Scientific Highlights 2011

The following selection of scientific highlights comprises either publications with NUM staff members as part of the list of authors or publications based on experiments, which were performed at one of the three user facilities operated by NUM: SINQ, SμS or Particle Physics.
We report on the recovery of the short-range static magnetic order and on the concomitant degradation of the superconducting state in optimally F-doped SmFe1-xRuxAsO0.85F0.15 for 0.1≤x≲0.5. The two reduced order parameters coexist within nanometer-size domains in the FeAs layers and eventually disappear around a common critical threshold xc∼0.6. Superconductivity and magnetism are shown to be closely related to two distinct well-defined local electronic environments of the FeAs layers. The two transition temperatures, controlled by the isoelectronic and diamagnetic Ru substitution, scale with the volume fraction of the corresponding environments. This fact indicates that superconductivity is assisted by magnetic fluctuations, which are frozen whenever a short-range static order appears, and totally vanish above the magnetic dilution threshold xc.
Facility: SμS

Reference: S. Sanna et al, Physical Review Letters 107, 227003 (2011)

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Microscopic Coexistence of Superconductivity and Magnetism in Ba1-xKxFe2As2

It is widely believed that, in contrast to its electron-doped counterparts, the hole-doped compound Ba1-xKxFe2As2 exhibits a mesoscopic phase separation of magnetism and superconductivity in the underdoped region of the phase diagram. Here, we report a combined high-resolution x-ray powder diffraction and volume-sensitive muon spin rotation study of Ba1-xKxFe2As2 showing that this paradigm does not hold true in the underdoped region of the phase diagram (0≤x≤0.25). Instead we find a microscopic coexistence of the two forms of order. A competition of magnetism and superconductivity is evident from a significant reduction of the magnetic moment and a concomitant decrease of the magnetoelastically coupled orthorhombic lattice distortion below the superconducting phase transition.
Facility: SμS

Reference: E. Wiesenmayer et al, Physical Review Letters 107, 237001 (2011)

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Muon spin rotation investigation of the pressure effect on the magnetic penetration depth in YBa2Cu3Ox

The pressure dependence of the magnetic penetration depth λ in polycrystalline samples of YBa2Cu3Ox with different oxygen concentrations x=6.45, 6.6, 6.8, and 6.98 was studied by muon spin rotation (μSR). The pressure dependence of the superfluid density ρs∝1/λ2 as a function of the superconducting transition temperature Tc is found to deviate from the usual Uemura line. The ratio (∂Tc/∂P)/(∂ρs/∂P) is smaller by a factor of ≃2 than that of the Uemura relation. In underdoped samples, the zero-temperature superconducting gap Δ0 and the BCS ratio Δ0/kBTc both increase with increasing external hydrostatic pressure, implying an increase of the coupling strength with pressure. The relation between the pressure effect and the oxygen isotope effect on λ is also discussed. In order to analyze reliably the μSR spectra of samples with strong magnetic moments in a pressure cell, a special model was developed and applied.
Featured as PRB Synopsis article, more information
Facility: SμS

Reference: A. Maisuradze et al, Physical Review B 84, 184523 (2011)

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Standard model Higgs-boson branching ratios with uncertainties

We present an update of the branching ratios for Higgs-boson decays in the Standard Model. We list results for all relevant branching ratios together with corresponding uncertainties resulting from input parameters and missing higher-order corrections. As sources of parametric uncertainties we include the masses of the charm, bottom, and top quarks as well as the QCD coupling constant. We compare our results with other predictions in the literature.
Reference: LHC Higgs Cross Section Working Group, A. Denner et al, European Physical Journal 71, 1753 (2011)

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The MEG experiment at PSI reaches an unprecedented level of precision in the search for the μ → e γ decay

The international collaboration forming the MEG-experiment, one of PSI’s “flagship” particle physics experiments at the proton accelerator facility in Switzerland, is publishing its latest results in the search for “New Physics” beyond the Standard Model (SM), in the journal Physical Review Letters. The goal of the experiment is the search for the existence of the decay μ → e γ, a quest that started more than sixty years ago using cosmic-ray muons and now uses one of the world’s most intense surface muon beams. Data collected by MEG in 2009-2010 shows no evidence, as yet, of an excess of events above the expected background. However, even while collecting thirty-million decays a second in the detector, the experiment will require about two more years of data-taking to reach the expected sensitivity. This new MEG result does however exclude, with 90% confidence, that more than one muon in approximately five-hundred billion, does decay into a positron (positive electron) and photon. This therefore translates into the most stringent constraint on the existence of the μ → e γ decay to date (Branching Ratio B ≤ 2.4⋅10-12, 90% C.L.) and improves the previous best limit by a factor of five.
Facility: Particle Physics, MEG

Reference: J. Adam et al., MEG collaboration, Physical Review Letters 107, accepted for publication (October 2011)

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Surfactant mediated growth of Ti/Ni multilayers

The surfactant mediated growth of Ti/Ni multilayers is studied. They were prepared using ion beam sputtering at different adatom energies. It was found that the interface roughness decreased significantly when the multilayers were sputtered with Ag as surfactant at an ion energy of 0.75 keV. On the other hand, when the ion energy was increased to 1 keV, it resulted in enhanced intermixing at the interfaces and no appreciable effect of Ag surfactant could be observed. On the basis of the obtained results, the influence of adatom energy on the surfactant mediated growth mechanism is discussed.
Facility: SINQ

Reference: M. Gupta et al et al, Applied Physics Letters 98, 101912 (2011)

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Pressure Effects in the Isoelectronic REFe0.85Ir0.15AsO System

The effect of chemical and hydrostatic pressure has been studied systematically in a selected system belonging to the 1111 family of iron pnictide high-temperature superconductors. The results show a surprising similarity between the trend of critical temperature vs hydrostatic pressure for isoelectronic samples with different rare earths (RE) on the RE site and samples of the SmFeAsO1-xFx series with different doping levels. These results open new questions about the underlying mechanism for superconductivity in iron pnictides.
Reference: B. Maroni et al, Journal of the American Chemical Society 133, 3252 (2011)

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Dimerization and Charge Order in Hollandite K2V8O16

The metal-insulator transition occurring in hollandite K2V8O16 has been studied by means of neutron and x-ray diffraction as well as by thermodynamic and electron-spin resonance measurements. The complete analysis of the crystal structure in the distorted phase allows us to identify dimerization as the main distortion element in insulating K2V8O16. At low-temperature, half of the V chains are dimerized perfectly explaining the suppression of magnetic susceptibility due to the formation of spin singlets. The dimerization is accompanied by the segregation of charges into chains.
Facility: SINQ

Reference: A.C. Komarek et al, Physical Review Letters 107, 027201 (2011)

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Three-dimensional visualization and quantification of water content in the rhizosphere

It has long been known that roots alter the soil in their immediate vicinity, where other microorganisms live and the chemical composition is altered compared to that further away from the roots. An international research team has now demonstrated in experiments at the Paul Scherrer Institute that the soil in the vicinity of roots also contains more water – contrary to the earlier belief that there must be less water in this region, as the plant takes up water from the soil. Apparently, however, plants create a small water reserve that helps to tide them over through short periods of drought. These findings could help, in the long term, in the breeding of plants to cope better during periods of drought or in support of the development of efficient irrigation systems. These results were obtained from experiments carried out with the benefit of neutron tomography at the Paul Scherrer Institute, using a method that makes it possible to exactly show the distribution of water to a fraction of a millimetre, without having to remove a plant from the soil.
Facility: SINQ

Reference: AB Moradi et al, New Phytologist 192, 653 (2011)

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Search for Bs → μ+μ- decays in pp collisions at √s = 7 TeV

Bs mesons, composed of a 'beauty' and a 'strange' quark, are produced copiously at the LHC. The fraction that subsequently decays (known as the 'branching fraction') to a pair of muons is highly suppressed in the Standard Model - only about three such decays are expected per billion Bs meson produced. Several extensions of the SM, for instance supersymmetric models, predict (significant) enhancements of this branching fraction. CMS has searched for the decays of Bs to muon pairs using proton-proton collision data collected up to June 2011. The CMS (barrel) pixel detector, developed and constructed at PSI, has a central role in reconstructing the signal with high efficiency while strongly reducing the background. The number of candidate decays observed in the available data sample is consistent with the Standard Model expectations for signal and background. CMS has excluded (at 95% confidence level) branching fractions larger than 1.9e-8 for the decay Bs → μ+μ-. This result is among the most stringent exclusion limits achieved until now and implies significant constraints on extensions of the Standard Model. The CMS result has been accepted for publication in PRL and has been combined with the corresponding result of the LHCb collaboration.
Facility: Particle Physics, CMS

Reference: CMS collaboration (author), Physical Review Letters 107, accepted for publication (2011)

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Direct Observation of Local Mn-Mn Distances in the Paramagnetic Compound CsMnxMg1-xBr3

We introduce a novel method for local structure determination with a spatial resolution of the order of 0.01 Å. It can be applied to materials containing clusters of exchange-coupled magnetic atoms. We use neutron spectroscopy to probe the energies of the cluster excitations which are determined by the interatomic coupling strength J. Since for most materials J is related to the interatomic distance R through a linear relation dJ/dR = α (for dR/R << 1), we can directly derive the local distance R from the observed excitation energies. This is exemplified for the mixed one-dimensional paramagnetic compound CsMnxMg1-xBr3 (x=0.05,0.10) containing manganese dimers oriented along the hexagonal c axis. Surprisingly, the resulting Mn-Mn distances R do not vary continuously with increasing internal pressure but lock in at some discrete values.
Facility: SINQ

Reference: A. Furrer et al, Physical Review Letters 107, 115502 (2011)

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SANS and UV-vis Spectroscopy of Resultant Structure from Lysozyme Adsorption on Silica Nanoparticles

The interaction of lysozyme protein (M.W. 14.7kD) with two sizes of silica nanoparticles (16 and 25nm) has been examined in aqueous solution using UV-vis spectroscopy and small-angle neutron scattering (SANS). The measurements were perfumed on fixed concentration (1 wt %) of nano particles and varying concentration of protein in the range 0 to 2 wt %...
Facility: SINQ

Reference: S. Kumar et al, Langmuir 27, 10167 (2011)

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Unconventional Magnetism in a Nitrogen-Containing Analog of Cupric Oxide

We have investigated the magnetic properties of CuNCN, the first nitrogen-based analog of cupric oxide CuO. Our muon-spin relaxation, nuclear magnetic resonance, and electron-spin resonance studies reveal that classical magnetic ordering is absent down to the lowest temperatures. However, a large enhancement of spin correlations and an unexpected inhomogeneous magnetism have been observed below 80 K. We attribute this to a peculiar fragility of the electronic state against weak perturbations due to geometrical frustration, which selects between competing spin-liquid and more conventional frozen states.
Facility: SμS

Reference: A. Zorko et al, Physical Review Letters 107, 047208 (2011)

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Effect of the Molecular Structure on the Hierarchical Self-Assembly of Semifluorinated Alkanes at the Air/Water Interface

Semifluorinated alkanes (CnF2n+1CmH2m+1), short FnHm display local phase separation of mutually incompatible hydrocarbon and fluorocarbon chain moieties, which has been utilized as a structure-forming motif in supramolecular architectures. The packing of semifluorinated alkanes, nominally based on dodecyl subunits, such as perfluoro(dodecyl)dodecane (F12H12) and perfluoro(dodecyl)eicosane (F12H20), as well as a core extended analogue, 1,4-dibromo-2-((perfluoroundecyl)methoxy)-5-(dodecyloxy)benzene) (F11H1-core-H12), was studied at the air/water interface. Langmuir monolayers were investigated by means of neutron reflectivity directly at the air/water interface and scanning force microscopy after transfer to silicon wafers...
Facility: SINQ

Reference: L. de Viguerie et al, Langmuir 27, 8776 (2011)

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The effect of hydrostatic pressure over the swelling of microgel particles

We review our recent results on the use of hydrostatic pressure to change the size and the structure of microgel particles based on poly-(N-isopropylacrylamide). These changes are brought about through changes in the miscibility of the polymer in the solvent. Swelling induced by hydrostatic pressure can thus be thought of as an alternative to swelling induced by temperature, an interesting fact that can be exploited in the study of fundamental problems in soft condensed matter.
Facility: SINQ

Reference: JJ Lietor-Santos et al, Soft Matter 7, 6370 (2011)

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Two-Dimensional Magnetic and Superconducting Phases in Metal-Insulator La2-xSrxCuO4 Superlattices Measured by Muon-Spin Rotation

We show, by means of low-energy muon-spin rotation measurements, that few-unit-cells thick La2CuO4 layers synthesized digitally by molecular beam epitaxy are antiferromagnetically ordered. Below a thickness of about 5 CuO2 layers the long-range ordered state breaks down, and a magnetic state appears with enhanced quantum fluctuations and a reduced spin stiffness. This magnetic state can exist in close proximity (few Å) to high-temperature superconducting layers, without transmitting supercurrents.
Facility: SμS

Reference: A. Suter et al, Physical Review Letters 106, 237003 (2011)

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Dimensionality Control of Electronic Phase Transitions in Nickel-Oxide Superlattices

The competition between collective quantum phases in materials with strongly correlated electrons depends sensitively on the dimensionality of the electron system, which is difficult to control by standard solid-state chemistry. We have fabricated superlattices of the paramagnetic metal lanthanum nickelate (LaNiO3) and the wide-gap insulator lanthanum aluminate (LaAlO3) with atomically precise layer sequences. We used optical ellipsometry and low-energy muon spin rotation to show that superlattices with LaNiO3 as thin as two unit cells undergo a sequence of collective metal-insulator and antiferromagnetic transitions as a function of decreasing temperature, whereas samples with thicker LaNiO3 layers remain metallic and paramagnetic at all temperatures. Metal-oxide superlattices thus allow control of the dimensionality and collective phase behavior of correlated-electron systems.
Facility: SμS

Reference: A.V. Boris et al, Science 332, 937 (2011)

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Magnetic and non-magnetic phases of a quantum spin liquid

A quantum spin-liquid phase is an intriguing possibility for a system of strongly interacting magnetic units in which the usual magnetically ordered ground state is avoided owing to strong quantum fluctuations. It was first predicted theoretically for a triangular-lattice model with antiferromagnetically coupled S = 1/2 spins. Recently, materials have become available showing persuasive experimental evidence for such a state. Although many studies show that the ideal triangular lattice of S = 1/2 Heisenberg spins actually orders magnetically into a three-sublattice, non-collinear 120° arrangement, quantum fluctuations significantly reduce the size of the ordered moment. This residual ordering can be completely suppressed when higher-order ring-exchange magnetic interactions are significant, as found in nearly metallic Mott insulators. The layered molecular system κ-(BEDT-TTF)2Cu2(CN)3 is a Mott insulator with an almost isotropic, triangular magnetic lattice of spin-1/2 BEDT-TTF dimers that provides a prime example of a spin liquid formed in this way. Despite a high-temperature exchange coupling, J, of 250 K, no obvious signature of conventional magnetic ordering is seen down to 20 mK. Here we show, using muon spin rotation, that applying a small magnetic field to this system produces a quantum phase transition between the spin-liquid phase and an antiferromagnetic phase with a strongly suppressed moment. This can be described as Bose–Einstein condensation of spin excitations with an extremely small spin gap. At higher fields, a second transition is found that suggests a threshold for deconfinement of the spin excitations. Our studies reveal the low-temperature magnetic phase diagram and enable us to measure characteristic critical properties. We compare our results closely with current theoretical models, and this gives some further insight into the nature of the spin-liquid phase.
Facility: SμS

Reference: F.L. Pratt et al, Nature 471, 612 (2011)

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The Meissner effect in a strongly underdoped cuprate above its critical temperature

The Meissner effect and associated perfect 'bulk' diamagnetism together with zero resistance and gap opening are characteristic features of the superconducting state. In the pseudogap state of cuprates, unusual diamagnetic signals and anomalous proximity effects have been detected, but a Meissner effect has never been observed. Here we probe the local diamagnetic response in the normal state of an underdoped La1.94Sr0.06CuO4 layer (Tc'≤5 K), which is brought into close contact with two nearly optimally doped La1.84Sr0.16CuO4 layers (Tc≈32 K). We show that the entire 'barrier' layer of thickness, much larger than the typical c axis coherence lengths of cuprates, exhibits a Meissner effect at temperatures above Tc' but below Tc. The temperature dependence of the effective penetration depth and superfluid density in different layers indicates that superfluidity with long-range phase coherence is induced in the underdoped layer by the proximity to optimally doped layers, but this induced order is sensitive to thermal excitation.
Facility: SμS

Reference: E. Morenzoni et al, Nature Communications 2, 272 (2011)

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Triggered Release from Liposomes through Magnetic Actuation of Iron Oxide Nanoparticle Containing Membranes

The ideal nanoscale drug delivery vehicle allows control over the released dose in space and time. We demonstrate that this can be achieved by stealth liposomes comprising self-assembled superparamagnetic iron oxide nanoparticles (NPs) individually stabilized with palmityl-nitroDOPA incor- porated in the lipid membrane. Alternating magnetic fields were used to control timing and dose of repeatedly released cargo from such vesicles by locally heating the membrane, which changed its permeability without major effects on the environment.
Facility: SINQ

Reference: E. Amstad et al, Nano Letters 11, 1664 (2011)

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Coexistence of Magnetism and Superconductivity in the Iron-Based Compound Cs0.8(FeSe0.98)2

We report on muon-spin rotation and relaxation (μSR), electrical resistivity, magnetization and differential scanning calorimetry measurements performed on a high-quality single crystal of Cs0.8(FeSe0.98)2. Whereas our transport and magnetization data confirm the bulk character of the superconducting state below Tc=29.6(2) K, the μSR data indicate that the system is magnetic below TN=478.5(3) K, where a first-order transition occurs. The first-order character of the magnetic transition is confirmed by differential scanning calorimetry data. Taken all together, these data indicate in Cs0.8(FeSe0.98)2 a microscopic coexistence between the superconducting phase and a strong magnetic phase. The observed TN is the highest reported to date for a magnetic superconductor.
Facility: SμS

Reference: Z. Shermadini et al, Physical Review Letters 106, 117602 (2011)

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Magnetic structure and spin dynamics of the quasi-one-dimensional spin-chain antiferromagnet BaCo2V2O8

Neutron diffraction and muon spin relaxation studies have revealed intriguing static and dynamical magnetic properties of BaCo2V2O8, a material where the Co magnetic moments lie in an array of one-dimensional chains. We infer a detailed model for the antiferromagnetic AF structure that sets in below a critical temperature, TN = 4.5 K. Our model includes reduced ordered Co moments arranged antiferromagnetically within the chains. This AF state is dominated by quantum fluctuations, a behavior consistent with the expectation of a three-dimensional anisotropic quantum-mechanical spin system. Muons probe different local environments in the crystal structure. The results at high temperatures reveal a homogeneous paramagnetic system, as expected for usual three-dimensional magnetic materials. This situation, however, changes dramatically as the temperature decreases and approaches TN. The magnetic system becomes gradually inhomogeneous, Co moment fluctuations drastically slow down and the low-dimensionality of the magnetic system assumes an increasingly important role with decreasing temperature. However, upon the phase transition at TN the effective dimensionality of the magnetic system abruptly changes to three.
Facility: SINQ, SμS

Reference: Y. Kawasaki et al, Physical Review B 83, 064421 (2011)

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Magnetic vortex lattice in HgBa2CuO4+δ, observed by small-angle neutron scattering

We report the direct observation of the magnetic vortex lattice in the model high-temperature superconductor HgBa2CuO4+δ. Using small-angle neutron scattering on high-quality crystals, we observe two equal domains of undistorted triangular vortex lattices well aligned with the tetragonal crystallographic axes. The signal decreases rapidly with increasing magnetic field and vanishes above 0.4T, which we attribute to a crossover from a three-dimensional to a two-dimensional vortex system, similar to previous results for the more anisotropic compound Bi2.15Sr1.95CaCu2O8+δ. Our result indicates that a triangular vortex lattice (with or without distortion) at low magnetic fields is a generic property of cuprates with critical temperatures above 80K.
Facility: SINQ

Reference: Y. Li et al, Physical Review B 83, 054507 (2011)

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Measurement of the Positive Muon Lifetime and Determination of the Fermi Constant to Part-per-Million Precision

We report a measurement of the positive muon lifetime to a precision of 1.0 ppm; it is the most precise particle lifetime ever measured. The experiment used a time-structured, low-energy muon beam and a segmented plastic scintillator array to record more than 2×1012 decays. Two different stopping target configurations were employed in independent data-taking periods. The combined results give τμ+(MuLan)=2 196 980.3(2.2)  ps, more than 15 times as precise as any previous experiment. The muon lifetime gives the most precise value for the Fermi constant: GF(MuLan)=1.166378(7)×10-5GeV-2 (0.6 ppm). It is also used to extract the μ-p singlet capture rate, which determines the proton’s weak induced pseudoscalar coupling gP.
Facility: Particle Physics

Reference: D.M. Webber et al, Physical Review Letters 106, 041803 (2011)

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Tuning the Structure and the Magnetic Properties of Metallo-supramolecular Polyelectrolyte−Amphiphile Complexes

Self-assembly of Fe2+ ions and the rigid ditopic ligand 1,4-bis(2,2′:6′,2′′-terpyridin-4′-yl)benzene results in metallo-supramolecular coordination polyelectrolytes (MEPE). Sequential self-assembly of MEPE and dialkyl phosphoric acid esters of varying chain length via electrostatic interactions leads to the corresponding polyelectrolyte−amphiphile complexes (PAC), which have liquid−crystalline properties. The PACs have a stratified architecture where the MEPE is embedded in between the amphiphile layers. Upon heating above room temperature, the PACs show either a reversible or an irreversible spin-crossover (SCO) in a temperature range from 360 to 460 K depending on the architecture of the amphiphilic matrix. As the number of amphiphiles per metal ion is increased in the sequence 1:2, 1:4, and 1:6, the temperature of the SCO is shifted to higher values whereas the amphiphile chain length does not have a significant impact on the SCO temperature. In summary, we describe in this article how the structure and the magnetic response function of PACs can be tailored through the design of the ligand and the composition. To investigate the structure and the magnetic behavior, we use X-ray scattering, X-ray absorption spectroscopy, differential scanning calorimetry, faraday-balance, and superconducting quantum interference measurements in combination with molecular modeling.
Facility: SINQ

Reference: G. Schwarz et al, Journal of the American Chemical Society 133, 547 (2011)

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