On the detection of nuclear spin waves by inelastic neutron scattering

The possibility of measuring nuclear spin waves (NSW) by inelastic neutron scattering is discussed. The differential cross section and scattered state polarization for the scattering of thermal neutrons from systems described by the Suhl-Nakamura Hamiltonian are developed in the Van Hove correlation function formalism; the relevant correlation functions for the Suhl-Nakamura system are computed. The implications of these calculations for the feasibility of detecting nuclear spin wave modes in neutron scattering experiments are discussed.     

Neutron-photon studies of condensed matter

A theory of simultaneous photon absorption and inelastic neutron scattering is developed by treating the photon and neutron-matter interactions perturbatively. The leading-order mixing between the interactions shows that the neutron scattering cross-section is proportional to the dynamic structure factor (or Van Hove function) evaluated at an energy that is enhanced by the photon energy. The photon induced modification of the scattering vector is negligible. Thus, the proposed technique affords the possibility of measuring the dynamic structure factor at large energies and modest wavevectors which is a domain that is usually difficult to access because of kinematic constraints. The theory is developed in detail for some models of nuclear and magnetic systems. The results show that, in most cases, the experiments are likely to demand the use of very high intensity light sources. A particularly promising application appears to be in the study of electron plasmas since, using readily available pulsed lasers, the neutron cross-section is comparable with that for pure magnetic scattering.     

Time-dependent triplet correlations in dense nitrogen gas

The inelastic neutron scattering cross section for dense nitrogen gas at several pressures near 240 atmospheres and room temperature has been measured in the region of the first diffraction peak. The Van Hove self-correlation function is calculated from models and subtracted from these cross sections to yield ?S _d(Q, ω)/?p. This derivative is related to an integral over the time-dependent triplet correlation function and models for the triplet function are tested. A simple model, based on pair functions, gives moderate agreement.     

Kinetic equations without “memory” for the time-displaced correlation functions

The relations between the kinetic equations with and without convolution in time are discussed on the basis of the kinetic equation for the Van Hove self-correlation function. Formal equivalence of both the equations is shown, and approximate scattering operators for the dilute-gas case and for the Brownian particle are considered.     

Translational light scattering in simple liquids

Summary A short review of translational light scattering of simple liquids is reported. The microscopic cross-section is compared with the equivalent expression for neutron scattering experiments (Van Hove, 1954). A survey of the fundamental experiments is reported and the possible application to the study of the dynamics of simple dense systems is analysed. The relevant approximations are critically discussed and the limits of the technique are established. The review mainly deals with classical or almost classical liquids, but also some extensions to quantum systems are considered. paper presented at the workshop ?Highlights on Simple Liquids?, held in Turin at ISI on 1–3 May 1989.     

Determination of the phonon spectrum from coherent neutron scattering by polycrystals

As proposed by Bredovet al. [2, 3] the phonon spectrum can be obtained approximately from coherent neutron scattering by polycrystals if suitable averages over scattering angles are considered. The accuracy of this method is estimated by comparison with analytical results for simple lattice models (discussed here in particular for Aluminium). The errors are about 5% for low order moments and about 50% near van Hove singularities for “cold” neutrons (wavelength of the order of the nearest-neighbour-distance).     

Linear photoelectron diffraction: application of a rapid approximation for surface structural studies

The linear superposition approximation proposed by Wander, Pendry and Van Hove for efficient low-energy electron diffraction calculations (“linear LEED”) is applied to photoelectron diffraction (“linear PD”). As with linear LEED, linear PD works very well for calculating the effect of displaced atoms. However, due to strong forward scattering at higher energies, linear PD requires that atoms do not move into or out of alignment. This limitation can be removed by suitable simple adjustments to the basic approximation, promising to make the method effective for structural searches of complex surfaces.     

Robustness of the Van Hove scenario for high-T(c) superconductors

The pinning of the Fermi level to the Van Hove singularity and the formation of flat bands in the two-dimensional t-t' Hubbard model is investigated by the renormalization group technique. The "Van Hove" scenario of non-Fermi-liquid behavior for high-T(c) compounds can take place in a broad enough range of the hole concentrations. The results are in qualitative agreement with the recent angle-resolved photoemission spectroscopy data on La 2CuO (4).     

High-spin states in 166Lu

It is shown that the discrepancy between the results obtained for different neutron energy ranges, when neutron polarizability is derived from the neutron scattering data, can be removed if one assumes that at the fast neutron scattering a strong-interaction long-range potential of Van der Waals (～ r ^?6 ) or Casimir-Polder (～ r ^?7 ) is observed. This strong-interaction long-range potential has possibly some experimental confirmation in the elastic p-p scattering.     

Dynamic heterogeneity of a colloidal solution near the sol-gel transition

The results of the simulation of the dynamics of particles in a colloidal solution in the vicinity of the sol-gel transition are presented. The Van Hove correlation function, incoherent scattering function, and mean-square displacement of particles at different temperatures and volume densities have been calculated from the simulation data. The effects of dynamic heterogeneity have been evaluated numerically and the gelation temperature at different volume densities has been determined using the non-Gaussian parameter. It has been shown that the specific features observed in the dynamics of particles in the colloidal solution near the sol-gel transition are explained by the conventional separation of the particles in the system into fast particles, which contribute to translational diffusion, and slow particles, which participate predominantly in vibrational processes.     

Impurity-induced anisotropic semiconductor-semimetal transition in monolayer biased black phosphorus

Taking into account the electron-impurity interaction within the continuum approximation of tight-binding model, the Born approximation, and the Green's function method, the main features of anisotropic electronic phase transition are investigated in monolayer biased black phosphorus (BP). To this end, we concentrated on the disordered electronic density of states (DOS), which gives useful information for electro-optical devices. Increasing the impurity concentration in both unbiased and biased impurity-infected single-layer BP, in addition to the decrease of the band gap, independent of the direction, leads to the midgap states and an extra Van Hove singularity inside and outside of the band gap, respectively. Furthermore, strong impurity scattering potentials lead to a semiconductor-semimetal transition and one more Van Hove singularity in x-direction of unbiased BP and surprisingly, this transition does not occur in biased BP. We found that there is no phase transition in y-direction. Since real applications require structures with modulated band gaps, we have studied the influence of different bias voltages on the disordered DOS in both directions, resulting in the increase of the band gap.     

Solution de l'equation de Boltzmann pour des batonnets durs unidimensionnels

We present the exact solution of the linear Boltzmann equation for one-dimensional hard rods. In particular, we compute the velocity correlation function and the Van Hove self-correlation function.     

Combined Scheme of Reconstruction of the Particle Size Distribution Function Using Small-Angle Scattering Data

JETP Letters - To analyze polydisperse systems of nanoparticles, the particle size distribution function can be determined from small-angle X-ray and neutron scattering data using some algorithms....     

Properties of electrons near a Van Hove singularity

The Fermi surface of most hole-doped cuprates is close to a Van Hove singularity at the M point. A two-dimensional electronic system, whose Fermi surface is close to a Van Hove singularity, shows a variety of weak coupling instabilities. It is a convenient model to study the interplay between antiferromagnetism and anisotropic superconductivity. The renormalization group approach is reviewed with emphasis on the underlying physical processes. General properties of the phase diagram and possible deformations of the Fermi surface due to the Van Hove proximity are described.     

Quantum corrections to liquid correlations

Sudarshan's semi-classical treatment of correlation functions is applied to the study of quantum corrections to the Van Hove function. In its generalised form, it enables one to choose the best correlation function for a given potential. Higher-order correlations are also sketched briefly and the details are similar to those considered by Oppenheim and Bloom.     

Complementary application of neutron and synchrotron X-ray scattering to the determination of the magnetic microstructure of exchange-coupled layered nanoheterostructures

Complementary neutron and synchrotron X-ray scattering techniques have been considered as applied to the investigation of an inhomogeneous magnetic microstructure typical of exchange-coupled nanoheterostructures consisting of alternating ferromagnetic and antiferromagnetic metal layers. It has been demonstrated that changes in magnetic moments in both the magnitude and the direction within ferromagnetic layers can be determined by combining polarized neutron and resonant X-ray magnetic reflectometry, and those within antiferromagnetic layers, by combining X-ray and neutron diffractometry.     

Quantum corrections to liquid correlations

Sudarshan's semi-classical treatment of correlation functions is applied to the study of quantum corrections to the Van Hove function. In its generalised form, it enables one to choose the best correlation function for a given potential. Higher-order correlations are also sketched briefly and the details are similar to those considered by Oppenheim and Bloom.     

Structure and dynamic properties of liquids confined in MCM-41 mesopores

The microscopic structure and dynamic properties of water, methanol, and acetonitrile confined in mesoporous MCM-41 materials have been investigated under monolayer and capillary-condensation conditions as a function of pore size and temperature by in situ FTIR and X-ray diffraction and quasi-elastic neutron scattering techniques. Both interfacial and confinement effects on the structure and dynamics of the liquids in hydrophilic pores are discussed at the molecular level.     

On the enhancement of superconductivity due to Van Hove singularity

We have rigorously solved the standard BCS gap equation with the density of states of Van Hove type. The result shows that the enhancement ofT _c is not as effective as previously estimated. It seems unlikely that Van Hove singularity inN(E) can be theT _c -enhancement mechanism for the high-T _c Cu oxides.     

Kinematics of electrons near a van hove singularity

A two-dimensional electronic system, where the Fermi surface is close to a Van Hove singularity, shows a variety of weak coupling instabilities. It is a convenient model to study the interplay between antiferromagnetism and anisotropic superconductivity. We present a detailed analysis of the kinematics of electron scattering in this model. The similarities and differences between a standard renormalization group approach and previous work based on parquet summations of log (2) divergences are analyzed, with emphasis on the underlying physical processes. General properties of the phase diagram are discussed.