Showing papers in "Australian Journal of Physics in 1994"
TL;DR: In this article, the phase diagram of electrons in two dimensions at T = 0 was investigated by means of accurate diffusion Monte Carlo simulations within the fixed-node approximation, and it was shown that inter-layer correlation first stabilises the crystalline phase at distances of the order of the in-plane interparticle spacing.
Abstract: We investigate the phase diagram of electrons in two dimensions at T = 0 by means of accurate diffusion Monte Carlo simulations within the fixed-node approximation At variance with previous studies, we find that in an isolated layer Slater-Jastrow nodes yield stability of the fully polarised fluid at intermediate coupling, before freezing into a triangular crystal sets in We have also studied coupled layers of electrons and of electrons and holes Preliminary results show that at large coupling, as two layers are brought together from infinity, inter-layer correlation first stabilises the crystalline phase at distances of the order of the in-plane inter-particle spacing As the distance is further decreased the effect of correlation, as expected, turns into an enhanced screening, which disrupts the crystalline order in favour of liquid phases
123 citations
TL;DR: In this paper, the authors review the Landau theory of interacting Fermi liquids such as mobile electrons in solids or helium-3 and present the rigorous microscopic basis of the theory.
Abstract: This article reviews the Landau theory of interacting Fermi liquids such as mobile electrons in solids or helium-3. It starts with Landau's original formulation which takes advantage of the existence of a Fermi surface to map the strongly interacting single-particle excitations near the Fermi surface into a system of weakly interacting quasiparticle excitations. The theory relates microscopic parameters for the quasiparticle energies and scattering strengths to experimental observables. The resulting low lying collective modes of the system, such as zero sound in helium-3, are then discussed. Next the rigorous microscopic basis of the theory is presented. Finally there is an outline of a recent modification of the theory which may resolve some of the puzzles about the nature of the electron states in materials exhibiting high transition temperature superconductivity.
43 citations
TL;DR: In this article, acceleration transfer theory is used to derive approximate expressions for transport properties of electrons and ions in a gas in crossed electric and magnetic fields, including the ratio D II / D1.
Abstract: Momentum-transfer theory is used to derive approximate expressions for transport properties of electrons and ions in a gas in crossed electric and magnetic fields. Included in the formal discussion are the generalised Einstein relations, negative differential conductivity, Tonks' theorem and the equivalent reduced electric field concept. Specific topics dealt with include the ratio D II / D1. for electrons, which 'flips over' as the magnetic field is increased, the enhancement of negative differential conductivity through increased BIN, and a discussion of anisotropic scattering.
32 citations
TL;DR: In the late 1950s and early 1960s, radio sky surveys were the centre of an intense and public debate-Big-Bang versus Steady-State cosmology-the arguments revolving about source counts and statistical interpretations in the face of instrumental complications as discussed by the authors.
Abstract: In the late 1950s and early 1960s, radio sky surveys were the centre of an intense and public debate-Big-Bang versus Steady-State cosmology-the arguments revolving about source counts and statistical interpretations in the face of instrumental complications. The 1965 discovery of the microwave background took the fire from the debate, but left the momentum in place for large-area radio surveys at different frequencies, and for extensive identification/redshift-measurement programs. By the 1970s the data enabled us to start disentangling the different populations of extragalactic radio sources. We could refine our taxonomy, and we could view the possibility of delineating individual cosmic histories and evolutions. We could at least describe a goal to elucidate the birth-life-death cycles of the objects involved [quasi-stellar objects (QSOs) and radio galaxies: together the 'active galactic nuclei' (AGNs)] whose unaccountably prodigious energies somehow produce the beautifully aligned radio structures with which we are now familiar. One part of John Bolton's vision to see how distorted a view of the AGN universe the original long-wavelength surveys provided. One legacy is thus the 'short-wavelength survey' for extragalactic radio sources, which has done so much to balance our picture of the radio sky. And indeed the legacy continues in the form of the immense sky surveys at present under way, complete with their sub-industries of radio-positioning and identification. From these, yet further results are emerging on spatial distribution and the skeleton structure of the universe. It is the purpose of this paper to outline something of this current view of the populations, their differences, similarities and unifying concepts.
30 citations
TL;DR: In this article, the gravitoelectric version of the Aharonov-Bohm effect was examined in the weak-field approximation to general relativity and a matter wave interferometry experiment, based on a modification of the gravity-induced quantum interference experiment of Colella, Overhauser and Werner (COW), was proposed.
Abstract: The gravitational Aharonov-Bohm (AB) effect is examined in the weak-field approximation to general relativity. In analogy with the electromagnetic AB effect, we find that a gravitoelectromagnetic 4-vector potential gives rise to interference effects. A matter wave interferometry experiment, based on a modification of the gravity-induced quantum interference experiment of Colella, Overhauser and Werner (COW), is proposed to explicitly test the gravitoelectric version of the AB effect in a uniform gravitational field.
27 citations
TL;DR: In this article, the possible applicability of coarse-graining or weighted density approximations to the theory of the liquid-crystal transition is discussed in terms of physical implications stemming from the crucial change in symmetry that occurs.
Abstract: Microscopically inhomogeneous states such as crystals and amorphous phases are wide-spread in nature, and it has become relatively common to attempt to describe the thermodynamic functions of such structured systems by utilising the corresponding properties of their homogeneous counterparts One route is via density functional theory, some aspects of which are reviewed and developed here for single-component systems In particular the possible applicability of coarse-graining or weighted density approximations to the theory of the liquid–crystal transition are discussed in terms of physical implications stemming from the crucial change in symmetry that occurs Some insight into the apparent inconsistency of weighted-density approaches can be gained by examination of the role of anharmonic terms in the structured phase, and their relation to the nature of the interactions which also control the range of the weight-functions
24 citations
TL;DR: In this paper, a model for positron-alkali atom scattering is presented, where a core term is added to the core potential to describe the rearrangement process of the positron interaction with the residual ion.
Abstract: The close coupling equatious for positron-alkali atom scattering are written as a set of coupled momentum-space Lippmann-Schwinger equations. The alkali atom is represented by a frozen-core model based upon the Hartree-Fock approximation. The interaction between the positronium and the residual ion is modified by the inclusion of a core potential. Similarly, a core term is present in the interaction describing the rearrangement process. Close coupling calculations of positron scattering from sodium are performed in a model containing multiple sodium (3s, 3p, 4s, 3d, 4p) and positronium (Is, 2s, 2p) states. Cross sections are reported for an energy range from threshold to 50·eV; the total cross sections are in agreement with experimental data.
24 citations
TL;DR: In this paper, the existence of topological quantum fields in high-T c superconductors and in Heisenberg quantum antiferromagnets was established. And the long distance physics of quantum Hall fluids, as well as their topological order parameter were discussed.
Abstract: We discuss quantum-mechanical many-body systems interacting with a topological field, known as topological quantum fields. Several topics on the theory of quantum fluids are examined. First we establish the existence of topological gauge fields in high-T c superconductors and in Heisenberg quantum antiferromagnets. Then we consider typical topological quantum fluids, known as quantum Hall fluids, which are systems exhibiting the fractional quantum Hall effect (FQHE). A theoretical model of these fluids is described in detail. We also discuss the long distance physics of topological quantum fluids, their topological order parameter and possible experimental tests of the theory.
23 citations
TL;DR: In this article, the transition to a periodic steady state for an ion swarm in a gas was investigated using the BGK model kinetic equation, and exact expressions for transport coefficients and the velocity distribution function were obtained and the latter was compared with experimental observations of ions in their parerit gases undergoing predominantly charge transfer collisions.
Abstract: The transition to a periodic steady state for an ion swarm in a gas is investigated using the BGK model kinetic equation. Exact expressions for transport coefficients and the velocity distribution function are obtained and the latter is compared with experimental observations of ions in their parerit gases undergoing predominantly charge-transfer collisions.
19 citations
TL;DR: In this article, the authors presented detailed radio and optical parameters for the stronger radio sources in 58 Abell cluster fields observed with the VLA using scaled arrays at 1·5 and 4·9 GHz.
Abstract: We present detailed radio and optical parameters for the stronger radio sources in 58 Abell cluster fields observed with the Very Large Array (VLA) using scaled arrays at 1·5 and 4·9 GHz. These sources comprise a complete sample with 1· 5 GHz flux density :;::20 mJy and cover a combined sky area of 3·5 X 10-5 sr. The cluster fields were distributed over 24 h of RA and between declinations +350 and -300 Contour maps at two frequencies are presented and source parameters such as position, angular size, spectral index, linear polarisation and core flux density are tabulated. We also derive the emitted power and linear size for those sources close to the cluster centres and therefore highly likely to be cluster members. We attempt to identify all these radio sources with optical images on the Palomar and SERC survey plates and give their accurate optical coordinates, morphologies and apparent magnitudes.
17 citations
TL;DR: In this paper, the density functional theory for classical (equilibrium) statistical mechanics is generalized so that one can discuss various dynamical processes associated with density fluctuations in liquids, which is effected by deriving a Langevin-diffusion equation for the density field.
Abstract: The density functional theory for classical (equilibrium) statistical mechanics is generalized so that one can discuss various dynamical processes associated with density fluctuations in liquids. This is effected by deriving a Langevin-diffusion equation for the density field, which satisfies a novel H-theorem. As applications of our theory we consider density fluctuations in both supercooled liquids and molecular liquids, interface dynamics and transport coefficients.
TL;DR: The field-trip to New Zealand by John Bolton and Gordon Stanley in 1948 was a crucial step in discovering the nature of discrete radio sources as discussed by the authors, and the circumstances surrounding the field trip and also looked at other investigations in radio astronomy in New Zealand at that time.
Abstract: The field-trip to New. Zealand by John Bolton and Gordon Stanley in 1948 was a crucial step in discovering the nature of discrete radio sources. This paper describes the circumstances surrounding the field-trip and also looks at other investigations in radio astronomy in New Zealand at that time.
TL;DR: The first radio spectra, angular sizes and source variability measurements were made during the first eight years of his astronomical career as discussed by the authors, and his collaboration with John Bolton, Gordon Stanley, Kevin West and Dick McGee at the Dover Heights field station resulted in the detection and optical identification of the first radio sources outside the solar system
Abstract: The author describes the first eight years of his astronomical career. His collaboration with John Bolton, Gordon Stanley, Kevin West fold and Dick McGee at the Dover Heights field station resulted in the detection and optical identification of the first radio sources outside the solar system. The first radio spectra, angular sizes and source variability measurements were made during this interval.
TL;DR: In this article, the authors show that parity-violating terms and well-defined finite topological actions via quantum loops can be obtained in 2+ 1 and 4+ 1 dimensions.
Abstract: One of the interesting features about field theories in odd dimensions is the induction of parity-violating terms and well-defined finite topological actions via quantum loops if a fermion mass term is originally present and conversely. Aspects of this issue are illustrated for electrodynamics in 2+ 1 and 4+ 1 dimensions.
TL;DR: Personal recollections are presented by the author of the commencement of galactic and extragalactic radio astronomy at Dover Heights and the design and construction of the California Institute of Technology's Owens Valley Interferometer as mentioned in this paper.
Abstract: Personal recollections are presented by the author of the commencement of galactic and extragalactic radio astronomy at Dover Heights and the design and construction of the California Institute of Technology's Owens Valley Interferometer.
TL;DR: In this paper, the authors present a brief derivation of the periodic orbit expansion for simple dynamical systems, and then apply it to the study of a classical statistical mechanical model, the Lorentz gas, both at equilibrium and in a nonequilibrium steady state.
Abstract: In this work we present a brief derivation of the periodic orbit expansion for simple dynamical systems, and then we apply it to the study of a classical statistical mechanical model, the Lorentz gas, both at equilibrium and in a nonequilibrium steady state The results are compared with those obtained through standard molecular dynamics simulations, and they are found to be in good agreement The form of the average using the periodic orbit expansion suggests the definition of a new dynamical partition function, which we test numerically An analytic formula is obtained for the Lyapunov numbers of periodic orbits for the nonequilibrium Lorentz gas Using this formula and other numerical techniques we study the nonequilibrium Lorentz gas as a dynamical system and obtain an estimate of the upper bound on the external field for which the system remains ergodic
TL;DR: In this article, the Hohenberg-Kohn-Sham-type existence theorems were extended to superconducting analogue of the normal-state Kohn Sham formalism and a gradient expansion technique was used to calculate an approximation of the non-interacting kinetic energy functional Ts[n, △] for superconductors.
Abstract: The basic idea of density functional theory is to map an interacting many-particle system on an effective non-interacting system in such a way that the ground-state densities of the two systems are identical The non-interacting particles move in an effective local potential which is a functional of the density The central task of density functional theory is to find good approximations for the density dependence of this local single-particle potential An overview of recent advances in the construction of this potential (beyond the local-density approximation) will be given along with successful applications in quantum chemistry and solid state theory We then turn to the extension of density functional theory to superconductors and first discuss the Hohenberg-Kohn-Sham-type existence theorems In the superconducting analogue of the the normal-state Kohn-Sham formalism, a local single-particle potential is needed which now depends on two densities, the ordinary density n(r) and the anomalous density △(r,r/) As a first step towards the construction of such a potential, a gradient expansion technique for superconductors is presented and applied to calculate an approximation of the non-interacting kinetic energy functional Ts[n, △] We also obtain a Thomas-Fermi-type variational equation for superconductors
TL;DR: In this article, the authors presented the results of multi-epoch, 8.4 GHz, very long baseline interferometry (VLBI) imaging observations of the Centaurus A nucleus made over the past three years.
Abstract: The bright, peculiar elliptical galaxy Centaurus A (NGC 5128, PKS 1322-427) was one of the first extragalactic radio sources to be optically identified (Bolton et al. 1949). At a distance of 4 Mpc, Centaurus A is the closest active radio galaxy and affords the highest linear imaging resolution (1 mas approximately equal to 0.02 pc) and hence the best prospects for studying an active nucleus close to the central radio source. We present the results of multi-epoch, 8.4-GHz, very long baseline interferometry (VLBI), imaging observations of the nucleus made over the past three years. The nucleus possesses a core-jet structure where the inner portion of the jet shows apparent linear motion with a velocity substantially less than the speed of light.
TL;DR: In this paper, the P3/2 optical absorption spectrum of boron impurity in silicon has been re-examined at high resolution, and the precise transition energies measured agree with energies previously reported.
Abstract: The P3/2 optical absorption spectrum of boron impurity in silicon has been re-examined at high resolution. The precise transition energies measured agree with energies previously reported. In addition, energies for several previously unrecognised transitions are given as well as values for the absorption strengths and line widths. The measured transition energies and absorption strengths correlate very well with several recent calculations of binding energies and oscillator strengths, respectively. This excellent agreement between experiment and theory motivates a renumbering of the spectral lines which is not expected to require future modification. High-resolution piezospectroscopy of the P3/2 series has also been undertaken. Small stresses were used to minimise the effect of interactions and permit accurate determination of the deformation potential constants. The deformation potential constants are found to be in fair agreement with previous experimental values and good agreement with recent theory. Experimental values for several of these are given for the first time, as are isotropic deformation potential constants of several excited states relative to the ground state.
TL;DR: In this article, a partial wave expansion of three-particle continuum states using hyperspherical coordinates was developed for electron-hydrogen atom ionisation studies, which may be easily extended for application to other three-body ionisation problems.
Abstract: A partial wave expansion of three-particle continuum states has been developed using hyperspherical coordinates. An approximate three-particle continuum state appears which may be useful in electron-hydrogen atom ionisation studies. Further improvement in the result is also possible. The analysis may be easily extended for application to other three-body ionisation problems.
TL;DR: The elastic and nonlinear acoustic vibrational properties of terbium metaphosphate glasses (Tb2O3)x(P2O5)1?x with x = 0·226,0·247, 0·263 and 0·271 (x is the mole fraction) have been determined from measurements of the effects of temperature, hydrostatic pressure, and uniaxial stress on' ultrasonic wave velocity as discussed by the authors.
Abstract: The elastic and nonlinear acoustic vibrational properties of terbium metaphosphate glasses (Tb2O3)x(P2O5)1?x with x = 0·226,0·247,0·263 and 0·271 (x is the mole fraction) have been determined from measurements of the effects of temperature, hydrostatic pressure, and uniaxial stress on' ultrasonic wave velocity. At temperatures below about 140 K, the elastic stiffness of' (Tb2O3)x(P2O5)1?x glasses becomes anomalously dependent upon temperature, a behaviour usually associated with interactions between acoustic phonons and two-level systems. Except for the (Tb2O3)0·271(P205)0·729 glass, the hydrostatic pressure derivatives of the elastic stiffness and also of the bulk modulus BS of terbium metaphosphate glasses are small and negative. The third-order elastic stiffness tensor components CIJK of the (Tb2O3)0·247(P2O5)0·753 glass between 77 K and 400 K have also been determined. At room temperature, C112, C123 and C144 are positive while C111, C155 and C456 are negative. Both longitudinal and shear acoustic mode Gruneisen parameters are small and negative: the application of pressure softens the long-wavelength acoustic phonon mode frequencies. The mode softening is enhanced as the temperature is reduced.
TL;DR: In this paper, the role of relativistic jets in radio galaxies and quasars was discussed and the evolutionary connection between Fanaroff~Riley Class I and Class II radio galaxies was investigated.
Abstract: This paper summarises some of the ideas surrounding the role of relativistic jets in radio galaxies and quasars and describes work presented in two recent papers (Bicknell 1994a,b) relating relativistic jets to the Fanaroff~Riley classification of radio galaxies. I conclude with some speculation on the evolutionary connection between Fanaroff~Riley Class I and Class II radio galaxies and the relationship between mergers and radio galaxies, an idea which was discussed at the time of the discovery of Cygnus A and Ccntaurus A.
TL;DR: Variations of the helicon wave-induced radial plasma transport are presented depending on values of the plasma radius, magnetostatic field, plasma density and the frequency of the helical wave as mentioned in this paper.
Abstract: Variations of the helicon wave-induced radial plasma transport are presented depending on values of the plasma radius, magnetostatic field, plasma density and the frequency of the helicon wave. It is shown that the value of the helicon wave-induced transport may be significant for plasma confinement; this is demonstrated, for the experiments BASIL and SHEILA. Whereas m = +1 helicons induce an inward-directed transport and thus improve the confinement, m = -1 helicons induce an outward-directed transport velocity.
TL;DR: In this paper, the analytic structure of the chiral loop corrections to the pion-nucleon sigma term arising from coupling to the lowest mass baryon decuplet was examined.
Abstract: We examine the analytic structure, in powers of the quark mass (m), of the chiral loop corrections to the pion-nucleon sigma term arising from coupling to the lowest mass baryon decuplet. The leading non-analytic term is found to go like m2 In(m/2a) (with a the delta-nucleon mass splitting), and to be quite significant numerically.
TL;DR: In this article, the manifestation of solar activity on radio noise records at 28.6 MHz is discussed with special emphasis on Type-I noise storms and the associated coronal magnetic fields above the active regions in time.
Abstract: The manifestation of solar activity on radio noise records at 28.6 MHz is discussed with special emphasis on Type-I noise storms and the associated coronal magnetic fields above the active regions in time. Magnetic fields are estimated, assuming that the Type-I radio emission at decametre wavelengths is due to shock waves, by making use of the observed shock velocity. The results are comparable with the existing estimates.
TL;DR: In this paper, the triple differential cross section for electron impact ionisation of helium atoms has been calculated in the coplanar asymmetric geometry following the multiple scattering approach of Das, where the impact energy of the incident electron is taken to be 150 and 250 eV in an intermediate energy range where there are still some discrepancies between theory and experiment.
Abstract: The triple differential cross section for electron impact ionisation of helium atoms has been calculated in the coplanar asymmetric geometry following the multiple scattering approach of Das. The method has already been successfully employed to describe the electron impact ionisation problem for the hydrogen atom. Here the impact energy of the incident electron is taken to be 150 and 250 eV in an intermediate energy range where there are still some discrepancies between theory and experiment. Present results are compared with the available experimental data and with two of the most recent calculations in some cases, and are found to be in reasonable accord with experiment, particularly in the binary peak region. The present calculation for 250 e V incident energy reproduces the experimental results in some cases better than other theories.
TL;DR: In the presence of a staggered magnetic field, the plaquette expansion of the Lanczos matrix elements is obtained for the antiferromagnetic 2D Heisenberg model up to order 1/Np (Np is the number of plaquettes on the lattice) as mentioned in this paper.
Abstract: In the presence of a staggered magnetic field, the plaquette expansion of the Lanczos matrix elements are obtained for the antiferromagnetic 2D Heisenberg model up to order 1/Np (Np is the number of plaquettes on the lattice). The resulting approximate tri-diagonal form of the Hamiltonian is diagonalised for various values of the field strength in the -> 00 limit for the ground state energy density. From the behaviour of the ground-state energy density at weak fields, the staggered magnetisation at this order in the plaquette expansion is found to be 0·71 (in units where the Neel state staggered magnetisation is 1· 0).
TL;DR: In this paper, the modulational instability of two pump Langmuir modes is considered for the case of comparatively small instability rates, when'renormalised' expressions for linear and nonlinear plasma polarisation responses provide the maximum effect on the instability development.
Abstract: The modulational instability of Langmuir waves in unmagnetised plasmas is reviewed for the cases when a pump consist of two monochromatic or a large number of random modes. It is demonstrated that the correct theory for the modulational instability operates with 'renormalised' equations for the linear dielectric function as well as for the effective third-order plasma response. This renormalisation is due to so-called interference terms. The appearance of interference terms is a specific feature of the multi-mode modulational instability in comparison with the well-known instability of a single mode. All calculations use a simple and universal formalism including new methods developed for description of the modulational effects in arbitrary media. The modulational instability of two pump Langmuir modes is considered for the case of comparatively small instability rates, when 'renormalised' expressions for linear and nonlinear plasma polarisation responses provide the maximum effect on the instability development. For instabilities of the broad spectra of random waves, the integral equations are presented for perturbations of wave field correlation functions. In the description of the modulational instability of random wave packets these equations play the same role as the set of coupled equations for the fields of modulational perturbations in the case of two monochromatic pumps. Rates and thresholds of the instabilities are found in various limits.
TL;DR: In this paper, the momentum transfer cross section for electrons in xenon, am, has been studied using electron transport coefficient measurements for a dilute hydrogen-xenon mixture (0·47% H2- 99 ·53% Xe).
Abstract: The momentum transfer cross section for electrons in xenon, am, has been studied using electron transport coefficient measurements for a dilute hydrogen-xenon mixture (0·47% H2- 99 ·53% Xe). Drift velocity measurements were made using the Bradbury-Nielsen time-of-flight method at E/N values from 0 ·12 to 2·50 Td, pressures from 10·3 to 94·0 kPa and a temperature of 295 K (E is the electric field strength and N the gas number density; 1 Td == 10-17 Vcm2). The ratio DT/f.t (where DT is the transverse diffusion coefficient and {£ the electron mobility) was measured using a Townsend-Huxley diffusion chamber at E/N values from 0·035 to 1· 70 Td, pressures from 13·4 to 40·3 kPa, and a temperature of 294 K. Three recently published Urn values for xenon have been tested and shown to be incompatible with both the present drift velocity and DT / f.t measurements.