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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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

Since its beginnings in Oak Ridge and Argonne in the late 1940's, neutron scattering has been established as the premier tool to study matter in its various states. Since the thermal neutron wavelength is of the same order of magnitude as typical atomic spacings and because they have comparable energies to those of atomic excitations in solids, both structure and dynamics of matter can be studied via neutron scattering. The High Flux Isotope Reactor (HFIR) provides an intense source of neutrons with which to carry out these measurements. This paper summarizes the available neutron scattering facilities at the HFIR.

The magnetization M and heat capacity of clean, single crystal YNi2B2C superconductor have been studied with magnetic field H||c-axis. The magnetization M(H) deviates from conventional predictions, but is accurately described by recent non-local London theory, with well-behaved parameters. We show that the Ginzburg-Landau parameters κ1 and κ2 increase considerably as T decreases, similar to niobium, but with κ(Tc)≈7.

We present magnetoresistance and Hall-effect measurements on single-crystal magnetite (Fe3O4) close to the Verwey transition TV=123.8K. We show that the formation of grain boundaries accompanying the reduction in crystal symmetry plays a significant role in the electron-scattering mechanism, and that grain boundaries can account for the apparent change in sign of the charge carriers below TV.

We have performed detailed SANS investigations on a large, high quality, twinned YBCO single crystal. The crystal was oriented with H 51 degrees to c to reduce the effect of twin plane pinning. The diffracted intensity was found to disappear at a temperature well below Hc2(T). The data is consistent with a Lindemann melting criterion giving a physically reasonable value forcL of 0.15 that is in good agreement with previous work. The disappearance of the diffracted intensity coincides with the irreversibility lines as measured by an inductance technique.