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This paper reports lattice dynamical measurements of Cu 1.8 Se superionic conductor in the superionic α-phase at ambient temperature. Measurements of phonon dispersion curves were performed with the new triple-axis spectrometer, TAIPAN, at the OPAL reactor. We found that TA [100], TA [111] and TA 1 [110] phonon branches demonstrate a decrease in frequency at wavevectors q > 0.5 rather than the flattening observed previously. Results are compared with calculated density functional theoretical calculations showing the presence of unstable soft mode related to ordering of Cu atoms observed in Cu 1.8 Se at room temperature followed by α–β phase transition at a lower temperature. Superstructure arising from the ordering causes effects similar to the folding of the Brillouin zone, although phonon intensities at new Brillouin zone centres are weak. The coupling of low-energy phonon modes with displacement of mobile ions can explain the strong damping of phonons at q > 0.5 observed in the experiment.

Recent experiments have indicated that the high-temperature properties of α-uranium may be strongly influenced by the formation of randomly distributed intrinsically localized vibrational modes, just a few atoms across in size. One observation was a loss of mechanical ductility that coincided with the formation of the intrinsically localized mode (ILM). Here, we consider this observation in more detail. In particular, we use the anisotropic thermal expansion behaviour to estimate the strains associated with each ILM and consider the implications for the forces between ILMs and the defects responsible for mechanical deformation. In the process we also suggest that an unusual transition from positive to negative thermal expansion along the [0 1 0] direction may be caused by the formation of ILMs.

Recent inelastic neutron scattering measurements on magnetite (Fe 3 O 4 ) below the metal–insulator (Verwey) transition reveal a large gap (7 meV) forming in the middle of the acoustic spin wave branch at q =(0,0,1/2) and E =43 meV. The wavevector (0,0,1/2) corresponds to the main superlattice reflection of the low symmetry monoclinic structure below T V and has been described as a charge-ordering wavevector. Detailed studies of Heisenberg models for the spin wave spectrum were performed assuming that the superexchange is modified to reflect crystallographic symmetry lowering due to either atomic distortions or charge ordering. None of the models studied introduced a significant gap in the acoustic spin wave branch. Another possible source is large spin–phonon coupling that results in the mixing of a phonon and spin wave near (0,0,1/2). We have evidence for the existence of such a phonon mode. Our results show a flat optical phonon branch that cuts through the acoustic spin wave precisely at the gap. The presence of strong spin-phonon coupling below the Verwey transition may further complicate our understanding of the Verwey problem.

Ca 2 Y 2 Cu 5 O 10 is a quasi-one-dimensional magnet, which consists of the ferromagnetic edge-sharing CuO 2 chains. It was previously reported from neutron inelastic scattering experiments in Ca 2 Y 2 Cu 5 O 10 up to ∼14 meV that in the magnetically ordered state there is an anomalous broadening of spin-wave excitations along the chain, which is caused mainly by the antiferromagnetic interchain interactions [M. Matsuda et al. : Phys. Rev. B 63 (2001) 180403(R)]. In this study we extended an energy range of the measurement up to ∼25 meV. The experimental result suggests that there exist two excitation modes, which is consistent with a theoretical result qualitatively. One mode corresponds to the relatively sharp spin-wave excitations, which broaden with increasing q chain and disappear around q chain ∼0.2 r.l.u. and ω∼10 meV. Another one corresponds to the very broad excitations apparent at q chain ∼0.2–0.3 r.l.u. and ω∼12–25 meV.

Phonons have been generally considered to be irrelevant to the high-temperature superconductivity in the cuprates. However, such a bias is usually based upon the assumption of conventional electron–phonon coupling, while in the cuprates the coupling can be rather unconventional because of strong electron correlation. We present the results of our recent measurements of phonon dispersion in YBa2Cu3O6+x by inelastic neutron scattering. These suggest certain phonon modes interact strongly with electrons and are closely involved in the superconductivity phenomenon with possible contribution to pairing.

We measured the phonon dispersions of YBa2Cu3O6.15 and YBa2Cu3O6.95 by time-of-flight inelastic neutron scattering. The in-plane bond-stretching modes in the metallic phase showed a distinct a-b plane anisotropy beyond what is expected for structural origin. Such anisotropy in the longitudinal optical modes, which is absent in the TO, suggests strong in-plane anisotropy in the underlying electronic structure. Apical oxygen bond-stretching modes showed a large frequency change between the insulating and the metallic phases. This large softening also is beyond structural origin, and suggests the effect of local electronic environment.

Flux lines repel each other, thus a hexagonal lattice which maximizes the distance between them is generally the lowest energy configuration. In the rare-earth nickel borocarbide superconductor YNi2B2C, square lattices of flux lines have been observed when the field is parallel to the c-axis. Using the c-axis penetration depth as a gauge, we find that the cross-section of an individual flux line has a four-fold symmetry, which makes square lattices energetically favored over the more prosaic hexagonal ordering. The dependence of the penetration depth within the basal plane is discussed.

In the high-{kappa} cubic superconductor V{sub 3}Si , phase transitions of the flux lattice structure occur as a function of applied field and temperature. With the field parallel to the fourfold [001] axis, the flux lattice transforms from triangular to square symmetry at approximately 1thinspthinspT. With the field parallel to the twofold [110] axis, the lattice, which is a nearly perfect hexagonal array at the lowest fields, distorts as the field is increased; the rate of increase in this distortion changes abruptly at 1.3thinspthinspT. As T{sub c} is approached, the system tends towards a more isotropic hexagonal array of the flux lines. These transitions are largely but not completely in agreement with a recent theory of the effects on the flux line arrangements of nonlocal electrodynamics in the London limit. {copyright} {ital 1999} {ital The American Physical Society }

In gadolinium gallium garnet (GGG) there is no long-range magnetic order down to 25 mK even though the Curie—Weiss temperature is !2 K. Instead, over a wide temperature range GGG supports short-range magnetic order (spin liquid), while long-range order can be induced by an applied magnetic field. It is believed that the geometrical frustration of the magnetic interactions is responsible for this behaviour: in GGG the magnetic Gd ions are located on a triangular lattice. We have investigated the magnetic properties of GGG by neutron di⁄raction measurements. The results show clearly the development of short-range magnetic order without an applied field and the appearance of long range order in an applied magnetic field. A comparison of the experimental data with classical Monte Carlo simulations results provides us with firm conclusions about the relative strength of the magnetic interactions in GGG and their importance in the formation of a magnetic ground state. ( 1999 Elsevier Science B.V. All rights reserved.

Inelastic neutron scattering measurements of La1.85Sr0.15CuO4 show that a discontinuity in dispersion develops in the middle of the highest energy LO phonon branch at low temperatures. The result suggests dynamic short range unit cell doubling in the CuO2 plane along the direction of the Cu-O bond, such as charge ordering on every other row of oxygen. Such charge ordering is related to, but is different from, the stripe charge ordering observed for nonsuperconducting cuprates. The question of the influence of the electron-lattice interaction in the normal and superconducting electronic properties of high temperature superconductors remains open. Perhaps the most intriguing experimental evidence of a large and anomalous electron-lattice interaction comes from inelastic neutron scattering experiments on various high temperature superconducting materials [1‐ 6]. A signature of these materials is the strong renormalization of longitudinal high frequency (,80 meV) oxygen bond-stretching phonons in the CuO2 plane when holes are doped into the insulating compounds. The doping causes a large decrease in the frequency of the zone boundary s0.5, 0, 0d half-breathing phonon mode (15%‐ 20% softening compared to the insulating compound), indicating that there is a strong coupling of these phonons to the doped holes. This effect seems to be ubiquitous, and has been observed in single-crystal inelastic neutron scattering measurements of La22xSrxCuO4 [1], La2CuO41y [3], YBa2Cu3O61y [4,5], and even Ba12xKxBiO3 [6]. There are also indications of this effect from the observed softening of oxygen phonon bands in inelastic neutron scattering measurements of the phonon density of states in Pb2Sr2sCa,YdCu3O8 [7] and Li11xTi22xO4 [8]. It has proven quite difficult to interpret these results in terms of normal electron-phonon coupling in a metal, especially since the other measured phonon modes are, for the most part, unaffected (especially the oxygen breathing and quadrupole modes). Typical indications of electronphonon coupling, such as Kohn anomalies, seem to be inconsistent with the wave vector of the affected phonon. The Fermi surface should have strong nesting at 2kf which is close to q › s0.5, 0.5d, but not at s0.5, 0d, as demonstrated by the local-density approximation calculation [9] and ARPES measurements [10]. We have chosen to investigate these phonon phenomena more closely and have discovered that the anomalous nature of the bond-stretching branch is even more mysterious at low temperatures. The sample studied consisted of two single crystals of La1.85Sr0.15CuO4 which were comounted in an aluminum can filled with He exchange gas. Both samples were grown by the floating zone method and were obtained from the same batch. The total size of the sample is approximately 3.8 3 1.5 3 0.8 cm 3 . Previ

Magnetic fields penetrate a type-II superconductor via quantized flux lines. Why these flux lines sometimes form square arrays, as in YNi{sub 2}B{sub 2}C with the field parallel to the {ital c} axis, rather than the expected hexagonal ordering, has long fascinated physicists and has eluded a simple explanation. Our latest measurements on YNi{sub 2}B{sub 2}C prove conclusively that the London penetration depth within the {ital a-b} plane is not isotropic. This anisotropy of the London depths implies that the cross section of an individual flux line has square symmetry, which makes a square lattice energetically favored over the more prosaic hexagonal ordering. {copyright} {ital 1998} {ital The American Physical Society}

A neutron small-angle scattering study of the flux-line lattice in heavily twinned YBa2Cu3O72x is presented. It is found that the diffraction signal disappears at temperatures well below Tc , associated with a melting of the flux lattice. The shape of the melting line is consistent with both a Lindemann criterion and the scaling expected for a vortex-glass transition with the superconducting parameters from the three-dimensional XY model. The influence of twin planes on the structure of the vortex lattice and its melting is studied by applying the field at different angles to the c axis. The results are compared with recent specific heat measurements on similar crystals. @S0163-1829~98!03022-7#

High resolution, neutron small angle scattering studies have been performed to investigate the low field $(Bl250\mathrm{mT})$ region of the phase diagram for the vortex lattice in the superconducting state of ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$. The data present clear evidence for a vortex lattice reorientation transition from a state with the diagonal of the rhombic unit cell along a $[110]$ direction to a $[100]$ direction. Above this transition the lattice distorts under the influence of the applied field until the apex angle becomes constant at some higher field. For $\mathbf{B}\ensuremath{\parallel}\mathbf{c}$ a square lattice configuration is formed. These experiments confirm qualitatively many of the predictions of a general model based on nonlocal corrections to the London model as applied to this and similar materials.

We observe a flux lattice with square symmetry in the superconductor YNi{sub 2}B{sub 2}C when the applied field is parallel to the {ital c} axis of the crystal. A square lattice observed previously by Yaron {ital et al.} in the isostructural magnetic analog ErNi{sub 2}B{sub 2}C was attributed to the interaction between magnetism in that system and the flux lattice. Since the Y-based compound does not order magnetically, it is clear that the structure of the flux lattice is unrelated to magnetic order. We do not observe any change in the mosaic or direction of the flux lines with applied field; these phenomena may be related to magnetism. In YNi{sub 2}B{sub 2}C, the strength of the flux lattice signal was seen to fall off rapidly with applied field; the results imply considerable disorder in the arrangement of the flux lines at 2.5T, but are not consistent with melting. We see a markedly anisotropic flux lattice when the applied field is at 60{degree} to the {ital c} axis. In this case, the flux lattice is observed to be a distorted hexagon. {copyright} {ital 1997} {ital The American Physical Society}

Polarized and unpolarized triple-axis neutron measurements were performed on an underdoped crystal of YBa2Cu3O7-x (x = 0.4 +/- 0.02, T-c = 62.7 K). Our results indicate, contrary to earlier evidence, that the spin excitations in the superconducting state are essentially the same as those in the fully doped material except that the unusual 41 meV resonance is observed at 34.8 meV. The normal state spin excitations are characterized by a weakly energy-dependent continuum whose temperature dependence shows the clear signature of a superconducting gap at T-c. The enhancement at the resonance is accompanied by a suppression of magnetic fluctuations at both higher and lower energies.

Neutron diffraction measurements have been carried out to investigate the nature of magnetic ordering in Ho1−xYxNi2B2C (x = 0, 0.1 and 0.2) compounds. HoNi2B2C shows a complex type of magnetic ordering below the superconducting transition, with a commensurate antiferromagnetic ordering of the Ho moments below 5 K and a modulation of the magnetic ordering along a- and/or c-direction between 5 K < T < 8.5 K. The substitution of Y for Ho alters the commensurate nature of the Ho ordering. In Ho0.9Y0.1Ni2B2C, satellite peaks corresponding to c-modulation appear below TN and exist down to 2 K, together with the principal Bragg peaks. However, when the Y concentration is increased to 20%, the principal Bragg peaks disappear and only the c-modulated satellite magnetic peaks are observed below TN.

Neutron diffraction measurements have been performed to study the nature of magnetic ordering in the compounds HoNiBC and ErNiBC. HoNiBC exhibits antiferromagnetic ordering of the Ho moments with a simple commensurate magnetic structure below TN D 10 K. The magnetic unit cell, doubled along the c-direction with respect to the chemical unit cell, consists of ferromagnetic planes antiferromagnetically coupled along the c-axis. On the other hand, the Er moments in ErNiBC show ferromagnetic ordering with a Curie temperature of 4.5 K. In contrast to the case for the structurally related superconducting counterparts (HoNi 2B2C and ErNi2B2C) no modulated magnetic structures were observed for any temperatures above 1.4 K.

A detailed study of the properties of ${\mathrm{Pr}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{MnO}}_{3}$ shows that for a range of compositions (0.3\ensuremath{\le}x0.5) there is a first-order magnetic-field-induced insulator-metal transition. For the x=0.4 composition the magnetic and charge ordering (CO) effects are decoupled with antiferromagnetic (AFM) ordering, ${\mathit{T}}_{\mathit{N}}$=170 K, developing at considerably lower temperatures than the CO state, ${\mathit{T}}_{\mathrm{CO}}$=250 K. Below the CO temperature, metamagnetic transitions exist that transform the magnetic correlations from either paramagnetic or AFM to ferromagnetic. Metastable conducting states are formed below 25 K.

A flux‐line lattice (FLL) was observed in a single crystal of Bi2.15Sr1.95CaCu2O8+x (BSCCO) using small‐angle neutron scattering methods. The sample has a superconducting transition at 85 K. The flux‐line lattice is observed to melt, evidenced by the rapid disappearance of diffracted intensity as the temperature is increased above a field‐dependent melting temperature. Diffracted intensity due to the vortex lattice also falls off as the applied field is increased. It is believed that this is a manifestation of the transition of the three‐dimensional flux lines into two‐dimensional pancake vortices. The Bragg intensity of the FLL peak is inversely proportional to the fourth power of the London penetration depth (λL). Hence, the temperature (T) dependence of the order parameter can be measured quite accurately from the intensity of the Bragg spots at different temperatures. In BSCCO with an applied field of 50 mT, the measured T dependence appears linear. The low‐T behavior is of great interest for an under...

In earlier studies of phonons in V3Si, a gradual softening of the (hh0) branch was observed and attributed to the martensitic transition in this compound, which precedes the onset of superconductivity by a few degrees K. In this work, the temperature dependence of the transverse acoustic (TA) branch along the hh0 direction was studied in greater detail and it is shown that while the TA hh0 mode starts to soften at relatively high temperatures (>200 K), an anomalous softening, which is rather localized in q, occurs just below Tc. The intensity of this soft-phonon peak correlates extremely well with the onset of the superconducting phase and appears to vary as the order parameter with temperature and applied field. The similarities in the phonon softening and Fermi-surface anisotropy between this compound and the rare-earth nickel borocarbides suggest the existence of a common mechanism for the superconducting transition.

This paper reports single-crystal inelastic neutron scattering measurements of the phonon-dispersion relations of the aluminium silicate minerals andalusite and sillimanite, Al2SiO5. The measurements were undertaken using the triple-axis spectrometer at the Dhruva reactor, Trombay for andalusite and at the Oak Ridge National Laboratory, USA for sillimanite. The phonon-dispersion relations (up to 50 meV) along various high-symmetry directions have been measured and have been analyzed on the basis of lattice dynamics shell model calculations. The calculated structure factors based on the model calculations were used as guides for planning these single-crystal measurements and have been used to identify regions in reciprocal space with large cross sections. The calculated phonon-dispersion relations are in good agreement with the measured data. The model has been successfully used to derive the high pressure–temperature thermodynamic properties like the specific heat and the equation of state.

We present the results of inelastic neutron-scattering measurements of a twinned crystal of YBa2Cu3O6.95 using the MAPS spectrometer of the ISIS facility of the Rutherford–Appleton Laboratory as well as the HFIR of the Oak Ridge National Laboratory. The dispersion of the Cu–O bond-stretching modes indicates a strong dielectric anisotropy within the plane and associated electronic anisotropy, supporting the dynamic stripe model. The results suggest a possibility that phonons play a major role in the mechanism of high-temperature superconductivity.

We report on small angle neutron scattering observations of the flux line lattice (FLL) in a single crystal of YBa[sub 2]Cu[sub 3]O[sub 7]. To probe the mass anisotropy ratio, [ital m][sub 3]/[ital m][sub 1], measurements were made as a function of angle, [Theta], between the 8 kOe applied field and the crystallographic (001) axis for 0[degree][le][Theta][le]80[degree]. With the rotation about an [ital a]/[ital b] (or [ital y]) axis, two symmetry-related distorted hexagonal FLL domains formed. Contrary to theoretical prediction, the lattices formed are consistent with a rotation of the short basis vector, [bold a][sub 1], from the [ital x] axis by 15[degree], after the effects of anisotropy are removed. The mass ratio is 20[plus minus]2, which is slightly lower than published values. The temperature dependence of the intensity is not conventional.

Polarization analysis has been used to identify the magnetic excitations in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. The dominant feature in the spectra is a peak at the (\ensuremath{\pi},\ensuremath{\pi}) reciprocal lattice position and centered at 41 meV. The behavior of the peak is shown to change dramatically at ${\mathit{T}}_{\mathit{c}}$, so that the magnetic excitations responsible for the peak must be strongly coupled to the superconductivity. Below an energy of about 35 meV, superconductivity suppresses the continuum scattering found in addition to the 41-meV peak.

Neutron diffraction has been used to measure the vortex lattice scattering for YBa2Cu3O7. A square pattern is found when the field is applied along the c axis, while a triangular pattern is found when the field is applied well away from the c axis. High‐resolution measurements for the square pattern show that the vortex lattice has long‐range orientational order but only short‐range positional order. The temperature dependence of the penetration depth is not that expected for a superconductor with a conventional s‐wave BCS‐type gap. Preliminary measurements for temperatures near the irreversibility line are consistent with the occurrence of melting of the vortex lattice or glass phase.

A small-angle neutron study of vortex lattices in YBa 2 Cu 3 O 7 showed a diffraction pattern with square symmetry when the applied field was along the c axis with the four main spots aligned with the crystallographic {110} directions. With the field 30 o from the c axis, a hexagonal lattice exists. At intermediate angles, a fraction of the flux lines follow the c axis and the two lattices coexist. The temperature dependence of the intensity deviates markedly from conventional predictions

An inelastic pulsed neutron scattering study was performed on the dependence of the dispersion and spectral intensity of the in-plane Cu-O bond-stretching LO phonon mode on doped charge density. The measurements were made in the time-of-flight mode with the multiangle position sensitive spectrometer of the ISIS facility on single crystals of YBa{sub 2}Cu{sub 3}O{sub 6+x} (x=0.15, 0.35, 0.6, 0.7, and 0.95). The focus of the study is the in-plane Cu-O bond-stretching LO phonon mode, which is known for strong electron-phonon coupling and unusual dependence on composition and temperature. It is shown that the dispersions for the samples with x=0.35, 0.6, and 0.7 are similar to the superposition of those for x=0.15 and 0.95 samples, and cannot be explained in terms of the structural anisotropy. It is suggested that the results are consistent with the model of nanoscale electronic phase separation, with the fraction of the phases being dependent on the doped charge density.

Vanadium sesquioxide (V{sub 2}O{sub 3}) undergoes a number of phase transitions as a function of temperature and doping. These phase transformations have been studied extensively using numerous experimental techniques. In this article, the author attempts to briefly review the experimental data on the pure and doped V{sub 2}O{sub 3} system.

We present neutron scattering data on $\mathrm{Ce}{({\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})}_{2}{\mathrm{Ge}}_{2}$ that show that magnetic ordering in the vicinity of a quantum critical point is restricted to short length scales even though all moments that are present have lined up with their neighbors. We argue that order is not limited by crystallographic defects but rather that quantum fluctuations disorder the system and dilute the magnetic moments to such an extent that a percolation network forms. The latter naturally explains how $E∕T$ scaling is possible in a system whose apparent dimensionality is above the upper critical dimension.

Magnetite (Fe$_{3}$O$_{4}$) is a mixed valent system where electronic conductivity occurs on the B-site (octahedral) iron sublattice of the spinel structure. Below $T_{V}=122$ K, a metal-insulator transition occurs which is argued to arise from the charge ordering of 2+ and 3+ iron valences on the B-sites (Verwey transition). Inelastic neutron scattering measurements show that optical spin waves propagating on the B-site sublattice ($\sim$80 meV) are shifted upwards in energy above $T_{V}$ due to the occurrence of B-B ferromagnetic double exchange in the mixed valent metallic phase. The double exchange interaction affects only spin waves of $\Delta_{5}$ symmetry, not all modes, indicating that valence fluctuations are slow and the double exchange is constrained by electron correlations above $T_{V}$., Comment: 4 pages, 5 figures

Inelastic neutron scattering results on magnetite (Fe3O4) show a large splitting in the acoustic spin wave branch, producing a 7 meV gap midway to the Brillouin zone boundary at q = (0,0,1/2) and E = 43 meV. The splitting occurs below the Verwey transition temperature, where a metal-insulator transition occurs simultaneously with a structural transformation, supposedly caused by the charge ordering on the iron sublattice. The wavevector (0,0,1/2) corresponds to the superlattice peak in the low symmetry structure. The dependence of the magnetic superexchange on changes in the crystal structure and ionic configurations that occur below the Verwey transition affect the spin wave dispersion. To better understand the origin of the observed splitting, we have constructed a series of Heisenberg models intended to reproduce the pairwise variation of the magnetic superexchange arising from both small crystalline distortions and charge ordering. We find that none of the models studied predicts the observed splitting, whose origin may arise from charge-density wave formation or magnetoelastic coupling., Comment: 62 pages, 17 figures, submitted to Phys. Rev. B

Phonon dispersion curves were obtained from inelastic x-ray and neutron scattering measurements on alpha-uranium single crystals at temperatures from 298 to 573 K. Both measurements showed a softening and an abrupt loss of intensity in the longitudinal optic branch along [00zeta] above 450 K. Above the same temperature a new dynamical mode of comparable intensity emerges along the [01zeta] zone boundary with energy near the top of the phonon spectrum. The new mode forms without a structural transition but coincides with an anomaly in the mechanical deformation behavior. We argue that the mode is an intrinsically localized vibration and formed as a result of a strong electron-phonon interaction.

The detailed flux line lattice (FLL) structure in YNi2B2C was investigated using small angle neutron scattering and the complete H‐T phase diagram was determined. The FLL in YNi2B2C shows a change of symmetry only in the low magnetic field region between 0.05 to 0.2 T. The observed square lattice is governed by an anisotropic Fermi velocity. Contrary to the theoretical prediction, a square lattice driven by an anisotropic superconducting gap does not appear below 5 T.

Neutron diffraction from a powder sample of Gd3Ga5O12 (GGG) has been measured. Due to the severe degree of magnetic frustration, GGG supports a short-range magnetic order (spin-liquid) over a wide temperature range, 140 mK −5 K. Neutron diffraction shows that at temperatures below 140 mK the magnetic system undergoes a partial transition to a phase with a much longer correlation length. However, even at the lowest temperature (43 mK) true long-range magnetic order is absent.

We have studied the effects of a travelling surface acoustic wave (SAW) disturbance on symmetric Bragg case neutron diffraction from a perfect crystal of lithium niobate. Generally, the reflectivity is increased as the SAW distortion creates an effective moving mosaic crystal in the near surface region. Unlike the corresponding X-ray situation in which the crystalline distortion is effectively stationary, thermal neutron and SAW velocities are comparable, giving rise to significant Doppler effects that modify the reflected neutron intensity. No quantitative picture exists of the interaction between the neutron and the coherent surface phonons in the present situation. Several of our observations contradict even the qualitative expectations of a moving mosaic crystal analogy.

Magnetic excitations of the spin-Peierls compound CuGeO3 under applied pressure of 2 GPa have been studied. The dispersion along the chain direction up to the zone boundary has been obtained. The spin-Peierls gap energy increases to 4.2 meV and the zone boundary energy decreases to 14.1 meV. The pressure dependence of dispersion relation can be interpreted by the increase of the next-nearest-neighbor intra-chain interaction under applied pressure causing the increase of both the spin-Peiels gap energy and transition temperature.

In ${\mathrm{V}}_{3}\mathrm{Si}$, a first-order structural phase transition from hexagonal to square flux-line lattice occurs at approximately 1 T with $H\ensuremath{\Vert}$ to the $a$ axis. In this paper, we demonstrate the reentrant structural transition in the flux-line lattice, which reverts to hexagonal symmetry as the magnetic field approached ${H}_{c2}(T)$. This behavior is described very well by a nonlocal London theory with thermal fluctuations. The phase diagram of the flux lattice topology is mapped out for this geometry.

Small-angle neutron diffraction measurements from the flux lattice in a single crystal of the cubic Laves' phase superconductor, CeRu2, are reported. The mixed state is described in terms of aligned rigid bundles of vortices. The bundle diameters decrease above 1 2 H c 2 (consistent with collective weak pinning theory) and become comparable with the penetration length at a field at which a ‘peak effect’ is seen in magnetisation measurements. A clear memory of field histories that pass through the ‘peak effect’ region is also found; however, some of the induced disorder can be removed by subsequently cycling the field.

RENi 2 B 2 C (RE = rare-earth) are quaternary compounds which exhibit a considerable degree of interaction between their superconducting and magnetic properties. The Ho variant is found to become superconducting at T c = 9 K, but anomalous behaviour is observed in the low-field magnetic properties at T < 7 K (increasing susceptibility with decreasing temperature); at temperatures less than 5.6 K more conventional behaviour is regained. Neutron diffraction measurements demonstrate that at low temperatures (T < 5 K) the magnetic order consists of ferromagnetic planes antiferromagnetically coupled up the c-axis with a Ho moment of 8.9 ± 0.2 μ B canted at 15 ± 5° from the a-b plane. At high temperatures (5 K < T < 8.5 K) satellites, together with the principal Bragg reflections, are observed which correspond to a modulation of the magnetic order with a wavelength of ∼136 A up the c-axis. In the intermediate temperature (5 K < T < 7 K) range further Bragg reflections become evident. An additional modulation along the a-axis becomes important in this temperature range.

Neutron small-angle diffraction has been used to investigate the flux-line lattice structure within single crystals of the high-temperature superconductor Bi 2.15 Sr 1.95 CaCu 2 O 8 + x . The diffracted intensity goes rapidly to zero as the magnetic field or the temperature is increased. Melting at low fields as a function of temperature coincides with the appearence of finite resistance within the superconducting state. At low temperatures the diffracted intensity disappears in fields greater than ∼ 70 mT, probably due to the decomposition of the flux-line lattice into randomly pinned 2d “pancake” vortices.

The peak effect in critical current density ${J}_{c}$ is investigated by studying the flux dynamics in ${\mathrm{V}}_{3}\mathrm{Si}$ using bulk magnetometry, small-angle neutron scattering, and transport measurements on clean single-crystal samples from the same ingot. For a field-cooled history, well-defined structure in the vortex lattice was found for fields and temperatures (H,T) below the peak-effect line ${H}_{P}(T);$ above this line, the structure disappeared. History-dependent, metastable disorder is found only for (H,T) below ${H}_{P}(T)$ but the vortex system is reproducibly re-ordered either by field cooling or a low-frequency, pulsed ``shaking'' transport current. The latter is shown to attain Bardeen-Stephen flux flow. In addition, flux flow is observed at ${H}_{P}(T)$ at high current levels. The results support the traditional picture of ${H}_{P}(T)$ as an order-disorder transition due to the competition between elasticity and pinning.

Inelastic pulsed neutron scattering measurements on YBa2Cu3O6.95 single crystals indicate that the sample has a distinct a-b plane anisotropy in the oxygen vibrations. The Cu-O bond-stretching-type phonons, which are suspected to interact strongly with charge, are simultaneously observed along the a and b directions due to a 7-meV splitting arising from the orthorhombicity, even though the sample is twinned. The bond-stretching LO branch with the polarization along a ~perpendicular to the chain! loses intensity beyond the middle of the zone, indicating branch splitting as seen in doped nickelates, with the second branch being located at 10 meV below. The mode along b has a continuous dispersion. These modes show temperature dependence, which parallels that of superconductive order parameter, suggesting significant involvement of phonons in the superconductivity of this compound.

The possibility of magnetic-order induced phonon anisotropy in single crystals of MnO and NiO is investigated using inelastic neutron scattering. Below Tn both compounds exhibit a splitting in their transverse optical phonon spectra of approximately 10%. This behavior illustrates that, contrary to general assumption, the dynamic properties of MnO and NiO are substantially non-cubic., Comment: 4 pages, 4 figures

High resolution, neutron small angle scattering experiments have been performed on the flux-line lattice within the mixed state of the type-II superconductors YNi2B2C and TmNi2B2C. In the Y compound clear evidence is found for a transition at low fields from a distorted hexagonal lattice with its diagonal along a [110] direction to a [100] direction as the field is increased. At higher fields the lattice is found to be square. These results are in

Small-angle neutron scattering and muon spin rotation have been used to obtain direct information about flux structures in the mixed state within the bulk of crystals of superconductors, including BSCCO-2212, niobium and YBCO. In highly anisotropic BSCCO, we see evidence of vortex fluctuations, melting, and decomposition of flux lines into ‘pancake’ vortices, which give a more twodimensional vortex structure. Our results from pure niobium show no sign of flux-lattice melting — in disagreement with recent claims. In YBCO the flux lattice is much more stable than in BSCCO, due to a much lower anisotropy, but it shows interesting flux structures as a function of field direction.

Electrical, magnetic, and structural properties of the vortex lattice (VL) in single crystal V 3 Si were studied by transport, bulk magnetometry and small-angle neutron scattering. Studies focused on the ‘peak effect’ in critical current density just below the upper critical field of this weak-pinning system. The overall picture is a slightly disordered VL easily re-ordered by transport-current ‘shaking’ and ultimately softening at the peak effect.

The thermal triple-axis spectrometer TAIPAN is the first instrument for inelastic neutron scattering at the new Australian research reactor OPAL. TAIPAN started operation in February 2009 and is in full user service since November 2010. Conceptually, it is similar to the triple-axis spectrometers IN8 (ILL) and PANDA (FRM-II) with variable incident and final energies and a secondary spectrometer with a single detector. The instrument can be operated either in a high flux mode with a double-focusing monochromator and analyser, or with Soller collimators - gaining resolution at the expense of intensity. Presently the PG (002) double-focusing monochromator and analyser are in use. The incident energy range on the TAIPAN TAS is from ~5 meV up to ~100 meV with neutron flux at sample position of 2.4≤107 n/cm2/s at incident energy of 14.8 meV. First experiments were performed with superionic conductor Cu2−δSe. The measurements reveal the presence of a soft mode related to ordering of Cu atoms followed by α - β phase transition at a lower temperature. The evolution of the magnetic structure with temperature in a magnetically modulated FePt3 thin film was investigated in the diffraction mode of TAIPAN. The results show that the film fabricated by modulation of the chemical order parameter consists of a magnetic FM/AFM superlattice in single-crystalline FePt3. The spin wave and phonon dispersion was recently investigated in TbVO3 single crystal. The acoustic and optical magnon branches were observed in the same energy range. This indicates that the 'orbital Peiers state' also exists in TbVO3.