| 1. |
- Aghion, S., et al.
(författare)
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A moiré deflectometer for antimatter
- 2014
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Ingår i: Nature Communications. - : Nature Publishing Group. - 2041-1723. ; 5
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Tidskriftsartikel (refereegranskat)abstract
- The precise measurement of forces is one way to obtain deep insight into the fundamental interactions present in nature. In the context of neutral antimatter, the gravitational interaction is of high interest, potentially revealing new forces that violate the weak equivalence principle. Here we report on a successful extension of a tool from atom optics—the moiré deflectometer—for a measurement of the acceleration of slow antiprotons. The setup consists of two identical transmission gratings and a spatially resolving emulsion detector for antiproton annihilations. Absolute referencing of the observed antimatter pattern with a photon pattern experiencing no deflection allows the direct inference of forces present. The concept is also straightforwardly applicable to antihydrogen measurements as pursued by the AEgIS collaboration. The combination of these very different techniques from high energy and atomic physics opens a very promising route to the direct detection of the gravitational acceleration of neutral antimatter.
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| 2. |
- Ahmed, Fareed, et al.
(författare)
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Manufacturing Poly(3,4-Ethylenedioxythiophene) Electrocatalytic Sheets for Large-Scale H2O2 Production
- 2022
-
Ingår i: Advanced Sustainable Systems. - : John Wiley and Sons Inc. - 2366-7486. ; 6:1
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Tidskriftsartikel (refereegranskat)abstract
- Producing thick films of conducting polymers by a low-cost manufacturing technique would enable new applications. However, removing huge solvent volume from diluted suspension or dispersion (1–3 wt%) in which conducting polymers are typically obtained is a true manufacturing challenge. In this work, a procedure is proposed to quickly remove water from the conducting polymer poly(3,4-ethylenedioxythiophene:poly(4-styrene sulfonate) (PEDOT:PSS) suspension. The PEDOT:PSS suspension is first flocculated with 1 m H2SO4 transforming PEDOT nanoparticles (≈50–500 nm) into soft microparticles. A filtration process inspired by pulp dewatering in a paper machine on a wire mesh with apertures dimension between 60 µm and 0.5 mm leads to thick free-standing films (≈0.5 mm). Wire mesh clogging that hinders dewatering (known as dead-end filtration) is overcome by adding to the flocculated PEDOT:PSS dispersion carbon fibers that aggregate and form efficient water channels. Moreover, this enables fast formation of thick layers under simple atmospheric pressure filtration, thus making the process truly scalable. Thick freestanding PEDOT films thus obtained are used as electrocatalysts for efficient reduction of oxygen to hydrogen peroxide, a promising green chemical and fuel. The inhomogeneity of the films does not affect their electrochemical function. © 2021 The Authors.
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| 3. |
- Andersson, Malte, 1941, et al.
(författare)
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”Minskande befolkning är inte problemet”
- 2020
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Ingår i: Dagens Nyheter. ; :1 augusti, DN-debatt
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
- Nätverket Population Matters Sweden: En uppmärksammad studie i The Lancet pekar mot en lägre befolkningsökning i världen än tidigare prognoser. Men en miljard människor till är fortfarande långt över vad jorden klarar. Befolkningstrenden måste snarare vända neråt, och det kräver åtgärder för att stärka kvinnors rättigheter världen över.
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| 4. |
- Ariga, T., et al.
(författare)
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Measuring GBAR with emulsion detector
- 2014
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Ingår i: International Journal of Modern Physics, Conference Series. - : World Scientific. - 2010-1945. ; 30
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Tidskriftsartikel (refereegranskat)
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| 5. |
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| 6. |
- Bankel, Johan, et al.
(författare)
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The CDIO syllabus: a comparative study of expected student proficiency
- 2003
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Ingår i: European Journal of Engineering Education. - : Taylor & Francis. - 0304-3797 .- 1469-5898. ; 28:3, s. 297-315
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Tidskriftsartikel (refereegranskat)abstract
- Engineering students must graduate with command of a vast body of technical knowledge. They must possess personal, interpersonal and system-building skills to function in teams, and be prepared to produce products and systems. Their education must have been structured under a curriculum blending ability to combine technical expertise with ethical, innovative, philosophical and humanistic acumen. This paper describes a unique international collaboration among four universities to reform engineering education. The collaborators agreed to a statement of goals, which includes descriptions of knowledge, skills and attitudes vital to an effective education, and codifies proficiency levels expected of graduates. We developed and utilized unique stakeholder surveys both to validate our prototype and to determine desired proficiency levels. This collaboration resulted in The CDIO Syllabus, A Statement of Goals for Undergraduate Engineering Education. The syllabus is both a template and a process that can be used to customize the syllabus to others' programmes. It can define new educational initiatives/and be employed as the basis for rigorous assessment. This paper details how, with the input of industry, academia and others, we employed an engineering problem-solving paradigm to effect our redesign. It outlines the syllabus and the unique process employed to create it.
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| 7. |
- Batson, Emma K., et al.
(författare)
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Effects of Helium Ion Exposure on the Single-Photon Sensitivity of MgB $_{2}$ and NbN Detectors
- 2024
-
Ingår i: IEEE Transactions on Applied Superconductivity. - 1558-2515 .- 1051-8223. ; In Press
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Tidskriftsartikel (refereegranskat)abstract
- Improving the scalability, reproducibility, and operating temperature of superconducting nanowire single photon detectors (SNSPDs) has been a major research goal since the devices were first proposed. The recent innovation of helium-ion irradiation as a post-processing technique for SNSPDs could enable high detection efficiencies to be more easily reproducible, but is still poorly understood. Additionally, fabricating detectors at micron-wide scales from high-T$_{\mathrm{C}}$ materials could improve scalability and operating temperature, respectively. At the same time, fabrication of successful devices in wide wires and from higher-T$_{\mathrm{C}}$ materials like magnesium diboride has proven challenging. In this work, we compare helium ion irradiation in niobium nitride and magnesium diboride detectors with different material stacks in order to better understand the mechanics of irradiation and practical implications of encapsulating layers on effective dose. We examine the effects of experimental effective dose tests and compare these results to the damage per ion predicted by simulations in corresponding material stacks. In both materials, irradiation results in an increase in count rate, though for niobium nitride this increase has not fully saturated even at the highest tested dose of $2.6\times 10^{17}$ ions/cm$^{2}$, while for resist-encapsulated magnesium diboride even the lowest tested dose of $1\times 10^{15}$ ions/cm$^{2}$ appears higher than optimal. Our results demonstrate the general applicability of helium ion irradiation to vastly different devices and material stacks, albeit with differing optimal doses, and show the reproducibility and effectiveness of this post-processing technique in significantly improving SNSPD efficiency.
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| 8. |
- Belaineh, Dagmawi, et al.
(författare)
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Controlling the Organization of PEDOT:PSS on Cellulose Structures
- 2019
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Ingår i: ACS APPLIED POLYMER MATERIALS. - : AMER CHEMICAL SOC. - 2637-6105. ; 1:9, s. 2342-2351
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Tidskriftsartikel (refereegranskat)abstract
- Composites of biopolymers and conducting polymers are emerging as promising candidates for a green technological future and are actively being explored in various applications, such as in energy storage, bioelectronics, and thermoelectrics. While the device characteristics of these composites have been actively investigated, there is limited knowledge concerning the fundamental intracomponent interactions and the modes of molecular structuring. Here, by use of cellulose and poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), it is shown that the chemical and structural makeup of the surfaces of the composite components are critical factors that determine the materials organization at relevant dimensions. AFM, TEM, and GIVVAXS measurements show that when mixed with cellulose nanofibrils, PEDOT:PSS organizes into continuous nanosized beadlike structures with an average diameter of 13 nm on the nanofibrils. In contrast, when PEDOT:PSS is blended with molecular cellulose, a phase-segregated conducting network morphology is reached, with a distinctly relatively lower electric conductivity. These results provide insight into the mechanisms of PEDOT:PSS crystallization and may have significant implications for the design of conducting biopolymer composites for a vast array of applications.
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| 9. |
- Belaineh Yilma, Dagmawi, et al.
(författare)
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Printable carbon-based supercapacitors reinforced with cellulose and conductive polymers
- 2022
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Ingår i: Journal of Energy Storage. - : Elsevier Ltd. - 2352-152X .- 2352-1538. ; 50
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Tidskriftsartikel (refereegranskat)abstract
- Sustainable electrical energy storage is one of the most important scientific endeavors of this century. Battery and supercapacitor technologies are here crucial, but typically the current state of the art suffers from either lack of large-scale production possibilities, sustainability or insufficient performance and hence cannot match growing demands in society. Paper and cellulosic materials are mature scalable templates for industrial roll-to-roll production. Organic materials, such as conducting polymers, and carbon derivatives are materials that can be synthesized or derived from abundant sources. Here, we report the combination of cellulose, PEDOT:PSS and carbon derivatives for bulk supercapacitor electrodes adapted for printed electronics. Cellulose provides a mesoscopic mesh for the organization of the active ingredients. Furthermore, the PEDOT:PSS in combination with carbon provides superior device characteristics when comparing to the previously standard combination of activated carbon and carbon black. PEDOT:PSS acts as a mixed ion-electron conducting glue, which physically binds activated carbon particles together, while at the same time facilitating swift transport of both electrons and ions. A surprisingly small amount (10%) of PEDOT:PSS is needed to achieve an optimal performance. This work shows that cellulose added to PEDOT:PSS-carbon enables high-performing, mechanically stable, printed supercapacitor electrodes using a combination of printing methods.
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| 10. |
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| 11. |
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| 12. |
- Berggren, Johanna, et al.
(författare)
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Reperfusion of Free Full-Thickness Skin Grafts in Periocular Reconstructive Surgery Monitored Using Laser Speckle Contrast Imaging
- 2021
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Ingår i: Ophthalmic Plastic and Reconstructive Surgery. - 1537-2677. ; 37:4, s. 324-328
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Tidskriftsartikel (refereegranskat)abstract
- PURPOSE: Free skin grafts are frequently used in reconstructive surgery. However, little is known about the course of reperfusion due to the previous lack of reliable perfusion monitoring techniques. The aim of this study was to use state-of-the-art laser speckle contrast imaging to monitor free skin grafts in the periocular area.METHODS: Seven patients needing surgery due to tumor removal or cicatricial ectropion in the periocular region underwent reconstructive surgery using free skin grafts from either the contralateral upper eyelid or the upper inner arm. The free skin grafts measured 10-30 mm horizontally and 9-30 mm vertically. Blood perfusion was monitored using laser speckle contrast imaging immediately postoperatively (0 weeks) and at follow-up after 1, 3, and 7 weeks.RESULTS: All grafts were reperfused gradually during healing, the median value being 46% in the central part of the graft after 1 week and 79% after 3 weeks. The grafts were completely reperfused after 7 weeks. No difference was observed in the rate of reperfusion between the center and periphery of the grafts (p = not significant). The cosmetic and functional outcome was excellent in all but 1 patient, who developed ectropion that had to be surgically corrected.CONCLUSIONS: Skin grafts in the periorbital area are fully reperfused after 7 weeks. The periocular area is known to be well-vascularized and thus forgiving to reconstructive surgery. Future investigations of the reperfusion of free skin grafts in other parts of the body or in higher-risk populations should be carried out.
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| 13. |
- Berggren, Johanna, et al.
(författare)
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Revascularization After H-plasty Reconstructive Surgery in the Periorbital Region Monitored With Laser Speckle Contrast Imaging
- 2021
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Ingår i: Ophthalmic Plastic and Reconstructive Surgery. - 1537-2677. ; 37:3, s. 269-273
-
Tidskriftsartikel (refereegranskat)abstract
- BACKGROUND: H-plasty reconstructive surgery is commonly used to close defects after tumor excision in the periorbital region. Revascularization of the bipedicle skin flaps is essential for healing. However, it has not previously been possible to study this revascularization in humans due to the lack of noninvasive perfusion monitoring techniques. The aim was to monitor perfusion in H-plasty flaps during surgery and during postoperative follow-up, using laser speckle contrast imaging.METHOD: H-plasty, i.e., bipedicle random advancement skin flaps, was used for reconstruction of the eyelids after tumor removal in 7 patients. The median length and width of the skin flaps were 13 mm (range, 8-20 mm) and 10 mm (range, 5-11 mm), respectively. Blood perfusion was measured using laser speckle contrast imaging during surgery and at follow up 1, 3, and 6 weeks postoperatively, to monitor revascularization.RESULTS: Immediately postoperatively, the perfusion in the distal end of the flaps had fallen to 54% (95% CI, 38%-67%). The perfusion then quickly increased during the healing process, being 104% (86%-124%) after 1 week, 115% (94%-129%) after 3 weeks, and 112% (96%-137%) after 6 weeks. There was no clinically observable ischemia or tissue necrosis.CONCLUSIONS: Revascularization of the H-plasty procedure flaps occurs quickly, within a week postoperatively, presumably due to the existing vascular network of the flap pedicle, and was not dependent on significant angiogenesis. This perfusion study confirms the general opinion that H-plasty is a good reconstructive technique, especially in the periorbital region with its rich vascular supply.
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| 14. |
- Berggren, Johanna, et al.
(författare)
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Revascularization of Free Skin Grafts Overlying Modified Hughes Tarsoconjunctival Flaps Monitored Using Laser-Based Techniques
- 2019
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Ingår i: Ophthalmic Plastic and Reconstructive Surgery. - 1537-2677. ; 35:4, s. 378-382
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: It has recently been shown that the flap pedicle does not supply blood to a tarsoconjunctival graft in the modified Hughes procedure in patients. This raises questions concerning the rate of revascularization of the free skin graft commonly used to reconstruct the anterior lamella. The aim of this study was, thus, to monitor the course of revascularization in free skin grafts overlying modified Hughes tarsoconjunctival flaps, using laser-based techniques.Methods: Free skin grafts from the upper eyelid or upper arm in 9 patients were used to cover a tarsoconjunctival flap according to the modified Hughes procedure. Blood perfusion was monitored using laser speckle contrast imaging, and vascular reactivity was studied with laser Doppler velocimetry after heating the tissue to 44°C. Measurements were made at the time of surgery (baseline) and at 1, 3, 8, and 16 weeks postoperatively.Results: The gradual increase in perfusion of the free skin grafts during the healing process indicates revascularization. A slight increase in perfusion was seen already after 1 week. Perfusion reached 50% of the baseline after 3 weeks, and complete restoration of perfusion was seen after 8 weeks. The vascular function monitored with heat-induced hyperemia increased in a similar fashion.Conclusions: Full-thickness skin grafts revascularize within 3 to 8 weeks, despite overlying a tarsoconjunctival flap, which has recently been reported to be avascular. This provides further evidence that it should be possible to repair large eyelid defects using free full-thickness eyelid grafts.
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| 15. |
- Berggren, Johanna V, et al.
(författare)
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Laser Speckle Contrast Imaging of a Rotational Full-Thickness Lower Eyelid Flap Shows Satisfactory Blood Perfusion
- 2021
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Ingår i: Ophthalmic Plastic and Reconstructive Surgery. - 1537-2677. ; 37:4, s. 139-141
-
Tidskriftsartikel (refereegranskat)abstract
- Full-thickness eyelid flaps from the lower eyelid are frequently used to repair larger upper eyelid defects. Perfusion monitoring has recently been implemented in several reconstructive surgical procedures, however, perfusion monitoring of a rotational eyelid flap has not yet been described. The authors' employed laser speckle contrast imaging to monitor blood perfusion in a rotational flap from the lower eyelid, used to cover a large tumor defect in the upper eyelid. Perfusion in the flap decreased by only 50% during surgery and was almost completely restored 5 weeks later at flap division (91%). The excellent surgical outcome in the present case is deemed to be the result of satisfactory blood perfusion of the flap.
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| 16. |
- Berggren, Johanna V, et al.
(författare)
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Laser Speckle Contrast Imaging of the Blood Perfusion in Glabellar Flaps Used to Repair Medial Canthal Defects
- 2022
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Ingår i: Ophthalmic Plastic and Reconstructive Surgery. - 1537-2677. ; 38:3, s. 274-279
-
Tidskriftsartikel (refereegranskat)abstract
- BACKGROUND: The glabellar flap is a common technique for surgical repair after tumor excision in the medial canthal area. However, the outcome may be affected by partial flap necrosis. Little is known about the impact of surgery on blood perfusion and the postoperative course of reperfusion due to the absence of reliable and noninvasive perfusion monitoring techniques. The aim of this study was to use a modern imaging technique to assess blood perfusion in glabellar flaps.METHODS: Glabellar flaps were used to repair medial canthal defects following tumor excision in 7 patients. Blood perfusion was monitored using laser speckle contrast imaging: during surgery, immediately postoperatively (0 weeks), and at follow-up, 1, 3, and 6 weeks after surgery.RESULTS: Perfusion decreased gradually along the length of the flap, and reached a minimum 15 mm from the flap base. Perfusion in the proximal 20 mm of the flap was completely restored after 1 week, while the distal part of the flap was gradually reperfused over 6 weeks. Both the functional and esthetic surgical outcomes were excellent.CONCLUSIONS: The rapid reperfusion of the glabellar flap may be explained by its connection to the vascular network via the flap pedicle. In flaps longer than 20 mm, the distal part can be considered a free skin transplant, and a combination of a glabellar flap and a free skin graft could then be considered.
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| 17. |
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| 18. |
- Berggren, Karl, 1989-, et al.
(författare)
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Characterization of photon-counting multislit breast tomosynthesis
- 2018
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Ingår i: Medical Physics. - : John Wiley & Sons. - 2473-4209 .- 0094-2405.
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: It has been shown that breast tomosynthesis may improve sensitivity and specificity compared to two-dimensional mammography, resulting in increased detection-rate of cancers or lowered call-back rates. The purpose of this study is to characterize a spectral photon-counting multislit breast tomosynthesis system that is able to do single-scan spectral imaging with multiple collimated x-ray beams. The system differs in many aspects compared to conventional tomosynthesis using energyintegrating flat-panel detectors. Methods: The investigated system was a prototype consisting of a dual-threshold photon-counting detector with 21 collimated line detectors scanning across the compressed breast. A review of the system is done in terms of detector, acquisition geometry, and reconstruction methods. Three reconstruction methods were used, simple back-projection, filtered back-projection and an iterative algebraic reconstruction technique. The image quality was evaluated by measuring the modulation transfer-function (MTF), normalized noise-power spectrum, detective quantum-efficiency (DQE), and artifact spread-function (ASF) on reconstructed spectral tomosynthesis images for a total-energy bin (defined by a low-energy threshold calibrated to remove electronic noise) and for a high-energy bin (with a threshold calibrated to split the spectrum in roughly equal parts). Acquisition was performed using a 29 kVp W/Al x-ray spectrum at a 0.24 mGy exposure. Results: The difference in MTF between the two energy bins was negligible, that is, there was no energy dependence on resolution. The MTF dropped to 50% at 1.5 lp/mm to 2.3 lp/mm in the scan direction and 2.4 lp/mm to 3.3 lp/mm in the slit direction, depending on the reconstruction method. The full width at half maximum of the ASF was found to range from 13.8 mm to 18.0 mm for the different reconstruction methods. The zero-frequency DQE of the system was found to be 0.72. The fraction of counts in the high-energy bin was measured to be 59% of the total detected spectrum. Scantimes ranged from 4 s to 16.5 s depending on voltage and current settings. Conclusions: The characterized system generates spectral tomosynthesis images with a dual-energy photon-counting detector. Measurements show a high DQE, enabling high image quality at a low dose, which is beneficial for low-dose applications such as screening. The single-scan spectral images open up for applications such as quantitative material decomposition and contrast-enhanced tomosynthesis.
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| 19. |
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| 20. |
- Berggren, Karl-Fredrik
(författare)
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All-Time High
- 2010
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Ingår i: Fysikaktuellt. - 0283-9148. ; :4, s. 3-3
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Tidskriftsartikel (populärvet., debatt m.m.)
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| 21. |
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| 22. |
- Berggren, Karl-Fredrik, et al.
(författare)
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Basic Modeling of Openness, Quantum States and Transport in Two- and Three-Dimensional Ballistic Cavities
- 2010
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Ingår i: Journal of Siberian Federal University. Mathematics & Physics. ; 3, s. 280-288
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Tidskriftsartikel (refereegranskat)abstract
- A basic model for particle states and current flow in open quantum dots/billiards are investigated. The model is unconventional and extends the use of complex potentials first introduced in phenomenological nuclear inelastic scattering theory (the optical model). Attached leads/source drain are represented by complex potentials. Probability densities and currents flows for open 2D quantum dots/billiards are calculated and the results are compared with microwave measurements used to emulate the dot. We also apply the model to a recangular enclosure and report on helical flows guided by nodal lines and disc-like accumulations of flow lines. The model is of conceptual as well as practical and educational interest.
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| 23. |
- Berggren, Karl-Fredrik, et al.
(författare)
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Basic modelling of transport in 2D wave-mechanical nanodots and billiards with balanced gain and loss mediated by complex potentials
- 2018
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Ingår i: Journal of Physics. - : IOP PUBLISHING LTD. - 0953-8984 .- 1361-648X. ; 30:20
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Tidskriftsartikel (refereegranskat)abstract
- Non-Hermitian quantum mechanics with parity-time (PT) symmetry is presently gaining great interest, especially within the fields of photonics and optics. Here, we give a brief overview of low-dimensional semiconductor nanodevices using the example of a quantum dot with input and output leads, which are mimicked by imaginary potentials for gain and loss, and how wave functions, particle flow, coalescence of levels and associated breaking of PT symmetry may be analysed within such a framework. Special attention is given to the presence of exceptional points and symmetry breaking. Related features for musical string instruments and wolf-notes are outlined briefly with suggestions for further experiments.
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| 24. |
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| 25. |
- Berggren, Karl-Fredrik, et al.
(författare)
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Chaos in a quantum dot with spin-orbit coupling
- 2001
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Ingår i: Foundations of physics. - 0015-9018 .- 1572-9516. ; 31:2, s. 233-242
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Tidskriftsartikel (refereegranskat)abstract
- Level statistics are nodal point distribution in a rectangular semiconductor quantum dot are studies for different degrees of spin-orbit coupling. The chaotic features occurring from the spin-orbit coupling have no classical counterpart. Using experimental values of GaSb/InAs/GaSb semiconductor quantum wells we find that level repulsion can lead to the semi-Poisson distribution for nearest level separations. Nodal lines and nodal points are also investigated. Comparison is made with nodal point distributions for fully chaotic states.
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| 26. |
- Berggren, Karl-Fredrik, et al.
(författare)
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“Correlation and disorder effects on electron transport in modulation-doped semiconductor quantum point contacts” (invited)
- 2013
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- ABSTRACT We simulate effects of electron localization and quantum correlations in realistic GaAs/AlGaAs quantum point contacts (QPCs) in the presence of randomly distributed donors (1) using spin-relaxed density functional theory (DFT/LSDA). Two different configurations of gates defining the QPCs were studied: a split gate and a top gate in addition to the split gate. In both cases we recover the conventional fluctuation-free parabolic electrostatic potential when the distance between the donor layer and the 2D electron gas exceeds 70 nm. Hence we also find ballistic phenomena such as integer conductance steps as well as the 0.7 anomaly. The electrostatic potential changes dramatically,however, when the random donors are placed closer to the 2D gas. Electron localization is then increased and conductance fluctuations and resonance peaks appear. At the same time the usual conductance steps vanish. By charging asymmetrically the split gates voltage we have found that conductance fluctuations caused by random donors are shifted while the anomalies caused by interaction effects may remain. Resonance peaks in the conductance derive from localized states inside within the QPC. The nature of electron localization has been discussed in our previous study (2) where we stress the crucial role of confinement potential on the formation of electron localization. In the present study we have shown that electron localization may be caused by randomly distributed donors and play an important role in electron transport, especially near the pinch-off regime. The results of our numerical simulations agree with recent experimental studies (3). (1) J.A. Nixon, J.H. Davies, and H.U Baranger, Phys. Rev. B 43, 12638 (1991) (2) I. I. Yakimenko, V. S. Tsykunov and K.-F. Berggren, J. Phys. Condens. Matter 25, 072201 (2013)(3) L.W. Smith, K. J. Thomas, M. Pepper, D. A. Ritchie, I. Farrer, J.P. Griffiths, G.A.C. Jones, J. of Phys.: Conf. Series 376, 012018, (2012)
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| 27. |
- Berggren, Karl-Fredrik, et al.
(författare)
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Crossover from regular to irregular behavior in current flow through open billiards
- 2002
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Ingår i: Physical Review E - Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755. ; 66:1
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Tidskriftsartikel (refereegranskat)abstract
- We discuss signatures of quantum chaos in terms of distributions of nodal points, saddle points, and streamlines for coherent electron transport through two-dimensional billiards, which are either nominally integrable or chaotic. As typical examples of the two cases we select rectangular and Sinai billiards. We have numerically evaluted distribution functions for nearest distances between nodal points and found that there is a generic form for open chaotic billiards through which a net current is passed. We have also evaluated the distribution functions for nodal points with specific vorticity (winding number) as well as for saddle points. The distributions may be used as signatures of quantum chaos in open systems. All distributions are well reproduced using random complex linear combinations of nearly monochromatic states in nominally closed billiards. In the case of rectangular billiards with simple sharp-cornered leads the distributions have characteristic features related to order among the nodal points. A flaring or rounding of the contact regions may, however, induce a crossover to nodal point distributions and current flow typical for quantum chaos. For an irregular arrangement of nodal points, as for example in the Sinai billiard, the quantum flow lines become very complex and volatile, recalling chaos among classical trajectories. Similarities with percolation are pointed out. ©2002 The American Physical Society.
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| 28. |
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| 29. |
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| 30. |
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| 31. |
- Berggren, Karl-Fredrik, 1937-, et al.
(författare)
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Effects of electron interactions at crossings of Zeeman-split subbands in quantum wires
- 2005
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 71:11, s. 115303-1-115303-5
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Tidskriftsartikel (refereegranskat)abstract
- Recent experimental studies of Zeeman-split one-dimensional subbands in ballistic quantum wires in an in-plane magnetic field show that additional nonquantized conductance structures occur as subbands cross at low electron densities [A. C. Graham et al., Phys. Rev.Lett. 91, 136404 (2003)]. These structures are called 0.7 analogs. We analyze the experimental transconductance data within the Kohn-Sham spin-density-functional method, including exchange and correlation effects for an infinite split-gate quantum wire in a parallel, in-plane magnetic field B∥. Energy levels are found to rearrange abruptly as they cross due to polarization effects driven by exchange and Coulomb interactions. Experimental qualitative features are explained well by this model. ©2005 The American Physical Society.
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| 32. |
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| 33. |
- Berggren, Karl-Fredrik, et al.
(författare)
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Effects of exchange and electron correlation on conductance and nanomagnetism in ballistic semiconductor quantum point contacts
- 2002
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 66:8
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Tidskriftsartikel (refereegranskat)abstract
- The spontaneous magnetization of a quantum point contact (QPC) formed between two large quantum dots by a lateral confinement of a high-mobility two-dimensional electron gas is studied for a realistic GaAs/AlxGa1-xAs heterostructure. The model of the device incorporates the contributions from a patterned gate, doping, surface states, and mirror charges. To explore the magnetic properties, the Kohn-Sham local spin-density formalism is used with exchange and correlation potentials that allows for local spin polarization. Exchange is the dominant mechanism behind local magnetization within the QPC, while the correlation part is less prominent. However, the correlation potential gives rise to an important correction in the QPC potential. Below the first conduction plateau we thus find a magnetized regime corresponding approximately to a single electron spin. Using an approximate separable saddle potential we compute the conductance and recover the so-called similar to0.7 (2e(2)/h) conduction anomaly plus an additional anomaly at similar to0.4 (2e(2)/h) below which the magnetization collapses.
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| 34. |
- Berggren, Karl-Fredrik, 1937-, et al.
(författare)
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Electrical Resonance Circuits as Analogs to Quantum Mechanical Billiards
- 2006
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Ingår i: Acta Physica Polonica. A. - 0587-4246 .- 1898-794X. ; 109, s. 33-42
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Tidskriftsartikel (refereegranskat)abstract
- We propose that a two-dimensional electric network may be used for fundamental studies of wave function properties, transport, and related statistics. Using Kirchhoff 's current law and the jw-method we find that the network is analogous to a discretized Schrodinger equation for quantum billiards and clots. Thus the complex electric potentials play the role of quantum rnechanical wave functions.
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| 35. |
- Berggren, Karl-Fredrik, 1937-, et al.
(författare)
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Electron transport and Nanomagnetism
- 2008
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Ingår i: Nordforsk Network in Nanospintronics Theory and Simulations,2008.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Electron states, local magnetization and conductance in realistic quantum point contacts (QPCs) with different geometry and applied gate voltages will be discussed for a model GaAs/AlGaAs device. Using the local spin density approximation (LSDA) we recover ferromagnetic spatially split solutions in the pinch-off regime as well as antisymmetric solutions that occur with increasing voltage. These kind of spin states, which may appear in a repeated fashion in the few electron regime, are precursors to an extended ferromagnetic state that may be associated with the 0.7 (2e2/h) conductance anomaly. As the length of the QPC increases, so does the spin polarization and the difference in conductance between up-spin and down-spin electrons, resulting in movement of 0.7 anomaly towards the value of 0.5 (2e2/h). Due to the strong ferromagnetic spin polarization in a long QPC it will effectively work as a spin filter which may be useful for spintronics application.
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| 36. |
- Berggren, Karl-Fredrik, et al.
(författare)
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Electrons in one dimension
- 2010
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Ingår i: Philosophical Transactions. Series A. - : The Royal Society. - 1364-503X .- 1471-2962. ; 368, s. 1141-1162
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Forskningsöversikt (refereegranskat)abstract
- In this article, we present a summary of the current status of the study of the transport of electrons confined to one dimension in very low disorder GaAs–AlGaAs heterostructures. By means of suitably located gates and application of a voltage to ‘electrostatically squeeze’ the electronic wave functions, it is possible to produce a controllable size quantization and a transition from two-dimensional transport. If the length of the electron channel is sufficiently short, then transport is ballistic and the quantized subbands each have a conductance equal to the fundamental quantum value 2e2/h, where the factor of 2 arises from the spin degeneracy. This mode of conduction is discussed, and it is shown that a number of many-body effects can be observed. These effects are discussed as in the spin-incoherent regime, which is entered when the separation of the electrons is increased and the exchange energy is less than kT. Finally, results are presented in the regime where the confinement potential is decreased and the electron configuration relaxes to minimize the electron–electron repulsion to move towards a two-dimensional array. It is shown that the ground state is no longer a line determined by the size quantization alone, but becomes two distinct rows arising from minimization of the electrostatic energy and is the precursor of a two-dimensional Wigner lattice.
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| 37. |
- Berggren, Karl-Fredrik
(författare)
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Entanglement väcker språkdebatt
- 2010
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Ingår i: Fysikaktuellt. - 0283-9148. ; :2, s. 5-5
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Tidskriftsartikel (refereegranskat)
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| 38. |
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| 39. |
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| 40. |
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| 41. |
- Berggren, Karl-Fredrik, 1937-
(författare)
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Fysik
- 2012. - 1
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Ingår i: Naturvetarguiden. - Nacka : Naturvetarna. - 9789163704130
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Bokkapitel (populärvet., debatt m.m.)
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| 42. |
- Berggren, Karl-Fredrik, et al.
(författare)
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Fysikersamfundet 90 år
- 2010
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Ingår i: Fysikaktuellt. - 0283-9148. ; :2, s. 12-12
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Tidskriftsartikel (populärvet., debatt m.m.)
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| 43. |
- Berggren, Karl-Fredrik, 1937-
(författare)
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Förolämpande och konspiratorisk - debattinlägg
- 2005
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Ingår i: Sydsvenska Dagbladet. - Malmö : Sydsvenska dagbladets AB. ; 28 maj 2005, s. C2-C3
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Tidskriftsartikel (populärvet., debatt m.m.)
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| 44. |
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| 45. |
- Berggren, Karl-Fredrik
(författare)
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Hur förstår man fysik?
- 2010
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Ingår i: Fysikaktuellt. - : Svenska Fysikersamfundet. ; :3, s. 3-3
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
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| 46. |
- Berggren, Karl-Fredrik
(författare)
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Hur står det till med SFS?
- 2011
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Ingår i: Fysikaktuellt. - 0283-9148. ; :4, s. 5-5
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Tidskriftsartikel (populärvet., debatt m.m.)
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| 47. |
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| 48. |
- Berggren, Karl-Fredrik
(författare)
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Jakten på liv i universum
- 2013
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Ingår i: Fysikaktuellt. - 0283-9148. ; :4, s. 26-26
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Recension (populärvet., debatt m.m.)
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| 49. |
- Berggren, Karl-Fredrik, et al.
(författare)
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Modeling of open quantum dots and wave billiards using imaginary potentials for the source and the sink
- 2010
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 12:073005
-
Tidskriftsartikel (refereegranskat)abstract
- A heuristic model for particle states and current flow in open ballistictwo-dimensional (2D) quantum dots/wave billiards is proposed. The modelmakes use of complex potentials first introduced in phenomenological nuclearinelastic scattering theory (the optical model). Here we assume that externalinput and output leads connecting the system to the source and the drain regionsmay be represented by complex potentials. In this way, a current may be set upbetween the two ‘pseudo-leads’. Probability densities and current flows for anopen quantum dot are analyzed here numerically and the results are comparedwith the microwave measurements used to emulate the system. The model isof conceptual as well as practical interest. In addition to quantum billiards, itmay be used as a tool per se to analyze transport in classical wave analogues,such as microwave resonators, acoustic resonators, effects of leakage on suchsystems, etc.
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| 50. |
- Berggren, Karl-Fredrik, 1937-, et al.
(författare)
-
Nature of electron states and symmetry breaking in quantum point contacts according to the local spin density approximation
- 2008
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 20:16
-
Tidskriftsartikel (refereegranskat)abstract
- Electron states and local magnetization in quantum point contacts (QPCs) with different geometries and applied gate voltages are examined for a model GaAs/AlGaAs device. Using the local spin density approximation (LSDA) we recover ferromagnetic spatially split solutions in the pinch-off regime as well as antisymmetric solutions that occur with decreasing gate voltage. These kinds of spin states, which may appear in a repeated fashion in the few-electron regime, are precursors to an extended ferromagnetic state that may be associated with the 0.7 conductance anomaly. We briefly comment on some recent experiments indicating the presence of bound states (Yoon et al 2007 Phys. Rev. Lett. 99 136805). We have not found any indication of such states but suggest that the accumulations of spin and charge at the two ends of a QPC and associated singlet and triplet states are relevant in this context. © 2008 IOP Publishing Ltd.
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