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.