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Values of ( delta beta / delta P)_T derived thermodynamically from the zero pressure ultrasonic data of Niksch and co-workers (1980) indicate that the absolute magnitude of the volumetric thermal expansion coefficient beta should be initially enhanced by pressure both at very low temperatures, where beta is negative, and at higher temperatures when beta becomes positive. Values of ( delta ²(C_P/T)/ delta P²)_T, similarly derived, are in qualitative agreement with measurements of C_P under pressure above 0.5 K, but at lower temperatures exacerbate an apparent inconsistency with thermal expansion data noted earlier by Brodale and co-workers (1986); it is unlikely that this anomaly is due solely to sample variation.

The authors consider the electronic momentum distribution in systems displaying spin-charge separation. In particular, then consider the t-J model in the large-N limit. Assuming that there is no Bose condensation of holons at low temperatures in one and two dimensions, the authors find that the momentum distribution of the electron shows no Migdal discontinuity in one and two dimensions. In three dimensions, the Migdal discontinuity prevails as a signature of a Fermi-liquid state due to the presence of a holon condensate.

Atomic-scale structural changes with composition in sputtered amorphous a-Si_1-yNi_y:H alloys with y varying from 0 to 1 have been studied by the X-ray absorption fine structure (EXAFS) technique. Complementary structural information from infrared (IR) spectroscopy has also been obtained. The EXAFS results indicate that there is a significant change in the local environment of the Ni atoms as their concentration is changed and the system appears to be chemically ordered; in the sense that the number of Ni-Ni bonds is minimized. The Si-Ni distance shows no significant variation with concentration and is the same as that found in crystalline nickel silicide. The Si-Ni Debye-Waller factors are also almost independent of composition. A correlation between these results and the infrared spectra is established.

Single crystals of CH₃NH₃CdBr₃ (MACB) have been studied as a function of temperature by means of differential scanning calorimetry, X-ray diffraction and Raman scattering measurements. A first-order structural phase transition is evidenced by all three techniques at 170 K. The high-temperature phase (I) is hexagonal, with space group P6₃/mmc and Z=2 formula units per unit cell. The X-ray diffraction patterns of the low-temperature phase (II) are consistent with a complex orthorhombic unit cell with lattice parameters such as a_II approximately=4a_hex, b_II approximately=2 square root 3a_hex and c_II approximately=3c_hex, i.e. corresponding to Z=96; however, it is shown that the system can be described in a simplified manner by means of an orthorhombic 'projection' unit cell onto the hexagonal plane with parameters such as a_p approximately=a_hex, b_p approximately= square root 3a_hex and c_p approximately=3c_hex. The Raman data clearly indicate that the phase transition is of the order-disorder type, owing to the reorientational processes of the methylammonium groups.

The authors present a new approach, based on the effective-medium theory of metallic cohesion, to the study of alloy phase stability. The efficiency and applicability of the method is demonstrated by studying ordering in Cu-Au alloys. The ground-state properties of the three stoichiometric Cu-Au structures are found to be in excellent agreement with experiment. Monte Carlo simulations have been carried out in an effort to understand the finite-temperature phase transitions. These simulations give a very good description of the complete temperature-concentration phase diagram, and illustrate the importance of going beyond a fixed-lattice Ising model in describing phase transitions in these alloys.

In structurally incommensurate systems phason and amplitudon thermal fluctuations of the frozen-in modulation wave provide for two different nuclear magnetic resonance (NMR) spin-lattice relaxation mechanisms. These two relaxation mechanisms influence different parts of the inhomogeneously broadened NMR absorption spectrum differently. They can be resolved in principle by a conventional one-dimensional measurement of the spin-lattice relaxation time T₁ over the NMR lineshape. Here the authors show that a two-dimensional NMR spin-lattice relaxation technique provides a very convenient way of determining the variation of T₁ over the lineshape by the use of normalized contour plots. The results for the central transition of ⁸⁷Rb in Rb₂ZnCl₄ show that the measured phason-induced spin-lattice relaxation time is temperature-independent. The much less efficient amplitudon relaxation mechanism is bypassed in part by the cross-relaxation process of the amplitudon-relaxed parts to the phason-relaxed parts of the NMR lineshape. The theoretical prediction for the total variation of T₁ over the lineshape for the cross-relaxation process agrees with the experimental value of three.

The magnetoresistance (MR) of two re-entrant spin-glass (SG) alloys in (Fe_0.65Ni_0.35)_1-xMn_x with x=9.1 at.% and 11.36 at.% have been measured over the temperature range 4.2-300 K using a new ultra-sensitive MR apparatus enabling operation in magnetic fields comparable with that of the Earth. In this temperature range, the MR exhibits three distinct features: (i) a hysteresis effect over the whole so-called ferromagnetic (FM) region which generally saturates in only a few Gauss followed by reversible behaviour at higher fields; (ii) predominantly positive MR under low field conditions indicative of FM-type coupling between moments; but (iii) in the x=11.36 at.% sample near the FM-paramagnetic (PM) and FM-SG boundaries, the MR changes sign in higher fields and becomes negative. The results are discussed in terms of a recently proposed magnetic phase diagram for this system. The results support the existence of a mixed FM-like and SG phase but it is proposed that the 'FM' phase is mixed also with little or no long-range ferromagnetism. Instead, a system of FM-coupled clusters and a system of weakly coupled moments is indicated by the results, the latter behaving paramagnetically near the so-called Curie temperature but freezing into an SG state at lower temperatures.

The interplay of a direct ferromagnetic exchange interaction I>0 between local and band states and an indirect antiferromagnetic exchange -J<0 via hybridization V is studied using an expanded version of the Anderson impurity model in the highly correlated regime. A simple scaling idea is put forward for the dependence of the free energy, one-particle spectra, T-matrix and susceptibility on the size of hybridization and direct exchange. It describes the many-body regime J>>I in terms of a universal picture as well as the crossover near J approximately=I to the trivial regime J<<I, the latter being characterized by an effective elastic one-particle scattering problem. These physical arguments are supported by extensive numerical calculations in the framework of perturbation theory with respect to I and V. It is also indicated how I may be included into new post-NCA theories of the Kondo effect.

The magnetic properties of amorphous (Fe_1-xCo_x)_77.5Nd₄B_18.5 alloys before and after crystallization were studied. The Fe atom moment mu _Fe is found to increase with increasing x from 2.0 mu _B for x=0 to a saturation value of 2.3 mu _B for x=0.7 and then to decrease, when the Co atom moment mu _Co is assumed to be constant for all x. The variation in Fe moment is in good agreement with the Co concentration dependence of the average hyperfine field. The Curie temperature T_C is found first to increase and then to decrease, attaining a maximum value of 895 K at about x=0.7. This variation in T_C with x is calculated using a phenomenological model described by Kouvel (1969), the exchange interaction temperatures T_Fe-Fe, T_Fe-Co and T_Co-Co are calculated to be 584 K, 1010 K and 825 K, respectively. With increasing Co concentration x, the crystallization temperature is approximately constant for x<or=0.7 and then increases slightly with increasing x. The crystalline phases in crystallized samples were identified by X-ray diffraction. A high room-temperature coercive field H_c and a high-energy product (BH)_max for the optimally annealed samples are obtained.

The authors present self-consistent spin-polarized electronic structure calculations for realistic models of amorphous transition metal alloys. The atomic structure is prepared by a simulated molecular-dynamic quench, based on interatomic forces calculated using hybridized nearly-free-electron tight-binding-bond theory. The electronic structure is calculated in the local spin-density approximation, using a linear muffin-tin orbital (LMTO) supercell approach. Detailed results for crystalline and amorphous alloys of Ni, Co and Fe with Zr are presented. Ni_xZr_1-x alloys are predicted to be paramagnetic for x<or=0.85, both in the crystalline and in the amorphous state. In Co_xZr_1-x the onset of magnetic ordering occurs at x approximately=0.67 for crystalline and at x approximately=0.50 for amorphous alloys. The Co-rich alloys are predicted to be strong ferrimagnets. The formation of the negative Zr moments is related to a covalent coupling of Co- and Zr-d states, which is strongest for the Co minority-spin states. The enhancement of magnetism in the glassy alloys is related to an increase of the density of states at the Fermi level induced by structural disorder. In both crystalline and amorphous Fe_xZr_1-x alloys, the onset of ferrimagnetic ordering occurs at x approximately=0.33. In contrast to the Co-based alloys, Fe_xZr_1-x alloys are weak magnets. For x>or=0.75 the competition between ferromagnetic and antiferromagnetic exchange interactions leads to the formation of negative moments on isolated Fe sites. The number of negative Fe moments increases strongly for x>or=0.90, leading to a decrease of the average moment in the Fe-rich limit. The authors show that the formation of negative local Fe moments is related to a large number of contracted Fe-Fe pairs. The predictions of local spin-density theory are found to be in excellent agreement with experiment. For all crystalline and amorphous alloys the local exchange splitting of the 3d states is shown to be correlated linearly with the local magnetic moment, with a slope of approximately=1 eV mu _B^-1. This relation holds for all types of magnetic order and also for the crystalline and amorphous pure metals.

The electron paramagnetic resonance (EPR), optical absorption and luminescence spectra of Ti³⁺ ions in YAlO₃ have been measured at low temperatures. The optical absorption spectrum is composed of two broad bands with peak wavelengths of 434 and 492 nm, the energy separation of which is due to splitting of the excited ²E state. The broad-band luminescence with a peak at a wavelength of 605 nm at low temperatures is accompanied by a single sharp zero-phonon line at 540 nm. The absorption and luminescence spectra are strongly polarized. The spin-Hamiltonian parameters of Ti³⁺ ions substituting for Al³⁺ ions in sites with orthorhombic symmetry were determined from the orientation dependence of the EPR spectra to be g_x=1.795(5), g_y=1.850(5) and g_z=1.950(5). The experimental g-tensor is compared with that calculated in terms of the mixing of the higher components of the ²T₂ ground states and the ²E excited states into the lowest ²T₂ ground state by spin-orbit interaction, the energies of these levels having been estimated from the broad-band absorption spectrum and the zero-phonon lines of the emission spectrum.

The ENDOR spectra of Cr³⁺ ions in natural MgAl₂O₄ spinel have been measured at 4.2 K and the hyperfine and quadrupole interactions with the nearest-neighbour ²⁷Al nuclei and with the ⁵³Cr isotope have been investigated. The results show that Cr³⁺ has the point symmetry 3m, being subjected to the original trigonal expansion at the B sites, but inducing some distortion effects on its surroundings. Further, the isotropic hyperfine constant of the ²⁷Al nuclei has been analysed in an independent bonding model, showing that there is appreciable transfer of unpaired spin to the aluminium ion due to covalency.

This paper discusses line broadening by compositional disorder and by spin-spin exchange interaction between the paramagnetic Cr³⁺ and Gd³⁺ ions in antiferromagnetic Gd₃Sc₂Al₃O₁₂ (GSAG) probed using fluorescence line narrowing (FLN) in the absence and presence of a magnetic field. The most intense R₁ lines in Cr³⁺:GSAG, occurring at lambda =691.2 nm and 692.4 nm, have halfwidths of approximately=32 cm^-1. The linewidths of the FLN spectra at 1.6 K are respectively 19 cm^-1 and 13 cm^-1, much larger than the width of the excitation laser beam (<0.2 cm^-1): thus the contributions to the R₁-line halfwidth from mechanisms of homogeneous and inhomogeneous broadening are approximately equal. In addition, the peaks of the FLN lines are shifted to lower energy from the laser excitation line by approximately=8 cm^-1. As the temperature is increased gradually from 1.6 K to 10 K, the FLN peak is shifted to higher energy and is resonant with the excitation above 10 K. In the presence of an external magnetic field the broad FLN spectrum becomes resolvable into three sharp lines with the width of approximately=3 cm^-1 at T=1.6 K. The peak of the line with maximum intensity is coincident with the excitation energy. The FLN spectrum at T=10 K and B=3.46 T is similar to that at T=10 K in the absence of a magnetic field. The temperature dependence and magnetic field dependence of the FLN spectra are discussed in terms of spin-spin exchange interaction between Cr³⁺ and Gd³⁺ ions in antiferromagnetic GSAG.