Electron transport through a quantum interferometer with side-coupled quantum dots: Green's function approach

We study electron transport through a quantum interferometer with side-coupled quantum dots. The interferometer, threaded by a magnetic flux $\phi$, is attached symmetrically to two semi-infinite one-dimensional metallic electrodes. The calculations are based on the tight-binding model and the Green's function method, which numerically compute the conductance-energy and current-voltage characteristics. Our results predict that under certain conditions this particular geometry exhibits anti-resonant states. These states are specific to the interferometric nature of the scattering and do not occur in conventional one-dimensional scattering problems of potential barriers. Most importantly we show that, such a simple geometric model can also be used as a classical XOR gate, where the two gate voltages, viz, $V_a$ and $V_b$, are applied, respectively, in the two dots those are treated as the two inputs of the XOR gate. For $\phi=\phi_0/2$ ($\phi_0=ch/e$, the elementary flux-quantum), a high output current (1) (in the logical sense) appears if one, and only one, of the inputs to the gate is high (1), while if both inputs are low (0) or both are high (1), a low output current (0) appears. It clearly demonstrates the XOR gate behavior and this aspect may be utilized in designing the electronic logic gate.Comment: 8 pages, 5 figure

NAND gate response in a mesoscopic ring: An exact study

NAND gate response in a mesoscopic ring threaded with a magnetic flux $\phi$ is investigated by using Green's function formalism. The ring is attached symmetrically to two semi-infinite one-dimensional metallic electrodes and two gate voltages, namely, $V_a$ and $V_b$, are applied in one arm of the ring those are treated as the two inputs of the NAND gate. We use a simple tight-binding model to describe the system and numerically compute the conductance-energy and current-voltage characteristics as functions of the gate voltages, ring-to-electrode coupling strength and magnetic flux. Our theoretical study shows that, for $\phi=\phi_0/2$ ($\phi_0=ch/e$, the elementary flux-quantum) a high output current (1) (in the logical sense) appears if one or both the inputs to the gate are low (0), while if both the inputs to the gate are high (1), a low output current (0) appears. It clearly exhibits the NAND gate behavior and this feature may be utilized in designing an electronic logic gate.Comment: 8 pages, 5 figure

Quantum transport through molecular wires

We explore electron transport properties in molecular wires made of heterocyclic molecules (pyrrole, furan and thiophene) by using the Green's function technique. Parametric calculations are given based on the tight-binding model to describe the electron transport in these wires. It is observed that the transport properties are significantly influenced by (a) the heteroatoms in the heterocyclic molecules and (b) the molecule-to-electrodes coupling strength. Conductance ($g$) shows sharp resonance peaks associated with the molecular energy levels in the limit of weak molecular coupling, while they get broadened in the strong molecular coupling limit. These resonances get shifted with the change of the heteroatoms in these heterocyclic molecules. All the essential features of the electron transfer through these molecular wires become much more clearly visible from the study of our current-voltage ($I$-$V$) characteristics, and they provide several key informations in the study of molecular transport.Comment: 8 pages, 4 figure

Metal-insulator transition in an aperiodic ladder network: an exact result

We show, in a completely analytical way, that a tight binding ladder network composed of atomic sites with on-site potentials distributed according to the quasiperiodic Aubry model can exhibit a metal-insulator transition at multiple values of the Fermi energy. For specific values of the first and second neighbor electron hopping, the result is obtained exactly. With a more general model, we calculate the two-terminal conductance numerically. The numerical results corroborate the analytical findings and yield a richer variety of spectrum showing multiple mobility edges.Comment: 4 pages, 3 figure

Selective spin transport through a quantum heterostructure: Transfer matrix method

In the present work we propose that a one-dimensional quantum heterostructure composed of magnetic and non-magnetic atomic sites can be utilized as a spin filter for a wide range of applied bias voltage. A simple tight-binding framework is given to describe the conducting junction where the heterostructure is coupled to two semi-infinite one-dimensional non-magnetic electrodes. Based on transfer matrix method all the calculations are performed numerically which describe two-terminal spin dependent transmission probability along with junction current through the wire. Our detailed analysis may provide fundamental aspects of selective spin transport phenomena in one-dimensional heterostructures at nano-scale level.Comment: 12 pages, 15 figures (Accepted for Publication in: International Journal of Modern Physics B

Electron transport through multilevel quantum dot

Quantum transport properties through some multilevel quantum dots sandwiched between two metallic contacts are investigated by the use of Green's function technique. Here we do parametric calculations, based on the tight-binding model, to study the transport properties through such bridge systems. The electron transport properties are significantly influenced by (a) number of quantized energy levels in the dots, (b) dot-to-electrode coupling strength, (c) location of the equilibrium Fermi energy $E_F$ and (d) surface disorder. In the limit of weak-coupling, the conductance ($g$) shows sharp resonant peaks associated with the quantized energy levels in the dots, while, they get substantial broadening in the strong-coupling limit. The behavior of the electron transfer through these systems becomes much more clearly visible from our study of current-voltage ($I$-$V$) characteristics. In this context we also describe the noise power of current fluctuations ($S$) and determine the Fano factor ($F$) which provides an important information about the electron correlation among the charge carriers. Finally, we explore a novel transport phenomenon by studying the surface disorder effect in which the current amplitude increases with the increase of the surface disorder strength in the strong disorder regime, while, the amplitude decreases in the limit of weak disorder. Such an anomalous behavior is completely opposite to that of bulk disordered system where the current amplitude always decreases with the disorder strength. It is also observed that the current amplitude strongly depends on the system size which reveals the finite quantum size effect.Comment: 12 pages, 7 figure

Antibiotic usage pattern among inpatients of a paediatric ward in a tertiary care hospital in Eastern India

Background: The inevitable consequence of the widespread use of antimicrobial agents has been the emergence of antibiotic resistant pathogens. The rising incidence of bacterial resistance to common antibiotics, particularly, multi-drug resistant pneumococci, has prompted the need to use antibiotics judiciously in paediatric practice. The present study thus attempted to understand the antibiotic usage pattern among inpatients in a paediatric ward of a tertiary care hospital.Methods: A cross-sectional study was carried out for three months among the inpatients in the Department of Pediatrics in a tertiary care teaching hospital, Kolkata. The data regarding patient’s demographics and antibiotic use was collected daily in a pre-structured proforma. A descriptive statistical analysis of the data was performed.Results: A total of 124 patients were screened of which males and females represented 58.06% and 41.94% of the cases respectively with age group of 2-14 years presenting in majority. Among various causes of hospitalization, prevalence of respiratory disorders were maximum (25.81%), followed by hematological disorders (25%). Among various categories of prescribed drugs, antibiotics were found to be maximum (39.25%). Beta-lactum antibiotics were prescribed in majority (64.41%) with 56.91% being of cephalosporin group. The preferred route of administration was found to be parenteral (64%). 61.29 % cases were ordered laboratory investigations prior to the antimicrobial therapy initiation.  About 63.16% of the total samples for culture & sensitivity tested were reported positive. 39.58% of these positive cases showed resistance to empirical antibiotic therapy on lab reports.Conclusions: The study enables to obtain information on the antibiotic usage pattern in the pediatric population, focusing on prevalence of antibiotic misuse in our set-up and suggesting strategies of its minimization.

XOR gate response in a mesoscopic ring with embedded quantum dots

We address XOR gate response in a mesoscopic ring threaded by a magnetic flux $\phi$. The ring, composed of identical quantum dots, is symmetrically attached to two semi-infinite one-dimensional metallic electrodes and two gate voltages, viz, $V_a$ and $V_b$, are applied, respectively, in each arm of the ring which are treated as the two inputs of the XOR gate. The calculations are based on the tight-binding model and the Green's function method, which numerically compute the conductance-energy and current-voltage characteristics as functions of the ring-electrodes coupling strengths, magnetic flux and gate voltages. Quite interestingly it is observed that, for $\phi=\phi_0/2$ ($\phi_0=ch/e$, the elementary flux-quantum) a high output current (1) (in the logical sense) appears if one, and only one, of the inputs to the gate is high (1), while if both inputs are low (0) or both are high (1), a low output current (0) appears. It clearly demonstrates the XOR behavior and this aspect may be utilized in designing the electronic logic gate.Comment: 7 pages, 5 figure

NOR gate response in a double quantum ring: An exact result

NOR gate response in a double quantum ring, where each ring is threaded by a magnetic flux $\phi$, is investigated. The double quantum ring is sandwiched symmetrically between two semi-infinite one-dimensional metallic electrodes and two gate voltages, namely, $V_a$ and $V_b$, are applied, respectively, in lower arms of the two rings those are treated as the two inputs of the NOR gate. A simple tight-binding model is used to describe the system and all the calculations are done through the Green's function formalism. Here we exactly calculate the conductance-energy and current-voltage characteristics as functions of the ring-to-electrode coupling strengths, magnetic flux and gate voltages. Our numerical study predicts that, for a typical value of the magnetic flux $\phi=\phi_0/2$ ($\phi_0=ch/e$, the elementary flux-quantum), a high output current (1) (in the logical sense) appears if both the inputs to the gate are low (0), while if one or both are high (1), a low output current (0) results. It clearly demonstrates the NOR gate behavior and this aspect may be utilized in designing an electronic logic gate.Comment: 8 pages, 5 figure

Electron transport in a double quantum ring: Evidence of an AND gate

We explore AND gate response in a double quantum ring where each ring is threaded by a magnetic flux $\phi$. The double quantum ring is attached symmetrically to two semi-infinite one-dimensional metallic electrodes and two gate voltages, namely, $V_a$ and $V_b$, are applied, respectively, in the lower arms of the two rings which are treated as two inputs of the AND gate. The system is described in the tight-binding framework and the calculations are done using the Green's function formalism. Here we numerically compute the conductance-energy and current-voltage characteristics as functions of the ring-to-electrode coupling strengths, magnetic flux and gate voltages. Our study suggests that, for a typical value of the magnetic flux $\phi=\phi_0/2$ ($\phi_0=ch/e$, the elementary flux-quantum) a high output current (1) (in the logical sense) appears only if both the two inputs to the gate are high (1), while if neither or only one input to the gate is high (1), a low output current (0) results. It clearly demonstrates the AND gate behavior and this aspect may be utilized in designing an electronic logic gate.Comment: 8 pages, 5 figure