Muon spin rotation (muSR) and point contact tunneling (PCT) are used since several years for bulk niobium studies. Here we present studies on niobium thin film samples of different deposition techniques (diode, magnetron and HIPIMS) and compare the results with RF measurements and bulk niobium results. It is consistently found from muSR and RF measurements that HIPIMS can be used to produce thin films of high RRR. Hints for magnetic impurities are found on HIPIMS and dcms samples. The Meissner effect is linear on all tested samples.

1. Low energy muon spin rotation and point contact tunneling on niobium thin films The sixth international workshop on thin films and new ideas for RF superconductivity
Tobias.Junginger@cern.ch 1
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Sample preparation: G. Terenziani & S. Calatroni (CERN)
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PCT: T. Proslier (ANL) & J. Zasadzinzki (Illinois Institute of Technology, Chicago)
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LEmuSR: A. Suter (PSI) 06/10/2014

2. Motivation The sixth international workshop on thin films and new ideas for RF superconductivity
Tobias.Junginger@cern.ch 2
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Niobium on copper is the material of choice for many accelerator cavities
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Despite many advantages compared to bulk niobium the technology is limited by a stronger Q-slope. Origin and correlation to the surface properties is not yet understood
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Low energy muon spin rotation (LEmuSR) and point contact tunnelling (PCT) have been used for bulk niobium studies
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Our aim is to use these techniques on niobium on copper films in order to find out, whether these techniques can reveal differences compared to bulk niobium 06/10/2014

3. Content The sixth international workshop on thin films and new ideas for RF superconductivity
Tobias.Junginger@cern.ch 3
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Point Contact Tunnelling (PCT)
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Review of measurements on bulk niobium
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Results on niobium films
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Low Energy Muon Spin Rotation (LEmuSR)
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Review of measurements on bulk niobium
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Results on niobium films
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Conclusions 06/10/2014

4. PCT on clean niobium 4
E.L Wolf - Principles of Electron Tunneling Spectroscopy - Oxford Scholarship Online 2012 06/10/2014
The sixth international workshop on thin films and new ideas for RF superconductivity
Tobias.Junginger@cern.ch

5. PCT on bulk niobium - Low temperature baking effect 5
unbaked
125°C baked (24h)
Observations:
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Zero-bias conductance
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Broadened DOS
The low temperature baking gives
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Reduced zero bias conductance
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Less broadened DOS
T. Proslier et al. – Tunneling study of cavity grade Nb: Possible magnetic scattering at the surface – APL 2008 06/10/2014
The sixth international workshop on thin films and new ideas for RF superconductivity
Tobias.Junginger@cern.ch

6. PCT on bulk niobium – High temperature baking effect 6
Blue: 1000°C Baking Red: 1400°C Baking
P. Dhakal et al. - Effect of high temperature heat treatments on the quality factor of a large-grain superconducting radio-frequency niobium cavity – PRSTAB 2013
Higher baking temperature (better RF performance) shows:
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less low energy gaps
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smaller Γ values (sharper distributions)

7. 
Few zero bias peaks
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Very few small gaps measured (<1 meV)
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Sharpest Distributions
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Very low Gamma/Delta values (< 10%)
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Homogeneous sample
PCT on niobium on copper (dcms)

8. 
Zero bias peaks -> mag impurities
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Small gaps measured (~1.3 meV)
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Distributions fairly sharp
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Good Gamma/Delta values (< 10%)
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Homogeneous sample
PCT on niobium on copper (HIPIMS)
Zero Bias Peaks
For RF measurements see Giovanni’s talk

9. 
No Zero bias peaks
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Small gaps measured (<1.3 meV)
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Widest Distributions
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High Gamma/Delta values
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Inhomogeneous sample
PCT on niobium on copper (strongly oxidized dcms)
For RF measurements see T. Junginger, PhD thesis University of Heidelberg (2012)

10. Point Contact Tunnelling – Conclusion 10
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Investigated by PCT Nb on Cu samples can look as good as the best performing bulk niobium samples
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HIPIMS sample showed zero bias peaks -> Magn. Impurities?
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A sample with a huge residual resistance showed small gaps, a wide distribution and high Gamma/Delta values but no zero bias peaks
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There is a correlation to the low field residual resistance but the stronger Q-slope of the Nb on Cu technology cannot be explained by the DC properties measured with PCT
RF≠DC 06/10/2014
The sixth international workshop on thin films and new ideas for RF superconductivity
Tobias.Junginger@cern.ch

11. LEmuSR on bulk niobium 06/10/2014 11
The sixth international workshop on thin films and new ideas for RF superconductivity
Tobias.Junginger@cern.ch
A. Romanenko et al. - Strong Meissner screening change in superconducting radio frequency cavities due to mild baking – APL 2014
Local (London)
Non-local (Pippard)
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Non exponential penetration
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Taking non-locality into account exponential fit possible
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Cold region of 120°C baked sample shows a strong change in Meissner screening with depth

12. LEmuSR on niobium on copper 06/10/2014
The sixth international workshop on thin films and new ideas for RF superconductivity
Tobias.Junginger@cern.ch 12
λ=38.8 nm
λ=21.5 nm
λ=26.4 nm
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Penetration depth almost twice as deep in dcms compared to HIPIMS
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Local London theory applicable to Nb films
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No change in Meissner screening with depth

13. LEmuSR on niobium on copper
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Zero field spectra of HIPIMS sample shows clear signs of magnetization at the surface and at 105 nm depth
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For dcMS sample weaker signs are found at the surface and no signs at 105 nm depth
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Further experiments are necessary to rule out trapped flux and muon diffusion

14. Conclusions The sixth international workshop on thin films and new ideas for
RF superconductivity Tobias.Junginger@cern.ch 14
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With PCT we find zero bias peaks and muSR gives direct evidence of magnetization of the HIPIMS sample (Magn. Impurities?)
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No change in Meissner screening with depth
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Higher RRR does not necessarily result in a less pronounced Q-slope (see also talks of Sarah and Giovanni) 06/10/2014

15. Backup Slides 06/10/2014
The sixth international workshop on thin
films and new ideas for RF
superconductivity
Tobias.Junginger@cern.ch
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16. Muon-spin rotation (μSR) technique
Sμ(0)
Bμ = (2π/γμ) νμ

17. Depth dependent μSR measurements
λB(z) zBext0superconductor
B(z)=Bextexp(−z/λ)
Jackson et al., Phys. Rev. Lett. 84, 4958 (2000)

18. More precise: use known implantation profile
Jackson et al., Phys. Rev. Lett. 84, 4958 (2000)

19. Standard DCMS
Temp dependence

20. HIPIMS 86 Hz
Statistic
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Zero bias peaks -> mag impurities
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Small gaps measured (<1.3 meV)
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Broadest Distributions
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Large Gamma/Delta values (> 10%)
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Inhomogeneous sample

21. HIPIMS 86 Hz
Temp dependence

22. HIE e9
Statistic
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Few Zero bias peaks –
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Small gaps measured (<1.3 meV), broad distribution
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Distributions fairly sharp in gamma
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Good Gamma/Delta values (< 10%) with few “hot spots”.

23. HIE e9
Temp dependence

24. HIE i9
Statistic
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Small gaps measured (<1 meV), broad distribution
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Distributions fairly sharp in gamma
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Good Gamma/Delta values (< 10%) with few “hot spots”.

25. HIE i9
Temp dependence

26. 06/10/2014
The sixth international workshop on thin
films and new ideas for RF
superconductivity
Tobias.Junginger@cern.ch
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27. 06/10/2014
The sixth international workshop on thin
films and new ideas for RF
superconductivity
Tobias.Junginger@cern.ch
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M.2.7
M.2.3

28. 06/10/2014 28
Tested
Substrate
Surface Treatment
Coating
RBCS [nΩ]
Rres [nΩ]
RRR
Δ/kB [K]
Remarks
4/12
M1.1
SUBU
HIPIMS 140 A
400 (530)
310
3.7
17.8
Peel off already at this test?
9/12
M2.1
SUBU
dcms
515 (6861)
3640
(1)
-
Peel off at iris
12/12
H8.6
EP in 1999
SUBU
dcms
526
91
-
-
Peel off on cut-off tube
02/13
H8.7
EP in 1999
SUBU
dcms
491
131
-
-
Millimetre sized defects observed
05/13
H8.8
EP in 1999
SUBU
HIPIMS 200 A
520
15
(8)
-
Slow cool down
07/13
M.1.2
SUBU
HIPIMS 140 A
?
13000
-
-
Peel off
08/13
M2.2
SUBU
HIPIMS 200 A
313 (4171)
55
(15.7)
-
11/13
M2.3
EP+SUBU
HIPIMS 200A
352 (4691)
4.4
13.1
18.0
EP; Field emission
12/13
M2.4
EP+SUBU
HIPIMS
240 A
378
(5031)
13.3
22.1
18.0
Q-switch/Field emission
03/14
M.1.3
EP+SUBU
HIPIMS
55 A
376
53
-
-
Q-switch/Field emission/Strong Q-slope
04/14
M.2.5
EP+SUBU
HIPIMS
240 A
413
123
-
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Residual resistance increased by processing
[1] Quadratically scaled to 1.5 GHz for comparison
[2] Values in parenthesis have only been obtained from frequency shift