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Grassellino - Application of Muon Spin Rotation to studies of cavity performance limitations

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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.

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    Grassellino - Application of Muon Spin Rotation to studies of cavity performance limitations Grassellino - Application of Muon Spin Rotation to studies of cavity performance limitations Presentation Transcript

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

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

    • 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)
    • 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]

    • HFQS:
early
flux
penetra'on?

 RF
Side
 16
 Outer
Side

    • 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

    • 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

    • 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

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


    • 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