| 1. |
- Csontos, Dan, et al.
(författare)
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Characteristics of electron transport through vertical double-barrier quantum-dot structures: Effects of symmetric and asymmetric variations of the lateral confinement potentials
- 2002
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 66:20
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Tidskriftsartikel (refereegranskat)abstract
- We report on a theoretical study of the electron transport through laterally-confined, vertical double-barrier resonant-tunneling (DBRT) structures, defined as one-dimensional (1D)-0D-1D systems, with a tunable lateral confinement. The current and the differential conductance of the systems are calculated and the influence caused by varying the lateral confinement on the device characteristics is investigated. Three representative systems are studied. First of all, a 1D-0D-1D device, symmetric with respect to the current flow, with a variable lateral confinement in the double-barrier quantum-well (DBQW) region, is investigated. This device would in an experimental setup correspond to the structure in which a thin, lateral metallic gate is placed in the DBQW region. Subsequently, calculations are performed for two asymmetric 1D-0D-1D devices, in which the strongest, but varying, lateral confinement is placed either in the collector or in the emitter region. In experiments, these two devices would correspond to the situations where a lateral metallic gate is positioned below or on top of the DBQW structure. The calculations predict several phenomena for the device characteristics. It is shown that as the lateral confinement increases, in addition to those normally observed current onsets and pinch-offs that move toward higher bias voltages, several current onsets and pinch-offs move towards lower bias voltages. These negative shifts of the current onsets and pinch-offs with increasing of the lateral confinement have so far not been expected for gated DBRT devices. It is also found that the threshold voltages, at which the current onsets and pinch-offs appear, depend strongly on the strength and position of the lateral confinement and on the Fermi levels in the collector and the emitter. The models that explain these predictions are presented and discussed.
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| 2. |
- Csontos, Dan
(författare)
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Modelling of Quantum Transport in Nanostructures
- 2002
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, theoretical studies of the transport properties of three nanoscale systems: one-dimensional (1D) quantum wires (QWRs), zero-dimensional (0D), laterally confined, double-barrier resonant tunnelling structures (DBRTSs) and three-terminal ballistic junctions (TBJs), have been performed. In the first part of the thesis, an overview of the realization and properties of such systems is given along with a description of modelling tools used in the calculations. The second and main part of the thesis contains the original research results, summarized into seven papers. The conductance of QWRs with corrugated boundaries is investigated in Paper I with respect to the nature of the boundary roughness, geometrical parameters of the QWR and temperature. It is shown that, due to the structural imperfections, the conductance exhibits rapid fluctuations, strong, broad dips between adjacent conductance plateaus at very low temperatures and, in general, a suppression of the conductance below the values expected for an ideal QWR. The results agree with existing experimental results. Experimental studies of the transport properties of 0D quantum dots obtained by laterally confining vertical DBRTSs by means of metallic gates have shown complex, gate-dependent fine structure in the measured current-voltage (I-V) characteristics. The origin of this fine structure is theoretically studied and explained (Papers II-V) in terms of quasi-1D-0D-1D systems with a tunable lateral confinement. It is shown that, due to the low dimensionality of the emitter, dot and collector regions, complex fine structure, which is strongly dependent on Fermi energy, source-drain voltage, and gate voltage, is formed in the I-V characteristics, which may explain the experimentally observed results. A tentative comparison between experiments and theory is made in Paper IV. Three-terminal junction systems have very recently emerged as excellent candidates for use as building blocks in the formation of nanoscale electronic devices. A general formalism for the calculation of electron transport through three- terminal quantum structures is presented in Paper VI. Using this method, the transport through Y-shaped TBJ structures is studied in Paper VII. Quantum effects are shown to influence the transport properties of TBJs at low temperatures, possibly enabling new device functionality.
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| 3. |
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| 4. |
- Csontos, Dan, et al.
(författare)
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Quantum interference effects in electron transport through Y-branches
- 2002
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Ingår i: 7th International Conference on Nanometer-Scale Science and Technology and 21st European Conference on Surface Science.
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Konferensbidrag (refereegranskat)abstract
- Recently, nonlinear properties of three-terminal ballistic junctions (TBJs) were predicted by theoretical calculations done by Xu, and potential applications such as rectification and logic function proposed. In that analysis the TBJ was modeled by connecting three quantum point contacts via a ballistic cavity with adiabatic boundaries, thus neglecting any backscattering of electrons. In this paper we develop a realistic model based on the scattering-matrix method for the calculation of transport through TBJs with arbitrary potential profiles. Temperature-dependent linear and nonlinear transport has been calculated and analyzed for a number of Y-shaped devices with different geometries, sizes, impurity concentrations and various degrees of boundary roughness. Interference effects are shown to influence the characteristics of the calculated transport in a strong way at low temperature where new quantum phenomena are predicted
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| 5. |
- Csontos, Dan, et al.
(författare)
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Scattering-matrix formalism of electron transport through three-terminal quantum structures: formulation and application to Y-junction devices
- 2002
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Ingår i: Journal of Physics: Condensed Matter. - : IOP Publishing. - 1361-648X .- 0953-8984. ; 14:47, s. 12513-12528
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we present a formalism for the calculation of electron transport through three-terminal junction devices, which have received attention due to their recently demonstrated non-linear electrical properties. The formalism, which is based on the scattering-matrix method, takes quantum interference effects fully into account. Furthermore, the formalism provides numerical stability in the calculations as well as large flexibility in the modelling of arbitrary potential profiles due to the common basis approach used in the formulation. The method is used to calculate the transport properties for Y-shaped three-terminal ballistic junction (TBJ) structures with configurations typical of recently performed experiments. Quantum interference effects are shown to strongly influence the transport characteristics of TBJ structures due to complex scattering of the electrons in the cavity-like coupling window between the three arms of the device. The theoretical approach presented in this paper provides a flexible tool for the study of such quantum interference effects, which may play an important role in the design and functionality of future nanoscale devices based on three-terminal junctions.
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| 6. |
- Gustafson, Boel, et al.
(författare)
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Coupling between lateral modes in a vertical resonant tunneling structure
- 2002
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Ingår i: Physica E: Low-Dimensional Systems and Nanostructures. - 1386-9477. ; 13:2-4, s. 950-953
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Tidskriftsartikel (refereegranskat)abstract
- We present experimental results and theoretical calculations of the vertical electron transport through a laterally constricted resonant tunneling transistor. Current-voltage measurements at 4.2 K show numerous current peaks that exhibit a complex dependence on the applied gate voltage. A scattering-matrix approach combined with the Landauer formalism was used to perform quantum mechanical calculations of the electron transport through a quantum dot structure with laterally confined emitter and collector regions. The simulations qualitatively reproduce the measured data, suggesting a strong coupling between the lateral modes in the quantum dot and the collector
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| 7. |
- Xu, Hongqi, et al.
(författare)
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Reflection phase of scattering electrons in a single-channel atomic wire
- 2003
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 68:24
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Tidskriftsartikel (refereegranskat)abstract
- A theoretical study of the phase property of the reflection coefficient of a single-channel quantum system is presented. This reflection phase information is required for a complete characterization of the scattering problem in the system. It is found that the phase of the reflection coefficient of the system shifts abruptly by pi when the reflection probability passes through a zero. It is also found that in certain systems this phase discontinuity can appear even when no zero and no phase discontinuity are present in the transmission coefficient. Possible experiments for the observation of the reflection phase discontinuity are discussed.
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