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Among the large class of $A$-site ordered perovskites of stoichiometry $A{C}_{3}{B}_{4}{\mathrm{O}}_{12}$, the rare-earth $(R)$ ruthenates $R{\mathrm{Cu}}_{3}{\mathrm{Ru}}_{4}{\mathrm{O}}_{12}$ $(R=\mathrm{La}, \mathrm{Pr}, \mathrm{Nd})$ are interesting compounds due both to $\mathrm{Ru}\text{\ensuremath{-}}4d$-derived electronic correlations and to unconventional crystal-electric-field effects of the $R$ ions. Here we report on detailed investigations of these compounds utilizing x-ray diffraction, neutron scattering, magnetic susceptibility, and electrical resistivity measurements as well as heat capacity and nuclear resonance experiments. A broad range of external parameters is scanned and depending on the specific technique, temperatures range from 100 mK to 730 K in external magnetic fields up to 14 T. In this work ${\mathrm{LaCu}}_{3}{\mathrm{Ru}}_{4}{\mathrm{O}}_{12}$ serves as reference compound with a nonmagnetic $A$ site, characterized in detail recently [S. Riegg et al., Phys. Rev. B 93, 115149 (2016)]. All compounds investigated reveal heavy-fermion behavior with a ${T}^{2}$ dependence of the low-temperature electrical resistivity and significantly enhanced Sommerfeld coefficients. Toward low temperatures, the compounds with $R=\mathrm{Pr}$ and Nd are dominated by the magnetic moments of the $R$ ions, which occupy crystallographic positions with point-group symmetry ${T}_{h}$. The crystal-electric-field effects are clearly visible especially in heat capacity and inelastic neutron scattering data from which the crystal-electric-field parameters are derived. The ground state of the ${\mathrm{Pr}}^{3+}$ ion is identified as a triplet $({\mathrm{\ensuremath{\Gamma}}}_{4}^{(1)})$, whereas for ${\mathrm{Nd}}^{3+}$ it is a quartet $({\mathrm{\ensuremath{\Gamma}}}_{67})$. Evidence for lowering of the ${T}_{h}$ symmetry is observed at the Pr site at temperatures below 10 K, suggesting the formation of orbital order. Moreover, the spin-lattice relaxation derived from $^{63}\mathrm{Cu}$ nuclear quadrupole resonance indicates characteristic temperatures close to 7 K and 350 mK, probably related to orbital and magnetic order, respectively.

The Ising triangular lattice remains the classic test-case for frustrated magnetism. Here we report neutron scattering measurements of short range magnetic order in CuMnO$_2$, which consists of a distorted lattice of Mn$^{3+}$ spins with single-ion anisotropy. Physical property measurements on CuMnO$_2$ are consistent with 1D correlations caused by anisotropic orbital occupation. However the diffuse magnetic neutron scattering seen in powder measurements has previously been fitted by 2D Warren-type correlations. Using neutron spectroscopy, we show that paramagnetic fluctuations persist up to $\sim$25 meV above TN= 65 K. This is comparable to the incident energy of typical diffractometers, and results in a smearing of the energy integrated signal, which hence cannot be analysed in the quasi-static approximation. We use low energy XYZ polarised neutron scattering to extract the purely magnetic (quasi)-static signal. This is fitted by reverse Monte Carlo analysis, which reveals that two directions in the triangular layers are perfectly frustrated in the classical spin-liquid phase at 75 K. Strong antiferromagnetic correlations are only found along the b-axis, and our results hence unify the pictures seen by neutron scattering and macroscopic physical property measurements.

The instrumental divergence parameters and resolution for the D7 neutron diffuse scattering spectrometer at the Institut Laue-Langevin, France, are presented. The resolution parameters are calibrated against measurements of powders, single crystals, and the incoherent scattering from vanadium. We find that the powder diffraction resolution is well described by the Cagliotti function, the single crystal resolution function can be parameterized using the Cooper-Nathans formalism, and that in time-of-flight mode the energy resolution is consistent with monochromatic focussing.

We present an experimental study of the quantum spin ice candidate pyrochlore compound ${\mathrm{Pr}}_{2}{\mathrm{Zr}}_{2}{\mathrm{O}}_{7}$ by means of magnetization measurements, specific heat, and neutron scattering up to 12 T and down to 60 mK. When the field is applied along the $[111]$ and $[1\overline{1}0]$ directions, $\mathbf{k}=0$ field-induced structures settle in. We find that the ordered moment rises slowly, even at very low temperature, in agreement with macroscopic magnetization. Interestingly, for $H\ensuremath{\parallel}[1\overline{1}0]$, the ordered moment appears on the so-called $\ensuremath{\alpha}$ chains only. The spin excitation spectrum is essentially inelastic and consists in a broad flat mode centered at about 0.4 meV with a magnetic structure factor which resembles the spin ice pattern. For $H\ensuremath{\parallel}[1\overline{1}0]$ (at least up to 2.5 T), we find that a well-defined mode forms from this broad response, whose energy increases with $H$, in the same way as the temperature of the specific-heat anomaly. We finally discuss these results in the light of mean field calculations and propose an interpretation where quadrupolar interactions play a major role, overcoming the magnetic exchange. In this picture, the spin ice pattern appears shifted up to finite energy because of those interactions. We then propose a range of acceptable parameters for ${\mathrm{Pr}}_{2}{\mathrm{Zr}}_{2}{\mathrm{O}}_{7}$ that allow to reproduce several experimental features observed under field. With these parameters, the actual ground state of this material would be an antiferroquadrupolar liquid with spin-ice-like excitations.

We report the synthesis and structural characterisation of a family of finite molecular chains, specifically [{[R2NH2](3) [Cr6F11 (O2CCMe3)(10)]}(2)] (in which R=nPr 1, Et 2, nBu 3), [{Et2NH}(2) {[Et2NH2](3)[Cr7F12(O2C-CMe3)(12)][HO2CCMe3](2)}(2)] (4), [{[Me2NH2](3)[Cr6F11(O2CCMe3)(10)]center dot 2.5 H2O}(4)] (5) and [I{iPr(2)NH(2)](3)[Cr7F12(O2CCMe3)(12)]}(2)] (6). The structures all contain horseshoes of chromium centres, with each Cr center dot center dot center dot Cr contact within the horseshoe bridged by a fluoride and two pivalates. The horseshoes are linked through hydrogen bonds to the secondary ammonium cations in the structure, leading to di- and tetrahorseshoe structures. Through magnetic measurements and inelastic neutron scattering studies we have determined the exchange coupling constants in 1 and 6. In 1 it is possible to distinguish two exchange interactions, J(A) = -1.1 meV and J(B) = -1.4 meV; J(A) is the exchange interactions at the tips of the horseshoe and JB is the exchange within the body of the horseshoe (1 meV = 8.066 cm(-1)). For 6 only one interaction was needed to model the data: J=-1.18meV The single-ion anisotropy parameters for Cr-III were also derived for the two compounds as: for 1, D-Cr = -0.028 meV and vertical bar ECr vertical bar = 0.005meV; for 6, D-Cr=-0.031meV. Magnetic-field-dependent inelastic neutron scattering experiments on I allowed the Zeeman splitting of the first two excited states and level crossings to be observed. For the tetramer of horseshoes (5), quantum Monte Carlo calculations were used to fit the magnetic susceptibility behaviour, giving two exchange interactions within the horseshoe (-1.32 and -1.65 meV) and a weak inter-horseshoe coupling of +0.12meV. Multi-frequency variable-temperature EPR studies on 1, 2 and 6 have also been performed, allowing further characterisation of the spin Hamiltonian parameters of these chains.

A magnetic insulator, LiInCr${}_{4}$O${}_{8}$ realizes a so-called ``breathing'' pyrochlore lattice of $S=3/2$ spins on corner-sharing tetrahedra that alternate in size. In this manuscript, a collaboration of researchers from Institut Laue-Langevin in Grenoble and Institute for Solid State Physics in Tokyo present a detailed neutron scattering study that elucidates the sequence of structural and magnetic phase transitions that occur at low temperatures in this material.

This volume “Neutrons, Sciences and Perspectives” presents a series of articles that emphasize and demonstrate the universality of the opportunities offered by neutron scattering techniques in wide domains of fundamental research and applications. These papers make good use of the specific properties of neutrons, such as their high penetration in most materials, their interaction with magnetisation, their ability to probe all nuclei and their selectivity between isotopes of the same atom.The different contributions focus on a few significant cases where the contribution of neutron scattering has been essential. Often written in a pedagogic style, the texts address researchers and students interested in this important experimental technique, easily accessible at several large facilities operated at the European level.

Intercalation of Li in TiO2 anatase results in a phase separation in a Li-poor and a Li-rich phase. The local lithium configuration in the coexisting crystallographic phases is resolved by detailed analysis of neutron diffraction data. In each of the phases, two distinct positions within the octahedral interstices are found, with a temperature-dependent occupancy. A combination of quasi-elastic neutron scattering and force field molecular dynamics simulations shows that Li is hopping on a picosecond time scale between the two sites in the octahedral interstices. The results also suggest a specific Li arrangement along the crystallographic a direction, albeit without long range order. It is likely that multiple discrete Li sites within a distorted oxygen octahedron occur not only in intercalated TiO2 anatase but also in other (transition metal) oxides.

Several ${\mathrm{Cu}}^{2+}$ hydroxide minerals have been recently identified as candidate realizations of the $S=1/2$ kagome Heisenberg model. In this context, we have studied the distorted system ${\mathrm{KCu}}_{3}{\mathrm{As}}_{2}{\mathrm{O}}_{7}$(OD)${}_{3}$ using neutron scattering and bulk measurements. Although the distortion favors magnetic order over a spin liquid ground state, refinement of the magnetic diffraction pattern below ${T}_{N1}=7.05(5)$ K yields a complex helical structure with $\mathbf{k}=(0.77,0,0.11)$. This structure, as well as the spin excitation spectrum, are well described by a classical Heisenberg model with ferromagnetic nearest neighbor couplings. Multiferroicity is observed below ${T}_{N1}$, with an unusual crossover between improper and pseudoproper behavior occurring at ${T}_{N2}=5.5$ K. The polarization at $T=2$ K is $P=1.5\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}$Cm${}^{\ensuremath{-}2}$. The properties of ${\mathrm{KCu}}_{3}{\mathrm{As}}_{2}{\mathrm{O}}_{7}$(OD)${}_{3}$ highlight the variety of physics which arise from the interplay of spin and orbital degrees of freedom in ${\mathrm{Cu}}^{2+}$ kagome systems.

At low temperatures, Tb2Ti2O7 enters a spin liquid state, despite expectations of magnetic order and/or a structural distortion. Using neutron scattering, we have discovered that in this spin liquid state an excited crystal field level is coupled to a transverse acoustic phonon, forming a hybrid excitation. Magnetic and phononlike branches with identical dispersion relations can be identified, and the hybridization vanishes in the paramagnetic state. We suggest that Tb2Ti2O7 is aptly named a "magnetoelastic spin liquid" and that the hybridization of the excitations suppresses both magnetic ordering and the structural distortion. The spin liquid phase of Tb2Ti2O7 can now be regarded as a Coulomb phase with propagating bosonic spin excitations.

Recently a novel cooling technique has been introduced based on the barocaloric effect. The effect occurs in rare earth compounds where the crystalline electric field splitting of the rare earth ion can easily be tuned by external pressure or uniaxial stress. As the positions of the low lying crystalline electric field levels have direct influence upon the thermodynamic properties of the system and hence its entropy, adiabatic cooling techniques can be implemented. The monopnictide HoAs shows a barocaloric effect resulting from both a structural as well as a magnetic phase transition. Direct measurements of the barocaloric effect in HoAs and inelastic neutron scattering measurements of the crystalline electric field are presented. The observed barocaloric effect can well be explained by a model taking into account the effects of pressure upon the magnetism as well as upon the structure of the compound.

The concept of ‘rattling modes’ has been brought up in the context of several recent studies concerning thermoelectric materials as well as a new class of oxide superconductors. We have initiated projects for the study of such phenomena, with the aim of better understanding of the ‘rattling mode’ scenario and its role in the physical properties in question. The experiments using Time-of-Flight (T-o-F) investigation of polycrystalline samples with optimized resolution are complemented with ab-initio -based phonon calculations, including the powder averaged coherent inelastic response. Accordingly, we can follow the wave-vector transfer dependence of intensities, in consequence the phase coherence of the motion between particular atomic species. The results we have obtained point out the power of the Powder Averaged Lattice Dynamics (PALD) approach using T-o-F experiments. Detailed modeling for this type of study is especially interesting for materials in polycrystalline form and, therefore, well suitable for material science investigations.

The temperature dependence of the relaxation rate of crystal-field excitations in the slightly underdoped HoBa2Cu4O8 and Er2Ba4Cu7O14.92 high-temperature superconductors has been investigated by means of neutron scattering measurements. Our data show clear evidence for the opening of an electronic gap in the normal state at T* ≈ 200 K far above Tc. The temperature evolution of the relaxation rate appears to be extremely dependent upon the energy at which the susceptibility is being probed. This points to a strongly anisotropic gap function which is tentatively modelled to be of the form s + id3, with Δs/Δd ≈ 0.2.

The molecular spin cluster V15 is a polyoxovanadate with 15 spins s=1/2 but total spin ground state S=1/2 due to strong antiferromagnetic (AFM) interactions. Below 1 K, the relaxation of the magnetisation exhibits a very peculiar behaviour in connection of the multi-spin character of the cluster and the ground state is composed of two non-degenerate S=1/2 Kramers doublets separated by a gap of about 0.3 K. The mechanism leading to such gap formation in the ground state has been elucidated using inelastic neutrons scattering (INS) under magnetic field. The observation and characterisation of several field-dependent INS transitions enable a comprehensive understanding of the low-energy quantum spin states. The energy gap between the two Kramers doublets is caused by slight deviations from the trigonal symmetry (symmetry lowering) of the cluster. The INS data are successfully mapped onto an S=1/2 AFM Heisenberg triangle with scalene distortion and a quantitative description of the wavefunction mixing (entanglement) within the ground state is derived. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Structurally, the two mixed-valence manganese disks Mn${}_{7}$-11 and Mn${}_{7}$-16 differ only in the peripheral ligand but, as a result of a subtle interplay of intramolecular exchange interactions, differ strongly in their magnetic properties, e.g., Mn${}_{7}$-11 possesses a ground-state spin of $S=11$ and Mn${}_{7}$-16 of $S=16$. The exchange interactions in the disks were studied by inelastic neutron scattering. The analysis of the $Q$ dependence of the observed magnetic transition intensities reveals that ferromagnetic cluster spin-wave excitations were observed. In this framework, it was possible to successfully model the experimental data and provide a physical understanding of the magnetism in the two disks.

SrYb${}_{2}$O${}_{4}$ is a geometrically frustrated rare-earth magnet, which presents a variety of interrelated magnetic phenomena. The magnetic Yb${}^{3+}$ ions ($J=7/2$) form potentially frustrated ``zigzag'' chains along the $c$ axis, arranged in a honeycomb fashion in the $ab$ plane. Heat capacity reveals a magnetic phase transition at ${T}_{\mathrm{N}}=0.9$ K. The magnetic structure was solved by polarized neutron diffraction and found to be noncollinear with a reduction of the ordered spin moment from the full ionic moment. The low-energy excitations, which were measured by inelastic neutron scattering reveal diffuse scattering both above and below ${T}_{\mathrm{N}}$. Heat capacity and magnetocaloric effect were performed to map out the magnetic phase diagram as a function of magnetic field and temperature and show a complicated series of states. Altogether, the results suggest that the magnetic interactions in SrYb${}_{2}$O${}_{4}$ compete with each other and with the single-ion anisotropy to produce a highly degenerate ground state manifold that suppresses the magnetic order, broadens the excitations and gives rise to a complex phase diagram.

International audience; [(iC3H7)2NH2][Cr8NiF9(O2CCMe)18], or Cr8Ni, is a prominent example of an odd-membered antiferromagnetic "wheel." A detailed characterization of the magnetic properties of Cr8Ni has been conducted. Inelastic neutron scattering (INS) is used to investigate the energy and momentum transfer dependence of the low-lying spin excitations, including excited states inaccessible by other experimental techniques. The richness of the INS data, in conjunction with microscopic spin Hamiltonian simulations, enables an accurate characterization of the magnetic properties of Cr8Ni. Nearest-neighbor exchange constants of JCrCr = 1.31 meV and JCrNi = 3.22 meV are determined, and clear evidence of axial single-ion anisotropy is found. The parameters determined by INS are shown to fit magnetic susceptibility. The spectroscopic identification of several successive S=1 excited total spin states and lowest spin band excitations show that the rotational band picture, valid for bipartite AFM wheels, breaks down for this odd-numbered wheel. The exchange constants determined here differ from previous efforts based on bulk measurements, and possible reasons are discussed. The large JCrNi/JCrCr ratio in Cr8Ni puts this wheel into a regime with strong quantum fluctuations in which the ground state can be described with a valence bond solid state picture.

We report a combination of physical property and neutron scattering measurements for polycrystalline samples of the one-dimensional spin-chain compound CoV2O6. Heat capacity measurements show that an effective S = 1/2 state is found at low temperatures and that magnetic fluctuations persist up to similar to 6T(N). Above T-N = 6.3 K, measurements of the magnetic susceptibility as a function of T and H show that the nearest-neighbor exchange is ferromagnetic. In the ordered state, we have discovered a crossover from a metamagnet with strong fluctuations between 5 K and T-N to a state with a 1/3 magnetization plateau at 2 < T < 5 K. We use neutron powder diffraction measurements to show that the antiferromagnetic state has incommensurate long-range order and inelastic time-of-flight neutron scattering to examine the magnetic fluctuations as a function of temperature. Above T-N, we find two broad bands between 3.5 and 5 meV and thermally activated low-energy features which correspond to transitions within these bands. These features show that the excitations are deconfined solitons rather than the static spin reversals predicted for a uniform ferromagnetic Ising spin chain. Below T-N, we find a ladder of states due to the confining effect of the internal field. A region of weak confinement below T-N, but above 5 K, is identified which may correspond to a crossover between two- and three-dimensional magnetic ordering. (Less)

The vibrational dynamics of the ternary Ca${}_{1\ensuremath{-}x}$Fe${}_{4}$Sb${}_{12}$, Ba${}_{1\ensuremath{-}x}$Fe${}_{4}$Sb${}_{12}$, Sr${}_{1\ensuremath{-}x}$Fe${}_{4}$Sb${}_{12}$, and Yb${}_{1\ensuremath{-}x}$Fe${}_{4}$Sb${}_{12}$ compounds is studied for its temperature dependence, dispersion relations, and material properties by experiments and ab initio powder-averaged lattice-dynamics (PALD) calculations. The experimental techniques used are high-resolution inelastic neutron scattering (INS) and inelastic x-ray scattering (IXS). Vibrational properties of polycrystalline material have been mapped out in an extensive energy-momentum phase space facilitating the study of powder-averaged phonon-dispersion relations within the first and second Brillouin zones. Qualitatively, the experimental dynamic structure factor $S(Q,\ensuremath{\omega})$ matches, to a high degree, PALD calculations allowing different schemes of collective vibrational eigenmodes to be evaluated. From the PALD calculations, quantitative values of velocity of sound, Debye temperatures, and elastic, bulk, and Young's moduli are computed. The variation of the generalized density of states $G(\ensuremath{\omega})$ with temperature is studied experimentally in the range from 2 to 600 K. No particular anomalies in the collective dynamics are observed apart from a global softening of $G(\ensuremath{\omega})$ upon heating.

First-principles density-functional theory and lattice-dynamics calculations were performed to study the vibrational dynamics and related observables of the ternary compounds ${\text{Ca}}_{1\ensuremath{-}x}{\text{Fe}}_{4}{\text{Sb}}_{12}$, ${\text{Sr}}_{1\ensuremath{-}x}{\text{Fe}}_{4}{\text{Sb}}_{12}$, ${\text{Ba}}_{1\ensuremath{-}x}{\text{Fe}}_{4}{\text{Sb}}_{12}$. and ${\text{Yb}}_{1\ensuremath{-}x}{\text{Fe}}_{4}{\text{Sb}}_{12}$. The calculation results are supported by experimental data, which were obtained from neutron inelastic scattering, neutron-diffraction, and heat-capacity measurements. Within the calculation approach based on the theory of harmonic solids all observables are linked to the phonon density of states $Z(\ensuremath{\omega})$. The good agreement with experimental data shows that the vibrational dynamics of the ternary skutterudite structures can be described by a set of normal modes. Features in the experimentally obtained density of states $G(\ensuremath{\omega})$ reflecting the variation in properties (mass, ionic radius) of the cations Ca, Sr, Ba, and Yb are reproduced by the calculations. The quality of the inelastic neutron experiments enables the detection of at least two mode peaks at 4.9 and 5.7 meV with a pronounced spectral weight of ytterbium.

Detailed knowledge of the dynamical spin–spin correlation function S(κ, ω) turns out to be a necessity in characterizing unconventional quantum spin systems — in particular if a finite spin excitation gap separates a singlet ground state from excited states. Basic predictions relying on a Heisenberg dimer model are presented in the context of neutron scattering investigations on the S= 1 2 Cu 2+ compound KCuCl3. Expressions concerning dispersion relation, intensity and field dependence of the observed singlet–triplet modes above the gap are derived and compared to the experimental results. The agreement between theory and experiment is found to be remarkably good, supporting the characterization of KCuCl3 as a quantum spin liquid near to the dimer limit.

International audience; Inelastic neutron scattering has been used to determine the microscopic Hamiltonian describing two high-spin variants of the high-anisotropy Mn-6 nanomagnet. The energy spectrum of both systems is characterized by the presence of several excited total-spin multiplets partially overlapping the S = 12 ground multiplet. This implies that the relaxation processes of these molecules are different from those occurring in prototype giant-spin nanomagnets. In particular, we show that both the height of the energy barrier and resonant tunnelling processes are greatly influenced by low-lying excited total-spin multiplets.

We have studied magnetically frustrated ${\text{Tb}}_{2}{\text{Sn}}_{2}{\text{O}}_{7}$ by neutron diffraction and high-resolution energy-resolved neutron scattering. At 0.1 K, we observed short-range magnetic correlations with a typical scale of $4\text{ }\text{\AA{}}$, close to the near-neighbor distance between ${\text{Tb}}^{3+}$ ions. This short-range order coexists with ferromagnetic correlations and long-range spin ice order at the scales of 18 and $190\text{ }\text{\AA{}}$, respectively. Spin dynamics was investigated at a time scale down to ${10}^{\ensuremath{-}9}\text{ }\text{s}$ by energy-resolved experiments on a backscattering spectrometer. We observed a freezing of the spin dynamics for all length scales with a strong slowing down of the spin fluctuations when long-range order settles in. We discuss the spin fluctuations remaining in the ground state, in comparison with previous data obtained by muon spectroscopy.

A field-dependent inelastic neutron scattering (INS) experiment on an array of oriented single crystals of Mn12-acetate is reported. The experimental results provide clear evidence that the minority species observed in the INS spectra is the fast-relaxing species observed in, for instance, AC-susceptibility measurements.

International audience; This paper aims at showing the complementarity between time-of-flight and triple-axis neutron scattering experiments, on the basis of two topical examples in the field of geometrical magnetic frustration. Rare earth pyrochlore magnets R 2 Ti 2 O 7 (R is a rare earth) play a prominent role in this field, as they form model systems showing a rich variety of ground states, depending on the balance between dipolar, exchange interactions and crystal field. We first review the case of the XY antiferromagnet Er 2 Ti 2 O 7. Here a transition towards a Néel state is observed, possibly induced by an order-by-disorder mechanism. Effective exchange parameters can be extracted from S(Q, ω). We then examine the case of the spin liquid Tb 2 Ti 2 O 7. Recent experiments reveal a complex ground state characterized by " pinch points " and supporting a low energy excitation. These studies demonstrate the existence of a coupling between crystal field transitions and a transverse acoustic phonon mode.

The low-lying magnetic excitations in the dimers of single-molecule magnets [Mn{sub 4}O{sub 3}Cl{sub 4}(O{sub 2}CEt){sub 3}(py){sub 3}]{sub 2}, or (Mn{sub 4}){sub 2}, are studied by inelastic neutron scattering as a function of hydrostatic pressure. The anisotropy parameters D and B{sub 0}{sup 4}, which describe each Mn{sub 4} subunit, are essentially pressure independent, while the antiferromagnetic exchange coupling J between the two Mn{sub 4} subunits strongly depends on pressure, with an increase of 42% at 17 kbar. Additional pressure-dependent powder x-ray measurements allow a structural interpretation of the findings.

The metallic thiospinel CuV 2 S 4 shows an enhanced value of the low-temperature specific heat with γ = C / T ≈ 60 mJ / mol K 2 , suggesting magnetic or electronic correlations. To elucidate the nature of this low-temperature phase with respect to possible magnetic fluctuations, inelastic neutron scattering measurements were performed on polycrystalline CuV 2 S 4 and CuTi 2 S 4 . Both compounds show an inelastic response dominated by phonons. The low-energy phonon response of CuTi 2 S 4 is centered around 13 meV and depends only weakly on temperature. The corresponding phonon intensity in the low energy transfer region of CuV 2 S 4 is centered at around 6 meV ( T = 120 K ). The center of gravity of this intensity shifts to about 10 meV ( T = 1.8 K ) below the structural phase transition at T ≈90 K .The significant difference in the low-energy phonon response between both compounds, which display the same lattice symmetry at 120 K, and the strong renormalization of the phonon energies below the structural transition might reflect the influence of strong electron-phonon coupling in CuV 2 S 4 .

The generalized density of states of LaFe4Sb12 and CeFe4Sb12 has been determined by inelastic neutron scattering and its main features are found to be in agreement with recently published calculations (J. L. Feldman, D. L. Singh, C. Kendziora, D. Mandrus and B. C. Sales, Phys. Rev. B, 2003, 68, 094301). In both compounds a localized vibrational contribution appears superposed on the low-energy Debye response. The distinct inelastic response of La in LaFe4Sb12 is obtained by subtraction of the data for the Ce filled compound and it shows even more clearly the resolution limited peak at 7 meV, attributed to the localized mode of La-atoms.

An external magnetic field suppresses the spin-energy gap in singlet ground state S=1/2 TlCuCl3. The system becomes quantum-critical at Hc≈5.7 T, where the energy of the lowest Zeeman-split triplet excitation crosses the nonmagnetic ground state. Antiferromagnetic ordering is reported above Hc, which underlines the three-dimensional nature of the observed quantum phase transition. The intrinsic parameters of S=1/2 TlCuCl3 allow us to access the critical region microscopically by neutron scattering. A substantial study of the spin dynamics in the high-field phase of TlCuCl3 at T=1.5 K up to H=12 T was performed for the first time. The results possibly indicate two dynamical regimes, which can be understood within characteristically renormalized triplet modes and a low-lying dynamics of potentially collective origin.

Elastic and inelastic neutron scattering has been employed to study the quadrupolar and dipolar ordering phenomena in DyPd 3 S 4 . At T Q =3.4 K the Dy quadrupole moments undergo a phase transition to an ordered lattice, reflected by a distinct change of the crystal-field level scheme observed by neutron spectroscopy. At T N1 ≈1 K a phase transition of the Dy dipole moments to a long-range canted antiferromagnetic state is observed, followed by a second magnetic phase transition at T N2 ≈0.75 K characterized by a change of the canting angle.

The fully overhauled IN5 was the first cold neutron time-of-flight spectrometer equipped with position sensitive detectors. It has been in scheduled operation since end of 2008. The experience of use has confirmed the expectations in terms of both a qualitative and quantitative efficiency increase, allowing to open new opportunities for time-of-flight spectroscopy. Using the full potential of the position sensitive detectors has brought up an increasing demand for single crystal spectroscopy while the enhanced count-rate and better signal-to-noise allow studying smaller samples or weaker scatterers in the more conventional field of quasi-elastic scattering. Additional effort with sample environment – dedicated magnet and polarization analysis option – are expected to improve further the potential for study of strongly correlated electron systems.

The anhydrous alum KTi(SO4)(2), where the Ti3+ (d(1), S = 1/2) ions form an anisotropic triangular lattice, has been prepared by a new hydrothermal route and characterized by magnetic susceptibility and neutron scattering measurements. Contrary to expectations, fits to the magnetic susceptibility indicate that the spins are isotropic (i.e. Heisenberg) and that the frustrating couplings are weak; indeed, the system is well modelled by nearly isolated chains. The inelastic neutron scattering data furthermore shows excellent agreement with an exact theoretical calculation for the one-dimensional spinon continuum. The unexpected magnetic properties of KTi(SO4)(2) are explained in the light of density functional calculations, which reveal an unusual orbital ground state for the Ti3+ ion.