tobias junginger low energy muon spin rotation and point contact tunneling on niobium thin films
DESCRIPTION
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.TRANSCRIPT
Low energy muon spin rotation and point contact tunneling on niobium thin films
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• 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
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• 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
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• 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
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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
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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
LEmuSR on bulk niobium
11 06/10/2014 The sixth international workshop on thin films and new ideas for RF superconductivity [email protected]
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
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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
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• 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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superconductivity [email protected]
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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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superconductivity [email protected]
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06/10/2014
The sixth international workshop on thin films and new ideas for RF
superconductivity [email protected]
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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