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MgB 2 Thin Film and Its Application to RF Cavities

MgB 2 Thin Film and Its Application to RF Cavities. Xiaoxing Xi Department of Physics and Department of Materials Science and Engineering Penn State University, University Park, PA. May 24, 2007 Workshop on SRF Materials Batavia, IL. Supported by ONR, NSF, PRF . B. Mg. π gap.

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MgB 2 Thin Film and Its Application to RF Cavities

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  1. MgB2 Thin Film and Its Application to RF Cavities Xiaoxing Xi Department of Physics and Department of Materials Science and Engineering Penn State University, University Park, PA May 24, 2007 Workshop on SRF Materials Batavia, IL Supported by ONR, NSF, PRF

  2. B Mg π gap σ gap MgB2: A Two-Band Superconductor Structure • Tc = 40 K • Low normal-state resistivity • A BCS Superconductor • Two bands with weak interband scattering: σ band (2D) and π band (3D) • Two gaps with weak but finite interband coupling R-T of a MgB2 Film Fermi Surface T-Dependence of Gaps Energy Gaps

  3. T = 4.2 K, f = 0.5 GHz Nb Nb3Sn Rsfromπ Gap Rsfromσ Gap Potential Low BCS Rs for RF Cavity Rs (BCS) versus (ρ0, Tc) Pickett, Nature 418, 733 (2002) Vaglio, Particle Accelerators 61, 391 (1998) BCS Rs for MgB2 presented in the same coordinates as in the figure.

  4. // ^ Progresses in Applications of MgB2 • ELECTRONICS • No reproducible, uniform HTS Josephson junctions yet, may be easier for MgB2 • 25 K operation, much less cryogenic requirement than LTS Josephson junctions • Superconducting digital circuits HIGH FIELD • High performance in field (Hc2 over 60 T) • Low material cost, easy manufacturing • High field magnets for NMR/MRI; high-energy physics, fusion, MAGLEV, motors, generators, transformers

  5. First MgB2 MRI System On November 23, 2006, ASG Superconductors, Paramed Medical Systems and Columbus Superconductors announce the successful operation of MR-Open, their first MRI system based on the new Magnesium Diboride superconductor MR-Open at the Radiological Society of the North America Convention in November 2006 First brain image acquired by Paramed Medical Systems on the MR-Open system

  6. High- and Intermediate-Temperature In-Situ Deposition B, Mg High and Intermediate Temperature Epitaxial Films Mg pressure where MgB2 is the thermodynamically stable phase, is very high: For example, for 600°C, 0.9 mTorr Mg vapor pressure, or Mg flux of 500 Å/s is required Zeng et al., Nature materials 1, 35 (2002) Schneider et al., APL 85, 5290 (2004) Moeckly & Ruby, SC Sci Tech 19, L21 (2006)

  7. Schematic View get rid of oxygen prevent oxidation make high Mg pressure possible H2 (~100 Torr) B2H6 (~ 5 - 250 sccm) pure source of B B supply (B2H6 flow rate) controls growth rate generate high Mg pressure: required by thermodynamics Mg Substrate 550–760 °C Pure source of Mg high enough T for epitaxy Susceptor Hybrid Physical-Chemical Vapor Deposition

  8. Very Clean HPCVD MgB2 Films: RRR > 80 Mean free length is limited by the film thickness. Xi et al, Physica C 456, 22 (2007)

  9. Clean MgB2: Weak Pinning and Low Hc2 Hc2(0) = F0/2πxab(0)2 xab(0) ≈ 7 nm Jc (0 K) ~3.5 x 107 A/cm2

  10. Low Rs and Short λ in Clean Films Microwave measurement: sapphire resonator technique at 18 GHz. Surface Resistance @ 18 GHz Penetration Depth k = l/x ≈ 6 Hc = √2Hc2/k ≈ 1.65 T Hsh ≈ 0.75 Hc ≈ 1.24 T Surface resistance and penetration depth decrease with residual resistivity. Clean HPCVD films show low surface resistance and short penetration depth. Jin et al, SC Sci. Tech. 18, L1 (2005)

  11. Effects of Two Gaps on Microwave Nonlinearity Nonlinear Coefficient of MgB2 • It has been predicted theoretically that • nonlinearity in MgB2 is large due to existence of two bands. • compares favorably with HTS at low temperature • Manipulation of interband and intraband scattering could improve nonlinearity. MgB2 of Different Intraband Scattering YBCO, MgB2, & 40-K BCS SC Dahm & Scalapino, APL 85, 4436 (2004)

  12. theoretical one-band s wave theoretical d wave MgB2 theoretical two-band s wave Гπ/Гσ=2 Nb YBCO Microwave Nonlinearity of HPCVD MgB2 Films • Result in agreement with Dahm – Scalapino prediction. • Modification of interband and intraband scattering key to low nonlinearity. Cifariello et al, APL 88, 142510 (2006)

  13. Defects in Epitaxial HPCVD Films Low-Magnification TEM High-Resolution TEM There are more defects at the film/substrate interface than in the top part of the film. Pogrebnyakov et al.PRL 93, 147006 (2004)

  14. Coalescence of Islands in MgB2 Films • Small islands grow together, giving rise to larger ones and a flat surface for further growth. • The boundaries between islands are clean. Wu et al.APL 85, 1155 (2004)

  15. Granularity: Rowell Model of Connectivity ρ REC Film Rowell, SC Sci. Tech. 16, R17 (2003) HPCVD Film • Residual resistivity: impurity, surface, and defects • Δρ≡ρ(300K) - ρ(50K): electron-phone coupling, roughly8 μΩcm • If Δρ is larger : actual area A’ smaller than total area A Δρ ~ 8 μΩcmgrains well connected High-T Annealed Film Bu et al., APL 81, 1851 (2002)

  16. Smooth Surface of HPCVD Films Small amount of N2 added in the deposition atmosphere Pure MgB2 RMS Roughness =3.64 nm RMS Roughness =0.96 nm

  17. Absence of Dendritic Magnetic Instability in Clean HPCVD Films Flux Entry Remnant State (Ye et al.APL 85, 5285 (2004))

  18. HPCVD MgB2 Films on Metal Substrates High Tc has been obtained in polycrystalline MgB2 films on stainless steel, Nb, TiN, and other substrates.

  19. Polycrystalline MgB2 Films on Flexible YSZ Rs measured by A. Findikoglu (LANL) Low Rs similar to epitaxial films on sapphire substrate.

  20. Integrated HPCVD System CVD #2 Transfer Chamber Sputtering CVD #1 System capable of depositing multilayers consisting of MgB2 and other materials.

  21. High-Temperature Ex-Situ Annealing B Low Temperature Mg ~ 850 °C in Mg Vapor Kang et al, Science 292, 1521 (2001) Eom et al, Nature 411, 558 (2001) Ferdeghini et al, SST 15, 952 (2001) Berenov et al, APL 79, 4001 (2001) Vaglio et al, SST 15, 1236 (2001) Moon et al, APL 79, 2429 (2001) Fu et al, Physica C377, 407 (2001) Epitaxial Films

  22. Previous MgB2 Films by High-TEx-Situ Annealing Bu et al, APL 81, 1851 (2002) Kang et al, Science 292, 1521 (2001)

  23. MgB2 Film by Reaction of CVD B Film Clean B precursor layer leads to clean MgB2 film.

  24. Coating SRF Cavity with a Two-Step Process H2, B2H6 Mg vapor Coating cavity with B layer at ~400-500°C using CVD Reacting with Mg to form MgB2 at ~ 850-900 °C in Mg vapor

  25. Conclusion • High Tc and low resistivity in clean MgB2 films promise low BCS Rs • Clean HPCVD MgB2 thin films have excellent properties: • low resistivity (<0.1 μΩ) and long mean free path • high Tc ~ 42 K (due to tensile strain), high Jc(10% depairing current) • low surface resistance, short penetration depth • smooth surface (RMS roughness < 10 Å with N2 addition) • well connected grains and clean grain boundaries • good thermal conductivity (free from dendritic magnetic instability) • Nonlinearity properties can be tuned by changing scattering in the two bands, e.g. by carbon doping • Films on some metallic substrates, polycrystalline films maintain good properties • The new integrated HPCVD system offers multilayer capability • MgB2 films prepared by reacting CVD boron films with Mg vapor show good properties. Technique compatible to coating of cavities.

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