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We report neutron scattering, magnetic susceptibility and Monte Carlo theoretical analysis to verify the short-range nature of the magnetic structure and spin-spin correlations in a ${\mathrm{Yb}}_{3}{\mathrm{Ga}}_{5}{\mathrm{O}}_{12}$ single crystal. The quantum spin state of ${\mathrm{Yb}}^{3+}$ in ${\mathrm{Yb}}_{3}{\mathrm{Ga}}_{5}{\mathrm{O}}_{12}$ is verified. The quantum spins organize into a short-ranged emergent director state for $Tl0.6$ K derived from anisotropy and near-neighbor exchange. We derive the magnitude of the near-neighbor exchange interactions $0.6l{J}_{1}l0.7\phantom{\rule{4pt}{0ex}}\mathrm{K},\phantom{\rule{4pt}{0ex}}{J}_{2}=0.12$ K and the magnitude of the dipolar exchange interaction, $D$, in the range $0.18lDl0.21$ K. Certain aspects of the broad experimental dataset can be modeled using a ${J}_{1}D$ model with ferromagnetic near-neighbor spin-spin correlations while other aspects of the data can be accurately reproduced using a ${J}_{1}{J}_{2}D$ model with antiferromagnetic near-neighbor spin-spin correlation. As such, although we do not quantify all the relevant exchange interactions, we nevertheless provide a strong basis for the understanding of the complex Hamiltonian required to fully describe the magnetic state of ${\mathrm{Yb}}_{3}{\mathrm{Ga}}_{5}{\mathrm{O}}_{12}$.

By employing quasielastic neutron scattering, we studied the atomic-scale relaxation dynamics and transport mechanism of ${\mathrm{La}}_{50}{\mathrm{Ni}}_{15}{\mathrm{Al}}_{35}$ and ${\mathrm{Ce}}_{70}{\mathrm{Cu}}_{19}{\mathrm{Al}}_{11}$ metallic glass melts in the temperature range of $g200\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ above their liquidus temperatures. The results show that both liquids exhibit stretched exponential relaxation and Arrhenius-type temperature dependence of the effective diffusion coefficient. The ${\mathrm{La}}_{50}{\mathrm{Ni}}_{15}{\mathrm{Al}}_{35}$ melt exhibits an activation energy of 0.545 \ifmmode\pm\else\textpm\fi{} 0.008 eV and a stretching exponent \ensuremath{\sim}0.77 to 0.86 in the studied temperature range; no change of activation energy, as suggested in previous reports, associated with liquid-liquid phase transition was observed. In contrast, the ${\mathrm{Ce}}_{70}{\mathrm{Cu}}_{19}{\mathrm{Al}}_{11}$ melt exhibits larger diffusivity with a much smaller activation energy of $0.201\ifmmode\pm\else\textpm\fi{}0.003\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ and a smaller stretching exponent \ensuremath{\sim}0.51 to 0.60, suggestive of more heterogeneous dynamics.

We study single crystals of the magnetic superconductor $\mathrm{Eu}\mathrm{Rb}{\mathrm{Fe}}_{4}{\mathrm{As}}_{4}$ by magnetization, electron spin resonance (ESR), angle-resolved photoemission spectroscopy, and electrical resistance in pulsed magnetic fields up to 63 T. The superconducting state below 36.5 K is almost isotropic and is only weakly affected by the development of ${\mathrm{Eu}}^{2+}$ magnetic order at 15 K. On the other hand, for the external magnetic field applied along the $c$ axis the temperature dependence of the ESR linewidth reveals a Berezinskii-Kosterlitz-Thouless topological transition below 15 K. This indicates that ${\mathrm{Eu}}^{2+}$ planes are a good realization of a two-dimensional $\mathit{XY}$ magnet, which reflects the decoupling of the ${\mathrm{Eu}}^{2+}$ magnetic moments from superconducting FeAs layers.

We employed inelastic neutron scattering (INS), specific heat, and magnetization analysis to study the magnetism in a synthetic polymorph of the quasi-two-dimensional natural metal-organic framework material, stepanovite $\mathrm{Na}\mathrm{Mg}\mathrm{Fe}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{3}\ifmmode\cdot\else\textperiodcentered\fi{}9{\mathrm{H}}_{2}\mathrm{O}$. No long-range magnetic order can be observed down to 0.5 K. The INS spectra show two dispersionless excitations at energy transfer 0.028(1) and 0.050(1) meV at base temperature, which are derived from the magnetic transitions between zero-field splitting (ZFS) of $S\phantom{\rule{4pt}{0ex}}=5/2$ ground state multiplets of ${\mathrm{Fe}}^{3+}$ ion. Further analysis of the INS results shows that the ${\mathrm{Fe}}^{3+}$ ion has an easy-axis anisotropy with axial ZFS parameter $D=\ensuremath{-}0.0128(5)$ meV and rhombic parameter $E=0.0014(5)$ meV. The upward behavior at zero field and Schottky-like peak under magnetic field of the low-temperature magnetic specific heat further support the INS results. Our results clearly reveal the magnetic ground and excited state of this stepanovite polymorph.

Magnetic correlation in ${\mathrm{YBaCo}}_{4}{\mathrm{O}}_{7}$ with kagome and triangular lattices was studied through inelastic-neutron-scattering measurements on a single crystal sample. The broad magnetic excitation was observed below 45 meV and showed dispersions both within the $c$ plane and along the $c$ axis. The wave vector with the minimum energy of the magnetic excitation was totally different from the magnetic Bragg point. Our analysis with the first-moment-sum rule suggests that Co ions in ${\mathrm{YBaCo}}_{4}{\mathrm{O}}_{7}$ form a unique three-dimensional network with the geometrically frustration.

We investigate the magnetic dynamics of the orthorhombic perovskite ${\mathrm{TmFeO}}_{3}$ at low temperatures, below the spin reorientation transition at ${T}_{\mathrm{SR}}\ensuremath{\approx}80$ K, by means of time-of-flight neutron spectroscopy. We find that the magnetic excitation spectrum combines two emergent collective modes associated with different magnetic sublattices. The Fe subsystem orders below ${T}_{\mathrm{N}}\ensuremath{\sim}632$ K into a canted antiferromagnetic structure and exhibits sharp, high-energy magnon excitations. We describe them using linear spin-wave theory, and reveal a pronounced anisotropy between in- and out-of-plane exchange interactions, which was mainly neglected in previous reports on the spin dynamics in orthoferrites. At lower energies, we find two crystalline electrical field (CEF) excitations of ${\mathrm{Tm}}^{3+}$ ions at energies of $\ensuremath{\sim}2$ and 5 meV. In contrast to the sister compound ${\mathrm{YbFeO}}_{3}$, where the ${\mathrm{Yb}}^{3+}$ ions form quasi-one-dimensional chains along the $c$ axis, the Tm excitations show dispersion along both directions in the $(0KL)$ scattering plane. Analysis of the neutron scattering polarization factor reveals a longitudinal polarization of the 2 meV excitation. To evaluate the effect of the CEF on the ${\mathrm{Tm}}^{3+}$ ions, we perform point-charge model calculations, and their results quantitatively capture the main features of Tm single-ion physics, such as energies, intensities, and polarization of the CEF transitions, and the type of magnetic anisotropy.

We present thermodynamic and neutron-scattering measurements on the quantum spin ice candidate ${\mathrm{Nd}}_{2}{\mathrm{Zr}}_{2}{\mathrm{O}}_{7}$. The parametrization of the anisotropic exchange Hamiltonian is refined based on high-energy-resolution inelastic neutron-scattering data together with thermodynamic data using linear spin-wave theory and numerical linked-cluster expansion. Magnetic phase diagrams are calculated using classical Monte Carlo simulations with fields along [100], [110], and [111] crystallographic directions which agree qualitatively with the experiment. Large hysteresis and irreversibility for [111] is reproduced and the microscopic mechanism is revealed by mean-field calculations to be the existence of metastable states and domain inversion. Our results shed light on the explanations of the recently observed dynamical kagome ice in ${\mathrm{Nd}}_{2}{\mathrm{Zr}}_{2}{\mathrm{O}}_{7}$ in [111] fields.

Using high-resolution inelastic neutron scattering, we examine the spin dynamics of ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{WO}}_{4}$ in the collinear AF1, the $ac\text{\ensuremath{-}}b$ spiral AF2, and the $ac$ cycloidal AF5 phases. The spin wave excitations are well described by a Heisenberg model with competing long-range exchange interactions $({J}_{i}$ up to 12th nearest neighbors) and the single-ion anisotropy $K$ 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 $(Kg0)$ in the collinear AF1 phase to easy plane $(Kl0)$ in the two multiferroic phases.

This work presents both the parametrization procedure of a self-consistent charge density functional based tight-binding scheme (SCC-DFTB) for hafnium oxide and the structural as well as the electronic results of the simulations of amorphous hafnium oxide $a\text{-HfO}{}_{2}$ using the data resulting during the parametrization. While aiming at describing amorphous hafnium oxide, the created parameters could be successfully compared to ab initio DFT data for hafnium and hafnium oxide crystalline structures. Furthermore the atomic structure and electronic properties obtained in the DFTB simulations of $a\text{-HfO}{}_{2}$ compare well to other theoretical studies using classical many-body as well as quantum-mechanical approaches.

A large broadening ($\ensuremath{\sim}0.4\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$) of quasielastic neutron scattering (QENS) signal in ${\mathrm{H}}_{2}\mathrm{O}$ ice ${I}_{h}$ at $T=5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ was observed by Bove et al. [Phys. Rev. Lett. 103, 165901 (2009)] and explained by a model of concerted proton tunneling. This result was rather unexpected, as prior studies never showed significant mobility in water at low temperatures and ambient pressure. There were a few attempts of theoretical understanding of the effect. Recent path-integral simulations as well as quantum lattice-gauge theory supported the possibility of the collective tunneling of protons in ice ${I}_{h}$, however other studies stated that concerted tunneling in ice ${I}_{h}$ should have very low frequency. Here, we report on QENS measurements of ${\mathrm{H}}_{2}\mathrm{O}$ ice ${I}_{h}$ at 1.8 and 5 K by using neutron scattering spectrometers with the energy resolution similar to and four times better than the energy resolution in the original experiment of Bove and co-workers. We did not observe any QENS broadening, and the measured spectra for the ice ${I}_{h}$ and the reference vanadium sample were almost identical. Therefore, we conclude that there is no proton tunneling in ice ${I}_{h}$ at temperatures down to 1.8 K measurable on an energy scale of 3.5 $\ensuremath{\mu}\mathrm{eV}$ and above. The literature data on low-temperature heat capacity of ice ${I}_{h}$ support this conclusion.

The physical properties of the spinel ${\mathrm{LiGaCr}}_{4}{\mathrm{S}}_{8}$ have been studied with neutron diffraction, x-ray diffraction, magnetic susceptibility, and heat capacity measurements. The neutron diffraction and synchrotron x-ray diffraction data reveal negative thermal expansion (NTE) below 111(4) K. The magnetic susceptibility deviates from Curie-Weiss behavior with the onset of NTE. At low temperature a broad peak in the magnetic susceptibility at 10.3(3) K is accompanied by the return of normal thermal expansion. First-principles calculations find a strong coupling between the lattice and the simulated magnetic ground state. These results indicate strong magnetoelastic coupling in ${\mathrm{LiGaCr}}_{4}{\mathrm{S}}_{8}$.

The phonon dispersion in terbium iron borate ${\mathrm{TbFe}}_{3}{({\mathrm{BO}}_{3})}_{4}$ has been measured by inelastic neutron scattering in a temperature range $180lTl350$ K through the displacive structural transition at ${T}_{\mathrm{S}}=192.5$ K and studied by ab initio calculations. Significant, but not complete, softening of the transverse acoustic (TA) branch has been observed at the corner of the Brillouin zone ($\mathrm{\ensuremath{\Lambda}}$ point) at temperatures $T⪆{T}_{\mathrm{S}}$, in full agreement with theoretical calculations. The TA soft mode undergoes considerable broadening at the $\mathrm{\ensuremath{\Lambda}}$ point near the transition temperature that can be attributed to the anharmonic interference between transverse acoustic and optical modes.

Motivated by dimensional crossover in layered organic ${\kappa}$ salts, we determine the phase diagram of a system of four periodically coupled Hubbard chains with frustration at half filling as a function of the interchain hopping ${t_{\perp}/t}$ and interaction strength ${U/t}$ at a fixed ratio of frustration and interchain hopping ${t'/t_{\perp}=-0.5}$. We cover the range from the one-dimensional limit of uncoupled chains (${t_{\perp}/t=0.0}$) to the isotropic model (${t_{\perp}/t=1.0}$). For strong ${U/t}$, we find an antiferromagnetic insulator; in the weak-to-moderate-interaction regime, the phase diagram features quasi-one-dimensional antiferromagnetic behavior, an incommensurate spin-density wave, and a metallic phase as ${t_{\perp}/t}$ is increased. We characterize the phases through their magnetic ordering, dielectric response, and dominant static correlations. Our analysis is based primarily on a variant of the density-matrix renormalization-group algorithm based on an efficient hybrid-real-momentum-space formulation, in which we can treat relatively large lattices albeit of a limited width. This is complemented by a variational cluster approximation study with a cluster geometry corresponding to the cylindrical lattice allowing us to directly compare the two methods for this geometry. As an outlook, we make contact with work studying dimensional crossover in the full two-dimensional system., Comment: 16 pages, 16 figures

We present experimental evidence for a quantum phase transition in the easy-axis $S=3/2$ anisotropic quasi-one-dimensional ferromagnet ${\mathrm{CoCl}}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}2{\mathrm{D}}_{2}\mathrm{O}$ in a transverse field. Elastic neutron scattering shows that the magnetic order parameter vanishes at a transverse critical field ${\ensuremath{\mu}}_{0}{H}_{c}=16.05$(4) T, while inelastic neutron scattering shows that the gap in the magnetic excitation spectrum vanishes at the same field value, and reopens for $Hg{H}_{c}$. The field dependence of the order parameter and the gap are well described by critical exponents $\ensuremath{\beta}=0.45\ifmmode\pm\else\textpm\fi{}0.09$ and $z\ensuremath{\nu}$ close to $1/2$, implying that the quantum phase transition in ${\mathrm{CoCl}}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}2{\mathrm{D}}_{2}\mathrm{O}$ differs significantly from the textbook version of a $S=1/2$ Ising chain in a transverse field. We attribute the difference to weak but finite three-dimensionality of the magnetic interactions.

Inelastic neutron and x-ray scattering measurements of phonons and spin waves were performed in the delafossite compound ${\mathrm{CuCrO}}_{2}$ over a wide range of temperature, and complemented with first-principles lattice dynamics simulations. The phonon dispersions and density of states are well reproduced by our density functional calculations, and reveal a strong anisotropy of Cu vibrations, which exhibit low-frequency modes of large amplitude parallel to the basal plane of the layered delafossite structure. The low frequency in-plane modes also show a systematic temperature dependence of neutron and x-ray scattering intensities. In addition, we find that spin fluctuations persist above 300 K, far above the N\'eel temperature for long-range antiferromagnetic order, ${T}_{N}\ensuremath{\simeq}24\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Our modeling of the thermal conductivity, based on our phonon measurements and simulations, reveals a significant anisotropy and indicates that spin fluctuations above ${T}_{N}$ constitute an important source of phonon scattering, considerably suppressing the thermal conductivity compared to that of the isostructural but nonmagnetic compound ${\mathrm{CuAlO}}_{2}$.

Here we present inelastic neutron scattering measurements from the frustrated, quantum spin-1/2 chain material linarite, PbCuSO_4(OH)_2. Time of flight data, taken at 0.5K and zero applied magnetic field reveals low-energy dispersive spin wave excitations below 1.5meV both parallel and perpendicular to the Cu-chain direction. From this we confirm that the interchain couplings within linarite are around 10% of the nearest neighbour intrachain interactions. We analyse the data within both linear spin-wave theory and density matrix renormalisation group theories and establish the main magnetic exchange interactions and the simplest realistic Hamiltonian for this material.

The pressure effect on the tunneling mode and vibrational spectra of hydrogen in $\ensuremath{\alpha}\ensuremath{-}{\text{MnH}}_{0.07}$ has been studied by inelastic neutron scattering. Applying hydrostatic pressure of up to 30 kbar is shown to shift both the hydrogen optical modes and the tunneling peak to higher energies. First-principles calculations show that the potential for hydrogen in $\ensuremath{\alpha}$-Mn becomes overall steeper with increasing pressure. At the same time, the barrier height and its extent in the direction of tunneling decrease and the calculations predict significant changes of the dynamics of hydrogen in $\ensuremath{\alpha}$-Mn at 100 kbar, when the estimated tunneling splitting of the hydrogen ground state exceeds the barrier height.

Neutron diffraction measurements were carried out on single crystals and powders of ${\mathrm{Yb}}_{2}{\mathrm{Pt}}_{2}\mathrm{Pb}$, where Yb moments form two interpenetrating planar sublattices of orthogonal dimers, a geometry known as Shastry-Sutherland lattice, and are stacked along the $c$ axis in a ladder geometry. ${\mathrm{Yb}}_{2}{\mathrm{Pt}}_{2}\mathrm{Pb}$ orders antiferromagnetically at ${T}_{N}=2.07\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, and the magnetic structure determined from these measurements features the interleaving of two orthogonal sublattices into a $5\ifmmode\times\else\texttimes\fi{}5\ifmmode\times\else\texttimes\fi{}1$ magnetic supercell that is based on stripes with moments perpendicular to the dimer bonds, which are along $(110)$ and $(\ensuremath{-}110)$. Magnetic fields applied along $(110)$ or $(\ensuremath{-}110)$ suppress the antiferromagnetic peaks from an individual sublattice, but leave the orthogonal sublattice unaffected, evidence for the Ising character of the Yb moments in ${\mathrm{Yb}}_{2}{\mathrm{Pt}}_{2}\mathrm{Pb}$ that is supported by point charge calculations. Specific heat, magnetic susceptibility, and electrical resistivity measurements concur with neutron elastic scattering results that the longitudinal critical fluctuations are gapped with $\mathrm{\ensuremath{\Delta}}E\ensuremath{\simeq}0.07\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$.

In this paper, we report inelastic neutron scattering measurements of the magnetic excitations of green dioptase ${\mathrm{Cu}}_{6}[{\mathrm{Si}}_{6}{\mathrm{O}}_{18}]\ifmmode\cdot\else\textperiodcentered\fi{}6{\mathrm{H}}_{2}\mathrm{O}$. The observed spectrum contains two magnetic modes and a prominent spin gap that is consistent with the ordered ground state of Cu moments coupled antiferromagnetically in spiral chains along the $c$ axis and ferromagnetically in $ab$ planes on the hexagonal cell. The data are in excellent agreement with a spin-$\frac{1}{2}$ Hamiltonian that includes antiferromagnetic nearest-neighbor intrachain coupling ${J}_{c}=10.6(1)$ meV, ferromagnetic interchain coupling ${J}_{ab}=\ensuremath{-}1.2(1)$ meV, and exchange anisotropy $\mathrm{\ensuremath{\Delta}}{J}_{c}=0.14(1)$ meV. We calculated the sublattice magnetization to be strongly reduced, $\ensuremath{\sim}0.39{\ensuremath{\mu}}_{\mathrm{B}}$. This appears compatible with a reduced N\'eel temperature, ${T}_{\mathrm{N}}=14.5\phantom{\rule{0.28em}{0ex}}\text{K}\ensuremath{\ll}{J}_{c}$, and can be explained by a presence of quantum spin fluctuations.

We know the ground state of the quantum spin ice candidate magnet Yb2Ti2O7 to be sensitive to weak disorder at the similar to 1% level which occurs in single crystals grown from the melt. Powders produced by solid state synthesis tend to be stoichiometric and display large and sharp heat capacity anomalies at relatively high temperatures, T-C similar to 0.26 K. We have carried out neutron elastic and inelastic measurements on well characterized and equilibrated stoichiometric powder samples of Yb2Ti2O7 which show resolution-limited Bragg peaks to appear at low temperatures, but whose onset correlates with temperatures much higher than T-C. The corresponding magnetic structure is best described as an icelike splayed ferromagnet. In the spin dynamics of Yb2Ti2O7 we see the gapless on an energy scale