M. McElfresh

Bio: M. McElfresh is an academic researcher from Purdue University. The author has contributed to research in topics: Superconductivity & Magnetic field. The author has an hindex of 13, co-authored 28 publications receiving 1394 citations.

Magnetization and transport currents in thin superconducting films.

Abstract: The critical-state behavior of an infinitely long type-II superconducting thin-film strip is theoretically analyzed for an arbitrary sequence of applied transport currents and perpendicular magnetic fields. Included are solutions for applied field only, transport current only, transport current applied to a sample initially in the remanent critical state, ac applied field, ac transport current, and simultaneously applied field and transport current. The results are compared side by side with corresponding solutions for the more famililar slab geometry; there are striking differences in behavior.

Flux creep characteristics in high‐temperature superconductors

Abstract: We describe the voltage‐current characteristics of YBa2Cu3O7−δ epitaxial films within the flux creep model in a manner consistent with the resistive transition behavior. The magnitude of the activation energy, and its temperature and magnetic field dependences, are readily derived from the experimentally observed power law characteristics and show a (1−T/Tc)3/2 type of behavior near Tc. The activation energy is a nonlinear function of the current density and it enables the determination of the shape of the flux line potential well.

Electronic properties of the metallic perovskite LaNiO3: Correlated behavior of 3d electrons.

Abstract: Low-temperature electrical resistivity, specific heat, and magnetic measurements have been performed on the distorted perovskite ${\mathrm{LaNiO}}_{3}$. The electrical resistivity shows a linear temperature dependence at high temperatures and an ${\mathit{AT}}^{2}$ dependence below \ensuremath{\sim}50 K. The static paramagnetic susceptibility is nearly Pauli-like with an additional small Curie-law contribution in the range 100--300 K, and shows stronger temperature dependence at lower temperatures. The specific heat can be represented as \ensuremath{\gamma}T+\ensuremath{\beta}${\mathit{T}}^{3}$+\ensuremath{\delta}${\mathit{T}}^{3}$lnT at low temperatures. Both the Pauli magnetic susceptibility \ensuremath{\chi}=5.1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ emu/mol and the linear temperature coefficient of the specific heat \ensuremath{\gamma}=13.7 mJ/${\mathrm{K}}^{2}$ mol are enhanced well above their electron gas values. The effective Stoner enhancement factor is S=0.58, i.e., far from the ferromagnetic instability. These results are interpreted in terms of a Fermi liquid composed of almost localized Ni 3d electrons. Thus, ${\mathrm{LaNiO}}_{3}$ represents a correlated metallic system.

Irreversibility line in YBa2Cu3O7 thin films: Correlation of transport and magnetic behavior.

Abstract: Using magnetic and magnetotransport measurements, we show that a ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ thin-film sample has a single irreversibility line (IRL), which is the same as the vortex-glass-transition phase boundary. Results suggest that the effects of pinning persist above the glass-transition temperature ${\mathit{T}}_{\mathit{g}}$, and that it is the loss of critical-current density ${\mathit{J}}_{\mathit{c}}$ rather than an onset of reversible magnetic behavior that characterizes the vortex-glass transition. Measurements of dc magnetization are shown to confirm a theoretical model that explains why the determination of ${\mathit{T}}_{\mathit{g}}$ requires that field-cooled data be collected on cooling (FCC) rather than on warming (FCW), as is frequently done. The use of the ac susceptibility response for measuring ${\mathit{T}}_{\mathit{g}}$ is shown to be a valid measure only at low frequency. This is because both the fundamental-frequency and third-harmonic ac susceptibilities measure the onset of ac flux penetration rather than the onset of irreversibility. The frequency dependence of the onset temperature of ac flux penetration (${\mathit{T}}_{\mathrm{on}}$) is shown to follow the vortex-glass-model dependence ${\mathit{T}}_{\mathrm{on}}$=C(2\ensuremath{\pi}f${)}^{\mathrm{{}1/[(\mathit{z}\mathrm{\ensuremath{-}}1)\ensuremath{ u}]\mathrm{}}}$+${\mathit{T}}_{\mathit{g}}$, with values of z and \ensuremath{ u} consistent with those determined from scaling of the magnetotransport data.

Ferromagnetic materials

Abstract: In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Chemical structures and performance of perovskite oxides.

Abstract: II. Structure of Perovskites 1982 A. Crystal Structure 1982 B. Nonstoichiometry in Perovskites 1983 1. Oxygen Nonstoichiometry 1983 2. Cation Nonstoichiometry 1984 C. Physical Properties 1985 D. Adsorption Properties 1986 1. CO and NO Adsorption 1986 2. Oxygen Adsorption 1987 E. Specific Surface and Porosity 1987 F. Thermal Stability in a Reducing Atmosphere 1989 III. Acid−Base and Redox Properties 1990 A. Acidity and Basicity 1990 B. Redox Processes 1991 1. Kinetics and Mechanisms 1992 2. Reduction−Oxidation Cycles 1993 C. Ion Mobility 1993 1. Oxygen Transport 1993 2. Cation Transport 1994 IV. Heterogeneous Catalysis 1995 A. Oxidation Reactions 1995 1. CO Oxidation 1995 2. Oxidation of Hydrocarbons 1996 B. Pollution Abatement 2001 1. NOx Decomposition 2001 2. Exhaust Treatment 2002 3. Stability 2004 C. Hydrogenation and Hydrogenolysis Reactions 2004 1. Hydrogenation of Carbon Oxides 2004 2. Hydrogenation and Hydrogenolysis Reactions 2006

Electronic structure calculations with dynamical mean-field theory

Abstract: We present a review of the basic ideas and techniques of the spectral density functional theory which are currently used in electronic structure calculations of strongly{correlated materials where the one{electron description breaks down. We illustrate the method with several examples where interactions play a dominant role: systems near metal{insulator transition, systems near volume collapse transition, and systems with local moments.

"Coulomb Staircase" at Room Temperature in a Self-Assembled Molecular Nanostructure

Abstract: Double-ended aryl dithiols [α,α′-xylyldithiol (XYL) and 4,4′-biphenyldithiol] formed self-assembled monolayers (SAMs) on gold(111) substrates and were used to tether nanometer-sized gold clusters deposited from a cluster beam. An ultrahigh-vacuum scanning tunneling microscope was used to image these nanostructures and to measure their current-voltage characteristics as a function of the separation between the probe tip and the metal cluster. At room temperature, when the tip was positioned over a cluster bonded to the XYL SAM, the current-voltage data showed “Coulomb staircase” behavior. These data are in good agreement with semiclassical predictions for correlated single-electron tunneling and permit estimation of the electrical resistance of a single XYL molecule (∼18 ± 12 megohms).

The flux-line lattice in superconductors

Abstract: Magnetic flux can penetrate a type-II superconductor in the form of Abrikosov vortices (also called flux lines, flux tubes, or fluxons) each carrying a quantum of magnetic flux phi 0=h/2e. These tiny vortices of supercurrent tend to arrange themselves in a triangular flux-line lattice (FLL), which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-Tc superconductors (HTSCs) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSCs the FLL is very soft mainly because of the large magnetic penetration depth lambda . The shear modulus of the FLL is c66~1/ lambda 2, and the tilt modulus c44(k)~(1+k2 lambda 2)-1 is dispersive and becomes very small for short distortion wavelengths 2 pi /k<< lambda . This softness is enhanced further by the pronounced anisotropy and layered structure of HTSCs, which strongly increases the penetration depth for currents along the c axis of these (nearly uniaxial) crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may `melt` the FLL and cause thermally activated depinning of the flux lines or ofthe two-dimensional `pancake vortices` in the layers. Various phase transitions are predicted for the FLL in layered HTSCs. Although large pinning forces and high critical currents have been achieved, the small depinning energy so far prevents the application of HTSCs as conductors at high temperatures except in cases when the applied current and the surrounding magnetic field are small.