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Geldart, Wally

Permanent URI for this collectionhttps://hdl.handle.net/10222/22137

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  • ItemOpen Access
    Critical magnetic susceptibility of gadolinium
    (1994-11/15) Dunlap, R. A.; Fujiki, N. M.; Hargraves, P.; Geldart, D. J. W.
    No abstract available.
  • ItemOpen Access
    Contribution of water vapor dimers to clear sky absorption of solar radiation
    (Swedish Geophys. Soc, 1999-03) Chylek, P.; Fu, Q.; Tso, H. C. W.; Geldart, D. J. W.
    Using previously derived-values of the water vapor dimer spectral absorption cross section we calculate the water vapor dimer contribution to the clear sky absorption of solar radiation. We consider the water dimer concentrations deduced from local equilibrium thermodynamics, and we account for the overlap of water vapor dimer and monomer absorption bands. We estimate that water vapor dimer contribution to the clear sky broad band column absorption of solar radiation is around 3 and 6 W/m2 for the cases of midlatitude summer and tropical atmosphere, respectively, at the solar zenith angle of 30. For the cases of midlatitude winter, the US standard atmosphere and subarctic winter atmosphere, the dimer contribution to the absorption of solar radiation is less than 1 W/m2. Due to an uncertainty of the dimer's concentration within clouds, we are not able to deduce, at this time, the dimer absorption in a cloudy atmosphere
  • ItemOpen Access
    Analytical structure of the wave-number-dependent susceptibility of many-fermion systems at low temperature and long wavelength. II
    (1980-10/15) Geldart, D. J. W.; Rasolt, M.
    For pt.I see ibid., vol.15, p.1523 (1977). It is shown that the nonanalytic structure of the wave-number-dependent susceptibility noted by Geldart and Rasolt is not removed by including repeated particle-particle scattering
  • ItemOpen Access
    Two-component scaling near the metal-insulator bifurcation in two dimensions
    (APS through AIP, 2003-05/15) Geldart, D. J. W.; Neilson, D.
    We consider a two-component scaling picture for the resistivity of two-dimensional (2D) weakly disordered interacting electron systems at low temperature with the aim of describing both the vicinity of the bifurcation and the low resistance metallic regime in the same framework. We contrast the essential features of one-component and two-component scaling theories. We discuss why the conventional lowest order renormalization group equations do not show a bifurcation in 2D, and a semiempirical extension is proposed which does lead to bifurcation. Parameters, including the product zN, are determined by least squares fitting to experimental data. An excellent description is obtained for the temperature and density dependence of the resistance of silicon close to the separatrix. Implications of this two-component scaling picture for a quantum critical point are discussed
  • ItemOpen Access
    Fourth-order gradient contributions in extended Thomas-Fermi theory for noninteracting fermions at finite temperature
    (1985-12/15) Geldart, D. J. W.; Sommer, E.
    Nonlocal corrections to the free energy of a system of noninteracting fermions are considered within the framework of extended Thomas-Fermi theory. The density and temperature dependence of gradient corrections is given through fourth order in powers of the density gradient. The calculation is based on long-wavelength expansions of linear and nonlinear response functions of a nearly uniform system. Results are given in terms of Fermi-Dirac integrals and convenient forms are given which span the full density and temperature range from the degenerate limit to the classical limit
  • ItemOpen Access
    Surface-induced quantum density oscillations in the presence of an external magnetic field
    (1985-03/15) Geldart, D. J. W.; Gumbs, G.; Glasser, M. L.
    The electron number density and spin density near the surface of a model metal, semimetal, or semiconductor are calculated in the presence of a uniform magnetic field. The magnetic field is applied in a direction perpendicular to the surface. Numerical results are presented for the limit of a semi-infinite medium. The effect is a magnetic field on the electron density is calculated when the confining potential forming the surface is either an infinite barrier or a finite barrier. The thermal smearing effect of finite temperature is also considered. The authors conclude that experiments designed to probe the magnetic field dependence of the spin polarization in the surface region would be useful
  • ItemOpen Access
    Effect of finite mean free path on spin-flip scattering rates near the critical point of magnetically ordered systems
    (1985) Geldart, D. J. W.; De'Bell, K.
    The finite electron mean free path of conduction electrons is found to play an important role in determining spin-flip scattering rates, 1/sf, near a magnetic critical point. Small-momentum-transfer processes are suppressed and the temperature dependence of 1/sf is dominated by large-momentum scattering. For TTc, 1/sf is monotonically increasing and its leading singular temperature dependence varies at t1-, where t=(T-Tc)/Tc. This temperature dependence is also present for TTc, but for Heisenberg systems, of present interest, the leading singular term varies as (-t)2--, where is the crossover exponent
  • ItemOpen Access
    Density dependence of critical magnetic fields at the metal-insulator bifurcation in two dimensions
    (APS through AIP, 2003-01/15) Geldart, D. J. W.; Neilson, D.
    The density dependence of the critical in-plane magnetic field Bc at the bifurcation of the resistivity of two-dimensional electron systems with low levels of disorder is determined using the spin-polarization dependence of the electron exchange-correlation hole. Recent numerical simulation results for ground-state energies also permit determination of the magnetic field Bpol(n) needed to saturate the spin polarization. The resulting picture gives a good account of reported experimental results for Bc as a function of electron density in p-type GaAs systems and indicates that the interactions between electrons play a crucial role in the bifurcation phenomenon
  • ItemOpen Access
    Convergence properties as a function of spatial dimensionality of gradient expansions for the ground-state energy of an inhomogeneous electron gas
    (1986-02/15) Geldart, D. J. W.; Gumbs, G.
    The extended Thomas-Fermi approximation for the ground-state energy of a many-fermion system is generalized to arbitrary spatial dimension. The authors' objective is a better understanding of convergence properties of such gradient expansions with a view to applications to systems of reduced dimensionality or esoteric geometry. The convergence is tested and found to be adequate by comparing to an exact result for the surface kinetic energy of a semi-infinite system. Both local and nonlocal contributions to the exchange energy are also given for arbitrary dimension. The extension to the thermodynamic free energy at finite temperature for arbitrary dimension is also discussed
  • ItemOpen Access
    Quantum critical behavior in the insulating region of the two-dimensional metal-insulator transition
    (American Physical Society by AIP, 2007-11/15) Geldart, D. J. W.; Neilson, D.
    We show that the quantum critical point associated with the metal-insulator transition phenomenon in two dimensions controls an extended critical region encompassing not only the usual quantum critical sector but also a range of the low-temperature insulator region. The extended range of criticality permits a unified analysis of data from the insulating region and quantum critical sector, allowing us to determine both the dynamical critical exponent z and the correlation length critical exponent N from published data from a single experiment in the insulator critical region. We show that the critical exponents determined from the insulator sector consistently describe the temperature dependence of the resistance data from the same experiment in the quantum critical sector. This provides evidence for the presence of a quantum critical point in these systems.
  • ItemOpen Access
    Dipole-dipole interactions and the critical resistivity of gadolinium
    (1987-06/01) Geldart, D. J. W.; De'Bell, K.; Cook, J.; Laubitz, M. J.
    Results are presented for the electrical resistivity of a c-axis single crystal of high-purity gadolinium metal in the immediate vicinity of the Curie temperature. Numerical analysis reveals that the data are not well described by the usual power laws in (T-Tc)/Tc but tend to exhibit a change in effective slope at Tc which can be described by classical Landau theory with logarithmic corrections. This is interpreted in terms of the dipole-dipole interactions, which are shown to become important at about 1 K above Tc. The authors suggest that gadolinium is a uniaxial dipolar ferromagnet in the asymptotic critical-behaviour regime
  • ItemOpen Access
    Temperature dependent resistivity in the low-resistance region for diffusive transport in two dimensions
    (APS through AIP, 2004-12/15) Geldart, D. J. W.; Neilson, D.
    The interpretation of the metal-insulator transition phenomena in disordered two-dimensional electron systems in terms of density-dependent scaling variables suggests the existence of a quantum critical point at some critical electron density. However a first principles scaling theory based on renormalization group (RG) methods predicts a strong temperature dependence of the dimensionless resistivity R(T), even at small R(T), that is not observed. The observed properties are in fact consistent with a weakly disordered Fermi liquid, and there are no indications of strong temperature dependence induced by scaling. While the RG expansion in a power series in R(T) has only been evaluated to lowest order, this should be sufficient to describe experiments in the region of very small R. A further apparent anomaly is a return from metal-like to insulating-like behavior for increasing density. We explain these fundamental discrepancies between the first principles theory and experiment. We find that the R1 data in the currently attainable temperature range are in a weak scaling regime described by the logarithmic approximation. We independently determine the density dependent prefactor of the logarithm using data for the spin susceptibility and effective mass. We find good agreement between theory and experiment for R(T) in the diffusive regime. We point out that there are corrections to the leading logarithm approximation that should be observable at still lower temperatures
  • ItemOpen Access
    Anisotropy of the critical magnetic susceptibility of gadolinium
    (1989-06/05) Geldart, D. J. W.; Hargraves, P.; Fujiki, N. M.; Dunlap, R. A.
    The magnetic susceptibility along the c axis (xc) and in the basal plane (xb) has been measured on a single crystal of Gd in the reduced temperature range 410-4t1.310-2. Uniaxial anisotropy is observed. Magnetic dipole-dipole interactions are shown to account for the magnitude of the uniaxial anisotropy and also to lead to complex crossover effects. The observed temperature dependence of xc(T) and xb(T) is described by effective exponents eff=1.230.02 and 1-eff=1.010.03, respectively
  • ItemOpen Access
    Lattice sums for dipolar systems
    (1987-12/01) Fujiki, N. M.; De'Bell, K.; Geldart, D. J. W.
    A general method for the evaluation of lattice sums determining the effective parameters in the Hamiltonian of a dipolar magnetic system is described. The anisotropy of the Hamiltonian is examined for crystal structures of tetragonal and hexagonal type. The results are of particular relevance in systems where exchange and any other nondipolar interactions are isotropic. Applications to gadolinium are considered
  • ItemOpen Access
    Short-distance expansion for the spin-spin correlation function of uniaxial dipolar systems
    (1992-03/01) Fujiki, N. M.; De'Bell, K.; Geldart, D. J. W.
    Motivated by recent work on the critical resistivity of gadolinium, a detailed study has been made of the temperature dependence of the two-point vertex function in the large-momentum regime. The operator-project expansion is used to calculate the temperature dependence in uniaxial systems at their critical dimension. Explicit forms are presented for systems with short-range interactions and with dipolar interactions
  • ItemOpen Access
    Influence of the envelope function on the ground-state energy of quasi-two-dimensional Wigner solids
    (1992-10/15) Fujiki, N. M.; Geldart, D. J. W.
    At sufficiently low density, quasi-two-dimensional electron systems in their ground state form Wigner solids. In previous studies of the ground-state energy of such systems (in the absence of an applied magnetic field), the finite extension of electron wave functions in the direction perpendicular to the interface plane has not been considered. The effect of the finite width of envelope functions on the ground-state energy of quasi-two-dimensional Wigner solids is studied in this paper. The authors evaluate the total Coulomb energy using numerically efficient Jacobi -function methods, for a variety of assumed crystal structures. The ground-state energy is found to be rather sensitive to the envelope wave-function width but the crystal structure remains hexagonal
  • ItemOpen Access
    Diamagnetic susceptibility of a dense electron gas
    (1988-02/15) Vignale, G.; Rosolt, M.; Geldart, D. J. W.
    The authors calculate the diamagnetic susceptibility of a uniform interacting electron gas in the random-phase approximation. From this the exact high-density expansion of the diamagnetic susceptibility is obtained
  • ItemOpen Access
    Motion of a point dipole in an infinite hole through a superconductor
    (1993-08/01) Yang, Z. J.; Dunlap, R. A.; Geldart, D. J. W.
    The authors consider the system of a magnetic point dipole placed in an infinite square hole through a superconductor. Using the method of images they obtain the potential and the field distribution in the hollow. Using Lagrangian mechanics, the authors study the motion of the point dipole in the nonrelativistic regime. Relevant applications of this problem are discussed
  • ItemOpen Access
    Negative electron-electron drag between narrow quantum Hall channels
    (APS through AIP, 1998-03/15) Tso, H. C. W.; Geldart, D. J. W.; Vasilopoulos, P.
    Momentum transfer due to Coulomb interaction between two parallel, two-dimensional, narrow, and spatially separated layers, when a current Idrive is driven through one layer, is studied in the presence of a perpendicular magnetic field B. The current induced in the drag layer Idrag is evaluated self-consistently with Idrive as a parameter. Idrag can be positive or negative depending on the value of the filling factor N of the highest occupied bulk Landau level (LL). For a fully occupied LL, Idrag is negative, (i.e., it flows opposite to Idrive), whereas it is positive for a half-filled LL. When the circuit is opened in the drag layer, a voltage Vdrag develops in it; it is negative for a half-filled LL and positive for a fully occupied LL. This positive Vdrag expressing a negative Coulomb drag, results from energetically favored near-edge inter-LL transitions that occur when the highest occupied bulk LL and the LL just above it become degenerate