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

• Sample preparation: G. Terenziani & S. Calatroni (CERN) • PCT: T. Proslier (ANL) & J. Zasadzinzki (Illinois Institute of

Technology, Chicago) • LEmuSR: A. Suter (PSI)

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Motivation

The sixth international workshop on thin films and new ideas for RF superconductivity Tobias.Junginger@cern.ch 2

• Niobium on copper is the material of choice for many accelerator cavities

• 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

• Low energy muon spin rotation (LEmuSR) and point contact tunnelling (PCT) have been used for bulk niobium studies

• 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

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Content

The sixth international workshop on thin films and new ideas for RF superconductivity Tobias.Junginger@cern.ch 3

• Point Contact Tunnelling (PCT) • Review of measurements on bulk niobium • Results on niobium films

• Low Energy Muon Spin Rotation (LEmuSR) • Review of measurements on bulk niobium • Results on niobium films

• Conclusions

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PCT on clean niobium

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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

PCT on bulk niobium - Low temperature baking effect

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unbaked

125°C baked (24h)

Observations: – Zero-bias conductance – Broadened DOS

The low temperature baking gives – Reduced zero bias conductance – 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

PCT on bulk niobium – High temperature baking effect

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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: • less low energy gaps • smaller Γ values (sharper

distributions)

Few zero bias peaks Very few small gaps measured (<1 meV) Sharpest Distributions Very low Gamma/Delta values (< 10%) Homogeneous sample

PCT on niobium on copper (dcms)

Zero bias peaks -> mag impurities Small gaps measured (~1.3 meV) Distributions fairly sharp Good Gamma/Delta values (< 10%) Homogeneous sample

PCT on niobium on copper (HIPIMS)

Zero Bias Peaks

For RF measurements see Giovanni’s talk

No Zero bias peaks Small gaps measured (<1.3 meV) Widest Distributions High Gamma/Delta values Inhomogeneous sample

PCT on niobium on copper (strongly oxidized dcms) For RF measurements see T. Junginger, PhD thesis University of Heidelberg (2012)

Point Contact Tunnelling – Conclusion

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• Investigated by PCT Nb on Cu samples can look as good as the best performing bulk niobium samples

• HIPIMS sample showed zero bias peaks -> Magn. Impurities? • A sample with a huge residual resistance showed small gaps, a

wide distribution and high Gamma/Delta values but no zero bias peaks

• 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

LEmuSR on bulk niobium

11 06/10/2014 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)

• Non exponential penetration • Taking non-locality into account exponential fit possible • Cold region of 120°C baked sample shows a strong change in Meissner screening with depth

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

• Penetration depth almost twice as deep in dcms compared to HIPIMS

• Local London theory applicable to Nb films

• No change in Meissner screening with depth

LEmuSR on niobium on copper

• Zero field spectra of HIPIMS sample shows clear signs of magnetization at the surface and at 105 nm depth

• For dcMS sample weaker signs are found at the surface and no signs at 105 nm depth

• Further experiments are necessary to rule out trapped flux and muon diffusion

Conclusions

The sixth international workshop on thin films and new ideas for RF superconductivity Tobias.Junginger@cern.ch 14

• With PCT we find zero bias peaks and muSR gives direct evidence of magnetization of the HIPIMS sample (Magn. Impurities?)

• No change in Meissner screening with depth • Higher RRR does not necessarily result in a

less pronounced Q-slope (see also talks of Sarah and Giovanni)

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Backup Slides

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The sixth international workshop on thin films and new ideas for RF

superconductivity Tobias.Junginger@cern.ch

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Muon-spin rotation (μSR) technique

Sµ(0)

Bμ = (2π/γμ) νμ

Depth dependent µSR measurements

λ

B(z)

z

Bext

0

superconductor

B(z) = Bext exp(−z /λ)

Jackson et al., Phys. Rev. Lett. 84, 4958 (2000)

More precise: use known implantation profile

Jackson et al., Phys. Rev. Lett. 84, 4958 (2000)

Standard DCMS Temp dependence

HIPIMS 86 Hz Statistic

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

HIPIMS 86 Hz Temp dependence

HIE e9 Statistic

Few Zero bias peaks – Small gaps measured (<1.3 meV),

broad distribution Distributions fairly sharp in gamma Good Gamma/Delta values (< 10%)

with few “hot spots”.

HIE e9 Temp dependence

HIE i9 Statistic

Small gaps measured (<1 meV), broad distribution

Distributions fairly sharp in gamma Good Gamma/Delta values (< 10%)

with few “hot spots”.

HIE i9 Temp dependence

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The sixth international workshop on thin films and new ideas for RF

superconductivity Tobias.Junginger@cern.ch

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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

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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 - - 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