1. University of Wisconsin-Madison Applied Superconductivity Center Conductors of MgB 2 : What are the Key Issues David Larbalestier Applied Superconductivity Center Department of Materials Science and Engineering Department of Physics University of Wisconsin General references: L. D. Cooley, C. B. Eom, E. E. Hellstrom, and D. C. Larbalestier, “Potential application of Magnesium Diboride for accelerator magnet applications”, Proceedings of the 2001 Particle Accelerator Conference to appear. P Canfield and S Bud’ko “MgB 2 : one year on” Physics World January 2002, p 29-34 C. Buzea and T. Yamashita, “Review of the Superconducting Properties of MgB 2 ”, Sup. Sci and Tech. 2001

2. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 2 of 41 David Larbalestier CERN Academic Training January 17, 2002 MgB 2 : Announcement January 2001 39 K Tc - January Can be made easily in connected form - February Wires possible by PIT method - March CERN has lots of 20 K refrigeration What next? Contrast to talks 2 and 3 Nb-Ti: well understood Bi-2223: not so well understood but nature of issues clear MgB 2 : possibilities clear, true potential unclear

3. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 3 of 41 David Larbalestier CERN Academic Training January 17, 2002 Topics to be covered I.Basic properties II.Critical fields and temperatures III.Jc, connectivity and flux pinning IV.Wires and tapes V.Open questions

4. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 4 of 41 David Larbalestier CERN Academic Training January 17, 2002 I. Discovery: Akimitsu Jan. 2001 J. Nagamatsu et al. Nature 410, 63 (2001) a = 3.1432 A c = 3.5193 A r = 2.55 g/cm 3

5. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 5 of 41 David Larbalestier CERN Academic Training January 17, 2002 I. A discovery missed…….. ? 39 ? 32 Citation courtesy Paul Grant (EPRI)

6. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 6 of 41 David Larbalestier CERN Academic Training January 17, 2002 I. Basic Properties (Ames)  (Tc) = 0.4 .cm, l = 60 nm (RR) ~ 25, v F = 4.8 x 10 7 cm/sec, n = 6.7 x 10 22 e/cm 3 (2  /unit cell)  C = 79-84 mJ/mol.K  = 26 (0) = 140 nm,  (0) = 5.6 nm H c1 (0) ~ 200 mT H c2 (0) = 15-16 T Budko et al. PRL 86, 1877 (2001), Finnemore et al PRL 86, 2420 (2001), Canfield et al. PRL 86, 2423 (2001)

7. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 7 of 41 David Larbalestier CERN Academic Training January 17, 2002 I. Clean limit effects? Ames group showed low resistivity behavior already in February 2001 H c2 = 3.11  n  T c Might this mean that Hc2 would be depressed? –Nb alloyed with Ti –Nb 3 Sn alloyed with Ti, Ta Budko et al. PRL 86, 1877 (2001), Finnemore et al PRL 86, 2420 (2001), Canfield et al. PRL 86, 2423 (2001)

8. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 8 of 41 David Larbalestier CERN Academic Training January 17, 2002 I. Issues early 2001 The H-T (magnetic field - temperature) plane defines opportunities –Superconductivity is destroyed at the upper critical field Hc2 –Bulk Critical current densities disappear at a lower irreversibility field H* Critical current densities above ~10 5 A/cm 2 are really attractive –All useful superconductors attain this over a wide H-T range –BUT HTS superconductors are very sensitive to obstacles that interrupt current flow!!! All HTS conductors must be textured to minimize grain boundary effects Two present classes of material –Metallic low temperature superconductors (LTS) with low Tc (<23 K) –Ceramic high temperature superconductors (HTS) with higher Tc up to 132 K Cost/Performance ratio (metric $/kA.m) drives electric power prospects –Mg and B are both cheap and abundant (Grant – EPRI) Refrigeration possibilities –Liquid cryogens –Single stage refrigerators down to ~25K Is MgB 2 an LTS or an HTS?

9. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 9 of 41 David Larbalestier CERN Academic Training January 17, 2002 I. First UW Issue: Are grain boundaries obstacles to current? Bulk samples –Sintered from MgB 2 powder (Princeton, UW, NRIM) –Directly reacted from Mg and B (Ames, UW) Thin Films UW (Chang-Beom Eom) –Pulsed Laser Deposition at RT and subsequent annealing in Mg vapor –Sputtered

10. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 10 of 41 David Larbalestier CERN Academic Training January 17, 2002 I. “January Bulk” Samples cd MO Image LM Image SEM

11. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 11 of 41 David Larbalestier CERN Academic Training January 17, 2002 I. Nanostructure of “January Bulk samples” Many bulk samples – even today – are not dense and contain oxide phases such as MgO and B 2 O 3 which are insulating and obstruct good sintering 500 nm Amorphous C C C Song et al. SuST 15, 581 2002

12. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 12 of 41 David Larbalestier CERN Academic Training January 17, 2002 1 mm ZFC T=11.7K H=1600 OeZFC T=35K H=1600 Oe ZFC T=40.2K H=400 Oe Remnant H=0 (H=1600 Oe) Ames GC-248 10 B I. MO Flux Maps of Dense Ames Sample I. MO Flux Maps of Dense Ames Sample Polyanskii et al SuST 14, 811 (2001)

13. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 13 of 41 David Larbalestier CERN Academic Training January 17, 2002 T=40K J c =5.3x10 3 A/cm 2 Slab Geometry T=16K J c =2.2x10 5 A/cm 2 I. Flux Profiles Polyanskii et al, SuST 14, 811 2001 ZFC H=1200 Oe T=20K MgB 2 has benign grain boundaries –Does not have major flaw of HTS

14. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 14 of 41 David Larbalestier CERN Academic Training January 17, 2002 II. Critical Field Issues Bulk MgB 2 seems to be in clean limit –Low Hc2 ?? Anisotropy of crystal structure –How much Hc2 anisotropy?

15. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 15 of 41 David Larbalestier CERN Academic Training January 17, 2002 II. Linear Kramer plots common Cooley, Naus, Larbalestier Princeton bulk 4.2 to 38 K

16. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 16 of 41 David Larbalestier CERN Academic Training January 17, 2002 II. Common Bulk Properties despite big sample differences? Temperature (K) Cerac powder H c2 Cerac Powder H K Princeton H c2 Princeton H K Princeton (Mg,Al) H c2 Princeton (Mg,Al) H K Ames (Budko) H c2 Ames (Budko) H* NRIM H c2 NRIM H K 0 5 10 15 20 010203040 Field (T) H c2 (T) H*, H K (T) Larbalestier, Cooley et al. Nature 410, 186 (2001)

17. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 17 of 41 David Larbalestier CERN Academic Training January 17, 2002 II. Thin Film Growth (Eom) First films PLD (now sputtering) STO substrates, reaction layer evident T c is low, 30-34 K Resistivity can be high Films had access to oxygen during anneal – film 2 more so

18. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 18 of 41 David Larbalestier CERN Academic Training January 17, 2002 Patnaik, Cai, Cooley, Larbalestier II. Resistivity of Films

19. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 19 of 41 David Larbalestier CERN Academic Training January 17, 2002 Song, Babcock II. Nanostructure by TEM ~30% MgO Pattern (b): normal to film, rings indicate random orientation Pattern (c): small angle to film normal, uneven rings imply fiber texture

20. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 20 of 41 David Larbalestier CERN Academic Training January 17, 2002 250 nm MgB 2 Amorphous SrTiO 3 5  m MgB 2 Film SrTiO 3 Substrate normal (a) (b) 20 nm (a) c a (b) II. Nanoscale and c-axis textured!

21. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 21 of 41 David Larbalestier CERN Academic Training January 17, 2002 Cooley, Naus, Larbalestier II. Magnetization of Films Alloyed  H* increases as resistivity increases! Eom et al. Nature 411, 558 (2001)

22. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 22 of 41 David Larbalestier CERN Academic Training January 17, 2002 II. Resistive In-field Transitions Patnaik et al, SuST 14, 315 (2001)

23. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 23 of 41 David Larbalestier CERN Academic Training January 17, 2002 II. Upper Critical Fields: parallel and perpendicular Film 2 Film 3 Film 4 Patnaik et al, SuST 14, 315 (2001)

24. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 24 of 41 David Larbalestier CERN Academic Training January 17, 2002 II. Upper critical and Irreversibility Fields Patnaik et al, SuST 14, 315 (2001)

25. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 25 of 41 David Larbalestier CERN Academic Training January 17, 2002 II. Global H-T phase diagram Patnaik et al, SuST 14, 315 (2001)

26. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 26 of 41 David Larbalestier CERN Academic Training January 17, 2002 II. Critical Fields - Summary MgB 2 is anisotropic –H c2 17-18 T, and mass anisotropy 1.8-2.0 –Perpendicular H c2 is half that of parallel, and H* lies below H c2  –Texture matters! MgB 2 may be a line compound in Mg-B system, but –MgB 2 can be alloyed to MgB 2-y X (??), X = O ?? –resistivity raised by about 200x compared to clean Ames samples - clean to extreme dirty transition? –T c reduced to ~30K, H c2 (0)  doubled to 39 T

27. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 27 of 41 David Larbalestier CERN Academic Training January 17, 2002 III. Critical Current Density Issues What is flux pinning? Is low density an obstacle to high Jc? How do bulk and thin films compare?

28. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 28 of 41 David Larbalestier CERN Academic Training January 17, 2002 III. Temperature Scaling of F p for Bulk Cooley, Larbalestier   F p / F p max One dominant pinning site?

29. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 29 of 41 David Larbalestier CERN Academic Training January 17, 2002 III. Thin Film Current Density Cooley, Larbalestier 25 K 10 K 20 K15 K Film 1 4.2 K Film 3 4.2 K Film 2 4.2 K J c = 30·  m·V –1 ·r –1 Perpendicular field

30. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 30 of 41 David Larbalestier CERN Academic Training January 17, 2002 III. Kramer plots of film 2

31. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 31 of 41 David Larbalestier CERN Academic Training January 17, 2002 Cooley, Larbalestier III. Pinning Force of Film does not scale! Perpendicular field Film contains ~30 % MgO

32. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 32 of 41 David Larbalestier CERN Academic Training January 17, 2002 Cooley, Larbalestier F P max Nb47Ti (4.2K) = 18 GN/m 3, Nb 3 Sn ~ 15-30 GN/m 3 III. Pinning Force of Films

33. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 33 of 41 David Larbalestier CERN Academic Training January 17, 2002 III. Thin Film Jc values from Literature Cooley, Lee and DCL PAC 2001

34. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 34 of 41 David Larbalestier CERN Academic Training January 17, 2002 IV. Tapes and Wires See work from groups of Flukiger (GE), Goldacker (ZfK), Grasso (Genoa), Glowacki (Cambridge), Jin (AT&T) and Togano (NIMS) in particular. Cross sections of MgB2/Nb/Cu/SS wires with 33, 51, 64% steel content Goldacker ISS 2001 Cond Mat 0110231

35. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 35 of 41 David Larbalestier CERN Academic Training January 17, 2002 IV. Jc of Tapes and Wires Cooley, Lee and DCL PAC 2001

36. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 36 of 41 David Larbalestier CERN Academic Training January 17, 2002 H c2 -T plane for LTS 0 10 20 30 40 010203040 Temperature (K) Field (T) Helium Hydrogen Neon Cooling liquids

37. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 37 of 41 David Larbalestier CERN Academic Training January 17, 2002 H c2 -T plane for LTS 0 10 20 30 40 010203040 Temperature (K) Field (T) Nb- Ti Nb 3 Sn MgB 2 bulk Helium Hydrogen Neon Cooling liquids ()() Alloyed MgB 2 film (||)

38. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 38 of 41 David Larbalestier CERN Academic Training January 17, 2002 H*-T plane for LTS 0 10 20 30 40 010203040 Temperature (K) Field (T) Nb- Ti Nb 3 Sn MgB 2 bulk Helium Hydrogen Neon Cooling liquids Alloyed MgB 2 film Below the red lines is usable ()() (||)

39. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 39 of 41 David Larbalestier CERN Academic Training January 17, 2002 H*-T plane for LTS and HTS 0 10 20 30 40 0306090120 Temperature (K) Field (T) Helium Nb- Ti Nb 3 Sn MgB 2 film MgB 2 bulk Hydrogen Neon Nitrogen BSCCO YBCO Cooling liquids

40. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 40 of 41 David Larbalestier CERN Academic Training January 17, 2002 V. Summary and Open Questions MgB2 is neither LTS nor HTS Does it have niches that can make it supplementary to both? Advantages: –Round wires, high Jc and Tc twice Nb 3 Sn –Cheap raw materials –CERN has lots of 20 K cooling –Much excitement about it Disadvantages –Very hard powder not yet easy to sinter to full density –Anisotropic and rather low Hc2

41. University of Wisconsin-Madison Applied Superconductivity Center Lecture 4 MgB2 conductors Slide 41 of 41 David Larbalestier CERN Academic Training January 17, 2002 Collaborators and Acknowledgements X.Y. Cai, J, Choi, L.D. Cooley, A. Gurevich, J.Y. Jiang, M. Naus, S. Patnaik, A.A. Polyanskii, M. Rikel, X. Song, S. E. Babcock, C.B. Eom, E.E. Hellstrom, and D.C. Larbalestier, Applied Superconductivity Center, University of Wisconsin–Madison, Supported by: NSF MRSEC, AFOSR, DOE S.L. Bud’ko, C. Petrovic, G. Lapertot, C.E. Cunningham, and P.C. Canfield, Ames National Laboratory. Supported by: DOE R. J. Cava, K.A. Regan, N. Rogado, M.A. Hayward, T. He, J.S. Slusky, P. Khalifah, K. Inumaru, and M. Haas ‡ Department of Chemistry and Princeton Materials Institute, Princeton University, Princeton NJ 08544 Supported by: NSF, DOE Additional sample from Takano and Kumukura NRIM