Based on the SU ( n )-algebra the Markoff master equation in discrete product space is reformulated to explicitly deal with composite systems. The resulting local (single node) and nonlocal (multi-node) state parameters allow a systematic approach to non-classical features of the state, like variance and covariance tensors. For local optical driving forces, inter-node interactions, and local damping channels the solution of the master equation is unraveled into stochastic quantum trajectories. Sampling leads to a joint distribution function in terms of those state parameters. Its linear moments define the ensemble-density matrix. The average variance and covariance are in terms of non-linear moments, which should be distinguished from their entirely statistical counterpairs. Non-classicality of the network dynamics is shown to reflect itself in the luminescence-photonstatistics.
We report a detailed synchrotron x-ray scattering study of the phases and phase transitions in the mixed Ising-XY magnetic system with quenched randomness: Fe x Co 1− x TiO 3 ( x =0.35, 0.50, 0.65 and 0.75). For concentrations x =0.35, 0.50 and 0.65, we observe at high resolution a breakup of both the magnetic and the atomic structures of the crystal into domains, as well as a uniform lattice distortion following the ordering of the XY spin components. We argue that this breakup into domains in the XY phases results from random anisotropy, random field and magnetoelastic effects in Fe x Co 1− x TiO 3 . In particular, we find that in random anisotropy XY magnets, there exists a novel phase transition which is critical, but involves no long-range ordered phase. In addition to the XY behavior, the Ising spin component in the mixed phase ( x =0.65) is found to break into domains following the (short range) ordering of the XY spin components. Specifically, the scattering profiles of the low temperature mixed states are well described by a Lorentzian squared cross-section, which in three dimensions corresponds to exponential decay of the real space spin-spin correlations. This loss of the long-range order of the Ising order due to the ordering of the XY spin components after initial establishment of the Ising order on cooling is difficult to understand within our current picture of the random field Ising model. We have also carried out a detailed study of the magnetic field effects on phase transitions in the mixed Ising random magnet Fe 0.75 Co 0.25 TiO 3 , for fields up to 2.9T. It is found, as in the diluted Ising antiferromagnets Mn x Zn 1− x F 2 and Fe x Zn 1− x F 2 , that when the sample is cooled in the presence of a field, it evolves from the high temperature paramagnetic phase to a low temperature domain state. The low temperature scattering profiles are well described by a Lorentzian squared cross-section. However, if the sample is cooled below the Neél temperature T N in the absence of a field, and a magnetic field is subsequently applied, the long range magnetic order persists on warming, up to a well defined field-dependent metastability temperature, T M ( H ). The shedding of this LRO in the metastable region is consistent with the “ trompe l’oeil critical behavior” description, with a β ZFC ∼0.17. The depression of the metastability temperature in magnetic fields can be well described by the form T M ( H )= T N (0)− bH 2 − aH 2/φ , with the best fit value for the crossover exponent φ =1.2(1). This smaller value (than the theoretical value φ =1.4) for φ probably arises from the close proximity of a multicritical point at higher fields. At the superlattice reciprocal lattice point (1, 1,–1.5), we observe a drastic field-dependence of the x-ray, but not the neutron, scattering intensity. This additional x-ray intensity is believed to arise from an induced staggered charge density. In particular, the quadratic magnetic field dependence of the additional intensity is consistent with a lattice and magnetism coupling of the form, ρ s M s M , where ρ ss is a staggered charge density, M s the staggered magnetization and M the uniform magnetization.
Thermal conductivity (κ) of two series of high-temperature superconductors with general formula Sm1+x Ba2-x Cu3Oy have been measured. Both series begin with the same sample of T c = 90.4 K and extend to nonsuperconducting phases. The first series is of 123 cation stoichiometry and variable oxygen content y, the second is a series of solid solutions with variable x. The temperature dependences of κ are very similar for superconducting partner samples from both series (i.e. with the same T c), whereas the nonsuperconducting samples reveal dramatic differences. We propose to attribute the huge increase of κ and the change of its temperature characteristics of insulating oxygen deficient sample to some additional heat carriers, supposedly of magnetic origin. Absence of this additional heat transport mechanism for insulating solid solution allows to treat it as a proper reference for estimation of phonon contribution in the superconducting 123 compounds.
The magnetic ordering of Er ions in Er 2 Ba 4 Cu 7 O 15-δ ( δ = 0:7 and δ = 0:08) has been studied by low temperature heat capacity measurements. The Néel temperature T N of Er 2 Ba 4 Cu 7 O 15-δ is almost independent of the oxygen concentration (TN = 0:54K for δ = 0:08 and T N = 0:50K for δ = 0:7). While an anisotropic two-dimensional (2D) Ising model describes the experimental data very well for Er 2 Ba 4 Cu 7 O 14:92 , this model cannot be applied to the oxygen reduced sample Er 2 Ba 4 Cu 7 O 14:3 . By using a model consisting of 1D-Ising chains and of 2D-Ising clusters of Er 3+ ions we could fit the specific heat data of Er 2 Ba 4 Cu 7 O 14:3 accurately. Our measurements show clearly that with the reduction of the oxygen content the in-plane anisotropy of the magnetic exchange coupling increases.
The Hubbard model on a semi-infinite three-dimensional lattice is considered to investigate electroncorrelation effects at single-crystal surfaces. The standard second-order perturbation theory in the interaction U is used to calculate the electronic self-energy and the quasi-particle density of states (QDOS) in the bulk as well as in the vicinity of the surface. Within a real-space representation we fully account for the non-locality of the self-energy and examine the quality of the local approximation. Numerical results are presented and discussed for the three different low-index surfaces of the simple-cubic lattice. Compared with the bulk significant differences can be found for the top-layer local self-energy, the imaginary part of which is energetically narrowed and has a reduced total weight. The non-local parts of the self-energy Σ ij(E) decrease with increasing distance between the sites i and j. At the surface and for the three-dimensional bulk their decrease is faster than for a two-dimensional lattice. For all surfaces considered the effects of the non-local parts of the self-energy on the QDOS are found to be qualitatively the same as for the bulk: The weight of the quasi-particle resonance at the Fermi energy is lowered while the high-energy charge-excitation peaks become more pronounced. The main structures in the layer-dependent spectra are already recovered within the local approximation; taking into account the nearest-neighbor non-local parts turns out to be an excellent approximation. Due to the reduced coordination number for sites at the very surface, the top-layer QDOS is narrowed. Contrary to the the free (U = 0) system, quasi-particle damping results in a comparatively weak layer dependence of the QDOS generally. Pronounced surface effects that are related to surface Friedel oscillations show up as a fine structure within the resonance around the Fermi level.
The screened electron-electron interaction in a multi-band electron system is calculated within the random phase approximation and in the tight-binding representation. The obtained dielectric matrix contains, beside the usual site-site correlations, also the site-bond and bondbond correlations, and thus includes all physically relevant polarization processes. The arguments are given that the bond contributions are negligible in the long wavelength limit. We analyse the system with two non-overlapping bands in this limit, and show that the corresponding dielectric matrix reduces to a 2 × 2 form. The intra-band and inter-band contributions are represented by diagonal matrix elements, while the off-diagonal elements contain the mixing between them. The latter is absent in insulators but may be finite in conductors. Performing the multipole expansion of the bare long-range interaction, we show that this mixing is directly related to the symmetry of the atomic orbitals participating in the tight-binding electronic states. In systems with forbidden atomic dipolar transitions, the intra-band and inter-band polarizations are separated. However, when the dipolar transitions are allowed, the off-diagonal elements of the dielectric matrix are of the same order as diagonal ones, due to a finite monopole-dipole interaction between the intra-band and inter-band charge fluctuations. We also calculate the macroscopic dielectric function and obtain an expression which interpolates between the well-known limits of oneband conductors and pure insulators. In particular, it is shown that the microscopic origin of the so-called selfpolarization corrections is the on-site interaction which exchanges two electrons at different orbitals, combined with a finite tunneling between neighboring sites.
The magnetic properties of Ce(Ru0.85Rh0.15)2Si2 were studied by neutron scattering and measurements of magnetization, susceptibility, specific heat and thermal expansion as a function of temperature. We observe a crossover from a high temperature localized spin to a low temperature heavy electron state. Spin density wave (SDW) behavior appears in the heavy electron state below TN = 5.5 K and the volume change due to spin quantum fluctuations associated with the SDW and the Kondo screening is reminiscent of moment-volume instabilities of the INVAR and anti-INVAR behavior of 3 d transition metal alloys.
We present the first measurements of magnetisation and Raman light scattering on alpha l- NaV2O5 single crystals. Below 34 K, we observe a pronounced isotropic decrease of the susceptibility indicating the opening of a spin gap. The transition temperature is slightly field dependent. Raman experiments reveal a crystallographic distortion at the transition. Our results clearly establish alpha' - NaV2O5 to be the second inorganic spin-Peierls system.
The (1 × 1) and (√3 × √3)R30° (T4) structures of Ga and As adatoms on the Ge(111) and Si(111) surfaces are studied using first-principles calculations. The surface energetics predicts, in some cases, a transformation of the T4 structure (surface covered with 1/3 monolayer (ML) of adatoms) into domains of the 1-ML covered (1 × 1) structure and areas of clean reconstructed suface. For As adatoms, such phase separation is favored on both substrates, while for Ga adatoms, it is only preferred on the Ge(111) surfaces. These results compare well with experimental observations.
The question of the universality of the longitudinal peak conductivity at the integer quantum Hall transition is considered. For this purpose, a system of 2D Dirac fermions with random mass characterised by variance g is proposed as a model which undergoes a quantum Hall transition. Whilst for some specific models the longitudinal peak conductivity σ xx was found to be universal (in agreement with the conjecture of Lee et al. as well as with some numerical work), we find that σ xx is reduced by a factor (1 + g /2π) −1 , at least for small g . This provides some theoretical evidence for the non-universality of σ xx , as observed in a number of experiments.
The single particle dynamics of a rigid NH 3 -molecule in an anharmonic mean crystal potential is analysed. The potential parameters used have been derived earlier from experimental neutron diffraction data on single crystals of Ni(NH 3 ) 6 X 2 ( X = I , NO 3 , PF 6 ): in all these compounds the ammonia molecules show dynamical orientational disorder. The mean crystal potential which is experienced by the three protons of one ammonia molecule is given by a two-dimensional anharmonic four-well potential, which leads to a coupling of the rotation of the molecule around its threefold axis to the translational motion of the molecular center of mass. Thus the dynamical problem is restricted to three degrees of freedom. The corresponding Hamiltonian equations of motion are solved numerically. Fourier analysis, reconstruction of trajectories in the six dimensional phase space and next-amplitude-maps from the simulated time series reveal either multiple periodic or chaotic solutions, depending on the potential parameters and the energy of the system. The anharmonic potential produces, as a generic property, three different kinds of proton orbits. At low energy, i. e. low temperatures, closed orbits related to hypocycloid functions occur. At intermediate temperatures the orbits are chaotic. High temperature simulations show circular orbits with a week high frequency jitter superimposed. Thus a crossover from weak localization via chaos to nearly free rotation is obtained by a variation of the energy in the simulation.
Optical mode crossing is not a plausible explanation for the new broad Brillouin doublet nor for the strong acoustic anomalies observed at low temperatures in SrTiO 3 . Data presented to support that explanation are also inconclusive.
We show that wearless atomic friction, recently observed by AFM techniques, is well desribed by the mechanism of “plucking of atoms” proposed by Tomlinson in 1929. Our results for a single-asperity contact yield a satisfactory fit of the experimental data. For extended contact areas, the relative orientation of the crystal lattices and the resulting misfit becomes important. The misfit gives rise to the formation of domains where the two surface structures are approximately in registry. The domains are separated by two sets of shift lines, crossing each other in topological defects. During quasistatic sliding, the whole domain pattern moves perpendicular to the driving force. Frictional behaviour occurs if the ratio of the coupling strength of the surface atoms to the bulk of the body to the potential barrier for sliding is less than a critical value. Dissipation and friction hysteresis are caused by irreversible jumps of these topological defects. The friction force depends strongly on both the sliding direction and the misfit angle.
The fascinating properties like giant magnetoresistance (GMR) effect, metal-insulator transition, charge ordering phenomenon etc. have made the divalent ion doped RMnO3 (R = rare-earth elements) an attractive system for investigation. Resistivity of these compounds shows a peak near the ferromagnetic transition temperature (T c ). The application of magnetic field inhibits the spin-disorder scattering and the resistivity decreases drastically. Keeping electrondoped superconductor Nd2−x Ce x CuO4 in mind we have doped RMnO3 (R = La, Pr, Nd) with tetravalent Ce ion. These compounds are very susceptible to the annealing treatment and belong to the orthorhombic perovskite phase. They show a very high value of resistivity at the peak and under the magnetic field the GMR effect is observed. For La0.7Ce0.3MnO3 and Pr0.7Ce0.3MnO3 the magnetoresistance ratio reaches about 54% and 82.5% respectively at 7.7 T. With the increase of the temperature the magnetic state changes from ferromagnetic to paramagnetic regime. This magnetic transition is not very sharp and the resistivity peak appears at a temperature higher than T c .
We relate the measurements by Ming Lei and Hassel Ledbetter (1991) of elastic moduli in strontium titanate near 37K to a dynamical anharmonic mode coupling model. It is suggested that the effects are purely dynamic and not indicative of a true structural phase transition and that neither macroscopic coherence nor second sound is involved.
The radiative decay of an exciton bound to a substitutional iodine ion in AgBr is investigated theoretically. The vibronic spectrum showing multiphonon replicas is calculated under the assumptions of linear electron-phonon coupling. The experimental intensity of the one-phonon spectrum as a function of frequency could satisfactorily be reproduced only by considering coupling coefficients up to fifth neighbors. This result indicates that the bound exciton influences the dynamics of the lattice over an extended neighborhood of the iodine.
CePd2Ga3, a Kondo lattice exhibiting ferromagnetic order below T C = 6.3 K, has been studied using Ga NMR technique. Measurements of the magnetic susceptibility on an oriented sample proved the strong anisotropy of this quantity, whose major component is in the basal plane. From the analysis of NMR spectra of differently oriented samples, the quadrupole parameters and the temperature-dependent anisotropic Knight-shift have been determined. While the anisotropy of the susceptibility can sufficiently account for the axial anisotropy of the 71Ga Knight shift, the in-plane anisotropy of the shift points towards dipolar effects enhanced by hybridization of the Ce-4f and Ga-s electrons.
The spin polarization of the ground state wave function of free uniaxial XH 3 rotors leads to a strong, angle dependent background in the energy integrated neutron scattering even at moderately low temperatures: the large incoherent cross section of individual protons becomes coherent. In the static approximation the powder average of this modulated background is given by a spherical Bessel function j 0 ( QR ) with R the proton-proton distance.
Recent studies have shown that structural instabilities play a vital role in segregation of charge carriers in the perovskites. We have used extended x-ray absorption fine structure (EXAFS) to investigate the instability of the CuO 6 octahedra of the La 1:85 Sr 0:15 CuO 4 (LSCO) system showing high Tc superconductivity and the MnO 6 octhahedra of the La 0:75 Ca 0:25 MnO 3 (LCMO) system showing GMR (giant-magneto resistance) properties. Temperature dependent Cu K-edge EXAFS spectra on the LSCO and Mn K-edge on the LCMO systems are reported. The results show a similarity in the lattice instability of the two systems providing evidence for a similar phase segregation of localized and itinerant charge carriers in the two families of perovskite systems.
