| 1. |
- Vos, M., et al.
(författare)
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Determination of the energy-momentum densities of aluminium by electron momentum spectroscopy
- 1999
-
Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 11:18, s. 3645-3661
-
Tidskriftsartikel (refereegranskat)abstract
- The energy-resolved momentum densities of thin polycrystalline aluminium films have been measured using electron momentum spectroscopy (EMS), for both the valence band and the outer core levels. The spectrometer used for these measurements has energy and momentum resolutions of around 1.0 eV and 0.15 atomic units, respectively. These measurements should, in principle, describe the electronic structure of the film very quantitatively, i.e. the dispersion and the intensity can be compared directly with theoretical spectral momentum densities for both the valence band and the outer core levels. Multiple scattering is found to hamper the interpretation somewhat. The core-level intensity distribution was studied with the main purpose of setting upper bounds on these multiple-scattering effects. Using this information we wish to obtain a full understanding of the valence band spectra using different theoretical models of the spectral function. These theoretical models differ significantly and only the cumulant expansion calculation that takes the crystal lattice into account seems to describe the data reasonably well.
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| 2. |
- Sohler, D., et al.
(författare)
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Yrast states of the proton drip line nucleus 106 Sb
- 1999
-
Ingår i: Physical Review C - Nuclear Physics. - 2469-9985 .- 2469-9993 .- 0556-2813 .- 1089-490X. ; 59:3, s. 1324-1327
-
Tidskriftsartikel (refereegranskat)abstract
- Yrast states of [Formula Presented] have been investigated in the [Formula Presented] reaction using in-beam [Formula Presented]-spectroscopic methods and in the [Formula Presented] reaction performing delayed [Formula Presented] and conversion electron studies. A new isomeric state was found at 103 keV with [Formula Presented] The number of states and transitions in the proposed level scheme have been doubled. The experimental results are discussed within the framework of the shell model. © 1999 The American Physical Society.
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| 3. |
- Karlsson, Magnus, 1967, et al.
(författare)
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Geometrically induced non-linear phenomena of the TE11-mode in a rectangular optical waveguide
- 1995
-
Ingår i: Physica Scripta. - : IOP Publishing. - 1402-4896 .- 0031-8949. ; 52:3, s. 313-317
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Tidskriftsartikel (refereegranskat)abstract
- Retaining all significant terms of Maxwell equations, we derive a gener-alized governing equation for waves in a Kerr-nonlinear, rectangulai optical waveguide. As a specific example, we show that the field com-ponents of the TEn-mode in such a waveguide will achieve oscillations in both amplitude and phase during propagation. The nonlinearity appeal only in a characteristic length scale of these phenomena, whereas tht overall properties of the oscillations are determined by the waveguidt geometry.
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| 4. |
- Kawamura, J., et al.
(författare)
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Low noise NbN lattice-cooled superconducting hot-electron bolometric mixers at submillimeter wavelengths
- 1997
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 70:12, s. 1619-1621
-
Tidskriftsartikel (refereegranskat)abstract
- Lattice-cooled superconducting hot-electron bolometric mixers are used in a submillimeter-wave waveguide heterodyne receiver. The mixer elements are niobium nitride film with 3.5 nm thickness and ∼ 10 μm2 area. The local oscillator power for optimal performance is estimated to be 0.5 μW, and the instantaneous bandwidth is 2.2 GHz. At an intermediate frequency centered at 1.4 GHz with 200 MHz bandwidth, the double sideband receiver noise temperature is 410 K at 430 GHz. The receiver has been used to detect molecular line emission in a laboratory gas cell.
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| 5. |
- Fjelstad, Jens, 1971-, et al.
(författare)
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Equivalence of Chern-Simons gauge theory and WZNW model using a BRST symmetry
- 1999
-
Ingår i: Physics Letters B. - : Elsevier. - 0370-2693 .- 1873-2445. ; 466:2-4, s. 227-233
-
Tidskriftsartikel (refereegranskat)abstract
- The equivalence between the Chern-Simons gauge theory on a three-dimensional manifold with boundary and the WZNW model on the boundary is established in a simple and general way using the BRST symmetry. Our approach is based on restoring gauge invariance of the Chern-Simons theory in the presence of a boundary. This gives a correspondence to the WZNW model that does not require solving any constraints, fixing the gauge or specifying boundary conditions.
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| 6. |
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| 7. |
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| 8. |
- Magnusson, Charlotte
(författare)
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The effect of the presence of quadrupole pairing on the properties of Kπ = 1+ states in doubly even nuclei
- 1991
-
Ingår i: Physica Scripta. - : IOP Publishing. - 0031-8949 .- 1402-4896. ; 43:5, s. 460-468
-
Tidskriftsartikel (refereegranskat)abstract
- The influence of a quadrupole pair interaction on Kπ = 1+ states in rare earth nuclei is investigated using the Quasiparticle Random Phase Approximation. Comparison of the obtained results with available experimental data indicates a rather weak quadrupole pair interaction in these nuclei. The calculation of the B(M1) strength is extended to higher energies to account for the recently observed higher lying M1-strength. The differential cross section of inelastic electron scattering is calculated within DWBA for lower lying Kπ = 1+ states and is seen to be consistent with the interpretation that the observed states around 3 MeV are of isovector convection current character and have their main M1-strength coming from natural parity proton components.
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| 9. |
- Nilsson, Thomas, 1965
(författare)
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Excursions Along the Drip Line
- 1995
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The drip lines are the outer boundaries of the chart of the nuclides, terminating the isobaric chains on both sides of the valley of .beta.-stability with respect to stability against particle emission. The neutron drip line is, at the moment, only experimentally accessible for the very lightest elements. When approaching the neutron drip line, the structure of the nuclei starts to show anomalies compared to stable nuclei. The best known case is 11Li, having an extremely dilute structure with two loosely bound neutrons surrounding a 9Li core with normal density. This kind of configuration has been labelled a neutron halo and can be found in several nuclei along the drip line. This work regards experimental studies of drip-line nuclei through dissociation reactions at high beam energies and spectroscopy of .beta.-delayed particles. The effect of cross-talk between neutron detectors has been measured and compared to the results of a simulation code. The measurements confirmed the validity of the code and showed a negligible contribution of cross-talk for the neutron-detector arrays used in experiments at GANIL. The .beta.-delayed charged particles following the precursors 6,8He have been studied at ISOLDE, CERN with a newly developed gas-detector telescope. The branching ratios for .beta.-delayed deuteron emission from 6He and .beta.-delayed triton emission from 8He were measured with improved accuracy and decreased low-energy cut-off compared to earlier experiments. In an experiment at GSI, dissociation of 8He, 11Li and 11Be, with energies of 240 MeV/u, 280 MeV/u and 460 MeV/u respectively, was studied. From the experimental data, cross sections and momentum distributions could be extracted. The reaction mechanisms could be probed by studying various reaction channels, revealing an invariability regarding the width of the neutron momentum distribution when concentrating upon the core-breakup reactions. The role of the unbound resonance 10Li has been elucidated by population through one-neutron stripping from 11Li and one-proton stripping from 11Be. The longitudinal and transverse momentum distributions from the reaction 11Li -> 9Li + X have been measured at the same beam energy. The widths were found to be identical, and the lack of Coulomb broadening in the transverse case indicates that the halo-removal reactions predominantly take place at large impact parameters.
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| 10. |
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| 11. |
- Schnack-Petersen, H, et al.
(författare)
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Superdeformed triaxial bands in Lu-163,165
- 1995
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Ingår i: Nuclear Physics A. - : Elsevier BV. - 0375-9474. ; 594:2, s. 175-202
-
Tidskriftsartikel (refereegranskat)abstract
- An experimental investigation of the nucleus 165Lu, using the reactions 138Ba(31P,4n) 165Lu and 150Sm(19F,4n) 165Lu at beam energies of E = 155 and 95 MeV, respectively, has been performed. Among other additions to the existing level scheme, a new band, with transition energies almost identical to a strongly deformed (β2 0.42) πi13/2[660 1/2+] band recently discovered in 163Lu has been established. A theoretical analysis of the structure of the two Lu isotopes, 165Lu and 163Lu is carried out by detailed calculations of total potential energy surfaces for specific configurations. By a diabatic treatment of crossings specific proton configurations as πi13/2[660 1/2+] are identified throughout the deformation space and as a function of spin. It is found as a general feature that well deformed local minima of considerable nonaxial symmetry coexist with a normal deformed global minimum. The depth of these local minima depend on configuration. The structure of the different global and local minima found in these surfaces are analysed and discussed in terms of occupation of available basis configurations and their orientation relative to the rotation axis. The strongly deformed minima are found to belong to a group of superdeformed triaxial structures, expected to appear at low energies for certain favourable combinations of proton and neutron numbers.
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| 12. |
- Svensson, C. E., et al.
(författare)
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Observation and Quadrupole Moment Measurement of the First Superdeformed Band in the A~60 Mass Region
- 1997
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Ingår i: Physical Review Letters. - 1079-7114. ; 79:7, s. 1233-1236
-
Tidskriftsartikel (refereegranskat)abstract
- A high-spin rotational cascade of six gamma rays has been observed in Zn-62. The quadrupole moment of 2.7(-0.5)(+0.7) e b measured for this band corresponds to a deformation beta(2) = 0.45(-0.07)(+0.10). The properties of this band are in excellent agreement with calculations that predict high-spin superdeformed bands in Zn-62 with deformations beta(2) = 0.41-0.49. These results establish a new region of superdeformation for nuclei with neutron and proton numbers N, Z approximate to 30-32.
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| 13. |
- Aleksandrovskii, A. N., et al.
(författare)
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Negative thermal expansion of fullerite C60 at helium temperatures
- 1997
-
Ingår i: Low temperature physics (Woodbury, N.Y., Print). - : AIP Publishing. - 1063-777X .- 1090-6517. ; 23:11, s. 943-946
-
Tidskriftsartikel (refereegranskat)abstract
- The thermal expansion of fullerite C60 has been measured in the temperature range 2–9 K. A compacted fullerite sample with a diameter of about 6 mm and height of 2.4 mm was used. It was found that at temperatures below ~ 3.4 K the linear thermal expansion coefficient becomes negative. At temperatures above 5 K our results are in good agreement with the available literature data. A qualitative explanation of the results is proposed
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| 14. |
- Andersson, Ove, et al.
(författare)
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Reorientational relaxation in C60 following a pressure induced change in the pentagon/hexagon equilibrium ratio
- 1995
-
Ingår i: Physics Letters A. - : Elsevier Science B.V.. - 0375-9601 .- 1873-2429. ; 206:3-4, s. 260-264
-
Tidskriftsartikel (refereegranskat)abstract
- The orientational structure of C60 depends on pressure and temperature. Pressurization below the glass transition temperature Tg can freeze in non-equilibrium orientational structures. The relaxation of such structures on heating through Tg has been studied through thermal conductivity measurements and the effects observed are explained in a simple model.
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| 15. |
- Andersson, Ove, et al.
(författare)
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Thermal conductivity of C60 at pressures up to 1 GPa and temperatures in the range 50-300 K
- 1996
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 54:5, s. 3093-3100
-
Tidskriftsartikel (refereegranskat)abstract
- The thermal conductivity λ of C60 shows anomalies near 260 K and 90 K which are associated with the well-established phase transition and glass transition, respectively. Both transition temperatures increase with pressure, at the rates 120 K GPa-1 and 62 K GPa-1, respectively. With increasing temperature, λ of the simple cubic (sc) phase increased below 170 K (glasslike behavior) but decreased above. The glasslike behavior of λ is probably due to a substantial amount of lattice defects. Possible reasons for the change of sign of dλ/dT near 170 K are discussed. In the face centered cubic (fcc) phase (T≳260 K at atmospheric pressure) λ was almost independent of temperature, a behavior which is far from that of an ordered crystal (λ∝T-1 for T≳Debye temperature). This result can be attributed to the molecular orientational disorder of the fcc phase. The relaxation behavior associated with the glassy state and its unusually strong dependence on thermal history are discussed briefly, and data which support a previously reported relaxation model are presented. At room temperature, the density dependencies of λ, (∂ lnλ/∂ lnρ)T, were 5.5 and 9.5 for the fcc and sc phases, which are values typical for an orientationally disordered phase and a normal crystal phase, respectively.
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| 16. |
- Chen, W., et al.
(författare)
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Electronic and geometric structure of clean Pt3Ti(111)
- 1994
-
Ingår i: Physical Review B. - 0163-1829 .- 1095-3795. ; 50:8, s. 5620-5627
-
Tidskriftsartikel (refereegranskat)abstract
- Photoemission spectra and scanning tunneling microscopy (STM) images of the clean Pt3Ti(111) surface are presented. Grazing-emission core-level spectra show that the topmost layer is pure platinum, modified compared with the Pt(111) surface. The Pt 4f levels at the surface are shifted 0.4 eV toward the Fermi level relative to bulk Pt3Ti while the Pt 4f and Ti 2p levels in the bulk are shifted 0.4 and 1.3 eV to higher binding energy relative to pure bulk platinum and titanium, respectively, Tunneling measurements show a surface with only metallic atoms and a small p(2×2) buckling. Our observations of molecularly adsorbed CO are not compatible with metallic titanium atoms at the surface and the STM data thus indirectly confirm that only platinum atoms are present in the topmost layer. Linear muffin-tin-orbital calculations of the bulk band structure and valence-band photoemission spectra reveal highly hybridized electron states between the Pt d and Ti d levels. The calculations give a minor charge transfer from Ti to Pt, 0.37 electrons per Ti atom, but the large core-level shifts reflect the stability of the alloy and the response to the excitation rather than the amount of charge transfer. The observed segregation of platinum to the surface and the altered electronic structure of the topmost layer, due to interaction with the underlying alloy, are in full agreement with earlier conclusions based on low-energy electron-diffraction measurements and on the chemical properties of the surface. © 1994 The American Physical Society.
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| 17. |
- Chen, W., et al.
(författare)
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Redox properties of titanium oxides on Pt3Ti
- 1995
-
Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 99:34, s. 12892-12895
-
Tidskriftsartikel (refereegranskat)abstract
- The morphology and electronic structure of surface-segregated titanium oxides on Pt3Ti(111) are presented. Core level photoemission spectra at grazing emission reveal two states of oxidation: a dominant and reducible four-valent oxide together with a small amount of a three-valent oxide is produced by oxidation in 0 2 at and below 400°C; an irreducible three-valent oxide by oxidation in 02 at and above 450 °C. The ratio between the active four-valent and the inactive three-valent oxides decreases with increasing oxidation temperature. The probability for reduction by CO is almost unity for the Ti 4+ oxide, and the conclusion must be that the four-valent oxide plays an active role for catalytic reactions. Scanning tunneling measurements relate these observations to changes in the dispersion and nucleation of the oxide overlayer. The four-valent oxide grows as islands with remaining areas open for CO adsorption while the three-valent oxide spreads on and blocks the crystal surface. Photoemission spectra relate these dispersion effects to an electronic interaction between the Ti 3+ oxide and adjacent Pt atoms. The above observations are in accordance with the common picture of dispersion effects in titania-supported SMSI catalysts and prove that interfacial energies play a crucial role whether the dominant phase is metallic or an oxide.
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| 18. |
- Lundin, Anders, et al.
(författare)
-
Compressibility and Structure of C70
- 1995
-
Ingår i: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 30:8, s. 469-474
-
Tidskriftsartikel (refereegranskat)abstract
- The compressibility of C70 has been investigated by means of direct piston and cylinder measurements in the temperature range 150 to 365 K and up to 1 GPa. At 296 and 343 K we find a very rapid change of volume with pressure below 0.15 GPa. We tentatively interpret this as a continuous molecular reorientation with pressure, probably resulting in a transformation from a mixture of face-centred cubic (f.c.c.) and rhombohedral (r.h.) phases to mainly pure r.h. phase. At 365 K as well as at 236 K and below we see no anomalies indicating any structural or rotational anomalies in the pressure range investigated. The zero-pressure bulk modulus decreases with increasing temperature, from 13.1 GPa at 185 K to 7.9 GPa at 365 K. Our volumetric measurements confirm a recently proposed phase diagram for C70.
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| 19. |
- Lundin, Anders, et al.
(författare)
-
Compressibility of C61D2 up to 1 GPa in the temperature range 175 - 345 K
- 1996
-
Ingår i: Carbon. - : Elsevier Science Ltd.. - 0008-6223 .- 1873-3891. ; 34:9, s. 1119-1121
-
Tidskriftsartikel (refereegranskat)abstract
- We have measured the bulk modulus K for C61D2 up to 1 GPa in the temperature range 175–343 K. For face-centered cubic C61D2 above 290 K, we find an anomalously low value for K below about 0.15 GPa, possibly indicating pressure-induced changes in the structure. The (extrapolated) zero-p bulk modulus K(0) decreases with increasing T from 6.7 GPa at 175 K to 5.2 GPa at 343 K. A comparison with hypothetical expanded f.c.c. C60 with the same lattice constant shows that K(0) values are similar, indicating that the main intermolecular interactions are still between molecular bellies, not the sidegroups.
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| 20. |
- Nagel, Peter, et al.
(författare)
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C60 one- and two-dimensional polymers, dimers, and hard fullerite: Thermal expansion, anharmonicity, and kinetics of depolymerization
- 1999
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 60:24, s. 16920-16927
-
Tidskriftsartikel (refereegranskat)abstract
- We report on high-resolution thermal expansion measurements of high-temperature-pressure treated C60 [one-dimensional (1D) and (2D) polymers and “hard fullerite”], as well as of C60 dimers and single crystal monomer C60 between 10 and 500 K. Polymerization drastically reduces the thermal expansivity from the values of monomeric C60 due to the stronger and less anharmonic covalent bonds between molecules. The expansivity of the “hard” material approaches that of diamond. The large and irreversible volume change upon depolymerization between 400 and 500 K makes it possible to study the kinetics of depolymerization, which is described excellently by a simple activated process, with activation energies of 1.9±0.1 eV (1D and 2D polymers) and 1.75±0.05 eV (dimer). Although the activation energies are very similar for the different polymers, the depolymerization rates differ by up to four orders of magnitude at a given temperature, being fastest for the dimers. Preliminary kinetic data of C70 polymers are presented as well.
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| 21. |
- Persson, Per-Axel, et al.
(författare)
-
High-pressure polymerized C60
- 1996
-
Ingår i: Extended Abstracts of Carbon´96, European Carbon Conference, Newcastle-upon-Tyne 1996, volume 2. - : The British Carbon Group. ; , s. 746-747, s. 746-747
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 22. |
- Persson, Per-Axel, et al.
(författare)
-
NMR and Raman characterization of pressure polymerized C-60
- 1996
-
Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 258:5-6, s. 540-546
-
Tidskriftsartikel (refereegranskat)abstract
- Bulk C60 has been treated at 1.1 GPa and 550–585 K, producing a dense insoluble material which on heating to above 600 K reverts to normal C60. Raman and IR studies on modified material show a large number of new lines, and the Raman pentagon pinch mode shifts from 1469 to 1458 cm−1 as on photopolymerization. MAS NMR shows one broadened line at the original C60 shift 144 ppm and a small peak at about 77 ppm due to the bridging carbons. None of the new resonances observed for C60 polymerized by other methods were observed. The results verify previously suggested polymeric structures where the fullerence cages are connected by four-membered rings.
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| 23. |
- Persson, Per-Axel, et al.
(författare)
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The physical properties of high-pressure polymerized C60
- 1997
-
Ingår i: Journal of Physics and Chemistry of Solids, volume 58, issue 11. - : Elsevier B.V.. ; 58:11, s. 1881-1885
-
Konferensbidrag (refereegranskat)abstract
- We have studied the structural, thermophysical, and spectroscopic properties of polymeric C60 obtained by high pressure treatment at pressures and temperatures near 1 GPa and 600 K. We present here a brief overview of our results for the structural and thermophysical properties and a more detailed report on recent results obtained by Raman spectroscopy on both thin films, polycrystalline, and single crystal material. The results presented include a comparison between Raman results for photopolymerized and pressure polymerized thin films and a preliminary estimate of the binding energy of polymeric C60.
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| 24. |
- Schröder, Elsebeth, 1966, et al.
(författare)
-
Diffusion von Teilchen in turbulenten Oberflächenwellen
- 1999
-
Ingår i: C. Nowak, I. Wilke, U Wollmer (eds.) ``Kiss the Future! Physikerinnen stellen sich vor'', Tagungsband der Deutschen Physikerinnentagung 1998, Kirchlinteln: Hoho-Verlag Hoffmann und Hoyer. - 9783929120127 ; , s. 171-175
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Wir haben die Diffusion von Teilchen in turbulenten Oberflächenwellen theoretisch und experimentell untersucht.Es ist seit langem bekannt, daß man mit Messungen der relativen Bewegung von Teilchenpaaren andere Informationen hervorhebt als mit Messungen der Bewegung einzelner Teilchen. Wir haben beides untersucht, hier wird aber hauptsächlich über unsere Ergebnisse der relativen Diffusion berichtet. Oberflächenwellen können durch eine Hamiltonsche Theorie beschrieben werden.Von dieser Theorie leiten wir quantitative Voraussagenfür die Korrelation von Geschwindigkeiten in Kapillar- undSchwerewellen her. Die theoretischen Voraussagen stimmen sehr gut mit unseren experimentellen Befunden überein.
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| 25. |
- Sing, M., et al.
(författare)
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Electronic structure studies of pressure-polymerized C60
- 1999
-
Ingår i: Synthetic Metals vol. 103. - : Elsevier B.V.. ; 103:1-3, s. 2454-2455
-
Konferensbidrag (refereegranskat)abstract
- We have investigated the electronic structure of one and two-dimensional C60 polymers with regard to both their low-lying excitations and C 1s excitation spectra by means of electron energy-loss spectroscopy in transmission. We compare the results with those for pristine C60. In general, the spectra for the polymers resemble those for pristine C60 but show a broadening due to both the lowering of the symmetry and the increased intermolecular overlap. This is also reflected by a reduction of the optical gap in comparison with pristine C60.
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| 26. |
- Soldatov, Alexander, et al.
(författare)
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Molecular rotation in C70 at high pressures: a thermal conductivity study
- 1996
-
Ingår i: Journal of Physics and Chemistry of Solids. - : Elsevier Science Ltd. - 0022-3697 .- 1879-2553. ; 57:9, s. 1371-1375
-
Tidskriftsartikel (refereegranskat)abstract
- We have measured the thermal conductivity ε and the heat capacity per unit volume varrhocp of highly pure C70 in the temperature interval 100–450 K under pressures up to 1 GPa. Anomalies indicating freezing of uniaxial molecular rotation were observed in λ and varrhocp upon both cooling and increasing pressure. The phase boundary for this transition has an approximate slope dT/dp = 70 K Gpa−1. The temperature and pressure dependence of λ indicate a substantial amount of structural defects in the sample and strong metastability effects.
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| 27. |
- Soldatov, Alexander, et al.
(författare)
-
Polymeric fullerenes: from C60 to C70
- 1999
-
Ingår i: AIP Conference Proceedings vol. 486. - Melville, NY : American Institute of Physics (AIP). - 1563969009 ; , s. 12-15
-
Konferensbidrag (refereegranskat)abstract
- For the first time polymerization of both powder and single crystals of C70 fullerene was established after their subjection to high pressure (1.1 - 2 GPa) at elevated temperature (500 - 580 K). High-resolution capacitance dilatometry, FTIR/Raman spectroscopy and thermal conductivity were employed to characterise the polymeric phase of C70. The results demonstrate drastic changes in the physical properties of C70 on polymerization. We report on a reverse transformation to the monomeric state on heating the polymer to 500 K at ambient pressure. The activation energy of depolymerization was determined to be 1.8(1) eV. We discuss our results in terms of existing structural models for polymerization of C70 and compare the physical properties of C70 and C60 polymers.
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| 28. |
- Soldatov, Alexander, et al.
(författare)
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Thermal conductivity of pressure polymerized C60
- 1997
-
Ingår i: Applied Physics A. - : Springer. - 0947-8396 .- 1432-0630. ; 64:3, s. 227-229
-
Tidskriftsartikel (refereegranskat)abstract
- We have studied the kinetics of C60 polymerization in the temperature interval 450-500 K at pressures below 1 GPa by measurements of the time dependence of the thermal conductivity. It has been found at 450 K that the polymerization process at 0.8 GPa is slower than the reverse transformation from ''polymeric'' to ''monomeric'' phase at 0.08 GPa. The thermal conductivity λ of polymerized C60 was measured in the temperature range 100-430 K and found to increase with increasing temperature, which reflects strong phonon scattering. Both the presence of non-bonded C60 molecules and a high degree of structural disorder in the crystalline lattice of the polymeric phase might be responsible for the behaviour of λ(T). The results for λ(T) are qualitatively similar to those reported previously for C60 polymerized at higher p, T but an order of magnitude smaller.
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| 29. |
- Soldatov, Alexander, et al.
(författare)
-
Transport and vibrational properties of pressure polymerized C60
- 1998
-
Ingår i: Molecular Materials (Molecular Crystals and Liquid Crystals, Section C), vol. 11. - Amsterdam : Overseas Publishers Association. ; 11:1-2, s. 1-6
-
Konferensbidrag (refereegranskat)abstract
- The vibrational spectrum of polycrystalline C60 polymerized at 1.1 GPa and 585 K was studied by inelastic neutron scattering. We find drastic changes in the spectrum compared to the vibrational spectrum of pristine C60: the appearance of a new, broad vibrational band at low energies, and splitting and significant changes in the peak positions of various modes. The thermal conductivity lambda of polymerized C60 was measured in the temperature range 150-320 K and was found to increase with a rise in temperature, which reflects strong phonon scattering. A high degree of structural disorder in the crystalline lattice of the polymeric phase is most probably responsible for the glass-like beahviour of lambda(T).
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| 30. |
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| 31. |
- Sundqvist, Bertil, et al.
(författare)
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Phase diagram, structure, and disorder in C60 below 300 K and 1 GPa
- 1995
-
Ingår i: Solid State Communications. - : Elsevier Science Ltd. - 0038-1098 .- 1879-2766. ; 93:2, s. 109-112
-
Tidskriftsartikel (refereegranskat)abstract
- Earlier structural studies have shown that the pentagon-to-hexagon orientation ratio in the orientationally ordered simple cubic phase of C60 decreases under pressure. From anomalies observed in the compressibility and thermal conductivity of C60 under pressure we have deduced a pressure-temperature phase diagram for this substance in the range below 300 K and 1 GPa (10 kbar). We conclude that C60 forms a new, completely “hexagon” ordered structural phase above about 0.6 GPa at 150 K (1.2 GPa at 300 K), and that the glass transition shifts upwards in T under pressure by 54 K GPa−1. However, above 0.1 GPa, pentagon-to-hexagon orientation relaxation seems to occur on heating at an almost pressure independent temperature near 100 K.
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| 32. |
- Sundqvist, Bertil, et al.
(författare)
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Structural and physical properties of pressure polymerized C60
- 1998
-
Ingår i: Carbon, vol. 36 issue 5-6. - : Elsevier B.V.. ; 36:5-6, s. 657-660
-
Konferensbidrag (refereegranskat)abstract
- We discuss the structural and dynamic properties of C60 polymerized under low-P, low-T conditions, and suggest that the disordered mixed orthorhombic-tetragonal-rhombohedral phases produced under these conditions arise from nucleation of molecular chains in random directions because of the quasi-free molecular rotation under standard reaction conditions in the fcc phase of C60. Polymerization in He gives results qualitatively different from those obtained in other media.
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| 33. |
- Winter, Johannes, et al.
(författare)
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Charge transfer in alkali-metal-doped polymeric fullerenes
- 1996
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 54, s. 17486-17492
-
Tidskriftsartikel (refereegranskat)abstract
- We present room-temperature Raman measurements of pressure-polymerized C60 and compare them with the spectra of RbC60 in the orthorhombic phase. Although both materials were prepared according to two completely different routes the spectra show a surprising similarity with respect to mode positions and line splitting. We concluded from this that both materials, the uncharged pressure-polymerized C60 and the rubidium-doped orthorhombic compound, have the same overall structure and the AC60 compounds can be considered as the doped species of the C60, polymerized using moderate low pressure and high temperatures. From a detailed comparison between both spectra mode shifting and line broadening as a consequence of the charge transfer was determined and electron-phonon coupling constants were estimated for the high-frequency Hg(7) and Hg(8) modes. The low values for the coupling constants compared to the ones in the K3C60 can explain the lack of superconductivity in the AC60 compounds.
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| 34. |
- Winter, Johannes, et al.
(författare)
-
Experimental access to doped fullerene polymers
- 1997
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Ingår i: Applied Physics A. - : Springer. - 0947-8396 .- 1432-0630. ; 64:3, s. 257-262
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Tidskriftsartikel (refereegranskat)abstract
- We present Raman measurements performed to study the experimental access to doped fullerene polymers. The polymeric orthorhombic AC60 compound could be obtained by slow cooling the high temperature fcc phase and by quenching with subsequent annealing. The various phases after quenching and during annealing were studied in detail. No evidence for a direct doping of undoped C60 to the polymeric AC60 phase was found. Due to the local character of the doping process the formation of A3C60 clusters is observed. The same results were obtained from doping experiments performed with undoped polymeric structures like phototransformed and pressure polymerized C60.
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| 35. |
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| 36. |
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| 37. |
- Chakalov, R, et al.
(författare)
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Fabrication and investigation of YBa2Cu3O7-delta/Ba0.05Sr0.95TiO3 thin film structures for voltage tunable devices
- 1998
-
Ingår i: Physica C: Superconductivity and its Applications. - 0921-4534. ; 308:3-4, s. 279-288
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Tidskriftsartikel (refereegranskat)abstract
- High-temperature superconducting/ferroelectric thin film structures were deposited by laser ablation. Three types of voltage tunable devices were fabricated and investigated-trilayer capacitor, planar interdigital capacitor and coplanar waveguide. As ferroelectric the solid solution BaxSr1-xTiO3 was chosen with barium content x = 0.05 because its Curie temperature is close to the liquid nitrogen boiling point (77 K). Temperature and voltage dependences of the Ba0.05Sr0.95TiO3 dielectric constant epsilon(r,BSTO) were studied. High epsilon(r,BSTO) values were determined-up to 3000 at 75 K, 20 GHz and zero de bias. Efficient voltage tunability was demonstrated (up to 40%) at loss level tan delta=0.01-0.1. Compact YBa2Cu3O7-delta/Ba0.05Sr0.95TiO3 coplanar waveguide with as narrow gap as 18 mu m was tested as electrically tunable phase-shifter and field-induced phase shifts of more than 180 degrees were obtained by 35 V de bias at 20 GHz. This improvement was attained by proper choice of the ferroelectric material, accomplished epitaxial growth of the films and decrease of the specific dimensions. (C) 1998 Published by Elsevier Science B.V. All rights reserved.
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| 38. |
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| 39. |
- Gevorgian, Spartak, 1948, et al.
(författare)
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HTS/ferroelectric devices for microwave applications
- 1997
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Ingår i: IEEE Transactions on Applied Superconductivity. - : Institute of Electrical and Electronics Engineers (IEEE). - 1558-2515 .- 1051-8223. ; 7:2, s. 2458 - 2461
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Tidskriftsartikel (refereegranskat)abstract
- High Temperature Superconducting (HTS, e.g. YBCO) microwave devices based on bulk or thin film ferroelectrics (e.g. Strontium Titanate-STO) are studied theoretically and experimentally. YBCO/STO/YBCO parallel-plate resonators and based on bulk single crystal STO are for electrically tunable high power filters, phase shifters and other devices in the frequency band 0.5-2.0 GHz of advanced microwave communication systems. Thin film YBCO/STO Inter-digital Capacitors (IDC), Coplanar Waveguides (CPW) phase shifters/delay lines are also designed and studied experimentally for low microwave power applications. Modelling problems of these devices and microwave losses in ferroelectrics are also discussed
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| 40. |
- Gevorgian, Spartak, 1948, et al.
(författare)
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The Optical Power Distribution and Scattering Losses in Symmetric Channel Waveguide X-Junction
- 1991
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Ingår i: Optical and Quantum Electronics. - 0306-8919 .- 1572-817X. ; 23, s. 649-656
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Tidskriftsartikel (refereegranskat)abstract
- It is shown theoretically and experimentally that there is sufficient light radiation at the crossing points of wave-guides, and sufficient coupling between input branches and output branches of integrated optic symmetric crossing channel wave-guides. For straight X-junctions the crosstalk could be changed in a wide range varying the crossing angle, while for widened X-junction with a fixed crossing angle the crosstalk could be changed by varying the length of the central straight waveguide section.
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| 41. |
- Gevorgian, Spartak, 1948, et al.
(författare)
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Tunable microwave devices based on bulk and thin film ferroelectrics
- 1998
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Ingår i: Integrated Ferroelectrics. - : Informa UK Limited. - 1058-4587 .- 1607-8489. ; 22:1-4, s. 245-257
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Tidskriftsartikel (refereegranskat)abstract
- Electrically tunable microwave resonators and filters based on bulk single crystal SrTiO3 and KTaO3 with superconducting plates are presented and discussed. Filters are designed for operation at frequencies 0.2 – 2.0 GHz. Thin film varactors and phase shifters, designed for cryogenic or room temperature operation and based on epitaxial SrTiO3 and BaSrTiO3 are also discussed. The frequency band for the thin film devices is 0.5 - 40 GHz.
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| 42. |
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| 43. |
- Karlsson, Krister, 1961-
(författare)
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Electronic Structure Calculations: Materials with Weak and Strong Correlations
- 1992
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is devoted to studies of various electronic properties which can be extracted from photoemission spectroscopy. Both weakly and strongly correlated systems are discussed, ranging from free electron metal surfaces to strongly correlated copper oxide compounds. In order to study such a variety of systems, different calculational schemes have to be employed. This work is mainly based on band structure calculations using the local density approximation. However, for strongly correlated systems the need for model Hamiltonians is crucial. Subjects discussed are: * inverse photoemission spectra from the Cd(0001) surface obtained by a multiple scattering formalism. The results are in good agreement with recent experimental data. Especially, the one-step model is used to demonstrate image states excitations by secondary beams. * artificial structures, such as semiconductor heterojunctions and short-period superlattices, which are of vital importance for modern electronic devices. A method for tuning the band offset at semiconductor interfaces is proposed; by varying (n,m) for the GaAs/(AlAs)n(GaAs)m interface, the magnitude of the valence band offset can be designed. The metallic superlattice Mo/V was also studied, with emphasis on the application of hydrogen storage. * Schottky barrier heights. Final state screening effects can influence the Schottky barrier height, as deduced from measured core level shifts. According to our calculations, such effects may be of the order 0.1-0.2 eV. * chemical shifts in Cu2O, CuO and NaCuO2, where the Cu atom is formally mono-, di- and trivalent. We find similar chemical shifts between Cu atoms in YBa2Cu3O6.5 as between the Cu atoms in the model compounds. This suggests that different Cu atoms in YBa2Cu3O6.5 are formally mono-, di- and trivalent. The Cu 2p core level spectra were calculated using the Anderson impurity model, with parameters obtained from ab initio calculations. The results are related to the formal valence of Cu, and can be explained by a charge counting argument.
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| 44. |
- Magnuson, Martin, 1965-
(författare)
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Process Control Methods for Operation of Superconducting Cavities at the LEP Accelerator at CERN
- 1992
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The aim of this work is to analyse the cryogenic process for cooling superconducting radio-frequency accelerator prototype cavities in the Large Electron-Positron collider (LEP) at CERN. A liquefaction cryoplant is analysed, including the production of liquid helium at 4.5 K, the systems for distribution and control of liquid helium, and the radio-frequency system used for accelerating particles. The cryogenic control problems of the cavities based on operation experiences of the prototype modules in 1991 and different solutions and modifications for the future cavity installations are presented in detail. In addition, different techniques for specifying the control programs for new cavities are evaluated. Various diagramming techniques, standards and methodologies, and ComputerAided Software Engineering (CASE) tools, are compared regarding their practical usefulness for the cryogenic process control.
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| 45. |
- Strupinski, W., et al.
(författare)
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New method for determination of the peak-velocity in epitaxial semiconductor structures by dc measurements on microbridges
- 1991
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 59:24, s. 3151-3153
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Tidskriftsartikel (refereegranskat)abstract
- A simple current-voltage measurement technique using microbridge patterns is described as a fast method for the determination of the effective peak electron velocity in III-V semiconducting materials. The method is tested for GaAs samples. Microbridge patterns with different geometries were investigated and the influence of ''self-heating'' by power dissipation was examined. Some other potential sources of errors deteriorating the accuracy of measurements were determined.
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| 46. |
- Apostolakis, A, et al.
(författare)
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A determination of the CP violation parameter η+- from the decay of strangeness-tagged neutral kaons
- 1999
-
Ingår i: Physics Letters B. - : Elsevier. - 0370-2693 .- 1873-2445. ; 458:4, s. 545-552
-
Tidskriftsartikel (refereegranskat)abstract
- LEAR offered unique opportunities to study the symmetries which exist between matter and antimatter. At variance with other approaches at this facility, CPLEAR was an experiment devoted to the study of CP, T and CPT symmetries in the neutral-kaon system. A variety of measurements allowed us to determine with high precision the parameters which describe the time evolution of the neutral kaons and their antiparticles, including decay amplitudes, and the related symmetry properties. Limits concerning quantum-mechanical predictions (EPR, coherence of the wave function) or the equivalence principle of general relativity have been obtained. An account of the main features of the experiment and its performances is given here, together with the results achieved.
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| 47. |
- Beideck, DJ, et al.
(författare)
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Lifetimes of 5d96p and 5d86s6p levels in Hg iii
- 1993
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Ingår i: Physical Review A (Atomic, Molecular and Optical Physics). - 1050-2947. ; 47:2, s. 884-889
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Tidskriftsartikel (refereegranskat)abstract
- Experimental and theoretical studies of lifetimes and transition probabilities for doubly ionized mercury, Hg III, are reported. Such data are currently of great astrophysical interest because observations with the Goddard high resolution spectrograph on board the Hubble space telescope have shown some 5d(9)6s-5d(9)6p transitions of Hg III in the spectrum of the chemically peculiar star chi Lupi. We now report experimental lifetimes for some selected 5d(9)6p and 5d(8)6s6p levels, determined by beam-foil spectroscopy, and compare the results with theoretical transition probabilities based on relativistic multiconfiguration Dirac-Fock calculations.
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| 48. |
- Andersson, Dan, 1966, et al.
(författare)
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Wave-breaking-free pulses in nonlinear-optical fibers
- 1993
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Ingår i: Journal of the Optical Society of America B: Optical Physics. - 1520-8540 .- 0740-3224. ; 10:7, s. 1185-1190
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Tidskriftsartikel (refereegranskat)abstract
- A qualitative as well as quantitative investigation is made of the conditions for avoiding wave breaking during pulse propagation in optical fibers. In particular, it is shown that pulses having a parabolic intensity variation are approximate wave-breaking-free solutions of the nonlinear Schrodinger equation in the high-intensity limit. A simple expression for the compression factor of a fiber-grating compressor based on parabolic pulses is also derived.
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| 49. |
- Boyer, Gilbert, et al.
(författare)
-
Four-wave mixing of femtosecond pump-probe pulses in optical fibers
- 1999
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Ingår i: Nonlinear Guided Waves and Their Applications.
-
Konferensbidrag (refereegranskat)abstract
- The nonlinear propagation of a pump-probe pulse pair in an optical fiber has been extensively studied both experimentally, numerically and theoretically. In particular, it has been shown that the probe can attain a spectral shift under the influence of cross-phase modulation (XPM) from the pump, an effect having important applications in e. g. optical switching and demultiplexing.
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| 50. |
- Karlsson, Magnus, 1967
(författare)
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Four-wave mixing in fibers with randomly varying zero-dispersion wavelength
- 1998
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Ingår i: Journal of the Optical Society of America B: Optical Physics. - 1520-8540 .- 0740-3224. ; 15:8, s. 2269-2275
-
Tidskriftsartikel (refereegranskat)abstract
- The effect of random fluctuations in the zero-dispersion wavelength is considered for fiber four-wave mixing. Theoretical expressions for average parametric gain, phase-conjugation conversion efficiency, and gain bandwidth are obtained and found to be in good agreement with experiments. Possible limitations on the noise figure in phase-sensitive amplifiers based on fiber four-wave mixing are also discussed.
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