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  • 1
    Digitale Medien
    Digitale Medien
    A modified dual-slope method for heat capacity measurements of condensable gases (2000)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 71 (2000), S. 3841-3845 
    Details
    ISSN: 1089-7623
    Quelle: AIP Digital Archive
    Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik
    Notizen: A high resolution nonadiabatic method for measuring the heat capacity (CP) of bulk samples of condensable gases in the range of 7.5–70 K is described. In this method CP is evaluated by directly comparing the heating and cooling rates of the sample temperature for two algebraically independent heat pulse sequences without explicit use of the thermal conductance between sample and thermal bath. A fully automated calorimeter for rapid measurement of CP of molecular solids utilizing this technique is presented. The technique along with the automated calorimeter with a provision to apply external electric and magnetic fields is particularly useful for the study of continuous phase transitions in molecular solids as well as field induced changes in the heat capacity. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1290040
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  • 2
    Digitale Medien
    Digitale Medien
    Very high sensitivity ac capacitance bridge for the dielectric study of molecular solids at low temperatures (1999)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 70 (1999), S. 4055-4058 
    Details
    ISSN: 1089-7623
    Quelle: AIP Digital Archive
    Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik
    Notizen: An ac capacitance bridge with excellent stability and two parts per billion resolution in the real part of the dielectric constant at temperatures ranging from 4.2 to 80 K and in the audio frequency range (0.2–20 kHz) is reported. The method uses commercially available components as well as few homemade cables and capacitors. Construction details and data on the resolution are presented. © 1999 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1150035
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  • 3
    Digitale Medien
    Digitale Medien
    Ultralow power low noise ultrahigh frequency magnetic resonance spectrometer (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 157-162 
    Details
    ISSN: 1089-7623
    Quelle: AIP Digital Archive
    Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik
    Notizen: We describe a simple marginal oscillator that operates at ultralow power dissipation (≥20 μW at 750 MHz) for use in ultrasensitive low noise magnetic resonance detection at very low temperatures. We have operated the circuit down to 0.6 K with the sample maintained below 20 mK and in magnetic fields up to 4.0 T. The samples are maintained at mK temperatures by physically separating the sample from the rf coil.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1142980
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  • 4
    Digitale Medien
    Digitale Medien
    Experimental studies of hydrogen on boron nitride: II. NMR studies of orientational ordering of H2 (1995)
    Springer
    Journal of low temperature physics 100 (1995), S. 551-574 
    Details
    ISSN: 1573-7357
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract We report the results of NMR studies of thin films of hydrogen adsorbed on hexagonal boron nitride. Orientational ordering is observed below 1 K but the ordering is not complete, and a clear two-component ordering is observed. Molecules are either (i) almost completely ordered with local order parameters σ = 〈1 − 3/2Jz 2〉 clustered close to a maximum value of a σ ≅0.94 (comparable to the values for long range ordering in bulk samples at high ortho concentrations), and (ii) a large fraction of the molecules that remain nearly disordered with σ ⩽ 0.25. The degree of Orientational ordering depends on the number of hydrogen layers and on the ortho-hydrogen concentration, and these studies indicate that ordering occurs principally in the first four layers closest to the substrate, with weaker Orientational ordering in the outer layers near the free surface even at temperatures as low as 210 mK.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00751525
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  • 5
    Digitale Medien
    Digitale Medien
    Imperfect wetting of hydrogen in zeolite (1995)
    Springer
    Journal of low temperature physics 101 (1995), S. 367-372 
    Details
    ISSN: 1573-7357
    Schlagwort(e): 68.35.Md ; 76.20.+q ; 67.80.Gb
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract We have considered the theoretical dependencies of the amount of supercooling of liquid hydrogen as a function of pore size for constrained geometries in order to compare the different mechanisms for supercooling that can be observed: notably the inhibition of nucleation in small geometries and the use of surfaces for which there is imperfect wetting. Analysis of the dependence of the observed supercooling reported elsewhere for molecular hydrogen and hydrogen deutende on the pore size in zeolites supports the view that imperfect wetting occurs for small pores that have rough surfaces and dimension less than approximately 10Å.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00753322
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  • 6
    Digitale Medien
    Digitale Medien
    The Orientational Ordering in Dilute Solid N2–Ar Mixtures (1998)
    Springer
    Journal of low temperature physics 111 (1998), S. 365-370 
    Details
    ISSN: 1573-7357
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract NMR studies have been carried out to determine the nature of the orientational ordering of N 2 in dilute solid N 2–Ar mixtures. The orientational ordering for cubic lattices (x(N 2)≍50%) was compared to that observed for the hexagonal lattice structures for 57 〈 x(N 2) 〈 75%. The NMR line shapes for the cubic structures were found to be sharply defined and could only be explained in terms of a narrow distribution of order parameters. This is in contrast to the quadrupolar glass state with a very broad order parameter distribution observed for the hexagonal structures.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1022287720183
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  • 7
    Digitale Medien
    Digitale Medien
    Nuclear spin-lattice relaxation times of metallic antimony at low temperatures (1995)
    Springer
    Journal of low temperature physics 101 (1995), S. 611-615 
    Details
    ISSN: 1573-7357
    Schlagwort(e): 07.20 Dt ; 76.60.-k ; 72.15.Lh
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract We have used pulsed Nuclear Quadrupole Resonance (NQR) techniques to measure the nuclear spin-lattice relaxation times in antimony at low temperatures. High quality echoes with strong signal/noise ratios were only observed for finely powdered samples of high purity (99.9999%). The samples were carefully annealed and diluted with fine silica to below the percolation limit to minimize RF heating. The powder mixture was immersed in liquid3He to ensure good thermal contact to a sintered silver heat exchanger attached to a copper nuclear demagnetization refrigerator. We report the temperature dependence of the nuclear spin-lattice relaxation times for the multiple level nuclear spin system.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00753362
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  • 8
    Digitale Medien
    Digitale Medien
    Theory of NMR multiple echoes in solid hydrogen (1995)
    Springer
    Journal of low temperature physics 101 (1995), S. 671-675 
    Details
    ISSN: 1573-7357
    Schlagwort(e): 76.60.Lz ; 67.90.+z ; 66.30.-h
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract We have examined the theory of NMR multiple echoes developed for solid3He to determine whether multiple echoes could be observed in solid hydrogen. We were particularly interested in the possibility of testing for low frequency quantum tunneling motions in solid hydrogen by the observation of multiple echoes. We find that for easily accessible nuclear spin polarizations, P 〉 12%, multiple echoes would be observed for HD impurities in solid parahydrogen if motional narrowing is effective in increasing the HD nuclear spin-spin relaxation time T2 to the order of 1 msec. These values for T2, which have been observed for HD impurity concentrations of the order of 1%, are larger than the calculated rigid lattice values and can be attributed to quantum tunneling at frequencies of the order of 1kHz.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00753372
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  • 9
    Digitale Medien
    Digitale Medien
    Ultra-Low Temperature Thermometry Using Zeeman Perturbed NQR (1998)
    Springer
    Journal of low temperature physics 110 (1998), S. 763-778 
    Details
    ISSN: 1573-7357
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract We report the results of measurements of the magnetic field perturbed nuclear quadrupole resonance (NQR) lines of powdered Sb immersed in liquid 3He at low temperatures. The magnetic field was used to separate the frequencies of the allowed transitions between the $$I_Z = \pm \frac{1}{2}$$ and $$\frac{3}{2}$$ levels that correspond to the zero field NQR absorption at 11.5289 MHz for121 Sb. The spectrum for the powder sample consists of two sharp lines with known separation. The ratios of the intensities of these two lines depends only on the difference in thermal population of the nuclear spin levels and can be used for thermometry at very low temperatures. Pulsed NQR measurements of this intensity ratio from 0.25 to 1.45 mK were in good agreement with the expected Boltzmann distributions. Temperature resolutions of 20% could be achieved for signal integration times of ∼ 1 hour.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1022610726294
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  • 10
    Digitale Medien
    Digitale Medien
    Nuclear Spin Relaxation of Powdered Metallic Antimony in Liquid 3He (1998)
    Springer
    Journal of low temperature physics 112 (1998), S. 21-45 
    Details
    ISSN: 1573-7357
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract Nuclear spin relaxation studies have been carried out to determine the surface relaxation of powdered Sb immersed in liquid 3 He. The surface relaxation mechanism was observed to be significantly more effective than the bulk Korringa relaxation rate for temperatures T 〈 75 mK. This surface relaxation is attributed to the modulation of the dipole-dipole interaction between the Sb spins at the metal surface and the 3 He spins in the solid-like layer on the surface by the quantum zero-point motion of these 3 He atoms. The total relaxation has been analyzed in terms of the surface relaxation and the spin diffusion through the bulk of the Sb particles. The experimental results are in excellent agreement with the theory developed for this system. Replacement of the 3 He by 4 He removes the surface relaxation component and restores the relaxation rates to the bulk value.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1022289811309
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