arXiv subjects

Kamil Walczak

Publications and source records attributed to Kamil Walczak.

24 records · Page 2Linked to original sources

arXiv2004

Kinetic equation approach to the problem of rectification in asymmetric molecular structures

Transport properties of asymmetric molecular structure are studied within the kinetic equation approach, taking into consideration the electron interaction in the self-consistent manner (SCF procedure). The device is made of a molecule (modeled as a quantum dot with discrete energy levels) which is asymmetrically coupled to the two metallic electrodes through the tunnel junctions. Electrical rectification follows from the combined effect of the geometric asymmetry in the molecular structure and simultaneously in the electrostatic potential spatial profile. Primarily the influence of broadening and charging effects on the charge and transport characteristics are investigated. Consequence of the broadening is to smooth the charge-voltage (N-V) and current voltage (I-V) functions, while the charging effect is rather responsible for the change of the slope of both N-V and I-V curves. Inclusion of the electron-electron repulsion results in significant enlargement of rectification ratio.

cond-mat.mes-hall↗

arXiv2004

The Role of Quantum Interference in Determining Transport Properties of Organic Molecules

An analytic approach to the electron transport phenomena in molecular devices is presented. Analyzed devices are composed of organic molecules attached to the two semi-infinite electrodes. Molecular system is described within the tight-binding model, while the coupling to the electrodes is analyzed through the use of Newns-Anderson chemisorption theory. The current-voltage (I-V) characteristics are calculated through the integration of transmission function in the standard Landauer formulation. The essential question of quantum interference effect of electron waves is discussed in three aspects: (i) the geometry of molecular bridge and (ii) the presence of an external magnetic field and (iii) the strength of the molecule-to-electrodes coupling.

cond-mat.mes-hall↗

arXiv2004

Sequential toy model for electron transfer in donor-bridge-acceptor systems

Sequential tunneling model is used to study electron transport in molecular rectifiers based on structures of donor-bridge-acceptor type. The device is made of two metallic electrodes connected by a molecule, which contains acceptor and donor subunits, separated by insulating bridge. Both subunits are modeled as quantum dots with discrete energy levels, isolated from each other by potential barrier and weakly coupled to both electrodes through tunnel junctions. Intervalence donor-acceptor tunneling process is treated as a superexchange mechanism or equivalently through the use of Marcus theory. Analytic formula for the current is found in the case of the Aviram-Ratner ansatz of rectification and current-voltage characteristic is obtained at an arbitrary strength of the potential drop over the tunneling region. It is shown that rectification current depends on the position of the acceptor's LUMO and donor's HOMO levels with respect to the Fermi energy of the electrodes before bias is applied, and their shift due to the bias voltage.

cond-mat.mes-hall↗

arXiv2004

Current fluctuations of polymeric chains

Coherent electron transport is investigated in a molecular device made of polymeric chain sandwiched between two metallic electrodes. Molecular system is described by a simple Huckel model, while the coupling to the electrodes is treated through the use of Newns-Anderson chemisorption theory. Transport characteristics and noise power are calculated in two response regimes: linear and nonlinear, respectively. Here is shown a strong dependence of the shot noise on: (i) the length of the polymeric chain and (ii) the strength of the molecule-to-electrodes coupling. In particular, detailed discussion of Poissonian to sub-Poissonian crossover in the noise spectra is included. Presented algorithm allows to calculate the lowest possible level of current fluctuations (due to Pauli exclusion principle) in designing molecular devices.

cond-mat.mes-hall↗

arXiv2004

Importance of quantum interference in molecular-scale devices

Electron transport is theoretically investigated in a molecular device made of anthracene molecule attached to the electrodes by thiol end groups in two different configurations (para and meta, respectively). Molecular system is described by a simple Huckel-like model (with non-orthogonal basis set of atomic orbitals), while the coupling to the electrodes is treated through the use of Newns-Anderson chemisorption theory (constant density of states within energy bandwidth). Transport characteristics (current-voltage and conductance-voltage) are calculated from the transmission function in the standard Landauer formulation. The essential question of quantum interference is discussed in detail. The results have shown a striking variation of transport properties of the device depending on the character of molecular binding to the electrodes.

cond-mat.mes-hall↗

arXiv2003

Simulations of molecular logic gates

Here we propose four-terminal molecular devices as functional logic gates (AND, NOR and XOR, respectively). Such devices are composed of single organic molecule connected to gold electrodes and located in between gate terminals. Specifically, the operation principle of molecular logic gates is discussed in detail. The input signal of the gate voltages can modify the output signal of the current carried through the device (for concrete value of applied source-drain bias) to simulate classical (Boolean) logical operations. Calculational scheme for current-voltage (I-V) characteristics is based on the Landauer transport theory, where molecule is described with the help of tight-binding model, gold electrodes are treated within an effective Newns-Anderson theory and gate terminals are modeled as capacitor plates.

cond-mat.mes-hall↗