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High-resolution neutron diffraction results for KH₂PO₄ (KDP) and its deuterated form (DKDP) have been used to re-examine evidence that the large increase in the phase transition temperature, T_c, on deuteration can be explained simply by the increase in the phase transition temperature, T_c, on deuteration can be explained simply by the increase in the O...O hydrogen-bond length, 2R, without invoking proton tunnelling effects. It is shown that deuteration at constant 2R still gives rise to an increase in T_c by about 40 K. This may represent the direct proton tunnelling effect on T_c. However, there are other geometric changes on deuteration at constant 2R, and the effect of these needs to be assessed before any more definite conclusion can be reached.

The authors present experimental results showing that the quantised nature of the ballistic resistance in narrow channels is preserved when the electrons pass ballistically through two narrow constrictions. As the width of each narrow channel is varied independently, the resistance of the pair is equal to the resistance of the narrowest; this is explained by the conservation of quantum (sub-band) number. The absolute quantisation is not as accurate as observed in a single constriction and is modified by an anomalous resistance whose origin the authors discuss.

The authors present results on the quantum conduction properties of a one-dimensional channel, 0.7 mu m long and 0.3 mu m wide, containing two potential barriers. The barriers are 0.5 mu m apart and 0.18 mu m wide and are defined by a gate on a high-mobility GaAs-AlGaAs heterojunction. The device behaves like a one-dimensional ballistic point contact with two narrow barriers in the middle. Reducing the width by electrostatic squeezing reveals quasi-periodic peaks in the resistance due to the resonance. At higher temperatures the effect of the barriers is removed so that the peaks disappear leaving the quantised plateaus associated with a one-dimensional ballistic point contact. A magnetic field removes the structure when the level broadening becomes comparable to the level separation.

Polarised neutron reflection measurements are reported for Ag/Cr/Ag(001) sandwich structures. For a Cr layer equivalent to a coverage of 0.33 monolayers, the data suggest long-range ferromagnetic order with a magnetic moment per atom that is significantly greater than the bulk value. For a 3.3-monolayer-thick layer, no ferromagnetism is detected.

In YBa₂Cu_2.985Fe_0.015O₇, the iron substitution and (consequently) the effect on superconductivity (SC) are low. Magnetically split Mossbauer spectra are obtained in the presence of an external field of 5.5 T at low temperatures, and they have been fitted to spin relaxation spectra appropriate for Fe³⁺ ions in the high-spin state. The analyses show near-perfect diamagnetic shielding of a component spectrum at low temperatures, characterised by a low spin relaxation frequency and consequently a larger SC band gap Delta . It appears that planes containing Cu2 ions are mainly superconducting, though SC extends to a smaller degree to the Cu1 sites. The authors argue that the results do not imply coexistence of magnetism and SC but of isolated magnetic ions and SC.

Positron lifetime experiments have been performed on as-grown samples which had been isochronally annealed up to 820 K and plastically deformed and these experiments yield a constant lifetime of 282+or-2 ps which is attributed to bulk positron states in InSe. Electron-irradiated samples exhibit a two-component spectrum, revealing the presence of positron traps which anneal out at about 330 K. The nature of the native shallow donors in InSe is discussed in the light of the results, which support the idea that native donor centres are probably interstitial In atoms rather than Se vacancies. Positron trapping observed in the electron-irradiated samples is attributed to defects related to In vacancies.

To calculate the lattice dynamics of a crystal from a microscopic point of view, it is necessary to know the change in electronic charge density when phonons of a particular wavevector q are present in the crystal. It is shown how information about this change can be obtained from phenomenological models such as the movable bond-charge model. From these models, tensors T_mj, which relate the movement delta r_m of the subsystem m in the model to the movement delta R_j of an ion, can be calculated. The contribution of the subsystem to the pseudo-atom at R_j can be calculated from T_mj and the microscopic charge distribution which the subsystem represents. It is shown how the calculation simplifies when q is in a symmetry direction. When pseudo-atoms cannot be defined, it is better to calculate the Fourier components of the change directly. The theory is applied to the calculation of the tensors for the (100) direction of diamond-type crystals, using a version of the movable bond-charge model which incorporates a more realistic valence bond force.

Highly accurate specific heat capacity measurements were used in order to study the phase transition sequence of tetramethylammonium tetrachloromanganate (TMATC-Mn) between 50 and 330 K. This study allowed the authors to confirm that the incommensurate phase that had been previously found at pressures higher than 38 MPa is also present at atmospheric pressure. At this pressure the phase transition sequence is given by I-(292.6 K)-II-(292.2 K)-III-(267.0 K)-IV-(175.0 K)-V, where phase II is incommensurate and covers a very narrow temperature interval. Heating and cooling measurements allowed them to study the thermal hysteresis associated with the lock-in (II-III) phase transition. The other phase transitions were also characterised within the authors' general calorimetric study. The position of the compounds of the general family (N(CH₃)₄)₂MX₄ in its common pressure-temperature phase diagram could be related to the thermodynamic function values of their phase transition sequences.

Recent Los Alamos data obtained by Buchanan, Swift and Wheatley (see Phys. Rev. Lett., vol.57, p.341, 1986) on the propagation of the superfluid ³He A-B phase front in hypercooled ³He-A are here first suggested to manifest a novel phase transition-associated with a change in the discrete symmetry and the energy-gap topology of the interface. Symmetry analysis and numerical computations on the moving A-B border support such an interpretation and provide a tentative identification of the moving phase boundaries as functions of the A-B phase-boundary velocity. Further experiments are proposed, especially in high magnetic fields-where the possibility for yet another superfluid A-B interface transition is pointed out-and under rotation.

For pt.I see Aust. J. Phys., vol.36, p.321 (1983). An effective-Hamiltonian method that was introduced previously by Brown is further developed in order to deal with electron propagation in crystals whose distortion is manifestly non-uniform. Hermitian operators depending explicitly on the local distortion emerge naturally from an expansion of the one-electron wavefunction in terms of suitably defined Wannier functions centred on each atom of an inhomogeneously distorted structure. They represent spatially averaged dynamical variables and serve to determine semi-classical particle dynamics in an approximation whereby the discrete crystal structure is subsumed into the components of the elastic distortion tensor and a few effective-mass coefficients. It is shown that a satisfactory Hermitian theory can still be constructed when distortion gradients are explicitly considered and that both inter-band effects and the effects of elastic rotation can be included. Finally, and most importantly, an acceleration operator is derived having a reasonably simple structure and expressing the effects of non-uniform distortions on electron dynamics.

Polaronic conductivity in ternary oxides NbO_2.5-x-WO_3-y follows the Mott T^-1/4 law. The transport properties differ from those of the end-member binary oxides in the higher degree of localisation of carriers (2r_A approximately=6.4 AA). Comparison between transport in different crystal structures with various Nb to W ratios shows that the plasma frequencies omega _p depend on the total carrier concentration N via, omega _p²=( epsilon _x/4 pi e²)(m*/N), where m*=0.65m_e is the effective mass. The polaronic ground state is correlated mainly with W⁵⁺ states; Franck-Condon transitions occur at 0.7 eV (W⁵⁺ to W⁶⁺) and 1.1 eV (W⁵⁺ to Nb⁵⁺). An increasing degree of chemical reduction leads to an increasing carrier concentration. The polaronic part of the carrier concentration saturates at a critical Anderson point with N_A=6*10²¹ cm^-1, with a simultaneous appearance of polarons and quasi-free carriers for higher concentrations.

The author reports detailed electron spin resonance (ESR) studies (300-77 K) of the high-temperature superconductor YBa₂Cu₃O_7-x for samples (type A) have a broad transition (T_c0-T_conset=13.5 K) as well as for samples (type B) with a sharp transition (T_c0-T_conset=3 K). The room-temperature axial symmetry around the Cu²⁺ ion in both cases is maintained down to T_c0+or-2 K (T_c0 equals 86.5 K for type A samples and 96 K for type B samples) below which the symmetry changes gradually to orthorhombic at 77 K. In type B samples the orthorhombicity in the g tensor is larger than in type A samples. When the sample undergoes as many as five repeated cooling and heating cycles (within a week for case II), the superconducting transition becomes broader with T_conset=96 K and T_c0 is correlated with the superconducting properties. The infrared spectrum of case II is significantly different from that of case I. The EST linewidth in both cases (types A and B) is almost temperature independent; this is a rather surprising result for the ESR of Cu complexes. The nearly linear variation in ESR signal intensity I_S with temperature in type A samples shows the possible existence of Curie susceptibility whereas in type B samples the weak dependence of I_S on T may indicate possible Pauli paramagnetism.

Considering interactions to be purely of dipolar origin, the ground-state energy and magnetic ordering of some rare-earth trifluoromethane sulphonates (RTFMS) are obtained. The extended Luttinger and Tisza approximation for magnetic ions in crystals interacting through dipole-dipole and exchange interactions is considered. The predicted results agree with the existing experimental information for R=Tb, Dy and Ho, and for PrTFMS a limit for the next-nearest-neighbour exchange constant is obtained that will yield the antiferromagnetic ordered state as observed experimentally.