Muon Spectroscopy

Koji YokoyamaSchool of Physics and Astronomy, QMUL

(on behalf of Dr. Alan Drew)

MRI Spectroscopy Workshop29th May, 2014

What is muon?

• ~1 muon every second on the size of a palm

• Charge ±e• 200 times heavier than e-

• 1/9 of proton mass• 2.2 μs lifetime• μ+ for muon spectroscopy

Producing muons

Muon beam

• 100% spin polarized• Opposite to propagation

• Monochromatic, kinetic energy of 4 MeV

• Pulsed or continuous beam source

• ISIS: 2,500 μ+/pulse

“Surface” muon beam

Implanted muons in sample

• Stopping range: 100 mg/cm2

• Thin sample, but probe of bulk material

(Thermalisation)

Final form of muons

• In insulator, often Mu• In metal, usually bare muon μ+

Muon decays

• Preferentially emits positron in the direction of its spin• Positron penetrates through e.g. sample holders

Muon spectroscopy

Muon time spectrum:

FB

spinμ

0 2 4 6 8 10 12 14 16

0

10000

20000

30000

40000

50000

60000

Backward Detector

Pos

itron

Cou

nts

Time (s)

Forward Detector

• If no relaxation, A(t) is just flat:

0 2 4 6 8 10 12 14 160.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24

0.26

Asym

met

ry

Time (s)

μSR

Muon Spin Relaxation

Rotation

Resonance

An example of detecotor: CHRONUS

Three Helmholtz coils forLongitudinalTransverseZero

Field

Changing Ts, BsSometimes with excitations: RF, Light

μSR Muons at ISIS

Chronus

Example 1: Ion diffusion

Lithium ion battery:cathode material is a crucial part for battery performance

• LixCoO2

• Layered structure: TMO || Li || TMO || …• Self-diffusion coefficient D of Li ions

J. Sugiyama et al. PRL 103, 147601 (2009)

Example 1: Ion diffusion

• Implanted muons bind to oxygen atoms• Muons are sensitive probes of local internal fields• Muons feel random nuclear magnetic fields of Li

Example 1: Ion diffusion

• Spin relaxation in isotropic internal field with Gaussian distribution (no external field)

• Kubo-Toyabe function describes its behavior

• First proposed as a “Toy” problem• Turned out to be quite useful in μSR• Dynamic KT function to include fluctuation in field

Time

Example 1: Ion diffusion

• Ion hopping ν: dynamic contribution• From T-dependence ν(T) the ion

diffusion coeff D is determined

Example 2: Level Crossing Resonance

• Low field relaxation rates are a superposition of several different relaxation rates from different Mu states

• Difficult to interpret low field relaxation rates• ALC resonances, on the other hand, are on individual Mu state

Example 2: Level Crossing Resonance

• Consider Mu in condensed matter

B-field

|+ +>|+ −>

|− +>|− −>

|e- μ+>

Polarization

• The magnitude of the hfcc is characteristic of the binding site

Example 2: Level Crossing Resonance

• Muonium substituted free radicals• Addition of Mu to unsaturated hydrocarbons• Unpaired electron distributed over the molecule• Hyperfine interactions with the muon and nuclear spins

Example 2: Level Crossing Resonance

What if the molecule is in the excited state?Can we see any change in the ALC signal?How about its dynamics?

… Project MuSES may discover the answer !

More applications

Inorganic magnetismSuperconductors

Organic magnetismHydrogen studies of semiconductorsCharge transport and diffusion

Chemistry and molecular studies

• 2 calls per year (deadlines 16 April and 16 October)• All submissions via ISIS website

• ~50 proposals submitted per round• Oversubscription ~1.8

• 6 weeks after the deadline, the selection panel meets• Results a few weeks after that (with comments)• Instrument scientist will then ask for preferred dates • Schedule produced, local contacts assigned• Run experiments!

The Proposal Process

2 pages proposal

Other types of ProposalsRapid Access

• For rapidly-moving science areas, new sample discoveries, other urgent studies• Proposals can be submitted any time• Rapidly reviewed by FAP Chair and one other FAP member• If awarded time, scheduled as soon as possible• Must be a clear case as to why the measurement is urgent

Xpress Access

• For initial characterisation of samples or feasibility checks on samples for future beamtime

• Proposals are short, and can be submitted any time• Reviewed internally• Awarded up to 5 hours of beamtime on either MuSR or EMU.• Users need not come for the measurement – can send the sample in

Further details on website

http://www.isis.stfc.ac.uk/apply-for-beamtime/

• Many thanks to:Prof. Roberto De RenziDr. Adrian HillierDr. Jamie PeckDr. Martin Mansson