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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 6323-6330 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The effects of 27Al (I=5/2) nuclear quadrupole interaction on the electron spin echo modulation (ESEM) in the case of S=1/2 in disordered systems are analyzed using exact diagonalization of the spin Hamiltonian. The Hamiltonian parameters used in the calculations are those expected in Cu+2-exchanged zeolites. The g anisotropy is taken into account and the ESEM obtained using the spherical model approximation is compared with a trigonal correlated structure. It is found that in the 0.32–0.34 nm range of distances between the paramagnetic center and the Al nuclei all parameters related to the quadrupole tensor, namely the quadrupole interaction constant, the asymmetry parameter and the relative orientation with respect to the g tensor, have a substantial effect on the modulation depth.
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 58 (1987), S. 624-627 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: An established method for measuring complex rf magnetic permeability is based on the change in inductance and resistance of a coaxial transmission line upon insertion of a sample toroid. It is not necessary to wind coils on the toroid or correct for geometric demagnetization factors. The use of modern commercial impedance analyzers, as described in this paper, makes measurements from 1 kHz to 1 GHz particularly easy, fast, and accurate.
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 5155-5163 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Deuterium NMR spectra are reported for several specifically deuterated hexaalkanoyloxytriphenylenes in their corresponding liquid crystalline phases. The higher homologs of this series are polymorphic and exhibit a variety of discotic mesophases, including both biaxial (D0 and D1) and uniaxial (D2) columnar phases. The ordering characteristics of these phases are studied using the quadrupolar splittings of the aromatic and aliphatic deuterons. The results show that during the transition from the biaxial D1 to the axial D2 phase the major susceptibility tensor switch orientation, apparently due to strong tilting of the molecules with respect to the columnar axis in the biaxial phase. Characteristic features which appear in the spectra of these phases are interpreted in terms of intercolumnar jumps of mesogen molecules.
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 64 (1988), S. 5914-5914 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Sintered solid copper-oxide superconductors generally have an intergranular coupling component that is evident in curves of ac susceptibility versus temperature: a broad peak in the imaginary (loss) part, χ‘, and a corresponding inflection in the real part, χ'.1 Polycrystalline Bi-Sr-Ca-Cu-O samples have been prepared from the melt. They are very different from typical sintered materials. Some have a single superconducting phase, while others have two superconducting phases. Most of the superconducting platelets are partially surrounded by nonsuperconducting material.2 An unusual feature of the susceptibility curves is that they show almost no coupling losses. This might be explained if the superconducting grains were isolated from each other, as is the case in susceptibility measurements of powdered superconductors.3 However, the characteristics of the melt-cast samples differ from those of powdered samples in three ways. (1) The resistivity goes to zero at low temperatures, which indicates some connectivity between crystallites. (2) χ' approaches −1 at low temperature, indicating substantial bulk shielding. (3) χ' decreases in magnitude with increasing field at low temperature, an effect not seen in powdered samples. We therefore believe that there is substantial superconductor grain-to-grain coupling, even though the susceptibility curves show little evidence of the usual intergranular coupling losses. We conclude that the coupling component in these melt-cast samples is virtually lossless.
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 74 (1993), S. 2501-2506 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Thin films of alternating Au, Ag, and Cu wedge-shaped metal layers were made by magnetron sputtering in order to develop a method for investigation of a multicomponent systems. X-ray-diffraction patterns of all as-deposited samples revealed satellites in the vicinity of (111) reflections, evidence of composition-modulated artificial superlattice formation. The experimental spectra are well explained by the kinematical diffraction theory for imperfect superlattices. Computer simulations have demonstrated extremely high sensitivity to the fluctuations ΔH/H of the superlattice period H. As a result, a rapid and simple method of ΔH/H determination is proposed.
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 2115-2118 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Interfilament contact in Nb3Sn wires made by the internal-tin-diffusion process causes excess hysteresis loss beyond the intrinsic magnetic hysteresis loss of the filaments. In analogy with eddy-current and proximity-effect coupling losses, the excess contact loss can be reduced by decreasing the twist-pitch length of the filaments in the wire. One consequence of interfilament contact is that volume magnetization measurements are strongly dependent on sample length below about one twist pitch. We define a characteristic length whose reciprocal is equal to the sum of the reciprocals of the sample length and the twist pitch. Hysteresis loss is a universal function of characteristic length for different sample lengths and twist pitches. We discuss several experimental parameters for the magnetic determination of hysteresis loss.
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  • 7
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 67 (1990), S. 5476-5476 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An improved flux-integration magnetometer was used to measure the magnetization of a toroidal sample of sintered YBa2Cu3Ox high-critical-temperature (high-Tc) superconductor as a function of field amplitude, frequency, and temperature. The sample was wound with copper drive and sense windings, and electronic integration was used to obtain magnetic induction. Drive fields up to 144 kA/m (1800 Oe) were generated with a burst of ac current at 0.1–1000 Hz through the field coil. The integrated wave forms were digitally recorded and manipulated to yield calibrated hysteresis loops of induction B and magnetization M versus field H with no need to correct for demagnetization factor. Several interesting effects were observed in the hysteresis loops, illustrating the usefulness of this high-resolution measurement technique for the study of high-Tc materials. As a function of increasing field amplitude, hysteresis first appears at the lower critical field Hc1 of the intergranular coupling component. There is a change in the slope of the magnetization curve as the grains decouple at higher fields. Above a certain temperature, the grains are decoupled even at the lowest fields. Frequency effects were observed at temperatures just below the transition temperature.
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 7180-7182 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The translational-to-vibrational and rotational (T-V,R) excitation of H2O with 2.2 eV hydrogen atoms is studied by time-resolved Fourier-transform infrared spectroscopy. Up to 2900 cm−1 of rotational excitation is observed in (001), with a strong preference for in-plane rather than out-of-plane rotation for J≥7, which implies a unique collision process leading to rotational alignment.
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 6464-6476 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Electron spin echo envelope modulation (ESEEM) induced by 27Al nuclei can be used to characterize the interactions of paramagnetic transition metal cations with the framework of various aluminosilicates. The quantitative analysis of such systems is complex due to the 27Al nuclear quadrupole interaction which is often large and unknown. In order to obtain a better understanding of the various spectral features of the 27Al modulation, the ESEEM of a S= (1)/(2) I= (5)/(2) spin system in orientationally disordered systems was investigated in the frequency domain. The relative contributions of the various electron-nuclear double resonance (ENDOR) frequencies to the Fourier transform (FT) ESEEM spectrum were studied as a function of the size of the nuclear quadrupole coupling constant and of the relative orientation of the nuclear quadrupole tensor with respect to the g-tensor. The parameters range investigated is that expected from Cu2+ interacting with framework Al in zeolites. Two approaches were employed in the calculations, exact diagonalization of the nuclear Hamiltonian and a second order perturbation treatment of the quadrupole interaction. It is found that the second order perturbation approach applies up to e2qQ/h〈7 MHz. The conditions under which the FT-ESEEM spectrum is dominated by the || (1)/(2) 〉−||− (1)/(2) 〉 ENDOR transitions were explored as well. This occurs when e2qQ/h is relatively large and it strongly depends on the orientation of the quadrupole tensor. When the FT-ESEEM can be described only by the || (1)/(2) 〉−||− (1)/(2) 〉 ENDOR transitions, the simulations are significantly simplified since these can be calculated using the analytical expressions obtained by perturbation theory also for a quadrupole coupling constant as large as 10 MHz.
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  • 10
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 69 (1998), S. 3357-3364 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A versatile high power X-band (8.5–9.5 GHz) pulsed EPR/ENDOR (electron-nuclear double resonance) spectrometer which can generate hundreds of microwave (MW) and rf pulses is described. The pulse programmer is constructed from a word generator with 32 channels and 4 ns resolution, coupled to five digital delay generators which can produce a total of ten pulses with a resolution better than 1 ns. The spectrometer contains two MW and two rf channels that allow independent variation of the frequency, amplitude, and phase of the MW and rf pulses. The ENDOR probe head is based on a bridged loop gap (BLG) resonator, coupling is achieved via a coupling loop connected to a waveguide, and the rf coil serves as a MW shield as well. The adjustment of the coupling is done by an up/down motion of the of the resonator assembly with respect to the fixed coupling loop. A flexible and user friendly data acquisition program written in C++ (Borland version 4.5), which uses the Windows-95 Multiple Document Interface (MDI) programming model, was developed to run the spectrometer. This program allows easy programming of any pulse sequence with sophisticated phase cycling. The performance of the spectrometer is demonstrated by two experiments. The first is the triple resonance hyperfine-selective (HS) ENDOR experiment carried out on a frozen solution of the copper protein laccase. The second is the two-dimensional hyperfine-ENDOR (HYEND) correlation experiment performed on a single crystal of γ-irradiated malonic acid. © 1998 American Institute of Physics.
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