LNS - Scientific Highlights

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.

Thin plasma polymer films were deposited in hexamethyldisiloxane (HMDSO) and HMDSO/O₂ low-pressure discharges and their chemical structures analyzed using infrared (IR) spectroscopy and neutron reflectometry (NR). The (plasma-polymerized) ppHMDSO film exhibits hydrophobic, poly(dimethylsiloxane)-like properties, while the retention of carbon groups is reduced by O₂ addition, yielding a more inorganic, hydrophilic ppSiO_x film.

Materials with interacting magnetic degrees of freedom display a rich variety of magnetic behaviour that can lead to novel collective equilibrium and out-of-equilibrium phenomena. In equilibrium, thermodynamic phases appear with the associated phase transitions providing a characteristic signature of the underlying collective behaviour.

The recent discovery of pressure (p) induced superconductivity in the binary helimagnet CrAs has raised questions on how superconductivity emerges from the magnetic state and on the mechanism of the superconducting pairing. In the present work the suppression of magnetism and the occurrence of superconductivity in CrAs were studied by means of muon spin rotation.

Following the early prediction of the skyrmion lattice (SkL) - a periodic array of spin vortices - it has been observed recently in various magnetic crystals mostly with chiral structure. Although non-chiral but polar crystals with C_nv symmetry were identified as ideal SkL hosts in pioneering theoretical studies, this archetype of SkL has remained experimentally unexplored.

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.

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.

We report the low-temperature magnetic properties of Ce₂Sn₂O₇, a rare-earth pyrochlore. Our suscep- tibility and magnetization measurements show that due to the thermal isolation of a Kramers doublet ground state, Ce₂Sn₂O₇ has Ising-like magnetic moments of ∼1.18 μ_B. The magnetic moments are confined to the local trigonal axes, as in a spin ice, but the exchange interactions are antiferromagnetic.

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.

Skyrmions, topologically protected vortex-like nanometric spin textures in magnets, have been attracting increasing attention for emergent electromagnetic responses and possible technological applications for spintronics. In particular, metallic magnets with chiral and cubic/tetragonal crystal structure may have high potential to host skyrmions that can be driven by low electrical current excitation.

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.

Motifs of periodic modulations are encountered in a variety of natural systems, where at least two rival states are present. In strongly correlated electron systems, such behaviour has 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.

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 manipulation of domains by external fields in ferroic materials is of major interest for applications. In multiferroics with strongly coupled magnetic and electric order, however, the magnetoelectric coupling on the level of the domains is largely unexplored. We investigated the field-induced domain dynamics of TbMnO₃ in the multiferroic ground state and across a first-order spin-flop transition.

Materials with broad absorption bands are highly desirable for electromagnetic filtering and processing applications, especially if the absorption can be externally controlled. Here, a new class of broadband-absorption materials is introduced. Namely, layered metamagnets exhibit an electromagnetic excitation continuum in the magnetic-field-induced mixed ferro- and antiferromagnetic phase.

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%).

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%).

We use nuclear magnetic resonance (NMR), high-resolution x-ray, and neutron scattering studies to study structural and magnetic phase transitions in phosphorus-doped BaFe₂(As_1-xP_x)₂. Previous transport, NMR, specific heat, and magnetic penetration depth measurements have provided compelling evidence for the presence of a quantum critical point (QCP) near optimal superconductivity at x=0.3.

Calcium silicate hydrates (C-S-H) are the most abundant hydration products in ordinary Portland cement paste. Yet, despite the critical role they play in determining mechanical and transport properties, there is still a debate about their density and exact composition. Here, the site-specific mass density and composition of C-S-H in hydrated cement paste are determined with nanoscale resolution in a nondestructive approach.

Inhomogeneity in the ground state is an intriguing, emergent phenomenon in magnetism. Recently, it has been observed in the magnetostructural channel of the geometrically frustrated α-NaMnO₂, for the first time in the absence of active charge degrees of freedom. Here we report an in-depth numerical and local-probe experimental study of the isostructural sister compound CuMnO₂ that emphasizes and provides an explanation for the crucial differences between the two systems.

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.

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.

We have studied the magnetic and superconducting properties of Ba(Fe_0.95Co_0.05)₂As₂ as a function of temperature and external magnetic field using neutron scattering and muon spin rotation. Below the superconducting transition temperature the magnetic and superconducting order parameters coexist and compete. A magnetic field can significantly enhance the magnetic scattering in the superconducting state, roughly doubling the Bragg intensity at 13.5T.

Quantum magnets have occupied the fertile ground between many-body theory and low-temperature experiments on real materials since the early days of quantum mechanics. However, our understanding of even deceptively simple systems of interacting spin-1/2 particles is far from complete. The quantum square-lattice Heisenberg antiferromagnet, for example, exhibits a striking anomaly of hitherto unknown origin in its magnetic excitation spectrum.

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.

Using angle-resolved photoemission spectroscopy, we show that the recently discovered surface state on SrTiO₃ consists of nondegenerate t_2g states with different dimensional characters.

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.

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.

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 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.