http://www.surfacetreatments.it/thinfilms Application of Muon Spin Rotation to studies of cavity performance limitations (Anna Grassellino - 20') Speaker: Anna Grassellino - TRIUMF - Vancouver, Canada | Duration: 20 min. Abstract In this contribution a new experiment to investigate magnetic flux entry in Nb coupons and HFQS limited cutout samples will be presented. The experimental technique, called muSR (muon spin rotation), utilizes a probe magnetic moment to reveal local magnetic fields in the sample under study. Through the use of low energy spin polarized muons, the experiment can probe near surface local magnetic fields with extreme sensitivity. Being a ‘local’ rather than external and global technique, it offers a different and precise way to measure the field of first penetration in type-II superconductors. The experiment will study the nature of the transition from superconducting to mixed state in the marginal type II superconductor Nb, for samples with different treatment and grain size, and for RF characterized (via thermometry) HFQS limited cutout samples. Studying the latest will provide an opportunity to look for correlation of the onset of HFQS with the appearance of flux entry into the sample, detectable via the extremely sensitive muSR probe. Models for HFQS and MFQS which muSR can help probing will be discussed.

1. Applica'on of Muon Spin Rota'on to
studies of cavity performance limita'ons
Anna Grassellino
University of Pennsylvania, TRIUMF

2. Some of the most powerful tools available in
condensed matter physics and materials science are
instrumental methods that utilize a Magnetic
Moment and/or Electric Quadrupole Moment to
probe the local magnetic, electronic or structural
properties of matter:
Conventional Methods Nuclear Beam Methods
Intrinsic probe: Implanted probe:
Nuclei Electrons Muons Radioactive
Nuclei

4. General Procedure:
1. Produce a non-equilibrium polarization
2. Monitor how the polarization evolves
in time or changes with frequency

5. Pion Decay: π+ → µ+ + νµ
A pion resting on the downstream side of the primary production
target has zero linear momentum and zero angular momentum.
Conservation of Linear Momentum: µ+ emitted with momentum equal and opposite
to that of the νµ
Conservation of Angular Momentum: µ+ and the νµ have equal and opposite spin
Weak Interaction: only “left-handed”
νµ are created. Therefore the
emerging µ+ has its spin pointing
antiparallel to its momentum
direction

6. µ+-Decay Asymmetry
Angular distribution of positrons from the µ+-decay. The asymmetry
is a = 1/3 when all positron energies are sampled with equal probability.

8. The muon is sensitive to the vector sum of the local
magnetic fields at its stopping site. The local fields
consist of:
• those from nuclear magnetic moments
• those from electronic moments
(100-1000 times larger than from nuclear moments)
• external magnetic fields
As a local probe, µSR can be used to deduce
Magnetic volume fractions.

10. Major Advantages of and
• can be implanted into any sample (gas, solid or liquid)
• polarization independent of sample and sample environment
• greater sensitivity enables studies of dilute or isolated impurities
• magnetism can be studied in zero external magnetic field
• can study dynamical ranges not accessible with conventional
methods
10-4 10-2 100 102 104 106 108 1010 1012 1014
Fluctuation Rate (Hz)

11. Conventional Methods Nuclear Beam Methods
Probe: host nuclei host electrons muons radioactive
nuclei
Lifetime: infinite infinite 2.2 µs 100 ms - hours
Polarization Method: apply large apply large natural optical
field field pumping
Polarization (max.): << 1 % << 1 % 100 % 80 %
Detection: absorbed absorbed anisotropic anisotropic
RF radiation microwave decay of decay of
radiation muon nucleus
Sensitivity: 1017 spins 1017 spins 107 spins 107 spins

12. Transverse-Field µSR
The time evolution of the muon spin
polarization is described by:
where G(t) is a relaxation function describing
the envelope of the TF-µSR signal that is
sensitive to the width of the static field
distribution or temporal fluctuations.

13. and for Q-slope studies
• “Local” extremely sensi've magne'c field probe
(vs magne'za'on etc)
• Can implant at different depths: can study
surface vs bulk
• Can be used to study thin films, surfaces and
mul'‐layered compounds
• Can answer several ques'ons:
1. Is HFQS due to early flux penetra7on?
2. Role of trapped flux on HFQS and/or MFQS ?
3. Field dependence of penetra7on depth and
coherence length?
4. Magne7c impuri7es?

14. HFQS: early flux penetra'on?
• Measure Hp, Hc2, Tc
• Vibra'ng sample
magnetometer and 3
experimental protocols
(ZFC warming, FCC,FCW)
• Samples: S1 (pris'ne as
received by vendor), S2
(BCP+10h 600C), S3 (S2
plus 10h 120C)
Roy, Myneni et Sahni, Supercond. Sci. Technol. 22 (2009)
105014 (6pp)

15. HFQS: early flux penetra'on?
Decrease in average disloca'on density observed by EBSD in cutout samples afer
120C baking (which removes the HFQS in cavi'es)
Before baking Afer baking
Working hypothesis – surface disloca'ons provide sites for early flux penetra'on
(below bulk Hc1) resul'ng in the HFQS due to fluxoid mo'on in Nb
[A Romanenko and H Padamsee 2010 Supercond. Sci. Technol. 23 045008]

16. HFQS: early flux penetra'on?
RF Side
16
Outer Side

17. HFQS and MFQS: trapped flux?
• Oscilla'ng fluxoids
can cause losses in
medium and high
field regimes
(Gurevich,
Rabinowitz)
• Look with muSR for
correla'on hot/cold
spots with higher/
lower trapped flux Ciova', Gurevich – Evidence of high field
radio frequency hot spots due to trapped
vor'ces in Nb cavi'es, PRST AB, 11, 122001,
2008

18. Descrip'on of the experiment
• Measure field of first penetra'on in RF characterized (via
thermometry) HFQS limited samples, and compare with HFQS RF
field onset:
– Hot vs cold
– Unbaked vs baked
• Study the nature of the transi'on: intermediate mixed state? Any
correla'on between IMS and hot spots?
• Trapped flux (hot/cold, baked/unbaked): zero field muSR
• Field range 0‐200 mT, Temperature range 1K‐4.2K
• 5 samples:
– Pris'ne Nb –from vendor
– Hot/cold spot cutout from large grain cavity (before and afer bake) –
provided by Alexander Romanenko, Hasan Padamsee (Cornell)
• Beam'me approved: ~1 day per sample  12 shifs, star'ng Oct
27th

19. MFQS, HFQS: field dependence of
fundamental superconduc'ng parameters?
• Field dependence of penetra'on depth and coherence
length?
• Determined by gap structure: double gap in Nb?
• Study vortex core size with muSR

20. 200
NbSe2 100 0.6
T = 0.02 K 0.5
T = 2.5 K 80
! /!n
T = 4.2 K
" (Å)
! (Å)
150 0.4
60
e
ab
ab
0.3
100 40
0.2
20 0.1
50
0.0 0.1 0.2 0.3 0.4 0 0.0
0 5 10 15 20 25
H/Hc2 H (kOe)
Freeze out thermal excitations of
quasiparticle core states to reveal
multiband vortices.

21. “Effec've” Magne'c Penetra'on
Depth: Magne'c Field Dependence
2000
NbSe2 • V3Si fully gapped
• LuNi2B2C anisotropic gap
1500
! (Å)
YBa2Cu3O6.95 • YBa2Cu3O6.95 dx2-y2-wave gap
ab
V3Si • NbSe2 multiband
1000
LuNi2B2C
500
0.0 0.1 0.2
H/Hc2

22. Pure Vanadium (marginal type‐II)
Laulajainen, Callaghan, Kaiser & Sonier PRB 74, 054511 (2006)

23. Impact on low, medium and high field
Q‐slope
• Field dependence of
penetra'on depth
• Field dependent losses
due to increased volume
where dissipa'on occurs
E(z,t)
H(t) Ermolov, Marchenko, Chizov, 1986
Rs ∼ (µ02ω2λ3σnΔ/T)exp(-Δ/T)
µ0ω 2 λ4 Δn 0   Δ 
2   Δ 
λ(H) Rs ∝ ln  + C0  exp − 
k B TpF   ω    kB T 

24. Impact on low, medium and high field
Q‐slope
• Field dependence of
coherence length can cause
‘gain’ (sta'onary trapped
flux)
• Sum of gain (coherence
length) and losses
(penetra'on depth) could
explain also low field Q‐
slope

25. Descrip'on of the second experiment
• Determine the field dependence of the effec've
penetra'on depth (and vortex core size) in the
vortex and intermediate mixed states. Will do this
at several temperatures to inves'gate the
possibility of two SC gaps.
• Take advantage of muSR unique sofware for
measurements of the vortex larce in a marginal
type‐II
• TF‐muSR, dilu'on refrigerator
• Pris'ne single crystal sample
• Beam'me approved: 12 shifs