The spin-dependent electronic transport is investigated in a paramagnetic resonant tunnelling diode formed from Zn_{1 - x}Mn_{x}Se quantum well between two ZnBeSe barrier layers. The spin-dependent current-voltage characteristics have been obtained in the presence of magnetic fields by solving the quantum kinetic equation for the Wigner distribution function and the Poisson equation in the self-consistent procedure. We have obtained two distinct current peaks due to the giant Zeeman splitting of electronic levels in a qualitative agreement with experiment. We have shown that the sign of spin current polarization can be reversed by tuning the bias voltage. Moreover, we have found the bias voltage windows with a nearly constant polarization.
We investigate the electronic transport properties of a bilayer graphene flake contacted by two monolayer nanoribbons. This finite-size bilayer flake can be built by overlapping two semi-infinite ribbons. We study and analyze the electronic behavior of this structure by means of a tight-binding method and a continuum Dirac model. We have found that the conductance oscillates markedly between zero and the maximum value of the conductance, allowing for the design of electromechanical switches.
A review of recent theoretical studies on a single-electron tunneling in quantum dots is presented. This effect underlies the transport spectroscopy performed on the vertical gated quantum dots and the capacitance spectroscopy on the self-assembled quantum dots. The conditions of the single-electron tunneling are formulated in terms of electrochemical potentials of the electrons in the leads and in the quantum dot. The electrochemical potentials for the electrons confined in the quantum dots can be calculated by solving the many-electron Schrödinger equation. The results obtained by the Hartree-Fock method are presented. For the vertical gated quantum dot, the realistic confinement potential is obtained from the Poisson equation. The application of the self-consistent procedure to the solution of the Poisson-Schrödinger problem is discussed. The calculated positions of the current peaks at zero bias and boundaries of the Coulomb diamonds for non-zero bias are in good agreement with experiment. The influence of an external magnetic field on the single-electron tunneling is also discussed. The spin-orbital configurations of the electrons confined in the quantum dots change with the magnetic field, which leads to features observed in the current-voltage and capacitance-voltage characteristics.
The electronic structure and quantum conductance of rotationally invariant (6,6)/(12,0) and rotationally non-invariant (5,5)/(8,2) superlattices made of metallic carbon nanotubes are investigated. It is shown that, except in the limit of very large periods, the quantum conductance of such superlattices does not critically depend on their rotational invariance, although it does in case of quantum dots and single junctions made of these nanotubes.
Using ab initio methods, we study transport and thermoelectric phenomena of magnetic organic chains functionalized with nitroxide groups. We predict very good thermoelectric performance of the structure, as the conventional and spin Seebeck coefficients are remarkably enhanced. Our results suggest that magnetic organic chains would have a great potential for applications in spintronic devices.
Thermoelectric properties of silicene nanoribbons doped with magnetic impurity atoms are investigated theoretically for both antiparallel and parallel orientations of the edge magnetic moments. Spin density distribution and transport parameters have been determined by ab-initio numerical methods based on the density functional theory. Doping with magnetic atoms considerably modifies the spin density distribution, leading to a ground state with a non-zero magnetic moment. Apart from this, the spin thermopower can be considerably enhanced by the impurity atoms.
We consider theoretically a system composed of a quantum dot coupled to a topological superconducting wire. The dot, being in Coulomb blockade (CB) regime is additionally coupled to the normal leads. The topological wire hosts Majorana states, which, as we show, characteristically modifies conductance through the dot. An unpaired Majorana state in the wire causes a unique temperature dependence of zero bias conductance vs. gate voltage. It decreases in-between CB peaks and on the sides of the peaks from the plateau at ~ e²/2h when temperature increases. At the same time conductance increases at the CB peak positions. It is accompanied by zero bias anomaly in differential conductance. For finite overlap of Majorana states in the wire the zero bias anomaly disappears. Instead, two characteristic Fano resonances of opposite symmetry appear, positioned mirror-symmetrically with respect to zero bias.
A simple model of disorder in fractional quantum Hall systems is combined with the standard exact diagonalisation technique. Electron-density-dependent gaps at filling factors 1/3,2/3,2/5, and 3/5 measured by activated transport can then be fitted with a single reasonable value of d which has the meaning of the separation of ionized donors from the quasi-2D electron gas.
An introduction into the area of inverse problems for the Schrödinger operators on metric graphs is given. The case of metric finite trees is treated in detail with the focus on matching conditions. For graphs with loops we show that for almost all matching conditions the potential on the loop is not determined uniquely by the Titchmarsh-Weyl function. The class of all admissible potentials is characterized.
In the present work we employ the master-equation approach to describe the transport through a molecule located in the central region between two external electrodes. In contrast to the transport through a quantum dot, electron-phonon coupling should be taken into account for tunnelling through a molecule. The coupling results in the appearance of additional effects such as vibrational sidebands or, for the case of strong coupling, a suppression of the current at low bias voltage (Franck-Condon blockade). In contrast to previous studies, the transport properties are described by the density matrix calculated explicitly with diagonal and off-diagonal elements. The observed phenomena are discussed and compared to previous studies.
Spin Hall effect in a two-dimensional electron gas with uniform Dresselhaus and random Rashba spin-orbit interactions is considered theoretically. Using Kubo formalism we derive some analytical formula for the spin Hall conductivity. It is shown that the contribution due to randomly fluctuating Rashba field disappears in the limit of strong Dresselhaus coupling.
The negative differential conductivity and electric instabilities are found to appear in type-II InAs/AlSb superlattices. The origin of the nonlinear effects is discussed.
Quantum graphs having one cycle are considered. It is shown that if the cycle contains at least three vertices, then the potential on the graph can be uniquely reconstructed from the corresponding Titchmarsh-Weyl function (Dirichlet-to-Neumann map) associated with graph's boundary, provided certain non-resonant conditions are satisfied.
We studied narrow (submicron) constrictions in the layers of ferromagnetic semiconductor (Ga,Mn)As. We have demonstrated a contribution of the quantum localization effects to the magnetoresistance of the constricted samples. We have also found a negative contribution of a domain wall trapped in the constriction to the resistance, due presumably to the erasing of the localization effects by the domain wall.
In this paper we present experimental investigations of carbon nanotubes deposited on highly orientated pyrolytical graphite using scanning tunneling microscopy and scanning tunneling spectroscopy. The aforementioned methods apart from detailed topographic data provided us with information about local density of state. We also show the I-V and dI/dV characteristics, which display the metallic and semiconducting characters of investigated carbon nanotubes. All measurements were taken in the air and at room temperature.
We consider a fractional Josephson junction mediated by a quantum dot in which the Zeeman field arising from the magnetic fields driving left and right wires into topological phase can be tuned. Both fields, forming an angle Θp, can be rotated in the common plane perpendicular to the spin-orbit field in the wires. For Θp=0 the dot can be regarded as effectively non-interacting due to the large Zeeman splitting, whereas for Θp ≤sssimπ electron interactions are switched on the dot, affecting Majorana states. The tunnel electrode, weakly coupled to the dot from the top, allows to probe their density of states via conductance measurement. We show that electron interactions renormalize Majorana peak and introduce characteristic asymmetry in the gate voltage dependence of the transverse zero-bias conductance through the dot.
The out-of-equilibrium transport properties of carbon nanotube quantum dot in the Kondo regime are studied by means of the non-equilibrium Green function. The equation of motion method is used. The influence of the polarization of electrodes and orbital level splitting, as well as left-right asymmetry, on the spin polarizations of differential conductance are discussed. For zero bias voltage and orbitally degenerate states the SU(4) symmetry of Kondo state is preserved for antiparallel configuration of polarizations of electrodes, whereas it is broken for parallel. In the former case a suppression of linear conductance with increasing polarization is observed. In the latter the behaviour is nonmonotonic due to splitting of the Kondo peak and bringing closer one of the peaks to the Fermi level with increasing polarization. This gives rise to giant tunnel linear magnetoresistance for large polarization.
We consider theoretically a junction between two topological superconducting wires, mediated by a quantum dot. The wires are modelled by the Kitaev chains tuned into topological phase, which possess unpaired Majorana states at their ends. We derive the low energy Hamiltonian of the model. The Majorana states closer to the dot convert into the Dirac fermion inside the dot, forming fractional Josephson junction. The dot is additionally weakly coupled to the normal tunneling probe allowing transport measurement through the dot. When the topological wires are short, the unpaired Majorana end-states can hybridize inside the wire forming an extended Dirac fermionic state. It yields the destruction of the extended state in the dot. We discuss the dependence of the spectral density of the dot and its conductance on superconducting phase. We show that the conservation of parity of the junction, crucial for successful measurement of the fractional effect, can be assured by the gate voltage manipulation of the dot level position and that in case of an unpaired Majorana state in the junction a half conductance quantum can be observed.
Motivated by recent advances in fabricating graphene nanostructures, we find that an electron can be trapped in Z-shaped graphene nanoconstriction with zigzag edges. The central section of the constriction operates as a single-level quantum dot, as the current flow towards the adjunct sections (rotated by 60°) is strongly suppressed due to mismatched valley polarization, although each section in isolation shows maximal quantum value of the conductance G_0 = 2e^2/h. We further show that the trapping mechanism is insensitive to the details of constriction geometry, except from the case when widths of the two neighboring sections are equal. The relation with earlier studies of electron transport through symmetric and asymmetric kinks with zigzag edges is also established.
We discuss lower and upper estimates for the spectral gap of the Laplace operator on a finite compact connected metric graph. It is shown that the best lower estimate is given by the spectral gap for the interval with the same total length as the original graph. An explicit upper estimate is given by generalizing Cheeger's approach developed originally for Riemannian manifolds.
JavaScript is turned off in your web browser. Turn it on to take full advantage of this site, then refresh the page.