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 Sw issFEL µ Sµ n S SI NQ

γ SLS PSI proton accelerator complex

SINQ, neutron spallation source

Dolly, µSR, GPS clone 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 proton therapy, irradiation facility 50 MHz proton cyclotron, 2.2 mA, 590 MeV, 1.3 MW beam power (2.4 mA, 1.4 MW test operation); Comet cyclotron (superconducting), most powerful cyclotron worldwide and most 250 MeV, 500 nA, 72.8 MHz intense muon beams (quasi-continuous); The PSI isochronous cyclotron

2.2 mA: ~1.4 x 1016 protons/sec @ 590 MeV (1.3 MW on 5x5 mm2 = 50 kW/mm2, 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 >108/sec Larmor frequency of protons: q/(2πm) = 15.25 MHz/T ν = q/(2πγm)∙B, γ = E /mc2 0 tot ν = n∙ν , frequency of accelerating radio-frequency rf 0

Isochronous cyclotron: B (R) ~ γ(R), constant ν ! 0 rf PSI cyclotron: B = 0.554 T, ν = 8.45 MHz, n = 6, 0 0 → ν = 50.7 MHz rf

“Decay” and “Surface” muon beams

Jess Brewer, UBC & TRIUMF: 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 105/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 “acci- dental” background in µ-decay histograms. Continuous beam µSR experiment

ωµ = γ µ · Β µ ( γ µ = 135.5 M H z/T)

P ( t = 0 ) P ( t ) + s µ p µ e

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 B e x t P o s i t r o n - C o u n t e r C o u n t e r

Accepted rate as a function of µ rate Continuous versus pulsed muon beams

● time resolution ≤ 1 ns, ν > 100 MHz Observable asymmetry as a function of time resolution σ : t ● fast depolarization observable µ -1 ω ω 2σ 2 ( >> 10 s ) A( ) = A0 x exp(- t /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 – 60o); 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 – 45o); 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 – 90o); 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. Muons On Request (MORE)

● 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

● if decay e+ is detected in GPS the next muon is requested from the kicker

Kicker (two electrodes, +-5kV)

“normal” MORE pulsed

B /N [10-5] 660 9 1 0 0 ∆t [ns] <1 <1 80 Events [106/h] 12 20 20-40 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 : 2 Knight shift components, very close 3 Dramatic and opposite T dependence around

TN (6K).

UPt3 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 σ -1 µ ~1.35 GHz < 0.1 µs @ 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

ν Sample Scintillators

ø

µ+ e+ Veto Ø (mm) N A F counter Muon ν counter 14 36’400 0.31 ± 0.01 59.1 60 20 30’800 0.33 ± 0.01 57.9 50

40 24 26’300 0.34 ± 0.01 55.1

30 30 19’900 0.39 ± 0.01 55.0 20 40 12’000 0.41 ± 0.01 44.9 10

0 50 7’300 0.41 ± 0.01 35.0 10 20 30 40 50 60 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 1 102 Cu “Surface Muons” from π+ bulk decay at rest ( ~ 4 MeV) µm 10 thin films, e generally used for bulk g 1 n multilayers.. studies: no depth resolu-

a 2

R 10 tion

nm 10 1 1 10 102 103 104 Energy [keV]

. “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

m 200 Stopping profiles calculated with Monte n Range Carlo code Trim.SP by W. Eckstein, MPI Variance 150 Garching, Germany.

100 Experimentally tested for muons: 50 E. Morenzoni, H. Glückler, T. Prokscha, R. YBa2Cu3O7 Khasanov, H. Luetkens, M. Birke, E. M. 0 Forgan, Ch. Niedermayer, M. Pleines, 0 5 10 15 20 25 30 35 NIM B192, 254 (2002). Energy [keV]

0,05 3.4 keV y t i s YBa2Cu3O7 n 0,04 6.9 keV e D 0,03 15.9 keV g 20.9 keV n i

24.9 keV p 0,02 p

o 29.4 keV t

S 0,01

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. Intensity: ~15 LE-µ +/sec Nagamine, M. Iwasaki, Y. Ikedo, K. Polarization: ~ 50% Shimomura, P. Strasser, S. Makimura Nucl. Instr Meth. B 266, 335 (2008) 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). Generation of polarized epithermal muons

„Surface“ Muons ~ 4 MeV ~ 100% polarized

~100 µm Ag ~500 nm 6 K s-Ne, Ar, s-N2 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

y

r

t

e

m

m

y

)

s

t

( L A

P

A P(0) ≅1

T i m e [ µ s ] E. Morenzoni, T. Prokscha, A. Suter, H. Luetkens, R. E. Morenzoni, F. Kottmann, D. Maden, B. Matthias, M. Meyberg, Khasanov, J.Phys.: Cond. Matt. 16, S4583 (2004). T. Prokscha, T. Wutzke, U. Zimmermann, PRL 72, 2793 (1994).

 suppression of electronic energy loss for E > Eg, large band gap Eg (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, > 108 µ+/s):

ε ∆Ω ∆ ∆ -4 -5 µ+ = Nepith/N4MeV ≈ (1-FMu ) L/ R ≈ 0.25 L/ R ≈ 10 – 10

∆Ω: probability to escape into vacuum (~50% for isotropic angular distribution) F : muonium formation probability Mu page 28 LE-µ+ Apparatus at µE4 beam line

At 2.2 mA proton current: ~4.6 • 108 µ +/s total, ∆p/p = 9.5% (FWHM) ~1.9 • 108 µ+/s on LEM moderator ~1.1 • 104 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 LN2 cooled ~1.9 •108 µ+/s rebuilt µE4 beam line

Polarized Low Energy Muon ~ 11000 µ +/s; accelerate Beam up to 20 keV Energy: 0.5-30 keV ∆E, ∆t: 400 eV, 5 ns Depth: 1 – 300 nm Polarization ~100 % Beam Spot: 12 mm (FWHM)

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

(LN2 cooled)

10-nm-thin C-foil Source Filter

INPC 2010, July-05 2010 Implantation time Sample Depth dependent LE-µSR measurements

July-05 2010 page 33 Magnetic field profiles in superconductors

 Direct, absolute measurement of “ l o c a l ” magnetic penetration depth

* m effective mass ξ λ(T) ∝ “ n o n - l o c a l ” density of ns(T) supercarriers ξ  Direct Test of theories (London, BCS) z − λ 0 → = ab (T) B(z) B0e

λ z B(z)

 Local, non-local response nonlocal local  Determination of the coherence length ξ, κ=λ/ξ z Magnetic field profiles in Pb and YBa Cu O 2 3 7-δ

Lead, T =7.0(2) K, h = 91.5(3)G, c ext ξ = 90(5)nm, λ = 58(3)nm YBa Cu O , T=20K, T =87.5K 0.01 0 0 2 3 7-δ c h = 91.5(3) G, 0.01 ext ξ = 1.5 nm fixed, λ = 137(10) nm 0 0 )

T 6.66K ( )

T B (

B

6.19K

h exp(-z/λ(T)) ext 3.4 keV 8.9 keV 15.9 keV 20.9 keV 29.4 keV 1E-3 1E-3 2.85K

0 50 100 150 0 50 100 150 z (nm) z (nm) Non-local: non- exponential 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 YBa2Cu3O6.92

samples produced by R. Liang, W. Hardy, D. Bonn, Univ. of British Columbia

m* effective mass λ(T) ∝ ns(T) density of super carriers

λ 2 ρ 1/ ~ ns/m* ≡ s, superfluid density Test of theories (London, BCS), symmetry of the sc gap Detwinned YBa2Cu3O6.92, Tc = 94.1 K, B = 9.47 mT

100 K

8 K

8 K

m* effective mass at low T for a d-wave SC: λ(T) ∝ ns(T) density of super carriers

λ λ a = 126(1.2)nm, b = 105.5(1.0) nm, λ λ a/ b = 1.19(1)

R.F. Kiefl et al, PRB 81, 180502(R), (2010)

page 37 Meissner effect in a strongly underdoped cuprate

La Sr CuO - La Sr CuO - La Sr Cu 1.84 0.16 4 1.94 0.06 4 1.84 0.16 4

T = 32 K T' < 5 K T = 32 K c c c

induced superfluid density disappears at T >>T' . This eff c 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 /LaAlO superlattices • 3 3

2 u.c. LaNiO : MI and AF transitions at T < 150 K TMI • 3 TMI 4 u.c. LaNiO : metallic and paramagnetic at all T • 3 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 metal- insulator transition at x ~ 0.03 muon decay asymmetry (10 mT TF top) and zero field (ZF) relaxation rate (bottom) as a function of • FM coupling between Mn before full emergence of temperature and doping itinerant hole carriers 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

Magnetoresistance vs B Long coherence time of injected Alq3 spins ~10-5 s  measurable static fields Spin diffusion length vs T corre- lates with magneto-resistance

Field distribution: I on - I off

First direct measurement of spin diffusion length in a Skewness working spin valve.

A.J. Drew et al., Nature Materials 8, 109 (2009)

INPC 2010, July-05 2010 “Cold” muonium from mesoporous SiO in vacuum 2

Motivated by recent results on Ps emission from mesoporous SiO films 2

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 Vacuum Mu fraction FV as a function of confirmed) 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) Superconductivity and T. Prokscha et al., Physcia B 404, 866 (2009) Magnetism in D.G. Eshchenko et al., Physica B 404, 873 (2009) T H.V. Alberto et al., Physica B 404, 870 (2009) N La2CuO4/La1.56Sr0.44CuO4 Superlattices Photo-induced A. Suter et al., PRL 106, effects in Tc 237003 (2011) semiconductors T. Prokscha et al. doping Superfluid density in high and

low Tc heterostructures Current effects on magnetism B. Wojek et al., PRB 85, 024505 (2012) and superconductivity in a thin

La1.94Sr0.06CuO4 wire Superconductivity and 300μ M. Shay et al., PRB 80, 144511 (2009) 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 <~2.0 K)

• External stimulus: ongoing developments on E-field, illumination, current

• Feasibility of a vector magnet at the sample

• Improve beam spot at sample, or active tracking detector system 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

H0 energy distribution H0 energy distribution

Transmission through SR > 90% TD MCP2 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