PSI Muons - ISIS

Wir schaffen Wissen – heute für morgen
Paul Scherrer Institut
Thomas Prokscha, Laboratory for Muon Spin Spectroscopy
The PSI continuous muon beam and low-energy µSR
facilities
ISIS Muon Spectroscopy Training School, March 21, 2012

RAL/ISIS
PSI

n SINQ
SwissFEL
µSµS
γ SLS

PSI proton accelerator complex
Dolly, µSR,
GPS clone
SINQ, neutron spallation source
Low-energy muon beam
facility (1-30 keV), LEM
GPD, high pressure µSR
High-Field (9.5 T, 20 mK) µSR, 2013
GPS/LTF, general purpose and
low T (20 mK) µSR facility
Ultra-cold
neutrons UCN
50 MHz proton cyclotron, 2.2 mA, 590 MeV, 1.3 MW
beam power (2.4 mA, 1.4 MW test operation);
most powerful cyclotron worldwide and most
intense muon beams (quasi-continuous);
proton therapy,
irradiation facility
Comet cyclotron (superconducting),
250 MeV, 500 nA, 72.8 MHz

The PSI isochronous cyclotron
2.2 mA: ~1.4 x 10 16 protons/sec @ 590 MeV (1.3 MW on 5x5 mm 2 = 50 kW/mm 2 ,
stainless steel melts in ~0.1 ms; electric power demand of 1000 households)
A MW proton beam allows to generate 100% polarized 4-MeV µ + beams with
rates >10 8 /sec
Larmor frequency of protons: q/(2πm) = 15.25 MHz/T
ν 0
= q/(2πγm)∙B, γ = E tot
/mc 2
ν rf
= n∙ν 0
, frequency of accelerating radio-frequency
Isochronous cyclotron: B 0
(R) ~ γ(R), constant ν rf
!
PSI cyclotron: B 0
= 0.554 T, ν 0
= 8.45 MHz, n = 6,
→ ν rf
= 50.7 MHz

Jess Brewer, UBC & TRIUMF:
“Decay” and “Surface” muon beams

PSI muon beam time structure
Pion decay time of 26 ns is “smearing” the
proton beam structure in the muon beam.
This results in a “continuous” muon beam.
A µ + rate R of 10 5 /s means: average time
between to µ + is 1/R = 10 µs.
Probability p to have the next µ + at time t:
p = 1 – exp(-Rt) (“pile-up”)
Single µ + can be detected with very good
time resolution (< 1ns), compared to a
bunch width of 50-100 ns at pulsed beams.
--> measurement of GHz frequencies and
fast relaxation rates (>100 µs -1 ). But “accidental”
background in µ-decay histograms.

Continuous beam µSR experiment
ω = γ · Β ( γ = 1 3 5 . 5 M H z / T )
µ µ µ µ
P ( t = 0 )
P ( t )
s e +
µ
p µ
µ + M y o n -
C o u n t e r
P o s i t r o n - S a m p l e
P o s i t r o n -
C o u n t e r
C o u n t e r
B e x t
Accepted rate as a function of µ rate

Continuous versus pulsed muon beams
● time resolution ≤ 1 ns, ν > 100 MHz
● fast depolarization observable
( >> 10 µs -1 )
Observable asymmetry as a function of
time resolution σ t
:
A(ω) = A 0
x exp(-ω 2 σ t2
/2)
● no segmentation/position
information for e + necessary
● start counter needed
● random background (can be reduced
by „Muons On REquest“, MORE)
● limited time window
● separator required for surface muons

µSR instruments at PSI
● GPS: General Purpose Surface muon instrument; πM3 permanent installation, shared
with LTF; 4 MeV µ + , Separator/Spin Rotator (6 – 60 o ); 0 – 0.6 T, 1.8 – 1000 K;
Muons On REquest (MORE).
● LTF: Low Temperature Facility; πM3 permanent installation, shared with GPS;
0 – 3 T, 0.01 K – 10 K.
● Dolly: GPS „clone“; πE1 permanent installation in 2012; 4 MeV µ + ;
Separator/Spin Rotator (6 – 45 o ); 0 – 0.5 T; 0.3 – 300 K.
● GPD: General Purpose Decay channel instrument; µE1 beam; 15 – 60 MeV µ + and µ - ;
0 – 0.65 T, 0.3 – 500 K; high pressure up to 26 kbar.
● ALC: Avoided Level Crossing instrument; 4 MeV µ + ; 0 – 5 T. Not in operation.
● High-Field µSR: πE3 permanent installation; 4 MeV µ + ; 2 Spin Rotators (0 – 90 o );
0.1 – 9.5 T; 0.02 – 300 K.
● LEM: Low Energy Muon beam & µSR instrument; µE4 beam; 1 – 30 keV µ +
(controllable implantation depth 5 – 300 nm), 0 – 0.3 T, 2.3 – 320 K.

● GPS “tells” kicker to send a muon to GPS
● If a muon is detected in GPS “tell”
kicker to send the beam to LTF
● no random beam background in GPS
Muons On Request (MORE)
● if decay e + is detected in GPS the
next muon is requested from the kicker
Kicker (two electrodes, +-5kV)
“normal” MORE pulsed
B 0
/N 0
[10 -5 ] 660 9 1
∆t [ns]

MORE example: UPt 3
MORE opens the possibility
● of studying slow relaxation phenomena
at high magnetic fields
● to resolve close spin-precession frequencies
In UPt 3
: 2 Knight shift components, very close
Dramatic and opposite T dependence around
T N
(6K).
UPt 3
is intrinsically inhomogenous.
A. Yaouanc et al, PRL84, 2702 (2000).

The new high-field µSR facility at SµS
Beamline: surface muon beam Δp/p < 5% FWHM: πE3
90° spin rotation!
Magnet: max. field ~10 T, homogeneity: 10 ppm, field drift < 1 ppm/hr.
ν µ
~1.35 GHz
σ < 0.1 µs -1 @ 10 T
Detector system: time resolution δt < 140 ps (RMS) for TF
Sample Environment: 2 K cryostat & DR, with integrated ‘Veto’
system
Difficulties: charged particles with finite momentum distribution
(spiraling radius of 30 MeV positron: 1 cm in 10 T)

Detector dimensions for a 10 T spectrometer
e + Sample
ν
ø
Scintillators
µ +
Veto
counter
ν
60
50
40
30
20
10
Muon
counter
0
10 20 30 40 50 60
Ø (mm) N A F
14 36’400 0.31 ± 0.01 59.1
20 30’800 0.33 ± 0.01 57.9
24 26’300 0.34 ± 0.01 55.1
30 19’900 0.39 ± 0.01 55.0
40 12’000 0.41 ± 0.01 44.9
50 7’300 0.41 ± 0.01 35.0
60 4’400 0.29 ± 0.01 19.1

Oxford Instrument 8/9.5 T
Recondensing System

BF-H400
0.01 – 4 K

Commissioning 2012, user operation 2013

Range of muons in matter
mm
1
Range
µm
10 2
10
1
10 2
Cu
thin films,
multilayers..
bulk
“Surface Muons” from π +
decay at rest ( ~ 4 MeV)
generally used for bulk
studies: no depth resolution
nm
10
1
1
10 10 2
10 3
Energy [keV]
10 4
“Low-energy muons”: 0.5 – 30 keV
Allows depth-dependent µSR investigations ( ~ 1 – 300 nm)
Extends the use of µSR to new objects of investigtions
New magnetic/spin probe for thin films, multilayers, surface regions, buried layers

Implantation Profiles of Low Energy Muons
nm
200
150
Range
Variance
Stopping profiles calculated with Monte
Carlo code Trim.SP by W. Eckstein, MPI
Garching, Germany.
100
50
YBa 2
Cu 3
O 7
0
0 5 10 15 20 25 30 35
Energy [keV]
Experimentally tested for muons:
E. Morenzoni, H. Glückler, T. Prokscha, R.
Khasanov, H. Luetkens, M. Birke, E. M.
Forgan, Ch. Niedermayer, M. Pleines,
NIM B192, 254 (2002).
Stopping Density
0,05
0,04
0,03
0,02
0,01
3.4 keV
6.9 keV
YBa 2
Cu 3
O 7
15.9 keV
20.9 keV
24.9 keV
29.4 keV
0,00
0 50 100 150 200
Depth [nm]

Generation of thermal µ + at a pulsed muon beam
Pulsed beam (25Hz) @ RIKEN-RAL:
P. Bakule,Y.Matsuda,Y.Miyake, K.
Nagamine, M. Iwasaki, Y. Ikedo, K.
Shimomura, P. Strasser, S. Makimura
Nucl. Instr Meth. B 266, 335 (2008)
Intensity: ~15 LE-µ + /sec
Polarization: ~ 50%
Beam spot: 4 mm

Generation of polarized epithermal muons
„Surface“ Muons
~ 4 MeV
~ 100% polarized
Energy spectrum after a degrader
Solid line: muon energy spectrum
Solid circles: energy spectrum of muonium
~100 µm Ag
Using a proper moderator:
motivated by experiments for
positron moderation, a solid
film of a rare-gas should work! T. Prokscha et al., Phys. Rev. A58, 3739 (1998).

„Surface“ Muons
~ 4 MeV
~ 100% polarized
Generation of polarized epithermal muons
~100 µm Ag ~500 nm
6 K
s-Ne, Ar,
s-N 2
motivated by
experiments for
positron moderation
T. Prokscha et al., Appl. Surf. Sci. 172, 235 (2001).
T. Prokscha et al., Phys. Rev. A58, 3739 (1998).
E. Morenzoni et al,. J. Appl. Phys. 81, 3340 (1997).
D. Harshmann et al., Phys. Rev. B36, 8850 (1987).

Characteristics of epithermal muons
L
A s y m m e t r y
AP(t)
P(0) ≅1
E. Morenzoni, T. Prokscha, A. Suter, H. Luetkens, R.
Khasanov, J.Phys.: Cond. Matt. 16, S4583 (2004).
T i m e [ µ s ]
E. Morenzoni, F. Kottmann, D. Maden, B. Matthias, M. Meyberg,
T. Prokscha, T. Wutzke, U. Zimmermann, PRL 72, 2793 (1994).
suppression of electronic energy loss for E > E g
, large band gap E g
(10-20 eV) „soft, perfect“
insulators
large escape depth L (10-100 nm), no loss of polarization during moderation (~10 ps)
moderation efficiency is low (requires high intensity µ + beam, > 10 8 µ + /s):
-4
ε µ+
= N epith
/N 4MeV
≈ ∆Ω (1-F Mu
) L/∆R ≈ 0.25 L/ ∆R ≈ 10 – 10 -5
∆Ω: probability to escape into vacuum (~50% for isotropic angular distribution)
F Mu
: muonium formation probability
page 28

LE-µ+ Apparatus at µE4 beam line
At 2.2 mA proton current:
~4.6 • 10 8 µ + /s total, ∆p/p = 9.5% (FWHM)
~1.9 • 10 8 µ + /s on LEM moderator
~1.1 • 10 4 µ + /s moderated (solid Ar)
T. Prokscha, E. Morenzoni, K. Deiters, F. Foroughi,
D. George, R. Kobler, A. Suter and V. Vrankovic
Nucl. Instr. Meth. A 595, 317 (2008).

LE-µ + beam and LE-µSR spectrometer
Moderator cryostat
QSM612, last quadrupole of µE4 beam
LE-µSR spectrometer

Low energy µ + beam and set-up for LE-µSR
- UHV system, 10 -10 mbar
- some parts LN 2
cooled
~1.9 •10 8 µ + /s
rebuilt µE4 beam line
Polarized Low Energy Muon
Beam
Energy: 0.5-30 keV
∆E, ∆t: 400 eV, 5 ns
Depth: 1 – 300 nm
Polarization ~100 %
Beam Spot: 12 mm (FWHM)
~ 11000 µ + /s; accelerate
up to 20 keV
up to ~ 4500 µ + /s
T = 2.5 – 320 K
B = 0 – 0.3 T ┴ , 0 – 0.03 T ║ sample surface
page 31

Low energy µ + beam and set-up for LE-µSR
• UHV:
p ∼ 10 -10 mbar
• Electrostatic
accel-decel and
transport system
(LN 2
cooled)
Filter
10-nm-thin C-foil
Source
INPC 2010, July-05 2010
Implantation time
Sample

Depth dependent LE-µSR measurements
July-05 2010 page 33

Magnetic field profiles in superconductors
“ l o c a l ”
Direct, absolute measurement of
magnetic penetration depth
ξ
“ n o n - l o c a l ”
λ(T)
∝
m
n (T)
s
*
effective mass
density of
supercarriers
0
ξ
Direct Test of theories (London, BCS)
z
−
λab
(T)
→ B(z) = B0e
λ
z
B(z)
Local, non-local response
Determination of the
coherence length ξ, κ=λ/ξ
local
nonlocal
z

Magnetic field profiles in Pb and YBa 2
Cu 3
O 7-δ
0.01
Lead, T c
=7.0(2) K, h ext
= 91.5(3)G,
ξ 0
= 90(5)nm, λ 0
= 58(3)nm
0.01
YBa 2
Cu 3
O 7-δ
, T=20K, T c
=87.5K
h ext
= 91.5(3) G,
ξ 0
= 1.5 nm fixed, λ 0
= 137(10) nm
B (T)
6.66K
B (T)
6.19K
1E-3
1E-3
h ext
exp(-z/λ(T))
3.4 keV
8.9 keV
15.9 keV
20.9 keV
29.4 keV
2.85K
0 50 100 150
z (nm)
Non-local: non- exponential
0 50 100 150
z (nm)
local: exponential
T.J. Jackson et al., Physical Review Letters 84, 4958 (2000).
A. Suter et al., Physical Review Letters 92, 087001 (2004).
A. Suter et al., Physical Review B 72, 024506 (2005).

In-plane Anisotropy in detwinned YBa 2
Cu 3
O 6.92
samples produced by R. Liang,
W. Hardy, D. Bonn, Univ. of
British Columbia
λ(T)
∝
m
n (T)
s
*
effective mass
density of super
carriers
1/λ 2 ~ n s
/m* ≡ ρ s
, superfluid density
Test of theories (London, BCS),
symmetry of the sc gap

Detwinned YBa 2
Cu 3
O 6.92
, T c
= 94.1 K, B = 9.47 mT
100 K
8 K
8 K
λ(T)
∝
m
n (T)
s
*
effective mass
density of super carriers
at low T for a d-wave SC:
λ a
= 126(1.2)nm, λ b
= 105.5(1.0) nm,
λ a
/λ b
= 1.19(1)
R.F. Kiefl et al, PRB 81, 180502(R), (2010)
page 37

Meissner effect in a strongly underdoped cuprate
La 1.84
Sr 0.16
CuO 4
- La 1.94
Sr 0.06
CuO 4
- La 1.84
Sr 0.16
Cu 4
T c
= 32 K
T' c
< 5 K T c
= 32 K
induced superfluid density disappears at T eff
>>T' c
. This
result is not expected within the conventional proximity
effect theory.
E. Morenzoni et al, Nature Communications 2, 272 (2011)

Dimensionality Control of Electronic Phase
Transitions in Nickel-Oxide Superlattices
A.V. Boris et al, Science 332, 937 (2011)
MPI Solid State Research - MPI Metals Research – Univ. Fribourg - PSI
• 100-nm-thick NxN u.c. LaNiO 3
/LaAlO 3
superlattices
• 2 u.c. LaNiO 3
: MI and AF transitions at T < 150 K
• 4 u.c. LaNiO 3
: metallic and paramagnetic at all T
T MI
T MI

Spatially homogeneous ferromagnetism of (Ga,Mn)As
• Mn-doped GaAs potential ‘spintronics’ material
• great interest in fundamental research: evolution from a
paramagnetic insulator to ferromagnetic metal
• controversy if FM is associated with intrinsic spatial
inhomogeneity
Low-energy µSR (in combination with conductivity and
DC/AC magnetization) results:
• sharp onset of FM order, developing homogeneously in
the full volume fraction, in both insulating and metallic
films.
• smooth evolution of ordered moment size across metalinsulator
transition at x ~ 0.03
• FM coupling between Mn before full emergence of
itinerant hole carriers
muon decay asymmetry (10 mT TF top) and zero
field (ZF) relaxation rate (bottom) as a function of
temperature and doping
S.R. Dunsiger, J.P. Carlo, T. Goko, G. Nieuwenhuys, T. Prokscha, A. Suter, E. Morenzoni, D. Chiba, Y.
Nishitani, F. Matsukara, H. Ohno, J. Ohe, S. Maekawa, Y.J. Uemura, Nature Materials 9, 299 (2010).

Spin diffusion length in organic spin valve
Alq 3
Magnetoresistance vs B
Long coherence time of injected
spins ~10 -5 s measurable
static fields
Spin diffusion length vs T correlates
with magneto-resistance
Field distribution: I on
- I off
Skewness
First direct measurement of
spin diffusion length in a
working spin valve.
A.J. Drew et al., Nature Materials 8, 109 (2009)
INPC 2010, July-05 2010

“Cold” muonium from mesoporous SiO 2
in vacuum
Motivated by recent results on Ps emission from mesoporous SiO 2
films
250 K, 5 keV: 40% emission
of thermal Mu in vacuum
100 K, 5 keV: 20% emission
of thermal Mu in vacuum;
expect 40% at 2 keV (to be
confirmed)
Vacuum Mu fraction F V as a function of
Mu
temperature and energy; solid lines are a
fit of a diffusion model to the data.
A. Antognini, P. Crivelli, T. Prokscha, K.S. Khaw, B. Barbiellini, L. Liszkay, K. Kirch, K. Kwuida, E. Morenzoni,
F.M. Piegsa, Z. Salman, A. Suter, arXiv:1112.4887v1, accepted by PRL (2012)

Some More Low Energy Muon Applications
Surface dynamics of polymers
Magnetic properties of monolayers
of single molecule magnets
Z. Salman et al.
F.L. Pratt et al., PRB 72, 121401(R) (2005)
Formation of hydrogen impurities
in semiconductors at low energies
T. Prokscha et al., PRL 98, 227401 (2007)
T. Prokscha et al., Physcia B 404, 866 (2009)
D.G. Eshchenko et al., Physica B 404, 873 (2009)
H.V. Alberto et al., Physica B 404, 870 (2009)
300μ
Photo-induced
effects in
semiconductors
T. Prokscha et al.
Current effects on magnetism
and superconductivity in a thin
La 1.94
Sr 0.06
CuO 4
wire
M. Shay et al., PRB 80, 144511 (2009)
Superconductivity and
Magnetism in
La 2
CuO 4
/La 1.56
Sr 0.44
CuO 4
Superlattices
T N
A. Suter et al., PRL 106,
T c
237003 (2011)
doping
Superfluid density in high and
low T c
heterostructures
B. Wojek et al., PRB 85, 024505 (2012)
Superconductivity and
magnetism in electron doped
cuprates and pnictide films
H. Luetkens et al.

Developments
• Spin-rotator for LF-µSR, commissioning started in Feb-2012; will give
factor 2 better time resolution (σ ~ 2.5 ns) compared to old setup
• Increasing LEM rate, solid Ne moderator
• A new APD positron spectrometer for the “B-parallel” magnet
• Extension to lower temperatures (from 2.7 K to

LEM spin-rotator for LF-µSR

LEM spin-rotator installation
Jan-17, 2012 Jan-18, 2012
Jan-27, 2012 Feb-27, 2012

LEM spin-rotator (SR): first tests with protons
H + Beamspot at sample position
H 0 energy distribution
H 0 energy distribution
TD
MCP2
Transmission through SR > 90%

LEM group at PSI
T. Prokscha, H. Saadaoui (MaNEP PostDoc) A. Suter, Z. Salman, H.P.
Weber (technician)
Part time: H. Luetkens, E. Morenzoni
Ph. D. students: G. Pascua (part time), E. Stilp (PSI/U Zurich)
Computing support: A. Raselli (part time)
All people of LMU (Laboratory for Muon Spin Spectroscopy):
http://lmu.web.psi.ch/people/people.html