About µSR : Muon Properties
Positive
muons (and their negative anti-particles) are leptons with spin 1/2 carrying
one elementary electric charge. From a particle-physics point of view they
are "heavy electrons". The properties of muons relevant for their use as
probes in solid state physics and chemistry are listed below.
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Mass:
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The
muon rest mass is about 207 times the rest mass of the electron or 1/9th
of the proton rest mass. From a solid-state-physics or chemistry point
of view positive muons are thus to be considered "light
protons" rather than heavy electrons.
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Magnetic
Moment:
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The muon magnetic moment is about three times larger than
that of the proton. Consequently, muons are very
sensitive magnetic probes. (In a magnetic field, the muon spin precesses
at a frequency of 13.5 kHz/Gauss). |
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Charge:
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In
condensed matter, positive muons are repelled
by the nuclei. Thus, they probe magnetic fields in the interstitial regions
between the atoms.
Negative muons are attracted by the nuclei,
form muonic atoms (i.e. atoms in which one electron is replaced by the negative
muon) and eventually get captured by the nucleus.
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Spin
Polarisation:
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Muons
result from the decay of positive or negative pions into a muon and a neutrino.
In the rest frame of the pion the muon magnetic moment is parallel or antiparallel
to the muon momentum for negative or positive muons, respectively, allowing
to produce muon beams with a very high degree of spin polarisation (nearly
100% when the muons are collected from pions decaying at rest).
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Decay:
-
Positive
(negative) muons decay into a positron (electron) and two neutrinos. Free
muons have a mean lifetime of 2.2 µs. (The lifetime of negative muons
in matter is reduced by nuclear capture and depends on the atomic number
Z of the capturing nucleus).
The positrons (electrons) are emitted preferentially in the direction of the
muon spin, allowing to measure the time evolution of the muon polarisation
by detecting the decay positrons (electrons).
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Muonium
Formation:
-
In
insulators, semiconductors, and in organic materials positive muons may
capture an electron, thus forming the hydrogen-like quasi-atom muonium
(Mu). Due to the hyperfine interaction between
muon and electron spin, muonium is an even more sensitive magnetic probe
than the bare muon.
Muonium can be used as a substitute for hydrogen in organic molecules or radicals,
giving information on the structure, dynamics and reactions of these species.
µSR Techniques and Applications
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An
excellent introduction to µSR techniques and their applications is
given by Jess H. Brewer in his article "Muon Spin Rotation / Relaxation
/ Resonance", published in
Encyclopedia
of Applied Physics, Vol. 11 (Mössbauer Effect to Nuclear Structure)
pp. 23-53 (VCH Publishers, Inc., 1994).
A
simplified HTML
version of this article may be found on the TRIUMF
µSR home page.
A brief
introduction to µSR techniques and applications may also be found
on the pages of ISIS
.
For
further reading, please refer to our
list of review articles
and books on µSR
and,
e.g.,
to the
Proceedings
of the International µSR Conferences:
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1978:
Rorschach, Switzerland, Hyperfine Interact. 6
(1979)
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1980:
Vancouver, Canada, Hyperfine Interact. 8 (1981)
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1983:
Shimoda, Japan, Hyperfine Interact. 17-19
(1984)
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1986:
Uppsala, Sweden, Hyperfine Interact. 31-32
(1986)
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1990:
Oxford, UK, Hyperfine Interact. 63-65 (1990)
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1993:
Maui, Hawaii, Hyperfine Interact. 85-87 (1994)
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1996:
Nikko, Japan, Hyperfine Interact. 104-106
(1997)
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1999:
Les Diablerets, Switzerland, Physica B 289-290
(2000)
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2002:
Williamsburg, Virginia, USA, Physica B 326
(2003)
Reviews on µSR Techniques and Applications
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The
references are ordered chronologically according to their date of publication.
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A.
Seeger:
-
POSITIVE
MUONS AS LIGHT ISOTOPES OF HYDROGEN
in:
Topics in Applied Physics, Vol. 28: Hydrogen in Metals I. Basic Properties,
Eds G. Alefeld and J. Völkl (Springer, Berlin-Heidelberg 1978), pp.
349-397
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D.C.
Walker:
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MUON
AND MUONIUM CHEMISTRY
(Cambridge
University Press, 1983)
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J.
Chappert:
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PRINCIPLES
OF THE µSR TECHNIQUE
in:
Muons and Pions in Materials Research, eds. J. Chappert and R.I. Grynszpan
(Elsevier, Amsterdam 1984), p. 35
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A.
Schenck:
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MUON
SPIN ROTATION SPECTROSCOPY
(Adam
Hilger, Bristol 1985)
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J.
Chappert and A. Yaouanc:
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MUON
SPECTROSCOPY
in:
Topics in Current Physics, edited by U. Gonser, Vol. 40: Microscopic Methods
in Metals (Springer, Heidelberg 1986), p. 297
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S.F.J.
Cox:
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IMPLANTED
MUON STUDIES IN CONDENSED MATTER SCIENCE
J.
Phys. C: Solid State Phys. 20 (1987) pp. 3187-3319
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E.
Roduner:
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THE
POSITIVE MUON AS A PROBE IN FREE RADICAL CHEMISTRY
Lecture
Notes in Chemistry No. 49 (Springer Verlag, Berlin 1988)
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B.D.
Patterson:
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MUONIUM
STATES IN SEMICONDUCTORS
Rev.
Mod. Phys. 60 (1988) pp. 69-159
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H.
Keller:
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MUON
SPIN ROTATION EXPERIMENTS IN HIGH-Tc SUPERCONDUCTORS
in:
Earlier and Recent Aspects of Superconductivity,
Springer Series in Solid-State Sciences, Vol. 90, Eds J.G. Bednorz and
K.A. Müller (Springer, Berlin-Heidelberg 1988), pp. 222-239
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A.
Seeger and L. Schimmele:
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µ+SR
IN MAGNETICALLY ORDERED METALS
in:
Perspectives of Meson Science, Eds T. Yamazaki, K. Nakai and K. Nagamine
(Elsevier, Amsterdam 1992), pp. 293-382
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E.
Roduner:
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POLARIZED
POSITIVE MUONS PROBING FREE RADICALS: A VARIANT OF MAGNETIC RESONANCE
Chem.
Soc. Rev. 22 (1993) 337
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J.H.
Brewer:
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MUON
SPIN ROTATION / RELAXATION / RESONANCE
in:
Encyclopedia of Applied Physics, Vol. 11: Mössbauer Effect to Nuclear
Structure (VCH Publishers, 1994), pp. 23-53
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V.P.
Smilga and Yu.M. Belousov:
-
THE
MUON METHOD IN SCIENCE
Proceedinges
of the Lebedev Physics Institute, Academy of Sciences of Russia, Vol. 219
(Nova Science, Commack, N.Y. 1994)
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A.
Schenck and F.N. Gygax:
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MAGNETIC
MATERIALS STUDIED BY MUON SPIN ROTATION SPECTROSCOPY
In:
Handbook of Magnetic Materials, edited by K.H.J. Buschow, Vol. 9 (Elsevier,
Amsterdam 1995) pp. 57-302
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E.
Karlsson:
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SOLID
STATE PHENOMENA
As
Seen by Muons, Protons, and Excited Nuclei
(Clarendon,
Oxford 1995)
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E.A.
Davis, S.F.J. Cox, editors:
PROTONS AND MUONS IN
MATERIALS SCIENCE
(Taylor&Francis,
1996)
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G.
Schatz and A. Weidinger:
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NUCLEAR
CONDENSED MATTER PHYSICS
Nuclear
Methods and Applications (John Wiley & Sons Ltd., Chichester 1996)
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P.
Dalmas de Réotier and A. Yaouanc:
-
MUON
SPIN ROTATION AND RELAXATION IN MAGNETIC MATERIALS
J.
Phys. Condens. Matter 9 (1997) pp. 9113-9166
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E.
Roduner, guest editor:
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APPLIED
MAGNETIC RESONANCE, special issue about µSR
Appl.
Magn. Resonance 13 (1997) pp. 1-229
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A.
Amato:
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HEAVY-FERMION
SYSTEMS STUDIED BY µSR TECHNIQUE
Rev.
Mod. Phys., Vol. 69, No. 4 (1997) pp. 1119-1179
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S.J.
Blundell:
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SPIN-POLARIZED
MUONS IN CONDENSED MATTER PHYSICS
Contemporary
Physics 40 (1999) pp. 175-192
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S.L.
Lee, S.H. Kilcoyne, R. Cywinski eds:
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MUON
SCIENCE: MUONS IN PHYSICS; CHEMISTRY AND MATERIALS
(IOP
Publishing, Bristol and Philadelphia, 1999)
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A.
Amato
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MUON
SPIN ROTATION: APPLICATIONS IN MAGNETISM
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in:
Encyclopedia of Materials: Science & Technology, edited by K.H.J. Buschow
et al. (Elsevier, Amsterdam,
2001) pp. 5888-5893.
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