Quantum capacitance

About: Quantum capacitance is a research topic. Over the lifetime, 954 publications have been published within this topic receiving 24165 citations.

Sensitive radiofrequency readout of quantum dots using an ultra-low-noise SQUID amplifier

Abstract: Fault-tolerant spin-based quantum computers will require fast and accurate qubit read out. This can be achieved using radiofrequency reflectometry given sufficient sensitivity to the change in quantum capacitance associated with the qubit states. Here, we demonstrate a 23-fold improvement in capacitance sensitivity by supplementing a cryogenic semiconductor amplifier with a SQUID preamplifier. The SQUID amplifier operates at a frequency near 200 MHz and achieves a noise temperature below 600 mK when integrated into a reflectometry circuit, which is within a factor 120 of the quantum limit. It enables a record sensitivity to capacitance of 0.07 aF / Hz. The setup is used to acquire charge stability diagrams of a gate-defined double quantum dot in a short time with a signal-to-noise ration of about 38 in 1 μ s of integration time.

Negative quantum capacitance in graphene nanoribbons with lateral gates

Abstract: We present numerical simulations of the capacitive coupling between graphene nanoribbons of various widths and gate electrodes in different configurations. We compare the influence of lateral metallic or graphene side gate structures on the overall back gate capacitive coupling. Most interestingly, we find a complex interplay between quantum capacitance effects in the graphene nanoribbon and the lateral graphene side gates, giving rise to an unconventional negative quantum capacitance. The emerging nonlinear capacitive couplings are investigated in detail. The experimentally relevant relative lever arm, the ratio between the coupling of the different gate structures, is discussed.

First-principles calculation of quantum capacitance of metals doped graphenes and nitrogen/metals co-doped graphenes: designing strategies for supercapacitor electrodes

Abstract: In this paper, we investigated the quantum capacitance and the integrated charge of graphene-based materials based on first-principles density functional theory calculations. Transition metals doped in a graphene plane will not move out of the graphene plane and will finally form an accurate symmetrical structure. Meanwhile, for transition metals doped out of plane, regardless of the positions that the metals were placed, they will conform to the same stable M–MV complexes eventually. Meanwhile, the introduction of N atoms makes the Al atom hybridization state tend to be consistent. Al3–NNN (the Al-embedded, nitrogen doped monovacancy graphene-based complexes, and the number of N atoms represents the number of N atoms surrounding the Al) and Al4–NNNN (Al-embedded, nitrogen doped divacancy graphene-based complexes) have the same Al atom hybridization, but it is different in the Al doped monovacancy graphene complexes (Al–MV) and the Al doped divacancy graphene complexes (Al–DV). In addition, we also found that both the Sc doped monovacancy graphene and the Al4–N (Al-embedded, nitrogen doped divacancy graphene-based complexes) can be used as candidates for the negative electrode materials of asymmetric supercapacitors. The results can provide valuable insights into the design and preparation of electrodes for high-performance supercapacitors.

TCAD Simulations of graphene field-effect transistors based on the quantum capacitance effect

Abstract: In this paper, the results of a comparative study between experimental measurements and technology computer-aided design (TCAD) simulations of graphene field-effect transistors (GFET) are presented. Our simulations were performed to study the electrical properties of few-layer graphene, and the physical approach to the simulation tools is described by using the basics of band theory, Poisson’s equation, the continuity equation and the drift diffusion equations that are suitable for devices with small active regions. A correct formulation of the carrier density was performed to take into account the quantum capacitance. The modeled current was compared to the measured results for a prototype and was shown to be accurate and to have a predictive behavior.

DFT Modeling of Bulk-Modulated Carbon Nanotube Field-Effect Transistors

Abstract: We report density-functional theory (DFT) atomistic simulations of the nonequilibrium transport properties of carbon nanotube (CNT) field-effect transistors (FETs). Results have been obtained within a self-consistent approach based on the nonequilibrium Green's functions (NEGF) scheme. We show that, as the current modulation mechanism is based on the local screening properties of the nanotube channel, a completely new, negative quantum capacitance regime can be entered by the device. We show how a well-tempered device design can be accomplished in this regime by choosing suitable doping profiles and gate contact parameters. At the same time, we detail the fundamental physical mechanisms underlying the bulk-switching operation, including them in a very practical and accurate model, whose parameters can be easily controlled in order to improve the device performance. The dependence of the nanotube screening properties on the temperature is finally explained by means of a self-consistent temperature analysis