Showing papers by "M. B. Maple published in 1993"
TL;DR: The first infrared and optical measurements of the absolute conductivity of FeSi are reported, showing that at low temperature most of the conductivity below 60 meV is depleted, consistent with an energy gap of that magnitude.
Abstract: We report the first infrared and optical measurements of the absolute conductivity of FeSi Above 200 K the ac conductivity of FeSi resembles that of a dirty metal, while at low temperature most of the conductivity below 60 meV is depleted, consistent with an energy gap of that magnitude Thus in contrast to what is expected from a picture based on simple thermal activation, the gap disappears at a temperature which is low relative to its size Moreover, the spectral weight lost below the gap does not reappear just above the gap, but is instead distributed over an energy range of order eV
234 citations
TL;DR: It is concluded that the metallic phase is stabilized by pressure because the bandwidth increases and hence the charge-transfer gap is reduced, in contrast to the dependence predicted and observed in the layered cuprates where the Charge-transfer energy dominates the bandwidth effects.
Abstract: We have investigated the pressure dependence (up to 20 kbar) of the metal-insulator (MI) transition displayed by the orthorhombic perovskites R${\mathrm{NiO}}_{3}$ (R=Nd, Pr, and ${\mathrm{Nd}}_{0.7}$${\mathrm{La}}_{0.3}$) by means of electrical resistance measurements. The transition temperature decreases under pressure, with a common rate of decrease of ${\mathit{dT}}_{\mathrm{MI}}$/dP=-4.2 K/kbar, in spite of differences in ${\mathit{T}}_{\mathrm{MI}}$ (100\char21{}200 K). On the basis of the structural effects associated with the application of pressure in these orthorhombic perovskites, we conclude that the metallic phase is stabilized by pressure because the bandwidth increases and hence the charge-transfer gap is reduced. This decrease with pressure of the charge-transfer gap in the R${\mathrm{NiO}}_{3}$ perovskites is in contrast to the dependence predicted and observed in the layered cuprates where the charge-transfer energy dominates the bandwidth effects. In addition, our experiments show that the first-order phase transformation occurring at ${\mathit{T}}_{\mathrm{MI}}$ is inhibited by pressure, and hence a larger proportion of metallic phase occurs, which leads to an apparent reentrant metallic behavior at low temperature.
131 citations
TL;DR: Magnetic relaxation measurements in a magnetic field of 5 kOe at temperatures 5 K − 90 K have been made on the system as discussed by the authors, where the measurements reveal two peaks in the normalized logarithmic decay of the magnetization S as a function of temperature.
Abstract: Magnetic relaxation measurements in a magnetic field of 5 kOe at temperatures 5 K \ensuremath{\le}T\ensuremath{\le}90 K have been made on the system ${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ for 0\ensuremath{\le}x\ensuremath{\le}0.4. The measurements reveal two peaks in the normalized logarithmic decay of the magnetization S as a function of temperature for the full range of Pr concentrations studied. The occurrence of the second peak in S could be a reflection of the softening of the flux lattice on approaching the melting line. Both the pinning energy at 27 K and the intragranular critical current density at 10 K exhibit maxima at x\ensuremath{\approxeq}0.1--0.2, with values that are about twice the corresponding values at x=0.
69 citations
TL;DR: Angle-resolved photoemission measurements for bulk single crystals show that the spectral weight which builds in the gap of the [ital x]=0 insulator displays a Luttinger Fermi surface, the first such observation for a material which is electron doped and which must be driven metallic by alloying.
Abstract: Angle-resolved photoemission measurements for bulk single crystals of ${\mathrm{Nd}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathit{y}}$ with x=0.15 show that the spectral weight which builds in the gap of the x=0 insulator displays a Luttinger Fermi surface, the first such observation for a material which is electron doped and which must be driven metallic by alloying. This finding greatly strengthens our previous ocnclusion that the metal's chemical potential lies in gap-filling spectral weight, and sharpens the distinction between theories in which this does or does not occur.
64 citations
TL;DR: The low-temperature effective mass is found to be m * /m b =325±50 in the zero-frequency limit and its temperature dependence is compared with that of the thermodynamic parameters.
Abstract: We report on optical conductivity measurements performed on ${\mathrm{CeAl}}_{3}$ spanning the photon energy range between approximately zero and 12 eV. From the measurements at micrometer- and millimeter-wave frequencies we obtained the detailed temperature dependence of both components of the complex conductivity. These results as well as the Kramers-Kronig analysis performed on the absorptivity lead, at low temperatures, to a strongly frequency- and temperature-dependent dynamic conductivity. We analyze the optical conductivity in terms of frequency-dependent scattering rate and dynamical mass. The low-temperature effective mass is found to be ${\mathit{m}}^{\mathrm{*}}$/${\mathit{m}}_{\mathit{b}}$=325\ifmmode\pm\else\textpm\fi{}50 in the zero-frequency limit and its temperature dependence is compared with that of the thermodynamic parameters.
54 citations
TL;DR: An intrinsic double resistive transition present in all superconducting compounds is attributed to the granularity of these polycrystalline samples.
Abstract: We report a systematic study of the double resistive superconducting transition in carefully prepared polycrystalline samples of the electron-doped superconducting series ${\mathrm{Sm}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathit{y}}$ for 0.12\ensuremath{\le}x\ensuremath{\le}0.20. From high-resolution x-ray-diffraction measurements, the solubility limit of Ce in this series is slightly greater than 0.16. An intrinsic double resistive transition present in all superconducting compounds is attributed to the granularity of these polycrystalline samples. There is a partial transition when the grains become superconducting at a temperature ${\mathit{T}}_{\mathit{c}1}$, and coupling between grains at a lower temperature ${\mathit{T}}_{\mathit{c}2}$ apparently completes the transition. Only the transition associated with coupling between grains is observed in magnetic-susceptibility measurements due to inhomogeneous grains and large penetration depths. From compositional dependences of resistive and magnetic measurements, the superconducting volume fraction increases linearly with increasing dopant concentration up to the solubility limit, while the best coupling between grains is at a dopant concentration of 0.15. Thus, there is a subtle relationship between doping and coupling in this series. Also, arguments for a true thermodynamic phase transition at ${\mathit{T}}_{\mathit{c}2}$ are given.
46 citations
TL;DR: In this article, high-resolution X-ray powder diffraction, electrical resistivity, and magnetic susceptibility measurements were performed on polycrystalline samples of Nd 1.85 Ce 0.15 CuO 4 y obtained from a sol-gel precursor.
29 citations
TL;DR: In this article, a study of Bi-Mo catalysts prepared from pure oxides (MoO3 and Bi2O3) by solid state reaction methods is presented, where the structure characterization by X-ray diffraction shows only the low temperature (koechlinite) and high temperature (γ(H)) phases in varying proportions depending on the calcination temperature.
Abstract: We present a study of Bi-Mo catalysts prepared from pure oxides (MoO3 and Bi2O3) by solid state reaction methods. The structure characterization by X-ray diffraction shows only the low temperature (koechlinite) and high temperature (γ(H)) phases in varying proportions depending on the calcination temperature. The carbon monoxide oxidation shows a synergetic effect for mixed oxides as compared to pure oxides. Significant differences in catalytic activity, surface morphology and surface concentration were observed when the molybdenum precursor was changed from molybdenum trioxide to ammonium heptamolybdate in the catalyst preparation.
20 citations
TL;DR: In this paper, the authors show that the position and shape of the anomaly is mainly controlled by the density of carriers in the superconducting CuO2 planes, and not by structural properties.
Abstract: Low‐temperature specific‐heat measurements were performed between 0.4 and 4 K on Nd–Ba–Cu–O‐samples doped with Ca and additional Nd. In these samples, the specific‐heat anomaly due to magnetic ordering of the Nd3+ ions as a function of hole concentration behaves similarly to that of oxygen‐deficient NdBa2Cu3Ox. In samples with 0
18 citations
TL;DR: In this article, heat capacity, resistance, and magnetic susceptibility have been measured on FeSi, Fe0.90Co0.10Si, and CoSi single crystals in the temperature range 1.5 K to 1.6 K.
Abstract: Heat capacity, resistance, and magnetic susceptibility have been measured on FeSi, Fe0.90Co0.10Si, Fe0.10Co0.10Si, and CoSi single crystals. From resistance experiments, the activation energy (Δ) shows a small variation from Δ≈310 K in FeSi to Δ≈325 K for the Fe0.90Co0.10Si compound. For the 90% Co compound the gap signature disappears completely. Heat capacity experiments performed in the temperature range 1.5 K
17 citations
TL;DR: The thermal expansion of the gapped intermetallic heavy-fermion compounds CeNi Sn, CeNi 0.9 Cu 0.1 Sn, and Ce 1-x La x NiSn is studied; and the transport and magnetic properties of CeNiSn are studied.
Abstract: We have studied the thermal expansion of the gapped intermetallic heavy-fermion compounds CeNiSn, CeNi 0.9 Cu 0.1 Sn, and Ce 1-x La x NiSn (x=0.03, 0.1) between 0.5 K and 25 K; and the transport (3
TL;DR: The pressure dependence of χ(T) is significantly weaker than that of the specific-heat coefficient C(T)/T; it is discussed whether this can be resolved by consideration of intersite antiferromagnetic interactions, or the quadrupolar Kondo-effect model, and it is concluded that the latter provides a stronger explanation of the results.
Abstract: The pressure dependence of the static magnetic susceptibility χ(T) of the heavy-electron superconductor UBe13 was investigated over temperatures ranging from 1.5 to 300 K and at pressures from 1 bar to 8 kbar. It is linear in pressure over the full temperature range, having a small constant pressure dependence at higher temperatures that increases rapidly below about T=150 K. The maximal value of the relative susceptibility change [1/χ(dχ/dP)] is about 1% per kbar. The Curie-Weiss law is obeyed in the temperature range 100 to 300 K with the effective magnetic moment peff=3.46μB independent of pressure, while the paramagnetic Curie temperature FTHETA∼-100 K becomes more negative with pressure. The lattice parameter of cubic UBe13 was measured as a function of pressure, permitting a determination of a compressibility value of -9.2×10-4 kbar-1 near ambient pressure. The pressure dependence of χ(T) is significantly weaker than that of the specific-heat coefficient C(T)/T; we discuss whether this can be resolved by (i) consideration of intersite antiferromagnetic interactions, or (ii) the quadrupolar Kondo-effect model, and conclude that the latter provides a stronger explanation of the results. © 1993 The American Physical Society.
TL;DR: The irreversibility line inferred from these measurements is sample dependent, and the resistivity for [ital H][perpendicular][ital c] does not exhibit any anomalous behavior or a maximum.
Abstract: Magnetoresistance measurements on superconducting Nd[sub 2[minus][ital x]]Ce[sub [ital x]]CuO[sub 4[minus][ital y]] single crystals with [ital T][sub [ital c]] varying from 7 to 19 K were made with magnetic fields [ital H] applied parallel ([ital H][parallel][ital c]) and perpendicular ([ital H][perpendicular][ital c]) to the tetragonal c axis. For [ital H][parallel][ital c], the resistive superconductive transition curves gradually develop structure at low temperatures and high magnetic fields as [ital T][sub [ital c]] decreases. For the crystals with [ital T][sub [ital c]][approx lt]11 K, this structure evolves into a double resistive transition with a resistivity maximum near [ital T][approx]1.1 K, followed by an extremely sharp transition into the superconducting state. In this region, the resistivity exhibits non-Ohmic behavior for low current densities and Ohmic behavior for larger measuring currents. The resistivity for [ital H][perpendicular][ital c] does not exhibit any anomalous behavior or a maximum. The irreversibility line inferred from these measurements is sample dependent.
TL;DR: In this article, a review of the electronic structure of Y1−xUxPd3 was presented along with new data on Fermi level tuning in alloy and in Th 1−x Uxpd3.
Abstract: This paper presents a review of the electronic structure of Y1−xUxPd3, along with new data on Fermi level tuning in alloy and in Th1−xUxPd3.
TL;DR: In this article, low-temperature AC magnetic susceptibility measurements on the heavy rare-earth antiferromagnetic cuprates R 2 CuO 4 which have a T' Nd 2CuO 4 -like crystal structure are reported, and a single susceptibility maximum is observed in all the compounds at different temperatures ranging from 1.5 K to 10 K.
Abstract: Low-temperature AC magnetic susceptibility measurements on the heavy rare-earth antiferromagnetic cuprates R 2 CuO 4 which have a T' Nd 2 CuO 4 -like crystal structure are reported. A single susceptibility maximum is observed in all the compounds at different temperatures ranging from 1.5 K to 10 K, which is interpreted as indicative of antiferromagnetic long-range ordering of the rare-earth ions. No double peak structure is observed in any of these weak ferromagnetic compounds, at variance with the weak ferromagnetic compound Gd 2 CuO 4 for which a second sudden transition, of uncertain origin, has been previously observed at T L ≈20 K. The overall dependence of the antiferromagnetic ordering temperature on the single ion rare-earth magnetic moments do not follow a simple scaling with the De Gennes factor but rather a scaling with the full rare-earth magnetic moments, suggesting a pseudo-dipolar origin for the leading magnetic interaction coupling the rare-earth ions. Some important deviations are observed, however, which may be indicative of strong crystal field effects. It is suggested that the Suhl-Nakamura mechanism may be a relevant mechanism to understand the rare-earth magnetic ordering in the R 2 CuO 4 series (R=Nd-Tm).
TL;DR: In this article, the effects of oxygen on the magnetic order of superconducting NdBa2Cu3O6+x powders were investigated. But the results were limited to a small reduction of oxygen to x=0.78.
Abstract: Neutron scattering and specific heat measurements have been carried out on a series of NdBa2Cu3O6+x powder samples with varying oxygen concentrations (x=0.94, 0.78, 0.45, 0.3, 0.13) in order to study the effects of oxygen on the magnetic order of Nd. For our highest oxygenated superconducting NdBa2Cu3O6.94 (Tc≂92 K), three‐dimensional (3D) long range order develops below TN≂0.53 K, with a spin configuration which is antiferromagnetic along all three crystallographic axes. For a small reduction of oxygen to x=0.78, we observe drastic effects on the Nd order; 3D order is inhibited and only short range 2D correlations are found at low temperature. However, the 2D correlations develop at a much higher temperature, near 1.5 K. At (nonsuperconducting) x=0.45, the 3D magnetic order is better developed at low temperature, yet long range order still does not occur. For x=0.3, 3D long range magnetic order of the Nd ions is reestablished, developing below TN≂1.5 K, three times that of the fully oxygenated material. ...
TL;DR: Inelastic neutron scattering measurements have been performed on a polycrystalline specimen of Y0.8U0.2Pd3 and on the nonmagnetic analog compound Y 0.8Th 0.2pd3 using unpolarized neutrons as discussed by the authors, and the integrated intensities are consistent with Γ5 and Γ4 first and second excited state triplets lying above a Γ3 doublet ground state for tetravalent U ions in a cubic CEF.
Abstract: Inelastic neutron scattering measurements have been performed on a polycrystalline specimen of Y0.8U0.2Pd3 and on the nonmagnetic analog compound Y0.8Th0.2Pd3 using unpolarized neutrons. The magnetic contribution to the inelastic spectrum at T=10 K exhibits two distinct peaks centered at energies E≃5 meV and 16 meV, attributed to broadened crystalline electric field (CEF) levels, with half widths at half maximum of ∼3 meV and ∼8 meV, respectively. The integrated intensities are consistent with Γ5 and Γ4 first and second excited state triplets lying above a Γ3 doublet ground state for tetravalent U ions in a cubic CEF. The absence of a pronounced low-energy (⩽4 meV≃kBTK) quasielastic response provides further evidence that the nonmagnetic γ3 doublet lies lowest in energy. These observations support the interpretation of the thermal, magnetic and transport properties in terms of the two-channel quadrupolar Kondo effect.
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TL;DR: In this article, the specific heat of diluted YbBiPt has been investigated and it was shown that in this material the Kondo and crystal-field energy scales are small and of comparable magnitude, placing it in the same class as many Uranium heavy-electron compounds.
Abstract: YbBiPt has a low temperature linear specific heat coefficient of 8J/mole-Yb K{sup 2} and a small specific-heat anomaly at 0.4K. We discuss new experiments on specific-heat of diluted YbBiPt, and magnetic field dependent effects and electrical resistivity in pure YbBiPt. We argue that in this material the Kondo and crystal-field energy scales are small and of comparable magnitude, placing YbBiPt in the same class as many Uranium heavy-electron compounds.
TL;DR: In this article, the nuclear spin lattice relaxation rate of 29Si was shown to decrease with Re substitution, and the system was in the Fermi liquid state for x ⩽ 0.3.
Abstract: In U(Ru1−xRex)2Si2, the nuclear spin lattice relaxation rate of 29Si shows that the density of states in the spin density wave state decreases with Re substitution, and that the system is in the Fermi liquid state for x ⩽ 0.3. The broadened NMR line for x ⩾ 0.2. Indicates the appearance of ferromagnetic ordering.
TL;DR: In this article, the structure and catalytic properties of Bi2MoO6 compounds for CO oxidation were characterized using X-ray diffraction and Auger analysis, showing the presence of a Koechlinite phase in low temperature, and a gamma phase (Watanabe) in high temperature.
Abstract: The aim of this study is to characterize the structure and catalytic properties of Bi2MoO6 compounds, for CO oxidation. These catalysts were prepared by solid state methods with an intended atomic ratio Bi/Mo=2. X-ray diffraction shows the presence of a Koechlinite phase in the samples calcined at low temperature, and a gamma phase (Watanabe) in the samples calcined at high temperature. However, all samples appeared to be bismuth deficient (compared to the stoichiometric ratio of two), as indicated by Auger analysis. The catalytic activity shows a CO2 yield better than 40% for these catalysts.
TL;DR: In this paper, strong evidence of granularity in polycrystalline Nd1.85Ce0.15CuO4−y specimens under applied magnetic fields as high as 8 kOe have been revealed.
Abstract: Systematic studies of transport and magnetic properties on polycrystalline Nd1.85Ce0.15CuO4−y specimens under applied magnetic fields as high as 8 kOe have revealed strong evidence of granularity in this system.Electrical resistivity measurements show two superconducting transitions. One of the transitions occurs at higher temperature and is almost insensitive to applied magnetic field and current density, while the other one occurs at lower temperature and is strongly modified by the application of magnetic field and changes in the current density. In the low‐field limit (H<100 Oe), a drastic broadening of the superconducting transition is observed. Magnetic susceptibility χac measurements show similar features. There are two well‐defined contributions to the diamagnetic signal below 20 K which are particularly evident in the resistive component of χac where two peaks can be identified. The application of an applied magnetic field shifts both peaks towards low temperatures. All the results can be satisfactorily explained in terms of weakly linked Josephson junctions between superconducting islands. The islands have a superconducting transition at higher temperature, and the long‐range order phase coherence is achieved through Josephson coupling at lower temperatures.
TL;DR: In this article, the average static random internal field H int at the muon site is found to decrease very rapidly with U concentration x, indicating that the ordered static U moment rapidly disappears with decreasing U concentration around x ≅0.2.
Abstract: Muon spin relaxation (μSR) measurements have been performed on polycrystalline specimens of Y 1− x U x Pd 3 with x =0.1, 0.2, 0.4 and on UPd 4 . Spin-glass-like magnetic order was observed in UPd 4 at T g ≅13 K and in Y 0.6 U 0.4 P d 3 at T g ≅12 K. The average static random internal field H int at the muon site is found to decrease very rapidly with U concentration x , indicating that the ordered static U moment rapidly disappears with decreasing U concentration around x ≅0.2. Our results are consistent with the conjecture that Kondo screening eliminates the magnetic moment around this concentration.
TL;DR: In this article, a direct method for determining the magnetic field Hfcs at which a superconductor enters the fully critical state (i.e., the field at which flux first penetrates throughout the entire sample) is described, and a formula for calculating the critical current density JcH from the value of HFcs is presented.
Abstract: A direct method for determining the magnetic field Hfcs at which a superconductor enters the fully critical state (i.e., the field at which flux first penetrates throughout the entire sample) is described, and a formula for calculating the critical current density JcH from the value of Hfcs is presented. The behavior of JcH(x) is compared to the variation of the fluxoid pinning energy U, inferred from magnetic relaxation measurements in the Y1−xPrxBa2Cu3O7−δ system.