L. Passell - Academia.edu (original) (raw)
Papers by L. Passell
Krypton in a triangular 2D solid phase was observed to form on the (100) surfaces of MgO crystall... more Krypton in a triangular 2D solid phase was observed to form on the (100) surfaces of MgO crystallites. This system is unique because the physisorbed inert gas overlayer is not only incommensurate, but of a symmetry entirely unrelated to that of the substrate. Melting of the adsorbate occurred in the neighborhood of 70K for all coverages studied.
AIP Conference Proceedings, 1975
The Hybrid Spectrometer (HYSPEC), under construction at the SNS on beam line 14B, is the only ine... more The Hybrid Spectrometer (HYSPEC), under construction at the SNS on beam line 14B, is the only inelastic scattering instrument designed to enable polarization of the incident and the scattered neutron beams. A Heusler monochromator will replace the graphite crystal for producing polarized neutrons. In the scattered beam it is planned to use a collimator--multi-channel supermirror bender array to analyze the polarization of the scattered beam over the final energy range from 5-20 meV. Other methods of polarization analysis under consideration such as transmission filters using He³, Sm, and polarized protons are considered. Their performance is estimated and a comparison of the various methods of polarization is made.
The divalent europium chalcogenides include the only known examples of isotropic Heisenberg ferro... more The divalent europium chalcogenides include the only known examples of isotropic Heisenberg ferromagnets. As such they offer unique opportunities for comparing the existing highly developed theory of simple Heisenberg ferromagnets with experiments. We have used neutrons to study the dynamic behavior of EuO at temperatures extending up to ths Curie temperature T c and over a fairly wide range of wave vectors. Since the exact form of the scattering cross section is not known, only limited comparisons with the existing theory were possible. Dynamic scaling predictions of spin wave renormalization appear to be qualitatively correct, but the observed line width variations with wave vector and temperature are in only fair agreement with "hydrodynamic" predictions. As was the case with isotropic metallic ferromagnets, the EuO line shapes show no indication of a diffusive mode near T c. Work performed under the auspices of the U. S. Atomic Energy Commission. Work done while a Guest Scientist at Brookhaven National Laboratory.
Physical Review B, 1979
... 19 q (A-1) 0.45 0.50 0.55 0.60 0.75 1.00 1.25 1.40 2.00 0.45 0.50 0.55 0.60 0.75 1.00 1.25 1.... more ... 19 q (A-1) 0.45 0.50 0.55 0.60 0.75 1.00 1.25 1.40 2.00 0.45 0.50 0.55 0.60 0.75 1.00 1.25 1.40 2.00 l JA TARVIN AND L ... J. Greytak and J. Jan, in Proceedings of the Twelfth Inter-national Conference on Low Temperature Physics, Kyoto, 1970 (Academic of Japan, Tokyo, 1971). ...
Canadian Journal of Chemistry, 1988
A neutron diffraction study of the low temperature structure and loss of orientational order was ... more A neutron diffraction study of the low temperature structure and loss of orientational order was performed for the low density phase of ethylene adsorbed on graphite. At temperatures below 30 K the ethylene molecules form a centered-rectangular herringbone structure with their C=C bond parallel to the surface. At T > 30 K we observe a gradual loss of orientational order and a simultaneous increase in the near-neighbor spacing. This correlates well with the onset of rotational motions about an axis perpendicular to the C=C bond which remains predominately parallel to the surface plane. We speculate that the transition to this rotator phase is accompanied by a structural change to a triangular lattice.
Journal of Nuclear Energy. Parts A/B. Reactor Science and Technology
This report summarizes the results obtained to date from a study of the properties of reflecting ... more This report summarizes the results obtained to date from a study of the properties of reflecting multi-channel collimators. The measurements have not yet been completed but enough information is available to give an indication of the capabilities of the system.
Physical Review Letters, 1988
Elastic and quasielastic neutron-scattering results for the melting of submonolayer ethylene film... more Elastic and quasielastic neutron-scattering results for the melting of submonolayer ethylene films on graphite are described and microscopic evidence is presented for a continuous melting transition. Comparisons are made with data for equivalent methane films and with molecular-dynamics simulations. Differences in melting behavior are attributed to different vacancy creation mechanisms in the films.
Physical Review Letters
to be published.
Physical Review Letters
the excitation spectrum in liquid 'He. Only a microscopic calculation of the particle-hole intera... more the excitation spectrum in liquid 'He. Only a microscopic calculation of the particle-hole interaction at finite temperature can lead to a proper description of the dynamical structure factor at finite values of Q and~over a wide temperature range.
Journal of Physics C: Solid State Physics
The authors have made a detailed numerical analysis of neutron diffraction lineshapes across the ... more The authors have made a detailed numerical analysis of neutron diffraction lineshapes across the constant-coverage melting transition of incommensurate monolayers of methane on graphite. They find that in the liquid phase the liquid lineshape is well fitted with a lorentzian structure factor. While the exact nature of the transition very close to Tm cannot be commented upon, the 'freezing' of
Physical Review
Measurements were made of the ratio, ao/~an~, of the carbon to the hydrogen coherent scattering a... more Measurements were made of the ratio, ao/~an~, of the carbon to the hydrogen coherent scattering amplitudes, using liquid hydrocarbon mirrors with various ratios of hydrogen to carbon atoms. In two of these measurements the rejected neutron intensity from different liquids was compared for a 6xed angle of reflection. In two other measurements the angles were determined for which the same reflected intensity was obtained. Our most accurate value for the ratio of the nuclear amplitudes, after various corrections, is (ac/~aII~) (nuclear) =1.775&0.004. Combining this value with the presently accepted value of the carbon nuclear scattering amplitude, ac= (6.64&0.02) &(10 "cm, we find the best value for the proton scattering amplitude is an = (-3.74+0.02) &(10 "cm. I. INTRODUCTION HE neutron-proton coherent scattering amplitude is an important parameter in the study of nuclear forces and has been measured by a number of different techniques in recent years. Teller' was the first to point out that measurement of the neutron scattering by hydrogen should lead to information concerning spindependent forces. The first experiments, which involved the comparison of the slow neutron cross sections of orthoand para-hydrogen, ' 'proved that the rt-p force was spin dependent but did not yieM a very precise value for the scattering amplitude because the parahydrogen cross section, from which the scattering amplitude is derived, is small compared to the orthohydrogen cross section. As a consequence the measurement is very sensitive to the presence of small amounts of orthohydrogen impurity. The best value obtained in these experiments was' aH-(-3.95&0.12))&10 " cm. Later Shull et at. analyzed the Bragg scattering of neutrons by hydrogen in crystals' ' and obtained the value aH= (-4.1+0.2) &&10 " cm. They were not able to improve on the precision of the earlier experiments, however, because of the background from thermal diffuse and spin incoherent scattering of hydrogen and the lack of a satisfactory theory of inelastic neutron scattering. Both results lead to values of the e-e force which were different from the value of the rt-p force *This work was done under the auspices of the U. S. Atomic Energy Commission.
Chemischer Informationsdienst
Physica B+C, 1986
CsFeS 2 is regarded as an example of a quasi one-dimensional alternating antiferromagnetic Heisen... more CsFeS 2 is regarded as an example of a quasi one-dimensional alternating antiferromagnetic Heisenberg chain above about 70 K. In accordance with this picture, an energy gap of 10 meV was observed for the singlet to triplet excitation at a zone center near the transition temperature by our neutron measurements. The large excitation width implies a strong coupling of excitons to phonons, and LA phonons along the (001) direction become ill-defined for q greater than 0.3. At about 70 K, a first order transition occurs, whereby the singlet ground state changes to a N6el state and a simultaneous structural transformation takes place, which is a martensitic transformation. A mechanism for such martensitic transformation is discussed based on the similarity of the magneic excitation and phonon properties of the present compound and the y-Mn alloys with Fe and Cu, which are itinerant electron magnetic systems.
AIP Conference Proceedings, 1975
The temperature dependence of the small-angle critical scattering of neutrons from cobalt has bee... more The temperature dependence of the small-angle critical scattering of neutrons from cobalt has been measured both above and below its Curie temperature, Tc. Below Tc the scattering is dominated by well-defined spin wave modes which exhibit nearly quadratic dispersion. As Tc is approached, the spin waves renormalized and broaden but no evidence of the longitudinal component of the susceptibility is observed. Above Tc, the exponents gamma=1.23+/-0.05 and nu=0.65+/-0.04, describing the power law dependences of the static susceptibility and the inverse correlation range, respectively, have been obtained from the small-angle scans after taking full account of th einelasticity of the scattering. The energy widths of the scattering above Tc have been measured and are well described by a dynamical scaling function over a range of temperature and wave vector extending throughout the critical region and well into the hydrodynamic regime.
Journal of Applied Physics
ABSTRACT
Review of Scientific Instruments, 1959
Krypton in a triangular 2D solid phase was observed to form on the (100) surfaces of MgO crystall... more Krypton in a triangular 2D solid phase was observed to form on the (100) surfaces of MgO crystallites. This system is unique because the physisorbed inert gas overlayer is not only incommensurate, but of a symmetry entirely unrelated to that of the substrate. Melting of the adsorbate occurred in the neighborhood of 70K for all coverages studied.
AIP Conference Proceedings, 1975
The Hybrid Spectrometer (HYSPEC), under construction at the SNS on beam line 14B, is the only ine... more The Hybrid Spectrometer (HYSPEC), under construction at the SNS on beam line 14B, is the only inelastic scattering instrument designed to enable polarization of the incident and the scattered neutron beams. A Heusler monochromator will replace the graphite crystal for producing polarized neutrons. In the scattered beam it is planned to use a collimator--multi-channel supermirror bender array to analyze the polarization of the scattered beam over the final energy range from 5-20 meV. Other methods of polarization analysis under consideration such as transmission filters using He³, Sm, and polarized protons are considered. Their performance is estimated and a comparison of the various methods of polarization is made.
The divalent europium chalcogenides include the only known examples of isotropic Heisenberg ferro... more The divalent europium chalcogenides include the only known examples of isotropic Heisenberg ferromagnets. As such they offer unique opportunities for comparing the existing highly developed theory of simple Heisenberg ferromagnets with experiments. We have used neutrons to study the dynamic behavior of EuO at temperatures extending up to ths Curie temperature T c and over a fairly wide range of wave vectors. Since the exact form of the scattering cross section is not known, only limited comparisons with the existing theory were possible. Dynamic scaling predictions of spin wave renormalization appear to be qualitatively correct, but the observed line width variations with wave vector and temperature are in only fair agreement with "hydrodynamic" predictions. As was the case with isotropic metallic ferromagnets, the EuO line shapes show no indication of a diffusive mode near T c. Work performed under the auspices of the U. S. Atomic Energy Commission. Work done while a Guest Scientist at Brookhaven National Laboratory.
Physical Review B, 1979
... 19 q (A-1) 0.45 0.50 0.55 0.60 0.75 1.00 1.25 1.40 2.00 0.45 0.50 0.55 0.60 0.75 1.00 1.25 1.... more ... 19 q (A-1) 0.45 0.50 0.55 0.60 0.75 1.00 1.25 1.40 2.00 0.45 0.50 0.55 0.60 0.75 1.00 1.25 1.40 2.00 l JA TARVIN AND L ... J. Greytak and J. Jan, in Proceedings of the Twelfth Inter-national Conference on Low Temperature Physics, Kyoto, 1970 (Academic of Japan, Tokyo, 1971). ...
Canadian Journal of Chemistry, 1988
A neutron diffraction study of the low temperature structure and loss of orientational order was ... more A neutron diffraction study of the low temperature structure and loss of orientational order was performed for the low density phase of ethylene adsorbed on graphite. At temperatures below 30 K the ethylene molecules form a centered-rectangular herringbone structure with their C=C bond parallel to the surface. At T > 30 K we observe a gradual loss of orientational order and a simultaneous increase in the near-neighbor spacing. This correlates well with the onset of rotational motions about an axis perpendicular to the C=C bond which remains predominately parallel to the surface plane. We speculate that the transition to this rotator phase is accompanied by a structural change to a triangular lattice.
Journal of Nuclear Energy. Parts A/B. Reactor Science and Technology
This report summarizes the results obtained to date from a study of the properties of reflecting ... more This report summarizes the results obtained to date from a study of the properties of reflecting multi-channel collimators. The measurements have not yet been completed but enough information is available to give an indication of the capabilities of the system.
Physical Review Letters, 1988
Elastic and quasielastic neutron-scattering results for the melting of submonolayer ethylene film... more Elastic and quasielastic neutron-scattering results for the melting of submonolayer ethylene films on graphite are described and microscopic evidence is presented for a continuous melting transition. Comparisons are made with data for equivalent methane films and with molecular-dynamics simulations. Differences in melting behavior are attributed to different vacancy creation mechanisms in the films.
Physical Review Letters
to be published.
Physical Review Letters
the excitation spectrum in liquid 'He. Only a microscopic calculation of the particle-hole intera... more the excitation spectrum in liquid 'He. Only a microscopic calculation of the particle-hole interaction at finite temperature can lead to a proper description of the dynamical structure factor at finite values of Q and~over a wide temperature range.
Journal of Physics C: Solid State Physics
The authors have made a detailed numerical analysis of neutron diffraction lineshapes across the ... more The authors have made a detailed numerical analysis of neutron diffraction lineshapes across the constant-coverage melting transition of incommensurate monolayers of methane on graphite. They find that in the liquid phase the liquid lineshape is well fitted with a lorentzian structure factor. While the exact nature of the transition very close to Tm cannot be commented upon, the 'freezing' of
Physical Review
Measurements were made of the ratio, ao/~an~, of the carbon to the hydrogen coherent scattering a... more Measurements were made of the ratio, ao/~an~, of the carbon to the hydrogen coherent scattering amplitudes, using liquid hydrocarbon mirrors with various ratios of hydrogen to carbon atoms. In two of these measurements the rejected neutron intensity from different liquids was compared for a 6xed angle of reflection. In two other measurements the angles were determined for which the same reflected intensity was obtained. Our most accurate value for the ratio of the nuclear amplitudes, after various corrections, is (ac/~aII~) (nuclear) =1.775&0.004. Combining this value with the presently accepted value of the carbon nuclear scattering amplitude, ac= (6.64&0.02) &(10 "cm, we find the best value for the proton scattering amplitude is an = (-3.74+0.02) &(10 "cm. I. INTRODUCTION HE neutron-proton coherent scattering amplitude is an important parameter in the study of nuclear forces and has been measured by a number of different techniques in recent years. Teller' was the first to point out that measurement of the neutron scattering by hydrogen should lead to information concerning spindependent forces. The first experiments, which involved the comparison of the slow neutron cross sections of orthoand para-hydrogen, ' 'proved that the rt-p force was spin dependent but did not yieM a very precise value for the scattering amplitude because the parahydrogen cross section, from which the scattering amplitude is derived, is small compared to the orthohydrogen cross section. As a consequence the measurement is very sensitive to the presence of small amounts of orthohydrogen impurity. The best value obtained in these experiments was' aH-(-3.95&0.12))&10 " cm. Later Shull et at. analyzed the Bragg scattering of neutrons by hydrogen in crystals' ' and obtained the value aH= (-4.1+0.2) &&10 " cm. They were not able to improve on the precision of the earlier experiments, however, because of the background from thermal diffuse and spin incoherent scattering of hydrogen and the lack of a satisfactory theory of inelastic neutron scattering. Both results lead to values of the e-e force which were different from the value of the rt-p force *This work was done under the auspices of the U. S. Atomic Energy Commission.
Chemischer Informationsdienst
Physica B+C, 1986
CsFeS 2 is regarded as an example of a quasi one-dimensional alternating antiferromagnetic Heisen... more CsFeS 2 is regarded as an example of a quasi one-dimensional alternating antiferromagnetic Heisenberg chain above about 70 K. In accordance with this picture, an energy gap of 10 meV was observed for the singlet to triplet excitation at a zone center near the transition temperature by our neutron measurements. The large excitation width implies a strong coupling of excitons to phonons, and LA phonons along the (001) direction become ill-defined for q greater than 0.3. At about 70 K, a first order transition occurs, whereby the singlet ground state changes to a N6el state and a simultaneous structural transformation takes place, which is a martensitic transformation. A mechanism for such martensitic transformation is discussed based on the similarity of the magneic excitation and phonon properties of the present compound and the y-Mn alloys with Fe and Cu, which are itinerant electron magnetic systems.
AIP Conference Proceedings, 1975
The temperature dependence of the small-angle critical scattering of neutrons from cobalt has bee... more The temperature dependence of the small-angle critical scattering of neutrons from cobalt has been measured both above and below its Curie temperature, Tc. Below Tc the scattering is dominated by well-defined spin wave modes which exhibit nearly quadratic dispersion. As Tc is approached, the spin waves renormalized and broaden but no evidence of the longitudinal component of the susceptibility is observed. Above Tc, the exponents gamma=1.23+/-0.05 and nu=0.65+/-0.04, describing the power law dependences of the static susceptibility and the inverse correlation range, respectively, have been obtained from the small-angle scans after taking full account of th einelasticity of the scattering. The energy widths of the scattering above Tc have been measured and are well described by a dynamical scaling function over a range of temperature and wave vector extending throughout the critical region and well into the hydrodynamic regime.
Journal of Applied Physics
ABSTRACT
Review of Scientific Instruments, 1959