Table of contents

The research on spin-Peierls transition and impurity-induced antiferromagnetic state(s) in CuGeO₃ is reviewed. In particular the recent progress of the studies of the compositional phase diagram will be discussed. The discovery of two antiferromagnetic phases in Mg-doped CuGeO₃ (Masuda T et al 1998 Phys. Rev. Lett.80 4566) opened a new aspect of this kind of research. The transition between dimerized antiferromagnetic and uniform antiferromagnetic phases is of the first order as a function of the impurity concentration. This phenomenon was studied in detail by the susceptibility, neutron diffraction, synchrotron x-ray diffraction and other methods. The existence of this transition was firmly confirmed and it was revealed that the impurity-induced phase diagram of CuGeO₃ is very complex. Another important problem is whether the impurity-induced ordered phase in the spin-gap system is unique to the S = ½ spin-gap system or not. As to this problem a new Haldane-gap compound PbNi₂V₂O₈ was found (Uchiyama Y et al 1999 Phys. Rev. Lett.83 632). This compound has relatively strong inter-chain interaction. It was also found that the substitution of Mg²⁺ ions (S = 0) for Ni²⁺ ions (S = 1) induces antiferromagnetic phase in PbNi₂V₂O₈. From these experimental results it was shown that the occurrence of the impurity-induced ordered phase is a common feature of quasi-one-dimensional spin systems with relatively strong inter-chain interaction.

In many disordered systems such as supercooled liquids the relaxation function does not decay exponentially with time. In this paper, it is shown from a consideration of the initial conditions that a system of identical molecules, each of which has the same non-exponential relaxation function, must be inhomogeneous. As a result, the relaxation function for the system is not the same as that of an individual molecule, contrary to what is often assumed. As an example, the general formulae are applied to a simple model system in which the dielectric relaxation function of the individual molecules is of the stretched exponential form. It is found that while the dielectric relaxation of the system can be well approximated by a function of this form, its stretching parameter and relaxation time differ from those for the individual molecules.

Transient gelation in colloid-polymer mixtures is an interesting but poorly understood non-equilibrium phenomenon which has recently attracted the attention of experiment and theory. In a transient gel the particles aggregate, by the depletion interaction, to form a space-spanning network which maintains its structural integrity for a finite period before suddenly collapsing. In this paper we present a study of the collapse process which provides new information on the way in which transient gels collapse. We have studied gel collapse in three ways. Firstly, we have observed the gel throughout its lifetime using dark-field optics and time-lapsed video recording. In a second experiment we have measured the concentration of gels throughout their entire height before, during and after collapse. This was performed using a non-intrusive ultrasonic technique which measures the speed of sound in the suspension. Finally, we have studied the effect of varying sample height and width on the lifetime of a gel. Our observations provide new information on the collapse process and have important implications for the formulation of a theoretical model. In discussing the effects of height and width we refer to the following companion paper which describes this particular effect.

Following a quench, colloidal systems with strong, short-ranged, attractive interactions can exhibit transient gelation, instead of the classical phase-ordering mechanisms of spinodal decomposition or nucleation. The particles aggregate into a tenuous, system-spanning network, which, for a time, remains robust to mechanical disturbance. Eventually, the network's ability to recover from destructive deformations becomes compromised, and the gel collapses. A detailed experimental study of gel collapse was reported in the preceding, companion article, leaving several open questions regarding the processes involved. We present a theoretical investigation into the factors affecting a gel's lifetime, concentrating in particular on the surprising influence of the size and shape of the container. We construct a model in which solvent dynamics are replaced by a dissipative coupling of the particulate network to a fixed frame and show that, in the absence of zero-frequency elasticity, such a coupling results in a novel class of matter in which stresses cannot propagate beyond a finite distance. We find our prediction of the characteristic length to be in quantitative agreement with the experimental data, and show how its ratio to the dimensions of the container controls the sedimentation. We discuss some aspects of the ageing mechanism, and suggest that a sudden collapse is ultimately due to erosion with positive feedback.

We have reported the electrical conductivity and the conductivity relaxation in fluoride glasses in the system 40ZrF₄-20BaF₂-10YF₃-(30-x)LiF-xNaF; 0⩽x⩽30. We have observed a mixed mobile ion effect in these mixed alkali fluoride glasses. Using modulus formalism we have shown that the conductivity relaxation also exhibits the mixed mobile ion effect as does the dc conductivity. We have observed that the relaxation mechanism is independent of temperature but dependent on the composition.

Resonant x-ray scattering spectra of UO₃ were measured at the U M₅ edge using monochromatic photon excitation. Large differences in inelastic scattering profiles were observed between two 90°-scattering geometries with the polarization vector of incident synchrotron radiation parallel and perpendicular to the scattering plane, respectively. For the latter case, a ~9.5 eV energy-loss structure, associated with the O 2p→U 5f charge-transfer satellite, shows a significant intensity enhancement. The observed resonance behaviour allows an accurate estimation of the energy of this satellite, which is obscured in the other experimental geometry, and indicates the charge-transfer origin of the U M₅ absorption structures at the respective excitation energies.

Calculations of one-electron spectral functions, optical conductivity and spin-wave energy in the Holstein double-exchange model are made using the many-body coherent potential approximation. Satisfactory agreement is obtained with angle-resolved photoemission results on La_1.2Sr_1.8Mn₂O₇ and optical measurements on Nd_0.7Sr_0.3MnO₃. A pseudogap in the one-electron spectrum at the Fermi level plays an important role in both systems, but a small-polaron band is only predicted to exist in the La system. A rigorous upper bound on spin-wave energies at T = 0 is derived. The spin-wave stiffness constant D decreases with increasing electron-phonon coupling g in a similar way to the Curie temperature T_C, but D/(k_BT_C) increases for large g (low T_C) as observed experimentally.

We report on the observation of a long-lasting phosphorescence phenomenon in GeO₂ and Er³⁺-doped GeO₂ glass samples fabricated by the sol-gel method. Absorption spectra showed that there were oxygen-deficient defects associated with Ge ions in the samples. Irradiation with ultraviolet light at 254 nm induced long-lasting phosphorescence with a peak at 465 nm for the GeO₂ glass sample. The intensity of the phosphorescence decreased with increasing temperature and in inverse proportion to the time after a quick decrease of the intensity. The long-lasting phosphorescence in these samples is considered to be due to the thermally activated electron-hole recombination at room temperature.

We have studied the evolution of the spin-Peierls temperature T_SP in pure and 2% Zn-doped CuGeO₃ as a function of hydrostatic pressure P up to 9 GPa. The transition temperature has been determined by ac-calorimetry measurements inside anvil cells under high-quality hydrostatic conditions achieved by using helium or argon as the transmitting medium. The pressure was tuned in a continuous way at low temperature, which enables us to explore the diagram T_SP = f(P) very precisely. We have shown that, in the two compounds, T_SP increases with P up to the highest pressure we reached. Moreover, two different linear stages are clearly evidenced. Both features could be consequences of the existing frustration due to the second-neighbour magnetic interaction.

We present calculations of the electronic and atomic structures of neutral and charged nitrogen vacancies in Al_xGa_1-xN alloys using a combination of first-principles methods. The treatment of the alloys is based on the generalized quasichemical approach to disorder and composition effects and a cluster expansion to account for the various configurations. The point defects are modelled by supercells which are multiples of the alloy clusters. The total-energy and electronic-structure calculations are performed within the density functional theory and the local spin density approximation. We study the atomic structure, the energetics and the charge-dependent vacancy states for the different clusters versus cation numbers and for the alloys versus composition x.

The DC transport properties of and microwave absorption (at 41 GHz) in a bulk ceramic sample and a film (220 nm thick) of La_0.5Sr_0.5CoO_3-δ are measured. The bulk sample was cut from a target from which the film was also produced, by pulsed-laser deposition. It is found that the temperature behaviours of the DC resistivity and magnetoresistance (MR) of the bulk sample are quite different from those of the film. This is attributed to oxygen depletion of the film as compared with the target. Below the Curie temperature T_C, the film behaves like a highly inhomogeneous system of weakly connected ferromagnetic grains (or clusters). The microwave study provides further data on the inhomogeneity of the samples. It is found that the surface layer of the bulk sample has very low conductivity compared with the bulk. This can be explained by the oxygen depletion of the surface layer. The most important feature of doped cobaltates revealed in this study is the following: the microwave conductivity, which should be related mainly to the conductivity within the poorly connected grains, increases by an order of magnitude at the transition to the ferromagnetic state. The increase is much greater than that found in the reported DC measurements on doped cobaltates of the highest crystal perfection. The microwave effect found is attributed to an inherent magnetically inhomogeneous state of the doped cobaltates. On the basis of the results obtained, it can be suggested that the rather low MR in the doped cobaltates as compared with that of the manganites is attributable to their more inhomogeneous magnetic state.

The mechanical behaviour of fully dense nanocrystalline Ni-19Fe (grain size \mbox{36-37} nm) was investigated by compression and deformation-trace observations with optical microscopy and atomic force microscopy in the temperature range from -162 to 600 °C. The results indicated that yield stress (σ_0.2) increased with increasing strain rate with strain-rate sensitivity exponent m = 0.01 at ambient temperature. At the temperatures T<100 °C or T>400 °C yield strength decreased gradually with increasing temperature, while it dropped catastrophically in the range 100 °C<T<400 °C. Low-temperature deformation produced two sets of macroscopic bandlike traces which oriented at around 55° to the compression axis and consisted of ultra-fine straight lines; in contrast, high-temperature deformation led to formation of microcavities accompanying which some nanograins arranged locally in regular form. The results on both mechanical behaviour and deformation morphology suggest that the deformation mechanism at low temperatures was different from that at high temperatures, with the transition temperature locating at T_c~340 °C.

In this paper we present calculations of thermodynamic functions within Zhang's SO(5) quantum rotor theory of high-T_c superconductivity. Using the spherical approach for three-dimensional quantum rotors we derive explicit analytical formulae for entropy and specific heat related to the lattice version of the SO(5) nonlinear quantum sigma model. We present the temperature dependence of these quantities for various settings of relevant control parameters (quantum fluctuations and chemical potential). We find our results in overall qualitative agreement with basic thermodynamics of high-T_c cuprates.

We have investigated pentagonal multi-shell-type Cu nanowires using classical molecular dynamics simulations. Each pentagonal multi-shell-type nanowire is composed of a central atomic strand and pentangular tubes made of five-times-folded {{ }100{} } sheets. Their structural properties are similar to those of face-centred-cubic structures, as each of the subunits has a quadrangular-pyramid shape oriented in the ⟨100⟩ direction. Our simulations show that, at room temperature, ultrathin pentagonal Cu nanowires are seldom found; however, pentagonal multi-shell-type Cu nanowires, with diameters of several nanometres, are found in the stable structures.

The effects of the supercell approximation in first-principles calculations for isolated, charged point defects in semiconductors and insulators are studied. The convergence of the Coulomb energy with respect to the supercell size is investigated. Quantitative numerical results for the standard uniform compensating charge and the newly proposed localized compensating charge scheme are presented for a prototypical defect, the doubly positive silicon self-interstitial.

The inelastic scattering of low-energy electrons with the mobile part of the electron density of free-electron-like materials is investigated. Based on the dielectric theory for the homogeneous electron gas, the concept of Bohm and Pines is adopted in order to separate the single-particle and collective basic channels of the total inelastic rate. An effective screened potential is introduced to describe the separated single-particle part. The role of the relative motion of electrons, a kind of dynamical correlation effect, is modelled in this potential via a physical argument. The results obtained show that the nontrivial correlated motion of electrons may have a measurable influence on the result of dynamical probing of a degenerate electron gas.

We derive explicit spin and charge correlation functions of the N×N Hubbard model from a recently obtained weak-coupling analytic ground state |Ψ^[0]_AF⟩. The spin correlation function shows an antiferromagnetic behaviour with different signs for the two sublattices and its Fourier tranform is peaked at Q = (π,π). The charge correlation function presents two valleys at 45° from the axes. Both functions behave in a smooth way with increasing N; the results agree well with the available numerical data.

In this detailed study we have reported on single-crystal and polycrystalline phases of V_1+xS₂; 0.10<x<0.25, prepared using excess sulfur in a sealed quartz tube and investigated their structural, transport and magnetic properties. The crystal flakes have trigonal symmetry with 2c and incommensurate {>}2(√3)a superlattice ordering in the a-b plane. As-grown flakes showed complex irreversible behaviour in resistivity on cycling to low temperatures, related to the metastability of the structure. The annealing of the crystal flakes and addition of 5-10 percent Al stabilizes the superstructure. We observe an anomalous contribution to the resistance of these crystal flakes with a maximum around 100-150 K. The contribution is more pronounced for better-ordered phases. The magnetic susceptibility and thermopower values of these compounds are large and vary smoothly on cooling around this temperature interval. The polycrystalline phase, obtained at a higher temperature, on the other hand, showed absence of superlattice distortions and gave a smooth behaviour in its resistance, but with a large T²-contribution. The structural and electronic properties of different phases are discussed in terms of disorder among the interstitial V atoms and the effect of in-plane vacancies on the charge-density-wave instability in these and similar compounds.

Specific features in the electronic structure of Al-transition metal quasicrystals are analysed by a combination of Auger and x-ray photoelectron spectroscopies. We first demonstrate that different degrees of asymmetry in the transition metals' 2p core-level lineshape observed across different types of surface structure correspond to variations in the density of states at the Fermi level, DOS(E_F). Using this effect, we explore the controversial issue of whether the quasicrystalline, decagonal AlNiCo system is electronically stabilized. We find strong evidence for the presence of a reduced DOS(E_F) in this system, as expected for electronically stabilized compounds, and as observed in the quasicrystalline, icosahedral AlPdMn and AlCuFe alloys. Finally, qualitative information on the nature of the electronic states in quasiperiodic structures extracted from the core-valence-valence Auger lines are presented and discussed.

An inelastic neutron scattering and ¹⁶⁶Er Mössbauer spectroscopy study was performed on ErCo₂Si₂ and a ¹⁶⁹Tm Mössbauer spectroscopy study was performed on TmT₂Si₂ (T = Co and Ni). The saturation Er moment in ErCo₂Si₂ was found to be 7.8 µ_B and the saturation Tm moment in TmCo₂Si₂ was determined to be 6.5 µ_B. A crystal field interpretation of ErCo₂Si₂ and TmT₂Si₂ (T = Co and Ni) is presented and, together with a new interpretation for existing data for T = Cu, trends in the crystal field parameters within the TmT₂Si₂ series are presented.

This paper (Diao et al 2002 J. Phys.: Condens. Matter14 L57-62) was submitted by Dr Diao without the knowledge of the other authors while the work was still at the research stage. The complete results will be given in a forthcoming publication after the results have been confirmed.

The research was a collaboration of The Institute of Physical and Chemical Research (RIKEN), Saitama, Japan and Shizuoka Institute of Science and Technology, Japan. None of the experimental work was carried out at Tianjin University, People's Republic of China.