Scientific Highlights 2016

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.

16 December 2016

Tuning magnetic spirals beyond room temperature with chemical disorder



Frustrated magnets with spiral magnetic orders are of high current interest due to their potential for spintronics and low-power magnetoelectric devices. However, their low magnetic order temperatures (typically <100K) greatly restrict their fields of application. Researchers of PSI have demonstrated that the stability domain of the spiral phase in the perovskite YBaCuFeO5 can be enlarged by more than 150K through a controlled manipulation of the Fe/Cu chemical disorder. Moreover, they showed that this novel mechanism can stabilize the magnetic spiral state of YBaCuFeO5 above the symbolic value of 25C. These findings demonstrate that the properties of a magnetic spiral, including its wavelength and stability range, can be engineered through the control of chemical disorder, offering a great potential for the design of materials with magnetoelectric properties beyond room temperature. This study, fully conducted at the PSI, was an output of the project 'Magnetoelectric coupling at RT: investigating novel routes' funded by the Swiss National Science Foundation.
Facility: SINQ and SLS

Reference: M.Morin et al, Nature Communications 7, 13758 (2016)

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2 December 2016

Magnetic Excitations and Electronic Interactions in Sr2CuTeO6: A Spin-1/2 Square Lattice Heisenberg Antiferromagnet



Sr2CuTeO6 presents an opportunity for exploring low-dimensional magnetism on a square lattice of S=1/2  Cu2+ ions. We employ ab initio multireference configuration interaction calculations to unravel the Cu2+ electronic structure and to evaluate exchange interactions in Sr2CuTeO6. The latter results are validated by inelastic neutron scattering using linear spin-wave theory and series-expansion corrections for quantum effects to extract true coupling parameters. Using this methodology, which is quite general, we demonstrate that Sr2CuTeO6 is an almost ideal realization of a nearest-neighbor Heisenberg antiferromagnet but with relatively weak coupling of 7.18(5) meV.
Facility: SINQ

Reference: P. Babkevich et al, Physical Review Letters 117, 237203 (2016)

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24 November 2016

Electromagnon dispersion probed by inelastic X-ray scattering in LiCrO2



Lattice vibrations (phonons) in crystals are typically weakly interacting with the electronic and magnetic degrees of freedom, such as charge and spin fluctuations. Researchers of PSI together with collaborators from EPF Lausanne, Japan and USA discovered an unexpectedly strong coupling between lattice vibrations and spin fluctuations in the quantum magnet LiCrO2. The observed magnetoelastic waves or electromagnons carry both electric and magnetic dipole moment. This was proven using complementary studies with non-resonant inelastic X-ray scattering at ESRF and on the EIGER thermal neutron triple-axis-spectrometer at SINQ, PSI. The experimental data together with model calculations revealed the underlying coupling mechanism. These results will help to develop better multiferroic materials and demonstrate that inelastic X-ray scattering can probe magnetism with high energy resolution in special systems in strong spin-lattice coupling.
Facility: SINQ

Reference: S. Toth et al, Nature Communications 7, 13547 (2016)

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23 November 2016

Intrinsic Ferromagnetism in the Diluted Magnetic Semiconductor Co:TiO2



Here we present a study of magnetism in Co0.05Ti0.95O2−δ anatase films grown by pulsed laser deposition under a variety of oxygen partial pressures and deposition rates. Energy-dispersive spectrometry and transmission electron microscopy analyses indicate that a high deposition rate leads to a homogeneous microstructure, while a very low rate or postannealing results in cobalt clustering. Depth resolved low-energy muon spin rotation experiments show that films grown at a low oxygen partial pressure (≈10−6 torr) with a uniform structure are fully magnetic, indicating intrinsic ferromagnetism. First principles calculations identify the beneficial role of low oxygen partial pressure in the realization of uniform carrier-mediated ferromagnetism. This work demonstrates that Co:TiO2 is an intrinsic diluted magnetic semiconductor.
Facility: SμS

Reference: H. Saadaoui et al, Physical Review Letters 117, 227202 (2016)

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22 November 2016

Bulk superconductivity at 84 K in the strongly overdoped regime of cuprates



By means of magnetization, specific heat, and muon-spin relaxation measurements, we investigate newly synthesized high-pressure oxidized Cu0.75Mo0.25Sr2YCu2O7.54, in which overdoping is achieved up to p ˜ 0.46 hole/Cu, well beyond the Tc-p superconducting dome of cuprates, where Fermi-liquid behavior is expected. Surprisingly, we find bulk superconductivity with Tc = 84 K and superfluid density similar to those of optimally doped YBa2Cu3O7-δ. On the other hand, specific heat data display a large electronic contribution at low temperature, comparable to that of nonsuperconducting overdoped La2−xSrxCuO4. These results point at an unusual high-Tc phase with a large fraction of unpaired holes. Further experiments may assess the Fermi-liquid properties of the present phase, which would put into question the paradigm that the high Tc of cuprates originates from a non-Fermi-liquid ground state.
Facility: SμS

Reference: A. Gauzzi et al, Physical Review B (Rapid Comm) 94, 180509(R) (2016)

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14 November 2016

The deuteron too poses a mystery

The deuteron — one of the simplest atomic nuclei, consisting of just one proton and one neutron — is considerably smaller than previously thought. This finding was arrived at by an international research group that carried out experiments at the Paul Scherrer Institute, PSI. The new result is consistent with a 2010 study by the same group, in which the researchers measured the proton and found a significantly smaller value than previous research using different experimental methods. The result from 2010 formed the basis for what has been known since then as the proton radius puzzle. The new measurement of the deuteron’s size has now given rise to an analogous mystery. It is possible that this will lead to an adjustment of the Rydberg constant, a fundamental quantity in physics. Another possible explanation is that a physical force as yet unknown is at work. For their experiments the researchers used laser spectroscopy to measure so-called muonic deuterium: an artificial atom consisting of a deuteron orbited by an exotic elementary particle known as a muon. The experiments took place at PSI because the world’s most powerful muon source, available here, was needed to produce sufficient muonic deuterium. The researchers have published their new study of the deuteron’s size in the renowned journal Science.
Facility: Particle Physics

Reference: R. Pohl, F. Nez, L.M.P. Fernandes, F.D. Amaro, F. Biraben, J.M.R. Cardoso, D.S. Covita, A. Dax, S. Dhawan, M.Diepold, A. Giesen, A.L. Gouvea, T. Graf, T.W. Hänsch, P. Indelicato, L. Julien, P. Knowles, F. Kottmann, E.-O. Le Bigot, Y.-W. Liu, J.A.M. Lopes, L. Ludhova, C.M.B. Monteiro, F. Mulhauser, T. Nebel, P. Rabinowitz, J.M.F. dos Santos, L.A. Schaller, K. Schuhmann, C. Schwob, D. Taqqu, J.F.C.A. Veloso, A. Antognini Science 12 August 2016: Vol. 353, no. 6300, page 669

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14 October 2016

Effect of disorder on a pressure-induced z = 1 magnetic quantum phase transition



Pressure-induced ordering close to a z = 1 quantum-critical point is studied in the presence of bond disorder in the quantum spin system (C4H12N2)Cu2(Cl1−xBrx)6 (PHCX) by means of muon-spin rotation and relaxation. As for the pure system (C4H12N2)Cu2Cl6, pressure allows PHCX with small levels of disorder (x ≤ 7.5%) to be driven through a quantum-critical point separating a low-pressure quantum paramagnetic phase from magnetic order at high pressures. However, the pressure-induced ordered state is highly inhomogeneous for disorder concentrations x > 1%. This behavior might be related to the formation of a quantum Griffiths phase above a critical disorder concentration 7.5% < xc < 15%. Br substitution increases the critical pressure and suppresses critical temperatures and ordered moment sizes.
Facility: SμS

Reference: A. Mannig et al, Physical Review B 94, 144418 (2016)

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4 October 2016

Magnetodielectric detection of magnetic quadrupole order in Ba(TiO)Cu4(PO4)4 with Cu4O12 square cupolas



In vortex-like spin arrangements, multiple spins can combine into emergent multipole moments. Such multipole moments have broken space-inversion and time-reversal symmetries, and can therefore exhibit linear magnetoelectric (ME) activity. Three types of such multipole moments are known: toroidal; monopole; and quadrupole moments. So far, however, the ME activity of these multipole moments has only been established experimentally for the toroidal moment. Here we propose a magnetic square cupola cluster, in which four corner-sharing square-coordinated metal-ligand fragments form a noncoplanar buckled structure, as a promising structural unit that carries an ME-active multipole moment. We substantiate this idea by observing clear magnetodielectric signals associated with an antiferroic ME-active magnetic quadrupole order in the real material Ba(TiO)Cu4(PO4)4. The present result serves as a useful guide for exploring and designing new ME-active materials based on vortex-like spin arrangements.
Facility: SINQ

Reference: K. Kimura et al, Nature Communications 7, 13039 (2016)

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5 October 2016

Nanoparticle-Based Magnetoelectric BaTiO3–CoFe2O4 Thin Film Heterostructures for Voltage Control of Magnetism



Multiferroic composite materials combining ferroelectric and ferromagnetic order at room temperature have great potential for emerging applications such as four-state memories, magnetoelectric sensors, and microwave devices. In this paper, we report an effective and facile liquid phase deposition route to create multiferroic composite thin films involving the spin-coating of nanoparticle dispersions of BaTiO3, a well-known ferroelectric, and CoFe2O4, a highly magnetostrictive material. This approach offers great flexibility in terms of accessible film configurations (co-dispersed as well as layered films), thicknesses (from 100 nm to several μm) and composition (5–50 wt % CoFe2O4 with respect to BaTiO3) to address various potential applications. A detailed structural characterization proves that BaTiO3 and CoFe2O4 remain phase-separated with clear interfaces on the nanoscale after heat treatment, while electrical and magnetic studies indicate the simultaneous presence of both ferroelectric and ferromagnetic order. Furthermore, coupling between these orders within the films is demonstrated with voltage control of the magnetism at ambient temperatures.
Facility: SLS

Reference: D. Erdem et al, ACS Nano 10, 9840 (2016)

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28 September 2016

Realizing topological stability of magnetic helices in exchange-coupled multilayers for all-spin-based system



Topologically stabilized spin configurations like helices in the form of planar domain walls (DWs) or vortex-like structures with magnetic functionalities are more often a theoretical prediction rather than experimental realization. In this paper we report on the exchange coupling and helical phase characteristics within Dy-Fe multilayers. The magnetic hysteresis loops with temperature show an exchange bias field of around 1.0 kOe at 10 K. Polarized neutron reflectivity reveal (i) ferrimagnetic alignment of the layers at low fields forming twisted magnetic helices and a more complicated but stable continuous helical arrangement at higher fields (ii) direct evidence of helices in the form of planar 2π-DWs within both layers of Fe and Dy. The helices within the Fe layers are topologically stabilized by the reasonably strong induced in-plane magnetocrystalline anisotropy of Dy and the exchange coupling at the Fe-Dy interfaces. The helices in Dy are plausibly reminiscent of the helical ordering at higher temperatures induced by the field history and interfacial strain. Stability of the helical order even at large fields have resulted in an effective modulation of the periodicity of the spin-density like waves and subsequent increase in storage energy. This opens broad perspectives for future scientific and technological applications in increasing the energy density for systems in the field of all-spin-based engineering which has the potential for energy-storing elements on nanometer length scales.
Facility: SINQ

Reference: S. Fust et al, Scientific Reports 6, 33986 (2016)

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27 September 2016

POLAR experiment successfully launched on Chinese spacecraft



The second Chinese space laboratory satellite Tian Gong 2 was successfully launched from the Jiuquan Satellite Launch Center on September 15th, 2016 at 22:04 BTC (UTC+8h). Among more than ten instruments onboard it also brought to space the only non-Chinese experiment POLAR - the hard X-ray polarimeter. This novel instrument was constructed in collaboration between University of Geneva, the Paul Scherer Institut in Switzerland, the Institute of High Energy Physics CAS in China and the National Center for Nuclear Research in Poland. POLAR is equipped with an array of 1600 plastic scintillators allowing for precise and efficient measurement of the linear polarization from the prompt emission of Gamma Ray Bursts (GRB) – the biggest explosions in the Universe.

The PSI laboratory of particle physics (LTP) has contributed with the initial concept, its verification and validation using the bread board model. In the main project stage we provided the full design, construction and qualification of all electronic subsystems except of power supplies as well as developed the firmware for front-end electronics and central computer.

The POLAR experiment onboard of Tian Gong 2 was switched on the 22nd September at 18:45 BCT. The polarimeter is now undergoing a 21 days long commissioning period followed by a calibration phase. Initial data show that POLAR works fully as expected and its operating conditions are stable. All detection channels are delivering useful data. The PSI POLAR Data Server receives preprocessed telemetry packets from the satellite with a delay of about 24 hours. POLAR onboard Tian Gong 2 is scheduled for two to three years operation in space. It anticipates detections of several tens of very strong GRBs events as well as dozens of intense Solar Flares. POLAR data are expected to provide final explanation for still puzzling physical mechanisms of GRBs. It will also be possible to conduct first precise measurements of the hard X-ray polarization in solar flares.

The first transient event from space was detected by POLAR already on September 23rd. Two other strong transient events were observed on September 25th. Their detection was confirmed by RHESSI satellite - another space mission sensitive to the hard X-rays. RHESSI is devoted mainly for solar observation and is operated by the FHNW in Brugg-Windisch – our collaborators for studies of solar flare eruption mechanisms.
Facility: Particle Physics

19 September 2016

Robust metastable skyrmions and their triangular–square lattice structural transition in a high-temperature chiral magnet



Skyrmions, topologically protected nanometric spin vortices, are being investigated extensively in various magnets. Among them, many structurally chiral cubic magnets host the triangular-lattice skyrmion crystal (SkX) as the thermo- dynamic equilibrium state. However, this state exists only in a narrow temperature and magnetic-field region just below the magnetic transition temperature Tc, while a helical or conical magnetic state prevails at lower temperatures. Here we describe that for a room-temperature skyrmion material, β-Mn-type Co8Zn8Mn4, a field-cooling via the equilibrium SkX state can suppress the transition to the helical or conical state, instead realizing robust metastable SkX states that survive over a very wide temperature and magnetic-field region. Furthermore, the lattice form of the metastable SkX is found to undergo reversible transitions between a conventional triangular lattice and a novel square lattice upon varying the temperature and magnetic field. These findings exemplify the topological robustness of the once-created skyrmions, and establish metastable skyrmion phases as a fertile ground for technological applications.
Facility: SINQ

Reference: K. Karube et al, Nature Materials 15, 1237 (2016)

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16 September 2016

Blocking Gastric Lipase Adsorption and Displacement Processes with Viscoelastic Biopolymer Adsorption Layers



Delayed fat digestion might help to fight obesity. Fat digestion begins in the stomach by adsorption of gastric lipases to oil/water interfaces. In this study we show how biopolymer covered interfaces can act as a physical barrier for recombinant dog gastric lipase (rDGL) adsorption and thus gastric lipolysis. We used β-lactoglobulin (β-lg) and thermosensitive methylated nanocrystalline cellulose (metNCC) as model biopolymers to investigate the role of interfacial fluid dynamics and morphology for interfacial displacement processes by rDGL and polysorbate 20 (P20) under gastric conditions. Moreover, the influence of the combination of the flexible β-lg and the elastic metNCC was studied. The interfaces were investigated combining interfacial techniques, such as pendant drop, interfacial shear and dilatational rheology, and neutron reflectometry. Displacement of biopolymer layers depended mainly on the fluid dynamics and thickness of the layers, both of which were drastically increased by the thermal induced gelation of metNCC at body temperature. Soft, thin β-lg interfaces were almost fully displaced from the interface, whereas the composite β-lg−metNCC layer thermogelled to a thick interfacial layer incorporating β-lg as filler material and therefore resisted higher shear forces than a pure metNCC layer. Hence, with metNCC alone lipolysis by rDGL was inhibited, whereas the layer performance could be increased by the combination with β-lg.
Facility: SINQ

Reference: N. Scheuble et al, Biomacromoecules 17, 3328 (2016)

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30 August 2016

Frequency-Induced Bulk Magnetic Domain-Wall Freezing Visualized by Neutron Dark-Field Imaging



We use neutron dark-field imaging to visualize and interpret the response of bulk magnetic domain walls to static and dynamic magnetic excitations in (110)-Goss textured iron silicon high-permeability steel alloy. We investigate the domain-wall motion under the influence of an external alternating sinusoidal magnetic field. In particular, we perform scans combining varying levels of dcoffset (0–30 A/m), oscillation amplitude Aac (0–1500 A/m), and frequency fac (0–200 Hz). By increasing amplitude Aac while maintaining constant values of dcoffset and fac, we record the transition from a frozen domain-wall structure to a mobile one. Vice versa, increasing fac while keeping Aac and dcoffset constant led to the reverse transition from a mobile domain-wall structure into a frozen one. We show that varying both Aac and fac shifts the position of the transition region. Furthermore, we demonstrate that higher frequencies require higher oscillation amplitudes to overcome the freezing phenomena. The fundamental determination and understanding of the frequency- induced freezing process in high-permeability steel alloys is of high interest to the further development of descriptive models for bulk macromagnetic phenomena. Likewise, the efficiency of transformers can be improved based on our results, since these alloys are used as transformer core material.
Facility: SINQ

Reference: B. Betz et al, Physical Review Applied 6, 024024 (2016)

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30 August 2016

Magnetization Response of the Bulk and Supplementary Magnetic Domain Structure in High-Permeability Steel Laminations Visualized In Situ by Neutron Dark-Field Imaging



Industrial transformer cores are composed of stacked high-permeability steel laminations (HPSLs). The performance and degree of efficiency of transformers are directly determined by the magnetic properties of each HPSL. In this article, we show how the neutron dark-field image (DFI) allows for the in situ visualization of the locally resolved response of the bulk and supplementary magnetic domain structures in HPSLs under the influence of externally applied magnetic fields. In particular, we investigate the domain formation and growth along the initial magnetization curve up to the saturated state. For decreasing field, we visualize the recurrence of the hysteretic domain structure down to the remanent state. Additionally, the DFI allows us to derive a correlation between the grain orientation and the corresponding volume and supplementary domain structure. Furthermore, we visualize the influence of the insulation coating, introducing desired tensile stresses on the domain structures. To compare our DFI findings to traditional methods, we perform complementary surface-sensitive magneto-optical Kerr-microscopy investigations.
Facility: SINQ

Reference: B. Betz et al, Physical Review Applied 6, 024023 (2016)

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23 August 2016

Search for the lepton flavour violating decay μ+→e+γ with the full dataset of the MEG experiment



The final results of the search for the lepton flavour violating decay μ+→e+γ based on the full dataset collected by the MEG experiment at the Paul Scherrer Institut in the period 2009–2013 and totalling 7.5×1014 stopped muons on target are presented. No significant excess of events is observed in the dataset with respect to the expected background and a new upper limit on the branching ratio of this decay of B(μ+→e+γ)<4.2×10−13 (90 % confidence level) is established, which represents the most stringent limit on the existence of this decay to date.
Facility: Particle Physics

Reference: Baldini, A.M., Bao, Y., Baracchini, e. et al, Eur. Phys. J. C 76 (2016), 434

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17 August 2016

Volume-wise destruction of the antiferromagnetic Mott insulating state through quantum tuning



RENiO3 (RE=rare-earth element) and V2O3 are archetypal Mott insulator systems. When tuned by chemical substitution (RENiO3) or pressure (V2O3), they exhibit a quantum phase transition (QPT) between an antiferromagnetic Mott insulating state and a paramagnetic metallic state. Because novel physics often appears near a Mott QPT, the details of this transition, such as whether it is first or second order, are important. Here, we demonstrate through muon spin relaxation/rotation (μSR) experiments that the QPT in RENiO3 and V2O3 is first order: the magnetically ordered volume fraction decreases to zero at the QPT, resulting in a broad region of intrinsic phase separation, while the ordered magnetic moment retains its full value until it is suddenly destroyed at the QPT. These findings bring to light a surprising universality of the pressure-driven Mott transition, revealing the importance of phase separation and calling for further investigation into the nature of quantum fluctuations underlying the transition.
Facility: SμS

Reference: B.A. Frandsen et al, Nature Communications 7, 12519 (2016)

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12 August 2016

On the Interaction between Digitonin and Cholesterol in Langmuir Monolayers



In this article, we describe the effect of a highly hemolytic saponin, digitonin, on model lipids cholesterol and dipalmitoylphosphatidylcholine (DPPC) using a combination of tensiometric (surface pressure and dilatational surface elasticity), spectroscopic (infrared reflection absorption spectroscopy, IRRAS), microscopic (fluorescence microscopy), and scattering techniques (neutron reflectivity, NR, and grazing incidence X-ray diffraction, GIXD). The monolayers of individual lipids and their 10:9 (mol/mol) mixture were exposed to an aqueous solution of digitonin (10−4 M) by subphase exchange using a setup developed recently in our laboratory. The results confirm that digitonin can adsorb onto both bare and lipid-covered water-air interfaces. In the case of DPPC, a relatively weak interaction can be observed, but the presence of cholesterol drastically enhances the effect of digitonin. The latter is shown to dissociate the weak cholesterol-DPPC complexes and to bind cholesterol in an additional layer attached to the original lipid monolayer.
Facility: SINQ

Reference: K. Wojciechowski et al, Langmuir 32, 9064 (2016)

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26 July 2016

Phononic Structure Engineering: the Realization of Einstein Rattling in Calcium Cobaltate for the Suppression of Thermal Conductivity



Phonons in condensed matter materials transmit energy through atomic lattices as coherent vibrational waves. Like electronic and photonic properties, an improved understanding of phononic properties is essential for the development of functional materials, including thermoelectric materials. Recently, an Einstein rattling mode was found in thermoelectric material Na0.8CoO2, due to the large displacement of Na between the [CoO2] layers. In this work, we have realized a different type of rattler in another thermoelectric material Ca3Co4O9 by chemical doping, which possesses the same [CoO2] layer as Na0.8CoO2. It remarkably suppressed the thermal conductivity while enhancing its electrical conductivity. This new type of rattler was investigated by inelastic neutron scattering experiments in conjunction with ab-initio molecular dynamics simulations. We found that the large mass of dopant rather than the large displacement is responsible for such rattling in present study, which is fundamentally di erent from skutterudites, clathrates as well as Na analogue. We have also tentatively studied the phonon band structure of this material by DFT lattice dynamics simulation, showing the relative contribution to phonons in the distinct layers of Ca3Co4O9.
Facility: SINQ

Reference: R. Tian et al, Scientific Reports 6, 30530 (2016)

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25 July 2016

Physical realization of a quantum spin liquid based on a complex frustration mechanism



Unlike conventional magnets where the magnetic moments are partially or completely static in the ground state, in a quantum spin liquid they remain in collective motion down to the lowest temperatures. The importance of this state is that it is coherent and highly entangled without breaking local symmetries. In the case of magnets with isotropic interactions, spin-liquid behaviour is sought in simple lattices with antiferromagnetic interactions that favour antiparallel alignments of the magnetic moments and are incompatible with the lattice geometries. Despite an extensive search, experimental realizations remain very few. Here we investigate the novel, unexplored magnet Ca10Cr7O28, which has a complex Hamiltonian consisting of several different isotropic interactions and where the ferromagnetic couplings are stronger than the antiferromagnetic ones. We show both experimentally and theoretically that it displays all the features expected of a quantum spin liquid. Thus spin-liquid behaviour in isotropic magnets is not restricted to the simple idealized models currently investigated, but can be compatible with complex structures and ferromagnetic interactions.
Facility: SμS

Reference: C. Balz et al, Nature Physics 12, 942 (2016)

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20 July 2016

Iridates from the molecular side



New exotic phenomena have recently been discovered in oxides of paramagnetic Ir4+ ions, widely known as ‘iridates’. Their remarkable properties originate from concerted effects of the crystal field, magnetic interactions and strong spin-orbit coupling, characteristic of 5d metal ions. Despite numerous experimental reports, the electronic structure of these materials is still challenging to elucidate, and not attainable in the isolated, but chemically inaccessible, [IrO6]8– species (the simplest molecular analogue of the elementary {IrO6}8– fragment present in all iridates). Here, we introduce an alternative approach to circumvent this problem by substituting the oxide ions in [IrO6]8– by isoelectronic fluorides to form the fluorido-iridate: [IrF6]2–. This molecular species has the same electronic ground state as the {IrO6}8– fragment, and thus emerges as an ideal model for iridates. These results may open perspectives for using fluorido-iridates as building-blocks for electronic and magnetic quantum materials synthesized by soft chemistry routes.
Facility: SμS

Reference: K.S. Pedersen et al, Nature communications 7, 12195 (2016)

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18 July 2016

POLAR detector developed at the PSI flies into orbit with a Chinese space mission

Researchers working with Wojciech Hajdas at the Paul Scherrer Institute PSI have developed a detector called POLAR. This instrument is expected to search out and investigate so-called gamma ray bursts coming from the depths of the universe. Gamma ray bursts are eruptions of high-energy light that despite being extremely strong remain, up to now, only poorly understood. Among other things, the origin of gamma ray bursts has not been resolved; it is possible that these strong flashes of light are emitted during the formation of black holes. To improve our understanding of gamma ray bursts, POLAR will measure a property of their light. POLAR was realised in cooperation with researchers at the University of Geneva and will be launched into orbit this coming September with a Chinese space mission.
Facility: Particle Physics

More information: here

13 July 2016

Ferromagnetic Quantum Critical Point Avoided by the Appearance of Another Magnetic Phase in LaCrGe3 under Pressure



The temperature-pressure phase diagram of the ferromagnet LaCrGe3 is determined for the first time from a combination of magnetization, muon-spin-rotation, and electrical resistivity measurements. The ferromagnetic phase is suppressed near 2.1 GPa, but quantum criticality is avoided by the appearance of a magnetic phase, likely modulated, AFMQ. Our density functional theory total energy calculations suggest a near degeneracy of antiferromagnetic states with small magnetic wave vectors Q allowing for the potential of an ordering wave vector evolving from Q=0 to finite Q, as expected from the most recent theories on ferromagnetic quantum criticality. Our findings show that LaCrGe3 is a very simple example to study this scenario of avoided ferromagnetic quantum criticality and will inspire further study on this material and other itinerant ferromagnets.
Facility: SμS

Reference: V. Taufour et al, Physical Review Letters 117, 037207 (2016)

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13 July 2016

Muon polarization in the MEG experiment: predictions and measurements



The MEG experiment makes use of one of the world’s most intense low energy muon beams, in order to search for the lepton flavour violating process μ+→e+γ . We determined the residual beam polarization at the thin stopping target, by measuring the asymmetry of the angular distribution of Michel decay positrons as a function of energy. The initial muon beam polarization at the production is predicted to be Pμ=−1Pμ=−1 by the Standard Model (SM) with massless neutrinos. We estimated our residual muon polarization to be Pμ=−0.86±0.02 (stat) +0.05−0.06 (syst)Pμ=−0.86±0.02 (stat) −0.06+0.05 (syst) at the stopping target, which is consistent with the SM predictions when the depolarizing effects occurring during the muon production, propagation and moderation in the target are taken into account. The knowledge of beam polarization is of fundamental importance in order to model the background of our μ+→e+γ search induced by the muon radiative decay: μ+→e+ν¯μνeγ .
Facility: Particle Physics

Reference: A.M. Baldini et al, European Physical Journal C 76, 223 (2016)

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12 July 2016

Momentum-Resolved Electronic Structure of the High-Tc Superconductor Parent Compound BaBiO3

We investigate the band structure of BaBiO3, an insulating parent compound of doped high-Tc superconductors, using in situ angle-resolved photoemission spectroscopy on thin films. The data compare favorably overall with density functional theory calculations within the local density approximation, demonstrating that electron correlations are weak. The bands exhibit Brillouin zone folding consistent with known BiO6 breathing distortions. Though the distortions are often thought to coincide with Bi3+/Bi5+ charge ordering, core level spectra show that bismuth is monovalent. We further demonstrate that the bands closest to the Fermi level are primarily oxygen derived, while the bismuth 6s states mostly contribute to dispersive bands at deeper binding energy. The results support a model of Bi-O charge transfer in which hole pairs are localized on combinations of the O 2p orbitals.
Facility: SLS

Reference: N.C. Plumb et al, Physical Review Letters 117, 037002 (2016)

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10 May 2016

Dimensional Reduction in Quantum Dipolar Antiferromagnets

We report ac susceptibility, specific heat, and neutron scattering measurements on a dipolar-coupled antiferromagnet LiYbF4. For the thermal transition, the order-parameter critical exponent is found to be 0.20(1) and the specific-heat critical exponent −0.25(1). The exponents agree with the 2D XY/h4 universality class despite the lack of apparent two-dimensionality in the structure. The order-parameter exponent for the quantum phase transitions is found to be 0.35(1) corresponding to (2+1)D. These results are in line with those found for LiErF4 which has the same crystal structure, but largely different TN, crystal field environment and hyperfine interactions. Our results therefore experimentally establish that the dimensional reduction is universal to quantum dipolar antiferromagnets on a distorted diamond lattice.
Facility: SINQ

Reference: P. Babkevich et al, Physical Review Letters 116, 197202 (2016)

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3 May 2016

Robust Magnetic Properties of a Sublimable Single Molecule Magnet

The organization of single-molecule magnets (SMMs) on surfaces via thermal sublimation is a prerequisite for the development of future devices for spintronics exploiting the richness of properties offered by these magnetic molecules. However, a change in the SMM properties due to the interaction with specific surfaces is usually observed. Here we present a rare example of a SMM system which can be thermally sublimated on gold surfaces while maintaining its intact chemical structure and magnetic properties. Muon spin relaxation and ac susceptibility measurements are used to demonstrate that, unlike other SMMs, the magnetic properties of this system in thin films are very similar to those in the bulk, throughout the full volume of the film, including regions near the metal and vacuum interfaces. These results exhibit the robustness of chemical and magnetic properties of this complex and provide important clues for the development of nanostructures based on SMMs.
Facility: SμS

Reference: E. Kiefl et al, ACS Nano (2016)

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28 April 2016

The role of ions in the self-healing behavior of soft particle suspensions

Understanding when a material crystallizes is of fundamental importance in condensed matter. In many materials, the presence of point defects suppresses crystallization. Surprisingly, colloidal hydrogels can overcome this limitation: A small number of large microgels can spontaneously deswell to fit in the crystal lattice of smaller microgels, thus avoiding the occurrence of point defects. We find that this unique particle deswelling is due to an osmotic pressure difference between the inside and the outside of the microgels resulting from the overlap of counterion clouds of neighboring particles. When this pressure difference exceeds the bulk modulus of the large microgels, these shrink, enabling crystallization without point defects.
Facility: SINQ, SLS

Reference: A. Scotti et al, Proceedings of the National Academy of Sciences, 1516011113 (2016)

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14 April 2016

Viscoelasticity Enhancement of Surfactant Solutions Depends on Molecular Conformation: Influence of Surfactant Headgroup Structure and Its Counterion

During the anisotropic growth from globular to wormlike micelles, the basic interactions among distinct parts of the surfactant monomer, its counterion, and additives are fundamental to tune molecular self-assembly. We investigate the addition of sodium salicylate (NaSal) to hexadecyltrimethylammonium chloride and bromide (CTAC and CTAB), 1-hexadecylpyridinium chloride and bromide (CPyCl and CPyBr), and benzyldimethylhexadecylammonium chloride (BDMC), which have the same hydrophobic tail. Their potential to enhance viscoelasticity by anisotropic micellar growth upon salt addition was compared in terms of (i) the influence of the headgroup structure, and (ii) the influence of surfactant counterion type. Employing proton nuclear magnetic resonance (1H NMR), we focused on the molecular conformation of surfactant monomers in the core and polar shell regions of the micelles and their interactions with increasing concentration of NaSal. The viscoelastic response was investigated by rotational and oscillatory rheology. We show that micellar growth rates can be tuned by varying the flexibility and size of the surfactant headgroup as well as the dissociation degree of the surfactant counterion, which directly influences the strength of headgroup−counterion pairing. As a consequence, the morphological transitions depend directly on charge neutralization by electrostatic screening. For example, the amount of salt necessary to start the rodlike-to-wormlike micelle growth depends directly on the number of dissociated counterions in the polar shell.
Reference: V. Lutz-Bueno et al, Langmuir 32, 4239 (2016)

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17 March 2016

Observation of Weyl nodes and Fermi arcs in tantalum phosphide

A Weyl semimetal possesses spin-polarized band-crossings, called Weyl nodes, connected by topological surface arcs. The low-energy excitations near the crossing points behave the same as massless Weyl fermions, leading to exotic properties like chiral anomaly. To have the transport properties dominated by Weyl fermions, Weyl nodes need to locate nearly at the chemical potential and enclosed by pairs of individual Fermi surfaces with non-zero Fermi Chern numbers. Combining angle-resolved photoemission spectroscopy and first-principles calculation, here we show that TaP is a Weyl semimetal with only a single type of Weyl fermions, topologically distinguished from TaAs where two types of Weyl fermions contribute to the low-energy physical properties. The simple Weyl fermions in TaP are not only of fundamental interests but also of great potential for future applications. Fermi arcs on the Ta-terminated surface are observed, which appear in a different pattern from that on the As-termination in TaAs and NbAs.
Facility: SLS

Reference: N. Xu et al, Nature Communications 7, 11006 (2016)

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15 March 2016

Semifluorinated Alkanes at the Air–Water Interface: Tailoring Structure and Rheology at the Molecular Scale

Semifluorinated alkanes form monolayers with interesting properties at the air–water interface due to their pronounced amphi-solvophobic nature and the stiffness of the fluorocarbons. In the present work, using a combination of structural and dynamic probes, we investigated how small molecular changes can be used to control the properties of such an interface, in particular its organization, rheology, and reversibility during compression–expansion cycles. Starting from a reference system perfluor(dodecyl)dodecane, we first retained the linear structure but changed the linkage groups between the alkyl chains and the fluorocarbons, by introducing either a phenyl group or two oxygens. Next, the molecular structure was changed from linear to branched, with four side chains (two fluorocarbons and two hydrocarbons) connected to extended aromatic cores. Neutron reflectivity at the air–water interface and scanning force microscopy on deposited films show how the changes in the molecular structure affect molecular arrangement relative to the interface. Rheological and compression–expansion measurements demonstrate the significant consequences of these changes in molecular structure and interactions on the interfacial properties. Remarkably, even with these simple molecules, a wide range of surface rheological behaviors can be engineered, from viscous over viscoelastic to brittle solids, for very similar values of the surface pressure.
Facility: SINQ

Reference: A. Theodoratou et al, Langmuir 32, 3139 (2016)

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11 March 2016

Spin Liquid State in the 3D Frustrated Antiferromagnet PbCuTe2O6: NMR and Muon Spin Relaxation Studies

PbCuTe2O6 is a rare example of a spin liquid candidate featuring a three-dimensional magnetic lattice. Strong geometric frustration arises from the dominant antiferromagnetic interaction that generates a hyperkagome network of Cu2+ ions although additional interactions enhance the magnetic lattice connectivity. Through a combination of magnetization measurements and local probe investigations by NMR and muon spin relaxation down to 20 mK, we provide robust evidence for the absence of magnetic freezing in the ground state. The local spin susceptibility probed by the NMR shift hardly deviates from the macroscopic one down to 1 K pointing to a homogeneous magnetic system with a low defect concentration. The saturation of the NMR shift and the sublinear power law temperature (T) evolution of the 1/T1 NMR relaxation rate at low T point to a nonsinglet ground state favoring a gapless fermionic description of the magnetic excitations. Below 1 K a pronounced slowing down of the spin dynamics is witnessed, which may signal a reconstruction of spinon Fermi surface. Nonetheless, the compound remains in a fluctuating spin liquid state down to the lowest temperature of the present investigation.
Facility: SμS

Reference: P. Khuntia et al, Physical Review Letters 116, 107203 (2016)

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5 March 2016

Spin excitations in copper selenate, a skyrmion host material

G.S. Tucker et al., Physical Review B 93, 054401 (2016). Inelastic neutron scattering measurements performed at EIGER and TASP have mapped the magnetic excitation spectrum along high-symmetry directions of the first Brillouin zone for the magnetic skyrmion host copper selenate, Cu2OSeO3. Most of the observed spectrum is consistent with a recently proposed model for the magnetic excitations in Cu2OSeO3, for which a new set of best-fit dominant exchange parameters has been found. Two regions of the observed spectrum contain extra modes unexpected by the model which likely arise from neglected anisotropic interactions, suggesting that anisotropy should be considered in future efforts to fully understand the emergence of the skyrmion state in copper selenate.

4 March 2016

Stratified Micellar Multilayers - Toward Nanostructured Photoreactors

Polyelectrolyte multilayers (PEMs) with stratification of the internal structure were assembled from statistical amphiphilic copolyelectrolytes of opposite charges. These polyelectrolytes organize in aqueous solutions into micellar structures with fluoroalkyl and aromatic nanodomains, respectively, that were also preserved after deposition as thin films via layer-by-layer (LbL) electrostatic self-assembly. The unimolecular micelles, formed due to statistical compositions of amphiphilic polyelectrolytes used, were shown to suppress chain interdiffusion between adjacent layers in resulting micellar PEMs, as evidenced by spectroscopic ellipsometry, atomic force microscopy (AFM), and neutron reflectometry (NR) measurements. Additionally, hydrophobic cores of the micelles were used as hosts for photoactive molecules, namely, ferrocene and perfluorinated magnesium phthalocyanine. Stratified micellar multilayers were then deposited as hollow capsules using CaCO3 microparticles as templates. Photoinduced electron transfer (PET) between ferrocene and phthalocyanine solubilized in the polymer micelles was demonstrated to occur efficiently inside the stratified, polyelectrolyte walls of the capsules, due to the polarity gradient created by the incompatible aromatic and fluoroalkyl domains. The obtained results present a new approach to construct well-organized, self-assembled nanostructured materials for solar energy conversion.
Facility: SINQ

Reference: M. Kopec et al, Chemistry of Materials 28, 2219 (2016)

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4 March 2016

Giant Controllable Magnetization Changes Induced by Structural Phase Transitions in a Metamagnetic Artificial Multiferroic

The realization of a controllable metamagnetic transition from AFM to FM ordering would open the door to a plethora of new spintronics based devices that, rather than reorienting spins in a ferromagnet, harness direct control of a materials intrinsic magnetic ordering. In this study FeRh films with drastically reduced transition temperatures and a large magneto-thermal hysteresis were produced for magnetocaloric and spintronics applications. Remarkably, giant controllable magnetization changes (measured to be as high has ~25%) are realized by manipulating the strain transfer from the external lattice when subjected to two structural phase transitions of BaTiO3 (001) single crystal substrate. These magnetization changes are the largest seen to date to be controllably induced in the FeRh system. Using polarized neutron reflectometry we reveal how just a slight in plane surface strain change at ~290C results in a massive magnetic transformation in the bottom half of the film clearly demonstrating a strong lattice-spin coupling in FeRh. By means of these substrate induced strain changes we show a way to reproducibly explore the effects of temperature and strain on the relative stabilities of the FM and AFM phases in multi-domain metamagnetic systems. This study also demonstrates for the first time the depth dependent nature of a controllable magnetic order using strain in an artificial multiferroic heterostructure.
Reference: S.P. Bennett et al, Scientific Reports 6, 22708 (2016)

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3 March 2016

Origin of the Spin-Orbital Liquid State in a Nearly J=0 Iridate Ba3ZnIr2O9

We show using detailed magnetic and thermodynamic studies and theoretical calculations that the ground state of Ba3ZnIr2O9 is a realization of a novel spin-orbital liquid state. Our results reveal that Ba3ZnIr2O9 with Ir5+ (5d4) ions and strong spin-orbit coupling (SOC) arrives very close to the elusive J 1⁄4 0 state but each Ir ion still possesses a weak moment. Ab initio density functional calculations indicate that this moment is developed due to superexchange, mediated by a strong intradimer hopping mechanism. While the Ir spins within the structural Ir2O9 dimer are expected to form a spin-orbit singlet state (SOS) with no resultant moment, substantial frustration arising from interdimer exchange interactions induce quantum fluctuations in these possible SOS states favoring a spin-orbital liquid phase down to at least 100 mK.
Facility: SμS, SINQ

Reference: A. Nag et al, Physical Review Letters 116, 097205 (2016)

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1 March 2016

Quasiparticle-continuum level repulsion in a quantum magnet

When the energy eigenvalues of two coupled quantum states approach each other in a certain parameter space, their energy levels repel each other and level crossing is avoided. Such level repulsion, or avoided level crossing, is commonly used to describe the dispersion relation of quasiparticles in solids. However, little is known about the level repulsion when more than two quasiparticles are present; for example, in a strongly interacting quantum system where a quasiparticle can spontaneously decay into a many-particle continuum. Here we show that even in this case level repulsion exists between a long-lived quasiparticle state and a continuum. In our fine-resolution neutron spectroscopy study of magnetic quasiparticles in the frustrated quantum magnet BiCu2PO6, we observe a renormalization of the quasiparticle dispersion relation due to the presence of the continuum of multi-quasiparticle states.
Reference: K.W. Plumb et al, Nature Physics 12, 224 (2016)

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19 February 2016

Dramatic pressure-driven enhancement of bulk skyrmion stability

The recent discovery of magnetic skyrmion lattices initiated a surge of interest in the scientic community. Several novel phenomena have been shown to emerge from the interaction of conducting electrons with the skyrmion lattice, such as a topological Hall-effect and a spin-transfer torque at ultra-low current densities. In the insulating compound Cu2OSeO3, magneto-electric coupling enables control of the skyrmion lattice via electric elds, promising a dissipation-less route towards novel spintronic devices. One of the outstanding fundamental issues is related to the thermodynamic stability of the skyrmion lattice. To date, the skyrmion lattice in bulk materials has been found only in a narrow temperature region just below the order-disorder transition. If this narrow stability is unavoidable, it would severely limit applications. Here we present the discovery that applying just moderate pressure on Cu2OSeO3 substantially increases the absolute size of the skyrmion pocket. This insight demonstrates directly that tuning the electronic structure can lead to a significant enhancement of the skyrmion lattice stability. We interpret the discovery by extending the previously employed Ginzburg- Landau approach and conclude that change in the anisotropy is the main driver for control of the size of the skyrmion pocket.
Reference: I. Levatic et al, Scientific Reports 6, 21347 (2016)

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4 February 2016

Coexistence of low-moment magnetism and superconductivity in tetragonal FeS and suppression of Tc under pressure

The family of iron-based superconductors has recently acquired a new member material, FeS. Theoretically, this compound has been shown to have electronic structure similar to that of the superconducting FeSe. However, contradictory ground states have been predicted for FeS. In this work, a collaboration of authors from Switzerland and Germany use muon spin rotation and relaxation to show that weak-moment magnetism microscopically coexists with bulk superconductivity. Additionally, in contrast with some earlier studies, the results suggest a fully gapped superconducting state of FeS.
Facility: SμS

Reference: S. Holenstein et al, Physical Review B (Rapid Communication and Editors' Suggestion) 93, 140506(R) (2016)

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21 January 2016

Small-Angle Neutron Scattering Study of Interplay of Attractive and Repulsive Interactions in Nanoparticle-Polymer System

The phase behavior of nanoparticle (silica)−polymer (polyethylene glycol) system without and with an electrolyte (NaCl) has been studied. It is observed that nanoparticle−polymer system behaves very differently in the presence of electrolyte. In the absence of electrolyte, the nanoparticle−polymer system remains in one-phase even at very high polymer concentrations. On the other hand, a re-entrant phase behavior is found in the presence of electrolyte, where one-phase (individual) system undergoes two-phase (nanoparticle aggregation) and then back to one-phase with increasing polymer concentration. The regime of two-phase system has been tuned by varying the electrolyte concentration. The polymer concentration range over which the two-phase system exists is significantly enhanced with the increase in the electrolyte concentration. These systems have been characterized by small-angle neutron scattering (SANS) experiments of contrast-marching the polymer to the solvent.
Facility: SINQ

Reference: S. Kumar et al, Langmuir 32, 1450 (2016)

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18 January 2016

Mechanically Enhanced Liquid Interfaces at Human Body Temperature Using Thermosensitive Methylated Nanocrystalline Cellulose

The mechanical performance of materials at oil/water interfaces after consumption is a key factor affecting hydrophobic drug release. In this study, we methylated the surface of nanocrystalline cellulose (NCC) by mercerization and dimethyl sulfate exposure to produce thermosensitive biopolymers. These methylated NCC (metNCC) were used to investigate interfacial thermogelation at air/water and medium-chain triglyceride (MCT)/water interfaces at body temperature. In contrast to bulk fluid dynamics, elastic layers were formed at room temperature, and elasticity increased significantly at body temperature, which was measured by interfacial shear and dilatational rheology in situ. This unique phenomenon depends on solvent quality, temperature, and polymer concentration at interfaces. Thus, by adjusting the degree of hydrophobicity of metNCC, the interfacial elasticity and thermogelation of the interfaces could be varied. In general, these new materials (metNCC) formed more brittle interfacial layers compared to commercial methylcellulose (MC A15). Thermogelation of methylcellulose promotes attractive intermolecular forces, which were reflected in a change in self-assembly of metNCC at the interface. As a consequence, layer thickness and density increased as a function of temperature. These effects were measured by atomic force microscopy (AFM) images of the displaced interface and confirmed by neutron reflection. The substantial structural and mechanical change of methylcellulose interfaces at body temperature represents a controllable encapsulation parameter allowing optimization of lipid-based drug formulations.
Facility: SINQ

Reference: N. Scheuble et al et al, Langmuir 32, 1396 (2016)

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13 January 2016

Self-Diffusion in Amorphous Silicon

The present Letter reports on self-diffusion in amorphous silicon. Experiments were done on 29Si/natSi heterostructures using neutron reflectometry and secondary ion mass spectrometry. The diffusivities follow the Arrhenius law in the temperature range between 550 and 700°C with an activation energy of (4.4 ± 0.3) eV. In comparison with single crystalline silicon the diffusivities are tremendously higher by 5 orders of magnitude at about 700°C, which can be interpreted as the consequence of a high diffusion entropy.
Facility: SINQ

Reference: F. Strauß et al, Physical Review Letters 116, 025901 (2016)

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5 January 2016

In-situ visualization of stress-dependent bulk magnetic domain formation by neutron grating interferometry

The performance and degree of efficiency of industrial transformers are directly influenced by the magnetic properties of high-permeability steel laminations (HPSLs). Industrial transformer cores are built of stacks of single HPSLs. While the insulating coating on each HPSL reduces eddy-current losses in the transformer core, the coating also induces favorable inter-granular tensile stresses that significantly influence the underlying magnetic domain structure. Here, we show that the neutron dark-field image can be used to analyze the influence of the coating on the volume and supplementary surfacemagnetic domain structures. To visualize the stress effect of the coating on the bulk domain formation, we used an uncoated HPSL and stepwise increased the applied external tensile stress up to 20 MPa. We imaged the domain configuration of the intermediate stress states and were able to reproduce the original domain structure of the coated state. Furthermore, we were able to visualize how the applied stresses lead to a refinement of the volume domain structure and the suppression and reoccurrence of supplementary domains.
Facility: SINQ

Reference: B. Betz et al, Applied Physics Letters 108, 012405 (2016)

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1 January 2016

Rate of Molecular Transfer of Allyl Alcohol across an AOT Surfactant Layer Using Muon Spin Spectroscopy

The transfer rate of a probe molecule across the interfacial layer of a water-in-oil (w/o) microemulsion was investigated using a combination of transverse field muon spin rotation (TF-μSR), avoided level crossing muon spin resonance (ALC-μSR), and Monte Carlo simulations. Reverse micro-emulsions consist of nanometer-sized water droplets dispersed in an apolar solvent separated by a surfactant monolayer. Although the thermodynamic, static model of these systems has been well described, our understanding of their dynamics is currently incomplete. For example, what is the rate of solute transfer between the aqueous and apolar solvents, and how this is influenced by the structure of the interface? With an appropriate choice of system and probe molecule, μSR offers a unique opportunity to directly probe these interfacial transfer dynamics.
Facility: SμS

Reference: U.A. Jayasooriya et al, Langmuir 32, 664 (2016)

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