Mg B 2 Thin Film and Its Application

Mg. B 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

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

Potential Low BCS Rs for RF Cavity Rs (BCS) versus (ρ0, Tc) T = 4. 2 K, f = 0. 5 GHz Nb Nb 3 Sn Pickett, Nature 418, 733 (2002) Vaglio, Particle Accelerators 61, 391 (1998) Rs from π Gap BCS Rs for Mg. B 2 presented in the same coordinates as in the figure. Rs from σ Gap

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

First Mg. B 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 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

High- and Intermediate-Temperature In-Situ Deposition B, Mg High and Intermediate Temperature Epitaxial Films Mg pressure where Mg. B 2 is thermodynamically stable phase, is very high: Zeng et al. , Nature materials 1, 35 (2002) Schneider et al. , APL 85, 5290 (2004) Moeckly & Ruby, SC Sci Tech 19, L 21 (2006) For example, for 600°C, 0. 9 m. Torr Mg vapor pressure, or Mg flux of 500 Å/s is required

Hybrid Physical-Chemical Vapor Deposition Schematic View get rid of oxygen prevent oxidation make high Mg pressure possible H 2 (~100 Torr) B 2 H 6 (~ 5 - 250 sccm) pure source of B generate high Mg pressure: required by thermodynamics B supply (B 2 H 6 flow rate) controls growth rate Mg Substrate 550– 760 °C Pure source of Mg high enough T for epitaxy Susceptor

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

Clean Mg. B 2: Weak Pinning and Low Hc 2(0) = F 0/2πxab(0)2 Jc (0 K) ~3. 5 x 107 A/cm 2 xab(0) ≈ 7 nm

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 Surface resistance and penetration depth decrease with residual resistivity. Clean HPCVD films show low surface resistance and short penetration depth. Hc = √ 2 Hc 2/k ≈ 1. 65 T Hsh ≈ 0. 75 Hc ≈ 1. 24 T Jin et al, SC Sci. Tech. 18, L 1 (2005)

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

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

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)

Coalescence of Islands in Mg. B 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)

Granularity: Rowell Model of Connectivity Rowell, SC Sci. Tech. 16, R 17 (2003) HPCVD Film — Residual resistivity: impurity, surface, and defects — Δρ ≡ ρ(300 K) - ρ(50 K): electron-phone coupling, roughly 8 μΩcm ρ — If Δρ is larger : actual area A’ smaller than total area A Δρ ~ 8 μΩcm grains well connected High-T Annealed Film REC Film Bu et al. , APL 81, 1851 (2002)

Smooth Surface of HPCVD Films Pure Mg. B 2 RMS Roughness = 3. 64 nm Small amount of N 2 added in the deposition atmosphere RMS Roughness = 0. 96 nm

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

HPCVD Mg. B 2 Films on Metal Substrates High Tc has been obtained in polycrystalline Mg. B 2 films on stainless steel, Nb, Ti. N, and other substrates.

Polycrystalline Mg. B 2 Films on Flexible YSZ Rs measured by A. Findikoglu (LANL) Low Rs similar to epitaxial films on sapphire substrate.

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

High-Temperature Ex-Situ Annealing B Mg 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 C 377, 407 (2001) Low Temperature ~ 850 °C in Mg Vapor Epitaxial Films

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

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

Coating SRF Cavity with a Two-Step Process H 2 , B 2 H 6 Coating cavity with B layer at ~400 -500°C using CVD Mg vapor Reacting with Mg to form Mg. B 2 at ~ 850900 °C in Mg vapor

Conclusion ― High Tc and low resistivity in clean Mg. B 2 films promise low BCS Rs ―Clean HPCVD Mg. B 2 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 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 ― Mg. B 2 films prepared by reacting CVD boron films with Mg vapor show good properties. Technique compatible to coating of cavities.