arXiv subjects

B. Szafran

Publications and source records attributed to B. Szafran.

At least 73 records · Page 4Linked to original sources

arXiv2010

Singlet-triplet avoided crossings and effective $g$ factor versus spatial orientation of spin-orbit-coupled quantum dots

We study avoided crossings opened by spin-orbit interaction in the energy spectra of one- and two-electron anisotropic quantum dots in perpendicular magnetic field. We find that for simultaneously present Rashba and Dresselhaus interactions the width of avoided crossings and the effective $g$ factor depend on the dot orientation within (001) crystal plane. The extreme values of these quantities are obtained for [110] and [1$\bar{1}$0] orientations of the dot. The width of singlet-triplet avoided crossing changes between these two orientations by as much as two orders of magnitude. The discussed modulation results from orientation-dependent strength of the Zeeman interaction which tends to polarize the spins in the direction of the external magnetic field and thus remove the spin-orbit coupling effects.

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arXiv2010

Time dependent configuration interaction simulations of spin swap in spin orbit coupled double quantum dots

We perform time-dependent simulations of spin exchange for an electron pair in laterally coupled quantum dots. The calculation is based on configuration interaction scheme accounting for spin-orbit (SO) coupling and electron-electron interaction in a numerically exact way. Noninteracting electrons exchange orientations of their spins in a manner that can be understood by interdot tunneling associated with spin precession in an effective SO magnetic field that results in anisotropy of the spin swap. The Coulomb interaction blocks the electron transfer between the dots but the spin transfer and spin precession due to SO coupling is still observed. The electron-electron interaction additionally induces an appearance of spin components in the direction of the effective SO magnetic field which are opposite in both dots. Simulations indicate that the isotropy of the spin swap is restored for equal Dresselhaus and Rashba constants and properly oriented dots.

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arXiv2010

Magnetic forces and stationary electron flow in three-terminal semiconductor quantum ring

We study stationary electron flow through a three-terminal quantum ring and describe effects due to deflection of electron trajectories by classical magnetic forces. We demonstrate that generally at high magnetic field ($B$) the current is guided by magnetic forces to follow a classical path which for $B>0$ leads via the left arm of the ring to the left output terminal. The transport to the left output terminal is blocked for narrow windows of magnetic field for which the interference within the ring leads to formation of wave functions that are only weakly coupled to the output channel wave functions. These interference conditions are accompanied by injection of the current to the right arm of the ring and by appearance of sharp peaks of the transfer probability to the right output terminal. We find that these peaks at high magnetic field are attenuated by thermal widening of the transport window. We also demonstrate that the interference conditions that lead to their appearance vanish when an elastic scattering within the ring is present. The clear effect of magnetic forces on the transfer probabilities disappears along with Aharonov-Bohm oscillations in a chaotic transport regime that is found for rings whose width is larger than the width of the channels.

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arXiv2010

Coupling of bonding and antibonding electron orbitals in double quantum dots by spin-orbit interaction

We perform a systematic exact diagonalization study of spin-orbit coupling effects for stationary few-electron states confined in quasi two-dimensional double quantum dots. We describe the spin-orbit-interaction induced coupling between bonding and antibonding orbitals and its consequences for magneto-optical absorption spectrum. The spin-orbit coupling for odd electron numbers (one, three) %only weakly perturbs the ground-state wave functions. %Nevertheless, %the spin-orbit interaction opens avoided crossings between low energy excited levels of opposite spin orientation and opposite spatial parity. For two-electrons the spin-orbit coupling allows for low-energy optical transitions that are otherwise forbidden by spin and parity selection rules. We demonstrate that the energies of optical transitions can be significantly increased by an in-plane electric field but only for odd electron numbers. Occupation of single-electron orbitals and effects of spin-orbit coupling on electron distribution between the dots are also discussed.

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arXiv2009

Electron transfer through a multiterminal quantum ring: magnetic forces and elastic scattering effects

We study electron transport through a semiconductor quantum ring with one input and two output terminals for an elastic scatterer present within one of the arms of the ring. We demonstrate that the scatterer not only introduces asymmetry in the transport probability to the two output leads but also reduces the visibility of the Aharonov-Bohm conductance oscillations. This reduction occurs in spite of the phase coherence of the elastic scattering and is due to interruption of the electron circulation around the ring by the potential defect. The results are in a qualitative agreement with a recent experiment by Strambini et al. [Phys. Rev. B {\bf 79}, 195443 (2009)]. We also indicate that the magnetic symmetry of the sum of conductance of both the output leads as obtained in the experiment can be understood as resulting from the invariance of backscattering to the input lead with respect to the magnetic field orientation.

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arXiv2009

Violation of Onsager symmetry for a ballistic channel Coulomb coupled to a quantum ring

We investigate a scattering of electron which is injected individually into an empty ballistic channel containing a cavity that is Coulomb coupled to a quantum ring charged with a single-electron. We solve the time-dependent Schrödinger equation for the electron pair with an exact account for the electron-electron correlation. Absorption of energy and angular momentum by the quantum ring is not an even function of the external magnetic field. As a consequence we find that the electron backscattering probability is asymmetric in the magnetic field and thus violates Onsager symmetry.

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arXiv2008

Pinning of electron densities in quantum rings by defects: symmetry constraints and distribution of persistent currents

We study the pinning of few-electron charge densities by weak perturbations to circular quantum ring potentials. We find that the pinning by weak defects is only allowed when the symmetry of the classical few-electron lowest-energy configuration agrees with the symmetry of the external potential. We indicate that whenever the pinning is allowed by the symmetry, its strength is an oscillatory function of the external magnetic field. In the magnetic fields for which the pinning is maximal the dipole moment generated by the persistent currents changes orientation from antiparallel to parallel to the external field in a continuous manner. For confinement potentials of a higher symmetry than the one of a classical Wigner molecule the pinning is forbidden and a discontinuous abrupt inversion of the dipole moments is observed. When the pinning of single-electron islands is absent or weak the current distribution resembles the one of a circular ring. For the maximal pinning instead of current loops running around the entire ring one observes formation of multiple current vortices circulating around each single-electron density island. We study the magnetic field generated by persistent currents and find that at the dipole moment reversal the currents tend to screen the external field in the region occupied by electrons. In consequence the magnetic field generated by the currents in the maximally pinned electron systems maps the charge distribution.

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arXiv2008

Few-electron artificial molecules formed by laterally coupled quantum rings

We study the artificial molecular states formed in laterally coupled double semiconductor nanorings by systems containing one, two and three electrons. An interplay of the interring tunneling and the electron-electron interaction is described and its consequences for the magnetization and charging properties of the system are determined. It is shown that both the magnetic dipole moment generated by the double ring structure and the chemical potential of the system as function of the external magnetic field strongly depend on the number of electrons and the interring barrier thickness. Both the magnetization and chemical potentials exhibit cusps at the magnetic fields inducing ground-state parity and / or spin transformations. The symmetry transformations are discussed for various tunnel coupling strengths: from rings coupled only electrostatically to the limit of coalesced rings. We find that in the ground-states for rings of different radii the magnetic field transfers the electron charge from one ring to the other. The calculations are performed with the configuration interaction method based on an approach of Gaussian functions centered on a rectangular array of points covering the studied structure. Electron-electron correlation is also discussed.

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arXiv2008

Gated combo nanodevice for sequential operations on single electron spin

An idea for a nanodevice in which an arbitrary sequence of three basic quantum single qubit gates - negation, Hadamard and phase shift - can be performed on a single electron spin. The spin state is manipulated using the spin-orbit coupling and the electron trajectory is controlled by the electron wave function self-focusing mechanism due to the electron interaction with the charge induced on metal gates. We present results of simulations based on iterative solution of the time dependent Schrödinger equation in which the subsequent operations on the electron spin can be followed and controlled. Description of the moving electron wave packet requires evaluation of the electric field within the entire nanodevice in each time step.

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arXiv2008

Spin rotations induced by electron running on closed trajectories in gated semiconductor nanodevices

A design for a quantum gate performing transformations of a single electron spin is presented. The spin rotations are performed by the electron going around the closed loops in a gated semiconductor device. We demonstrate the operation of NOT, phase-flip and Hadamard quantum gates, i.e. the single-qubit gates which are most commonly used in the algorithms. The proposed devices employ the self-focusing effect for the electron wave packet interacting with the electron gas on the electrodes and the Rashba spin-orbit coupling. Due to the self-focusing effect the electron moves in a compact wave packet. The spin-orbit coupling translates the spatial motion of the electron into the rotations of the spin. The device does not require microwave radiation and operates using low constant voltages. It is therefore suitable for selective single-spin rotations in larger registers.

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arXiv2008

Magnetic-field asymmetry of electron wave packet transmission in bent channels capacitively coupled to a metal gate

We study the electron wave packet moving through a bent channel. We demonstrate that the packet transmission probability becomes an uneven function of the magnetic field when the electron packet is capacitively coupled to a metal plate. The coupling occurs through a non-linear potential which translates a different kinetics of the transport for opposite magnetic field orientations into a different potential felt by the scattered electron.

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arXiv2007

Exciton spectra in vertical stacks of triple and quadruple quantum dots in an electric field

We study an electron-hole pair in a stack of multiple quantum dots in the presence of an external electric field using the configuration interaction approach. We find that the bright energy levels can be grouped into families which are associated with the hole localized in a specific dot of the stack. The exciton energy levels undergo avoided crossings as function of the external electric field with different pattern for each family. We show that the variation of the depths of the dots along the stack can be deduced from the exciton spectrum. In the strong confinement limit the families are mixed by a weak electric field due to hole tunneling. This results in a characteristic multiple avoided crossing of energy levels belonging to different families with an accompanying modulation of the recombination probabilities and an appearance of a single particularly bright state. We discuss the modification of the spectrum when dots are added to the stack.

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arXiv2007

Quantum dot defined in two-dimensional electron gas at n-AlGaAs/GaAs heterojunction: simulation of electrostatic potential and charging properties

We present a self-consistent Schroedinger-Poisson scheme for simulation of electrostatic quantum dots defined in gated two-dimensional electron gas formed at n-AlGaAs/GaAs heterojunction. The computational method is applied to a quantitative description of transport properties studied experimentally by Elzermann et al. [Appl. Phys. Lett. {\bf 84}, 4617 (2004)]. The three-dimensional model describes the electrostatics of the entire device with a quantum dot that changes shape and floats inside a gated region when the applied voltages are varied. Our approach accounts for the metal electrodes of arbitrary geometry and configuration, includes magnetic field applied perpendicular to the growth direction, electron-electron correlation in the confined electron system and its interaction with the electron reservoir surrounding the quantum dot. We calculate the electric field, the space charge distribution as well as energies and wave functions of confined electrons to describe opening of two transport channels between the reservoir and the confined charge puddle. We determine the voltages for charging the dot with up to 4 electrons. The results are in a qualitative and quantitative agreement with the experimental data.

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arXiv2007

Induced quantum dots and wires: electron storage and delivery

We show that quantum dots and quantum wires are formed underneath metal electrodes deposited on a planar semiconductor heterostructure containing a quantum well. The confinement is due to the self-focusing mechanism of an electron wave packet interacting with the charge induced on the metal surface. Induced quantum wires guide the transfer of electrons along metal paths and induced quantum dots store the electrons in specific locations of the nanostructure. Induced dots and wires can be useful for devices operating on the electron spin.

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arXiv2006

Stark effect on the exciton spectra of vertically coupled quantum dots: horizontal field orientation and non-aligned dots

We study the effect of an electric-field on an electron-hole pair in an asymmetric system of vertically coupled self-assembled quantum dots taking into account their non-perfect alignment. We show that the non-perfect alignment does not qualitatively influence the exciton Stark effect for the electric field applied in the growth direction, but can be detected by application of a perpendicular electric field. We demonstrate that the direction of the shift between the axes of non-aligned dots can be detected by rotation of a weak electric field within the plane of confinement. Already for a nearly perfect alignment the two-lowest energy bright exciton states possess antilocked extrema as function of the orientation angle of the horizontal field which appear when the field is parallel to the direction of the shift between the dot centers.

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arXiv2006

Magnetic-field-induced binding of few-electron systems in shallow quantum dots

Binding of few-electron systems in two-dimensional potential cavities in the presence of an external magnetic field is studied with the exact diagonalization approach. We demonstrate that for shallow cavities the few-electron system becomes bound only under the application of a strong magnetic field. The critical value of the depth of the cavity allowing the formation of a bound state decreases with magnetic field in a non-smooth fashion, due to the increasing angular momentum of the first bound state. In the high magnetic field limit the binding energies and the critical values for the depth of the potential cavity allowing the formation of a bound system tend to the classical values.

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arXiv2005

Time dependent simulations of electron transport through a quantum ring: effect of the Lorentz force

The time dependent Schrödinger equation for an electron passing through a semiconductor quantum ring of nonzero width is solved in the presence of a perpendicular homogenous magnetic field. We study the effects of the Lorentz force on the Aharonov-Bohm oscillations. Within the range of incident momentum for which the ring is transparent at zero magnetic field, the Lorentz force leads to a decrease of the oscillation amplitude, due to the asymmetry in the electron injection in the two arms of the ring. For structures in which the fast electrons are predominantly backscattered, the Lorentz force assists in the transport producing an initial increase of the corresponding oscillation amplitude. Furthermore, we discuss the effect of elastic scattering on a potential cavity within one of the arms of the ring. For the cavity tuned to shift maximally the phase of the maximum of the wave packet we observe a $π$ shift of the Aharonov-Bohm oscillations. For other cavity depths the oscillations with a period of half of the flux quantum are observed.

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arXiv2005

Dependence of the vortex structure in quantum dots on the range of the inter-electron interaction

The internal structure of a composite fermion is investigated for a two dimensional parabolic quantum dot containing three electrons. A Yukawa screened Coulomb interaction is assumed, which allows us to discuss the evolution of the electron-vortex correlations from the Coulomb interaction limit to the contact potential limit. The vortex structure approaches the Laughlin limit non-monotonically through the formation of intermediate composite fermions in which a flip of the spatial orientation of the vortices with respect to the position of the electrons is observed. Only when we limit ourselves to the lowest Landau level (LLL) approximation the flip appears through the formation of an intermediate giant vortex at specific values of the screening length. Beyond the LLL approximation antivortices appear in the internal structure of the intermediate composite fermions which prevent the nucleation of giant vortices. We also studied the system of five electrons and show that the mechanism of the flip of the vortex orientation found for three-electron system is reproduced for higher number of electrons.

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