Probability current density of an electron propagating through the wire with conic opening into two-dimensional electron reservoir with the energy of incoming electron of 4 meV.
The research in our unit deals with materials physics, quantum physics, non-linear optics and quantum chemistry.
Our unit’s research focuses on materials physics, quantum physics, and non-linear phenomena. We employ density functional theory (DFT), the nonlinear Schrödinger equation, and non-linear dynamics. Our results have applications in materials science, quantum information and quantum computing, spintronics, non-linear optics, and topological photonics. We use the facilities of the National Supercomputer Centre (NSC) in Linköping. Our staff teach basic and advanced courses in theoretical physics at the undergraduate and graduate levels.
Highlights
Injection of spinning electrons from quantum wires into 2D electron reservoirs
Probability current density of an electron propagating through the wire with conic opening into two-dimensional electron reservoir with the energy of incoming electron of 4 meV.
Ivan P Yakimenko, Irina I Yakimenko, Karl-Fredrik Berggren, Advances in Engineering
In mesoscopic physics, ballistic conduction is defined as the transport of electrons in a medium, having negligible electrical resistivity caused by scattering. Specifically, mesoscopic systems based on GaAs/AlGaAs heterostructures have been reported to allow the rich physics of ballistic electron transport. A favorite system for studying ballistic electrons is the high mobility two-dimensional electron gas (2DEG) which resides at the interface between GaAs and AlGaAs layered semiconductors. By proper gating on top of the heterostructure, one can create confined low-dimensional systems like quantum wires (QWs) and quantum point contacts (QPCs). It is possible to control the flow of electrons by applying electric and magnetic fields. At low temperature and small electric bias, only the electrons near the Fermi energy contribute to the current. It has been shown that, under certain conditions, the propagation of ballistic electrons in semiconductor quantum wires, of electromagnetic waves in wave guides, of sound waves in pipes with different geometry, light propagation in optical fibers for photonic applications are all described by the same type of Helmholtz equation. All the same, the precise control of spin states and spin dependent electron transport is required for applications in spintronics and quantum information processing.
Presently, the technique of transverse electron focusing has been proposed to study electron transmission through a QPC by means of investigation of the position, the shape and the height of the focusing peaks. Nonetheless, further research on the spin-splitting effect when a transverse magnetic field is applied in the 2D region is necessitated. To this end, a team of researchers from the Department of Physics, Chemistry and Biology at Linköping University in Sweden: Ivan Yakimenko, Irina Yakimenko and Karl-Fredrik Berggren designed a model illustrated in the figure for electron flow from a quantum wire into a 2D region through an opening having different geometries. Their work is currently published in Journal of Physics: Condensed Matter.
To begin with, the researchers studied the transport of electrons into the 2D reservoir from a quantum wire having a rectangular opening without a magnetic field using the proposed mode-matching on a grid method. They then studied the conic and rounded openings which are more typical in real semiconductor structures. Lastly, the spin-splitting effect in an applied transverse magnetic field in the 2D region was investigated.
For the examined cases, the team reported that the geometry of the opening did not play a crucial role for the electron propagation. In fact, it was seen that when a perpendicular magnetic field was applied, the electron paths in the 2D reservoir were curved. These observations were further analyzed both classically and quantum-mechanically. Consequently, it was established that the effect was clearly present for realistic choices of device parameters and consistent with observations.
In summary, the study presented the numerical cross-examination of electron transport from the injector wire into the open two-dimensional reservoir with and without magnetic field. Remarkably, the study demonstrated the development of the novel mode-matching technique and further studied electron flow through the coupled wire with rectangular, conic and rounded openings. In an interview with Advances in Engineering, Professor Karl-Fredrik Berggren further affirmed that the results of their study may be applied in designing magnetic focusing devices and spin separation.
Reference: Ivan P Yakimenko, Irina I Yakimenko, Karl-Fredrik Berggren. Basic modelling of effects of geometry and magnetic field for quantum wires injecting electrons into a two-dimensional electron reservoir. Journal of Physics: Condensed Matter, 31 (2019) 345302.
Study of spin polarization and spin-related transport in quantum point contacts coupled through a two-dimensional electron reservoir.
Spin polarization for symmetric injector and detector quantum point contacts.
Irina I. Yakimenko and Ivan P. Yakimenko 2025 J. of Phys.: Condens. Matter, 37 325301
This paper presents a theoretical study of electron transport in a split-gate device comprising two quantum point contacts (QPCs) connected by a wider two-dimensional electron gas (2DEG) region at the GaAs/AlGaAs interface. The QPCs, defined by split gates, act as an electron injector and detector, while the intermediate 2DEG region between them can be electrostatically tuned via a top gate. Electron transport is modeled using density-functional theory. Experimental observations suggest that when the injector QPC is asymmetrically biased, the broadening of the current peak detected at the detector QPC may be associated with spin polarization in the injector. Our simulations for both symmetric and asymmetric injector QPCs indicate that, although the overall shape of the detector current profile depends only weakly on the injector’s asymmetry, the width of the current peak increases with the current through the injector QPC. This trend is consistent with spin-related effects, such as the 0.7 × (2e²/h) conductance anomaly observed in the injector. These results provide insight into the electronic properties of the 2DEG in the proposed device and may be helpful for the future design of semiconductor structures for spintronics and quantum technologies.
Electronic properties of semiconductor quantum wires for shallow symmetric and asymmetric confinements.
Spin polarization in the conduction channels for symmetric, weak confinement. Irina I Yakimenko and Ivan P Yakimenko 2022 J. Phys.: Condens. Matter 34, 105302
This paper is motivated by experiments on a quantum wire (QW) formed in a two-dimensional electron gas (2DEG) at the GaAs/AlGaAs interface, with shallow symmetric and asymmetric confinements. In these experiments, additional zero-field conductance anomalies were observed. The proposed device consists of a quantum point contact (QPC) defined by split gates with a top gate between two large electron reservoirs. We focus on a theoretical study of electron transport through a wide, top-gated QPC in the low-density regime, using density-functional theory. Electron–electron interactions together with shallow confinement can split the conducting channel into two parallel channels. Each channel can become spin-polarized at certain split-gate and top-gate voltages and thus contribute to the conductance, giving rise to additional anomalies. With symmetrically biased split gates, the two channels contribute equally to the conductance. For asymmetric split-gate bias, conductance anomalies can occur between 0.25×(2e^2/h)and 0.7×(2e^2/h), depending on the degree of asymmetry. This behavior corresponds to different spin polarizations in the two channels, which then contribute unequally. In the strongly asymmetric case, one channel is pinched off and only the remaining channel conducts. We also find spin-polarized states localized around the perimeter of an antidot. If the antidot radius is sufficiently small, tunneling between these states may occur and contribute to the conductance. Spin-polarized states in an electrostatically tunable, shallow-confinement QPC may be useful for quantum technologies.
Our current projects
Schematic view of a split gate device containing three quantum point contacts.Theoretical modelling of electron states and spin poplarization in low-dimensional nanodevices fabricated from semiconductor GaAs/GaAlAs heterostructures
Iryna Yakymenko and Ivan Yakimenko
The present project aims to theoretically study electronic focusing devices embedded in a GaAs/AlGaAs semiconductor heterostructure. The system comprises three quantum point contacts (QPCs) and an intervening two-dimensional (2D) region. Two QPCs, defined by split gates, serve as the electron injector and detector, while the intermediate 2D region is electrostatically tunable via a top gate. The conductance of a third, asymmetrically loaded QPC is probed using the detector QPC. Special attention is paid to electron transport, spin polarization, and conductance anomalies in the asymmetrically loaded QPC within the experimentally proposed electron-focusing device. Electron transport is modelled using density functional theory. Experimental observations suggest that when the injector QPC is asymmetrically biased, the broadening of the current peak detected at the detector QPC may be associated with spin polarization in the injector. We investigate the correlation between conductance anomalies in the studied asymmetric QPC and the increase of the full width at half maximum (FWHM) of the current peak at the detector QPC. We also consider potential applications of electron states realized in semiconductor nanowires for quantum information processing and quantum computing. The project is supported by the Carl Trygger Foundation for Scientific Research. The proposed research is closely related to experiments performed by the group of Prof. Sir Michael Pepper and Dr. Sanjeev Kumar at the Nanotechnology Laboratory, University College London (UCL).
