| 1. |
- Aleksandrovskii, A. N., et al.
(author)
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Thermal expansion of single-crystal fullerite C60 at helium temperatures
- 2000
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In: Low temperature physics (Woodbury, N.Y., Print). - : AIP Publishing. - 1063-777X .- 1090-6517. ; 26:1, s. 75-80
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Journal article (peer-reviewed)abstract
- The thermal expansion of single-crystal fullerite C60 has been studied in the range of liquid-helium temperatures (2–10 K). At temperatures below ~4.5 K the thermal expansion of fullerite C60 becomes negative, in agreement with the previous results on polycrystalline materials. A qualitative explanation of the results is proposed.
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| 2. |
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| 3. |
- Wågberg, Thomas, et al.
(author)
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2D polymerization and doping of fullerenes under pressure
- 2000
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In: High Pressure Research vol. 18. - : Gordon & Breach. ; 18:1-6, s. 139-143
-
Conference paper (peer-reviewed)abstract
- Tetragonal polymeric C60 has been studied by Raman spectroscopy and other methods. Attempts have been made to transform samples from the tetragonal to the orthorhom-bic phase and vice versa. The results suggest that the transformation is direct with no intermediate stage with free molecules. Tetragonal C60 has also been intercalated by potassium metal.
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| 4. |
- Giangreco Marotta Puletti, Valentina, 1980-
(author)
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On string integrability : A journey through the two-dimensional hidden symmetries in the AdS/CFT dualities
- 2009
-
Doctoral thesis (other academic/artistic)abstract
- One of the main topics in the modern String Theory are the conjectured string/gauge (AdS/CFT) dualities. Proving such conjectures is extremely difficult since the gauge and string theory perturbative regimes do not overlap. In this perspective, the discovery of infinitely many conserved charges, i.e. the integrability, in the planar AdS/CFT has allowed us to reach immense progresses in understanding and confirming the duality.The first part of this thesis is focused on the gravity side of the AdS5/CFT4 duality: we investigate the quantum integrability of the type IIB superstring on AdS5 x S5. In the pure spinor formulation we analyze the operator algebra by computing the operator product expansion of the Maurer-Cartan currents at the leading order in perturbation theory. With the same approach at one loop order, we show the path-independence of the monodromy matrix which implies the charge conservation law, strongly supporting the quantum integrability of the string sigma-model. We also verify that the Lax pair field strength remains well-defined at one-loop order being free from UV divergences. The same string sigma-model is analyzed in the Green-Schwarz formalism in the near-flat-space (NFS) limit. Such a limit remarkably simplifies the string world-sheet action but still leaving interesting physics. We use the NFS truncation to show the factorization of the world-sheet S-matrix at one-loop order. This property defines a two-dimensional field theory as integrable: it is the manifestation of the higher conserved charges. Hence, we have explicitly checked their presence at quantum level. The second part is dedicated to the AdS4/CFT3 duality: in particular the type IIA superstring on AdS4 x CP3. We compute the leading quantum corrections to the string energies for string configurations with a large but yet finite angular momentum on CP3 and show that they match the conjectured all-loop Bethe Ansatz equations.
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| 5. |
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| 6. |
- Fritioff, T., et al.
(author)
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Purification of radioactive neutron-rich argon beams using an ion source in charge breeding mode
- 2006
-
In: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier BV. - 0168-9002 .- 1872-9576 .- 0167-5087. ; 556:1, s. 31-37
-
Journal article (peer-reviewed)abstract
- Measurements of nuclear properties of some neutron-rich asymmetric fission isotopes at an ISOL laboratory present a severe problem when a plasma ion source is used as the primary ion-source. The problem is associated with intense multi-charged symmetric fission contaminants that cannot be mass-separated from asymmetric fission products. In this work we demonstrate how this problem can be overcome on-line by using an electron cyclotron resonance ion source (ECRIS) as a charge-breeding secondary ion-source. Mixed Ar+/Kr2+/Xe3+ radioactive beams from an plasma ion-source were injected into the PHOENIX ECRIS that currently is installed at the ISOLDE facility at CERN in order to improve the ratio of Ar to the background contaminants of Kr and Xe. The signal to noise ratio was improved by more than two orders of magnitude after charge-breeding and mass-separation of the Ar8+, Ar9+ beams. The information on the decay of the Ar-48 nucleus obtained in these tests is useful for a better understanding of the origin of the Ca-48/Ca-46 meteorite isotopic anomaly. Potential applications of the method are discussed.
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| 7. |
- Baek, Seung Ki, 1979-, et al.
(author)
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Curvature-induced frustration in the XY model on hyperbolic surfaces
- 2009
-
In: Physical Review E - Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755. ; 79:6, s. 060106(R)-
-
Journal article (peer-reviewed)abstract
- We study low-temperature properties of the XY spin model on a negatively curved surface. Geometric curvature of the surface gives rise to frustration in local spin configuration, which results in the formation of high-energy spin clusters scattered over the system. Asymptotic behavior of the spin-glass susceptibility suggests a zero-temperature glass transition, which is attributed to multiple optimal configurations of spin clusters due to nonzero surface curvature of the system. It implies that a constant ferromagnetic spin interaction on a regular lattice can exhibit glasslike behavior without possessing any disorder if the lattice is put on top of a negatively curved space such as a hyperbolic surface.
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| 8. |
- Shiu, Jin-Yu, 1978, et al.
(author)
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DC and microwave performance of AlGaN/GaN HEMTs passivated with sputtered SiNx
- 2007
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In: Semicronductor Science and Technology. ; 22, s. 717-721
-
Journal article (peer-reviewed)abstract
- The effects of sputtered and room temperature plasma enhanced chemical vapour deposition (RT-PECVD) SiNx passivation on the dc and microwave performance of AlGaN/GaN high electron mobility transistors (HEMTs)are studied. The pulsed I–V characteristics from a class B quiescent bias point and transient measurements indicate that the sputtered SiNx passivation is more efficient in suppressing lag effects in AlGaN/GaN HEMTs. Dispersion-free sputtered SiNx passivated AlGaN/GaN HEMTs were obtained using this technique. Continuous-wave (CW) measurements without active cooling give a maximum output power density of 6.6 W mm−1 at Vgs=−4 V, Vds = 50 V and a maximum power added efficiency of 51.3% at Vgs=−4 V, Vds = 30 V at 3 GHz on 2 × 50 μmAlGaN/GaN HEMTs on the sapphire substrate, with a gate length of 2 μm and without field-plated gates. To the best of our knowledge, this is the highest level power density reported on the sapphire substrate without field-plate design. The extrinsic cut-off frequency ( ft) and maximumoscillation frequency ( fmax) are 51 GHz and 100 GHz, respectively, on 2 × 50 × 0.15 μm HEMTs. To our knowledge, the sputtered SiNx passivation for AlGaN/GaN HEMTs is a unique technique, which has never beenpublished before.
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| 9. |
- Onipko, Alexander, et al.
(author)
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Tunneling through a tilted tight-binding sand
- 2002
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In: Annals of the New York Academy of Sciences. - : Wiley. - 0077-8923 .- 1749-6632. ; 960, s. 143-152
-
Journal article (peer-reviewed)abstract
- In the single-band approximation, an explicit expression of the exponential factor, that governs tunneling through a thin crystal subjected to a constant electric field, is derived. The basic features of Wannier-Stark, Airy, and intermediate type of quantization, as they are displayed in the transmission spectrum and hence in tunneling current, are thus described analytically.
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| 10. |
- Soldatov, Alexander, et al.
(author)
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Topochemical polymerization of C70 controlled by monomer crystal packing
- 2001
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In: Science. - : American Association for the Advanement of Science. - 0036-8075 .- 1095-9203. ; 293:5530, s. 680-683
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Journal article (peer-reviewed)abstract
- Polymeric forms of C60 are now well known, but numerous attempts to obtain C70 in a polymeric state have yielded only dimers. Polymeric C70 has now been synthesized by treatment of hexagonally packed C70 single crystals under moderate hydrostatic pressure (2 gigapascals) at elevated temperature (300°C), which confirms predictions from our modeling of polymeric structures of C70. Single-crystal x-ray diffraction shows that the molecules are bridged into polymeric zigzag chains that extend along the c axis of the parent structure. Solid-state nuclear magnetic resonance and Raman data provide evidence for covalent chemical bonding between the C70 cages.
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| 11. |
- Wågberg, Thomas, et al.
(author)
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Spectroscopic study of phase transformations between orthorhombic and tetragonal C60 polymers
- 2006
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In: The European Physical Journal B: Condensed Matter and Complex Systems. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 49:1, s. 59-65
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Journal article (peer-reviewed)abstract
- We present a detailed study of transformations between the orthorhombic and tetragonal polymeric states of C60. The transformations are characterised by Raman spectroscopy and X-ray diffraction. We show that the transformation from the orthorhombic (O) phase to the tetragonal (T) phase is very fast and our results indicate that the transformation goes via an intermediate dimer (D) state in a two-stage process, O↦D and, D↦T transformations, where the second process is slower than the first. On the other hand, the transformation from the tetragonal to the orthorhombic phase is significantly slower, indicating a high-energy threshold to break the polymer bonds at the temperatures used. The results also support earlier suggestions that the tetragonal phase contains disordered dimers that can be viewed as lattice defects in the formation of higher polymers.
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| 12. |
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| 13. |
- Waltersson, Erik, 1979-, et al.
(author)
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Controlled operations in a strongly correlated two-electron quantum ring
- 2009
-
In: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 79:11, s. 115318-
-
Journal article (peer-reviewed)abstract
- We have analyzed the electronic spectrum and wave-function characteristics of a strongly correlated two-electron quantum ring with model parameters close to those observed in experiments. The analysis is based on an exact diagonalization of the Hamiltonian in a large B-spline basis. We propose a qubit pair for storing quantum information, where one component is stored in the total electron spin and one multivalued “quMbit” is represented by the total angular momentum. In this scheme the controlled-NOT quantum gate is demonstrated with near 100% fidelity for a realistic far-infrared electromagnetic pulse.
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| 14. |
- Höök, Mikael, 1982-
(author)
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Depletion and decline curve analysis in crude oil production
- 2009
-
Licentiate thesis (other academic/artistic)abstract
- Oil is the black blood that runs through the veins of the modern global energy system. While being the dominant source of energy, oil has also brought wealth and power to the western world. Future supply for oil is unsure or even expected to decrease due to limitations imposed by peak oil. Energy is fundamental to all parts of society. The enormous growth and development of society in the last two-hundred years has been driven by rapid increase in the extraction of fossil fuels. In the foresee-able future, the majority of energy will still come from fossil fuels. Consequently, reliable methods for forecasting their production, especially crude oil, are crucial. Forecasting crude oil production can be done in many different ways, but in order to provide realistic outlooks, one must be mindful of the physical laws that affect extraction of hydrocarbons from a reser-voir. Decline curve analysis is a long established tool for developing future outlooks for oil production from an individual well or an entire oilfield. Depletion has a fundamental role in the extraction of finite resources and is one of the driving mechanisms for oil flows within a reservoir. Depletion rate also can be connected to decline curves. Consequently, depletion analysis is a useful tool for analysis and forecasting crude oil production. Based on comprehensive databases with reserve and production data for hundreds of oil fields, it has been possible to identify typical behaviours and properties. Using a combination of depletion and decline rate analysis gives a better tool for describing future oil production on a field-by-field level. Reliable and reasonable forecasts are essential for planning and nec-essary in order to understand likely future world oil production.
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| 15. |
- Höök, Mikael, 1982-, et al.
(author)
-
A decline rate study of Norwegian oil production
- 2008
-
In: Energy Policy. - : Elsevier BV. - 0301-4215 .- 1873-6777. ; 36:11, s. 4262-4271
-
Journal article (peer-reviewed)abstract
- Norway has been a very important oil exporter for the world and an important supplier for Europe. Oil was first discovered in the North Sea in late 1960s and the rapid expansion of Norwegian oil production lead to the low oil prices in the beginning of the 1990s. In 2001 Norway reached its peak production and began to decline. The Norwegian oil production can be broken up into four subclasses; giant oil fields, smaller oil fields, natural gas liquids and condensate. The production of each subclass was analyzed to find typical behaviour and decline rates. The typical decline rates of giant oil fields were found to be -13% annually. The other subclasses decline equally fast or even faster, especially condensate with typical decline rates of -40% annually. The conclusion from the forecast is that Norway will have dramatically reduced export volume of oil by 2030.
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| 16. |
- Höök, Mikael, 1982-, et al.
(author)
-
Future Danish oil and gas export
- 2009
-
In: Energy. - : Elsevier BV. - 0360-5442 .- 1873-6785. ; 34:11, s. 1826-1834
-
Journal article (peer-reviewed)abstract
- Denmark possesses only a small share of the exploitation rights to North Sea oil and is a minor producer when compared to Norway and the UK. However, Denmark is still an oil exporter and a very important supplier of oil for certain countries, in particular Sweden. A field-by-field analysis of the Danish oil and gas fields, combined with estimated production contribution from new field developments, enhanced oil recovery and undiscovered fields, provides a future production outlook. The conclusion from this analysis is that by 2030 Denmark will no longer be an oil or gas exporter at all. Our results are also in agreement with the Danish Energy Authority’s own forecast, and may be seen as an independent confirmation of their general statements. Decreasing Danish oil production, coupled with a rapid decline in Norway’s oil output, will force Sweden to import oil from more distant markets in the future, dramatically reducing Swedish energy security. If no new gas suppliers are introduced to the Swedish grid, then Swedish gas consumption is clearly predestined to crumble alongside declining Danish production. Future hydrocarbon production from Denmark displays a clear link to Sweden’s future energy security.
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| 17. |
- Höök, Mikael, 1982-, et al.
(author)
-
Giant oil field decline rates and their influence on world oil production
- 2009
-
In: Energy Policy. - : Elsevier BV. - 0301-4215 .- 1873-6777. ; 37:6, s. 2262-2272
-
Journal article (peer-reviewed)abstract
- The most important contributors to the world's total oil production are the giant oil fields. Using a comprehensive database of giant oil field production, the average decline rates of the world's giant oil fields are estimated. Separating subclasses was necessary, since there are large differences between land and offshore fields, as well as between non-OPEC and OPEC fields. The evolution of decline rates over past decades includes the impact of new technologies and production techniques and clearly shows that the average decline rate for individual giant fields is increasing with time. These factors have significant implications for the future, since the most important world oil production base - giant fields - will decline more rapidly in the future, according to our findings. Our conclusion is that the world faces an increasing oil supply challenge, as the decline in existing production is not only high now but will be increasing in the future.
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| 18. |
- Höök, Mikael, 1982-, et al.
(author)
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The evolution of giant oil field production behaviour
- 2009
-
In: Natural Resources Research. - : Springer Science and Business Media LLC. - 1520-7439 .- 1573-8981. ; 18:1, s. 39-56
-
Journal article (peer-reviewed)abstract
- The giant oil fields of the world are only a small fraction of the total number of fields, but their importance is huge. Over 50% of the world oil production came from giants by 2005 and more than haft of the worlds ultimate reserves are found in giants. Based on this it is reasonable to assume that the future development of the giant oil fields will have a significant impact on the world oil supply. In order to better understand the giant fields and their future behaviour one must first understand their history. This study has used a comprehensive database on giant oil fields in order to determine their typical parameters, such as the average decline rate and life-times of giants. The evolution of giant oil field behaviour has been investigated to better understand future behaviour. One conclusion is that new technology and production methods have generally lead to high depletion rate and rapid decline. The historical trend points towards high decline rates of fields currently on plateau production. The peak production generally occurs before half the ultimate reserves have been produced in giant oil fields. A strong correlation between depletion-at-peak and average decline rate is also found, verifying that high depletion rate leads to rapid decline. Our result also implies that depletion analysis can be used to rule out unrealistic production expectations from a known reserve, or to connect an estimated production level to a needed reserve base.
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| 19. |
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| 20. |
- Evaldsson, Martin, 1977-
(author)
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Quantum transport and spin effects in lateral semiconductor nanostructures and graphene
- 2008
-
Doctoral thesis (other academic/artistic)abstract
- This thesis studies electron spin phenomena in lateral semi-conductor quantum dots/anti-dots and electron conductance in graphene nanoribbons by numerical modelling. In paper I we have investigated spin-dependent transport through open quantum dots, i.e., dots strongly coupled to their leads, within the Hubbard model. Results in this model were found consistent with experimental data and suggest that spin-degeneracy is lifted inside the dot – even at zero magnetic field.Similar systems were also studied with electron-electron effects incorporated via Density Functional Theory (DFT) in the Local Spin Density Approximation (LSDA) in paper II and III. In paper II we found a significant spin-polarisation in the dot at low electron densities. As the electron density increases the spin polarisation in the dot gradually diminishes. These findings are consistent with available experimental observations. Notably, the polarisation is qualitatively different from the one found in the Hubbard model.Paper III investigates spin polarisation in a quantum wire with a realistic external potential due to split gates and a random distribution of charged donors. At low electron densities we recover spin polarisation and a metalinsulator transition when electrons are localised to electron lakes due to ragged potential profile from the donors. In paper IV we propose a spin-filter device based on resonant backscattering of edge states against a quantum anti-dot embedded in a quantum wire. A magnetic field is applied and the spin up/spin down states are separated through Zeeman splitting. Their respective resonant states may be tuned so that the device can be used to filter either spin in a controlled way.Paper V analyses the details of low energy electron transport through a magnetic barrier in a quantum wire. At sufficiently large magnetisation of the barrier the conductance is pinched off completely. Furthermore, if the barrier is sharp we find a resonant reflection close to the pinch off point. This feature is due to interference between a propagating edge state and quasibond state inside the magnetic barrier.Paper VI adapts an efficient numerical method for computing the surface Green’s function in photonic crystals to graphene nanoribbons (GNR). The method is used to investigate magnetic barriers in GNR. In contrast to quantum wires, magnetic barriers in GNRs cannot pinch-off the lowest propagating state. The method is further applied to study edge dislocation defects for realistically sized GNRs in paper VII. In this study we conclude that even modest edge dislocations are sufficient to explain both the energy gap in narrow GNRs, and the lack of dependance on the edge structure for electronic properties in the GNRs.
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| 21. |
- Hollmark, Malin, et al.
(author)
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Analytical model for light ion pencil beam dose distributions: multiple scattering of primary and secondary ions
- 2008
-
In: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 53:13, s. 3477-3491
-
Journal article (peer-reviewed)abstract
- An analytical algorithm based on the generalized Fermi–Eyges theory, amended for multiple Coulomb scattering and energy loss straggling, is used for calculation of the dose distribution of light ion beams in water. Pencil beam energy deposition distributions are derived for light ions by weighting a Monte Carlo (MC) calculated planar integral dose distribution with analytically calculated multiple scattering and range straggling distributions. The planar integral dose distributions are calculated using the MC code SHIELD-HIT07, in which multiple scattering and energy loss straggling processes are excluded. The contribution from nuclear reactions is included in the MC calculations. Multiple scattering processes are calculated separately for primary and secondary ions and parameters of the initial angular and radial spreads, and the covariance of these are derived by a least-square parameterization of the SHIELD-HIT07 data. The results from this analytical algorithm are compared to pencil beam dose distributions obtained from SHIELD-HIT07, where all processes are included, as well as to experimental data. The presented analytical approach allows for the accurate calculation of the spatial energy deposition distributions of ions of atomic numbers Z = 1 − 8.
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| 22. |
- Lindén, Martin
(author)
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Decay times in turnover statistics of single enzymes
- 2008
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In: Physical review E. - 1539-3755 .- 1550-2376. ; 78:1, s. 010901-1-010901-4
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Journal article (peer-reviewed)abstract
- The first passage times for enzymatic turnovers in nonequilibrium steady state display a statistical symmetry property related to nonequilibrium fluctuation theorems, which makes it possible to extract the chemical driving force from single molecule trajectories in nonequilibrium steady state. Below, we show that the number of decay constants needed to describe the first passage time distribution of this system is not equal to the number of states in the first passage problem, as one would generally expect. Instead, the structure of the kinetic mechanism makes half of the decay times vanish identically from the turnover time distribution. The terms that cancel out correspond to the eigenvalues of a certain submatrix of the master equation matrix for the first exit time problem. We discuss how these results make modeling and data analysis easier for such systems, and how the turnovers can be measured.
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| 23. |
- Lindén, Martin, et al.
(author)
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Dwell Time Symmetry in Random Walks and Molecular Motors
- 2007
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In: Biophysical Journal. - : Elsevier BV. - 0006-3495 .- 1542-0086. ; 92:11, s. 3804-3816
-
Journal article (peer-reviewed)abstract
- The statistics of steps and dwell times in reversible molecular motors differ from those of cycle completion in enzyme kinetics. The reason is that a step is only one of several transitions in the mechanochemical cycle. As a result, theoretical results for cycle completion in enzyme kinetics do not apply to stepping data. To allow correct parameter estimation, and to guide data analysis and experiment design, a theoretical treatment is needed that takes this observation into account. In this article, we model the distribution of dwell times and number of forward and backward steps using first passage processes, based on the assumption that forward and backward steps correspond to different directions of the same transition. We extend recent results for systems with a single cycle and consider the full dwell time distributions as well as models with multiple pathways, detectable substeps, and detachments. Our main results are a symmetry relation for the dwell time distributions in reversible motors, and a relation between certain relative step frequencies and the free energy per cycle. We demonstrate our results by analyzing recent stepping data for a bacterial flagellar motor, and discuss the implications for the efficiency and reversibility of the force-generating subunits.
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| 24. |
- Tsygankov, Denis, et al.
(author)
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Back-stepping, hidden substeps, and conditional dwell times in molecular motors
- 2007
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In: Physical Review E. - 1539-3755 .- 1550-2376. ; 75:2, s. 021909-1-021909-16
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Journal article (peer-reviewed)abstract
- Processive molecular motors take more-or-less uniformly sized steps, along spatially periodic tracks, mostly forwards but increasingly backwards under loads. Experimentally, the major steps can be resolved clearly within the noise but one knows biochemically that one or more mechanochemical substeps remain hidden in each enzymatic cycle. In order to properly interpret experimental data for back-to-forward step ratios, mean conditional step-to-step dwell times, etc., a first-passage analysis has been developed that takes account of hidden substeps in N-state sequential models. The explicit, general results differ significantly from previous treatments that identify the observed steps with complete mechanochemical cycles; e.g., the mean dwell times tau(+) and tau(-) prior to forward and back steps, respectively, are normally unequal although the dwell times tau(++) and tau(- -) between successive forward and back steps are equal. Illustrative (N=2)-state examples display a wide range of behavior. The formulation extends to the case of two or more detectable transitions in a multistate cycle with hidden substeps.
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| 25. |
- Vestergren, Anders, et al.
(author)
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Effect of columnar disorder on the superconducting transition of a type-II superconductor in zero applied magnetic field
- 2004
-
In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 70:5
-
Journal article (peer-reviewed)abstract
- We investigate the effect of random columnar disorder on the superconducting phase transition of a type-II superconductor in zero applied magnetic field using numerical simulations of three dimensional XY and vortex loop models. We consider both an unscreened model, in which the bare magnetic penetration length is approximated as infinite, and a strongly screened model, in which the magnetic penetration length is of order the vortex core radius. We consider both equilibrium and dynamic critical exponents. We show that, as in the disorder free case, the equilibrium transitions of the unscreened and strongly screened models lie in the same universality class, however scaling is now anisotropic. We find for the correlation length exponent nu=1.2+/-0.1, and for the anisotropy exponent zeta=1.3+/-0.1. We find different dynamic critical exponents for the unscreened and strongly screened models.
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| 26. |
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| 27. |
- Höglund, Linda, 1974-, et al.
(author)
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Bias and temperature dependence of the escape processes in quantum dots-in-a-well infrared photodetectors
- 2008
-
In: Applied Physics Letters. - New York : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 93:10, s. 103501-
-
Journal article (peer-reviewed)abstract
- The performance of quantum dots-in-a-well infrared photodetectors (DWELL IPs) has been studied by means of interband and intersubband photocurrent measurements as well as dark current measurements. Using interband photocurrent measurements, substantial escape of electrons from lower lying states in the DWELL structure at large biases was revealed. Furthermore, a significant variation in the escape probability from energy states in the DWELL structure with applied bias was observed. These facts can explain the strong temperature and bias dependence of both photocurrent and dark currents in DWELL IPs.
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| 28. |
- Nilson, Katharina, et al.
(author)
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STM and XPS characterization of Zinc Phthalocyanine on InSb(001)
- 2008
-
In: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 602:2, s. 452-459
-
Journal article (peer-reviewed)abstract
- Zinc phthalocyanine (ZnPc) adsorbed on the InSb(0 0 1)-c(8 x 2) surface has been studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Coverages from sub-monolayer to monolayer (ML) have been investigated. The molecules form ordered structures on the reconstructed rows of the surface with the molecular plane parallel to the surface. A change in the electronic structure between the sub-ML and higher coverages has been observed. Moreover, in order to study the influence of annealing on the electronic and geometric structures, the samples have been heated to elevated temperatures (about 640 K). In addition, multi-layer ZnPc films have been characterized by XPS measurements.
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| 29. |
- Arvanitidis, J., et al.
(author)
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High pressure study of the 2D polymeric phase of C60by means of raman spectroscopy
- 2000
-
In: High Pressure Research. - : Taylor & Francis. - 0895-7959 .- 1477-2299. ; 18:1-6, s. 145-151
-
Journal article (peer-reviewed)abstract
- The effect of high hydrostatic pressure, up to 12 GPa, on the intramolecular phonon frequencies and the material stability of the two-dimensional tetragonal C-60 polymer has been studied by means of Raman spectroscopy in the spectral range of the radial intramolecular modes (200-800 cm(-1)). A number of new Raman modes appear in the spectrum for pressures similar to 1.4 and similar to 5.0 GPa. The pressure coefficients for the majority of the phonon modes exhibit changes to lower values at P=4.0 GPa, which may be related to a structural modification of the 2D polymer to a more isotropic phase. The peculiarities observed in the Raman spectra are reversible and the material is stable in the pressure region investigated.
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| 30. |
- Lebedkin, Sergei, et al.
(author)
-
Structure and Properties of the Fullerene Dimer C140 Produced by Pressure Treatment of C70
- 2000
-
In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 104:17, s. 4101-4110
-
Journal article (peer-reviewed)abstract
- A [2+2] cycloaddition cap-to-cap C70 dimer with C2h molecular symmetry was synthesized in high yield by pressure treatment of polycrystalline C70 at 1 GPa and 200 °C. It was separated from unreacted monomers by chromatography and characterized by 13C NMR, Raman, and infrared spectroscopy, and other methods. Remarkably, only one isomer was produced out of the five possible [2+2] cycloaddition products which have equally low formation energies according to semiempirical modeling calculations. The dimer obtained is the one favored when C70 molecules adopt an ordered packing with parallel D5 axes. The intercage bonding in C140, its thermal stability, and intercage vibrational modes are similar to those found for the C60 dimer, C120. Both dimers photodissociate to the monomers in solution, probably via excited triplet states. The UV absorption and fluorescence properties of C140 are not very different from those of C70, suggesting only weak electronic interactions between the two cages of C140. In comparison, the pressure-induced dimerization of C60, under the conditions used for C70, results mainly in C60 oligomers and polymeric chains, but the dimer C120 could be isolated at low yield when short reaction times (≤5 min) were used.
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| 31. |
- Barbero, Giovanni, et al.
(author)
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Continuum description of the interfacial layer of nematic liquid crystals in contact with solid surfaces
- 2009
-
In: J. Chem. Phys.. - : AIP Publishing. - 0021-9606. ; 130
-
Journal article (peer-reviewed)abstract
- We investigate when it is possible to introduce surface physical parameters characterizing the nematic/substrate interface. The analysis is performed by solving the problem assuming that the presence of the surface introduces a spatial variation, mainly localized close to the limiting surfaces, of the bulk properties of the nematic (delocalized model). The results of the calculation are compared to the prediction of a model in which the presence of the surface is taken into account by means of new physical parameters, localized to the surface (localized model). We show that if the viscous dissipative effects or the surface alignment effects are considered, the two models predict the same relaxation times and the same threshold for the Freedericksz transition is obtained. From these results we deduce that the localized models are equivalent to the delocalized ones. A continuum description of the interfacial layer of nematic liquid crystals in contact with solid surface in terms of surface properties is then correct, which makes the solution of this kind of problems simpler. Also a softening of the elastic constants near the surfaces can be represented by a localized surface energy term.
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| 32. |
- Borck, Øyvind, 1971, et al.
(author)
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Adsorption of Methanol on Aluminum Oxide: A Density Functional Study
- 2003
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In: ATB-Metallurgie. - 0365-7302. ; 43, s. 342-
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Conference paper (peer-reviewed)abstract
- Theoretical calculations based on density functional theory have made significant contributions to our understanding of metal oxides, their surfaces, and the binding of molecules at these surfaces. In this paper we investigate the binding of methanol at the alpha-Al2O3(0001) surface using first-principles density functional theory. We calculate the molecular adsorption energy of methanol to be E^g_ads=1.03 eV/molecule. Taking the methanol-methanol interaction into account, we obtain the adsorption energy E_ads=1.01 eV/molecule. Our calculations indicate that methanol adsorbs chemically by donating electron charge from the methanol oxygen to the surface aluminum. We find that the surface atomic structure changes upon adsorption, most notably the spacing between the outermost Al and O layers changes from 0.11 Angstrom to 0.33 Angstrom.
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| 33. |
- Zha, Chaolin, et al.
(author)
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Pseudo spin valves based on L10 (111)-oriented FePt fixed layers with tilted anisotropy
- 2009
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 94
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Journal article (peer-reviewed)abstract
- We demonstrate magnetoresistance (MR) in excess of 4% in FePt/CoFe/Cu/CoFe/NiFe pseudo spin valves based on L10 (111)-oriented FePt fixed layers with a 36 degrees out-of-plane tilted magnetization. The high MR is achieved by increasing the spin polarization at the Cu interfaces, using thin CoFe, and optimizing the FePt growth and Cu interface quality using Ta and Ta/Pt underlayers. We observe well-separated switching of the FePt/CoFe fixed layer and the CoFe/NiFe free layer, suggesting that CoFe is rigidly exchange coupled to FePt and NiFe in the respective layers.
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| 34. |
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| 35. |
- Bengtsson, Ingemar
(author)
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A Kochen-Specker Inequality
- 2009
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In: AIP Conference Proceedings. - : AIP. - 0094-243X .- 1551-7616. ; 1101, s. 241-245
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Journal article (other academic/artistic)abstract
- I review a paper by Klyachko, Can, Binicioğlu, and Shumovsky, and explain a little of the background as I see it
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| 36. |
- Björnham, Oscar, 1976-, et al.
(author)
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Methods and error estimations of uncertainties in single-molecule dynamic force spectroscopy
- 2009
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In: European Biophysics Journal. - : Springer Science and Business Media LLC. - 0175-7571 .- 1432-1017. ; 38:7, s. 911-922
-
Journal article (peer-reviewed)abstract
- In dynamic force spectroscopy, access to the characteristic parameters of single molecular bonds requires non-trivial measurements and data processing as the rupture forces are found not only to be distributed over a wide range, but are also dependent on the loading rate. The choice of measurement procedure and data processing methods has a considerable impact on the accuracy and precision of the final results. We analyse, by means of numerical simulations, methods to minimize and assess the magnitude of the expected errors for different combinations of experimental and evaluation methods. It was found that the choice of fitting function is crucial to extract correct parameter values. Applying a Gaussian function, which is a common practice, is equivalent to introducing a systematic error, and leads to a consequent overestimation of the thermal off-rate by more than 30%. We found that the precision of the bond length and the thermal off-rate, in presence of unbiased noise, were improved by reducing the number of loading rates for a given number of measurements. Finally, the results suggest that the minimum number of measurements needed to obtain the bond strength, with acceptable precision, exceeds the common number of ~100 reported in literature.
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| 37. |
- Hellberg, Magnus, et al.
(author)
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In situ scanning probe microscopy studies of morphology and growth kinetics in amyloid-peptide fibrillogenesis with custom designed tapping mode and pulsed force mode systems
- 2004
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In: Proceedings of 5th Nordic-Baltic Scanning Probe Microscopy Workshop. ; , s. 93-95
-
Conference paper (peer-reviewed)abstract
- We have used a commercial Nanoscope II atomic force microscope (AFM) with a custom designedtapping mode (TM) system to in situ monitor amyloid β-peptide aggregation related to Alzheimer’sdisease (AD). The custom tapping mode setup was successfully used to image the real timeaggregation behaviour of the arctic mutation amyloid β-peptide, Aβ(1-40), in vitro in aphysiologically relevant buffer and compare with the behaviour of the normal wild type of theAlzheimer’s amyloid peptide Aβ(1-40) at the same conditions. The investigation revealed distinctdifferences in fibrillogenesis behaviour for the two peptides. Our results demonstrate a previouslysuggested alternative fibrillogenesis pathway, of highly distinct aggregates with orderedmorphology as on-pathway. Moreover, additional investigations using a pulsed force mode (PFM)are under way.
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| 38. |
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| 39. |
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| 40. |
- Heydari, Hoshang
(author)
-
Different classes of quantum gates entanglers
- 2009
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In: International journal of quantum information. - : World scientific publishing company. - 0219-7499. ; 7:1, s. 279-285
-
Journal article (peer-reviewed)abstract
- We construct quantum gates entanglers for different classes of multipartite states. In particular, we construct entangler operators for W and GHZ classes of multipartite states based on the construction of the concurrence classes. We also discuss in detail these two classes of the quantum gates entanglers for three-partite states.
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| 41. |
- Heydari, Hoshang
(author)
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Generalized controlled phase quantum gates entanglers
- 2009
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In: International journal of quantum information. - : World scientific publishing. - 0219-7499. ; 7:6, s. 1211-1216
-
Journal article (peer-reviewed)abstract
- We construct a generalized controlled phased gate entangler for a multi-qubit state based on the geometrical structure of quantum systems. We also investigate the relation between the generalized controlled phase construction of a quantum gate entangler and graph state for two-qubit and three-qubit states.
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| 42. |
- Heydari, Hoshang
(author)
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Selective Phase Rotation Quantum Gate Entangler
- 2009
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In: Open systems & information dynamics. - : world scientific publishing. - 1230-1612 .- 1573-1324. ; 16:4, s. 407-412
-
Journal article (peer-reviewed)abstract
- We construct a quantum gate entangler for multi-qubit states based on a selective phase rotation transform. In particular, we establish a relation between the quantum integral transform and the quantum gate entangler in terms of universal controlled gates for multi-qubit states. Our result gives an effective way of constructing topological and geometrical quantum gate entanglers for multipartite quantum systems, which could also lead to a construction of geometrical quantum algorithms.
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| 43. |
- Johnson, Matthew C, et al.
(author)
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Field dynamics and tunneling in a flux landscape
- 2008
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In: Physical Review D. Particles and fields. - : American Physical Society. - 0556-2821 .- 1089-4918. ; 78:8, s. 0835341-08353423
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Journal article (peer-reviewed)
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| 44. |
- Liljequist, David, 1946-
(author)
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Limits of validity of trajectory simulation: correlation of the error with density of scatterers and particle wavelength
- 2009
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In: Nuclear Instruments and Methods in Physics Research Section B. - : Elsevier. - 0168-583X .- 1872-9584. ; 267, s. 3409-3419
-
Journal article (peer-reviewed)abstract
- To a first approximation, the elastic scattering of long wavelength particles in amorphous matter may be modelled as scattering in a volume filled with a density n of N point scatterers in random positions. For not too large N (up to about 2×103), the error in trajectory simulation (classical transport theory) due to the neglect of interference effects can then be determined in detail by means of a comparison with an exact quantum calculation of the plural or multiple scattering process. A relative error RE is defined and calculated for the scattering in different directions as well as for the distribution of scattering events inside the volume. A very strong correlation is found between the relative error and the ratio λ/dnn, where λ is the wavelength of the incident particle and dnn=n-1/3 is an average distance between nearest neighbour scatterers. For scattering in a volume of dimensions large compared to the particle wavelength, present calculations suggest that the correlation can be described as RE≈a·(λ/dnn)b, where the parameters a<0.05 and b∼2 depend on the s-wave phaseshift δ0 in the scattering process. The condition for validity of trajectory simulation, defined in terms of a limit of validity L (maximum acceptable relative error), may thus be written λ/dnn<ξ, where ξ=(L/a)1/b∼1. For λ/dnn<1, the relative error is generally less than 5%, and trajectory simulation may be regarded as valid with at least 95% accuracy. In the exact quantum calculation, two features of pronounced quantum character are observed in the distribution of scattering events: oscillations due to quantum interference in finite volumes, and, for small negative δ0, randomly localized peaks due to proximity resonance.
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| 45. |
- Marklund, Mattias, 1970-, et al.
(author)
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Quantum vacuum experiments using high intensity lasers
- 2009
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In: European Physical Journal D. - : Springer Science and Business Media LLC. - 1434-6060 .- 1434-6079. ; 55, s. 319-326
-
Journal article (peer-reviewed)abstract
- The quantum vacuum constitutes a fascinating medium of study, in particular since near-future laser facilities will be able to probe the nonlinear nature of this vacuum. There has been a large number of proposed tests of the low-energy, high intensity regime of quantum electrodynamics (QED) where the nonlinear aspects of the electromagnetic vacuum come into play, and we will here give a short description of some of these. Such studies can shed light, not only on the validity of QED, but also on certain aspects of nonperturbative effects, and thus also give insights for quantum field theories in general.
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| 46. |
- Nikolaou, Marios, et al.
(author)
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Critical scaling properties at the superfluid transition of He-4 in aerogel
- 2006
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 97:22, s. 225702-
-
Journal article (peer-reviewed)abstract
- We study the superfluid transition of He-4 in aerogel by Monte Carlo simulations and finite size scaling analysis. Aerogel is a highly porous silica glass, which we model by a diffusion limited cluster aggregation model. The superfluid is modeled by a three dimensional XY model, with excluded bonds to sites on the aerogel cluster. We obtain the correlation length exponent nu=0.73 +/- 0.02, in reasonable agreement with experiments and with previous simulations. For the heat capacity exponent alpha, both experiments and previous simulations suggest deviations from the Josephson hyperscaling relation alpha=2-d nu. In contrast, our Monte Carlo results support hyperscaling with alpha=-0.2 +/- 0.05. We suggest a reinterpretation of the experiments, which avoids scaling violations and is consistent with our simulation results.
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| 47. |
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| 48. |
- Rocha, Luis E C, 1981-
(author)
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Structural evolution of the Brazilian airport network
- 2009
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In: Journal of Statistical Mechanics. - Bristol : Institute of Physics. - 1742-5468. ; , s. 04020-
-
Journal article (peer-reviewed)abstract
- The aviation sector is profitable, but sensitive to economic fluctuations, geopolitical constraints and governmental regulations. As for other means of transportation, the relation between origin and destination results in a complex map of routes, which can be complemented with information associated with the routes themselves, for instance, frequency, traffic load and distance. The theory of networks provides a natural framework for investigating the dynamics on the resulting structure. Here, we investigate the structure and evolution of the Brazilian airport network (BAN) as regards several quantities: routes, connections, passengers and cargo. Some structural features are in accordance with previous results for other airport networks. The analysis of the evolution of the BAN shows that its structure is dynamic, with changes in the relative relevance of some airports and routes. The results indicate that the connections converge to specific routes. The network shrinks at the route level but grows in number of passengers and amount of cargo, which more than doubled during the period studied.
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| 49. |
- Stigner, Carl, 1980-
(author)
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A classifying algebra for CFT boundary conditions
- 2009
-
Licentiate thesis (other academic/artistic)abstract
- Conformal field theories (CFT) constitute an interesting class of twodimensionalquantum field theories, with applications in string theoryas well as condensed matter physics. The symmetries of a CFT can beencoded in the mathematical structure of a conformal vertex algebra.The rational CFT’s are distinguished by the property that the categoryof representations of the vertex algebra is a modular tensor category.The solution of a rational CFT can be split off into two separate tasks, apurely complex analytic and a purely algebraic part. The TFT-construction gives a solution to the second part of the problem.This construction gets its name from one of the crucial ingredients,a three-dimensional topological field theory (TFT). The correlators obtainedby the TFT-construction satisfy all consistency conditions of thetheory. Among them are the factorization constraints, whose implicationsfor boundary conditions are the main topic of this thesis. The main result reviewed in this thesis is that the factorization constraintsgive rise to a semisimple commutative associative complex algebrawhose irreducible representations are the so-called reflection coefficients.The reflection coefficients capture essential information aboutboundary conditions, such as ground-state degeneracies and Ramond-Ramond charges of string compactifications. We also show that the annuluspartition function can be derived fromthis classifying algebra andits representation theory.
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| 50. |
- Hidvegi, Attila, et al.
(author)
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A high-speed data acquisition system for segmented Ge-detectors
- 2007
-
In: 2007 IEEE Nuclear Science Symposium Conference Record. - : IEEE. - 1424409233 - 9781424409235 - 9781424409228 ; , s. 536-537
-
Conference paper (peer-reviewed)abstract
- When using segmented Ge-detectors for gamma ray tracking it is necessary to determine the segment pulse shapes with high accuracy. A high-speed data acquisition system with many channels, high precision and with high sampling rate is required. There are also many other applications for such a system. Our system uses high performance FPGAs (Xilinx Virtex-V [2]) to cope with the data rates delivered by the high speed ADC chosen (Atmel 2Gsps, 10 bits) and to make all the data processing onboard in real time. Each board contains four such ADCs, which can either handle four channels up to full speed, or achieve higher sampling rates with interleaving. The boards can communicate with each other over different types of high-speed communication links. Control and monitoring is implemented with embedded processors. The processed result will be transmitted over Ethernet to final storage. The project introduces many challenging issues: signal integrity, ADC performance, interfacing ADCs to the FPGA, synchronisation of ADCs across the entire system, implementing flexible processing algorithms, high speed interconnection between boards and managing the significant heat generation. This is an ongoing project with interesting potentials for the future.
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