Scientific Highlights

2014

Bipartite magnetic parent phases in the iron oxypnictide superconductor

High-temperature superconductivity appears as a consequence of doping charge carriers into an undoped parent compound exhibiting antiferromagnetic order; therefore, ground-state properties of the parent compound are highly relevant to the superconducting state. On the basis of this logic, spin fluctuations have been considered as the origin of pairing of the superconducting electrons in the cuprates. As possible pairing mechanisms, there is growing interest in unconventional spin fluctuations or advanced orbital fluctuations owing to the characteristic multi-orbital states in iron pnictides. Here, we report the discovery of an antiferromagnetic phase as well as a unique structural transition in electron-overdoped LaFeAsO1−xHx (x∼0.5), whereby a second parent phase is uncovered, albeit heavily doped. The unprecedented two-dome superconducting phases observed in this material can be interpreted as a consequence of the carrier doping starting from the original x∼0 and additional x∼0.5 parent phases towards the intermediate region. The bipartite parent phases with distinct physical properties in the second magnetic phase provide us with an interesting example to illustrate the intimate interplay between the magnetic interaction, structural change and orbital degree of freedom in iron pnictide superconductors.
Facility: SμS

Reference: M. Hiraishi et al, Nature Physics 10, 300 (2014)

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Hydride ions in oxide hosts hidden by hydroxide ions

The true oxidation state of formally ‘H􏰁-’ ions incorporated in an oxide host is frequently discussed in connection with chemical shifts of 1H nuclear magnetic resonance spectroscopy, as they can exhibit values typically attributed to H+. Here we systematically investigate the link between geometrical structure and chemical shift of H- 􏰁ions in an oxide host, mayenite, with a combination of experimental and ab initio approaches, in an attempt to resolve this issue. We demonstrate that the electron density near the hydrogen nucleus in an OH-􏰁 ion (formally H+ state) exceeds that in an H􏰁- ion. This behaviour is the opposite to that expected from formal valences. We deduce a relationship between the chemical shift of H-􏰁 and the distance from the H􏰁- ion to the coordinating electropositive cation. This relationship is pivotal for resolving H- species that are masked by various states of H+ ions.
Facility: SINQ

Reference: K. Hayashi et al, Nature Communications 5, 3515 (2014)

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Molecular Scale Dynamics of Large Ring Polymers

We present neutron scattering data on the structure and dynamics of melts from polyethylene oxide rings with molecular weights up to ten times the entanglement mass of the linear counterpart. The data reveal a very compact conformation displaying a structure approaching a mass fractal, as hypothesized by recent simulation work. The dynamics is characterized by a fast Rouse relaxation of subunits (loops) and a slower dynamics displaying a lattice animal-like loop displacement. The loop size is an intrinsic property of the ring architecture and is independent of molecular weight. This is the first experimental observation of the space-time evolution of segmental motion in ring polymers illustrating the dynamic consequences of their topology that is unique among all polymeric systems of any other known architecture.
Facility: SINQ

Reference: S. Goossen et al, Physical Review Letters 113, 168302 (2014)

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Short-Range Correlations in the Magnetic Ground State of Na4Ir3O8

The magnetic ground state of the Jeff = 1/2 hyperkagome lattice in Na4Ir3O8 is explored via combined bulk magnetization, muon spin relaxation, and neutron scattering measurements. A short-range, frozen state comprised of quasistatic moments develops below a characteristic temperature of TF = 6K, revealing an inhomogeneous distribution of spins occupying the entirety of the sample volume. Quasistatic, short- range spin correlations persist until at least 20 mK and differ substantially from the nominally dynamic response of a quantum spin liquid. Our data demonstrate that an inhomogeneous magnetic ground state arises in Na4Ir3O8 driven either by disorder inherent to the creation of the hyperkagome lattice itself or stabilized via quantum fluctuations.
Facility: SμS

Reference: R. Dally et al, Physical Review Letters 113, 247601 (2014)

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Measurement of the parameter ξ″ in polarized muon decay and implications on exotic couplings of the leptonic weak interaction

The muon decay parameter ξ″ has been determined in a measurement of the longitudinal polarization of positrons emitted from polarized and depolarized muons. The result, ξ″ = 0.981 ± 0.045stat ± 0.003syst, is consistent with the Standard Model prediction of unity, and provides an order of magnitude improvement in the relative precision of this parameter. This value sets new constraints on exotic couplings beyond the dominant V-A description of the leptonic weak interaction.
Facility: Particle Physics

Reference: R. Prieels et al, Physical Review D 90, 112003 (2014)

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k=0 Magnetic Structure and Absence of Ferroelectricity in SmFeO3

SmFeO3 has attracted considerable attention very recently due to its reported multiferroic properties above room temperature. We have performed powder and single crystal neutron diffraction as well as complementary polarization dependent soft X-ray absorption spectroscopy measurements on floating-zone grown SmFeO3 single crystals in order to determine its magnetic structure. We found a k=0 G-type collinear antiferromagnetic structure that is not compatible with inverse Dzyaloshinskii-Moriya interaction driven ferroelectricity. While the structural data reveal a clear sign for magneto-elastic coupling at the Néel-temperature of ∼675 K, the dielectric measurements remain silent as far as ferroelectricity is concerned.
Facility: SINQ

Reference: C.-Y. Kuo et al, Physical Review Letters 113, 217203 (2014)

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A high-pressure hydrogen time projection chamber for the MuCap experiment

The MuCap experiment at the Paul Scherrer Institute performed a high-precision measurement of the rate of the basic electroweak process of nuclear muon capture by the proton, μ- + p → n + νμ. The experimental approach was based on the use of a time projection chamber (TPC) that operated in pure hydrogen gas at a pressure of 10 bar and functioned as an active muon stopping target. The TPC detected the tracks of individual muon arrivals in three dimensions, while the trajectories of outgoing decay (Michel) electrons were measured by two surrounding wire chambers and a plastic scintillation hodoscope. The muon and electron detectors together enabled a precise measurement of the μp atom's lifetime, from which the nuclear muon capture rate was deduced. The TPC was also used to monitor the purity of the hydrogen gas by detecting the nuclear recoils that follow muon capture by elemental impurities. This paper describes the TPC design and performance in detail.
Facility: Particle Physics

Reference: J. Egger et al, European Physical Journal A 50, 163 (2014)

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A measurement of the neutron to 199Hg magnetic moment ratio

The neutron gyromagnetic ratio has been measured relative to that of the 199Hg atom with an uncertainty of 0.8 ppm. We employed an apparatus where ultracold neutrons and mercury atoms are stored in the same volume and report the result γnHg = 3.8424574(30).
Facility: UCN

Reference: S. Afach et al, Physics Letters B 739, 128 (2014)

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The μ → eγ decay in a systematic effective field

We implement a systematic effective field theory approach to the benchmark process μ → eγ, performing automated one-loop computations including dimension 6 operators and studying their anomalous dimensions. We obtain limits on Wilson coefficients of a relevant subset of lepton-flavour violating operators that contribute to the branching ratio μ → eγ at one-loop. In addition, we illustrate a method to extract further constraints induced by the mixing of operators under renormalisation-group evolution. This results in limits on the corresponding Wilson coefficients directly at the high scale. The procedure can be applied to other processes as well and, as an example, we consider also lepton-flavour violating decays of the τ.
Reference: G.M. Pruna et al, Journal of High Energy Physics 10, 014 (2014)

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Direct Spectroscopic Observation of a Shallow Hydrogenlike Donor State in Insulating SrTiO3

We present a direct spectroscopic observation of a shallow hydrogenlike muonium state in SrTiO3 which confirms the theoretical prediction that interstitial hydrogen may act as a shallow donor in this material. The formation of this muonium state is temperature dependent and appears below ∼70 K. From the temperature dependence we estimate an activation energy of ∼50 meV in the bulk and ∼23 meV near the free surface. The field and directional dependence of the muonium precession frequencies further supports the shallow impurity state with a rare example of a fully anisotropic hyperfine tensor. From these measurements we determine the strength of the hyperfine interaction and propose that the muon occupies an interstitial site near the face of the oxygen octahedron in SrTiO3. The observed shallow donor state provides new insight for tailoring the electronic and optical properties of SrTiO3-based oxide interface systems.
Facility: SμS

Reference: Z. Salman et al, Physical Review Letters 113, 156801 (2014)

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Controlling the near-surface superfluid density in under doped YBa2Cu3O6+x by photo-illumination

The interaction with light weakens the superconducting ground state in classical superconductors. The situation in cuprate superconductors is more complicated: illumination increases the charge carrier density, a photo-induced effect that persists below room temperature. Furthermore, systematic investigations in underdoped YBa2Cu3O6+x (YBCO) have shown an enhanced critical temperature Tc. Until now, studies of photo-persistent conductivity (PPC) have been limited to investigations of structural and transport properties, as well as the onset of superconductivity. Here we show how changes in the magnetic screening profile of YBCO in the Meissner state due to PPC can be determined on a nanometer scale utilizing low-energy muons. The data obtained reveal a strongly increased superfluid density within the first few tens of nanometers from the sample surface. Our findings suggest a non-trivial modification of the near-surface band structure and give direct evidence that the superfluid density of YBCO can be controlled by light illumination.
Facility: SμS

Reference: E. Stilp et al, Scientific Reports 4, 6250 (2014)

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Spin-lattice coupling induced weak dynamical magnetism in EuTiO3 at high temperatures

EuTiO3, which is a G-type antiferromagnet below TN = 5.5 K, has some fascinating properties at high temperatures, suggesting that macroscopically hidden dynamically fluctuating weak magnetism exists at high temperatures. This conjecture is substantiated by magnetic field dependent magnetization measurements, which exhibit pronounced anomalies below 200 K becoming more distinctive with increasing magnetic field strength. Additional results from muon spin rotation experiments provide evidence for weak fluctuating bulk magnetism induced by spin-lattice coupling which is strongly supported in increasing magnetic field.
Facility: SμS

Reference: Z. Guguchia et al, Physical Review B 90, 064413 (2014)

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Origin of Anomalous Slip in Tungsten

Low-temperature deformation of body-centered cubic metals shows a significant amount of plastic slip on planes with low shear stresses, a phenomenon called anomalous slip. Despite progress in atomistic modeling of the consequences of complex stress states on dislocation mobility, the phenomenon of anomalous slip remained elusive. Using in situ Laue microdiffraction and discrete dislocation dynamics in micrometer sized tungsten single crystals, we demonstrate the occurrence of significant anomalous slip. It occurs as a consequence of cross kinks, topological configurations generated by prior dislocation interactions. This clearly identifies anomalous slip as a multidislocation process and not a property of isolated dislocations. The cross-kink mechanism also explains the ambiguous reporting of anomalous slip traces in the past and directs us to ways of including anomalous slip in continuum crystal plasticity formulations.
Facility: SLS

Reference: C. Marichal et al, Physical Review Letters 113, 025501 (2014)

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Electric-Field-Induced Skyrmion Distortion and Giant Lattice Rotation in the Magnetoelectric Insulator Cu2OSeO3

Uniquely in Cu2OSeO3, the Skyrmions, which are topologically protected magnetic spin vortexlike objects, display a magnetoelectric coupling and can be manipulated by externally applied electric (E) fields. Here, we explore the E-field coupling to the magnetoelectric Skyrmion lattice phase, and study the response using neutron scattering. Giant E-field induced rotations of the Skyrmion lattice are achieved that span a range of ∼25°. Supporting calculations show that an E-field-induced Skyrmion distortion lies behind the lattice rotation. Overall, we present a new approach to Skyrmion control that makes no use of spin-transfer torques due to currents of either electrons or magnons.
Facility: SINQ

Reference: J.S. White et al, Physical Review Letters 113, 107203 (2014)

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Mixed Dimensionality of Confined Conducting Electrons in the Surface Region of SrTiO3

Using angle-resolved photoemission spectroscopy, we show that the recently discovered surface state on SrTiO3 consists of nondegenerate t2g states with different dimensional characters. While the dxy bands have quasi-2D dispersions with weak kz dependence, the lifted dxz/dyz bands show 3D dispersions that differ significantly from bulk expectations and signal that electrons associated with those orbitals permeate the near-surface region. Like their more 2D counterparts, the size and character of the dxz/dyz Fermi surface components are essentially the same for different sample preparations. Irradiating SrTiO3 in ultrahigh vacuum is one method observed so far to induce the "universal" surface metallic state. We reveal that during this process, changes in the oxygen valence band spectral weight that coincide with the emergence of surface conductivity are disproportionate to any change in the total intensity of the O 1s core level spectrum. This signifies that the formation of the metallic surface goes beyond a straightforward chemical doping scenario and occurs in conjunction with profound changes in the initial states and/or spatial distribution of near-EF electrons in the surface region.
Facility: SLS

Reference: N.V. Plumb et al, Physical Review Letters 113, 086801 (2014)

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Direct observation of the spin texture in SmB6 as evidence of the topological Kondo insulator

Topological Kondo insulators have been proposed as a new class of topological insulators in which non-trivial surface states reside in the bulk Kondo band gap at low temperature due to strong spin–orbit coupling. In contrast to other three-dimensional topological insulators, a topological Kondo insulator is truly bulk insulating. Furthermore, strong electron correlations are present in the system, which may interact with the novel topological phase. By applying spin- and angle-resolved photoemission spectroscopy, here we show that the surface states of SmB6 are spin polarized. The spin is locked to the crystal momentum, fulfilling time reversal and crystal symmetries. Our results provide strong evidence that SmB6 can host topological surface states in a bulk insulating gap stemming from the Kondo effect, which can serve as an ideal platform for investigating of the interplay between novel topological quantum states with emergent effects and competing orders induced by strongly correlated electrons.
Facility: SLS

Reference: N. Xu et al, Nature Communications 5, 4566 (2014)

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Correlated Decay of Triplet Excitations in the Shastry-Sutherland Compound SrCu2(BO3)2

The temperature dependence of the gapped triplet excitations (triplons) in the 2D Shastry-Sutherland quantum magnet SrCu2(BO3)2 is studied by means of inelastic neutron scattering. The excitation amplitude rapidly decreases as a function of temperature, while the integrated spectral weight can be explained by an isolated dimer model up to 10 K. Analyzing this anomalous spectral line shape in terms of damped harmonic oscillators shows that the observed damping is due to a two-component process: one component remains sharp and resolution limited while the second broadens. We explain the underlying mechanism through a simple yet quantitatively accurate model of correlated decay of triplons: an excited triplon is long lived if no thermally populated triplons are nearby but decays quickly if there are. The phenomenon is a direct consequence of frustration induced triplon localization in the Shastry-Sutherland lattice.
Facility: SINQ

Reference: M.E. Zayed et al, Physical Review Letters 113, 067201 (2014)

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Negative Oxygen Isotope Effect on the Static Spin Stripe Order in Superconducting La2−xBaxCuO4(x=1/8) Observed by Muon-Spin Rotation

Large negative oxygen-isotope (16O and 18O) effects (OIEs) on the static spin-stripe-ordering temperature Tso and the magnetic volume fraction Vm were observed in La2−xBaxCuO4(x=1/8) by means of muon-spin-rotation experiments. The corresponding OIE exponents were found to be αTso=-0.57(6) and αVm=-0.71(9), which are sign reversed to αTC=0.46(6) measured for the superconducting transition temperature Tc. This indicates that the electron-lattice interaction is involved in the stripe formation and plays an important role in the competition between bulk superconductivity and static stripe order in the cuprates.
Facility: SμS

Reference: Z. Guguchia et al, Physical Review Letters 113, 057002 (2014)

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Small-angle neutron scattering study of the mixed state of Yb3Rh4Sn13

Using the small angle neutron scattering (SANS) technique we investigated the vortex lattice (VL) in the mixed state of the stannide superconductor Yb3Rh4Sn13. We find a single domain VL of slightly distorted hexagonal geometry for field strengths between 350 and 18500G and temperatures between T=0.05 and 6.5K. We observe a clear in-plane rotation of the VL for different magnetic field directions relative to the crystallographic axes. We also find that the hexagonal symmetry of the VL is energetically favorable in Yb3Rh4Sn13 for external fields oriented along axes of different symmetries: twofold [110], threefold [111], and fourfold [100]. The observed behavior is different from other conventional and unconventional superconductors. The superconducting state is characterized by an isotropically gapped order parameter with an amplitude of Δ(0) = 1.57 ± 0.05 meV. At the lowest temperatures, the field dependence of the magnetic form factor in our material reveals a London penetration depth of λL = 2508 ± 17 Å and a Ginzburg coherence length of ξ = 100 ± 1.3 Å,i.e.,it is a strongly type-II superconductor, κ = λL/ξ = 25.
Facility: SINQ

Reference: D. Mazzone et al, Physical Review B (Rapid Comm) 90, 020507(R) (2014)

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Spin-Wave Spectrum of the Quantum Ferromagnet on the Pyrochlore Lattice Lu2V2O7

Neutron inelastic scattering has been used to probe the spin dynamics of the quantum (S=1/2) ferromagnet on the pyrochlore lattice Lu2V2O7. Well-defined spin waves are observed at all energies and wave vectors, allowing us to determine the parameters of the Hamiltonian of the system. The data are found to be in excellent overall agreement with a minimal model that includes a nearest-neighbor Heisenberg exchange J=8.22(2)meV and a Dzyaloshinskii-Moriya interaction (DMI) D=1.5(1)meV. The large DMI term revealed by our study is broadly consistent with the model originally used to explain the magnon Hall effect in this compound. However, our ratio of D/J=0.18(1) is roughly half of their value, and is much larger than those found in other theoretical studies.
Facility: SINQ

Reference: M. Mena et al, Physical Review Letters 113, 047202 (2014)

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Dual Spectrum Neutron Radiography: Identification of Phase Transitions between Frozen and Liquid Water

In this Letter, a new approach to distinguish liquid water and ice based on dual spectrum neutron radiography is presented. The distinction is based on arising differences between the cross section of water and ice in the cold energy range. As a significant portion of the energy spectrum of the ICON beam line at Paul Scherrer Institut is in the thermal energy range, no differences can be observed with the entire beam. Introducing a polycrystalline neutron filter (beryllium) inside the beam, neutrons above its cutoff energy are filtered out and the cold energy region is emphasized. Finally, a contrast of about 1.6% is obtained with our imaging setup between liquid water and ice. Based on this measurement concept, the temporal evolution of the aggregate state of water can be investigated without any prior knowledge of its thickness. Using this technique, we could unambiguously prove the production of supercooled water inside fuel cells with a direct measurement method.
Facility: SINQ

Reference: J. Biesdorf et al, Physical Review Letters 112, 248301 (2014)

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Muon Cooling: Longitudinal Compression

A 10 MeV/c positive muon beam was stopped in helium gas of a few mbar in a magnetic field of 5T. The muon 'swarm' has been efficiently compressed from a length of 16cm down to a few mm along the magnetic field axis (longitudinal compression) using electrostatic fields. The simulation reproduces the low energy interactions of slow muons in helium gas. Phase space compression occurs on the order of microseconds, compatible with the muon lifetime of 2μs. This paves the way for the preparation of a high- quality low-energy muon beam, with an increase in phase space density relative to a standard surface muon beam of 107. The achievable phase space compression by using only the longitudinal stage presented here is of the order of 104.
Facility: Particle Physics

Reference: Y. Bao et al, Physical Review Letters 112, 224801 (2014)

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Understanding the μSR spectra of MnSi without magnetic polarons

Transverse-field muon-spin rotation (μSR) experiments were performed on a single crystal sample of the noncentrosymmetric system MnSi. The observed angular dependence of the muon precession frequencies matches perfectly the one of the Mn-dipolar fields acting on the muons stopping at a 4a position of the crystallographic structure. The data provide a precise determination of the magnetic dipolar tensor. In addition, we have calculated the shape of the field distribution expected below the magnetic transition temperature TC at the 4a muon site when no external magnetic field is applied. We show that this field distribution is consistent with the one reported by zero-field μSR studies. Finally, we present ab initio calculations based on the density-functional theory which confirm the position of the muon stopping site inferred from transverse field μSR. In view of the presented evidence we conclude that the μSR response of MnSi can be perfectly and fully understood without invoking a hypothetical magnetic polaron state.
Facility: SμS

Reference: A. Amato et al, Physical Review B 89, 184425 (2014) (Editor's choice)

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Bulk superconductivity in undoped T'-La1.9Y0.1CuO4 probed by muon spin rotation

The Meissner effect has been directly demonstrated by depth-resolved muon spin rotation measurements in high-quality thin films of the T'-structured cup rate, T'-La1.9Y0.1CuO4, to confirm bulk superconductivity (Tc ≈ 21 K) in its undoped state. The gradual expelling of an external magnetic field is observed over a depth range of ∼ 100 nm in films with a thickness of 275(15) nm, from which the penetration depth is deduced to be 466(22) nm. Based on this result, we argue that the true ground state of the “parent” compound of the n-type cuprates is not a Mott insulator but a strongly correlated metal with colossal sensitivity to apical oxygen impurities.
Facility: SμS

Reference: K.M. Kojima et al, Physical Review B 89, 180508(R) (2014) (Editor's choice)

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Controlling Magnetic Order and Quantum Disorder in Molecule-Based Magnets

We investigate the structural and magnetic properties of two molecule-based magnets synthesized from the same starting components. Their different structural motifs promote contrasting exchange pathways and consequently lead to markedly different magnetic ground states. Through examination of their structural and magnetic properties we show that [Cu(pyz)(H2O)(gly)2](ClO4)2 may be considered a quasi-one- dimensional quantum Heisenberg antiferromagnet whereas the related compound [Cu(pyz)(gly)](ClO4), which is formed from dimers of antiferromagnetically interacting Cu2+ spins, remains disordered down to at least 0.03 K in zero field but shows a field-temperature phase diagram reminiscent of that seen in materials showing a Bose-Einstein condensation of magnons.
Facility: SμS

Reference: T. Lancaster et al, Physical Review Letters 112, 207201 (2014)

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Lattice-Site-Specific Spin Dynamics in Double Perovskite Sr2CoOsO6

Magnetic properties and spin dynamics have been studied for the structurally ordered double perovskite Sr2CoOsO6. Neutron diffraction, muon-spin relaxation, and ac-susceptibility measurements reveal two antiferromagnetic (AFM) phases on cooling from room temperature down to 2 K. In the first AFM phase, with transition temperature TN1=108K, cobalt (3d7, S=3/2) and osmium (5d2, S=1) moments fluctuate dynamically, while their average effective moments undergo long-range order. In the second AFM phase below TN2=67K, cobalt moments first become frozen and induce a noncollinear spin-canted AFM state, while dynamically fluctuating osmium moments are later frozen into a randomly canted state at T≈5K. Ab initio calculations indicate that the effective exchange coupling between cobalt and osmium sites is rather weak, so that cobalt and osmium sublattices exhibit different ground states and spin dynamics, making Sr2CoOsO6 distinct from previously reported double-perovskite compounds.
Facility: SμS

Reference: B. Yan et al, Physical Review Letters 112, 147202 (2014)

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Quantum and classical criticality in a dimerized quantum antiferromagnet

A quantum critical point (QCP) is a singularity in the phase diagram arising due to quantum mechanical fluctuations. The exotic properties of some of the most enigmatic physical systems, including unconventional metals and superconductors, quantum magnets, and ultracold atomic condensates, have been related to the importance of the critical quantum and thermal fluctuations near such a point. However, direct and continuous control of these fluctuations has been difficult to realise, and complete thermodynamic and spectroscopic information is required to disentangle the effects of quantum and classical physics around a QCP. Here we achieve this control in a high-pressure, high-resolution neutron scattering experiment on the quantum dimer material TlCuCl3. By measuring the magnetic excitation spectrum across the entire quantum critical phase diagram, we illustrate the similarities between quantum and thermal melting of magnetic order.
Reference: P. Merchant et al, Nature Physics 10, 373 (2014)

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Spin-Nematic Interaction in the Multiferroic Compound Ba2CoGe2O7

We demonstrate the existence of the spin-nematic interactions in an easy-plane type antiferromagnet Ba2CoGe2O7 by exploring the magnetic anisotropy and spin dynamics. The combination of neutron scattering and magnetic susceptibility measurements reveals that the origin of the in-plane anisotropy is an antiferro-type interaction of the spin-nematic operator. The relation between the nematic operator and the electric polarization in the ligand symmetry of this compound is presented. The introduction of the spin-nematic interaction is useful to understand the physics of spin and electric dipole in multiferroic compounds.
Facility: SINQ

Reference: M. Soda et al, Physical Review Letters 112, 127205 (2014)

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Strong Meissner screening change in superconducting radio frequency cavities due to mild baking

We investigate 'hot' regions with anomalous high field dissipation in bulk niobium superconducting radio frequency cavities for particle accelerators by using low energy muon spin rotation (LE-μSR) on corresponding cavity cutouts. We demonstrate that superconducting properties at the hot region are well described by the non-local Pippard/BCS model for niobium in the clean limit with a London penetration depth λL=23+/-2 nm . In contrast, a cutout sample from the 120C baked cavity shows a much larger λ>100nm and a depth dependent mean free path, likely due to gradient in vacancy concentration. We suggest that these vacancies can efficiently trap hydrogen and hence prevent the formation of hydrides responsible for rf losses in hot regions.
Facility: SμS

Reference: A. Romanenko et al, Applied Physics Letters 104, 072601 (2014)

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Direct Observation of Magnetic Metastability in Individual Iron Nanoparticles

X-ray photoemission electron microscopy combined with x-ray magnetic circular dichroism is used to study the magnetic properties of individual iron nanoparticles with sizes ranging from 20 down to 8 nm. While the magnetocrystalline anisotropy of bulk iron suggests superparamagnetic behavior in this size range, ferromagnetically blocked particles are also found at all sizes. Spontaneous transitions from the blocked state to the superparamagnetic state are observed in single particles and suggest that the enhanced magnetic energy barriers in the ferromagnetic particles are due to metastable, structurally excited states with unexpected life times.
Facility: SLS

Reference: A. Balan et al, Physical Review Letters 112, 107201 (2014)

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Comprehensive study of the spin-charge interplay in antiferromagnetic La2-xSrxCuO4

The origin of the pseudogap and its relationship with superconductivity in the cuprates remains vague. In particular, the interplay between the pseudogap and magnetism is mysterious. Here we investigate the newly discovered nodal gap in hole-doped cuprates using a combination of three experimental techniques applied to one, custom made, single crystal. The crystal is an antiferromagnetic La2-xSrxCuO4 with x=1.92%. We perform angle- resolved photoemission spectroscopy measurements as a function of temperature and find: quasi-particle peaks, Fermi surface, anti-nodal gap and below 45 K a nodal gap. Muon spin rotation measurements ensure that the sample is indeed antiferromagnetic and that the doping is close, but below, the spin-glass phase boundary. We also perform elastic neutron scattering measurements and determine the thermal evolution of the commensurate and incommensurate magnetic order, where we find that a nodal gap opens well below the commensurate ordering at 140K, and close to the incommensurate spin density wave ordering temperature of 30K.
Facility: SLS, SINQ, SμS

Reference: G. Drachuck et al, Nature Communications 5, 3390 (2014)

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Frustration-induced nanometre-scale inhomogeneity in a triangular antiferromagnet

Phase inhomogeneity of otherwise chemically homogenous electronic systems is an essential ingredient leading to fascinating functional properties, such as high-Tc superconductivity in cuprates, colossal magnetoresistance in manganites and giant electrostriction in relaxors. In these materials distinct phases compete and can coexist owing to intertwined ordered parameters. Charge degrees of freedom play a fundamental role, although phase-separated ground states have been envisioned theoretically also for pure spin systems with geometrical frustration that serves as a source of phase competition. Here we report a paradigmatic magnetostructurally inhomogenous ground state of the geometrically frustrated α-NaMnO2 that stems from the system’s aspiration to remove magnetic degeneracy and is possible only due to the existence of near-degenerate crystal structures. Synchrotron X-ray diffraction, nuclear magnetic resonance and muon spin relaxation show that the spin configuration of a monoclinic phase is disrupted by magnetically short-range-ordered nanoscale triclinic regions, thus revealing a novel complex state of matter.
Facility: SμS

Reference: A. Zorko et al, Nature Communications 5, 3222 (2014)

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Pressure-Induced Quantum Critical and Multicritical Points in a Frustrated Spin Liquid

The quantum spin-liquid compound (C4H12N2)Cu2Cl6 is studied by muon spin relaxation under hydrostatic pressures up to 23.6 kbar. At low temperatures, pressure-induced incommensurate magnetic order is detected beyond a quantum critical point at Pc ∼ 4.3 kbar. An additional phase transition to a different ordered phase is observed at P1 ∼ 13.4 kbar. The data indicate that the high-pressure phase may be a commensurate one. The established (P-T) phase diagram reveals the corresponding pressure-induced multicritical point at P1, T1 = 2.0 K.
Facilities: SμS

Reference: M. Thede et al, Physical Review Letters 112, 087204 (2014)

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Switching of magnetic domains reveals spatially inhomogeneous superconductivity

The interplay of magnetic and charge fluctuations can lead to quantum phases with exceptional electronic properties. A case in point is magnetically-driven superconductivity, where magnetic correlations fundamentally affect the underlying symmetry and generate new physical properties. The superconducting wavefunction in most known magnetic superconductors does not break translational symmetry. However, it has been predicted that modulated triplet p-wave superconductivity occurs in singlet d-wave superconductors with spin-density-wave (SDW) order. Here we report evidence for the presence of a spatially inhomogeneous p-wave Cooper pair-density wave in CeCoIn5. We show that the SDW domains can be switched completely by a tiny change of the magnetic field direction, which is naturally explained by the presence of triplet superconductivity. Further, the Q-phase emerges in a common magneto-superconducting quantum critical point. The Q-phase of CeCoIn5 thus represents an example where spatially modulated superconductivity is associated with SDW order.
Facility: SINQ

Reference: S. Gerber et al, Nature Physics 10, 126 (2014)

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Magnetoelastic Excitations in the Pyrochlore Spin Liquid Tb2Ti2O7

At low temperatures, Tb2Ti2O7 enters a spin liquid state, despite expectations of magnetic order and/or a structural distortion. Using neutron scattering, we have discovered that in this spin liquid state an excited crystal field level is coupled to a transverse acoustic phonon, forming a hybrid excitation. Magnetic and phononlike branches with identical dispersion relations can be identified, and the hybridization vanishes in the paramagnetic state. We suggest that Tb2Ti2O7 is aptly named a 'magnetoelastic spin liquid' and that the hybridization of the excitations suppresses both magnetic ordering and the structural distortion. The spin liquid phase of Tb2Ti2O7 can now be regarded as a Coulomb phase with propagating bosonic spin excitations.
Facilities: SINQ

Reference: T. Fennell et al, Physical Review Letters 112, 017203 (2014)

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