LNS - Scientific Highlights


17. October 2017

Coulomb spin liquid in anion-disordered pyrochlore Tb2Hf2O7

naturecomm8.png R. Sibille et al., Nature Communications 8, 892 (2017) (full article). The charge ordered structure of ions and vacancies characterizing rare-earth pyrochlore oxides serves as a model for the study of geometrically frustrated magnetism. The organization of magnetic ions into networks of corner-sharing tetrahedra gives rise to highly correlated magnetic phases with strong fluctuations, including spin liquids and spin ices. It is an open question how these ground states governed by local rules are affected by disorder. Here we demonstrate in the pyrochlore Tb2Hf2O7, that the vicinity of the disordering transition towards a defective fluorite structure translates into a tunable density of anion Frenkel disorder while cations remain ordered. Quenched random crystal fields and disordered exchange interactions can therefore be introduced into otherwise perfect pyrochlore lattices of magnetic ions. We show that disorder can play a crucial role in preventing long-range magnetic order at low temperatures, and instead induces a strongly fluctuating Coulomb spin liquid with defect-induced frozen magnetic degrees of freedom.

28. July 2017

4-spin plaquette singlet state in the Shastry–Sutherland compound SrCu2(BO3)2

naturephyszzz.png M.E. Zayed et al., Nature Physics, adv. online publication (July 2017). The study of interacting spin systems is of fundamental importance for modern condensed-matter physics. On frustrated lattices, magnetic exchange interactions cannot be simultaneously satisfied, and often give rise to competing exotic ground states. The frustrated two-dimensional Shastry–Sutherland lattice realized by SrCu2(BO3)2 is an important test to our understanding of quantum magnetism. It was constructed to have an exactly solvable 2-spin dimer singlet ground state within a certain range of exchange parameters and frustration. While the exact dimer state and the antiferromagnetic order at both ends of the phase diagram are well known, the ground state and spin correlations in the intermediate frustration range have been widely debated. We report here the first experimental identification of the conjectured plaquette singlet intermediate phase in SrCu2(BO3)2. It is observed by inelastic neutron scattering after pressure tuning at 21.5kbar. This gapped singlet state leads to a transition to an ordered Neel state above 40 kbar, which can realize a deconfined quantum critical point.

6. June 2017

Field-induced magnetic instability within a superconducting condensate

sciadv3.png D. Mazzone et al., Science Advances 3, e1602055 (2017). The application of magnetic fields, chemical substitution, or hydrostatic pressure to strongly correlated electron materials can stabilize electronic phases with different organizational principles. We present evidence for a field-induced quantum phase transition, in superconducting Nd0.05Ce0.95CoIn5, that separates two antiferromagnetic phases with identical magnetic symmetry. At zero field, we find a spin-density wave that is suppressed at the critical field μ0H* = 8 T. For H > H*, a spin-density phase emerges and shares many properties with the Q phase in CeCoIn5. These results suggest that the magnetic instability is not magnetically driven, and we propose that it is driven by a modification of superconducting condensate at H*.

28. April 2017

Bound States and Field-Polarized Haldane Modes in a Quantum Spin Ladder

prl118c.png S. Ward, M. Mena et al., Physical Review Letters 118, 177202 (2017). The challenge of one-dimensional systems is to understand their physics beyond the level of known elementary excitations. By high-resolution neutron spectroscopy in a quantum spin-ladder material, we probe the leading multiparticle excitation by characterizing the two-magnon bound state at zero field. By applying high magnetic fields, we create and select the singlet (longitudinal) and triplet (transverse) excitations of the fully spin-polarized ladder, which have not been observed previously and are close analogs of the modes anticipated in a polarized Haldane chain. Theoretical modeling of the dynamical response demonstrates our complete quantitative understanding of these states.

27. March 2017

Tuning the multiferroic mechanisms of TbMnO3 by epitaxial strain

srep7.png K. Shimamoto, S. Mukherjee et al., Scientific Reports 7, 44753 (2017). A current challenge in the field of magnetoelectric multiferroics is to identify systems that allow a controlled tuning of states displaying distinct magnetoelectric responses. Here we show that the multiferroic ground state of the archetypal multiferroic TbMnO3 is dramatically modified by epitaxial strain. Neutron diffraction reveals that in highly strained films the magnetic order changes from the bulk-like incommensurate bc-cycloidal structure to commensurate magnetic order. Concomitant with the modification of the magnetic ground state, optical second-harmonic generation (SHG) and electric measurements show an enormous increase of the ferroelectric polarization, and a change in its direction from along the c- to the a-axis. Our results suggest that the drastic change of multiferroic properties results from a switch of the spin-current magnetoelectric coupling in bulk TbMnO3 to symmetric magnetostriction in epitaxially-strained TbMnO3. These findings experimentally demonstrate that epitaxial strain can be used to control single-phase spin-driven multiferroic states.

17. March 2017

Sub-pixel correlation length neutron imaging: Spatially resolved scattering information of microstructures on a macroscopic scale

srep7b.png R.P. Harti et al., Scientific Reports 7, 44588 (2017). Neutron imaging and scattering give data of significantly different nature and traditional methods leave a gap of accessible structure sizes at around 10 micrometers. Only in recent years overlap in the probed size ranges could be achieved by independent application of high resolution scattering and imaging methods, however without providing full structural information when microstructures vary on a macroscopic scale. In this study we show how quantitative neutron dark-field imaging with a novel experimental approach provides both sub-pixel resolution with respect to microscopic correlation lengths and imaging of macroscopic variations of the microstructure. Thus it provides combined information on multiple length scales. A dispersion of micrometer sized polystyrene colloids was chosen as a model system to study gravity induced crystallisation of microspheres on a macro scale, including the identification of ordered as well as unordered phases. Our results pave the way to study heterogeneous systems locally in a previously impossible manner.

20. February 2017

Magnetic Field Dependence of Excitations Near Spin-Orbital Quantum Criticality

prl118b.png A. Biffin et al., Physical Review Letters 118, 067205 (2017). The spinel FeSc2S4 has been proposed to realize a near-critical spin-orbital singlet (SOS) state, where entangled spin and orbital moments fluctuate in a global singlet state on the verge of spin and orbital order. Here we report powder inelastic neutron scattering measurements that observe the full bandwidth of magnetic excitations and we find that spin-orbital triplon excitations of an SOS state can capture well key aspects of the spectrum in both zero and applied magnetic fields up to 8.5 T. The observed shift of low-energy spectral weight to higher energies upon increasing applied field is naturally explained by the entangled spin-orbital character of the magnetic states, a behavior that is in strong contrast to spin-only singlet ground state systems, where the spin gap decreases upon increasing applied field.

8. February 2017

Elastic properties revealed by thermal diffuse x-ray scattering

prl118.png B. Wehinger et al., Physical Review Letters 118, 035502 (2017). High-precision measurements of thermal diffuse x-ray scattering revealed that the full elasticity tensor can accurately be obtained in a single crystal diffraction experiment. The new method opens the perspective to determine elastic properties together with crystal structure under the same experimental conditions. The results published in Physical Review Letters show, that absolute values can be obtained within a model-free analysis with a precision comparable to standard methods. The advantage of the new method is its applicability to very small and opaque crystals of arbitrary shape and symmetry. It implies a broad applicability in material science, geophysics and in the study of sound wave anomalies due to fundamental interactions in condensed matter physics.

1. January 2017

Spiral spin-liquid and the emergence of a vortex-like state in MnSc2S4

naturephys2016.png S. Gao et al., Nature Physics 13, 157 (2017). In conventional paramagnets spins fluctuate randomly, which leads to a completely disordered state. This is not the case for spiral spin-liquids, where spins fluctuate as correlated spirals. Recently researchers of LNS together with collaborators from Germany and France experimentally observed the spiral spin-liquid state in MnSc2S4 validating the theoretical prediction made almost 10 years ago by Bergman et al. Besides, an emergent vortex-like triple-q phase was discovered under a magnetic field, establishing the A-site spinels as promising systems to realize the magnetic vortex lattice. This project is a part of a grant on Quantum Frustration in Model Magnets funded by the Swiss National Science Foundation.

24. November 2016

Electromagnons probed by inelastic X-ray scattering in LiCrO2

natcomm7.png S. Tóth et al., Nature Communications 7, 13547 (2016). 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.

26. October 2016

100 Hz neutron radiography at the BOA beamline using a parabolic focussing guide

methodsx.gif P. Trtik, et al., MethodsX 3, 535 (2016). The recent developments in scientific complementary metal oxide semiconductor (sCMOS) detector technology allow for imaging of relevant processes with very high temporal resolution with practically negligible readout time. However, it is neutron intensity that limits the high temporal resolution neutron imaging. In order to partially overcome the neutron intensity problem for the high temporal resolution imaging, a parabolic neutron focussing guide was utilized in the test arrangement and placed upstream the detector in such a manner that the focal point of the guide was positioned slightly behind the scintillator screen. In such a test arrangement, the neutron flux can be increased locally by about one order of magnitude, albeit with the reduced spatial resolution due to the increased divergence of the neutron beam. In a pilot test application, an in-situ titration system allowing for a remote delivery of well-defined volumes of liquids onto the sample stage was utilized. The process of droplets of water (H2O) falling into the container filled with heavy water (D2O) and the subsequent process of the interaction and mixing of the two liquids were imaged with temporal resolution of 0.01 s.

26. October 2016

Progress in High-resolution Neutron Imaging at the Paul Scherrer Institut – The Neutron Microscope Project

jphys746.png P. Trtik & E.H. Lehmann, Journal of Physics – Conference Series 746, 012004 (2016). The recent improvement on the capability of neutron imaging that allows acquiring neutron images with isotropic spatial resolution of about 5 micrometres is demonstrated. This is achieve by combining the tailor-made high-numerical aperture magnifying optics together with a thin isotopically-enriched 157Gd2O2S:Tb scintillator screens (see Trtik & Lehmann, NIM-A 788 (2015) 67-70). The newly achieved level of the spatial resolution represents about 30% enhancement compared to the first prototype (see Trtik et al, Physics Procedia 69 (2015) 169-176) and approximately six-fold enhancement in the spatial resolution capabilities available for the general users community at PSI before the start of the Neutron Microscope project.

4. May 2016

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

pnas2016.png A. Scotti et al., Proceedings of the National Academy of Sciences, 1516011113 (2016). 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.

15. April 2016

Phonon spectroscopy reveals important details of geophysical relevant materials

geophysreslett43b.png B. Wehinger et al., Geophysical Research Letters 43 (2016). Using inelastic x-ray scattering and parameter-free calculations the authors provide the first complete and fully consistent lattice dynamics description of MgSiO3-brigmanite, the main constituent of the Earth’s lower mantle and one of the most important Earth’s minerals. The present work constitutes an important milestone fully validating the lattice dynamics calculation against precise experimental evidence and marks a starting point to extend this kind of combined studies to the high-pressure and high-temperature conditions directly relevant for the physical properties and chemical composition of Earth's lower mantle.

5. March 2016

Spin excitations in copper selenate, a skyrmion host material

prb93.png 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

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

applphyslett108.gif B. Betz et al., Applied Physics Letters 108, 012405 (2016). The efficiency of industrial transformers is directly influenced by the magnetic properties of high-permeability steel laminations (HPSLs). These laminations are coated by insulating layers, to reduce eddy-current losses in the transformer core. In addition, the coating induces favorable inter-granular tensile stresses that significantly influence the underlying magnetic domain structure. In this work, we show how the neutron dark-field image can be used to analyze the influence of the coating on the volume and supplementary surface domain structures. For the visualization of the stress effect of the coating on the underlying 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 we were able to reproduce the original domain structure of the coated state. Furthermore, it is shown, how the applied stresses lead to a refinement of the volume domain structure and the suppression and reoccurrence of supplementary lance-leaf domains.

5. November 2015

Visualizing the morphology of vortex lattice domains in a bulk type-II superconductor

naturecomm6r.jpg T. Reimann et al., Nature Communications 6, 8813 (2015). Alike materials in the solid state, the phase diagram of type-II superconductors exhibit crystalline, amorphous, liquid and spatially inhomogeneous phases. The multitude of different phases of vortex matter has thence proven to act as almost ideal model system for the study of both the underlying properties of superconductivity but also of general phenomena such as domain nucleation and morphology. Here we show how neutron grating interferometry yields detailed information on the vortex lattice and its domain structure in the intermediate mixed state of a type-II niobium superconductor. In particular, we identify the nucleation regions, how the intermediate mixed state expands, and where it finally evolves into the Shubnikov phase. Moreover, we complement the results obtained from neutron grating interferometry by small-angle neutron scattering that confirm the spatially resolved morphology found in the intermediate mixed state, and very small-angle neutron scattering that confirm the domain structure of the vortex lattice.

31. August 2015

Lattice dynamics of α-cristobalite and the Boson peak in silica glass

cristobalite TDS3D movie.gif B. Wehinger et al., J. Phys.: Condens. Matter 27, 305401 (2015). This work marks a decisive step in the solution of the longstanding problem understanding the origin of the Boson peak in silica glass. The investigation by means of diffuse and inelastic x-ray scattering and lattice dynamics calculations from first principles allow for a direct comparison of the atomic motion in crystalline silica polymorphs and silica glass. The article was selected to illustrate the cover page of Journal of Physics: Condensed Matter, Vol. 27, Nr. 30. The animation shows the intensity distribution of thermal diffuse scattering in 3D reciprocal space.

13. July 2015

A new class of chiral materials hosting magnetic skyrmions beyond room temperature

naturecomm6w.png Y. Tokunaga et al., Nature Communications 6, 7638 (2015). Magnetic skyrmions are tiny, magnetic-spin vortices that can emerge in magnetic materials. Due to their nanometric size, skyrmions could be used to build extremely high density memory spintronics devices. However, stable skyrmions are not easy to find and control, and are usually only observed well below room temperature. Here we report the discovery that engineered alloys of cobalt, zinc and manganese are a new class of material that display chiral and stable skyrmions both at, and beyond room temperature. This breakthrough means that skyrmions can be readily integrated into new spintronics devices without needing complicated cooling systems. This project is led by scientists at RIKEN Center for Emergent Matter Science, Japan. Scientists from LNS and EPFL used small-angle neutron scattering at SINQ to prove that when a magnetic field was applied these materials indeed display skyrmions at high temperatures.

12. June 2015

Spin-stripe phase in a frustrated zigzag spin-1/2 chain

naturecomm6a.png M. Pregelj et al., Nature Communications 6, 7255 (2015). In strongly correlated electron systems periodic modulations on the nano-scale have typically been associated with competition between short- and long-range interactions, for example, between exchange and dipole–dipole interactions in the case of ferromagnetic thin films. Here we show that spin-stripe textures may develop also in antiferromagnets, where long-range dipole–dipole magnetic interactions are absent. We discover a magnetic stripe structure which appears at the transition between the spiral and collinear magnetic orders in β-TeVO4 compound, a nearly perfect realization of a frustrated (zigzag) ferromagnetic spin-1/2 chain. The presented system thus allows better understanding of the origin of the intriguing nanometer-sized modulation in other systems, e.g., high-temperature superconductors.

10. June 2015

Controllable Broadband Absorption in the Mixed Phase of Metamagnets

advfunctmater25c.png M. Pregelj et al., Advanced Functional Materials 25 (2015). Combination of neutron scattering, muon spin relaxation, specific heat, ac and dc magnetization measurements, and electron magnetic resonance, reveals the ability of metamagnetic materials to absorb the electromagnetic radiation in an extremely broad frequency range. The effect is controlled by the external magnetic field, which actuates a mixed ferro/antiferromagnetic phase, where the absorption in Cu3Bi(SeO3)2O2Br system extends over at least nine order of frequency scale. Considering that artificial metamagnets (magnetic multilayers) allow for a direct control over the required magnetic field, a novel way of tuning the material’s functional properties is imminent. The work is performed in the LNS and LMU laboratories of NUM and in the Jožef Stefan Institute (Slovenia) in close collaboration with partners from Germany and Moldova.

20. April 2015

Isotopically-enriched gadolinium-157 oxysulfide scintillator screens for the high-resolution neutron imaging

nim788d.jpg P. Trtik and E.H. Lehmann, Nuclear Instruments and Methods in Physics Research A 788, 67-70 (2015). High-resolution neutron imaging (Neutron Microscope project) requires highly efficient scintillator screens. Our aim is to achieve sub-5µm spatial resolution. Here, we demonstrate the feasibility of the production of isotopically-enriched gadolinium oxysulfide scintillator screens for the high spatial-resolution neutron imaging. Approximately 10 g of 157Gd2O2S:Tb was produced in the form of fine powder (the level of 157Gd enrichment above 88%). Approximately 2.5 µm thick 157Gd2O2S:Tb scintillator screens were produced and tested for the absorption power and the light output. The results are compared to the reference screens based on natGd2O2S:Tb. The isotopically-enriched screens provided increase by a factor of 3.8 and 3.6 for the absorption power and the light output, respectively. The project is supported by SNF R’Equip, CCMX and by internal PSI funding.

29. January 2015

Pressure dependence of the magnetic order in CrAs

prb91.png L. Keller et al., Phys. Rev. B 91, 020409(R) (2015). The suppression of magnetic order with pressure concomitant with the appearance of pressure-induced superconductivity was recently discovered in CrAs. Here we present a neutron diffraction study of the pressure evolution of the helimagnetic ground state towards and in the vicinity of the superconducting phase. Neutron diffraction on polycrystalline CrAs was employed from zero pressure to 0.65 GPa and at various temperatures. The helimagnetic long-range order is sustained under pressure and the magnetic propagation vector does not show any considerable change. The average ordered magnetic moment is reduced from 1.73(2) μB at ambient pressure to 0.4(1) μB close to the critical pressure Pc ≈ 0.7 GPa, at which magnetic order is completely suppressed, indicating a reduction of the magnetic volume fraction and a large coexistence region of magnetic order and superconductivity. The width of the magnetic Bragg peaks strongly depends on temperature and pressure, showing a maximum in the region of the onset of superconductivity. We interpret this as associated with competing ground states in the vicinity of the superconducting phase.

5. September 2014

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

prl113dd.png J.S. White et al., Phys. Rev. Lett. 113, 107203 (2014), Editor's Suggestion. Discovering fundamentally new ways to manipulate magnetic spins is crucial for research into advanced technologies. Magnetic Skyrmions, which are topologically stable whirls of magnetic spins, are promising candidates for new device components since those found in metallic host materials can be manipulated using electric currents. In this paper, we demonstrate that in the insulating material Cu2OSeO3, the inherent magnetoelectric coupling allows the Skyrmions to be controlled using electric fields. This coupling is immediately attractive for applications, since the control of Skyrmions by electric fields is in principle more efficient than by using electric currents. Using small-angle neutron scattering, we report the observation of giant electric field-induced Skyrmion lattice rotations, and explain our observations with supporting theory. The new physics revealed from our work is that the Skyrmion lattice rotations are caused by an electric field-induced distortion of the Skyrmion shape. This corresponds to an entirely new approach for controlling Skyrmions in general; in all other experimental work until now Skyrmion motion could only be initiated by harnessing the forces provided by dissipative flows of either conduction electrons or magnons. Therefore our study establishes the principles behind a new Skyrmion control paradigm that can be exploited for the development of energy efficient Skyrmion-based applications.

27. August 2014

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

prl113cc.png N.C. Plumb et al., Phys. Rev. Lett. 113, 086801 (2014). 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. This project is led by the Spectroscopy of Novel Materials Group of PSI.

6. August 2014

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

prl113b.png M.E. Zayed et al., Phys. Rev. Lett. 113, 067201 (2014). 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.

28. July 2014

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

prb90.png D. Mazzone et al., Phys. Rev. B 90, 020507(R) (2014). 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 18 500 G and temperatures between T=0.05 and 6.5 K. 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.

24. July 2014

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

prl113.png M. Mena et al., Phys. Rev. Lett. 113, 047202 (2014). 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 the previously published value and is much larger than those found in other theoretical studies.

1. July 2014

Identification of transitions between liquid water and ice with dual spectrum neutron imaging

prl112ice.jpg J. Biesdorf et al., Phys. Rev. Lett. 112, 248301 (2014) / PSI press release. The ability to start up at sub-zero Celsius temperatures is a prerequisite for the use of fuel cells in automotive applications, but specific measures need to be taken to prevent the product water to freeze and block the gas supply pathways. In this context, a method for imaging the distribution of liquid water and ice from neutron imaging experiments was developed. The differences in inelastic neutron scattering cross sections of ice and liquid water were used to distinguish between these two states in transmission imaging experiments. As the inelastic interactions become increasingly important for low energy neutrons, they are emphasized by using a Beryllium filter transmitting only the part of the neutron energy spectrum below 5.25 eV. To resolve the second unknown – the amount of water in each region of the cell – a second measurement is realized with the full energy spectrum. This “dual spectrum” imaging method has the advantage of a significantly increased flux compared to energy resolved imaging using monochromatic beams or velocity selectors. Although further work is necessary to make it an absolute detection method, dual spectrum neutron radiography was successfully applied in a small scale test cell to identify transitions of water from the super-cooled to the frozen state.

10. April 2014

Chloroplast remodeling during state transitions in green algae

pnas111.png G. Nagy et al., PNAS 111, 5042-5047 (2014) / PSI press release. In oxygenic photosynthesis photochemical reactions occur in two different photosystems (PSs) and the light–energy conversion is regulated by balancing their activity. Such a power balance requires a sophisticated regulatory mechanism called state transitions, which involve reversible phosphorylation of the light-harvesting complex proteins (LHCIIs) to redistribute absorbed excitation energy between the two photosystems. Using noninvasive techniques (small-angle neutron scattering, circular dichroism, and absorption transient spectroscopy) in the green alga Chlamydomonas reinhardtii, we have revealed that state transitions modify the chloroplast structure, affecting the stacking and periodicity of the photosynthetic membranes and altering protein–protein interactions within these membranes. These structural changes accompany the conversion of LHCII into an energy-dissipating mode with only minor displacements of phosphorylated LHCIIs from PSII to PSI, thereby allowing us to reevaluate the physiological significance of state transitions.

7. April 2014

Quantum and classical criticality in a dimerised quantum antiferromagnet

naturephys4.jpg P. Merchant et al., Nature Physics 10, 373-379 (2014) / PSI press release. 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.

15. January 2014

Switching of magnetic domains reveals spatially inhomogeneous superconductivity

naturephys.jpg S. Gerber et al., Nature Physics 10, 126-129 (2014) / PSI press release. 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. This project is led by the Laboratory for Developments and Methods LDM of PSI and supported by the Swiss National Science Foundation.

14. January 2014

Magnetoelastic Excitations in the Pyrochlore Spin Liquid Tb2Ti2O7

prl112small2.png T. Fennell et al., Phys. Rev. Lett. 112, 017203 (2014). Tb2Ti2O7 is often referred to as a spin liquid as it does indeed remain in a magnetically disordered phase with spin dynamics down to 0.05 K, but this itself is a surprise since there are strong expectations of magnetic order and/or a structural distortion. However, throughout the spin liquid regime there are also strong signs of magnetoelastic coupling, leading to the suggestion that both spin and structural degrees of freedom are frustrated. Using a combination of time-of-flight spectroscopy at the ILL and triple axis spectroscopy with (TASP+MuPAD at PSI) and without (EIGER at PSI) polarization analysis, we have discovered that 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.

25. July 2013

Strain-Induced Ferromagnetism in Antiferromagnetic LuMnO3 Thin Films

prl111a.png J.S. White et al., Phys. Rev. Lett. 111, 037201 (2013). Interfaces of transition metal oxides are a fertile ground for new physics, often showing novel electronic and magnetic properties that do not exist in the bulk form of the material. A relatively little-explored direction in this field concerns the interfacial properties of multifunctional materials such as the magnetoelectric multiferroics. A prototypical family of such materials are the orthorhombic rare-earth manganites (REMnO3, RE=Tb, Lu, Y) in which a symmetry-breaking magnetic transition causes the direct coupling of antiferromagnetic (AFM) and ferroelectric properties. Here, we report a study of single-phase LuMnO3 thin films grown on YAlO3 substrates. Remarkably, our experiments show ferromagnetic and antiferromagnetic orders to coexist in the LuMnO3 thin film. The large moment ferromagnetism (≈1μB), which is absent in bulk samples, displays a magnetic moment distribution that is peaked at the substrate-film interface. This observation indicates strongly that the ferromagnetism arises as a consequence of the interfacial strain induced by the mismatch between the film and substrate crystal lattices. We further show that the strain-induced ferromagnetism and the symmetry-breaking antiferromagnetic order are coupled via an exchange field, which demonstrates strained REMnO3 thin films as promising candidate systems for new multifunctional devices.

2. July 2013

1D to 2D Na+ Ion Diffusion Inherently Linked to Structural Transitions in Na0.7CoO2

prl110d.png M. Medarde et al., Phys. Rev. Lett. 110, 266401 (2013). We report the observation of a stepwise "melting" of the low-temperature Na-vacancy order in the layered transition-metal oxide Na0.7CoO2. High-resolution neutron powder diffraction analysis indicates the existence of two first-order structural transitions, one at T1 ≈ 290 K followed by a second at T2 ≈ 400 K. Detailed analysis strongly suggests that both transitions are linked to changes in the Na mobility. Our data are consistent with a two-step disappearance of Na-vacancy order through the successive opening of first quasi-1D (T1 > T > T2) and then 2D (T > T2) Na diffusion paths. These results shed new light on previous, seemingly incompatible, experimental interpretations regarding the relationship between Na-vacancy order and Na dynamics in this material. They also represent an important step towards the tuning of physical properties and the design of tailored functional materials through an improved control and understanding of ionic diffusion.

12. March 2013

Magnetic Cluster Excitations

furrer rmp.jpg A. Furrer et al., Reviews of Modern Physics 85, 367 (2013). Magnetic clusters, i.e., assemblies of a finite number (between two or three and several hundred) of interacting spin centers which are magnetically decoupled from their environment, can be found in many materials ranging from inorganic compounds and magnetic molecules to artificial metal structures formed on surfaces and metalloproteins. Their magnetic excitation spectra are determined by the nature of the spin centers and of the magnetic interactions, and the particular arrangement of the mutual interaction paths between the spin centers. Small clusters of up to four magnetic ions are ideal model systems in which to examine the fundamental magnetic interactions, which are usually dominated by Heisenberg exchange, but often complemented by anisotropic and/or higher-order interactions. In large magnetic clusters, which may potentially deal with a dozen or more spin centers, there is the possibility of novel many-body quantum states and quantum phenomena. In this review the necessary theoretical concepts and experimental techniques to study the magnetic cluster excitations and the resulting characteristic magnetic properties are introduced, followed by examples of small clusters, demonstrating the enormous amount of detailed physical information that can be retrieved. The current understanding of the excitations and their physical interpretation in the molecular nanomagnets which represent large magnetic clusters is then presented, with a section devoted to the subclass of single-molecule magnets, distinguished by displaying quantum tunneling of the magnetization. Finally, there is a summary of some quantum many-body states which evolve in magnetic insulators characterized by built-in or field-induced magnetic clusters. The review concludes by addressing future perspectives in the field of magnetic cluster excitations.

14. January 2013

Spin ladders and quantum simulators for Tomonaga–Luttinger liquids

jpcm25.jpg S. Ward et al., J. Phys.: Condens. Matter 25, 014004 (2013). Magnetic insulators have proven to be usable as quantum simulators for itinerant interacting quantum systems. In particular the compound (C5H12N)2CuBr4 (for short: (Hpip)2CuBr4) was shown to be a remarkable realization of a Tomonaga–Luttinger liquid (TLL) and allowed us to quantitatively test the TLL theory. Substitution weakly disorders this class of compounds and thus allows us to use them to tackle questions pertaining to the effect of disorder in TLL as well, such as that of the formation of the Bose glass. In this paper we present, as a first step in this direction, a study of the properties of the related (Hpip)2CuCl4 compound. We determine the exchange couplings and compute the temperature and magnetic field dependence of the specific heat, using a finite temperature density matrix renormalization group procedure. Comparison with the measured specific heat at zero magnetic field confirms the exchange parameters and Hamiltonian for the (Hpip)2CuCl4 compound, giving the basis needed to begin studying the disorder effects.

4. December 2012

Persistent Spin Dynamics Intrinsic to Amplitude-Modulated Long-Range Magnetic Order

prl109c.jpg M. Pregelj et al., Phys. Rev. Lett. 109, 227202 (2012). In geometrically and/or exchange frustrated materials spin fluctuations may endure down to lowest accessible temperatures – the phenomenon known as persistent spin dynamics. Since spin fluctuations hinder the onset of extended static correlations, persistent spin dynamics and long-range magnetic order are generally considered as mutually exclusive. Remarkably, their coexistence has been found in several frustrated magnetic systems but was lacking a suitable explanation.
Our work reports on a combined study of spherical neutron polarimetry and neutron diffraction, which reveal that in the frustrated coupled-spin-chain system FeTe2O5Br the incommensurate amplitude modulated magnetic structure persists to the lowest accessible temperatures (T/TN ~ 1/200). This is consistent with muon spin relaxation and specific-heat measurements, which in addition to static long-range order signify the presence of persistent spin dynamics. Our study, therefore, suggests that their coexistence is intrinsic to amplitude modulated magnetic structures and thus offers a well-defined framework and a coherent explanation for the coexistence of long-range order and persistent spin dynamics.

23. October 2012

Electric field control of the skyrmion lattice in Cu2OSeO3

jpcm24.jpg J.S. White et al., J. Phys.: Condens. Matter 24, 432201 (2012). Skyrmions are topologically protected magnetic spin 'whirls' that form a hexagonal 2D lattice in non-centrosymmetric magnets. Until recently, skyrmions had only been observed in itinerant metallic alloys such as MnSi, where they can also be manipulated by applied electric currents. Recently however, a skyrmion lattice (SkL) phase was also found in the chiral cubic insulator Cu2OSeO3, thus showing that skyrmion formation is a more general phenomenon to be expected of non-centrosymmetric systems, and that metallicity is not a prerequisite. Since Cu2OSeO3 is furthermore a magneto-electric, an important open question was to discover if the skyrmion lattice could be manipulated by applied electric (E-) fields. In our study, we report small-angle neutron scattering experiments that demonstrate successfully the first manipulation of skyrmions by applied E-fields in an insulator. Using a geometry with μ0H || [1-10] and E || [111], we discover that the effect of applying an E-field is to controllably rotate the SkL around the magnetic field axis in a manner dependent on both the size and sign of the E-field. Our results are an important first demonstration for a microscopic coupling between applied E-fields and the skyrmions in an insulator, and show that the new paradigm for skyrmion manipulation in this material is dependent on the emergent magneto-electric properties of the individual skyrmions.

31. August 2012

Power-Law Spin Correlations in the Pyrochlore Antiferromagnet Tb2Ti2O7

prl109b.png T. Fennell et al., Physical Review Letters 109, 017201 (2012). Spin correlations with power-law decay are usually associated with a critical point, but stable phases with power-law correlations may exist in frustrated magnets. Such phases are interesting, because they represent model materials where short-range interactions and local constraints lead to emergent symmetries and fractional quasiparticles. For example, in a spin ice, spin configurations that respect a local ice-rule constraint give rise to dipolar (i.e., 1/r3) spin correlations and emergent magnetic monopoles. Dipolar correlations are identified in scattering experiments by the existence of distinctive sharp and anisotropic features known as pinch points. We investigated the low-temperature state of the rare-earth pyrochlore Tb2Ti2O7 using polarized neutron scattering experiments performed on TASP (SINQ) and D7 (ILL). Tb2Ti2O7 is often described as an antiferromagnetic spin liquid with isotropic spin correlations extending over lengths comparable to individual tetrahedra of the pyrochlore lattice. We confirmed this picture at 20 K but find that at 0.05 K the data contain evidence of pinch-point scattering, suggesting that the low temperature state of Tb2Ti2O7 has power-law spin correlations and may be described by an as yet unknown emergent gauge theory.

27. August 2012

Two types of adjacent dimer layers in the low temperature phase of BaCuSi2O6

prb86.jpg D. Sheptyakov et al., Physical Review B 86, 014433 (2012). The interest in BaCuSi2O6 is motivated by its extraordinary phase diagram with field-induced Bose-Einstein condensation. Being a quantum paramagnet at zero magnetic field down to the lowest temperatures, the system displays a quantum phase transition into a magnetically ordered state at the critical value of magnetic field of ~23.5 T. Knowledge of the exact crystal structure of BaCuSi2O6 is of outmost importance for the accurate determination of the relevant exchange couplings, since its previously accepted room-temperature tetragonal crystal structure model failed to explain the high-resolution inelastic neutron scattering data at low temperature. Following the discovery of the low-temperature structural phase transition in this material by several research groups, we have now determined its average low-temperature crystal structure. It has been investigated with high-resolution synchrotron x-ray and neutron powder diffraction techniques and has been found to be orthorhombic, with the most probable space group Ibam. The Cu–Cu dimers in this material are forming two types of layers with distinctly different interatomic distances. Subtle changes also modify the partially frustrated interlayer Cu–Cu exchange paths. The present results corroborate the interpretation of low-temperature nuclear magnetic resonance and inelastic neutron scattering data in terms of distinct dimer layers.

15. June 2012

Dipolar Antiferromagnetism and Quantum Criticality in LiErF4

science2.jpg C. Kraemer et al., Science 336, 1416-1419 (2012). Magnetism has been predicted to occur in systems in which dipolar interactions dominate exchange. We present neutron scattering, specific heat, and magnetic susceptibility data for LiErF4, establishing it as a model dipolar-coupled antiferromagnet with planar spin-anisotropy and a quantum phase transition in applied field Hc|| = 4.0 ± 0.1 kilo-oersteds. We discovered non–mean-field critical scaling for the classical phase transition at the antiferromagnetic transition temperature that is consistent with the two-dimensional XY/h4 universality class; in accord with this, the quantum phase transition at Hc exhibits three-dimensional classical behavior. The effective dimensional reduction may be a consequence of the intrinsic frustrated nature of the dipolar interaction, which strengthens the role of fluctuations.

2. May 2012

Direct observation of the quantum critical point in heavy fermion CeRhSi3

phasediagram2.jpg N. Egetenmeyer et al., Physical Review Letters 108, 177204 (2012). In many heavy fermion materials the quantum critical point is masked by superconductivity and it can only be detected by use of a local probe. In the noncentrosymmetric heavy fermion CeRhSi3 the ground state at ambient pressure is antiferromagnetically ordered and superconductivity sets in above 12 kbar coexisting with antiferromagnetism. We have unraveled a magnetic quantum critical point hidden deep inside the superconducting state of CeRhSi3. Using the muon spin rotation technique we observed the suppression of the internal fields at the lowest measured temperature, upon increase of external pressure. Our data suggest that the ordered moments are gradually quenched with increasing pressure. At 23.6 kbar, the ordered magnetic moments are fully suppressed via a second-order phase transition, and TN is zero.

2. April 2012

Ellipsoidal hybrid magnetic microgel particles with thermally tunable aspect ratios

staedele2.jpg V. Städele et al., Soft Matter 8, 4427-4431 (2012). We report on the synthesis and characterization of multiresponsive hybrid microgel particles. The particles consist of ellipsoidal silica-coated maghemite cores subsequently coated with thermoresponsive poly (N-isopropylacrylamide) (PNIPAM) shells. The PNIPAM shell enables the hybrid particle to alter its size and ratio of long to small axis with increasing temperature while the core morphology remains unchanged. The maghemite core can be magnetically oriented along the long axis as evidenced by small-angle X-ray scattering (SAXS) and confocal microscopy. Dynamic light scattering techniques and confocal microscopy have been applied to study the particles' morphological evolution with increasing temperature in terms of their aspect ratio. The aspect ratio of the particles was found to vary from 1.25 to 1.45 within a temperature range from 20 °C to 44 °C.

20. February 2012

Directly coupled Ferromagnetism and Ferroelectricity in the Olivine Mn2GeO4

white.jpg J.S. White et al., Physical Review Letters 108, 077204 (2012). The olivine compound Mn2GeO4 is shown to feature both a ferroelectric polarization and a ferromagnetic magnetization that are directly coupled and point along the same direction. We show that a spin spiral generates ferroelectricity, and a canted commensurate order leads to weak ferromagnetism. Symmetry suggests that the direct coupling between the ferromagnetism and ferroelectricity is mediated by Dzyaloshinskii-Moriya interactions that exist only in the ferroelectric phase, controlling both the sense of the spiral rotation and the canting of the commensurate structure. Our study demonstrates how multicomponent magnetic structures found in magnetically frustrated materials like Mn2GeO4 provide a new route towards functional materials that exhibit coupled ferromagnetism and ferroelectricity.

8. September 2011

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

furrer.jpg A. Furrer et al., Physical Review Letters 107, 115502 (2011). We introduce a novel method for local structure determination with a spatial resolution of the order of 0.01 Å. It can be applied to materials containing clusters of exchange-coupled magnetic atoms. We use neutron spectroscopy to probe the energies of the cluster excitations which are determined by the interatomic coupling strength J. Since for most materials J is related to the interatomic distance R through a linear relation dJ/dR = α (for dR/R << 1), we can directly derive the local distance R from the observed excitation energies. This is exemplified for the mixed one-dimensional paramagnetic compound CsMnxMg1-xBr3 (x = 0.05, 0.10) containing manganese dimers oriented along the hexagonal c axis. Surprisingly, the resulting Mn-Mn distances R do not vary continuously with increasing internal pressure but lock in at some discrete values.
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