Jiang, Y. Huang, D. C. Larbalestier, and E. E. Hellstrom, "The Origin of the Pb-Rich Bi₃Sr₂Ca₂CuO_x (3221) Phase During Post Annealing of (Bi,Pb)₂Sr₂Ca₂Cu₃O_x,Ag Composite Conductors," accepted for IEEE Trans. On Appl. Superconductivity 15: 2005.

B. A. Zeitlin, E. Gregory, T. Pyon, R. M. Scanlan, A. A. Polyanskii, and P. J. Lee, "Progress on the Use of Internal Fins as Barriers to Reduce Magnetization in High Current Density Mono-Element Tin Conductors (MEIT)," Advances in Cryogenic Engineering, 50B: 417-424, 2004.

D. M. Feldmann, D. C. Larbalestier, T. Holesinger, R. Feenstra, A. A. Gapud, and E. D. Specht, "Evidence for Extensive Grain Boundary Meander and Overgrowth of Substrate Grain Boundaries in High Critical Current Density ex situ YBa₂Cu₃O_x Coated Conductors," submitted to Journal of Materials Research, 2005.