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

MgB 2 : A Two-Band Superconductor — T c = 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 B Mg StructureR-T of a MgB 2 Film π gap σ gap Fermi Surface Energy Gaps T-Dependence of Gaps

Potential Low BCS R s for RF Cavity BCS R s for MgB 2 presented in the same coordinates as in the figure. Pickett, Nature 418, 733 (2002) R s from π GapR s from σ Gap Nb T = 4.2 K, f = 0.5 GHz Nb 3 Sn Vaglio, Particle Accelerators 61, 391 (1998) R s (BCS) versus (ρ 0, T c )

Progresses in Applications of MgB 2 — High performance in field (H c2 over 60 T) — Low material cost, easy manufacturing — High field magnets for NMR/MRI; high- energy physics, fusion, MAGLEV, motors, generators, transformers ELECTRONICS — No reproducible, uniform HTS Josephson junctions yet, may be easier for MgB 2 — 25 K operation, much less cryogenic requirement than LTS Josephson junctions — Superconducting digital circuits HIGH FIELD  //

First brain image acquired by Paramed Medical Systems on the MR-Open system MR-Open at the Radiological Society of the North America Convention in November 2006 First MgB 2 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

High- and Intermediate-Temperature In-Situ Deposition Zeng et al., Nature materials 1, 35 (2002) Schneider et al., APL 85, 5290 (2004) Moeckly & Ruby, SC Sci Tech 19, L21 (2006) High and Intermediate Temperature Epitaxial Films B, Mg Mg pressure where MgB 2 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

Hybrid Physical-Chemical Vapor Deposition get rid of oxygen prevent oxidation make high Mg pressure possible generate high Mg pressure: required by thermodynamics pure source of B B supply (B 2 H 6 flow rate) controls growth rate Pure source of Mg high enough T for epitaxy Schematic View Substrate H 2 (~100 Torr) B 2 H 6 (~ sccm) Mg Susceptor 550–760 °C

Very Clean HPCVD MgB 2 Films: RRR > 80 Mean free length is limited by the film thickness. Xi et al, Physica C 456, 22 (2007)

Clean MgB 2 : Weak Pinning and Low H c2 J c (0 K) ~3.5 x 10 7 A/cm 2 H c2 (0) =  0 /2π  ab (0) 2  ab (0) ≈ 7 nm

Jin et al, SC Sci. Tech. 18, L1 (2005) Low R s and Short λ in Clean Films Surface 18 GHzPenetration Depth Surface resistance and penetration depth decrease with residual resistivity. Clean HPCVD films show low surface resistance and short penetration depth. Microwave measurement: sapphire resonator technique at 18 GHz.  = /  ≈ 6 H c = √2H c2 /  ≈ 1.65 T H sh ≈ 0.75 H c ≈ 1.24 T

Dahm & Scalapino, APL 85, 4436 (2004) Effects of Two Gaps on Microwave Nonlinearity Nonlinear Coefficient of MgB 2 YBCO, MgB 2, & 40-K BCS SC MgB 2 of Different Intraband Scattering — It has been predicted theoretically that nonlinearity in MgB 2 is large due to existence of two bands. compares favorably with HTS at low temperature — Manipulation of interband and intraband scattering could improve nonlinearity.

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

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

Coalescence of Islands in MgB 2 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)

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

Smooth Surface of HPCVD Films RMS Roughness = 3.64 nm Small amount of N 2 added in the deposition atmosphere Pure MgB 2 RMS Roughness = 0.96 nm

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

HPCVD MgB 2 Films on Metal Substrates High T c has been obtained in polycrystalline MgB 2 films on stainless steel, Nb, TiN, and other substrates.

Polycrystalline MgB 2 Films on Flexible YSZ Low R s similar to epitaxial films on sapphire substrate. R s measured by A. Findikoglu (LANL)

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

High-Temperature Ex-Situ Annealing 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) B Mg Low Temperature ~ 850 °C in Mg Vapor Epitaxial Films

Kang et al, Science 292, 1521 (2001) Bu et al, APL 81, 1851 (2002) Previous MgB 2 Films by High-T Ex-Situ Annealing

MgB 2 Film by Reaction of CVD B Film Clean B precursor layer leads to clean MgB 2 film.

Coating SRF Cavity with a Two-Step Process Coating cavity with B layer at ~ °C using CVD Reacting with Mg to form MgB 2 at ~ °C in Mg vapor H 2, B 2 H 6 Mg vapor

Conclusion ― High T c and low resistivity in clean MgB 2 films promise low BCS R s ―Clean HPCVD MgB 2 thin films have excellent properties:  low resistivity (<0.1 μΩ) and long mean free path  high T c ~ 42 K (due to tensile strain), high J c (10% depairing current)  low surface resistance, short penetration depth  smooth surface (RMS roughness < 10 Å with N 2 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 ― MgB 2 films prepared by reacting CVD boron films with Mg vapor show good properties. Technique compatible to coating of cavities.