arXiv subjects

Vidar Gudmundsson

Publications and source records attributed to Vidar Gudmundsson.

At least 91 records · Page 5Linked to original sources

arXiv2016

Single and double finger-gate controlled spin electronic transport with an in-plane magnetic field

A propagation matrix method is proposed to investigate spin-resolved electronic transport in single finger-gate (SFG) and double finger-gate (DFG) controlled split-gate quantum devices. We show how the interplay of the Rashba and Dresselhaus spin-orbit (SO) interactions as well as a Zeeman (Z) field influences the quantum transport characteristics. Without the Dresselhaus effect, the conductance reveals a mirror symmetry between the hole-like and the electron-like quasi-bound states in the SO-Z gap energy regime in the SFG system, but not for the DFG system. For the Dresselhaus interaction, we are able to analytically identify the binding energy of the SFG and DFG bound states. Furthermore, the DFG resonant states can be determined by tuning the distance between the finger gates.

cond-mat.mes-hall↗

arXiv2016

Regimes of radiative and nonradiative transitions in transport through an electronic system in a photon cavity reaching a steady state

We analyze how a multilevel many-electron system in a photon cavity approaches the steady state when coupled to external leads. When a plunger gate is used to lower cavity photon dressed one- and two-electron states below the bias window defined by the external leads, we can identify one regime with nonradiative transitions dominating the electron transport, and another regime with radiative transitions. Both transitions trap the electrons in the states below the bias bringing the system into a steady state. The order of the two regimes and their relative strength depends on the location of the bias window in the energy spectrum of the system and the initial conditions.

cond-mat.mes-hall↗

arXiv2016

Multi-domain electromagnetic absorption of triangular quantum rings

We present a theoretical study of the unielectronic energy spectra, electron localization, and optical absorption of triangular core-shell quantum rings. We show how these properties depend on geometric details of the triangle, such as side thickness or corners' symmetry. For equilateral triangles, the lowest six energy states (including spin) are grouped in an energy shell, are localized only around corner areas, and are separated by a large energy gap from the states with higher energy which are localized on the sides of the triangle. The energy levels strongly depend on the aspect ratio of the triangle sides, i.e., thickness/length ratio, in such a way that the energy differences are not monotonous functions of this ratio. In particular, the energy gap between the group of states localized in corners and the states localized on the sides strongly decreases with increasing the side thickness, and then slightly increases for thicker samples. With increasing the thickness the low-energy shell remains distinct but the spatial distribution of these states spreads. The behavior of the energy levels and localization leads to a thickness dependent absorption spectrum where one transition may be tuned in the THz domain and a second transition can be tuned from THz to the infrared range of electromagnetic spectrum. We show how these features may be further controlled with an external magnetic field. In this work the electron-electron Coulomb repulsion is neglected.

cond-mat.mes-hall↗

arXiv2016

Conductance oscillations of core-shell nanowires in transversal magnetic fields

We analyze theoretically electronic transport through a core-shell nanowire in the presence of a transversal magnetic field. We calculate the conductance for a variable coupling between the nanowire and the attached leads and show how the snaking states, which are low-energy states localized along the lines of vanishing radial component of the magnetic field, manifest their existence. In the strong coupling regime they induce Aharonov-Bohm-like conductance oscillations, which, by decreasing the coupling to the leads, evolve into well resolved peaks. These results show that the formation of snaking states in the nanowire affects magnetoconductance measurements irrespective of the strength of the contacts with the leads. We analyze theoretically electronic transport through a core-shell nanowire in the presence of a transversal magnetic field. We calculate the conductance for a variable coupling between the nanowire and the attached leads and show how the snaking states, which are low-energy states localized along the lines of vanishing radial component of the magnetic field, manifest their existence. In the strong coupling regime they induce Aharonov-Bohm-like conductance oscillations, which, by decreasing the coupling to the leads, evolve into well resolved peaks. The flux periodic oscillations arise due to interference of the snaking states, which is a consequence of backscattering at either the contacts with leads or magnetic/potential barriers in the wire.

cond-mat.mes-hall↗

arXiv2016

Optical switching of electron transport in a waveguide-QED system

Electron switching in waveguides coupled to a photon cavity is found to be strongly influenced by the photon energy and polarization. Therefore, the charge dynamics in the system is investigated in two different regimes, for off- and on-resonant photon fields. In the off-resonant photon field, the photon energy is smaller than the energy spacing between the first two lowest subbands of the waveguide system, the charge splits between the waveguides implementing a $\sqrt{\rm NOT}$-quantum logic gate action. In the on-resonant photon field, the charge is totally switched from one waveguide to the other due to the appearance of photon replica states of the first subband in the second subband region instigating a quantum-NOT transition.In addition, the importance of the photon polarization to control the charge motion in the waveguide system is demonstrated.The idea of charge switching in electronic circuits may serve to built quantum bits.

cond-mat.mes-hall↗

arXiv2015

Competition of static magnetic and dynamic photon forces in electronic transport through a quantum dot

We investigate theoretically the balance of the static magnetic and the dynamical photon forces in the electron transport through a quantum dot in a photon cavity with a single photon mode. The quantum dot system is connected to external leads and the total system is exposed to a static perpendicular magnetic field. We explore the transport characteristics through the system by tuning the ratio, $\hbarω_γ / \hbarω_c$, between the photon energy, $\hbarω_γ$, and the cyclotron energy, $\hbarω_c$. Enhancement in the electron transport with increasing electron-photon coupling is observed when $\hbarω_γ / \hbarω_c > 1$. In this case the photon field dominates and stretches the electron charge distribution in the quantum dot, extending it towards the contacts area for the leads. Suppression in the electron transport is found when $\hbarω_γ / \hbarω_c < 1$, as the external magnetic field causes circular confinement of the charge density around the dot.

cond-mat.mes-hall↗

arXiv2015

Cavity-photon contribution to the effective interaction of electrons in parallel quantum dots

A single cavity photon mode is expected to modify the Coulomb interaction of an electron system in the cavity. Here we investigate this phenomena in a parallel double quantum dot system. We explore properties of the closed system and the system after it has been opened up for electron transport. We show how results for both cases support the idea that the effective electron-electron interaction becomes more repulsive in the presence of a cavity photon field. This can be understood in terms of the cavity photons dressing the polarization terms in the effective mutual electron interaction leading to nontrivial delocalization or polarization of the charge in the double parallel dot potential. In addition, we find that the effective repulsion of the electrons can be reduced by quadrupolar collective oscillations excited by an external classical dipole electric field.

cond-mat.mes-hall↗

arXiv2015

Cavity-photon controlled thermoelectric transport through a quantum wire

We investigate the influence of a quantized photon field on thermoelectric transport of electrons through a quantum wire embedded in a photon cavity. The quantum wire is connected to two electron reservoirs at different temperatures leading to a generation of a thermoelectric current. The transient thermoelectric current strongly depends on the photon energy and the number of photons initially in the cavity. Two different regimes are studied, off-resonant and resonant polarized fields, with photon energy smaller than, or equal to the energy spacing between some of the lowest states in the quantum wire. We observe that the current is inverted for the off-resonant photon field due to participation of photon replica states in the transport. A reduction in the current is recorded for the resonant photon field, a direct consequence of the Rabi-splitting.

cond-mat.mes-hall↗

arXiv2015

Coupled collective and Rabi oscillations triggered by electron transport through a photon cavity

We show how the switching-on of an electron transport through a system of two parallel quantum dots embedded in a short quantum wire in a photon cavity can trigger coupled Rabi and collective electron-photon oscillations. We select the initial state of the system to be an eigenstate of the closed system containing two Coulomb interacting electrons with possibly few photons of a single cavity mode. The many-level quantum dots are described by a continuous potential. The Coulomb interaction and the para- and dia-magnetic electron-photon interactions are treated by exact diagonalization in a truncated Fock-space. To identify the collective modes the results are compared for an open and a closed system with respect to the coupling to external electron reservoirs, or leads. We demonstrate that the vacuum Rabi oscillations can be seen in transport quantities as the current in and out of the system.

cond-mat.mes-hall↗

arXiv2015

Electron localization and optical absorption of polygonal quantum rings

We investigate theoretically polygonal quantum rings and focus mostly on the triangular geometry where the corner effects are maximal. Such rings can be seen as short core-shell nanowires, a generation of semiconductor heterostructures with multiple applications. We show how the geometry of the sample determines the electronic energy spectrum, and also the localization of electrons, with effects on the optical absorption. In particular, we show that irrespective of the ring shape low-energy electrons are always attracted by corners and are localized in their vicinity. The absorption spectrum in the presence of a magnetic field shows only two peaks within the corner-localized state domain, each associated with different circular polarization. This picture may be changed by an external electric field which allows previously forbidden transitions, and thus enables the number of corners to be determined. We show that polygonal quantum rings allow absorption of waves from distant ranges of the electromagnetic spectrum within one sample.

cond-mat.mes-hall↗

arXiv2015

Symmetry dependent electron localization and optical absorption of polygonal quantum rings

We compare energy spectra, electron localization and optical absorption of square and diamond quantum rings and analyze how sample geometry affects those features. We show that low energy levels of diamond rings form two groups delocalized between opposite corners which results in increased number of optical transitions. We also show that contacts applied to corner areas allow for continuous change between square- and diamond-like behavior of the same sample, irrespective of its shape.

cond-mat.mes-hall↗

arXiv2015

Signature of snaking states in the conductance of core-shell nanowires

We model a core-shell nanowire (CSN) by a cylindrical surface of finite length. A uniform magnetic field perpendicular to the axis of the cylinder forms electron states along the lines of zero radial field projection, which can classically be described as snaking states. In a strong field, these states converge pairwise to quasidegenerate levels, which are situated at the bottom of the energy spectrum. We calculate the conductance of the CSN by coupling it to leads, and predict that the snaking states govern transport at low chemical potential, forming isolated peaks, each of which may be split in two by applying a transverse electric field. If the contacts with the leads do not completely surround the CSN, as is usually the case in experiments, the amplitude of the snaking peaks changes when the magnetic field is rotated, determined by the overlap of the contacts with the snaking states.

cond-mat.mes-hall↗

arXiv2014

Double-finger-gate controlled spin-resolved resonant quantum transport in the presence of a Rashba-Zeeman gap

We investigate double finger gate (DFG) controlled spin-resolved resonant transport properties in an n-type quantum channel with a Rashba-Zeeman (RZ) subband energy gap. By appropriately tuning the DFG in the strong Rashba coupling regime, resonant state structures in conductance can be found that is sensitive to the length of the DFG system. Furthermore, a hole-like bound state feature below the RZ gap and an electron-like quasi-bound state feature at the threshold of the upper spin branch can be found that is insensitive to the length of the DFG system.

cond-mat.mes-hall↗

arXiv2014

Cavity-photon-switched coherent transient transport in a double quantum waveguide

We study a cavity-photon-switched coherent electron transport in a symmetric double quantum waveguide. The waveguide system is weakly connected to two electron reservoirs, but strongly coupled to a single quantized photon cavity mode. A coupling window is placed between the waveguides to allow for electron interference or inter-waveguide transport. The transient electron transport in the system is investigated using a quantum master equation. We present a cavity-photon tunable semiconductor quantum waveguide implementation of an inverter quantum gate in which the output of the waveguide system may be selected via the selection of an appropriate photon number, or 'photon frequency' of the cavity. In addition, the importance of the photon polarization in the cavity that is either parallel or perpendicular to the direction of electron propagation in the waveguide system is demonstrated.

cond-mat.mes-hall↗

arXiv2014

Excitation spectra of a quantum ring embedded in a photon cavity

We explore the response of a quantum ring system coupled to a photon cavity with a single mode when excited by a classical dipole field. We find that the energy oscillates between the electronic and photonic components of the system. The contribution of the linear and the quadratic terms in the vector potential to the electron-photon interaction energy are of similar magnitude, but opposite signs stressing the importance of retaining both in the model. Furthermore, we find different Fourier spectra for the oscillations of the center of charge and the oscillations of the mean photon number in time. The Fourier spectra are compared to the spectrum of the many-body states and selection rules discussed. In case of the center of charge oscillations, the dipole matrix elements preselect the allowed Bohr frequencies of the transitions, while for the oscillations of the mean photon number, the difference of the photon content of the many-body states influences the selection rules.

cond-mat.mes-hall↗

arXiv2014

Coherent transient transport of interacting electrons through a quantum waveguide switch

We investigate coherent electron-switching transport in a double quantum waveguide system in a perpendicular static or vanishing magnetic field. The finite symmetric double waveguide is connected to two semi-infinite leads from both ends. The double waveguide can be defined as two parallel finite quantum wires or waveguides coupled via a window to facilitate coherent electron inter-wire transport. By tuning the length of the coupling window, we observe oscillations in the net charge current and a maximum electron conductance for the energy levels of the two waveguides in resonance. The importance of the mutual Coulomb interaction between the electrons and the influence of two-electron states is clarified by comparing results with and without the interaction. Even though the Coulomb interaction can lift two-electron states out of the group of active transport states the length of the coupling window can be tuned to locate two very distinct transport modes in the system in the late transient regime before the onset of a steady state. A static external magnetic field and quantum-dots formed by side gates (side quantum dots) can be used to enhance the inter-waveguide transport which can serve to implement a quantum logic device. The fact that the device can be operated in the transient regime can be used to enhance its speed.

cond-mat.mes-hall↗

arXiv2014

Excitation of radial collective modes in a quantum dot: Beyond linear response

We compare the response of five different models of two interacting electrons in a quantum dot to an external short lived radial excitation that is strong enough to excite the system well beyond the linear response regime. The models considered describe the Coulomb interaction between the electrons in different ways ranging from mean-field approaches to configuration interaction (CI) models, where the two-electron Hamiltonian is diagonalized in a large truncated Fock space. The radially symmetric excitation is selected in order to severely put to test the different approaches to describe the interaction and correlations of an electron system in a nonequilibrium state. As can be expected for the case of only two electrons none of the mean-field models can in full details reproduce the results obtained by the CI model. Nonetheless, some linear and nonlinear characteristics are reproduced reasonably well. All the models show activation of an increasing number of collective modes as the strength of the excitation is increased. By varying slightly the confinement potential of the dot we observe how sensitive the properties of the excitation spectrum are to the Coulomb interaction and its correlation effects. In order to approach closer the question of nonlinearity we solve one of the mean-field models directly in a nonlinear fashion without resorting to iterations.

cond-mat.mes-hall↗

arXiv2014

Spin and impurity effects on flux-periodic oscillations in core-shell nanowires

We study the quantum mechanical states of electrons situated on a cylindrical surface of finite axial length to model a semiconductor core-shell nanowire. We calculate the conductance in the presence of a longitudinal magnetic field by weakly coupling the cylinder to semi-infinite leads. Spin effects are accounted for through Zeeman coupling and Rashba spin-orbit interaction (SOI). Emphasis is on manifestations of flux-periodic (FP) oscillations and we show how factors such as impurities, contact geometry and spin affect them. Oscillations survive and remain periodic in the presence of impurities, noncircular contacts and SOI, while Zeeman splitting results in aperiodicity, beating patterns and additional background fluctuations. Our results are in qualitative agreement with recent magnetotransport experiments performed on GaAs/InAs core-shell nanowires. Lastly, we propose methods of data analysis for detecting the presence of Rashba SOI in core-shell systems and for estimating the electron g-factor in the shell.

cond-mat.mes-hall↗