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Magnetic-field-induced phase transitions are investigated in the frustrated gapped quantum paramagnet Rb2Cu2Mo3O12 through dielectric and calorimetric measurements on single-crystal samples. It is clarified that the previously reported dielectric anomaly at 8 K in powder samples is not due to a chiral spin liquid state as has been suggested, but rather to a tiny amount of a ferroelectric impurity phase. Two field-induced quantum phase transitions between paraelectric and paramagnetic and ferroelectric and magnetically ordered states are clearly observed. It is shown that the electric polarization is a secondary order parameter at the lower-field (gap closure) quantum critical point but a primary one at the saturation transition. Having clearly identified the magnetic Bose-Einstein condensation (BEC) nature of the latter, we use the dielectric channel to directly measure the critical divergence of BEC susceptibility. The observed power-law behavior is in very good agreement with theoretical expectations for three-dimensional BEC. Finally, dielectric data reveal magnetic presaturation phases in this compound that may feature exotic order with unconventional broken symmetries., Physical Review Research, 3 (3), ISSN:2643-1564

Magnetization, magnetocaloric, calorimetric, neutron and X-ray diffraction and inelastic neutron scattering measurements are performed on single crystals of BaCdVO(PO$_4$)$_2$. The low-temperature crystal structure is found to be of a lower symmetry than previously assumed. The result is a more complicated model spin Hamiltonian, which we infer from measurements of the spin wave dispersion spectrum. The main finding is a novel spin state which emerges in high magnetic fields after antiferromagnetic order is terminated at $H_{c1}\simeq 4.0$ T. It is a distinct thermodynamic phase with a well-defined phase boundary at $H_{c2}\simeq 6.5$ T and is clearly separate from the fully saturated phase. Yet, it shows no conventional (dipolar) magnetic long range order. We argue that it is fully consistent with the expectations for a quantum bond-nematic state.

We have used inelastic neutron scattering and muon-spin rotation to compare the low energy magnetic excitations in single crystals of superconducting Fe1.01Se0.50Te0.50 and non-superconducting Fe1.10Se0.25Te0.75. We confirm the existence of a spin resonance in the superconducting phase of Fe1.01Se0.50Te0.50, at an energy of 7 meV and a wavevector of (1/2,1/2,0). The non-superconducting sample exhibits two incommensurate magnetic excitations at (1/2,1/2,0)\pm(0.18,-0.18,0) which rise steeply in energy, but no resonance is observed at low energies. A strongly dispersive low-energy magnetic excitation is also observed in Fe1.10Se0.25Te0.75 close to the commensurate antiferromagnetic ordering wavevector (1/2-\delta,0,1/2) where \delta \approx 0.03. The magnetic correlations in both samples are found to be quasi-two dimensional in character and persist well above the magnetic (Fe1.10Se0.25Te0.75) and superconducting (Fe1.01Se0.50Te0.50) transition temperatures., Comment: 10 pages, 4 figures

Relaxor ferroelectrics were discovered in the 1950s but many of their properties are not understood. In this review, we shall concentrate on materials such as PMN (PbMg1/3Nb2/3O3), which crystallize in the cubic perovskite structure but with the Mg ion, charge 2+, and the Nb ion, charge 5+, randomly distributed over the B site of the perovskite structure. The peak of the dielectric susceptibility for relaxors is much broader in temperature than that of conventional ferroelectrics, while below the maximum of the susceptibility most relaxors remain cubic and show no electric polarization, unlike that observed for conventional ferroelectrics. Because of the large width of the susceptibility, relaxors are often used as capacitors. Recently, there have been many X-ray and neutron scattering studies of relaxors and the results have enabled a more detailed picture to be obtained. An important conclusion is that relaxors can exist in a random field state, as initially proposed by Westphal, Kleemann and Glinchuk, ...

The paper presents results of studies of the dielectric response and structure of the PbCo1/3Nb2/3O3 (PCN) crystal. Analysis of the dielectric response at frequencies ranging from 10 Hz to 50 kHz has revealed a considerable dispersion in a wide temperature interval. Anomalies in the temperature dependences of ϵ′ and ϵ″ in the vicinity of 350, 250, and 130 К have been observed. It is shown that the anomaly near 250 K can be attributed to the transition into a relaxor ferroelectric state, whereas the anomaly at 130 K occurs nearby to the antiferromagnetic phase transition. The behaviour of the dielectric response in the vicinity of 350 К requires further studies. The PCN crystal structure at room temperature has been refined.

The ‘Paul Scherrer Institut’ (PSI, http://www.psi.ch) operates three major user laboratories for condensed-matter research on one campus: a third generation X-ray synchrotron source (SLS), the world's most powerful continuous-beam μSR facility (SμS) and the only continuous spallation neutron-source worldwide (SINQ).

The results of the study of Mandelstam-Brillouin light scattering in single-crystal Na1/2Bi1/2TiO3 within a broad temperature interval covering successively the cubic, tetragonal, and rhombohedral phases are reported. Measurements conducted within the range 820–300 K have revealed quasi-elastic scattering and distortion of the phonon spectra in shape, which suggests possible coupling between the low-frequency transverse optical and acoustic modes. These two observed phenomena are believed to be closely interrelated.

The influence of isotope exchange of hydrogen with deuterium on the lysozyme dynamics was studied by incoherent inelastic neutron scattering. The generalized vibrational densities of states G(ω) were constructed from experimental results for protonated and deuterated protein samples at 200, 280, and 311 K. The major isotope effect was observed in G(ω) in the frequency region higher than 100 cm−1. At all temperatures, both the Debye mode and the region of G(ω), whose spectral dimension corresponds to the fracton mode, are observed in the low-frequency region of the densities of states of both protonated and deuterated lysozyme. The influence of the hydrogen isotope exchange on the low-frequency region of G(ω) is insignificant.

The dynamical properties of the nearly stoichiometric LiTaO 3 crystals are studied by neutron scattering and light scattering. The neutron scattering shows the high value of the Curie temperature Tc which is the typical feature of near stoichiometry. In the broadband Brillouin scattering the intense quasielastic scattering is observed in the vicinity of Tc and the critical slowing down is clearly observed in the vicinity of Tc. As to the inelastic neutron scattering in the vicinity of Tc, we found that it behaves as expected for the critical scattering at a phase transition. In addition, we observed scattering with complex shape in the wavevector-energy space which is present in a very broad temperature and energy transfers range.

The Tb0.1Tm0.9Co2 compound is investigated using neutron diffraction. It is shown that this compound undergoes an irreversible band metamagnetic transition induced by an external magnetic field. The magnetization of the Co sublattice increases from 0.2 to 0.6 μB. The critical field strength is approximately equal to 1 T at temperatures of 1.8 and 4.0 K. As the temperature increases, the effect of the magnetic field on the magnetic state of the sample weakens and, at 25 K, no noticeable changes are observed in an external field of 0.75 T. The metamagnetic transition at 1.8 K is accompanied by the disappearance of rhombohedral distortions and brings about a lattice expansion by approximately 1%.

Inelastic incoherent neutron scattering in hen egg white lysozyme, alpha-chymotrypsin from bovine pancreas, and human serum albumin has been investigated. The effect of the isotopic hydrogen-deuterium exchange on the vibrational spectrum of lysozyme has been studied by analyzing the experimentally obtained density of states (DOS). Phonon and fracton regimes have been identified in the low-frequency region of the DOS of deuterated lysozyme. It has been shown that deuteration of lysozyme does not lead to significant changes in the phonon and fracton dimensions. At the same time, the most pronounced differences in G(ω) of deuterated and protonated lysozyme were identified at frequencies above 100 cm − 1 . The DOS of alpha-chymotrypsin from bovine pancreas and human serum albumin at 285 K have been determined.

The behavior of acoustic phonons in crystals of a relaxor ferroelectric, namely, the lead scandium tantalate PbSc1/2Ta1/2O3 (PST), is studied in the vicinity of the diffuse phase transition. The behavior of longitudinal and transverse acoustic phonons in a PST single crystal is examined using Brillouin scattering. The phonon subsystem is found to behave anomalously in the vicinity of T = 297 K, which can probably be assigned to the existence of a phase transition. Analysis of the results obtained yields the values of the elastic moduli C11, C12, and C44 for the cubic phase of the crystal over a wide temperature range.

We have studied the ferromagnetic and antiferromagnetic fluctuations in the charge-ordered Pr0.65Ca0.35MnO3 antiferromagnet by triple-axis neutron spectrometry. Whereas ferromagnetic fluctuations are observed above and below the charge-ordering transition (TCO), the antiferromagnetic fluctuations develop only below TCO .T he dynamical exponent z of both ferromagnetic and antiferromagnetic fluctuations are determined. The ferromagnetic fluctuations are not completely suppressed below TCO and their correlation lengths are short-ranged at all temperatures. The results are discussed with respect to the Zener polaron model recently introduced to describe the charge-ordered state of Pr0.6Ca0.40MnO3.

YMnO3 exhibits a ferroelectric transition at high temperature (~ 900 K) and magnetic ordering at T_N ~ 70 K where the dielectric constant shows an anomaly indicative of the magneto-dielectric effect. Here we report powder neutron diffraction experiments in this compound that show that the magnetic moment at saturation is reduced by application of hydrostatic pressure. Our results yield further insight about the nature of the spin-lattice interaction in ferroic materials.

The low-energy part of the vibration spectrum of KNbO3 was studied by cold neutron inelastic scattering in the cubic phase. In addition to acoustic phonons, we observe strong diffuse scattering, which consists of two components. The first one is quasistatic and has a temperature-independent intensity. The second component appears as quasielastic scattering in the neutron spectrum, indicating a dynamic origin. From analysis of the inelastic data, we conclude that the quasielastic component and the acoustic phonon are mutually coupled. The susceptibility associated with the quasielastic component grows as the temperature approaches T C .

Specific heat of PbMg1/3Nb2/3O3, PbMg1/3Ta2/3O3, and BaMg1/3Ta2/3O3 complex perovskites has been measured from 2 K up to 295 K. At low temperatures a drastic difference of the behaviour of the specific heat of BaMg1/3 Ta2/3O3 with respect to other two compounds was found. Different approaches to describe the specific heat at the low temperatures of the PbMg1/3Nb2/3O3 relaxor ferroelectric are discussed.

The lattice dynamics of PbMg1/3Nb2/3O3 and PbMg1/3Ta2/3O3 relaxor ferroelectrics is investigated by inelastic neutron scattering. The relaxation mode in these crystals is revealed from an analysis of the low-frequency part of the vibrational spectrum. The possible reasons for the appearance of this mode are discussed, and different approaches to the description of the low-frequency spectrum of ferroelectrics are compared.

We studied the effect of temperature on density of vibrational states of model complex perovskites PbMg1/3Nb2/3O3 (PMN), PbMg1/3Ta2/3O3 (PMT), and BaMg1/3Ta2/3O3 (BMT) by inelastic neutron scattering. It has been found that PMN and PMT relaxor ferroelectrics exhibit an anomalous temperature evolution of the density of vibrational states G(E), while G(E) in the BMT compound does not demonstrate any temperature evolution. It is shown that there is a correlation between the temperature behaviors of the dielectric response and vibrational spectra of all the complex perovskites studied. Possible reasons for variation in G(E) in these materials are discussed.

This paper presents studies of lattice dynamics of a crystal of Cs 5 H 3 (SO 4 ) 4 · x H 2 O (PCHS) and its deuterated analog Cs 5 D 3 (SO 4 ) 4 · x D 2 O (DPCHS) in wide temperature and frequency ranges by dielectric spectroscopy and modulated temperature differential scanning calorimetry. It is shown that deuteration of PCHS gives rise to an additional structural phase transition at T c3 , 250 K. The temperature of the isostrctural phase transition T c2 (372 K for PCHS and 370 K for DPCHS) has been determined more accurately. It has been found that deuteration does not suppress phase transition at T c2 . The temperature dependence of dielectric response is analyzed.

We report a neutron scattering study of a ferroelectric phase transition in Sr0.61Ba0.39Nb2O6 (SBN-61). The ferroelectric polarization is along the crystallographic c-axis but the transverse acoustic branch propagating along the 〈1, 1, 0〉 direction does not show any anomaly associated with this transition. We find no evidence for a soft transverse optic phonon. We do, however, observe elastic diffuse scattering. The intensity of this scattering increases as the sample is cooled from a temperature well above the phase transition. The susceptibility associated with this diffuse scattering follows the anomaly of the dielectric permittivity of SBN-61 well. Below Tc the shape of this scattering is consistent with the scattering expected from ferroelectric domain walls. Our results suggest that despite apparent chemical disorder SBN-61 behaves as a classic order-disorder uniaxial ferroelectric with critical fluctuations in the range

The specific heat of the PbMg1/3Nb2/3O3 relaxor ferroelectric has been measured by the adiabatic calorimetry and the relaxation method in the temperature range 2–300 K. The low-temperature heat capacity data are compared with the experimental data on inelastic neutron scattering and discussed in the context of the present-day concepts. The analysis of various approaches to the description of the temperature dependence of the heat capacity showed that the experiment can be adequately described only within the framework of the fractal concept. The calculated value of the spectral dimension \(\tilde d = 1.6\) is consistent with the inelastic neutron-scattering data.

Measurements of specific heat of relaxor ferroelectric PbMg1/3Nb2/3O3 (PMN) in the temperature range from 2 to 300 K by adiabatic calorimetry and relaxation technique are reported. Comparison of different approaches to description of the temperature dependence of specific heat has shown that experimental data can be adequately described in terms of fractal formalism. Spectral dimension inferred from low temperature specific heat measurements was found to be in good agreement with spectral dimension estimated from inelastic neutron scattering data.

We describe damping of hypersonic and ultrasonic longitudinal acoustic (LA) phonons in crystals of Cs 5H (3)(SO (4))(4);xH 2O (PCHS) between 100 and 360 K. The damping of LA phonons exhibits strong dispersion caused by relaxation processes in the region of transformation into the glasslike phase (T(g) approximately 260 K). Near T(g) the damping of ultrasonic phonons propagating in the basal plane reflects the cooperative freezing of acid protons. The damping of LA phonons propagating perpendicular to the basal plane can be fit by the Debye model and is due to the interaction between protons and LA phonons. This suggests that the proton glass state that is realized at T

Russian Foundation for Basic Research Grant (99-02-18316); NSF Grant (DMR-9805272); ISSEP Grant

The sequence of phase transitions at temperatures from 290 to 420 K in the crystals of Cs5H3(SO4)4·0.5H2O (CTSM) and Cs5D3(SO4)4·0.5D2O (DCTSM) was studied by Brillouin light scattering and neutron diffraction. Anomalies in the temperature dependences of the velocity and damping of hypersonic acoustic phonons in the vicinity of 360 K were observed in the CTSM crystal. In contrast, no anomalies were found in the DCTSM crystal. Preliminary analysis of neutron diffraction data does not show symmetry changes in the temperature range studied. Thus it has been found that deuteration of the CTSM crystals leads to a change in the sequence of phase transitions.

This paper describes the results of Raman and inelastic neutron-scattering studies for the PbMg1/3Nb2/3O3 (PMN) relaxor ferroelectric. The comparative analysis of the low-frequency region of the vibrational spectrum has revealed the excitation at the frequency of the order of 50 cm−1 which we regard as a fracton. In the generalized density of vibrational states this excitation corresponds to the portion of the curve with the exponent d =1.5 at T=77 K. Thus, we have observed a fracton in dielectric crystals for the first time and therefore we believe that fractal ideology can be used to describe the physical properties of the PMN relaxor ferroelectric.

The recent studies of the PrNi system have shown that, at a temperature slightly higher than the ferromagnetic ordering temperature (TC ∼ 21 K), softening of some part of magnetic excitations occurs near the Γ point of the Brillouin zone. The magnetic modes observed have been qualitatively described by a model taking into account the crystal field and the exchang interaction within the mean-field random-phase approximation. To refine the model parameters and obtain a complete set of excitation modes, inelastic neutron scattering measurements on PrNi single crystals and polycrystals have been performed at T = 23 K. An acoustic branch related to the level with high excitation energy has been observed for the first time. The character of its dispersion suggests that the low-energy acoustic mode is responsible for the magnetic phase transition.

Raman light scattering and neutron scattering in the range of 20–295 K were studied in the relaxor ferroelectric of PbMg1/3Ta2/3O3. Anomalous temperature dependences of intensity of the Raman lines and of the neutron diffuse scattering were observed. Relation of these anomalies with the diffuse phase transition in PbMg1/3Ta2/3O3 is discussed.

The structure of BaMg(1/3)Ta(2/3)O(3) (BMT) has been studied using x-ray scattering. The phonons have been measured and the results are similar to those of other materials with a perovskite structure such as PbMg(1/3)Nb(2/3)O(3) (PMN). The acoustic and lowest energy optic branches were measured but it was not possible to measure the branches of higher energy, possibly this is because they largely consist of oxygen motions. High-resolution inelastic measurements also showed that the diffuse scattering was strictly elastic and not directly related to the phonon spectra. Diffuse scattering was observed in BMT near the (H ± 1/2, K ± 1/2, L ± 1/2) points in the Brillouin zone and these had a characteristic cube shape. This arises from ordering of the B-site ions in BMT. Additional experiments revealed the diffuse scattering in BMT similar in shape to Bragg reflections at wavevectors of the form (H ± 1/3, K ± 1/3, L ± 1/3). Such reflections were also observed by Lufaso (2004 Chem. Mater. 16 2148) from powders and suggest that this structure of BMT consists of four differently oriented domains of a trigonal structure and results from a different ordering of the B-site ions from that responsible for the scattering at the (H ± 1/2, K ± 1/2, L ± 1/2) points. The results lead us to suggest that for BMT single crystals the bulk has the properties of a cubic perovskite, whereas the surface may have quite different structure from that of the bulk. This difference resembles the behaviour of cubic relaxors like PMN and PMN doped by PbTiO(3), where significant surface effects have been reported.

The superconducting and magnetic properties of ${\text{Fe}}_{y}{\text{Se}}_{0.25}{\text{Te}}_{0.75}$ single crystals $(0.9\ensuremath{\le}y\ensuremath{\le}1.1)$ were studied by means of x-ray diffraction, superconducting quantum interference device magnetometry, muon-spin rotation, and elastic neutron diffraction. The samples with $yl1$ exhibit coexistence of bulk superconductivity and incommensurate magnetism. The magnetic order remains incommensurate for $y\ensuremath{\ge}1$ but with increasing Fe content superconductivity is suppressed and the magnetic correlation length increases. The results show that the superconducting and the magnetic properties of the ${\text{Fe}}_{y}{\text{Se}}_{1\ensuremath{-}x}{\text{Te}}_{x}$ can be tuned not only by varying the Se/Te ratio but also by changing the Fe content.

We report the discovery of a spin-glass ground state in the transition-metal monosilicides with the B20 crystallographic structure. Magnetic, transport, neutron, and muon investigations of the solid solution ${\text{Mn}}_{1\ensuremath{-}x}{\text{Co}}_{x}\text{Si}$ have revealed a new dome in the phase diagram with evidence of antiferromagnetic interactions. For Mn-rich compounds, a sharp decrease in the Curie temperature is observed upon Co doping and neutron elastic scattering shows that helimagnetic order of MnSi persists up to $x=0.05$ with a shortening of the helix period. For higher Co $(0.05lxl0.90)$ concentrations, the Curie-Weiss temperature changes sign and the system enters a spin-glass state upon cooling (${T}_{g}=9\text{ }\text{K}$ for ${x}_{\text{Co}}=0.50$), due to chemical disorder. In this doping range, a minimum appears in the resistivity, attributed to scattering of conduction electron by localized magnetic moments.

The effect of the ordering of Sc3+ and Ta5+ ions on the behavior of the phonon subsystem in the relaxor ferroelectric PbSc1/2Ta1/2O3 from the numerous AB′ x B″ x−1O3 family has been studied. The nature of an additional contribution to the light-scattering spectra has been analyzed on the experimental data on Raman scattering in PbSc1/2Ta1/2O3, SrAl1/2Ta1/2O3, SrAl1/2Nb1/2O3, and BaMg1/3Ta2/3O3 compounds and inelastic neutron scattering in PbSc1/2Ta1/2O3. It is shown that broadening of the lines in the Raman spectra of PbSc1/2Ta1/2O3 crystals is caused by additional scattering by phonons from various points of the Brillouin zone.

We have studied the superconducting and magnetic properties of ${\text{Fe}}_{1+y}{\text{Se}}_{x}{\text{Te}}_{1\ensuremath{-}x}$ single crystals $(0\ensuremath{\le}x\ensuremath{\le}0.5)$ by magnetic susceptibility, muon-spin rotation, and neutron diffraction. We find three regimes of behavior: (i) commensurate magnetic order for $x\ensuremath{\lesssim}0.1$, (ii) bulk superconductivity for $x\ensuremath{\sim}0.5$, and (iii) a range $x\ensuremath{\approx}0.25--0.45$ in which superconductivity coexists with incommensurate magnetic order. The results are qualitatively consistent with two-band mean-field models in which itinerant magnetism and extended $s$-wave superconductivity are competing order parameters.

We report an x-ray diffraction study on the charge-density-wave CDW LaTe3 and CeTe3 compounds as a function of pressure. We extract the lattice constants and the CDW modulation wave vector. We observe that the intensity of the CDW satellite peaks tend to zero with increasing pressure, thus providing direct evidence for a pressure-induced quenching of the CDW phase. Our findings further support the equivalence between chemical and applied pressures in RTe3, put forward by our previous optical investigations, but reveal some subtle differences. We offer a possible explanation for these differences.

The spatial structure and dynamics of hen egg white lysozyme have been investigated by small-angle and inelastic neutron scattering. Analysis of the results was carried using the fractal approach, which allowed determination of the fractal and fracton dimensions of lysozyme, i.e., consideration of the protein structure and dynamics by using a unified approach. Small-angle neutron scattering studies of thermal denaturation of lysozyme have revealed changes in the fractal dimension in the vicinity of the thermal denaturation temperature that reflect changes in the spatial organization of protein.

Relaxor ferroelectrics are difficult to study and understand. The experiment shows that at low energy scattering there is an acoustic mode, an optic mode, dynamic quasi-elastic scattering and strictly elastic scattering as well as Bragg peaks at the zone centre. We have studied the scattering using the TASP spectrometer at PSI and have analysed the data using a model with interactions between the different components particularly to determine the properties of the elastic scattering. The quasi-elastic scattering begins to become significant at the Burns temperature of 620 K. It steadily increases in intensity on cooling reaching a maximum at ~400 K. Below this temperature the strictly elastic scattering begins to increase and shows a broadened line shape characteristic of crystals in a random applied field. We show that all the results obtained from PMN for the elastic scattering are consistent with the crystal having a random field transition at ~400 K. We have obtained similar results for PMN-PT and PZN-PT suggesting that random fields of the nano-regions also play an important role in these materials.

Results of the Brillouin light scattering studies of elastic properties of a relaxor ferroelectric lead magnesium niobate, $\mathrm{Pb}{\mathrm{Mg}}_{1∕3}{\mathrm{Nb}}_{2∕3}{\mathrm{O}}_{3}$, crystals in the temperature range from $870\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are presented. By using the experimental results, temperature dependences of elastic ${C}_{11}$, ${C}_{12}$, and ${C}_{44}$ constants have been built. Analysis of the temperature dependences of elastic constants has revealed anomalies in the vicinity of the Burns temperature $({T}_{B}\ensuremath{\approx}620\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and a suppressed ferroelectric phase transition $({T}_{c}\ensuremath{\approx}210\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ that occurs when an external electric field is applied. A comparative analysis of the behaviors of elastic constants at different probe frequencies derived from our Brillouin scattering results and ultrasonic [G. A. Smolenskii et al., Sov. Phys. Solid State 27, 801 (1985)] measurements has been carried out. The remarkable dispersion in the relative variations of elastic ${C}_{11}$ and ${C}_{44}$ constants with decreasing temperature below the transition temperature into a cubic anisotropic phase $({T}_{M}\ensuremath{\approx}370\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ has been found. These anomalous elastic behaviors related to three typical transition temperatures are discussed as the peculiar natures of relaxor ferroelectrics.

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

The non-centrosymmetric magnetic structure of ErMn$_2$O$_5$ has been shown to be very similar to that of HoMn$_2$O$_5$ (Vecchini {\it et al.}, 2008 Phys. Rev. B {\bf 77} 134434). The magnetic modulation at 25 K has propagation vector $\vec k$=(1/2,0,1/4) and the symmetry imposes very few constraints on the magnetic configurations allowed. Only by combining the results of bulk magnetisation measurements, powder and single crystal neutron diffraction and spherical neutron polarization analysis was it possible to distinguish clearly between different models. The susceptibility measurements show that the erbium magnetic moments are aligned parallel to the c-axis indicating strong single-ion anisotropy. Spherical neutron polarimetry demonstrates the presence of two unequally populated chirality domains in ErMn$_2$O$_5$ single crystals. X-ray diffraction measurements on an ErMn$_2$O$_5$ powder using synchrotron radiation show that the buckling angles of the Mn-O-Mn bond change below the transition to the ferroelectric phase.

The compound BaCuSi2O6 is a quantum magnet with antiferromagnetic dimers of S=1/2 moments on a quasi-2D square lattice. We have investigated its spin dynamics by inelastic neutron scattering experiments on single crystals with an energy resolution considerably higher than in an earlier study. We observe multiple magnon modes, indicating clearly the presence of magnetically inequivalent dimer sites. The more complex spin Hamiltonian revealed in our study leads to a distinct form of magnon Bose-Einstein condensate phase with a spatially modulated condensate amplitude.

We report a neutron scattering study of the ferroelectric phase transition in Sr$_{0.585}$Ce$_{0.025}$Ba$_{0.39}$Nb$_2$O$_6$ (SBN-61:Ce). We find no evidence for a soft transverse optic phonon. We do, however, observe anisotropic diffuse scattering. This scattering has inelastic and elastic contributions. In the paraelectric phase the susceptibility associated with the elastic diffuse scattering follows well the anomaly of the dielectric susceptibility of SBN-61:Ce. In the ferroelectric phase the lineshape of the elastic scattering is consistent with the form expected for the ferroelectric domain walls. In contrast to the macroscopic observations, the scattering properties of Ce-doped crystal do not exhibit important changes with respect to those of pure Sr$_{0.61}$Ba$_{0.39}$Nb$_2$O$_6$.

The compound BaCuSi2O6 is a quantum magnet with antiferromagnetic dimers of S=1/2 moments on a quasi-2D square lattice. We have investigated its spin dynamics by inelastic neutron scattering experiments on single crystals with an energy resolution considerably higher than in an earlier study. We observe multiple magnon modes, indicating clearly the presence of magnetically inequivalent dimer sites. The more complex spin Hamiltonian revealed in our study leads to a distinct form of magnon Bose-Einstein condensate phase with a spatially modulated condensate amplitude.

The crystal structure of the PbMg{sub 1/3}Ta{sub 2/3}O{sub 3} relaxor ferroelectric was studied under hydrostatic pressure up to {approx}7 GPa by means of powder neutron diffraction. We find a drastic pressure-induced decrease of the lead displacement from the inversion center, which correlates with an increase by {approx}50% of the anisotropy of the oxygen temperature factor. The vibrations of the Mg/Ta are, in contrast, rather pressure insensitive. We attribute these changes being responsible for the previously reported pressure-induced suppression of the anomalous dielectric permittivity and diffuse scattering in relaxor ferroelectrics.

The substitution of Tb for Tm in the Laves phase compound $\mathrm{Tm}{\mathrm{Co}}_{2}$ leads to appearance of a magnetic moment on the Co atoms through the metamagnetic transition in the itinerant $d$-electron subsystem and gives rise to long-range ferrimagnetic order in ${\mathrm{Tm}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}{\mathrm{Co}}_{2}$ at $x\ensuremath{\geqslant}0.15$. The magnetic state of the compound ${\mathrm{Tm}}_{0.9}{\mathrm{Tb}}_{0.1}{\mathrm{Co}}_{2}$, i.e., just below the critical Tb concentration, is characterized by the presence of large regions with short-range magnetic order and localized spin fluctuations (LSFs) induced in the Co $3d$-electron subsystem by the fluctuating $f\text{\ensuremath{-}}d$ exchange due to the Tm-Tb substitution. The peculiar magnetic state of this compound is strongly influenced by an external magnetic field which produces a first-order magnetic phase transition to a long-range ferrimagnetic state with the magnetic moment on the Co atoms up to $(0.7--0.8){\ensuremath{\mu}}_{B}$. This field-induced transition in ${\mathrm{Tm}}_{0.9}{\mathrm{Tb}}_{0.1}{\mathrm{Co}}_{2}$ is found to be irreversible. It is accompanied by a giant and irreversible reduction of the electrical resistivity $(\ensuremath{\Delta}\ensuremath{\rho}∕\ensuremath{\rho}\ensuremath{\sim}\ensuremath{-}45%)$, specific heat (by about 3.7 times at $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$), and intensity of magnetic neutron scattering. Such behavior is associated with the field-induced metamagnetic transition in the itinerant $d$-electron subsystem mediated by the $f\text{\ensuremath{-}}d$ exchange. Significantly enhanced values of the residual resistivity and the coefficient $\ensuremath{\gamma}$ of the $T$-linear contribution to the specific heat in the compound with $x=0.1$ as well as their unusual behavior with temperature and under application of the magnetic field is ascribed to the presence of LSF.

The spectrum of spin fluctuations in the stacked-triangular antiferromagnet YMnO3 was studied above the Neel temperature using both unpolarized and polarized inelastic neutron scattering. We find an in-plane and an out-of-plane excitation. The in-plane mode has two components just above TN, a resolution-limited central peak and a Debye-like contribution. The quasi-elastic fluctuations have a line-width that increases with q like Dq^z and the dynamical exponent z=2.3. The out-of-plane fluctuations have a gap at the magnetic zone center and do not show any appreciable q-dependence at small wave-vectors., Comment: JETP LETTERS, in press

Raman scattering in relaxor ferroelectric PbMg{sub 1/3}Ta{sub 2/3}O{sub 3} (PMT) was investigated in the single-crystalline form in the temperature range of 20-295 K. An anomalous temperature dependence of the integrated intensity and the Raman line contours were found at the diffuse phase transition. A correlation between the anomalies in the integrated intensities and the dispersion of the dielectric response was observed. The distortions of Raman lines with decreasing temperature are discussed.

The low-energy part of vibration spectrum in ${\mathrm{PbMg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_{3}$ (PMN) relaxor ferroelectric was studied by inelastic neutron scattering. We observed the coexistence of a resolution-limited central peak with strong quasielastic scattering. The line width of the quasielastic component follows a ${\ensuremath{\Gamma}}_{0}{+Dq}^{2}$ dependence. We find that ${\ensuremath{\Gamma}}_{0}$ is temperature dependent. The relaxation time follows the Arrhenius law well. The presence of a relaxation mode associated with quasielastic scattering in PMN indicates that order-disorder behavior plays an important r\^ole in the dynamics of diffuse phase transitions.

We describe damping of hypersonic and ultrasonic longitudinal acoustic (LA) phonons in crystals of Cs 5H (3)(SO (4))(4);xH 2O (PCHS) between 100 and 360 K. The damping of LA phonons exhibits strong dispersion caused by relaxation processes in the region of transformation into the glasslike phase (T(g) approximately 260 K). Near T(g) the damping of ultrasonic phonons propagating in the basal plane reflects the cooperative freezing of acid protons. The damping of LA phonons propagating perpendicular to the basal plane can be fit by the Debye model and is due to the interaction between protons and LA phonons. This suggests that the proton glass state that is realized at TT(g) has a quasi-two-dimensional nature.

The recent study of PrNi shows a soft mode associated with the ferromagnetic transition ( T C ∼21 K). The observed magnetic modes are qualitatively described by the model of the crystal field with the exchange term within the framework of mean-field random phase approximation. To approve the model consideration and obtain a full set of excitation modes the inelastic neutron scattering measurements of PrNi single- and polycrystal has been performed at T =23 K. The “acoustic” mode related to level with “higher” energy in crystal field has been observed for the first time. The character of its dispersion allows to conclude that “acoustic” mode connected with “lower” energy level is involved in the magnetic ordering.