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Debasement of silver Roman coins is a well-known phenomenon and understanding the quality of ancient silver coinages can provide an idea about the underlying fiscal condition of the issuing states. These coins are made from a silver-copper alloy, the surfaces of which were deliberately enhanced at the mints by a process of surface-enrichment to give them the appearance of being made of pure silver. Therefore, any surface analysis would provide a composition of the silver-copper alloy that would not be representative of the original alloy from which the coin blank was made, the result would be too high in silver. However, the bulk of the sample, the interior, should provide a composition that is true to the original alloy. Elemental analysis using negative muons has been used to provide a depth dependent compositional, completely non-destructive analysis of a silver-copper alloy denarius of the empress Julia Domna datable to 211&ndash, 217 CE. The composition of the coin, beyond the surface enrichment layer, is 51 &plusmn, 1.8 % copper and 49 &plusmn, 1.9% silver, taken at a muon depth of 402 &plusmn, 61 &micro, m. The surface enrichment layer is approximately 190 &micro, m thick.

We report the design of a compact sapphire anvil cell for small angle neutron scattering. Based on turnbuckle-opposed anvil technique, the dimensions of this cell are minimized for use with a small-bore magnets using finite element analysis. This cell is the smallest sapphire anvil cell to date. The cell body is approximately 14 mm diameter cylinder and 13 mm length, the overall length of the assembled cell is less than 17 mm. Inside the cell, two spherical-shaped sapphire anvils are used to ensure compactness and anvil’s strength, and at the same time to provide sufficient sample volume for a neutron scattering experiment. The sample volume of the cell used in the SANS experiment was as large as 0.4 mm\(^{3}\), which is approximately 40 times larger than the conventional diamond anvil cell. The cell has been used in a SANS study of a niobium single crystal at D22 Institut Laue-Langevin with the maximum pressure of 4 GPa being achieved with the liquid pressure medium. During the tests the cell demonstrated that it was capable of achieving pressure to 6 GPa with a powder sample.

International audience; Damage evolution at room temperature in Ho(2)Ti(2)O(7) single crystals is studied under 1 MeV Au(2+) ion irradiation by Rutherford backscattering spectroscopy along the (0 0 1) direction. For a better determination of ion-induced disorder profile, an iterative procedure and a Monte Carlo code (McChasy) were used to analyze ion channeling spectra. A disorder accumulation model, with contributions from the amorphous fraction and the crystalline disorder, is fit to the Ho damage accumulation data. The damage evolution behavior indicates that the relative disorder on the Ho sublattice follows a nonlinear dependence on dose and that defect-stimulated amorphization is the primary amorphization mechanism. Similar irradiation behavior previously was observed in Sm(2)Ti(2)O(7). A slower damage accumulation rate for Ho(2)Ti(2)O(7), as compared with damage evolution in Sm(2)Ti(2)O(7), is mainly attributed to a lower effective cross section for defect-stimulated amorphization. (c) 2010 Elsevier B.V. All rights reserved.

Damage accumulation in Sm2Ti2O7 single crystals irradiated with Au2+ ions at 170, 300 and 700 K was studied by Rutherford backscattering spectrometry using a 2.0 MeV He+ beam along the 〈0 0 1〉 channeling direction. The relative disorder on the Sm sublattice follows a nonlinear dependence on ion fluence. The nonlinear behavior is described well by a disorder accumulation model that indicates a predominant role of a defect-stimulated amorphization process. The critical dose for amorphization at 300 K is ∼0.14 dpa, which is in good agreement with in situ transmission electron microscopy results for polycrystalline Sm2Ti2O7 irradiated with 600 keV Bi+ ions and with Gd2Ti2O7 doped with 244Cm. Despite the six orders of magnitude difference in damage rates, the good agreement between the amorphization doses in Sm2Ti2O7 at 300 K and 244Cm-doped Gd2Ti2O7 at 340 K indicates that damage accumulation at these temperatures is relatively independent of dose rate.

Damage evolution on the Sm and O sublattices in Sm2Ti2O7 single crystals irradiated with 1 MeV Au2+ ions at 170, 300, and 700 K was studied by Rutherford backscattering spectroscopy and 16O(d,p)17O nuclear reaction analysis along the 〈001〉 direction. The damage accumulation behavior at each irradiation temperature indicates that the relative disorder on the O sublattice is higher than that on the Sm sublattice, and the relative disorder, determined by ion channeling, on each sublattice follows a nonlinear dependence on dose that is well described by a disorder accumulation model. While there is little difference in damage accumulation behavior on the Sm sublattice at 170 and 300 K irradiation, the rate of damage accumulation decreases dramatically at 700 K due to dynamic recovery processes. The critical dose for amorphization at 170 and 300 K is 0.14 displacements per atom (dpa), and a higher dose of 0.22 dpa is observed under irradiation at 700 K. During thermal annealing in an 18O environment, a signifi...

We have studied the heat capacity of a single crystal PrCoAl4. The data indicate that Pr3+ in this compound has a non-magnetic singlet ground state. The compound becomes antiferromagnetically ordered below the Neel temperature of 17 K. The magnetocrystalline anisotropy in PrCoAl4 is relatively strong. Up to 300 K, the entropy contained in the specific heat is 15.2 J/Kmol, which is only 83% of 18.3 J/Kmol expected for the 2J+1-fold degeneracy of the ground multiplet of the Pr3+ ion.

Inelastic neutron scattering (INS) and specific heat measurements have been performed on the intermediate valence compound CeRuSi$_3$, which is isostructural to the noncentrosymmetric pressure-induced superconductors CeRhSi$_3$, CeIrSi$_3$ and CeCoGe$_3$. INS measurements at 7~K reveal a broad peak at (58.5~$\pm$~1.4)~meV, while at 300~K, broad quasielastic scattering is observed. This indicates a large Kondo temperature of $T_{\rm K}~\sim~680$~K. The magnetic contribution to the specific heat ($C_{\rm mag}$) has a value of $\gamma~=~62.5(1)$~mJ/mol~K$^2$ at low temperatures and above about 100~K can be well accounted for by the Coqlbin-Schrieffer model with a characteristic temperature of $T_0$~=~680~K, which is further evidence that CeRuSi$_3$ is in the intermediate valence regime., Comment: 6 pages, 7 figures. Accepted for publication in Physical Review B

The noncentrosymmetric superconductors LaPdSi${}_{3}$ and LaPtSi${}_{3}$ have been studied with magnetization, specific-heat, resistivity, and $\ensuremath{\mu}$SR measurements. These crystallize in the tetragonal BaNiSn${}_{3}$ structure and superconductivity is observed at ${T}_{\mathrm{c}}=2.65(5)$ K for LaPdSi${}_{3}$ and ${T}_{\mathrm{c}}=1.52(6)$ K for LaPtSi${}_{3}$. The results are consistent with both compounds being weakly coupled, fully gapped superconductors but $\ensuremath{\mu}$SR measurements reveal that LaPdSi${}_{3}$ is a bulk type-I superconductor while LaPtSi${}_{3}$ is a type-II material with a Ginzburg-Landau parameter of $\ensuremath{\kappa}=2.49(4)$. This is further supported by specific-heat measurements, where the transition in an applied field is first order in LaPdSi${}_{3}$ but second order in LaPtSi${}_{3}$. The electronic specific heat in the superconducting state was analyzed using an isotropic $s$-wave model that gave ${\ensuremath{\Delta}}_{0}/{k}_{\mathrm{B}}{T}_{\mathrm{c}}=1.757(4)$ for LaPdSi${}_{3}$ and 1.735(5) for LaPtSi${}_{3}$. The temperature dependence of the effective penetration depth [${\ensuremath{\lambda}}_{\mathrm{eff}}(T)$] of LaPtSi${}_{3}$ was extracted from $\ensuremath{\mu}$SR measurements and was fitted giving ${\ensuremath{\Delta}}_{0}/{k}_{\mathrm{B}}{T}_{\mathrm{c}}=1.60(8)$ and ${\ensuremath{\lambda}}_{\mathrm{eff}}(0)=239(3)$ nm. A critical field of ${B}_{\mathrm{c}}$(0) = 182.7 G was obtained for LaPdSi${}_{3}$ from $\ensuremath{\mu}$SR measurements, which is in good agreement with the calculated thermodynamic critical field.

The effect of pressure of up to 9 GPa on the magnetic and structural properties of layered $({\mathrm{La}}_{0.6}{\mathrm{Nd}}_{0.4}{)}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ manganite has been investigated. Pressure shifts the Mn atoms from the center of each bilayer towards the adjacent bilayer. The observed shift of the Mn atoms involves mainly a charge redistribution within the ${d}_{{3z}^{2}{\ensuremath{-}r}^{2}}$ axial orbital. The decrease in the interbilayer spacing and the shift of the Mn atoms away from the center of each bilayer result in increased magnetic and electronic coupling between the adjacent bilayers along the $c$ axis.

Studies of single crystal YNi{sub 2}B {sub 2}C have revealed a fourfold anisotropy of the equilibrium magnetization in the square crystallographic basal plane. This {pi}/2 periodicity occurs deep in the superconductive mixed state. In this crystal symmetry, an ordinary superconductive mass anisotropy (as in usual London theory) allows only a constant, isotropic response. In contrast, the experimental results are well described by generalized London theory incorporating nonlocal electrodynamics, as needed for this clean, intermediate-{kappa} superconductor. {copyright} {ital 1999} {ital The American Physical Society }

Neutron diffraction techniques have been used to investigate the influence of applied magnetic field and pressure on the structural and magnetic phase transitions in La1xSrxMnO3. The structural, ma...

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#

We report a neutron diffraction investigation of the role of pressure on the strong coupling between the structural and ferromagnetic phase transitions in La 0.835Sr0.165MnO3. The results are summarized in the form of a pressure-temperature phase diagram and reveal the dominant character of the structural changes with respect to the magnetic transitions, which results in a broad metastable area around the crossing point of the structural and magnetic phase transitions. The phase diagram contains a number of extremely unusual features including the pressure independence of the Curie temperature in the orthorhombic phase, a reentrance of the rhombohedral phase at low temperatures, and a change of the type of the magnetic phase transition from second order to first order due to the strong coupling between the structural and the magnetic properties of this material. @S0163-1829~98!52212-6#

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}

The point-contact (PC) spectra of the electron-phonon interaction (EPI) in the superconducting RNi{sub 2}B{sub 2}C (R=Ho,Y) compounds reveal strong EPI with low-energy (4{endash}9meV) phonons, unlike the case of nonsuperconducting LaNi{sub 2}B{sub 2}C. The PC-EPI spectral functions for all three compounds are determined and lower limits for the EPI interaction parameter {lambda} estimated. Besides, an unusual suppression of the PC-EPI spectral intensity by the external magnetic field was observed for the superconducting compounds, unlike the nonsuperconducting one. {copyright} {ital 1997} {ital The American Physical Society}

The magnetic states of the noncentrosymmetric pressure-induced superconductor CeCoGe3 have been studied with magnetic susceptibility, muon spin relaxation (?SR), single-crystal neutron diffraction, and inelastic neutron scattering (INS). CeCoGe3 exhibits three magnetic phase transitions at TN1=21,TN2=12, and TN3=8K. The presence of long-range magnetic order below TN1 is revealed by the observation of oscillations of the asymmetry in the ?SR spectra between 13 and 20 K and a sharp increase in the muon depolarization rate. Single-crystal neutron-diffraction measurements reveal magnetic Bragg peaks consistent with propagation vectors of k=(0,0,23) between TN1 and TN2,k=(0,0,58) between TN2 and TN3 and k=(0,0,12) below TN3. An increase in intensity of the (110) reflection between TN1 and TN3 also indicates a ferromagnetic component in these phases. These measurements are consistent with an equal moment two-up two-down magnetic structure below TN3 with a magnetic moment of 0.405(5)?B/Ce. Above TN2, the results are consistent with an equal moment two-up one-down structure with a moment of 0.360(6)?B/Ce. INS studies reveal two crystal-electric-field (CEF) excitations at ?19 and ?27meV. From an analysis with a CEF model, the wave functions of the J=52 multiplet are evaluated along with a prediction for the magnitude and direction of the ground-state magnetic moment. Our model correctly predicts that the moments order along the c axis, but the observed magnetic moment of 0.405(5)?B is reduced compared to the predicted moment of 1.0?B. This is ascribed to hybridization between the localized Ce3+ f electrons and the conduction band. This suggests that CeCoGe3 has a degree of hybridization between that of CeRhGe3 and the noncentrosymmetric superconductor CeRhSi3

We report the results of dc magnetization measurements of Bi2Sr2CaCu2O81d single crystals as a function of temperature. The evolution of the hysteresis loops with temperature is examined and the irreversibility line extracted. These loops can be scaled onto each other from 4.2 to 20 K, while the lower field part of the loops for the virgin curves continue to scale up to Tc . At intermediate temperatures, the scaling is broken by the appearance of the second peak or ‘‘arrowhead’’ which appears at around 20 K but gradually disappears above about 30 K. While the behavior is dominated by significant bulk pinning at low temperatures, the hysteresis and irreversibility fields at high temperatures are considerably modified by geometrical and surface barrier effects. We identify the onset, with increasing field, of bulk pinning and significant critical current from the emergence of the ‘‘arrowhead’’ feature from the background surface barrier-determined magnetization curve. Scaling in the low-temperature regime, and extrapolation, enables the irreversibility line at 0 K to be estimated at about 110 T. We propose a phase diagram up to high magnetic fields over a wide range of temperature. @S0163-1829~96!03521-7#

We present magnetic, calorimetric and transport properties of CeCuGa 3 , which characterise CeCuGa 3 as a magnetically ordered Kondo lattice system. The Kondo temperature T K is estimated to be between 5.6 and 6.9 K. For the first time this system is shown to exhibit long-range magnetic order at 1.9 K. The system shows negative magnetoresistance in fields of up to 8 T.

Low-field magnetization M(H) measurements are reported for single crystals of various high-${\mathit{T}}_{\mathit{c}}$ cuprates with different anisotropies (\ensuremath{\gamma}=${\ensuremath{\lambda}}_{\mathit{c}}$/${\ensuremath{\lambda}}_{\mathit{ab}}$, where ${\ensuremath{\lambda}}_{\mathit{c}}$ and ${\ensuremath{\lambda}}_{\mathit{ab}}$ are the c-axis and ab-plane penetration depths). For ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ (\ensuremath{\gamma}\ensuremath{\approxeq}5) and ${\mathrm{La}}_{1.89}$${\mathrm{Sr}}_{0.11}$${\mathrm{CuO}}_{4}$ (\ensuremath{\gamma}\ensuremath{\approxeq}12), qualitatively similar M(H) behavior is observed when the magnetic field is applied both perpendicular (H\ensuremath{\parallel}c) and parallel (H\ensuremath{\parallel}ab) to the ${\mathrm{CuO}}_{2}$ planes, consistent with a description of these compounds as anisotropic three-dimensional superconductors. ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CuO}}_{6+\mathrm{\ensuremath{\delta}}}$ (\ensuremath{\gamma}\ensuremath{\approxeq}25) and ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$O (\ensuremath{\gamma}\ensuremath{\approxeq}250), on the other hand, show significant differences in M(H) with H\ensuremath{\parallel}ab which can be attributed to the presence of Josephson screening currents across the layers. These observations imply that the nature of the interlayer coupling is determined primarily by the ratio of the out-of-plane coherence length ${\ensuremath{\xi}}_{\mathit{c}}$ to the interlayer spacing s. \textcopyright{} 1996 The American Physical Society.

In a type II superconductor, such as the Nb or the high temperature superconductors, there are two critical magnetic fields, HC1 and HC2. When a sample is cooled through the critical temperature for superconductivity, TC in a magnetic field of less than HC1 all magnetic flux is expelled from the material-the Meissner-Ochsenfeld effect. HC2 is the maximum magnetic field which can be applied to the superconductor without driving the sample back into the normal state. At intermediate magnitudes of magnetic field, HC1

Superconductivity has been observed in ${\mathrm{DyNi}}_{2}$${\mathrm{B}}_{2}$C single crystals at 6.4 K. ${\mathrm{DyNi}}_{2}$${\mathrm{B}}_{2}$C also orders antiferromagnetically at \ensuremath{\sim}10.5 K and is a unique member of R${\mathrm{Ni}}_{2}$${\mathrm{B}}_{2}$C compounds (R=Y or rare earth) in that the onset of superconductivity takes place in a magnetically ordered state. The crystals exhibit anomalous field-cooled magnetization in low applied fields of 1--5 Oe. Specific-heat measurements on the crystals give further evidence for the bulk nature of the magnetic order. The upper critical field ${\mathit{H}}_{\mathit{c}2}$ measured parallel and perpendicular to the c axis of the crystals, shows little or no anisotropy. ${\mathit{H}}_{\mathit{c}2}$(0) for ${\mathrm{DyNi}}_{2}$${\mathrm{B}}_{2}$C is deduced to be 0.7 T.

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...

We have used the technique of polarized-neutron reflectometry to study the magnetic-induction profile just beneath the surface of a 1-\ensuremath{\mu}m-thick film of lead with the magnetic field applied parallel to the surface. The sample was maintained at a temperature of 1.5 K throughout the experiment, and the applied field H was varied from below ${\mathit{H}}_{\mathit{c}}$, the bulk critical field, up to the critical field for surface superconductivity, ${\mathit{H}}_{\mathit{c}3}$. From the measurements with the lead film in the bulk superconducting phase (H${\mathit{H}}_{\mathit{c}}$), we found that the spin-dependent reflectivity profiles are consistent with an exponential decay of magnetic induction with a penetration depth \ensuremath{\lambda}=39\ifmmode\pm\else\textpm\fi{}1 nm. At higher applied fields (${\mathit{H}}_{\mathit{c}}$H${\mathit{H}}_{\mathit{c}3}$) we obtained information on the diamagnetism in the surface superconducting layer. We discuss the sensitivity of the measurements to the form of the magnetic-induction profile, in particular to nonlocal effects, and show that a good description of the surface diamagnetism, over the whole range of applied fields, can be achieved with the local Ginzburg-Landau theory if the Ginzburg-Landau parameter \ensuremath{\kappa} is allowed to vary with the applied field.

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.

We have compared the pinning and creep in two identically shaped single crystals, Tl 2 Ba 2 CaCu 2 O 8 (Tl 2:2:1:2) and Bi 2 Sr 2 CaCu 2 O 8 (Bi 2:2:1:2) using magnetometry. The critical current density, J c , deduced from the M-H hysteresis loops is found to be the highest in Bi 2:2:1:2 at low temperatures (T

This article reviews the use of neutron small-angle scattering to determine the spatial distribution of the magnetic field within the bulk of a high-temperature superconductor. Results will be presented for experiments using single crystals of YBa 2 Cu 3 O 7 . The influence of pinning and anisotropy on the experimental data will be discussed together with the anomalous temperature dependence of the integrated intensity. Future directions for such work will be suggested.

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

Muon-spin-spectroscopy measurements have been used to study the superconducting state of FeTe0.5Se0.5. The temperature dependence of the in-plane magnetic penetration depth, lambda(ab)(T), is found to be compatible with either a two-gap s+s-wave or an anisotropic s-wave model. The value for lambda(ab)(T) at T = 0 K is estimated to be lambda(ab)(0) = 534(2) nm.

Detailed neutron scattering measurements have been made for selected phonons for ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$. The temperature dependence of the linewidths indicates large anharmonic effects particularly for the oxygen modes. The superconductivity induced change in width below ${\mathit{T}}_{\mathit{c}}$ of the two highest energy oxygen modes is found to be nearly 2% of the phonon energy suggesting a large electron phonon coupling.

17 O NMR measurements have been performed on the Bi 2 Sr 2 Ca n −1 Cu n O 4+2 n ( n = 1, 2, 3) phases allowing the different oxygen sites to be identified. Detailed study of the n =3 phase in different applied magnetic fields has allowed extraction of the shift and quadrupole interaction parameters at the oxygen sites. For this phase the two distinct sets of copper planes have markedly different oxygen shifts, 1590 ppm and 1285 ppm, shift anisotropies, 1300 ppm and 2 plane having the smaller shift and anisotropy. This implies that the local susceptibility is lower in this plane and that the 2p occupancy is less. There is only a small shift difference for the CuO 2 plane oxygens between the superconducting n =2 phase and the nonsuperconducting (for our samples) n =1 phase. Only one resonance, at 265 ppm, is observed in all three phases for the SrO, BiO planes. We suggest that this is from the SrO plane and that the electric field gradient at the oxygen site produced by the incommensurate modulation of the BiO distances broadens the line from the BiO plane beyond detection.

We present a comprehensive analysis of Raman spectra in the ${\mathit{R}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ system (R=Gd,Sm,Nd,Pr) as a function of doping, temperature, rare-earth atomic radius, and Raman resonance conditions. Phonon frequencies as well as their temperature dependences behave anomalously for R=Pr: The ${\mathit{B}}_{1\mathit{g}}$ phonon, for instance, softens by as much as 11 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ when the crystal is cooled from room temperature to 10 K, while it hardens by 11 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for R=Nd. These observations are attributed to the fact that Pr, as the largest rare-earth atom that can give rise to the T' structure, is already close to the T' stability limit, yielding large phonon anharmonicities in this compound. The dependence of the phonon frequencies on doping is strong only for the oxygen ${\mathit{E}}_{\mathit{g}}$ mode, which can thus be used for sample characterization. We also present and discuss phonon resonance profiles for ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4}$ and ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4}$, which yield electronic structural information that should be compared with future calculations of the resonance profiles. Finally, we also discuss the origin of an additional large ${\mathit{A}}_{1\mathit{g}}$-symmetry peak that shows a dramatic, rare-earth-dependent resonance behavior and gives a possible explanation on the grounds of a partial T'\ensuremath{\rightarrow}T transition. We always observe several well-defined, unexpected vibrational peaks that seem to be intrinsic to the ${\mathit{R}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ system, indicating crystal distortions that have not been conclusively identified by x-ray- or neutron-diffraction experiments so far.

The effect of externally applied pressure on the magnetic behavior of ${\mathrm{Cu}}_{2}{\mathrm{Te}}_{2}{\mathrm{O}}_{5}({\mathrm{Br}}_{x}{\mathrm{Cl}}_{1\ensuremath{-}x}{)}_{2}$ with $x=0$, 0.73, and 1, is investigated by a combination of magnetic susceptibility, neutron diffraction, and neutron inelastic scattering measurements. The magnetic transition temperatures of the $x=0$ and 0.73 compositions are observed to increase linearly with increasing pressure at a rate of 0.23(2) and $0.04(1)\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕\mathrm{kbar}$, respectively. However, the bromide shows contrasting behavior with a large suppression of the transition temperature under pressure, at a rate of $\ensuremath{-}0.95(9)\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕\mathrm{kbar}$. In neutron inelastic scattering measurements of ${\mathrm{Cu}}_{2}{\mathrm{Te}}_{2}{\mathrm{O}}_{5}{\mathrm{Br}}_{2}$ under pressure only a small change to the ambient pressure magnetic excitations were observed. A peak in the density of states was seen to shift from $\ensuremath{\sim}5\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ in ambient pressure to $\ensuremath{\sim}6\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ under an applied pressure of $11.3\phantom{\rule{0.3em}{0ex}}\mathrm{kbar}$, which was associated with an increase in the overall magnetic coupling strength.

The transport properties of a single crystal of ${\mathrm{La}}_{1.2}{\mathrm{Ca}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ with the layered ${\mathrm{Sr}}_{3}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$-type perovskite structure have been studied under hydrostatic pressures of up to \ensuremath{\sim}9 kbar. It is found that ${\mathrm{La}}_{1.2}{\mathrm{Ca}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ undergoes a first-order phase transition from a paramagnetic insulating to a low-temperature ferromagnetic metallic state which is accompanied by a large decrease in the electrical resistivity. At ambient pressure the magnetic phase transition takes place at ${T}_{c}=242 \mathrm{K}$ and as the external pressure is applied the critical temperature increases linearly at a rate of 1.1 K/kbar. The present results reflect the competition between double and superexchange resulting from two-dimensional Mn-O-Mn networks and can be interpreted in terms of the exchange striction model.

Measurements of the thermal expansion of a single crystal of La{sub 0.835}Sr{sub 0.165}MnO{sub 3} were performed under hydrostatic pressure of up to 9 kbar. The P-T phase diagram is established for the structural and magnetic phase transitions. Pressure is found to enhance the ferromagnetic coupling and to weaken the stability of the low-temperature orthorhombic phase by reducing the bending of the Mn-O-Mn bond. The decrease in the temperature T{sub S} of the structural phase transition and the increase in the Curie temperature with increasing pressure leads to the two phase boundaries crossing at a pressure of about 3 kbar. The peculiar behavior of the T{sub S} and T{sub C} in the vicinity of the crossing point demonstrates a strong coupling between structural distortion and magnetic ordering in the La{sub 0.835}Sr{sub 0.165}MnO{sub 3} compound. {copyright} {ital 1997} {ital The American Physical Society}

We report on unusual field-induced magnetic phase transitions observed in a GdSi single crystal at low temperature. The magnetization curves initially show an anisotropic character between the b axis and the ac plane. With increasing applied field, the magnetization measured along the c axis merges with that measured along the b axis through a jumplike transition at field of about 2.5 T, whereas that measured along the a axis gradually joins the other two at about 3.8 T. Above 3.8 T, the magnetization behaves isotropically with applied magnetic field and then reaches saturation through a jumplike transition at a field of about 19.5 T. We discuss the anomalous behavior of the compound in terms of interplay between the localized 4f spins and the sitinerantd conduction electrons.

Disorder in ${\mathrm{Gd}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ is investigated by near-edge x-ray-absorption fine structure (NEXAFS) and x-ray photoelectron spectroscopy (XPS). NEXAFS shows ${\mathrm{Ti}}^{4+}$ ions occupy octahedral sites with a tetragonal distortion induced by vacant oxygen sites. $\mathrm{O}\phantom{\rule{0.3em}{0ex}}1s$ XPS spectra obtained with a charge neutralization system from ${\mathrm{Gd}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}(100)$ and the ${\mathrm{Gd}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ pyrochlore used by Chen et al. [Phys. Rev. Lett. 88, 105901 (2002)], both yielded a single peak, unlike the previous result on the latter that found two peaks. The current results give no evidence for an anisotropic distribution of Ti and O. The extra features reported in the aforementioned communication resulted from charging effects and incomplete surface cleaning. Thus, a result confirming the direct observation of simultaneous cation-anion antisite disordering and lending credence to the split vacancy model has been clarified.

We present an overview of the temperature, field and angular dependence of the irreversible magnetization of non-magnetic borocarbides (Y;Lu)Ni2B2C. We show that nonlocal electrodynamics influences pinning via the unusual behavior of the shear modulus in non-hexagonal lattices. On top of that, we observe that the pinning force density Fp exhibits a rich anisotropic behavior that sharply contrasts with its small mass anisotropy. When H⊥c, Fp is much larger and has a quite different H dependence, indicating that other pinning mechanisms are present.

Pressure effects on the stability of magnetic phases in La(1.4)Sr(1.6)Mn(2)O(7) have been studied using magnetization measurements and neutron diffraction. At ambient conditions this material is a quasi-two-dimensional ferromagnet. On cooling it becomes ordered three dimensionally: at 90 K La(1.4)Sr(1.6)Mn(2)O(7) it becomes an antiferromagnet, and at 65 K it undergoes a transition into a ferromagnetic phase. Using neutron diffraction techniques on a single crystal of La(1.4)Sr(1.6)Mn(2)O(7) it has been shown that these two magnetic phases belong to a single structural phase and do not coexist at low temperatures. The application of pressure enhances the antiferromagnetic correlations between the Mn(2)O(9) bilayers.

We have studied the magnetic properties of the frustrated triangular antiferromagnet Gd3Ga5O12 (GGG) by means of classical Monte Carlo simulations. Low-temperature specific heat, magnetization, susceptibility, autocorrelation function and neutron scattering function have been calculated for several models including different types of magnetic interactions and with the presence of an external magnetic field. In order to reproduce the experimentally observed properties of GGG, the simulation model must include nearest neighbor exchange interactions and also dipolar forces. In zero field there is a tendency to form incommensurate short-range magnetic order around positions in reciprocal space where antiferromagnetic Bragg peaks appear in an applied magnetic field.

We describe small-angle neutron scattering (SANS) studies of vortex structures in the high T c superconductor YBa 2 Cu 3 O 7 in the mixed state. The SANS technique is a unique tool for probing the crystallography of a flux line arrangement in the bulk sample that can measure the values of the characteristic lengths and the effect of defects on lattice arrangement and orientation extremely accurately. The effects of crystal anisotropy on the field distribution in a vortex line is the topic of this paper

We present results for the specific heat of a Nd1/2Sr1/2MnO3 crystal. The continuous phase transition from paramagnet to metallic ferromagnet is observed at 250K and a first order transition from metallic ferromagnet to the insulating charge ordered state is observed at 150K in zero field. Hysteresis is observed on warming and cooling, which increases in a magnetic field. The magnetic field is seen to drive the low temperature charged ordered state into the ferromagnetic state through a first order transition.

We have examined the vortex dynamics in both as-grown and columnar-defected BSCCO 2212 crystals where the latter have been irradiated to produce columns alongs the crystallographic c-axis. Both the magnetisation and the (dynamic) relaxation rate have been measured as a function of field and temperature. Using a scaling analysis we extract the form of the energy scales of the characteristic critical current and pinning.

We present a detailed study by the Bitter decoration technique of the mixed vortex chains-FLL phase found in tilted magnetic fields in BSCCO. We show that this new vortex phase can be understood if one assumes that the chain and FLL vortices have two different orientations, both not parallel to the applied field.

Specific heat data on the new quaternary magnetic superconductor ErNi 2 B 2 C is presented in the temperature range 20–300K. Crystalline Electric Feild Effects (CEF) are confirmed by the existence of a Schottky anomaly around 45K with a magnitude of 10 Jmol −1 K −1 . The Schottky peak is analysed in terms of a CEF levels scheme based on neutron data.

Neutron scattering has been used to observe diffraction from the flux-line l function of temperature(0–40K) and magnetic field(0–2T). An anomalous temperature and magnetic field dependence of the intensities of the Bragg reflections has been observed.

In type II superconductors, like the new high Tc oxide materials, if you cool acrystal below the superconducting transition in an applied magnetic field, B>Bc1, the system minimizes its free energy by distributing the flux, in units of single flux quanta, on a lattice. This should be a perfect experiment for the loveable neutron with its ability to detect variations of magnetic field in a sample. There exist several studies of these effects in conventional superconductors but, as usual for high Tc systems, weird and wonderful things are expected. The theoreticians predict: no flux lattice only a glassy state, a lattice at low T which will melt below Tc, entangled-disentangled configurations in the liquid state, a vortex-plasma of flux-lines (whatever that may be) and so on. Its the usual story, we need some experiments to separate fact from highly entertaining theoretical models. However, the experiments are not that easy…

Magnetism can be one of the most applied fields of physics, yet also one of the most interesting to study at a fundamental level, because there is often a strong interaction between a basic understanding of magnetic properties and everyday applications. A recent refinement of many previous studies has revealed a particular composition of lanthanum strontium manganite that has ferromagnetic ordering, a structural phase transition that is sensitive to the composition or magnetic field, and a colossal magnetoresistance (A Asamitsu et al. 1995 Nature 373 407).

A Comment on the Letter by J. W. Lynn, [ital et] [ital al]., Phys. Rev. Lett. [bold 72], 3413 (1994).

Neutron diffraction has been employed to study the magnetic ordering in the high-temperature superconducting compound GdBa/sub 2/Cu/sub 3/O/sub 7/. The compound is found to undergo an antiferromagnetic transition with the Gd magnetic moments ordering at 2.22 +- 0.07 K. The ordering is shown to be three dimensional with a doubling of the orthorhombic unit in all three directions. An analysis of the intensities of the observed magnetic Bragg reflections shows that the ordered magnetic moment per Gd atom is 7.4 +- 0.6..mu../sub B/ and lies along the c axis of the orthorhombic cell. It is unlikely that dipolar ordering can explain the occurrence of this magnetic phase transition where the system locks into a three-dimensional ordered arrangement at the Neel temperature, since the separation of the Gd atoms along the c axis is approximately three times that along the a,b axes.