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1

Digitale Medien

Elastic anomalies in iron-platinum invar alloys : The 5th Joint MMM−Intermag Conference (1991)

Kawald, U. ; Schulenberg, P. ; Bach, H. ; [weitere]

[S.l.] : American Institute of Physics (AIP)

Journal of Applied Physics 70 (1991), S. 6537-6539

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ISSN: 1089-7550

Quelle: AIP Digital Archive

Thema: Physik

Notizen: Elastic anomalies in an Fe75Pt25 single crystal have been investigated by ultrasonic pulse echo and inelastic neutron scattering measurements in the temperature range 200 to 800 K. The single crystal which was grown by the Bridgman–Stockbarger process undergoes a magnetic transition at Tc = 370 K and a martensitic transformation at Ms = 200 K corresponding to a degree of order of S=0.7. Both ultrasonics and neutron scattering reveal a softening of the transverse modes below Tc. The excess elastic constants obey an M2 law as expected from a local magnetoelastic interaction model. The longitudinal ultrasonic velocities already start to soften at 700 K whereas the neutron data for the same acoustic branch do not show any anomaly down to 200 K. The observed q dependences of the acoustic branches point towards a softening confined to the very center of the Brillouin zone.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.349899

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2

Digitale Medien

"Magic number'' carbon clusters: Ionization potentials and selective reactivity (1991)

Zimmerman, Jeffrey A. ; Eyler, John R. ; Bach, Stephan B. H. ; [weitere]

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 94 (1991), S. 3556-3562

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ISSN: 1089-7690

Quelle: AIP Digital Archive

Thema: Physik , Chemie und Pharmazie

Notizen: The ionization potentials (IPs) of several large carbon clusters Cn (n≥48), including the enhanced abundance ("magic number'') clusters C50, C60, and C70, have been determined by Fourier transform ion cyclotron resonance (FTICR) mass spectrometric charge transfer bracketing experiments. The IPs of C50, C60, and C70 were bracketed by the same two charge transfer compounds, leading to a common value of 7.61±0.11 eV. The IPs of even numbered clusters adjacent to these magic number clusters were found to be lower by as much as 0.5 eV and all clusters between C50 and C200 were determined to have IPs greater than 6.20 eV. The reaction rates of C+60 and C+70 with metallocenes were anomalously slow in comparison to the other large carbon cluster ions. IP and reactivity results suggest that C50, C60, and C70 may indeed have different or more stable structures than neighboring clusters, which supports the hypothesis of closed-shell, spherical species. The implications of these results for the mechanism of C+n formation by direct laser vaporization are also discussed.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.459778

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3

Digitale Medien

Determination of carbon cluster ionization potentials via charge transfer reactions (1990)

Bach, Stephan B. H. ; Eyler, John R.

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 92 (1990), S. 358-363

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ISSN: 1089-7690

Quelle: AIP Digital Archive

Thema: Physik , Chemie und Pharmazie

Notizen: Ionization potentials (IPs) for carbon clusters containing 6–24 atoms have been determined from charge transfer reactions of carbon cluster ions with compounds of known ionization potential in a Fourier transform ion cyclotron resonance mass spectrometer. Cluster IPs generally decrease with increasing cluster size, but the IPs for clusters containing 4n+3 atoms (n=1–5) are found to be ∼0.5 eV lower than those of neighboring clusters. The relationship between cluster IP and structure is discussed.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.458437

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4

Digitale Medien

Absolute measurements of the equations of state of low-Z materials in the multi-Mbar regime using laser-driven shocks (1997)

Cauble, R. ; Da Silva, L. B. ; Perry, T. S. ; [weitere]

[S.l.] : American Institute of Physics (AIP)

Physics of Plasmas 4 (1997), S. 1857-1861

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ISSN: 1089-7674

Quelle: AIP Digital Archive

Thema: Physik

Notizen: Although high intensity lasers offer the opportunity to explore the equations of state (EOSs) of materials under high energy density conditions, experimental difficulties have limited the application of laser-driven shocks to EOS measurements. However, we have recently performed absolute EOS measurements on the principal Hugoniot of liquid deuterium near one Mbar and of polystyrene from 10 to 40 Mbar. The D2 measurements were made with direct drive; the polystyrene experiments were indirectly driven. The data were sufficiently accurate to differentiate between existing EOS models and were surprising, particularly for D2. The results demonstrate that laser driven shocks can be used effectively to investigate high pressure EOSs. © 1997 American Institute of Physics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.872362

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5

Digitale Medien

A pneumatically powered mechanical translator–rotator for the direct laser vaporization of solid materials (1997)

Stone, Earle G. ; Bach, Stephan B. H.

[S.l.] : American Institute of Physics (AIP)

Review of Scientific Instruments 68 (1997), S. 1547-1552

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ISSN: 1089-7623

Quelle: AIP Digital Archive

Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik

Notizen: A pneumatically powered mechanical translator–rotator system has been designed and constructed for use in the direct laser vaporization (DLV) of materials. This translator–rotator was initially developed for the reproducible DLV production of refractory metal atoms to be reacted with small molecules and characterized in matrix isolation experiments, but has applications wherever a reproducible DLV stream of a material is required, such as matrix assisted laser desorption ionization. Key features of the new translator–rotator design are the employment of an inexpensive air ratchet to provide power for the translator–rotator mechanism, the elimination of magnetic relays and electrical limit switches through the use of an all mechanical gear and slot mechanism, and a triple O-ring gland capable of maintaining high vacuum, 10−7 Torr, while the translator–rotator is in operation. © 1997 American Institute of Physics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.1147939

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6

Digitale Medien

Polarization analysis of the magnetic excitations in Fe65Ni35 Invar : 38th Annual Conference on Magnetism and Magnetic Materials (1994)

Lynn, J. W. ; Rosov, N. ; Acet, M. ; [weitere]

[S.l.] : American Institute of Physics (AIP)

Journal of Applied Physics 75 (1994), S. 6069-6071

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ISSN: 1089-7550

Quelle: AIP Digital Archive

Thema: Physik

Notizen: Triple-axis inelastic polarized neutron measurements have been performed as a function of temperature on a single crystal of the Invar alloy Fe65Ni35 to distinguish longitudinal from transverse magnetic excitations in the magnetically ordered phase. Well below the Curie temperature of 501 K the magnetic excitation spectrum is dominated by conventional transverse spin-wave excitations, which in fact follow the predictions of spin-wave theory very well. In particular, we find no evidence for propagating longitudinal excitations in this system, in sharp contrast to the behavior observed in the amorphous Invar Fe86B14 material as well as the non-Invar amorphous system Fe40Ni40P14B6.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.355460

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7

Digitale Medien

Temperature dependence of the magnetic excitations in ordered and disordered Fe72Pt28 : 38th Annual Conference on Magnetism and Magnetic Materials (1994)

Rosov, N. ; Lynn, J. W. ; Kästner, J. ; [weitere]

[S.l.] : American Institute of Physics (AIP)

Journal of Applied Physics 75 (1994), S. 6072-6074

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ISSN: 1089-7550

Quelle: AIP Digital Archive

Thema: Physik

Notizen: We have performed inelastic neutron scattering measurements of the long-wavelength spin wave excitations of both ordered and disordered Fe72Pt28 single crystals below their critical temperatures, TC=510 and 375 K, respectively. The spin waves followed the expected E=Dq2 dependence, and the temperature-dependent spin stiffness D decreased as (T/TC)5/2, as expected for an isotropic ferromagnet. The extrapolated zero-temperature spin stiffness was D=98(4) meV A(ring)2 and 107(1) meV A(ring)2 for the disordered and ordered alloy, respectively. These values are significantly higher than the zero-temperature stiffness as determined by magnetization measurements.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.355461

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8

Digitale Medien

Accuracy of the Hartree–Fock approximation for the Hubbard model (1997)

Bach, Volker ; Poelchau, Jurij

College Park, Md. : American Institute of Physics (AIP)

Journal of Mathematical Physics 38 (1997), S. 2072-2083

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ISSN: 1089-7658

Quelle: AIP Digital Archive

Thema: Mathematik , Physik

Notizen: A lower bound 0≥(Egs(n)−Ehf(n))/|Λ|≥−const[n2/3U4/3(|ln U|+1) +Un1/2|Λ|−1/2d(|ln(|Λ|−1/2)|1)] to the difference of the ground state and the Hartree–Fock energy of the Hubbard model is derived. Here |Λ| is the lattice size, U is the coupling parameter, and n is the electron density per site. This estimate holds for all dimensions d≥2 and all densities. Thus the Hartree–Fock approximation becomes exact (even beyond terms of order U) for small U and large |Λ|. © 1997 American Institute of Physics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.531922

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9

Digitale Medien

Integrated wavelength demultiplexer-receiver on InP (1992)

Bornholdt, C. ; Trommer, D. ; Unterbörsch, G. ; [weitere]

Woodbury, NY : American Institute of Physics (AIP)

Applied Physics Letters 60 (1992), S. 971-973

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ISSN: 1077-3118

Quelle: AIP Digital Archive

Thema: Physik

Notizen: A detector stage comprising a photodiode, a field-effect transistor, and a load resistor, and a wavelength demultiplexer have been monolithically integrated in the GaInAsP/InP material system. Chips were mounted into complete modules and operated in a 1.3 μm/1.55 μm bidirectional transmission link. At 576 Mbit/s and 10−9 bit error rate the sensitivity of the module was −21 dBm, while the intrinsic sensitivity of the receiver was determined to be −28 dBm.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.107460

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10

Digitale Medien

First transitions in circular Couette flow with axial stratification (1997)

Hua, Bach Lien

[S.l.] : American Institute of Physics (AIP)

Physics of Fluids 9 (1997), S. 365-375

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ISSN: 1089-7666

Quelle: AIP Digital Archive

Thema: Physik

Notizen: The first flow regimes which have been observed experimentally for a circular Couette flow with a stable, axial stratification in density are investigated through direct numerical simulations of the three-dimensional Navier-Stokes equations for a Boussinesq fluid. The setup of two concentric cylinders has a nondimensional gap width of ε=(b−a)/a=0.289; the outer cylinder is fixed and the stratification in density in the axial direction is linear. The main effect of an axial density stratification is to reduce the height of the Taylor vortices and to cause the formation of density layers of small aspect ratio. For large enough Prandtl number, the primary bifurcation from circular Couette flow is found to be axisymmetric and of Hopf-type in the direct numerical simulations. An analytical solution for onset of instability and slightly different boundary conditions from the experimental ones agrees within 0.6% with numerical simulations at a Prandtl number of 700. The experimental flow regimes with well-defined density layers are well reproduced by the numerical simulations in the appropriate range of relative Reynolds number Re/Rec1, where Rec1 denotes the critical Reynolds number for the primary bifurcation from circular Couette flow. However, the increase of axial scale with Re/Rec1 is found to be continuous, whereas it is quantized in the laboratory experiments. Numerical results reveal that the first two transitions between the flow regimes are primarily due to the temporal behavior of the axially symmetric part of the flow. Onset of nonaxisymmetric motions appears at the same Re/Rec1(approximate)1.18 as in the homogeneous fluid case at the same η=a/b. Stratification precludes large axial displacements and the azimuthal modes patterns have a quite distinct appearance from the homogeneous wavy modes. At large enough Re, a destabilization of the jet-like outflow between pairs of vortices causes the suppression of the density front which is located at the same axial height. This nonaxisymmetric flow regime presents common features with the wavy outflow boundary (WOB) pattern, which is commonly observed in the homogeneous Couette-Taylor case. © 1997 American Institute of Physics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.869237

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