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6. Imaging correlated wave functions of few-electron quantum dots: Theory and scanning tunneling spectroscopy experiments

Author

Rontani, Massimo, Molinari, Elisa, Maruccio, Giuseppe, Janson, Martin, Schramm, Andreas, Meyer, Christian, Matsui, Tomohiro, Heyn, Christian, Hansen, Wolfgang, and Wiesendanger, Roland

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Strongly Correlated Electrons
Abstract

We show both theoretically and experimentally that scanning tunneling spectroscopy (STS) images of semiconductor quantum dots may display clear signatures of electron-electron correlation. We apply many-body tunneling theory to a realistic model which fully takes into account correlation effects and dot anisotropy. Comparing measured STS images of freestanding InAs quantum dots with those calculated by the full configuration interaction method, we explain the wave function sequence in terms of images of one- and two-electron states. The STS map corresponding to double charging is significantly distorted by electron correlation with respect to the non-interacting case., Comment: RevTeX 4.0, 5 pages, 3 B/W figures, 1 table. This paper is based on an invited talk presented by the authors at the 28th International Conference on the Physics of Semiconductors, which was held 24-28 July 2006, in Vienna, Austria

Published
2007
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7. Correlation Effects in Wave Function Mapping of Molecular Beam Epitaxy Grown Quantum Dots

Author

Maruccio, Giuseppe, Janson, Martin, Schramm, Andreas, Meyer, Christian, Matsui, Tomohiro, Heyn, Christian, Hansen, Wolfgang, Wiesendanger, Roland, Rontani, Massimo, and Molinari, Elisa

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Strongly Correlated Electrons
Abstract

We investigate correlation effects in the regime of a few electrons in uncapped InAs quantum dots by tunneling spectroscopy and wave function (WF) mapping at high tunneling currents where electron-electron interactions become relevant. Four clearly resolved states are found, whose approximate symmetries are roughly s and p, in order of increasing energy. Because the major axes of the p-like states coincide, the WF sequence is inconsistent with the imaging of independent-electron orbitals. The results are explained in terms of many-body tunneling theory, by comparing measured maps with those calculated by taking correlation effects into account., Comment: PDF file, 20 pages, 6 color figures

Published
2007
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10. The motivational drives of sickness

Author

Lekander, Mats, primary, Janson, Martin, additional, Sundelin, Tina, additional, Paues, Sofie, additional, Hansson, Lina S, additional, Olsson, Mats J, additional, Axelsson, John, additional, and Lasselin, Julie, additional

Published
2022
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12. The use of a mini-ridge filter with cyclotron-based pencil beam scanning proton therapy.

Author

O’Grady, Fionnbarr, Janson, Martin, Rao, Avani D., Chawla, Ashish K., Stephenson, Lisa, Jiajin Fan, and Peng Wang

Subjects
*PROTON beams, *PROTON therapy, *IONIZATION chambers
Abstract

Background: Pencil beam scanning (PBS) proton therapy allows for far superior dose conformality compared with passive scattering techniques. However, one drawback of PBS is that the beam delivery time can be long, particularly when treating superficial disease. Minimizing beam delivery time is important for patient comfort and precision of treatment delivery. Mini-ridge filters (MRF) have been shown to reduce beam delivery time for synchrotron-based PBS. Given that cyclotron systems are widely used in proton therapy it is necessary to investigate the potential clinical benefit of mini-ridge filters in such systems. Purpose: To demonstrate the clinical benefit of using a MRF to reduce beam delivery time for patients with large target volumes and superficial disease in cyclotron-based PBS proton therapy. Methods: A MRF beam model was generated by simulating the effect of a MRF on our clinical beam data assuming a fixed snout position relative to the isocenter. The beam model was validated with a series of measurements. The model was used to optimize treatment plans in a water phantom and on six patient DICOM datasets to further study the effect of the MRF and for comparison with physician-approved clinical treatment plans. Beam delivery time was measured for six plans with and without the MRF to demonstrate the reduction achievable. Plans with and without MRF were reviewed to confirm clinical acceptability by a radiation oncologist. Patient-specific QA measurements were carried out with a two-dimensional ionization chamber array detector for one representative patient’s plan optimized with the MRF beam model. Results: Results show good agreement between the simulated beam model and measurements with mean and maximum deviations of 0.06 mm (0.45%) and 0.61 mm (4.9%). The increase in Bragg peak width (FWHM) ranged from 2.7 mm at 226 MeV to 6.1 mm at 70 MeV. The mean and maximum reduction in beam delivery time observed per field was 29.1 s (32.2%) and 79.7 s (55.3%). Conclusion: MRFs can be used to reduce treatment time in cyclotron-based PBS proton therapy without sacrificing plan quality. This is particularly beneficial for patients with large targets and superficial disease such as in breast cancer where treatment times are generally long, as well as patients treated with deep inspiration breath hold (DIBH). [ABSTRACT FROM AUTHOR]

Published
2023
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13. Commissioning and validation of a novel commercial TPS for ocular proton therapy.

Author

Wulff, Jörg, Koska, Benjamin, Heufelder, Jens, Janson, Martin, Bäcker, Claus Maximilian, Siregar, Hilda, Behrends, Carina, Bäumer, Christian, Foerster, Andreas, Bechrakis, Nikolaos E., and Timmermann, Beate

Subjects
PROTON therapy, X-ray imaging, PROTON scattering, VISUAL accommodation, GEOMETRIC modeling, PROTON beams, PHOTON beams
Abstract

Background: Until today, the majority of ocular proton treatments worldwide were planned with the EYEPLAN treatment planning system (TPS). Recently, the commercial, computed tomography (CT)‐based TPS for ocular proton therapy RayOcular was released, which follows the general concepts of model‐based treatment planning approach in conjunction with a pencil‐beam‐type dose algorithm (PBA). Purpose: To validate RayOcular with respect to two main features: accurate geometrical representation of the eye model and accuracy of its dose calculation algorithm in combination with an Ion Beam Applications (IBA) eye treatment delivery system. Methods: Different 3D‐printed eye‐ball‐phantoms were fabricated to test the geometrical representation of the corresponding CT‐based model, both in orthogonal 2D images for X‐ray image overlay and in fundus view overlaid with a funduscopy. For the latter, the phantom was equipped with a lens matching refraction of the human eye. Funduscopy was acquired in a Zeiss Claus 500 camera. Tantalum clips and fiducials attached to the phantoms were localized in the TPS model, and residual deviations to the actual position in X‐ray images for various orientations of the phantom were determined, after the nominal eye orientation was corrected in RayOcular to obtain a best overall fit. In the fundus view, deviations between known and displayed distances were measured. Dose calculation accuracy of the PBA on a 0.2 mm grid was investigated by comparing between measured lateral and depth–dose profiles in water for various combinations of range, modulation, and field‐size. Ultimately, the modeling of dose distributions behind wedges was tested. A 1D gamma‐test was applied, and the lateral and distal penumbra were further compared. Results: Average residuals between model clips and visible clips/fiducials in orthogonal X‐ray images were within 0.3 mm, including different orientations of the phantom. The differences between measured distances on the registered funduscopy image in the RayOcular fundus view and the known ground‐truth were within 1 mm up to 10.5 mm distance from the posterior pole. No clear benefit projection of either polar mode or camera mode could be identified, the latter mimicking camera properties. Measured dose distributions were reproduced with gamma‐test pass‐rates of >95% with 2%/0.3 mm for depth and lateral profiles in the middle of spread‐out Bragg‐peaks. Distal falloff and lateral penumbra were within 0.2 mm for fields without a wedge. For shallow depths, the agreement was worse, reaching pass‐rates down to 80% with 5%/0.3 mm when comparing lateral profiles in air. This is caused by low‐energy protons from a scatter source in the IBA system not modeled by RayOcular. Dose distributions modified by wedges were reproduced, matching the wedge‐induced broadening of the lateral penumbra to within 0.4 mm for the investigated cases and showing the excess dose within the field due to wedge scatter. Conclusion: RayOcular was validated for its use with an IBA single scattering delivery nozzle. Geometric modeling of the eye and representation of 2D projections fulfill clinical requirements. The PBA dose calculation reproduces measured distributions and allows explicit handling of wedges, overcoming approximations of simpler dose calculation algorithms used in other systems. [ABSTRACT FROM AUTHOR]

Published
2023
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15. Experiments and Monte Carlo simulations on multiple Coulomb scattering of protons

Author

Verbeek, Nico, Wulff, Jörg, Janson, Martin, Bäumer, Christian, Timmermann, Beate, and Brualla, Lorenzo

Subjects
Physics::Medical Physics, Geant4, PENELOPE, TOPAS, Monte Carlo, multiple scattering, PENH, proton
Abstract

Background and purpose: Monte Carlo simulations as well as analytical computations of proton transport in material media require accurate values of multiple Coulomb scattering (MCS) angles. High-quality experimental data on MCS angles in the energy range for proton therapy are, however, sparse. In this work, MCS modeling in proton transport was evaluated employing an experimental method to measure these angles on a medical proton beamline in clinically relevant materials. Results are compared to Monte Carlo simulations and analytical models. Materials and methods: Aluminum, brass, and lucite (PMMA) scatterers of clinically relevant thicknesses were irradiated with protons at 100, 160, and 220 MeV. Resulting spatial distributions of individual pencil beams were measured with a scintillating screen. The MCS angles were determined by deconvolution and a virtual point source approach. Results were compared to those obtained with the Monte Carlo codes PENH, TOPAS, and RayStation Monte Carlo, as well as the analytical models RayStation Pencil Beam Algorithm and the Molière/Fano/Hanson variant of the Molière theory. Results: Experimental data obtained with the presented methodology agree with previously published results within 6%, with an average deviation of 3%. The combined average uncertainty of the experimental data yielded 1.8%, while the combined maximum uncertainty was below 4%. The obtained Monte Carlo results for PENH, TOPAS, and RayStation deviate on average for all considered energies, materials and thicknesses, by 2.5%, 3.4%, and 2.8% from the experimental data, respectively. For the analytical models, the average deviations amount to 4.5% and 2.9% for the RayStation Pencil Beam Algorithm and the Molière/Fano/Hanson model, respectively. Conclusion: The experimental method developed for the present work allowed to measure MCS angles in clinical proton facilities with good accuracy. The presented method permits to extend the database on experimental MCS angles which is rather limited. This work further provides benchmark data for lucite in thicknesses relevant for clinical applications. The data may serve to validate dose engines of treatment planning systems and secondary dose check software. The Monte Carlo and analytical algorithms studied are capable of reproducing MCS data within the required accuracy for clinical applications.

Published
2021

22. Imaging correlated wave functions of few-electron quantum dots: Theory and scanning tunneling spectroscopy experiments

Author

Rotani, Massimo, Molinari, Elisa, Maruccio, Giuseppe, Janson, Martin, Schramm, Andreas, Meyer, Christian, Heyn, Christian, Matsui, Tomohiro, Hansen, Wolfgang, and Wiesendanger, Roland

Subjects
Wave functions -- Analysis, Indium -- Atomic properties, Arsenic compounds -- Atomic properties, Physics
Abstract

Experiments are conducted to show that scanning tunneling spectroscopy (STS) images of semiconductor quantum dots display clear signatures of electron-electron correlation. The wave-function sequence is described in terms of images of one- and two-electron states by comparing measured STS images of freestanding InAs quantum dots with those calculated by the full configuration interaction method.

Published
2007

25. Ion Implantation Processing and Related Effects in SiC

Author

Svensson, B.G., primary, Hallén, Anders, additional, Wong-Leung, J., additional, Janson, Martin S., additional, Linnarsson, Margareta K., additional, Kuznetsov, Andrej Yu., additional, Alfieri, Giovanni, additional, Grossner, Ulrike, additional, Monakhov, Edouard V., additional, K.-Nielsen, H., additional, Jagadish, C., additional, and Grillenberger, Joachim, additional

Published
2006
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28. Consideration of the Bragg peak detector size in the modeling of proton PBS machines in the treatment planning system RayStation

Author

Janson, Martin

Abstract

Two sets of single spot integrated depth doses (IDDs) in the energy range 100-226 MeV were recorded using the IBA dedicated PBS nozzle in Gantry treatment room 4 of the West German Proton Therapy Centre in Essen (WPE). The first set used the Bragg Peak Chamberu00ae detector (BPC) from PTW, which has an electrode diameter of 81.6 mm. The other set of IDDs were recorded using the significantly larger StingRayu00ae detector from IBA dosimetry, with a diameter of 120 mm. In addition, central axis depth dose curves (DDCs) of 10x10 cm2 scanned fields in water were measured. Two beam models based on the two sets of measured IDDs are created in in the treatment planning system RayStation (v.8A), where the size of the detectors is input to the modeling. DDCs of the scanned fields are computed by the Monte Carlo dose engine in RayStation 8A using the two beam models and are compared to the measured DDCs.Excluding the high gradient parts around the Bragg peak, the relative difference between the measured IDDs of the two detectors was found to be up to 2.8%, with the largest differences at intermediate depths for the highest energies. However, the relative difference between the measured and the two sets of calculated DDCs were within 1% and was dominated by random noise from the measurements. No systematic dependence in depth, energy, or type of detector was found. This shows that the lateral extension of the IDD detector is accurately considered in the RayStation beam modeling and that the same degree of accuracy can be achieved using either of the BPC or StingRayu00ae detectors.

Published
2017

29. Localized interaction of single porphyrin molecules with oxygen vacancies on TiO2(110).

Author

Lackinger, Markus, Janson, Martin S., and Ho, W.

Subjects
*PORPHYRINS, *TITANIUM oxides, *ELECTRONIC structure, *SURFACE chemistry, *MOLECULAR orbitals, *TEMPERATURE effect
Abstract

Alterations in the electronic structure of adsorbed zinc(II) etioporphyrin I (ZnEtio) through interaction with bridging oxygen vacancies on TiO2(110) are studied by scanning tunneling microscopy and spectroscopy at cryogenic temperatures under ultrahigh vacuum (UHV) conditions. Upon lateral manipulation of ZnEtio molecules above surface oxygen vacancies, the highest occupied molecular orbital shifts away from the Fermi level. The magnitude of the shift rapidly decreases with increasing distance of the molecule from the vacancy, indicating a highly localized interaction. [ABSTRACT FROM AUTHOR]

Published
2012
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32. Hydrogen diffusion and ion implantation in silicon carbide

Author

Janson, Martin

Subjects
electronics and electrical, materials science, heat abd tgernidtbanucs
Abstract

Secondary ion mass spectrometry (SIMS) has been employed tostudy the spatial distributions resulting from mass transportby diffusion and ion implantation in single crystal siliconcarbide (SiC). By a systematic analysis of this data,fundamental processes that govern these phenomena have beenderived. The acceptor atoms Al and B are known to be electricallypassivated by H in SiC. By studying the thermally stimulatedredistribution of implanted deuterium (2H) in various acceptordoped structures, it is found that hydrogen forms complexeswith the doping atoms, and also interacts strongly withimplantation induced defects. A comprehensive understanding ofthe formation and dissociation kinetics of these complexes hasbeen obtained. The extracted effective capture radius for theformation of 2H-B complexes is in good agreement with thatexpected for a coulomb force assisted trapping mechanism. Thelarge difference of 0.9 eV in the extracted dissociationenergies for the 2H-Al and 2H-B complexes suggests that theatomic configurations of the two complexes are significantlydifferent. Furthermore, by studying the migration behavior of Hin the presence of built-in electric fields, it is concludedthat all of the mobile H is in the positive charge state inp-type SiC. A large number of implantations have been performed withrespect to ion mass, energy, fluence, and crystal orientation.The electronic stopping cross sections in the low velocityregime for ions with atomic numbers 1 ≤ Z1 ≤ 15have been extracted from the ion range distributions. Theydisplay both Z1-oscillations and a smaller than velocityproportional stopping for ions with Z1 ≤ 8, in agreementwith previous reports for other materials. Furthermore, thedegree of ion channeling in various major axial and planarchannels of the 6H and 4H-SiC crystal has been explored. Twotypes of ion implantation simulators have been developed. Onebased on a statistical, data-base approach, and one atomisticsimulator, based on the binary collision approximation (BCA).By fitting BCA simulated profiles to the experimental profiles,detailed information about the electronic stopping andimplantation induced damage is extracted. In addition, thevacancy-related damage caused by the implantations has beeninvestigated by positron annihilation spectroscopy (PAS). Twotypes of implantation induced positron traps have been isolatedand are tentatively identified as a Si vacancy (VSi) and a Si-Cdivacancy (VSiVC). The extension of detected VSi is in goodagreement with that predicted by BCA simulations, and forimplantations with heavier ions VSi are revealed at far greaterdepths than the mean projected ion range due to deeplypenetrating channeled ions. NR 20140805

Published
2003

35. Common variants in human CRC genes as low-risk alleles

Author

Picelli, Simone, primary, Zajac, Pawel, additional, Zhou, Xiao-Lei, additional, Edler, David, additional, Lenander, Claes, additional, Dalén, Johan, additional, Hjern, Fredrik, additional, Lundqvist, Nils, additional, Lindforss, Ulrik, additional, Påhlman, Lars, additional, Smedh, Kennet, additional, Törnqvist, Anders, additional, Holm, Jörn, additional, Janson, Martin, additional, Andersson, Magnus, additional, Ekelund, Susanne, additional, Olsson, Louise, additional, Lundeberg, Joakim, additional, and Lindblom, Annika, additional

Published
2010
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36. PepT1 oligopeptide transporter (SLC15A1) gene polymorphism in inflammatory bowel disease

Author

Zucchelli, Marco, primary, Torkvist, Leif, additional, Bresso, Francesca, additional, Halfvarson, Jonas, additional, Hellquist, Anna, additional, Anedda, Francesca, additional, Assadi, Ghazaleh, additional, Lindgren, Gunnar B., additional, Svanfeldt, Monika, additional, Janson, Martin, additional, Noble, Colin L., additional, Pettersson, Sven, additional, Lappalainen, Maarit, additional, Paavola-Sakki, Paulina, additional, Halme, Leena, additional, Färkkilä, Martti, additional, Turunen, Ulla, additional, Satsangi, Jack, additional, Kontula, Kimmo, additional, Löfberg, Robert, additional, Kere, Juha, additional, and DʼAmato, Mauro, additional

Published
2009
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46. The Effect of Hydrogen Diffusion in p- and n-Type SiC Schottky Diodes at High Temperatures

Author

Unéus, Lars, primary, Nakagomi, Shinji, additional, Linnarsson, Margareta K., additional, Janson, Martin S., additional, Svensson, Bengt Gunnar, additional, Yakimova, Rositza, additional, Syväjärvi, Mikael, additional, Henry, Anne, additional, Janzén, Erik, additional, Ekedahl, Lars-G., additional, Lundström, Ingemar, additional, and Lloyd Spetz, Anita, additional

Published
2002
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50. Precipitate Formation in Heavily Al-Doped 4H-SiC Layers

Author

Linnarsson, Margareta K., primary, Persson, P.O.Å., additional, Bleichner, H., additional, Janson, Martin S., additional, Zimmermann, Uwe, additional, Andersson, H., additional, Karlsson, S., additional, Yakimova, Rositza, additional, Hultman, L., additional, and Svensson, Bengt Gunnar, additional

Published
2001
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