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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.

CuCrS2 is a triangular lattice Heisenberg antiferromagnet with a rhombohedral crystal structure. We report on neutron and synchrotron powder diffraction results which reveal a monoclinic lattice distortion at the magnetic transition and verify a magnetoelastic coupling. CuCrS2 is therefore an interesting material to study the influence of magnetism on the relief of geometrical frustration., Comment: 6 pages, 6 figures, 1 table

The antiferromagnetic molecular wheel Fe18 of eighteen exchange-coupled Fe(III) ions has been studied by measurements of the magnetic torque, the magnetization, and the inelastic neutron scattering spectra. The combined data show that the low-temperature magnetism of Fe18 is very accurately described by the Neel-vector tunneling (NVT) scenario, as unfolded by semiclassical theory. In addition, the magnetic torque as a function of applied field exhibits oscillations that reflect the oscillations in the NVT tunnel splitting with field due to quantum phase interference., Comment: 5 pages, 4 figures, REVTEX4, to appear in PRL

The electronic structures of the compounds K[(5-Brsalen)(2)(H(2)O)(2)-Mn(2)M(III)(CN)(6)].2H(2)O (M(III) = Co(III), Cr(III), Fe(III)) have been determined by inelastic neutron scattering (INS) and magnetic susceptibility studies, revealing the manganese(III) single-ion anisotropy and exchange interactions that define the low-lying states of the Mn-M(III)-Mn trimeric units. Despite the presence of an antiferromagnetic intertrimer interaction, the experimental evidence supports the classification of both the Cr(III) and Fe(III) compounds as single-molecule magnets. The value of 17(2) cm(-1) established from AC susceptibility measurements for a spin-reversal barrier of K[(5-Brsalen)(2)(H(2)O)(2)-Mn(2)Cr(CN)(6)].2H(2)O may be readily rationalized in terms of the energy level diagram determined directly by INS. AC susceptibility measurements on samples of K[(5-Brsalen)(2)(H(2)O)(2)-Mn(2)Fe(CN)(6)].2H(2)O are contrary to those previously reported, exhibiting but the onset of peaks below temperatures of 1.8 K at oscillating frequencies in the range of 100-800 Hz. INS measurements reveal an anisotropic ferromagnetic manganese(III)-iron(III) exchange interaction, in accordance with theoretical expectations based on the unquenched orbital angular momentum of the [Fe(CN)(6)](3-) anion, giving rise to an M(s) approximately +/-9/2 ground state, isolated by approximately 11.5 cm(-1) from the higher-lying levels. The reported INS and magnetic data should now serve as a benchmark against which theoretical models that aim to inter-relate the electronic and molecular structure of molecular magnets should be tested.

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.

International audience; The low thermal conductivity in filled skutterudites has been ascribed to rattling atoms inducing a phonon glass. Experimental evidence now shows that the phonon glass description is incorrect, and provides essential insight for the development of microscopic models aimed at describing the thermoelectric properties of these materials.

Inelastic neutron scattering on powder samples has been employed to investigate the lattice dynamics of alkali-metal osmates. We have obtained the neutron weighted generalized phonon density-of-states from momentum-transfer averaged coherent inelastic response. We have paid special attention to the low-energy modes that have been assigned to the Einstein term of the lattice specific heat and from which a ``rattling'' behavior of the alkali atoms has been inferred. In all of the three compounds our results reveal a split band with two prominent peaks in the low-energy range associated with the vibrational modes of $A$ atoms. The temperature dependence of these bands has been followed in the range from $1.5\text{ }\text{K}lTl300\text{ }\text{K}$. The strongest temperature effect is visible in the case of ${\text{KOs}}_{2}{\text{O}}_{6}$, the split band peaking at $\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\approx}5.5\text{ }\text{meV}$, and 6.8 meV at $T=300\text{ }\text{K}$ softens on cooling resulting in a main band at about $\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\approx}3.4\text{ }\text{meV}$ at $T\ensuremath{\lesssim}50\text{ }\text{K}$, while a smaller softening of $\ensuremath{\Delta}(\ensuremath{\hbar}\ensuremath{\omega})l1\text{ }\text{meV}$ is seen in the other two compounds. In agreement with earlier work the temperature dependent modifications of the phonon spectrum can be attributed to the weak potential felt by the $A$ atom in the oversized cage where it is located. At low $T$ the characteristic energies for each of the compounds are well correlated with the specific-heat observations published earlier. The powder averaged phonon response shows $Q$ dependence indicating coherent coupling of the alkali atom vibrations with the rest of the lattice.

We report a detailed study of specific heat, electrical resistivity, and thermal expansion in combination with inelastic neutron and inelastic x-ray scattering to investigate the origin of superconductivity in the two silicon clathrate superconductors Ba8Si46 and Ba24Si100. Both compounds have a similar structure based on encaged barium atoms in oversized silicon cages. However, the transition temperatures are rather different: 8 and 1.5 K, respectively. By extracting the superconducting properties, phonon density of states, electron-phonon coupling function, and phonon anharmonicity from these measurements, we discuss the important factors governing Tc and explain the difference between the two compounds.

The Cu4 OCl6 daca4 system composed of isolated Cu2+ S=1/2 tetrahedra with antiferromagnetic exchange should exhibit the properties of a frustrated quantum spin system. Ab initio density functional theory calculations for electronic structure and molecular dynamics computations suggest a complex interplay between magnetic exchange, electron delocalization, and molecular vibrations. Yet, extensive experimental characterization of Cu4 OCl6 daca4 by means of synchrotron x-ray diffraction, magnetization, specific heat, and inelastic neutron scattering reveal that properties of the real material can be only partly explained by proposed theoretical models as the low-temperature properties seem to be governed by a manifold of molecular configurations coexisting at high temperatures. © 2008 The American Physical Society.

We follow the evolution of the elementary excitations of the quantum antiferromagnet TlCuCl3 through the pressure-induced quantum critical point, which separates a dimer-based quantum disordered phase from a phase of long-ranged magnetic order. We demonstrate by neutron spectroscopy the continuous emergence in the weakly ordered state of a low-lying but massive excitation corresponding to longitudinal fluctuations of the magnetic moment. This mode is not present in a classical description of ordered magnets, but is a direct consequence of the quantum critical point., 4 pages, 4 figures

We study the spin dynamics in two variants of the high-anisotropy Mn-6 nanomagnet by inelastic neutron scattering, magnetic resonance spectroscopy and magnetometry. We show that a giant-spin picture is completely inadequate for these systems and that excited S multiplets play a key role in determining the effective energy barrier for the magnetization reversal. Moreover, we demonstrate the occurrence of tunneling processes involving pair of states having different total spin.

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.

The antiferromagnetic molecular finite chain Cr6 was studied by inelastic neutron scattering. The observed magnetic excitations at 2.6 and 4.3 meV correspond, due to the open boundaries of a finite chain, to standing spin waves. The determined energy spectrum revealed an essentially classical spin structure. Hence, various spin-wave theories were investigated in order to assess their potential for describing the elementary excitations of finite spin systems., Comment: 6 pages, 6 figures, accepted for publication in Europhysics Letters

Energy resolved neutron diffraction elucidates the magnetic ordering process in the magnetic A-site thiospinel compound MnSc2S4, which reveals two magnetic transitions at TN1 = 2.3 K and K. The coexistence of long range magnetic order and remaining short range contributions with different propagation vectors for clarifies that the transition at K is first order. The ordering process can be described in the framework of single- and double-q short range order fluctuations around approaching the magnetic ordering transition at TN1 = 2.3 K from the high temperature paramagnetic state. The double-q short range contribution may serve as a precursor for the finally stabilized spiral structure with for . Anisotropic magneto-elastic coupling is suggested to induce the magnetic ordering and to lift a possible exchange frustration via an associated lattice distortion.

The Q dependence of the inelastic neutron scattering (INS) intensity of transitions within the ground-state spin multiplet of single-molecule magnets (SMMs) is considered. For these transitions, the Q dependence is related to the spin density map in the ground state, which in turn is governed by the Heisenberg exchange interactions in the cluster. This provides the possibility to infer the exchange-coupling constants from the Q dependence of the INS transitions within the spin ground state. The potential of this strategy is explored for the M = +-10 -> +- 9 transition within the S = 10 multiplet of the molecule Mn12 as an example. The Q dependence is calculated for powder as well as single-crystal Mn12 samples for various exchange-coupling situations discussed in the literature. The results are compared to literature data on a powder sample of Mn12 and to measurements on an oriented array of about 500 single-crystals of Mn12. The calculated Q dependence exhibits significant variation with the exchange-coupling constants, in particular for a single-crystal sample, but the experimental findings did not permit an unambiguous determination. However, although challenging, suitable experiments are within the reach of today's instruments., 11 pages, 6 figures, REVTEX4, to appear in PRB

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 $A$-site spinel compound $\mathrm{Mn}{\mathrm{Sc}}_{2}{\mathrm{S}}_{4}$ exhibits strong frustration effects with a low ordering temperature $({T}_{N}=2.3\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. Detailed neutron-scattering investigations have been performed to study the magnetic ordering process and the associated magnetic excitations. The magnetic structure found at $T=1.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is a spiral within the $a\text{\ensuremath{-}}b$ plane characterized by a propagation vector of $\mathbf{q}=(3∕4,3∕4,0)$. The magnetic ordering process is spectroscopically reflected by a change from a purely quasielastic to an inelastic response. The quasielastic line width corresponds to a magnetic energy scale of $\ensuremath{\approx}1\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ which is consistent with the spin wave stiffness as determined from the magnon dispersion at low temperatures.

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 .

We have applied high-resolution neutron time-of-flight spectroscopy and ab initio lattice dynamics calculations to study the guest dynamics of La and Ce in the filled skutterudite system xFe4 Sb12. Both the experiments and the calculation show that the guest dynamics is governed by collective vibrations, i.e. phonons. Unlike the case of localized rattlers the guest modes reveal dispersive character and a non-trivial Q-dependence of the signal intensity. The rather low-energy of the guest modes is due to their higher mass and weaker bonding with the host network

The compound TlCuCl3 represents a model system of dimerized quantum spins with strong interdimer interactions. We investigate the triplet dispersion as a function of temperature by inelastic neutron scattering experiments on single crystals. By comparison with a number of theoretical approaches we demonstrate that the description of Troyer, Tsunetsugu, and Wuertz [Phys. Rev. B 50, 13515 (1994)] provides an appropriate quantum statistical model for dimer spin systems at finite temperatures, where many-body correlations become particularly important., 4 pages, 4 figures, to appear in Physical Review Letters

Energy splittings resulting from anisotropy and exchange interactions in the dimer of single-molecule magnets [Mn4O3Cl4(O2CEt)3(py)3]2.8MeCN are determined for both an undeuterated and a partially deuterated sample using inelastic neutron scattering. The antiferromagnetic (AF) exchange coupling between the two Mn4 subunits strongly depends on their separation. The Cl...Cl distance between the two subunits can be modified either by exchanging the solvent of crystallization or by deuteration of the C-H...Cl hydrogen bonds. The exchange of acetonitrile for n-hexane leads to a five times greater shortening of the Cl...Cl separation than does full deuteration of all the hydrogen bonds. As a result, the AF exchange coupling constants between the subunits are 0.0073(4) and 0.0103(9) meV in the samples with acetonitrile and n-hexane solvent molecules, respectively, in the crystal structure. On the other hand, the effect of C-H...Cl deuteration on the AF exchange coupling is not detectable within the experimental accuracy of INS.

From inelastic neutron scattering experiments, we demonstrate quantum tunneling of the N\'eel vector in the antiferromagnetic molecular ferric wheel ${\mathrm{CsFe}}_{8}$. Analysis of the linewidth of the tunneling transition evidences coherent tunneling.

Inelastic neutron scattering results on the spin-orbital liquid in ${\mathrm{FeSc}}_{2}{\mathrm{S}}_{4}$ are presented. This sulfospinel reveals strong geometric frustration in the spin and in the orbital sector. In the present experiments the orbital liquid is evidenced by a clear spectroscopic signature of a dynamic Jahn-Teller effect with a vibronic splitting $3\ensuremath{\Gamma}\ensuremath{\approx}2\text{ }\text{ }\mathrm{meV}$ in agreement with theoretical estimates. The excitations of the spin liquid reveal strong dispersion and can be characterized as cooperative spin excitations in a supercooled paramagnet with a spin gap of $\ensuremath{\Delta}\ensuremath{\approx}0.2\text{ }\text{ }\mathrm{meV}$.

The magnetic exchange interactions in a C0(3)(11) moiety encapsulated in Na(17) [(NaOH(2))Co(3)(H(2)O)(P(2)W(15)O(56))(2)] (NaCo(3)) were studied by a combination of magnetic measurements (magnetic susceptibility and low-temperature magnetization), with a detailed Inelastic Neutron Scattering (INS) investigation. The novel structure of the salt was determined by X-ray crystallography. The ferromagnetic Co(3)O(14) triangular cluster core consists of three octahedrally oxo-coordinated Co(II) ions sharing edges. According to the single-ion anisotropy and spin-orbit coupling usually assumed for octahedral Co(II) ions, the appropiate exchange Hamiltonian to describe the ground-state properties of the isosceles triangular Co(3) spin cluster is anisotropic and is expressed as H = - 2sigma(alpha)(=)(x,y,z)(J(alpha)(12)S(1alpha)S(2alpha) + J(alpha)(23)S(2alpha)S(3alpha) + J(alpha)(13)S(1alpha)S(3alpha)), where J(alpha) are the components of the exchange interactions between the Co(II) ions. To reproduce the INS data, nonparallel anisotropic exchange tensors needed to be introduced, which were directly connected to the molecular symmetry of the complex. The following range of parameters (value +/- 0.5 cm(-1)) was found to reproduce all experimental information while taking magnetostructural relations into account: J(x)(12) = J(y)(13) = 8.6 cm(-1); J(y)(12) = J(x)(13) = 1.4 cm(-1); J(z)(12) = J(z)(13) = 10.0 cm(-1); J(x)(23) = J(y)(23) = 6.5 cm(-1) and = 3.4 cm(-1).

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.

A high resolution inelastic neutron scattering (INS) study of fully deuterated Mn$_{12}$-acetate provides the most accurate spin Hamiltonian parameters for this prototype single molecule magnet so far. The Mn$_{12}$-clusters deviate from axial symmetry, a non-zero rhombic term in the model Hamiltonian leading to excellent agreement with observed positions and intensities of the INS peaks. The following parameter set provides the best agreement with the experimental data: $D=-0.0570(1)$ meV, $B_{4}^0=-2.78(7)\cdot 10^{-6}$ meV, $B_{4}^4=-3.2(6)\cdot 10^{-6}$ meV and $\mid$\textit{E}$\mid =6.8(15)\cdot 10^{-4}$ meV. Crystal dislocations are not the likely cause of the symmetry lowering. Rather, this study lends strong support to a recently proposed model, which is based on the presence of several molecular isomers with distinct spin Hamiltonian parameters., 4 pages, 4 figures

The anisotropy splitting in the ground state of the single-molecule magnet ${\mathrm{Mn}}_{4}{\mathrm{O}}_{3}\mathrm{Br}{(\mathrm{O}\mathrm{Ac})}_{3}{(\mathrm{dbm})}_{3}$ is studied by inelastic neutron scattering as a function of hydrostatic pressure. This allows a tuning of the anisotropy and thus the energy barrier for slow magnetization relaxation at low temperatures. The value of the negative axial anisotropy parameter ${D}_{\text{cluster}}$ changes from $\ensuremath{-}0.0627(1)\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ at ambient to $\ensuremath{-}0.0603(3)\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ at $12\phantom{\rule{0.3em}{0ex}}\mathrm{kbar}$ pressure, and in the same pressure range the height of the energy barrier between up and down spins is reduced from $1.260(5)\phantom{\rule{0.3em}{0ex}}\mathrm{meV}\phantom{\rule{0.5em}{0ex}}\text{to}\phantom{\rule{0.5em}{0ex}}1.213(9)\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. Since the $\mathrm{Mn}\mathrm{Br}$ bond is significantly softer and thus more compressible than the $\mathrm{Mn}\mathrm{O}$ bonds, pressure induces a tilt of the single ion ${\mathrm{Mn}}^{3+}$ anisotropy axes, resulting in the net reduction of the axial cluster anisotropy.

The anisotropy splitting in the ground state of the single-molecule magnet [Mn4O3Br(OAc)3(dbm)3] is studied by inelastic neutron scattering as a function of hydrostatic pressure. This allows a tuning of the anisotropy and thus the energy barrier for slow magnetisation relaxation at low temperatures. The value of the negative axial anisotropy parameter $D_{\rm cluster}$ changes from -0.0627(1) meV at ambient to -0.0603(3) meV at 12 kbar pressure, and in the same pressure range the height of the energy barrier between up and down spins is reduced from 1.260(5) meV to 1.213(9) meV. Since the $\rm Mn-Br$ bond is significantly softer and thus more compressible than the $\rm Mn-O$ bonds, pressure induces a tilt of the single ion Mn$^{3+}$ anisotropy axes, resulting in the net reduction of the axial cluster anisotropy., 4 pages, 3 figures

The ground-state properties of a Co3II moiety encapsulated in a polyoxometalate anion were investigated by combining measurements of specific heat, magnetic susceptibility, and low-temperature magnetization with a detailed inelastic neutron scattering (INS) study on a fully deuterated polycrystalline sample of Na12[Co3W(D2O)2(ZnW9O34)2].40D2O (Co3). The ferromagnetic Co3O14 cluster core consists of three octahedrally oxo-coordinated CoII ions. According to the single-ion anisotropy and spin-orbit coupling of the octahedral CoII ions, the appropriate exchange Hamiltonian to describe the ground-state properties of the Co3 spin cluster is anisotropic and is expressed as H = -2 sigma a = x,y,z (Ja12 S1a S2a + Ja23 S2a S3a), where Ja are the components of the exchange interactions between the CoII ions. To reproduce the INS data, different orientations of the two anisotropic J tensors must be considered, and the following conditions had to be introduced: Jx12 = Jy23, Jy12 = Jx23, Jz12 = Jz23. This result was correlated with the molecular symmetry of the complex. The following set of parameters was obtained: Jx12 = Jy23 = 1.37, Jy12 = Jx23 = 0.218, and Jz12 = Jz23 = 1.24 meV. This set also reproduces in a satisfactory manner the specific heat, susceptibility, and magnetization properties of Co3.

The oxygen and copper isotope effects on the relaxation rate of crystal-field excitations in the optimally doped high-temperature superconductor ${\mathrm{La}}_{1.81}{\mathrm{Ho}}_{0.04}{\mathrm{Sr}}_{0.15}{\mathrm{CuO}}_{4}$ have been investigated by means of inelastic neutron scattering. For the ${}^{16}\mathrm{O}$ compound there is clear evidence for the opening of an electronic pseudogap in the normal state at ${T}^{*}\ensuremath{\approx}60\mathrm{K},$ far above ${T}_{c}\ensuremath{\approx}32\mathrm{K}.$ Upon substituting ${}^{16}\mathrm{O}$ by ${}^{18}\mathrm{O},$ ${T}^{*}$ is shifted to about 70 K. On the other hand, no shift is found for the copper isotope substitution, i.e., ${T}^{*}{(}^{63}\mathrm{Cu})\ensuremath{\approx}{T}^{*}{(}^{65}\mathrm{Cu})\ensuremath{\approx}60\mathrm{K}.$ These results, together with the large oxygen and copper isotope effects on ${T}^{*}$ for ${\mathrm{HoBa}}_{2}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8},$ give evidence that the pseudogap formation in the single-layer LSCO and bilayer YBCO type compounds is governed by an additional local mode in the latter.

Elementary excitations in valence bond magnets have a finite spin gap $\ensuremath{\Delta}=g{\ensuremath{\mu}}_{B}{H}_{c}$ to well-defined triplet states. The degeneracy of the triplet states is lifted in the presence of an external field, according to the Zeeman interaction term. The energy and intensity of the excitation spectra in $S=1/2$ valence bond ${\mathrm{KCuCl}}_{3}$ and ${\mathrm{TlCuCl}}_{3}$ are investigated by inelastic neutron scattering at finite external fields. Experimental observations along representative directions of reciprocal space are addressed up to ${H/H}_{c}\ensuremath{\sim}0.6$ and ${H/H}_{c}\ensuremath{\sim}0.9,$ respectively. A comprehensive analysis of the split Zeeman modes is reported, which shows excellent agreement with first principles. The obtained results extend former characterizations at $H=0$ and are of importance in the context of the field-driven quantum criticality realized in the sister compounds ${\mathrm{KCuCl}}_{3}$ and ${\mathrm{TlCuCl}}_{3}.$

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.

The magnetic exchange interactions in the mixed-valence dodecanuclear polyoxovanadate cluster compound (NHEt3)(4)[V12As8O40(H2O)] x H2O were investigated by a detailed inelastic neutron scattering study using cold neutrons. The data show clear evidence for the presence of a magnetic anisotropy within the cluster. Exchange parameters are accurately determined.

Neutron spectroscopy has been employed to observe the crystal field in the high- T c superconductor HoBa 2 Cu 3 O x (6 x

Charge density-waves (CDW) have been the subject of intensive research for about twenty years. Most often they are found in synthetic compounds issuing from the assiduous efforts of chemists. These fascinating materials, inorganic and organic quasi-one-dimensional and quasi-two-dimensional conductors, have metallic properties that are unusual in many ways [1–4]. The key feature is the unstable metallic state, apt to spontaneously form a charge-density modulation ρ CDW (r) and an associated lattice distortion. The modulation period is determined by the conduction electron density, i.e. related to the Fermi wave vector k F, $$\rho_{CDW}=\rho_osin(qr+\phi )=\rho_osin(2k_Fr+\phi )$$ in 1D presentation. As a consequence of this instability there is a rich variety of low temperature phase transitions driven by the temperature dependence of the CDW amplitude ρο and the interaction of its phase φ with the underlying lattice. One can find metal-insulator transitions, incommensurate and commensurate modulated structures for example. A central and most intriguing ingredient is the collective sliding mode conductivity [5–9] where the CDW condensate moves as a whole under an applied field. In practice this sliding is hindered by defects, since the phase φ of the modulation will have preferred values at defect sites, opposing the ideally free choice of position of the modulated charge density. This strong connection to defects influences the whole physics of charge density-waves with consequences that are manifest in a wide space and time scale from microscopic to macroscopic. Nevertheless the initial effects are found at the spatial scale of the defects and of the CDW wavelength. This is the reason why microstructural characterization of the CDW is of primary importance for understanding the CDW physics and of course not only with regard to the sliding CDW but also in the full scope of CDW related features.

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 copper salt S =1/2 TlCuCl 3 is a three-dimensional quantum spin system with a singlet ground-state of dimer origin. A spin energy gap Δ ≈0.7 meV separates the ground-state from the triplet waves, which propagate along all directions in reciprocal space. Across the quantum critical point at H c = Δ / gμ B Bose–Einstein condensation of the triplet quasi-particles has been observed. The inelastic neutron scattering spectra at H > H c proof the coexistence of a gapless Goldstone mode, which has a linear dispersion, with renormalized quadratic Zeeman modes.

A Si-analyzer neutron backscattering spectrometer, DNA , in MLF is the first indirect geometry instrument which chooses the coupled moderator as a pulsed neutron source in addition with a high speed disc chopper as a pulse-shaping device, aiming for high energy resolution with high intensity. The pulse-shaping has the advantage of obtaining a variety of slit opening times. On the other hand, it has the disadvantage of limiting the wavelength band width measured within one pulse-shaped neutron beam. This disadvantage is solved by employing 4 slit discs on the pulse-shaping chopper and generating multiple pulse-shaped incident beams to realize efficient measuring, the so called Repetition Rate Multiplication: RRM technique. The automatic phasing procedure for the RRM technique is firstly discussed.

An investigation into the physical consequences of including a Jahn-Teller distorted Cu(II) ion within an antiferromagnetically coupled ring, [R(2)NH(2)][Cr(7)CuF(8)((O(2)C(t)Bu)(16))] is reported. Inelastic neutron scattering (INS) and electron paramagnetic resonance (EPR) spectroscopic data are simulated using a microscopic spin Hamiltonian, and show that the two Cr-Cu exchange interactions must be inequivalent. One Cr-Cu exchange is found to be antiferromagnetic and the other ferromagnetic. The geometry of the Jahn-Teller elongation is deduced from these results, and shows that a Jahn-Teller elongation axis must lie in the plane of the Cr(7)Cu wheel; the elongation is not observed by X-ray crystallography, due to positional disorder of the Cu site within the wheel. An electronic structure calculation confirms the structural distortion of the Cu site.

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.