Abstract
Spin-orbit coupled honeycomb magnets with the Kitaev interaction have received a lot of attention due to their potential of hosting exotic quantum states including quantum spin liquids. Thus ...far, the most studied Kitaev systems are 4
d
/5
d
-based honeycomb magnets. Recent theoretical studies predicted that 3
d
-based honeycomb magnets, including Na
2
Co
2
TeO
6
(NCTO), could also be a potential Kitaev system. Here, we have used a combination of heat capacity, magnetization, electron spin resonance measurements alongside inelastic neutron scattering (INS) to study NCTO’s quantum magnetism, and we have found a field-induced spin disordered state in an applied magnetic field range of 7.5 T <
B
(⊥
b
-axis) < 10.5 T. The INS spectra were also simulated to tentatively extract the exchange interactions. As a 3
d
-magnet with a field-induced disordered state on an effective spin-1/2 honeycomb lattice, NCTO expands the Kitaev model to 3
d
compounds, promoting further interests on the spin-orbital effect in quantum magnets.
The conventional way in time-of-flight (TOF) neutron spectroscopy is to use neutron pulses of one monochromatic wavelength. By contrast, the Repetition Rate Multiplication (RRM) technique for pulsed ...neutron sources uses a set of several monochromatic wavelengths coming from one source pulse F. Mezei, J. Neutron Res. 6 (1997) 3, and references therein. Thus a major efficiency drawback of practical pulsed neutrons sources can be overcome, i.e. their too low pulse repetition rate for TOF spectroscopy. At the same time the pulse repetition rate at the sample can be chosen freely. Here we present the first experimental realization of this novel technique using the flexible disc chopper system of the TOF spectrometer NEAT at BENSC and report the first real life data collection by Repetition Rate Multiplication. The first choppers of the instrument emulated a pulsed source with a repetition rate of 20–40
Hz at a distance of 13.3
m from the sample. The secondary spectrometer was operated at a chopper pulse frequency of about 160
Hz. The spectra were detected by
3He counters placed 2.5
m from the sample. Several RRM frames with different incoming wavelengths coming from each “source” chopper pulse were recorded. The results show that information from different RRM frames can be efficiently combined and evaluated. This offers a more efficient use of the pulsed source by reducing the data collection dead time and allows for faster and enhanced-quality data collection by using several monochromatic wavelengths in parallel.
Here we report the first structural and dynamic investigation on ethylammonium nitrate, a representative protic Ionic liquid, and dimethylsulfoxide. By using joined x/ray and neutron diffraction, we ...exploit the EPSR approach to extract structural information at atomistic level. EAN/DMSO turns out to be homogeneous at microscopic scales and indications for the existence of a structural leit motiv with stoichiometric composition 2DMSO:1EAN are found. Dielectric spectroscopy is used to access the relaxation map of the DMSO:EAN=60:40 mixture. No crystallisation is detected and three relaxation processes could be characterised. Overall this study provides new indications of strict analogies between water and ethylammonium nitrate.
•X-ray and neutron diffraction from ethylammonium nitrate–DMSO mixtures are shown.•Mixtures are highly structured at mesoscopic level, but not phase segregated.•Intimate intermixing between EAN and DMSO through hydrogen bonding occurs.•Evidences for formation of stoichiometric complex EAN:DMSO=1:2 are found.•The relaxation map for dielectrically active processes is reported.
We present results from complementary characterizations of the primary relaxation rate of a room temperature ionic liquid (RTIL), 1-hexyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl} imide, ...C6mimTf2N, over a wide temperature range. This extensive data set is successfully merged with existing literature data for conductivity, viscosity, and NMR diffusion coefficients thus providing, for the case of RTILs, a unique description of the primary process relaxation map over more than 12 decades in relaxation rate and between 185 and 430 K. This unique data set allows a detailed characterization of the VTF parameters for the primary process, that are: B = 890 K, T 0 = 155.2 K, leading to a fragility index m = 71, corresponding to an intermediate fragility. For the first time neutron spin echo data from a fully deuteriated sample of RTIL at the two main interference peaks, Q = 0.76 and 1.4 Å−1 are presented. At high temperature (T > 250 K), the collective structural relaxation rate follows the viscosity behavior; however at lower temperatures it deviates from the viscosity behavior, indicating the existence of a faster process.
Highly ordered and highly cooperative water with properties of both solid and liquid states has been observed by means of neutron scattering in hydrophobic one-dimensional channels with van der Waals ...diameter of 0.78 nm. We have found that in the initial stages of adsorption water molecules occupy niches close to pore walls, followed later by the filling of the central pore area. Intensified by confinement, intermolecular water interactions lead to the formation of well-ordered hydrogen-bonded water chains and to the onset of cooperative vibrations. On the other hand, the same intermolecular interactions lead to two relaxation processes, the faster of which is the spontaneous position exchange between two water molecules placed 3.2–4 Å from each other. Self-diffusion in an axial pore direction is the result of those spontaneous random exchanges and is substantially slower than the self-diffusion in bulk water.
The dynamics of the THF molecule encapsulated in the type II clathrate hydrate matches the MD-QENS observation time (typically 0.1–10ps) between 100K and 270K. Spatial and time characteristics of the ...THF molecule’s dynamics obtained by means of MD simulations are in agreement with those experimentally determined by means of quasielastic neutron scattering. A detailed model of the THF dynamics is then proposed through the calculations of MD-derived properties. Reorientational relaxation has been observed on a timescale of 0.7±0.1ps at 270K with activation energy of 3.0±0.3kJ/mol in addition to a highly damped rotational excitation occurring in the plane of the THF molecule with a period of ca. 2ps. Moreover, the anisotropic cage energy landscape of the THF clathrate hydrate is revealed through a comprehensive investigation of THF orientational distribution functions, revealing the occurrence of preferred orientation of the THF molecule within the cage.
We present results of parallel quasielastic neutron scattering (QENS) experiments and molecular dynamics numerical simulations for the dynamics of a prototype ionic liquid, ...1-ethyl-3-methyl-imidazolium bromide. Differences and similarities with those from the crystal phase are also discussed. Both experiment and simulation demonstrate that, in the length and time scales being probed here (fractions of a nm and a few ps), the dynamics are dominated by activated translational diffusion in the liquid phase and reorientations of the ethyl groups in both solid and liquid.
The spin dynamics of {Cr8Y8}, a rare example of ferromagnetic molecular rings, has been studied by inelastic neutron scattering (INS) and heat capacity (HC) methods. Clear evidence of low-lying ...magnetic excitation has been found. Magnetic Schottky anomalies are observed in low-temperature (low-T) HC curves measured under various fields and can be well fitted with a multi-level Schottky term, giving important information on the energy gaps between spin levels. The INS results obtained on TOFTOF and NEAT time-of-flight spectrometers show INS peaks corresponding to the transitions within S = 12 ground states and between the S = 12 ground state and the S = 11 excited state. The single-J model with a unique exchange constant J = 0.151 meV can well reproduce the low-lying energy levels and their Zeeman splitting upon applied magnetic fields. This work shows that the single-J model can be a good approach for the low-T spin dynamics of {Cr8Y8} and may have general significance for other weak ferromagnetic molecular rings. Determination of the Cr3+-Cr3+ exchange constant in {Cr8Y8} will benefit the study on the complicated magnetic interactions in chromium lanthanide complexes. The zero-field splitting which is suggested by HC data still calls for EPR or high-resolution INS technique to verify.
In porous materials the molecular confinement is often realized by means of weak Van der Waals interactions between the molecule and the pore surface. The understanding of the mechanism of such ...interactions is important for a number of applications. In order to establish the role of the confinement size we have studied the microscopic dynamics of molecular hydrogen stored in the nanocages of clathrate hydrates of two different dimensions. We have found that by varying the size of the pore the diffusive mobility of confined hydrogen can be modified in both directions, i.e. reduced or enhanced compared to that in the bulk solid at the same temperatures. In the small cages with a mean crystallographic radius of 3.95 Å the confinement reduces diffusive mobility by orders of magnitude. In contrast, in large cages with a mean radius of 4.75 Å hydrogen molecules displays diffusive jump motion between different equilibrium sites inside the cages, visible at temperatures where bulk H2 is solid. The localization of H2 molecules observed in small cages can promote improved functional properties valuable for hydrogen storage applications.
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