MgB2 Thin Film and Its Application to RF Cavities

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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
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MgB2 A Two-Band Superconductor

• Tc 40 K
• Low normal-state resistivity
• A BCS Superconductor
• Two bands with weak interband scattering s band
  (2D) and p band (3D)
• Two gaps with weak but finite interband coupling

Structure
R-T of a MgB2 Film
Fermi Surface
T-Dependence of Gaps
Energy Gaps
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Potential Low BCS Rs for RF Cavity
Rs (BCS) versus (?0, 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.
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Progresses in Applications of MgB2

• High performance in field (Hc2 over 60 T)
• Low material cost, easy manufacturing

• ELECTRONICS
• No reproducible, uniform HTS Josephson junctions
  yet, may be easier for MgB2
• 25 K operation, much less cryogenic requirement
  than LTS Josephson junctions

HIGH FIELD
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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
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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 600C, 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)
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Hybrid Physical-Chemical Vapor Deposition
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Very Clean HPCVD MgB2 Films RRR gt 80
Mean free length is limited by the film thickness.
Xi et al, Physica C 456, 22 (2007)
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Clean MgB2 Weak Pinning and Low Hc2
Hc2(0) F0/2pxab(0)2 xab(0) 7 nm
Jc (0 K) 3.5 x 107 A/cm2
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Low Rs and Short ? in Clean Films
Microwave measurement sapphire resonator
technique at 18 GHz.
Surface Resistance _at_ 18 GHz
Penetration Depth
k l/x 6 Hc v2Hc2/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)
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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)
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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)
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Defects in Epitaxial HPCVD Films
High-Resolution TEM
Low-Magnification 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)
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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)
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Granularity Rowell Model of Connectivity
Rowell, SC Sci. Tech. 16, R17 (2003)
HPCVD Film

• Residual resistivity impurity, surface, and
  defects
• ?? ?(300K) - ?(50K) electron-phone coupling,
  roughly 8 µOcm
• If ?? is larger actual area A smaller than
  total area A

?? 8 µOcm ? grains well connected
High-T Annealed Film
Bu et al., APL 81, 1851 (2002)
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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
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Absence of Dendritic Magnetic Instability in
Clean HPCVD Films
Flux Entry
Remnant State
(Ye et al. APL 85, 5285 (2004))
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HPCVD MgB2 Films on Metal Substrates
High Tc has been obtained in polycrystalline MgB2
films on stainless steel, Nb, TiN, and other
substrates.
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Polycrystalline MgB2 Films on Flexible YSZ
Rs measured by A. Findikoglu (LANL)
Low Rs similar to epitaxial films on sapphire
substrate.
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Integrated HPCVD System
CVD 2
Transfer Chamber
Sputtering
CVD 1
System capable of depositing multilayers
consisting of MgB2 and other materials.
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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
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Previous MgB2 Films by High-T Ex-Situ Annealing
Bu et al, APL 81, 1851 (2002)
Kang et al, Science 292, 1521 (2001)
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MgB2 Film by Reaction of CVD B Film
Clean B precursor layer leads to clean MgB2 film.
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Coating SRF Cavity with a Two-Step Process
Mg vapor
Coating cavity with B layer at 400-500C using
CVD
Reacting with Mg to form MgB2 at 850-900 C in
Mg vapor
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Conclusion

• High Tc and low resistivity in clean MgB2 films
  promise low BCS Rs
• Clean HPCVD MgB2 thin films have excellent
  properties
• low resistivity (lt0.1 µO) 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 lt 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.