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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.

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 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.

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.

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.

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.

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.

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 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.

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

Fe3(PO4)2, sarcopside, displays unusual magnetic properties which have been investigated using time-of-flight powder neutron diffraction and magnetic susceptibility measurements. The structure contains chains of three edge-shared FeO6 octahedra with two inequivalent iron sites, the central M(1) site possessing a fairly regular symmetry and smaller volume compared with the two distorted M(2) sites of the terminal octahedra. In the absence of an applied field, the compound orders antiferromagnetically, with the M(2) sites in each chain having opposite spin directions along the [100] direction, leaving the central M(1) cation frustrated with no net spin. Under applied magnetic fields of 0.7 T, only the bicritical transition from ferrimagnetic to paramagnetic is observed at 44(1) K.