mgb 2 thin film and its application to rf cavities xiaoxing xi department of physics and department...
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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, 2007Workshop on SRF Materials
Batavia, IL
Supported by ONR, NSF, PRF
MgB2: A Two-Band Superconductor— 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
39.5 40.0 40.5 41.0 41.50.00
0.05
0.10
(
cm)
T (K)
B
Mg
Structure R-T of a MgB2 Film
π gap
σ gap
Fermi Surface
Energy GapsT-Dependence of Gaps
Potential Low BCS Rs for RF Cavity
BCS Rs for MgB2 presented in the same coordinates as in the figure.
Pickett, Nature 418, 733 (2002)
Rs from π Gap Rs from σ Gap
Nb
T = 4.2 K, f = 0.5 GHz
Nb3Sn
Vaglio, Particle Accelerators 61, 391 (1998)
Rs (BCS) versus (ρ0, Tc)
Progresses in Applications of MgB2
— 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
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
0 10 20 30 400
10
20
30
40
50
60
NbTi Nb3Sn
MgB2
Fie
ld (
T)
Temperature (K)
MgB2
//
-1.0
-0.5
0.0
0.5
1.0
-0.4 -0.2 0.0 0.2 0.4
-2 dBm
-9 dBm
V (mV)
I (m
A)
no RF
MgB2/TiB
2
planar junctionT = 28 KRF f = 29.5 GHz
15/11/2006
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 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
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
EpitaxialFilms
B, Mg
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
Hybrid Physical-Chemical Vapor Deposition
get rid of oxygenprevent oxidation
make high Mgpressure possible
generate high Mg pressure: required by thermodynamics
pure source of B
B supply (B2H6 flow rate) controls growth rate
Pure source of Mghigh enough T
for epitaxy
Schematic View
Substrate
H2 (~100 Torr)B2H6 (~ 5 - 250 sccm)
Mg
Susceptor
550–760 °C
Very Clean HPCVD MgB2 Films: RRR > 80
0 50 100 150 200 250 3000
2
4
6
8
39.5 40.0 40.5 41.0 41.50.00
0.05
0.10
(cm
)
T (K)
Res
istiv
ity (
cm)
Temperature (K)
053105aMgB
2/sapphire
Thickness 770 nm
Mean free length is limited by the film thickness.
0.0 5.0x10-4 1.0x10-30.0
0.5
1.0
1.5
Thickness (Å)4000 1000
(
cm
)
1/Thickness (1/Å)
2000
Xi et al, Physica C 456, 22 (2007)
0 5 10 15 20 25 30 35 40
104
105
106
107
108
Pure MgB2/6H-SiC
4
3
2
1
0.5
0.20.1
00.05
H(T)
Temperature (K)
J c (A
/cm
2 )
0 10 20 30 400
5
10
15
20
Hc2
(T)
T (K)
H // ab H // c
Clean MgB2: Weak Pinning and Low Hc2
Jc (0 K) ~3.5 x 107 A/cm2
Hc2(0) = 0/2πab(0)2
ab(0) ≈ 7 nm
Jin et al, SC Sci. Tech. 18, L1 (2005)
Low Rs and Short λ in Clean Films
Surface Resistance @ 18 GHz Penetration 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
Hc = √2Hc2/ ≈ 1.65 T
Hsh ≈ 0.75 Hc ≈ 1.24 T
Dahm & Scalapino, APL 85, 4436 (2004)
Effects of Two Gaps on Microwave NonlinearityNonlinear Coefficient of MgB2
YBCO, MgB2, & 40-K BCS SCMgB2 of Different Intraband
Scattering
— 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.
Microwave Nonlinearity of HPCVD MgB2 Films
theoretical d wave
theoretical one-band s wave
theoretical two-band s waveГπ/Гσ=2
YBCONb
MgB2
Cifariello et al, APL 88, 142510 (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, 147006 (2004)
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)
ρ
Granularity: Rowell Model of Connectivity
0
0A
A
— 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
0 50 100 150 200 250 3000
2
4
6
8
R
esis
tivity
(
cm)
Temperature (K)
Bu et al., APL 81, 1851 (2002)
High-T Annealed Film
HPCVD Film
0
2
4
6
8
10
0 50 100 150 200 250 300
M03044a
Resistivity
Res
istiv
ity (
c
m)
Temperature (K)
MgB2 on polycrystalline aluminaREC 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 N2 added in the deposition atmosphere
Pure MgB2
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))
0 50 100 150 200 250 3000.000
0.002
0.004
0.006
0.008
36 37 38 39 40 410.000
0.002
0.004
R ()
T (K)
Re
sist
an
ce (
Oh
ms)
Temperature (K)
MgB2/Stainless Steel
0 50 100 150 200 250 3000.0
0.5
1.0
1.5
2.0
36 37 38 39 40 410.00
0.05
0.10
0.15
R (
x 1
04
)
T (K)
Res
ista
nce
( x
104
)
Temperature (K)
MgB2/Nb
HPCVD MgB2 Films on Metal Substrates
High Tc has been obtained in polycrystalline MgB2 films on stainless steel, Nb, TiN, and other substrates.
Polycrystalline MgB2 Films on Flexible YSZ
Low Rs similar to epitaxial films on sapphire substrate.
Rs measured by A. Findikoglu (LANL)
Integrated HPCVD System
CVD #1
CVD #2
Sputtering
TransferChamber
System capable of depositing multilayers consisting of MgB2 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 °Cin Mg Vapor
Epitaxial Films
Kang et al, Science 292, 1521 (2001)Bu et al, APL 81, 1851 (2002)
Previous MgB2 Films by High-T Ex-Situ Annealing
MgB2 Film by Reaction of CVD B Film
Clean B precursor layer leads to clean MgB2 film.
0 50 100 150 200 250 3000
2
4
6
8
10
38 39 40 41 420.0
0.5
1.0
1.5
Tc
onset=41.0 K
Tc
zero =40.6 K
RRR=7.8
cm
)
Temperature (K)
cm
)
Tc
onset=41.0 K
Tc
zero =40.6 K
RRR=7.8
cm
)
T (K)
Coating SRF Cavity with a Two-Step Process
Coating cavity with B layer at ~400-500°C using CVD
Reacting with Mg to form MgB2 at ~ 850-900 °C in Mg vapor
H2, B2H6 Mg vapor
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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