arXiv subjects

Malati Dessai

Publications and source records attributed to Malati Dessai.

3 recordsLinked to original sources

arXiv2026

Image Force Effects on Tunneling Currents in an STM -- I `Point charge in the Barrier Region' - Model

In a Scanning Tunneling Microscope (STM), when a tunneling electron treated as a point charge enters the barrier region between the tip and the sample, it induces image charges on the conducting surfaces, which modifies the shape of the potential barrier it sees. In this paper, the effect of the modification in the barrier potential due to these induced charges on the tunneling current density and currents in an STM,is studied as a function of the tip-sample distance $d$ and the Bias Potential $eV_b$. The image potential is found to reduce the height and the effective width of the potential barrier, leading to a huge increase in the tunneling current densities. This huge increase (by several order of magnitudes) is however unreasonable, prompting a revisit of the assumption that the electron in the barrier region is a point particle.

cond-mat.other↗

arXiv2024

Theoretical Studies on the Scanning Tunneling Microscope

The thesis explores calculating tunneling current densities between planar conducting electrodes in an STM. It considers factors like bias voltages and the separation between electrodes, using Fermi energy and work functions. Pauli blocking effects on forward and reverse current densities are introduced, comparing Airy function solutions with WKB results. A 'Russell Potential' is defined for field lines, considering non-linearities and image force effects. A multi-slice method using the transfer matrix approach calculates tunneling currents for the Russell Potential and trapezoid + image force potentials. The Simmons image potential's unreasonable enhancement of tunneling currents prompts the construction of models with distributed charge, which show negligible image effects compared to trapezoidal potentials. Tunneling currents increase with bias voltage, decrease exponentially with tip-sample distance, and rise with increasing tip curvature radius. The resolving power of the STM degrades with blunter tips, higher bias voltages, and increased tip-sample distances. In essence, the thesis covers calculating tunneling current densities in STM, exploring different potential models and their effects. It discusses factors influencing tunneling behavior, like bias voltage, tip-sample distance, and tip curvature, revealing limitations in imaging capabilities under certain conditions.

cond-mat.other↗

arXiv2022

Calculation of tunneling current across Trapezoidal potential barrier in a Scanning Tunneling Microscope

The Planar Model of the Electrode-Vacuum-Electrode configuration for STM in which electrode surfaces are assumed to be infinite parallel planes, with atomic size separation and vacuum between them, is used to calculate tunneling current densities for both low and high bias voltages. Non WKB, Airy function solutions for the Schrödinger Equation for the trapezoidal barrier in the tunneling region are used to calculate the tunneling probability. Temperature dependent Fermi Factors for each electrode are introduced and the calculation involves integration over the electron energies. In order to convert the current densities obtained in the planar model to tunneling currents the tip and sample surfaces are modelled as confocal hyperboloids, and the tip sample distance is replaced by the length of the line of force (field line). The current is found by integrating the current density over a finite area of the tip. The calculated tunnel currents for a few electrode pairs at room temperature are plotted for several values of bias voltage, tip sample distances, and tip radii of curvature. Pauli Effects are studied as a function of bias voltage and tip-sample distance. Some estimate of lateral resolution and its dependence on bias voltage and tip radius is also presented.

cond-mat.other↗