arXiv subjects

J. Adamowski

Publications and source records attributed to J. Adamowski.

At least 19 recordsLinked to original sources

arXiv2017

Spin filtering effect generated by the inter-subband spin-orbit coupling in the bilayer nanowire with the quantum point contact

The spin filtering effect in the bilayer nanowire with quantum point contact is investigated theoretically. We demonstrate the new mechanism of the spin filtering based on the lateral inter-subband spin-orbit coupling, which for the bilayer nanowires has been reported to be strong. The proposed spin filtering effect is explained as the joint effect of the Landau-Zener inter-subband transitions caused by the hybridization of states with opposite spin (due to the lateral Rashba SO interaction) and the confinement of carriers in the quantum point contact region.

cond-mat.mes-hall↗

arXiv2016

Effect of the inter-subband spin-orbit interaction on the spin transistor operation

We consider the electron transport in the Datta-Das spin transistor within the two-subband model taking into account the intra- and inter-subband spin-orbit (SO) interaction and study the influence of the inter-subband SO coupling on the spin-transistor operation. Starting from the model, in which the SO coupling constants are treated as parameters, we show that the inter-subband SO interaction strongly affects the ordinary conductance oscillations predicted for the transistor with the single occupancy. Interestingly, we find that even in the absence of the intra-subband SO interaction, the conductance oscillates as a function of the inter-subband SO coupling constant. This phenomenon is explained as resulting from the inter-subband transition with spin-flip. Next, we consider the realistic spin transistor model based on the gated Al$_{0.48}$In$_{0.52}$As/Ga$_{0.47}$In$_{0.53}$As double quantum well, for which the SO coupling constants are determined by the Schr\"{o}dinger-Poisson approach. We show that the SO coupling constants rapidly change around $V_g=0$, which is desirable for the spin transistor operation. We demonstrate that for high electron densities the inter-subband SO interaction starts to play the dominant role. The strong evidence of this interaction is the reduction of the conductance for gate voltage $V_g=0$, which leads to the reduction of the on/off conductance ratio.

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arXiv2015

Electrically controlled spin-transistor operation in helical magnetic field

A proposal of electrically controlled spin transistor in helical magnetic field is presented. In the proposed device, the transistor action is driven by the Landau-Zener transitions that lead to a backscattering of spin polarized electrons and switching the transistor into the high-resistance state (off state). The on/off state of the transistor can be controlled by the all-electric means using Rashba spin-orbit coupling that can be tuned by the voltages applied to the side electrodes.

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arXiv2014

Resonant Landau-Zener transitions in helical magnetic fields

The spin-dependent electron transport has been studied in magnetic semiconductor waveguides (nanowires) in the helical magnetic field. We have shown that -- apart from the known conductance dip located at the magnetic field equal to the helical-field amplitude $B_h$ -- the additional conductance dips (with zero conductance) appear at magnetic field different from $B_h$. This effect occuring in the non-adiabatic regime is explained as resulting from the resonant Landau-Zener transitions between the spin-splitted subbands.

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arXiv2014

Non-ballistic spin separator based on Y-shaped nanostructure with a quantum point contact

A proposal of a spin separator based on the spin Zeeman effect in Y-shaped nanostructure with a quantum point contact is presented. Our calculations show that the appropriate tuning of the quantum point contact potential and the external magnetic field leads to the spin separation of the current: electrons with opposite spins flow through the different output branches. We demonstrate that this effect is robust against the scattering on impurities. The proposed device can also operate as a spin detector, in which -- depending on the electron spin -- the current flows through one of the output branches.

cond-mat.mes-hall↗

arXiv2013

All-electrical manipulation of electron spin by the spin-orbit interaction in a semiconductor nanotube: analytical results

A possibility of controlled manipulation of electron spin states has been investigated for a cylindrical two-dimensional electron gas confined in a semiconductor nanotube/cylindrical nanowire with the Rashba spin-orbit interaction. We present analytical solutions for the two limiting cases, in which the spin-orbit interaction results from (A) the radial electric field and (B) the electric field applied along the axis z of the nanotube. In case (A), the superposition of the two lowest-energy bands corresponding to the opposite spins leads to the precession of electron spin around the nanowire axis. We have found that the direction of the spin precession changes from clockwise to counterclockwise if the energy of the injected electron achieves the value corresponding to the crossing of energy levels associated with the two components of the superposition state. In case (B), we have obtained the damped oscillations of the z spin component with the period that changes as a function of the coordinate z. We have also shown that the damped oscillations of the average value of the z spin component form beats localized along the nanowire axis.

cond-mat.mes-hall↗

arXiv2013

Spin transistor operation driven by the Rashba spin-orbit coupling in the gated nanowire

The theoretical description has been proposed for the operation of the spin transistor in the gate-controlled InAs nanowire. The calculated current-voltage characteristics show that the current flowing from the source (spin injector) to the drain (spin detector) oscillates as a function of the gate voltage, which results from the precession of the electron spin caused by the Rashba spin-orbit interaction in the vicinity of the gate. We have studied two operation modes of the spin transistor: (A) the ideal operation mode with the full spin polarization of electrons in the contacts, the zero temperature, and the single conduction channel corresponding to the lowest-energy subband of the transverse motion and (B) the more realistic operation mode with the partial spin polarization of the electrons in the contacts, the room temperature, and the conduction via many transverse subbands taken into account. For mode (A) the spin-polarized current can be switched on/off by the suitable tuning of the gate voltage, for mode (B) the current also exhibits the pronounced oscillations but with no-zero minimal values. The computational results obtained for mode (B) have been compared with the recent experimental data and a good agreement has been found.

cond-mat.mes-hall↗

arXiv2013

Spin filter effect at room temperature in GaN/GaMnN ferromagnetic resonant tunneling diode

We have investigated the spin current polarization without the external magnetic field in the resonant tunneling diode with the emitter and quantum well layers made from the ferromagnetic GaMnN. For this purpose we have applied the self-consistent Wigner-Poisson method and studied the spin-polarizing effect of the parallel and antiparallel alignment of the magnetization in the ferromagnetic layers. The results of our calculations show that the antiparallel magnetization is much more advantageous for the spin filter operation and leads to the full spin current polarization at low temperatures and 35 % spin polarization of the current at room temperature.

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arXiv2011

Effect of current hysteresis on the spin polarization of current in a paramagnetic resonant tunneling diode

A spin-dependent quantum transport is investigated in a paramagnetic resonant tunneling diode (RTD) based on a Zn$_{1-x}$Mn$_x$Se/ZnBeSe heterostructure. Using the Wigner-Poisson method and assuming the two-current model we have calculated the current-voltage characteristics, potential energy profiles and electron density distributions for spin-up and spin-down electron current in an external magnetic field. We have found that -- for both the spin-polarized currents -- two types of the current hysteresis appear on the current-voltage characteristics. The current hysteresis of the first type occurs at the bias voltage below the resonant current peak and results from the accumulation of electrons in the quantum well layer. The current hysteresis of the second type appears at the bias voltage above the resonant current peak and is caused by the creation of the quasi-bound state in the left contact region and the resonant tunneling through this quasi-bound state. The physical interpretation of both the types of the current hysteresis is further supported by the analysis of the calculated self-consistent potential profiles and electron density distributions. Based on these results we have shown that -- in certain bias voltage and magnetic field ranges -- the spin polarization of the current exhibits the plateau behavior with the nearly full spin polarization. This property is very promising for possible applications in spintronics.

cond-mat.mes-hall↗

arXiv2009

XOR logic gate on electron spin qubits in quadruple coupled quantum dots

The spin-dependent localization of electrons in quadruple quantum dots (QD's) has been studied by the configuration interaction method. We have investigated two nanodevices that consist of laterally coupled quadruple QD's. We have shown that -- in both the nanodevices with suitably chosen parameters -- the exclusive OR (XOR) logic gate can be realized by all-electrical control with the readout of output via the spin-to-charge conversion. We have determined the nanodevice parameters that are optimal for the performance of the XOR logic gate.

cond-mat.mes-hall↗

arXiv2009

Self-consistent Wigner distribution function study of gate-voltage controlled triple-barrier resonant tunnelling diode

The electron transport through the triple-barrier resonant tunnelling diode (TBRTD) have been studied by the self-consistent numerical method for the Wigner-Poisson problem. The electron flow through the TBRTD can be controlled by the gate voltage applied to one of the potential well regions. For different gate voltage values we have determined the current-voltage characteristics, potential energy profiles, and electron density distribution. We have found the enhancement of the peak-to-valley ratio (up to $\sim$10), the appearance of the linear current versus bias voltage behaviour within the negative-differential resistance region, and the bistability of the current-voltage characteristics. The analysis of the self-consistent potential energy profiles and electron density distribution allowed us to provide a physical interpretation of these properties.

cond-mat.mes-hall↗

arXiv2007

Effect of confinement potential shape on exchange interaction in coupled quantum dots

Exchange interaction has been studied for electrons in coupled quantum dots (QD's) by a configuration interaction method using confinement potentials with different profiles. The confinement potential has been parametrized by a two-centre power-exponential function, which allows us to investigate various types of QD's described by either soft or hard potentials of different range. For the soft (Gaussian) confinement potential the exchange energy decreases with increasing interdot distance due to the decreasing interdot tunnelling. For the hard (rectangular-like) confinement potential we have found a non-monotonic behaviour of the exchange interaction as a function of distance between the confinement potential centres. In this case, the exchange interaction energy exhibits a pronounced maximum for the confinement potential profile which corresponds to the nanostructure composed of the small inner QD with a deep potential well embedded in the large outer QD with a shallow potential well. This effect results from the strong localization of electrons in the inner QD, which leads to the large singlet-triplet splitting. Implications of this finding for quantum logic operations have been discussed.

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arXiv2006

Controlled exchange interaction for quantum logic operations with spin qubits in coupled quantum dots

A two-electron system confined in two coupled semiconductor quantum dots is investigated as a candidate for performing quantum logic operations on spin qubits. We study different processes of swapping the electron spins by controlled switching on/off the exchange interaction. The resulting spin swap corresponds to an elementary operation in quantum information processing. We perform a direct time evolution simulations of the time-dependent Schroedinger equation. Our results show that -- in order to obtain the full interchange of spins -- the exchange interaction should change smoothly in time. The presence of jumps and spikes in the corresponding time characteristics leads to a considerable increase of the spin swap time. We propose several mechanisms to modify the exchange interaction by changing the confinement potential profile and discuss their advantages and disadvantages.

cond-mat.mes-hall↗

arXiv2005

Exciton and negative trion dissociation by an external electric field in vertically coupled quantum dots

We study the Stark effect for an exciton confined in a pair of vertically coupled quantum dots. A single-band approximation for the hole and a parabolic lateral confinement potential are adopted which allows for the separation of the lateral center-of-mass motion and consequently for an exact numerical solution of the Schrödinger equation. We show that for intermediate tunnel coupling the external electric field leads to the dissociation of the exciton via an avoided crossing of bright and dark exciton energy levels which results in an atypical form of the Stark shift. The electric-field-induced dissociation of the negative trion is studied using the approximation of frozen lateral degrees of freedom. It is shown that in a symmetric system of coupled dots the trion is more stable against dissociation than the exciton. For an asymmetric system of coupled dots the trion dissociation is accompanied by a positive curvature of the recombination energy line as a function of the electric field.

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arXiv2005

Stability of negative and positive trions in quantum wires

Binding energies of negative ($X^-$) and positive trions ($X^+$) in quantum wires are studied for strong quantum confinement of carriers which results in a numerical exactly solvable model. The relative electron and hole localization has a strong effect on the stability of trions. For equal hole and electron confinement, $X^+$ is more stable but a small imbalance of the particle localization towards a stronger hole localization e.g. due to its larger effective mass, leads to the interchange of $X^-$ and $X^+$ recombination lines in the photoluminescent spectrum as was recently observed experimentally. In case of larger $X^-$ stability, a magnetic field oriented parallel to the wire axis leads to a stronger increase of the $X^+$ binding energy resulting in a crossing of the $X^+$ and $X^-$ lines.

cond-mat.mes-hall↗

arXiv2004

Exact broken-symmetry states and Hartree-Fock solutions for quantum dots at high magnetic fields

Wigner molecules formed at high magnetic fields in circular and elliptic quantum dots are studied by exact diagonalization (ED) and unrestricted Hartree-Fock (UHF) methods with multicenter basis of displaced lowest Landau level wave functions. The broken symmetry states with semi-classical charge density constructed from superpositions of the ED solutions are compared to the UHF results. UHF overlooks the dependence of the few-electron wave function on the actual relative positions of electrons localized in different charge puddles and partially compensates for this neglect by an exaggerated separation of charge islands which are more strongly localized than in the exact broken-symmetry states.

cond-mat.mes-hall↗

arXiv2004

Spatial ordering of charge and spin in quasi one-dimensional Wigner molecules

ew-electron systems confined in quasi one-dimensional quantum dots are studied by the configuration interaction approach. We consider the parity symmetry of states forming Wigner molecules in large quantum dots and find that for the spin-polarized Wigner molecules it strictly depends on the number of electrons. We investigate the spatial spin-ordering in the inner coordinates of the quantum system and conclude that for small dots it has a short-range character and results mainly from the Pauli exclusion principle while the Wigner crystallization in large dots is accompanied by spin ordering over the entire length of the dot.

cond-mat.mes-hall↗

arXiv2003

Accuracy of the Hartree-Fock method for Wigner molecules at high magnetic fields

Few-electron systems confined in two-dimensional parabolic quantum dots at high magnetic fields are studied by the Hartree-Fock (HF) and exact diagonalization methods. A generalized multicenter Gaussian basis is proposed in the HF method. A comparison of the HF and exact results allows us to discuss the relevance of the symmetry of the charge density distribution for the accuracy of the HF method. It is shown that the energy estimates obtained with the broken-symmetry HF wave functions become exact in the infinite magnetic-field limit. In this limit the charge density of the broken-symmetry solution can be identified with the classical charge distribution.

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