Capacitance Bridge

Abstract

We are developing a home-made, programmable capacitance bridge circuit for determining the capacitance of low-dimensional bilayer material samples. The bridge circuit consists of one sample, one capacitor of known capacitance, a dual-output signal generator to excite the sample and capacitor, an eight-output DC voltage source to apply a gate voltage to the sample, and a lock-in amplifier to measure the voltage at the bridge’s central node. We will use this bridge to generate a capacitance spectrum of the sample as a function of an applied gate voltage and from this map the density of states of the sample. The density of states is a very important quantity to determine in condensed matter physics, since it can be used in tandem with a probability density function like the Fermi-Dirac distribution for fermionic systems or the Bose-Einstein distribution for bosonic systems to predict the expectation value of many energy-dependent observable quantities. Mapping the density of states experimentally generally requires very expensive equipment, so being able to do this with a home-made setup of relatively inexpensive equipment and equipment that can serve multiple purposes is very desirable from an economic standpoint. We will use this setup to probe the properties of topologically insulating low-dimensional twisted bilayer materials that can be used as advanced field-effect transistors (FETs) for quantum computers.