| 1. |
|
|
| 2. |
- Berggren, Karl-Fredrik
(författare)
-
All-Time High
- 2010
-
Ingår i: Fysikaktuellt. - 0283-9148. ; :4, s. 3-3
-
Tidskriftsartikel (populärvet., debatt m.m.)
|
|
| 3. |
- Berggren, Karl-Fredrik, et al.
(författare)
-
Basic Modeling of Openness, Quantum States and Transport in Two- and Three-Dimensional Ballistic Cavities
- 2010
-
Ingår i: Journal of Siberian Federal University. Mathematics & Physics. ; 3, s. 280-288
-
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.
|
|
| 4. |
- Berggren, Karl-Fredrik, et al.
(författare)
-
“Correlation and disorder effects on electron transport in modulation-doped semiconductor quantum point contacts” (invited)
- 2013
-
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)
|
|
| 5. |
|
|
| 6. |
- Berggren, Karl-Fredrik, et al.
(författare)
-
Electrons in one dimension
- 2010
-
Ingår i: Philosophical Transactions. Series A. - : The Royal Society. - 1364-503X .- 1471-2962. ; 368, s. 1141-1162
-
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.
|
|
| 7. |
- Berggren, Karl-Fredrik
(författare)
-
Entanglement väcker språkdebatt
- 2010
-
Ingår i: Fysikaktuellt. - 0283-9148. ; :2, s. 5-5
-
Tidskriftsartikel (refereegranskat)
|
|
| 8. |
|
|
| 9. |
- Berggren, Karl-Fredrik, 1937-
(författare)
-
Fysik
- 2012. - 1
-
Ingår i: Naturvetarguiden. - Nacka : Naturvetarna. - 9789163704130
-
Bokkapitel (populärvet., debatt m.m.)
|
|
| 10. |
- Berggren, Karl-Fredrik, et al.
(författare)
-
Fysikersamfundet 90 år
- 2010
-
Ingår i: Fysikaktuellt. - 0283-9148. ; :2, s. 12-12
-
Tidskriftsartikel (populärvet., debatt m.m.)
|
|
| 11. |
- Berggren, Karl-Fredrik
(författare)
-
Hur förstår man fysik?
- 2010
-
Ingår i: Fysikaktuellt. - : Svenska Fysikersamfundet. ; :3, s. 3-3
-
Tidskriftsartikel (populärvet., debatt m.m.)abstract
|
|
| 12. |
- Berggren, Karl-Fredrik
(författare)
-
Hur står det till med SFS?
- 2011
-
Ingår i: Fysikaktuellt. - 0283-9148. ; :4, s. 5-5
-
Tidskriftsartikel (populärvet., debatt m.m.)
|
|
| 13. |
- Berggren, Karl-Fredrik
(författare)
-
Jakten på liv i universum
- 2013
-
Ingår i: Fysikaktuellt. - 0283-9148. ; :4, s. 26-26
-
Recension (populärvet., debatt m.m.)
|
|
| 14. |
- Berggren, Karl-Fredrik, et al.
(författare)
-
Modeling of open quantum dots and wave billiards using imaginary potentials for the source and the sink
- 2010
-
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.
|
|
| 15. |
|
|
| 16. |
- Berggren, Karl-Fredrik, et al.
(författare)
-
Science at the ESS : a brief outline
- 2012
-
Ingår i: In Pursuit of a Promise. - : Arkiv förlag & tidskrift. - 9789179242473 ; , s. 31-47
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- On 28 May 2009, at a closed meeting in Brussels, ministers and state secretaries of education and science from several EU countries decided to build the European Spallation Source (ESS) in Lund, Sweden. Or did they?It is common for big European science projects to be surrounded by secrecy and political deceit, but the ESS is extraordinary in its elusiveness. There is a remarkable lack of concrete economic, political, technical and scientific underpinnings to the project but a boasting certainty in the promises of future paybacks.The ESS is an accelerator-based neutron spallation facility that will cost billions of Euros to build and run. It is expected to bring new knowledge in several fields including materials science, energy research, and the life sciences. But its financing is not yet certain, and future returns hard to predict. How then could the decision to build ESS occur? Why was there so little organized resistance?This book places the ESS project in its political and scientific context. It links the decisions taken to the history of Big Science in Europe and in Sweden. It looks at the dynamic political processes of establishing this megaproject in a small town in the south of Sweden. The eight chapters start from a paradoxical state of affairs: The ESS is not funded, and not formally decided in any binding agreements yet it is treated as a future reality, locally and nationally, loaded with promises of scientific, economic and social returns.The book makes a much-needed first contribution to the analysis of the ESS project and its political, environmental, and social ramifications. It should be read by scholars of science and technology studies, politicians and the interested general public.
|
|
| 17. |
- Berggren, Karl-Fredrik
(författare)
-
Signerat: Nya krafter vid rodret
- 2012
-
Ingår i: Fysikaktuellt. - Stockholm : Svenska fysikersamfundet. - 0283-9148. ; :4, s. 3-3
-
Tidskriftsartikel (populärvet., debatt m.m.)
|
|
| 18. |
- Berggren, Karl-Fredrik
(författare)
-
Signerat; Välkommen till år 2010
- 2010
-
Ingår i: Fysikaktuellt. - 0283-9148. ; :1, s. 3-3
-
Tidskriftsartikel (populärvet., debatt m.m.)
|
|
| 19. |
- Berggren, Karl-Fredrik
(författare)
-
Slow physics
- 2012
-
Ingår i: Fysikaktuellt. - 0283-9148. ; :3, s. 3-3
-
Tidskriftsartikel (populärvet., debatt m.m.)
|
|
| 20. |
|
|
| 21. |
|
|
| 22. |
- Berggren, Karl-Fredrik, et al.
(författare)
-
Timeline of major events
- 2012
-
Ingår i: In pursuit of a promise: Perspectives on the political process to establish the European Spallation Source (ESS) in Lund, Sweden. - Lund : Arkiv Förlag. - 9789179242473
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)
|
|
| 23. |
- Berggren, Karl-Fredrik, et al.
(författare)
-
Timeline of major events
- 2012
-
Ingår i: In pursuit of a promise. - : Arkiv förlag & tidskrift. - 9789179242473 ; , s. 21-30
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- On 28 May 2009, at a closed meeting in Brussels, ministers and state secretaries of education and science from several EU countries decided to build the European Spallation Source (ESS) in Lund, Sweden. Or did they?It is common for big European science projects to be surrounded by secrecy and political deceit, but the ESS is extraordinary in its elusiveness. There is a remarkable lack of concrete economic, political, technical and scientific underpinnings to the project but a boasting certainty in the promises of future paybacks.The ESS is an accelerator-based neutron spallation facility that will cost billions of Euros to build and run. It is expected to bring new knowledge in several fields including materials science, energy research, and the life sciences. But its financing is not yet certain, and future returns hard to predict. How then could the decision to build ESS occur? Why was there so little organized resistance?This book places the ESS project in its political and scientific context. It links the decisions taken to the history of Big Science in Europe and in Sweden. It looks at the dynamic political processes of establishing this megaproject in a small town in the south of Sweden. The eight chapters start from a paradoxical state of affairs: The ESS is not funded, and not formally decided in any binding agreements yet it is treated as a future reality, locally and nationally, loaded with promises of scientific, economic and social returns.The book makes a much-needed first contribution to the analysis of the ESS project and its political, environmental, and social ramifications. It should be read by scholars of science and technology studies, politicians and the interested general public.
|
|
| 24. |
- Berggren, Karl-Fredrik
(författare)
-
Åsikter om infrastruktursatsningarna
- 2012
-
Ingår i: Fysikaktuellt. - Stockholm : Svenska fysikersamfundet. - 0283-9148. ; :3, s. 5-5
-
Tidskriftsartikel (populärvet., debatt m.m.)
|
|
| 25. |
|
|
| 26. |
- Lind, Hans, et al.
(författare)
-
Electric-field control of magnetization in biased semiconductor quantum wires and point contacts
- 2011
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 83:075308
-
Tidskriftsartikel (refereegranskat)abstract
- true origin of the 0.25 and 0.85 conductance features which have been observed in biased split-gatequantum wires and quantum point contacts in semiconductor heterostructures is debated in the literature; onesuggestion is that they are caused by spontaneous spin polarization due to the electron-electron interactions. Thepresent work confirms that spontaneous spin splitting may occur within the system and is responsible for boththe 0.25 and 0.85 plateaux. We have also shown that the 0.25 plateau consists of two regions, one that is spinpolarized, and one that is degenerate with a conductance that remains essentially the same at both sides of thetransition. This finding could be of interest for semiconductor spintronics because it opens the possibility for spinmanipulation by electric means only.
|
|
| 27. |
- Ohlin, Kjell, et al.
(författare)
-
Att se toner bortom Faradays vågor
- 2013
-
Ingår i: Fysikaktuellt. - 0283-9148. ; :1, s. 14-16
-
Tidskriftsartikel (populärvet., debatt m.m.)
|
|
| 28. |
- Wahlstrand, B, et al.
(författare)
-
Wave transport and statistical properties of an open non-Hermitian quantum dot with parity-time symmetry
- 2014
-
Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - : American Physical Society. - 1539-3755 .- 1550-2376. ; 89:062910
-
Tidskriftsartikel (refereegranskat)abstract
- A basic quantum-mechanical model for wave functions and current flow in open quantum dots or billiards is investigated. The model involves non-Hertmitian quantum mechanics, parity-time (PT) symmetry, and PT-symmetry breaking. Attached leads are represented by positive and negative imaginary potentials. Thus probability densities, currents flows, etc., for open quantum dots or billiards may be simulated in this way by solving the Schrödinger equation with a complex potential. Here we consider a nominally open ballistic quantum dot emulated by a planar microwave billiard. Results for probability distributions for densities, currents (Poynting vector), and stress tensor components are presented and compared with predictions based on Gaussian random wave theory. The results are also discussed in view of the corresponding measurements for the analogous microwave cavity. The model is of conceptual as well as of practical and educational interest.
|
|
| 29. |
- Welander, Erik, et al.
(författare)
-
Localization of electrons and formation of two-dimentional Wigner spin lattices in a special cylindrical semiconductor stripe
- 2010
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:073307
-
Tidskriftsartikel (refereegranskat)abstract
- We consider a two-dimensional (2D)electron gas residing on the surface of a cylinder of a given radius R in the presence of a parabolic confinement along the axis of the cylinder. In this way the system of electrons forms a closed cylindrical stripe (wire). Using the local spin-density technique we first consider localization of electrons within of a potential barrier embedded in the wire. Barriers with sharp retangularlike features are populated in steps because of Coulomb blockade. The nature of a single bound state in a short soft barrier (quantum point contacts) at pinch-off is discussed in terms Coulomb blockade. For a shallow barrier-free wire we retrace the structural transitions at low electron densities from a single chain of localized states to double and triple chains (Wigner spin lattices). The present system is related to the model of a inhomogeneous quantum wire introduced recently by Güçlü et al. [Phys. Rev. B 80, 201302(R) (2009)]. An important aspect is, however, the present extension into higher electron densities as well as to the low-density regime and the formation of 2D Wigner microlattices.
|
|
| 30. |
- Yakimenko, Iryna, et al.
(författare)
-
Bound states, electron localization and spin correlations in low-dimensional GaAs/AlGaAs quantum constrictions
- 2013
-
Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 25:7, s. 072201-
-
Tidskriftsartikel (refereegranskat)abstract
- We analyze the occurrence of local magnetization and the effects of electron localization in different models of quantum point contacts (QPCs) using spin-relaxed density functional theory (DFT/LSDA) by means of numerical simulations. In the case of soft confinement potentials the degree of localization is weak and we therefore observe only traces of partial electron localization in the middle of the QPC. In the pinch-off regime there is, however, distinct accumulation at the QPC edges. At the other end, strong confinement potential, low-electron density in the leads and top or implant gates favor electron localization. In such cases one may create a variety of electron configurations from a single localized electron to more complex structures with multiple rows and Wigner lattices.
|
|
| 31. |
|
|
| 32. |
- Yakymenko, Iryna, et al.
(författare)
-
Spin magnetization and electron localization in semiconductor quantum wires and quantum point contacts
- 2012
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The physics of low-dimensional semiconductor structures such as quantum wires (QW) and quantum point contacts (QPCs) has developed into an important part of nanotechnology, especially in connection with spintronics and quantum information. In our current studies we focus on electronic states, effects of electron interactions and conductance anomalies in semiconductor quantum point contacts and wires with different geometries to a model GaAs/AlGaAs device [1]. Using the local spin density approximation (LSDA) we recover spin-polarized states in the middle of a conventional QPC results in the 0.7 (2e2/h) conductance anomaly as well as spin-split states near the two edges of the QPC that may be associated with the conductance anomaly in the vicinity of 0.25 (2e2/h). Effects of spontaneous magnetization might be also used for spintronics applications such as spin filters and all-electric QPC spin-polarizer. These systems are physically rich and constantly bring new experimental data such as the unusual conductance behavior in the case of shallow confinement potential in a low electron density regime, the anomalous properties of the 2DEG around the pinch-off regime and the formation of bound states with Kondo physics involved. This leads to the study of ballistic transport in low-dimensional semiconductor-based nanostructures in the ultralow electron density domain where the interaction-induced localization of electrons takes place. By means of LSDA we have shown that the localization of electrons within the barrier embedded in the wire and that the localization is highly dependent on the sharpness and length of the potential barrier. For a shallow barrier-free wire we retraced the structural transitions at low densities from a single chain of localized states to double and triple chains (Wigner spin lattices). We have found that the double chain appears as a double zig-zag co nfiguration [2] . Localized electrons may have interesting applications for semiconductor nanodevices. Indeed the formation of bound state and spin binding within QPC confinement potential opens possibilities to store information and to realize qubits in quantum circuits. We have studied the electron transport in a quantum wire in the presence of bias between source and drain [3]. Our work has confirmed that spontaneous spin splitting does occur within the wire and it is responsible for both the 0.25 and 0.85 plateaus. We have also shown that the 0.25 plateau consists of two regions, one that is spin polarized, and another that is degenerate with a conductance that remains essentially the same at both sides of the transition. This result is of potential interest for semiconductor spintronics since it opens a new possibility for spin manipulation by electric field. [1] K.-F. Berggren and I.I. Yakimenko. J. Phys.: Condens. Matter 20, 164203 (2008). [2] E. Welander, K.-F. Berggren, I. I. Yakimenko. Phys. Rev. B 82, 073307 (2010). [3] H. Lind, I. I. Yakimenko, K.-F. Berggren. Phys. Rev. B 83, 075308 (2011).
|
|
