CHRNS MACS - The Multi-Axis Crystal Spectrometer

Development of a new theoretical framework to model the local magnetic anisotropy of materials

Anisotropy in strongly correlated materials plays a key role in determining their electronic ground state and is controlled by the local crystalline electric field. In this study, we develop a method to understand the local magnetic anisotropy and apply it to neutron spectroscopy experiments on the antiferromagnetic CeRhIn₅ (with a Néel temperature of 3.8 K), which shares the same crystal structure as the superconducting CeCoIn₅ (with a critical temperature of 2.3 K). By solving the local crystal field Hamiltonian with a tetragonal symmetry (𝐶₄ point group) and coupling the results to the random phase approximation, we identify two distinct excitation modes, polarized along the crystallographic 𝑐-axis and the 𝑎−𝑏 plane, in agreement with experimental observations. These modes’ anisotropy and energy scale are tunable with a magnetic field, allowing us to distinguish between single and multiparticle excitations. This helps demonstrate the instability of excitations polarized within the 𝑎−𝑏 plane in CeRhIn₅. We also compare our findings to simpler models using effective spin-½ parameters and argue for extending traditional SU(2) magnetic excitation theories to account for the complex nature of the crystal-field states constrained by the 𝐶₄ symmetry.. Details

Introducing an experimental technique of time-resolved inelastic neutron scattering (TRINS)

Spectroscopy—the measurement of the energy-resolved rate of radiation-induced transitions between quantum states of atoms molecules, or electrons in solids—provides unique quantitative information about interactions and dynamical processes. With sensitivity to the anisotropic spatial character of excitations at the atomic scale, inelastic neutron scattering is the principal spectroscopic tool for magnetism. Here, we demonstrate an extension of this technique to probe time-dependent nonequilibrium dynamics in magnetic materials in the time domain beyond μ10 μs. We employ time-resolved inelastic neutron scattering to probe the evolution of a nonthermal state of a singlet ground state molecular magnet induced by pulsed microwaves tuned to resonance between its lowest-lying excited states. Details

Weyl-mediated helical magnetism in NdAlSi

Emergent relativistic quasiparticles in Weyl semimetals are the source of exotic electronic properties such as surface Fermi arcs, the anomalous Hall effect and negative magnetoresistance, all observed in real materials. Whereas these phenomena highlight the effect of Weyl fermions on the electronic transport properties, less is known about what collective phenomena they may support. Here, we report a Weyl semimetal, NdAlSi, that offers an example. Using neutron diffraction, we found a long-wavelength helical magnetic order in NdAlSi, the periodicity of which is linked to the nesting vector between two topologically non-trivial Fermi pockets, which we characterize using density functional theory and quantum oscillation measurements. We further show the chiral transverse component of the spin structure is promoted by bond-oriented Dzyaloshinskii–Moriya interactions associated with Weyl exchange processes. Our work provides a rare example of Weyl fermions driving collective magnetism. Details

Time-resolved elastic neutron scattering is employed to investigate monopolar and dipolar relaxation processes in the spin ice material Ho₂Ti₂O₇.

Ferromagnetically interacting Ising spins on the pyrochlore lattice of corner-sharing tetrahedra form a highly degenerate manifold of low-energy states. A spin flip relative to this “spin-ice” manifold can fractionalize into two oppositely charged magnetic monopoles with effective Coulomb interactions. To understand this process, we have probed the low-temperature magnetic response of spin ice to time-varying magnetic fields through stroboscopic neutron scattering and SQUID magnetometry on a new class of ultrapure Ho₂Ti₂O₇ crystals. Details

Novel State of Matter: Observation of a Quantum Spin Liquid

A novel and rare state of matter known as a quantum spin liquid has been empirically demonstrated in a monocrystal of the compound calcium-chromium oxide by team at HZB. What is remarkable about this discovery is that according to conventional understanding, a quantum spin liquid should not be possible in this material. A theoretical explanation for these observations has now also been developed. This work deepens our knowledge of condensed matter and might also be important for future developments in quantum information. The results have just been published in Nature Physics. Details

NIST Contributes to Discovery of Novel Quantum Spin-Liquid

An international team of researchers including scientists from the National Institute of Standards and Technology (NIST) has found what may be the first known example of a "spin-orbital liquid," a substance in a never-before-seen quantum mechanical state.  Details.

For Newly Discovered 'Quantum Spin Liquid', the Beauty Is in Its Simplicity.

A research team including scientists from the National Institute of Standards and Technology (NIST) has confirmed long-standing suspicions among physicists that electrons in a crystalline structure called a kagome (kah-go-may) lattice can form a "spin liquid," a novel quantum state of matter in which the electrons' magnetic orientation remains in a constant state of change.  Details.

Neutron scattering experiments clarify mechanism of piezoelectricity in  PMN.

Piezoelectrics—materials that can change mechanical stress to electricity and back again—are everywhere in modern life. Computer hard drives. Loud speakers. Medical ultrasound. Sonar. Though piezoelectrics are a widely used technology, there are major gaps in our understanding of how they work. Now researchers at the National Institute of Standards and Technology (NIST) and Canada's Simon Fraser University believe they've learned why one of the main classes of these materials, known as relaxors, behaves in distinctly different ways from the rest and exhibit the largest piezoelectric effect. And the discovery comes in the shape of a butterfly. .  Details.