µ-SR IN NUCLEI WITH SPIN

Jess H. Brewer

Canadian Institute for Advanced Research

and Dept. of Physics & Astronomy,

Univ. of British Columbia

Vancouver, BC, Canada

(NOT “µSSR ”!)

Huh? Wuzzat?

Visit http://musr.org

Transverse Field (TF) µ+SR

Typical time spectrum (histogram)

Brewer's List of µSR Acronyms

Transverse

Field

Zero Field

Longitudinal

Field

Avoided

Level

Crossing

Resonance

Muon

Spin

Echo

Muon

Spin

Resonance

Fourier

Transform

µ SR

Typical time spectrum (histogram)

µ+SR vs. µ-SR

Asymmetry spectrum

Single lifetime

τμ = 2.197 μs

Multiple lifetimes

(some very short!)

Large amplitudes

Small amplitudes

Nuclear
µ-
Capture

Suzuki, 1980

µ-p n µ in a nucleus:

Rate exceeds that of µ- e- µe for Z 11

Ne

Mg

Na

Typical time spectrum (histogram)

µ+SR vs. µ-SR

Asymmetry spectrum

Single lifetime

τμ = 2.197 μs

Multiple lifetimes

(some very short!)

Large amplitudes

Small amplitudes

View along µ-

momentum

Large impact parameters are more probable

 initial orbits tend to be circular.

LS couplings depolarize µ- spin unless fast Auger!

Atomic Capture & LS Depolarization of µ-

Typical time spectrum (histogram)

µ+SR vs. µ-SR

Asymmetry spectrum

Single lifetime

τμ = 2.197 μs

Multiple lifetimes

(some very short!)

Large amplitudes

Small amplitudes

Characteristic

precession

frequencies

of F±

hyperfine states

in selected

low-Z

muonic atoms

µ - Z

µ - Z

F+

F-

ΔEHF

JHB 1982

µ-SR in Teflon [(CF2)n]:

High frequency signal: μ-C

Low frequency signal:

F+ (triplet) state of 19Fμ-

F- (singlet) state

HF transition rate

RHF = 5.2(5) μs-1

fF •f+•P+(0) = 0.08(1)

µ-SR in Melamine (C3H6N6):

High frequency signal: μ-C

Low frequency signal: F+ state of 14Nμ-

fN •f+•P+(0) = 0.050(3)

µ-SR in

Melamine

(C3H6N6)

fN •f+•P+(0) decreases with B

Λ N decreases with B

and is much too large

to be caused by either

RHF or neighbouring

nuclear dipoles.

ΛC is also

anomalously fast.

“Coulomb Explosion” Leftovers

µ-Z

e-

R.

µ-

Auger

γ

e-

paramagnetic

ion or radical

free

electrons

muonic atom

Depolarization!

(later)

(t ≈ 0)

ONLY in NONMETALS!

First observation of µ-SR in Sodium Metal

fNa •f+•P+(0) = 0.025(4)

Λ Na = 13(3) µs-1

Finis

Linux
and
OpenOffice
RULE!

“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-

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

LS 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.

Nuclear µ- Capture

Suzuki, 1980

µ-p n µin a nucleus:

rate comparable to that of

µ- e- µe 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

µ-SR in 27Al:

Residual F- polarization

after initial precession

in F+ HF state followed

by spin-flip transition

to F- state

RHF due to Auger

of core electrons:

measured value

is consistent with

the calculation of

Winston (1963):

RHF ≈ 41 μs-1.

Relativistic Shift of µ- Frequency

Atomic Number

(ωμ- Z - ωμ+) / ωμ+

Taken at 7 kG

Pb

Cd

Al

C,N,O

Verbally acknowledge Khashayar Ghandi and the staff of the TRIUMF µSR User Facility; except for their help, “I did it all by myself!” [½ min]

If anyone here doesn't know what µSR is, please go to this site to find out. There you will find everything I know, including this talk. [½ min]

A VERY quick review of the familiar µ+SR technique. Parity violation in the weak interaction gives us 100% polarized muons and the asymmetric decay that makes their polarization easy to follow as a function of time. See Web site for details.

[1 min]

A reminder of the wealth of techniques available in µ+SR thanks to its intrinsic simplicity and high sensitivity. [½ min]

Basic differences between µ+SR and µ-SR due to nuclear capture, depolarization during atomic capture, and giant HF coupling to nuclear spin (if any). [1 min]

Very quick overview. [½ min]

Basic differences between µ+SR and µ-SR due to nuclear capture, depolarization during atomic capture, and giant HF coupling to nuclear spin (if any). [1 min]

Very quick overview. [1 min]

Basic differences between µ+SR and µ-SR due to nuclear capture, depolarization during atomic capture, and giant HF coupling to nuclear spin (if any). [1 min]

[½ min]

Examples (all in 500 G) showing different characteristic frequencies and (for Al) initial spin flip. [1 min]

Note keV (!) HF splittings deexciting via electronic Auger, rates predicted by Winston in 1963.

[1 ½ min]

[1 ½ min]

[1 ½ min]

Relaxation rates are much too fast to be due to HF transitions by electronic Auger. Best guess: initial “Coulomb explosion” of Auger electrons generates nearby paramagnetic species whose random local fields depolarize both 14Nµ- and 12Cµ-. [2 min]

Paramagnetic “junk” left over from ionization in initial cascade through muonic orbitals causes later relaxation of the muonic atom's spin. ONLY in insulators (and possibly semiconductors). [1 min]

In a metal, initial ionization “heals” quickly; thus relaxation rate is due to HF transition via electronic Auger, as predicted by Winston in 1963. [1 min]

[2 min]

How to beat down the problems (if we're lucky). I'll only discuss the last item today. [2 min]

[½ min]

Jess Brewer

2002-05-11T22:01:43

Jess H. Brewer