Download - µ - SR IN NUCLEI WITH SPIN
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µ-SR IN NUCLEI WITH SPIN
Jess H. Brewer
Canadian Institute for Advanced Researchand Dept. of Physics & Astronomy,
Univ. of British ColumbiaVancouver, BC, Canada
(NOT “µSSR ”!)
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Huh? Wuzzat?
Visit http://musr.org
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Transverse Field (TF) µ+SR
Typical time spectrum (histogram)
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Brewer's List of µSR Acronyms
TransverseField
Zero Field
LongitudinalField
AvoidedLevel
CrossingResonance
MuonSpinEcho
MuonSpin
Resonance
FourierTransform
µ SR
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Typical time spectrum
(histogram)
µ+SR vs. µ-SR
Asymmetry spectrum
Single lifetimeτμ = 2.197 μs
Multiple lifetimes(some very short!)
Large amplitudes
Small amplitudes
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Nuclearµ-
Capture
Suzuki, 1980
µ-p n µin a nucleus:Rate exceeds that of µ- e- µe for Z
Ne
Mg
Na
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Typical time spectrum
(histogram)
µ+SR vs. µ-SR
Asymmetry spectrum
Single lifetimeτμ = 2.197 μs
Multiple lifetimes(some very short!)
Large amplitudes
Small amplitudes
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View along µ- momentum
Large impact parameters are more probable initial orbits tend to be circular.
LS couplings depolarize µ- spin unless fast Auger!
Atomic Capture & LS Depolarization of µ-
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Typical time spectrum
(histogram)
µ+SR vs. µ-SR
Asymmetry spectrum
Single lifetimeτμ = 2.197 μs
Multiple lifetimes(some very short!)
Large amplitudes
Small amplitudes
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Characteristicprecession frequencies
of F± hyperfine states
in selected low-Z
muonic atoms
µ - Z µ - Z
F+
F-ΔEHF
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JHB 1982
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µ-SR in Teflon [(CF2)n]:High frequency signal: μ-C
Low frequency signal: F+ (triplet) state of 19Fμ- F- (singlet) stateHF transition rate
RHF = 5.2(5) μs-1
fF •f+•P
+(0) = 0.08(1)
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µ-SR in Melamine (C3H6N6):High frequency signal: μ-C Low frequency signal: F+ state of 14Nμ-
fN •f+
•P+(0) = 0.050(3)
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µ-SR inMelamine(C3H6N6)
fN •f+
•P+(0) decreases with
B
Λ N decreases with Band is much too largeto be caused by eitherRHF or neighbouring
nuclear dipoles.
ΛC is alsoanomalously fast.
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“Coulomb Explosion” Leftovers
µ-Z
e-
R.
µ-
Augerγe-
paramagneticion or radical
freeelectrons
muonic atom
Depolarization!
(later)(t ≈ 0)
ONLY in NONMETALS!
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First observation of µ-SR in Sodium Metal
fNa •f+
•P+(0) = 0.025(4)
Λ Na
= 13(3) µs-1
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Finis
Linuxand
OpenOfficeRULE!
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“Themes” in µ±SR
Muonium as light Hydrogen (Mu = µ+e-) (H = p+e-)
● Mu vs. H atom Chemistry: - gases, liquids & solids
- Best test of reaction rate theories.
- Study “unobservable” H atom rxns.
- Discover new radical species.
● Mu vs. H in Semiconductors:
- Until recently, µ +SR → only data on
metastable H states in semiconductors!
The Muon as a Probe
● Quantum Diffusion: µ + in metals (compare H+); Mu in nonmetals (compare H).
● Ultra-Heavy Hydrogen: neutral muonic helium (α++μ-e-) has m ≈ 4.11 mH
● Probing Magnetism: unequalled sensitivity - Local fields: electronic structure; ordering - Dynamics: electronic, nuclear spins
● Probing Superconductivity: (esp. HTcSC) - Coexistence of SC & Magnetism - Magnetic Penetration Depth λ - Coherence Length ξ
μ+ only (?) μ+ or μ-
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µ-
SR :Although most µSR is µ+SR, it is often desirable to use negative muons in the same way, albeit with more difficulty.
DRAWBACKS of µ-SR PROPOSED MITIGATIONS
LS Depolarization in the atomic cascade
Nuclear Muon Capture: short lifetimes, few decay e-
Giant Hyperfine Interaction with nonzero-spin nuclei
"Tag" events with specific muonic X-rays
Look for neutron asymmetries in heavier elements
Observe characteristic F precession signals
It is easy to get the impression that only positive muons are employed in µSR.
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Nuclear µ-
CaptureSuzuki, 1980
µ-p n µin a nucleus:rate comparable to
that of µ- e- µe for Z
Possible Help: Many times a fast neutron is emitted from nuclear µ- capture. Very few measurements have been made of the correlation of that neutron with the muon's spin direction. If cases are found where this neutron asymmetry is sizeable, we may be able to do neutron-triggered µ-SR , for which the event rate can be higher than in µ+SR .
PROBLEM
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µ-SR in 27Al:Residual F- polarization
after initial precession in F+ HF state followed by spin-flip transition
to F- stateRHF due to Augerof core electrons:measured valueis consistent withthe calculation of Winston (1963):
RHF ≈ 41 μs-1.
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Relativistic Shift of µ- Frequency
Atomic Number
(ωμ-
Z - ω
μ+) /
ωμ+
Taken at 7 kG
Pb
Cd
Al
C,N,O