Wir schaffen Wissen heute für morgen

Transcription

1 Wir schaffen Wissen heute für morgen Paul Scherrer Institut Thomas Prokscha, Laboratory for Muon Spin Spectroscopy Low energy muons at PSI Future Muon Sources, Univ. Huddersfield, January 13, 2015

2 Range of Muons in Matter mm 1 2 Range 10 m Cu thin films, multilayers.. Surface Muons from + decay at rest ( ~ 4 MeV) generally used for bulk studies: no depth resolution bulk 10 nm Energy [kev] Low-energy muons : kev Allows depth-dependent SR investigations ( ~ nm) Extends the use of SR to new objects of investigtions New magnetic/spin probe for thin films, multilayers, surface regions, buried layers

3 Implantation Profiles of Low Energy Muons nm 200 Stopping profiles calculated with Monte Carlo code Trim.SP by W. Eckstein, MPI Garching, Germany. Range Variance Stopping Density 0 YBa2Cu3O , Energy [kev] 3.4 kev 0, kev 15.9 kev 0,03 YBa2Cu3O kev 24.9 kev 0, kev 0,01 0, Depth [nm] 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).

4 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 m 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).

5 Generation of polarized epithermal muons Surface Muons ~ 4 MeV ~ 100% polarized ~100 m 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). A. Hofer, PhD thesis, U Konstanz (1998).

6 L AP(t) Characteristics of epithermal muons P(0) 1 T im e [ s ] E. Morenzoni, T. Prokscha, A. Suter, H. Luetkens, R. Khasanov, J.Phys.: Cond. Matt. 16, S4583 (2004). 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 > Eg, large band gap Eg (10-20 ev) soft, perfect insulators large escape depth L ( nm), no loss of polarization during moderation (~10 ps) moderation efficiency is low (requires high intensity + beam, > 108 +/s): + = Nepith/N4MeV (1 FMu ) L/ R 0.25 L/ R : probability to escape into vacuum (~50% for isotropic angular distribution) FMu: muonium formation probability page 7

7 High-intensity E4 beam for LE- + production Goal: >108 +/s on moderator target (3x3 cm2) no modification of the pion/muon target region no modification of the main shielding of proton beam, i.e. reconstruct existing beam line to obtain maximum acceptance at limited space: use a solenoid as the first focusing element (normal conducting, limiting p) use large aperture radius (200 mm) quadrupole triplets for subsequent transport to obtain large transmission use large vacuum tubes (diameter 500 mm) first solenoid and bending magnet: radiation hard coils Completed in LEM user operation since 2006

8 Solenoid versus quadrupole First order transfer matrix for static magnetic system with midplane symmetry: First order transfer matrix for a solenoid, mixing of horizontal and vertical phase space: Mixing of phase space might lead to an increase of beam spot size Rotation of phase space: 90 x-y PS exchanged Focusing powers PS,T of solenoid and triplet at same power dissipation in device: Azimuthal symmetry of solenoids leads to larger acceptance

9 Layout of the E4 high-intensity beam

10 2005: LE- + rebuilt E4 beam line At 2.2 ma proton current: ~ /s total, p/p = 9.5% (FWHM) ~ /s on LEM moderator ~ /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).

11 Low energy + beam and set-up for LE- SR (LEM) electrostatic mirror - UHV system, mbar surface + beam, ~4 MeV - some parts LN2 cooled Spin MCP detector Polarized Low Energy Muon Beam Energy: kev E, t: 400 ev, 5 ns Depth: nm Polarization ~100 % Beam Spot: 12 mm (FWHM) Spin rotator (E x B) Einzel lens (LN2 cooled) B = T, T sample surface T = K moderator ~ /s; accelerate up to 20 kev Einzel lens (LN2 cooled) Beam spot at sample Start detector (10 nm C foil) Sample environment: ~ /s Conical lens at sample: up to ~ /s Sample cryostat e+ detectors

12 LEM spin-rotator for LF- SR Muon momentum Muon spin E-Field B-Field (10 nm)

13 LEM spectrometer for TF/LF- SR 0.6m B-Field BC400 scintillators, coated with reflective paint Avalanche Photo Detectors (APDs) B = T. Positron (e+) scintillators split in upstream and downstream detectors; use wavelength shifting fibres (blue-to-green) to guide the light to green-sensitive APDs (1mm2, Photonique SSPM 0810G). Solid angle and e+ rates optimized with musrsim: Use 1-mm Ti vacuum tube containing the sample Shape of the scintillators Use carbon-fibre support structure for the e+ counters

14 LEM science, some selected topics

15 Depth dependent LE- SR measurements July page 16

16 Magnetic field profiles in Pb and YBa2Cu3O7- Lead, Tc=7.0(2) K, hext = 91.5(3)G, 0 = 90(5)nm, 0 = 58(3)nm 0.01 YBa2Cu3O7-, T=20K, Tc=87.5K hext = 91.5(3) G, 0 = 1.5 nm fixed, 0 = 137(10) nm 6.66K B (T) B (T) K hext exp(-z/ (T)) 3.4 kev 8.9 kev 15.9 kev 20.9 kev 29.4 kev 1E-3 1E K z (nm) Non-local: non- exponential z (nm) local: exponential T.J. Jackson et al., PRL 84, 4958 (2000). A. Suter et al., PRL 92, (2004). A. Suter et al., PRB 72, (2005); V. Kozhevnikov et al., PRB 87, (2013).

17 Low Energy Muon Applications Meissner effect in strongly underdoped cuprate Dimensionality control of electronic phase transition in Ni-oxide superlattices La1.84Sr0.16CuO4- La1.94Sr0.06CuO4- La1.84Sr0.16Cu4 100-nm-thick NxN u.c. LaNiO3/LaAlO3 superlattices 2 u.c. LaNiO3: MI and AF transitions at T < 150 K Tc = 32 K T'c < 5 K Tc = 32 K E. Morenzoni et al., Nat. Comm. 2, 272 (2011) 4 u.c. LaNiO3: metallic and paramagnetic at all T A.V. Boris et al., Science 332, 937 (2011)

18 Low Energy Muon Applications Spatially homogeneous ferromagnetism in (Ga,Mn)As Spin diffusion length in organic spin valves Field distribution: I First direct measurement of spin diffusion length in a working spin valve. S.R. Dunsiger et al., Nat. Mat. 9, 299 (2010) A.J. Drew et al., Nat. Mat. 8, 109 (2009) on - I off

19 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, (R) (2005) I. McKenzie et al., PRE 89, (2014) Formation of hydrogen impurities in semiconductors at low energies T. Prokscha et al., PRL 98, (2007) T. Prokscha et al., PRB 90, (2014) D.G. Eshchenko et al., Physica B 404, 873 (2009) Z. Salman et al., PRL 113, (2014) Photo-induced effects in semiconductors T. Prokscha et al. Sci. Rep. 3, 2569 (2013) 300μ Current effects on magnetism and superconductivity in a thin La1.94Sr0.06CuO4 wire M. Shay et al., PRB 80, (2009) Superconductivity and Magnetism in TN La2CuO4/La1.56Sr0.44CuO4 Superlattices A. Suter et al., PRL 106, Tc (2011) doping Superfluid density in high and low Tc heterostructures B. Wojek et al., PRB 85, (2012) Superconductivity and magnetism in electron doped cuprates films H. Saadaoui et al., Nat. Comm. accepted

20 Low Energy Muon Applications Superconducting Nb RF-cavities Photo-persistent change of Meissner screening in YBa2Cu3O6+x A. Romanenko et al., APL 104, (2014) E. Stilp et al. Sci. Rep. 4, 6250 (2014) Depth-dependent spin dynamics in TbPc2 thin films Photo-induced persistent inversion of Ge in a 200nm-deep surface region T. Prokscha et al., Sci. Rep. 3, 2569 (2013) A. Hofmann et al., ACS NANO 6, 8390 (2012)

21 Low Energy Muon Applications Mu states in CZTS solar cell material Magnetic phase diagram of low-doped La2-xSrxCu2O4 thin films E. Stilp et al., PRB 88, (2013) H.V. Alberto et al., J.Phys.: Conf. Ser. 551, (2014) Depth-dependent ionization energy of shallow hydrogen in ZnO and CdS Spatially homogeneous ferromagnetism in EuO1-x thin films T. Prokscha et al., PRB 90, (2014) P. Monteiro et al., PRL 110, (2013)

22 Cold muonium from mesoporous SiO2 in vacuum Motivated by recent results on Ps emission from mesoporous SiO2 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 FVMu as a function of temperature and energy; solid lines are a fit of a diffusion model to the data. A. Antognini et al., PRL 108, (2012): cold Mu enables cw precision spectroscopy on Mu 1S-2S transition.

23 Developments Increasing LEM rate, solid Ne moderator Sample transfer chamber Extension to lower temperatures (from 2.7 K to < 2.0 K) External stimulus: ongoing developments on E-field, illumination, current, RF Feasibility of a vector magnet at the sample Improve beam spot on moderator, at sample, tracking detector system Computing: use of graphics cards and MICs for faster fitting and simulation

24 LEM group at PSI T. Prokscha, A. Suter, Z. Salman, H.P. Weber (technician) Part time: H. Luetkens PhD student: E. Stilp (PSI/U Zurich), just finished Guest: R. Xiao (PhD student at USTC), U. Locans (PhD student U Riga) Computing support: A. Raselli (part time)