High-resolution Inelastic Neutron Research Articles

Direct Observation and Analysis of Low-Energy Magnons with Raman Spectroscopy in Atomically Thin NiPS3.

van der Waals (vdW) magnets have rapidly emerged as a fertile playground for fundamental physics and exciting applications. Despite the impressive developments over the past few years, technical limitations pose a severe challenge to many other potential breakthroughs. High on the list is the lack of suitable experimental tools for studying spin dynamics on atomically thin samples. Here, Raman scattering techniques are employed to directly observe the low-lying magnon (∼1 meV) even in bilayer NiPS3. The advantage is that it offers excellent energy resolutions far better on low-energy sides than most inelastic neutron spectrometers can offer. More importantly, with appropriate theoretical analysis, the polarization dependence of the Raman scattering by those low-lying magnons also provides otherwise hidden information on the dominant spin-exchange scattering paths for different magnons. By comparing with high-resolution inelastic neutron scattering data, these low-energy Raman modes are confirmed to be indeed of magnon origin. Because of the different scattering mechanisms involved in inelastic neutron and Raman scattering, this information is fundamental in pinning down the final spin Hamiltonian. This work demonstrates the capability of Raman spectroscopy to probe the genuine two-dimensional spin dynamics in atomically thin vdW magnets, which can provide insights that are obscured in bulk spin dynamics.

Neutron spin echo spectroscopy with a moving sample

Neutron spin echo spectroscopy is a high resolution inelastic neutron scattering method probing nanosecond dynamics. It is well suited to study the atomistic motion in polymer systems and contributes to our understanding of viscoelasticity. However, for samples under shear, or moving samples in general, Doppler scattering has to be considered. We compare the measured phase shift and depolarisation due to Doppler scattering from a rotating graphite disk to numerical and analytical calculations and find excellent agreement. This allows to take into account Doppler scattering during the data processing and makes longer Fourier times as well as higher shear rates and Q ranges possible with neutron spin echo spectroscopy, enabling for example the study of polymers under high shear.

Neutron scattering study of fluctuating and static spin correlations in the anisotropic spin glass Fe2TiO5

The anisotropic spin-glass transition, in which spin freezing is observed only along the $c$ axis in pseudobrookite ${\mathrm{Fe}}_{2}\mathrm{Ti}{\mathrm{O}}_{5}$, has long been perplexing because the ${\mathrm{Fe}}^{3+}$ moments $({d}^{5})$ are expected to be isotropic. Recently, neutron diffraction demonstrated that surfboard-shaped antiferromagnetic nanoregions coalesce above the glass transition temperature ${T}_{g}\ensuremath{\approx}55\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and a model was proposed in which the freezing of the surfboard magnetization fluctuations leads to the anisotropic spin-glass state. Given this model, we have carried out high-resolution inelastic neutron scattering measurements of the spin-spin correlations to understand the temperature dependence of the intrasurfboard spin dynamics on neutron (picosecond) timescales. Here, we report on the temperature-dependence of the spin fluctuations measured from single-crystal ${\mathrm{Fe}}_{2}\mathrm{Ti}{\mathrm{O}}_{5}$. Strong quasi-elastic magnetic scattering, arising from intrasurfboard correlations, is observed well above ${T}_{g}$. The spin fluctuations possess a steep energy--wave vector relation and are indicative of strong exchange interactions, consistent with the large Curie-Weiss temperature. As the temperature approaches ${T}_{g}$ from above, a shift in spectral weight from inelastic to elastic scattering is observed. At various temperatures between 4 and 300 K, a characteristic relaxation rate of the fluctuations is determined. Despite the freezing of most of the spin correlations, an inelastic contribution remains even at base temperature, signifying the presence of fluctuating intrasurfboard spin correlations to at least $T/{T}_{g}\ensuremath{\approx}0.1$, consistent with an energy landscape that is a hybrid between conventional and geometrically frustrated spin glasses.

Unraveling the Ordered Phase of the Quintessential Hybrid Perovskite MAPbI3─Thermophysics to the Rescue.

Hybrid perovskites continue to attract an enormous amount of attention, yet a robust microscopic picture of their different phases as well as the extent and nature of the disorder present remains elusive. Using specific-heat data along with high-resolution inelastic neutron scattering and ab initio modeling, we address this ongoing challenge for the case of the ordered phase of the quintessential hybrid-perovskite MAPbI3. At low temperatures, the specific heat of MAPbI3 reveals strong deviations from the Debye limit, a common feature of pure hybrid perovskites and their mixtures. Our thermophysical analysis demonstrates that the (otherwise ordered) structure around the organic moiety is characterized by a substantial lowering of the local symmetry relative to what can be inferred from crystallographic studies. The physical origin of the observed thermophysical anomalies is unequivocally linked to excitations of sub-terahertz optical phonons responsible for translational–librational distortions of the octahedral units.

Spin dynamics in the square-lattice cupola system Ba(TiO)Cu4(PO4)4

We report high-resolution single-crystal inelastic neutron scattering measurements on the spin-1/2 antiferromagnet Ba(TiO)Cu$_4$(PO$_4$)$_4$. This material is formed from layers of four-site \cupola" structures, oriented alternately upwards and downwards, which constitute a rather special realization of two-dimensional (2D) square-lattice magnetism. The strong Dzyaloshinskii-Moriya (DM) interaction within each cupola, or plaquette, unit has a geometry largely unexplored among the numerous studies of magnetic properties in 2D Heisenberg models with spin and spatial anisotropies. We have measured the magnetic excitations at zero field and in fields up to 5 T, finding a complex mode structure with multiple characteristic features that allow us to extract all the relevant magnetic interactions by modelling within the linear spin-wave approximation. We demonstrate that Ba(TiO)Cu$_4$(PO$_4$)$_4$ is a checkerboard system with almost equal intra- and inter-plaquette couplings, in which the intra-plaquette DM interaction is instrumental both in enforcing robust magnetic order and in opening a large gap at the Brillouin-zone center. We place our observations in the perspective of generalized phase diagrams for spin-1/2 square-lattice models and materials, where exploring anisotropies and frustration as routes to quantum disorder remains a frontier research problem.

Vibrational Modes and Terahertz Phenomena of the Large-Cage Zeolitic Imidazolate Framework-71.

The zeolitic imidazole framework ZIF-71 has the potential to outperform other well-studied metal–organic frameworks due to its intrinsic hydrophobicity and relatively large pore size. However, a detailed description of its complex physical phenomena and structural dynamics has been lacking thus far. Herein, we report the complete assignment of the vibrational modes of ZIF-71 using high-resolution inelastic neutron scattering measurements and synchrotron radiation infrared spectroscopy, corroborated by density functional theory (DFT) calculations. With its 816 atoms per unit cell, ZIF-71 is the largest system yet for which frequency calculations have been accomplished employing the CRYSTAL17 DFT code. We discover low-energy terahertz dynamics such as gate-opening and shearing modes that are central to the functions and stability of the ZIF-71 framework structure. Nanoscale analytical methods based on atomic force microscopy (near-field infrared spectroscopy and AFM nanoindentation) further unravel the local chemical and mechanical properties of ZIF-71 single crystals.

On the energetics of binding and hydride exchange in the RuH2(H2)2[P(C5H9)3)]2 complex as revealed by inelastic neutron scattering and DFT studies

Low temperature quantum rotation of dihydrogen in RuH2(H2)2[P(C5H9)3)]2 switched to a facile hydride exchange above 150 K.

Unequal sensitivities of energy levels in a high-symmetry Ho3+ complex towards lattice distortions.

High-resolution inelastic neutron scattering has been used to study low-energy magnetic transitions in a Ho3+ complex. This complex crystallises in the high-symmetry space group P4/m and has near-perfect D4d symmetry, which has allowed for the determination of all relevant spin-Hamiltonian parameters. Static and dynamic inhomogeneity in the crystal lattice manifests as a temperature-dependent broadening of the observed magnetic excitations. By implementing distributions in the spin-Hamiltonian parameters, it is possible to reproduce with great accuracy the observed magnetic transition spectrum. This reveals the range to which extraneous perturbations of the crystal field affect low-energy electronic states, such as those involved in quantum tunnelling of magnetisation, in atomic clock transitions, or in spintronics.

Order-by-disorder from bond-dependent exchange and intensity signature of nodal quasiparticles in a honeycomb cobaltate

Recent theoretical proposals have argued that cobaltates with edge-sharing octahedral coordination can have significant bond-dependent exchange couplings thus offering a platform in 3d ions for such physics beyond the much-explored realisations in 4d and 5d materials. Here we present high-resolution inelastic neutron scattering data within the magnetically ordered phase of the stacked honeycomb magnet CoTiO3 revealing the presence of a finite energy gap and demonstrate that this implies the presence of bond-dependent anisotropic couplings. We also show through an extensive theoretical analysis that the gap further implies the existence of a quantum order-by-disorder mechanism that, in this material, crucially involves virtual crystal field fluctuations. Our data also provide an experimental observation of a universal winding of the scattering intensity in angular scans around linear band-touching points for both magnons and dispersive spin-orbit excitons, which is directly related to the non-trivial topology of the quasiparticle wavefunction in momentum space near nodal points.

Order-disorder in room-temperature ionic liquids probed via methyl quantum tunneling

Room-temperature ionic liquids are promising candidates for applications ranging from electrolytes for energy storage devices to lubricants for food and cellulose processing to compounds for pharmaceutics, biotransformation, and biopreservation. Due to the ion complexity, many room-temperature ionic liquids readily form amorphous phases upon cooling, even at modest rates. Here, we investigate two commonly studied imidazolium-based room-temperature ionic liquids, 1-ethyl-3-methylimidazolium tetrafluoroborate and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, as well as their mixtures, to demonstrate how the complex interplay between the crystalline and amorphous phases is affected by the processing conditions, such as thermal history, liquid mixing, and applied pressure. We show that quantum tunneling in the cation methyl groups, measured by high-resolution inelastic neutron scattering, can be used to probe the order-disorder in room-temperature ionic liquids (crystalline vs amorphous state) that develops as a result of variable processing conditions.

Avoided quasiparticle decay and enhanced excitation continuum in the spin- 12 near-Heisenberg triangular antiferromagnet Ba3CoSb2O9

We explore the magnetic excitations of the spin-1/2 triangular antiferromagnet Ba3CoSb2O9 in its 120 degree ordered phase using single-crystal high-resolution inelastic neutron scattering. Sharp magnons with no decay are observed throughout reciprocal space, with a strongly renormalized dispersion and multiple soft modes compared to linear spin wave theory. We propose an empirical parametrization that can quantitatively capture the complete dispersions in the three-dimensional Brillouin zone and explicitly show that the dispersion renormalizations have the direct consequence that one to two magnon decays are avoided throughout reciprocal space, whereas such decays would be allowed for the unrenormalized dispersions. At higher energies, we observe a very strong continuum of excitations with highly-structured intensity modulations extending up at least 4x the maximum one-magnon energy. The one-magnon intensities decrease much faster upon increasing energy than predicted by linear spin wave theory and the higher-energy continuum contains much more intensity than can be accounted for by a two-magnon cross-section, suggesting a significant transfer of spectral weight from the high-energy magnons into the higher-energy continuum states. We attribute the strong dispersion renormalizations and substantial transfer of spectral weight to continuum states to the effect of quantum fluctuations and interactions beyond the spin wave approximation, and make connections to theoretical approaches that might capture such effects. Finally, through measurements in a strong applied magnetic field, we find evidence for magnetic domains with opposite senses for the spin rotation in the 120 degree ordered ground state, as expected in the absence of Dzyaloshinskii-Moriya interactions, when the sense of spin rotation is selected via spontaneous symmetry breaking.

Terahertz collective dynamics of DNA as affected by hydration and counterions

Local and propagating vibrations occurring in DNA in the terahertz domain are essential for processes at the basis of cellular metabolism. Here we report the results of a study on the B-DNA terahertz dynamics where we combined high-resolution inelastic x-ray experiments and incoherent inelastic neutron scattering. By using two different high-hydration conditions we could study the effect of packing interactions between double helices, while by selecting Na and Cs counterions we could inspect how the mass loading affects the DNA low-frequency modes. The pattern of coherent excitation energies is well represented in terms of two branches, an acoustic-like one, with an associated propagation velocity of 3000 ± 100 m/s, and the other almost dispersionless at ~2 meV. This picture is also supported by the vibrational density of states projected on the hydrogen atoms. The acoustic-like mode is assigned to DNA excitations, despite its intriguing similarity with the analogous acoustic mode of bulk water at low wave-vector transfers. We also provide evidence for the intrahelical optic-like character of the low-energy mode, that we ascribe to large scale relative motions of DNA sub-domains.

Experimental observation of magnetic dimers in dilutedYb:YAlO3

We present a comprehensive experimental investigation of Yb magnetic dimers in Yb$_{0.04}$Y$_{0.96}$AlO$_3$, an Yb-doped Yttrium Aluminum Perovskite (YAP) YAlO$_3$ by means of specific heat, magnetization and high-resolution inelastic neutron scattering (INS) measurements. In our sample, the Yb ions are randomly distributed over the lattice and $\sim 7$\% of Yb ions form quantum dimers due to nearest-neighbor antiferromagnetic coupling along the $c$-axis. At zero field, the dimer formation manifests itself in an appearance of an inelastic peak at $\Delta \approx 0.2$~meV in the INS spectrum and a Schottky-like anomaly in the specific heat. The structure factor of the INS peak exhibits a cosine modulation along the $L$ direction, in agreement with the $c$-axis nearest-neighbor intra-dimer coupling. A careful fitting of the low-temperature specific heat shows that the excited state is a degenerate triplet, which indicates a surprisingly small anisotropy of the effective Yb-Yb exchange interaction despite the low crystal symmetry and anisotropic magnetic dipole contribution, in agreement with previous reports for the Yb parent compound, YbAlO$_3$ [arXiv:1904.11513, arXiv:1902.04112], and in contrast to Yb$_2$Pt$_2$Pb [arXiv:1606.01309, arXiv:1907.01067]. The obtained results are precisely reproduced by analytical calculations for the Yb dimers.

Horizontal Line Nodes in Sr2RuO4 Proved by Spin Resonance

We investigated the low-energy incommensurate (IC) magnetic fluctuations in Sr$_2$RuO$_4$ by the high-resolution inelastic neutron scattering measurements and random phase approximation (RPA) calculations. We observed a spin resonance with energy of $\hbar\omega_\text{res}=0.56$~meV centered at a characteristic wavevector $\mathbf{Q}_\text{res}=(0.3, 0.3, 0.5)$. The resonance energy corresponds well to the superconducting gap $2\Delta=0.56$~meV estimated by the tunneling spectroscopy. The spin resonance shows the $L$ modulation with a maximum at around $L = 0.5$. The $L$ modulated intensity of the spin resonance and our RPA calculations indicate that the superconducting gaps regarding the quasi-one-dimensional $\alpha$ and $\beta$ sheets at the Fermi surfaces have the horizontal line nodes. These results may set a strong constraint on the pairing symmetry of Sr$_2$RuO$_4$. We also discuss the implications on possible superconducting order parameters.

Vibrational dynamics of the type-I clathrates A8Sn44□2 (A = Cs, Rb, K) from lattice-dynamics calculations, inelastic neutron scattering, and specific heat measurements

We report on a joint theoretical and experimental study of the vibrational dynamics of the type-I clathrates A8Sn44□2 (A = K, Rb, Cs, and □ stands for a vacancy) by high-resolution inelastic neutron scattering experiments combined with low-temperature specific heat measurements (2–300 K). Ab initio lattice dynamics calculations were performed on hypothetical vacancy-free A8Sn46 clathrates in order to determine the phonon dispersions and vibrational density of states Z(ω). The temperature dependence of the generalized vibrational density of states (GVDOS) was traced from 420 K down to 50 K, paying particular attention to the low-energy region of the GVDOS spectra. In the Cs and Rb compounds, the inelastic signal at low energies is dominated by several peaks mainly associated with the dynamics of the alkali metal atoms A in the polyhedral cages of the clathrate structure. In contrast, the low-energy spectrum of the K compound features a more pronounced contribution of the weighed modes of the framework Sn atoms. Upon cooling, the A-weighted modes soften regardless of the nature of the alkali element. The shift observed is similar for the three compounds and of small amplitude, suggestive of a dominant quasi-harmonic behavior above 50 K. The distinct dynamics of the K atoms in comparison to Cs and Rb is further demonstrated by the analyses of the low-temperature specific heat data, indicating that the low-energy Sn-weighted modes cannot be described by a Debye model with Einstein-like contributions.

Origin of magnetic anisotropy in the spin ladder compound (C5H12N)2CuBr4

The $S=1/2$ spin ladder compound (C$_5$H$_{12}$N)$_2$CuBr$_4$ (BPCB) is studied by means of high-resolution inelastic neutron scattering. In agreement with previous studies we find a band of triplet excitations with a spin gap of $\sim0.8$~meV and a bandwidth of $\sim0.6$~meV. In addition, we observe a distinct splitting of the triplet band of $50(1)$~$\mu$eV or $40(2)$~$\mu$eV at the band minimum or maximum, respectively. By comparison to a strong coupling expansion calculation of the triplet dispersion for a spin ladder with anisotropic exchange, weakly anisotropic leg interactions are identified as the dominant source of magnetic anisotropy in BPCB. Based on these results, we discuss the nature of magnetic exchange anisotropy in BPCB and in related transition-metal insulators.

Temperature dependence of nanoscale dynamic processes measured in living millipedes by high resolution inelastic and elastic neutron scattering

We have used high energy-resolution neutron scattering to probe nanoscale dynamic processes in living millipedes (Narceus americanus). We have measured the temperature dependence of the intensity of scattered neutrons that do not exchange energy with the living samples on the 1.5 ns time scale, thereby excluding the signal from the highly mobile intra- and extra-cellular bulk-like aqueous constituents in the sample. This measured “elastic” scattering intensity exhibits a non-monotonic temperature dependence, with a noticeable systematic decrease detected between 295 and 303 K on warming up from 283 to 310 K. This decrease demonstrates an excellent inverse correlation with the non-monotonic, as a function of temperature, increase in the slow diffusivity previously observed in planarian flatworms and housefly larvae. This correlation suggests the existence of a biological mechanism, possibly common between different classes (Insects and Myriapods) and even phyla (Arthropods and Platyhelminthes), that dampens the slow nanoscopic dynamics in ectothermic organisms in response to the temperature of the environment exceeding the physiologically optimal range.

Current status of the neutron resonance spin echo spectrometer on BL06 “VIN ROSE” at MLF, J-PARC

Neutron spin echo technique is essential for high resolution inelastic and quasi-elastic neutron scattering experiments. We have developed a neutron resonance spin echo (NRSE) spectrometer at BL06 in Materials and Life Science Experimental Facility (MLF), Japan Proton Accelerator Research Complex (J-PARC), and installed a pair of 900 mm long ellipsoidal focusing m = 5 supermirrors with 2.5 m focal length. In this manuscript, we report the first result of time-of-flight neutron spin echo signals by the NRSE instrument.

Comprehensive inelastic neutron scattering study of the multiferroic .

Using high-resolution inelastic neutron scattering, we examine the spin dynamics of in the collinear AF1, the spiral AF2, and the cycloidal AF5 phases. The spin wave excitations are well described by a Heisenberg model with competing long-range exchange interactions ( up to 12th nearest neighbors) and the single-ion anisotropy induced by the spin-orbit interaction. While the exchange constants are relatively unchanged, the dominant effect of doping is to change the single-ion anisotropy from easy axis in the collinear AF1 phase to easy plane in the two multiferroic phases.

Hyperfine interaction and electronic spin fluctuation study on Sr2−xLaxFeCoO6 ( x = 0, 1, 2) by high-resolution backscattering neutron spectroscopy

The study of hyperfine interaction by high-resolution inelastic neutron scattering is not very well known compared to the other competing techniques viz. NMR, M\"ossbauer, PACS etc. Also the study is limited mostly to magnetically ordered systems. Here we report such study on Sr$_{2-x}$La$_x$FeCoO$_6$ (x = 0, 1, 2) of which first (Sr$_2$FeCoO$_6$ with x = 0) has a canonical spin spin glass, the second (SrLaFeCoO$_6$ with x = 1) has a so-called magnetic glass and the third (La$_2$FeCoO$_6$ with x = 2) has a magnetically ordered ground state. Our present study revealed clear inelastic signal for SrLaFeCoO$_6$, possibly also inelastic signal for Sr$_2$FeCoO$_6$ below the spin freezing temperatures $T_{sf}$ but no inelastic signal at all for for the magnetically ordered La$_2$FeCoO$_6$ in the neutron scattering spectra. The broadened inelastic signals observed suggest hyperfine field distribution in the two disordered magnetic glassy systems and no signal for the third compound suggests no or very small hyperfine field at the Co nucleus due to Co electronic moment. For the two magnetic glassy system apart from the hyperfine signal due only to Co, we also observed electronic spin fluctuations probably from both Fe and Co electronic moments. \end{abstract}