T. Kwapinski

Double nonequivalent chain structure on a vicinal Si(557)-Au surface

We study electronic and topographic properties of the vicinal Si(557)-Au surface using scanning tunneling microscopy and reflection of high energy electron diffraction technique. STM data reveal double wire structures along terraces. Moreover behavior of the voltage dependent STM tip - surface distance is different in different chains. While the one chain shows oscillations of the distance which are sensitive to the sign of the voltage bias, the oscillations in the other chain remain unchanged with respect to the positive/negative biases. This suggests that one wire has metallic character while the other one - semiconducting. The experimental results are supplemented by theoretical calculations within tight binding model suggesting that the observed chains are made of different materials, one is gold and the other one is silicon chain.

Influence of microwave fields on electron transport through a quantum dot in the presence of direct tunneling between leads

We consider the time-dependent electron transport through a quantum dot coupled to two leads in the presence of the additional overdot (bridge) tunneling channel. By using the evolution operator method together with the wide-band limit approximation we derived the analytical formulas for the quantum dot charge and current flowing in the system. The influence of the external microwave field on the current and the derivatives of the current with respect to the gate and source-drain voltages has been investigated for a wide range of parameters. The most characteristic feature of including the additional overdot tunneling channel is an asymmetric behavior of the function describing the dependence of the average current vs the gate voltage or the differential conductance vs the source-drain voltage.

Photon-assisted electron transport through a three-terminal quantum dot system with nonresonant tunneling channels

We have studied the electron transport through a quantum dot coupled to three leads in the presence of external microwave fields supplied to different parts of the considered mesoscopic system. Additionally, we introduced a possible nonresonant tunneling channel between leads. The quantum dot charge and currents were determined in terms of the appropriate evolution operator matrix elements and under the wide-band limit the analytical formulas for time-averaged currents and differential conductance were obtained. We have also examined the response of the considered system on the rectangular-pulse modulation imposed on different quantum dot-lead barriers as well as the time dependence of currents flowing in response to suddenly removed or included connection of a quantum dot with one of the leads.

Conductance oscillations and charge waves in zigzag shaped quantum wires

Electron transport through a quantum wire (or coupled quantum dots) with time-dependent couplings between the nearest-neighbor and next-neighbor sites is studied by means of the evolution operator method and tight-binding Hamiltonian. Two geometries of a wire (linear and zigzag shaped) are considered in our calculations. Charge waves inside the wire and the conductance oscillation effect, i.e. the conductance as a function of the wire length, are analyzed. For a zigzag shaped wire with time-dependent couplings the conductance is characterized by a Fano-like resonance with many sideband peaks.

Conductance oscillations of a metallic quantum wire

The electron transport through a monatomic metallic wire connected to leads is investigated using the tight-binding Hamiltonian and the Green function technique. Analytical formulae for the transmittance are derived and M-atom oscillations of the conductance versus the length of the wire are found. Maxima of the transmittance function versus the energy, for a wire consisting of N atoms, determine the (N+1) period of the conductance. The periods of conductance oscillations are discussed and the local and average quantum wire charges are presented. The average charge of the wire is linked with the period of the conductance oscillations and for M-atom periodicity there are possible (M-1) average occupations of the wire states.

Spin and charge pumping in a quantum wire: the role of spin-flip scattering and Zeeman splitting

We investigate theoretically charge and spin pumps based on a linear configuration of quantum dots (quantum wire) which are disturbed by an external time-dependent perturbation. This perturbation forms an impulse which moves as a train pulse through the wire. It is found that the charge pumped through the system depends non-monotonically on the wire length, N. In the presence of the Zeeman splitting pure spin current flowing through the wire can be generated in the absence of charge current. Moreover, we observe electron pumping in a direction which does not coincide with the propagation direction of the pulse and the spin pumping direction (spin-charge separation). Additionally, on-site spin-flip processes significantly influence electron transport through the system and can also reverse the charge current direction.

Charge fluctuations in a perfect and disturbed quantum wire

The tight-binding Hamiltonian and retarded Green function formalism are used to study the local and total density of states of a monatomic quantum wire. An additional atom (adatom) can be coupled with the wire at its side. Also the local and average charges of the perfect and disturbed wire are shown and the analytical formula for the local density of states at each site is obtained. The local density of states at each atom in a wire which consists of an even number of atoms is characterized by minima at the Fermi level, in comparison with a wire disturbed by an adatom, where the density of states is characterized by local peaks. Charge fluctuations (waves) are observed in the wire for the case when the condition for the conductance oscillations is satisfied. For the single electron energies of a wire localized above the Fermi energy the local charge along the wire forms a wave curve which possesses one maximum less than for energies localized below the Fermi level.

Phase-dependent electron transport through a quantum wire on a surface.

Electron transport through a quantum wire in the presence of external periodic energy-level modulations with different on-site phases is studied within the time evolution operator method for a tight-binding Hamiltonian. It is found that in the presence of spatial symmetry of the system and no source-drain and static gate voltages the pumping current can be generated. Moreover, for a wire which is tunnel-coupled to the underlying substrate, the current flowing through an unbiased wire does not fade away but increases with the wire-surface coupling. For randomly chosen phases at every wire site two regimes of the phase-averaged current are found which are related to small and high wire density of states.

Electron transport across a quantum wire in the presence of electron leakage to a substrate

Abstract We investigate electron transport through a mono-atomic wire which is tunnel coupled to two electrodes and also to the underlying substrate. The setup is modeled by a tight-binding Hamiltonian and can be realized with a scanning tunnel microscope (STM). The transmission of the wire is obtained from the corresponding Green’s function. If the wire is scanned by the contacting STM tip, the conductance as a function of the tip position exhibits oscillations which may change significantly upon increasing the number of wire atoms. Our numerical studies reveal that the conductance depends strongly on whether or not the substrate electrons are localized. As a further ubiquitous feature, we observe the formation of charge oscillations.

STM tunneling through a quantum wire with a side-attached impurity

The STM tunneling through a quantum wire (QW) with a side-attached impurity (atom, island) is investigated using a tight-binding model and the non-equilibrium Keldysh Green function method. The impurity can be coupled to one or more QW atoms. The presence of the impurity strongly modifies the local density of states of the wire atoms, thus influences the STM tunneling through all the wire atoms. The transport properties of the impurity itself are also investigated mainly as a function of the wire length and the way it is coupled to the wire. It is shown that the properties of the impurity itself and the way it is coupled to the wire strongly influence the STM tunneling, the density of states and differential conductance.