Heather M. Hill

Ultraviolet radiation is becoming increasingly popular for applications such as germicidal disinfection and photo-polymer additive manufacturing. However, calibrations of UV detectors have been limited by relative expanded uncertainties typically larger than 2% and a lack of National Measurement Institutes that measure wavelengths from 200 nm to 250 nm because of the difficulty working in that range. The National Institute of Standards and Technology (NIST) has established a new facility that achieves diode-to-diode spectral power responsivity transfers with uncertainties of 0.1% or better across the 200 nm to 400 nm wavelength range and a relative expanded uncertainty ( k =2) of less than 1%. The Ultraviolet Spectral Comparator Facility (UVSCF) provides spectral power and spectral irradiance responsivity calibrations of detectors and filter radiometers from 200 nm to 1800 nm. Its low UV transfer uncertainties derive from a combination of common-mode rejection, nitrogen purging the ozone from the system, a careful selection of mirrors, and strategic usage of broadband optical radiation sources, including a laser-driven light source and a deuterium lamp. The measurement stability over long time periods enables spatial mapping of detectors’ responsivities using calibrated x-y motion stages, which allows the facility to directly calibrate the spectral irradiance responsivity. Spectral irradiance is a critical measurement in many UV applications, and legacy calibration methods for spectral irradiance require multiple facilities and measurements. Our detector-based spectral irradiance responsivity method determines the spectral irradiance responsivity of silicon carbide detectors with 1% relative expanded uncertainty ( k =2). Those results agreed to within 0.70% with spectrally integrated irradiance measurements using a calibrated deuterium lamp with an independent scale traceability. This novel comparison of UVC scales validates the performance and uncertainties of the UVSCF and the efficacy of scanning spectral irradiance calibrations down to 200 nm.

This work presents a substantial advancement on how one may develop high-resistance quantized Hall array resistance standards (QHARS) by using star-mesh transformations for element count minimization. More specifically, this work introduces a generalized mathematical reconciliation to recover exact effective quantized resistances found by simulation and measurement. Furthermore, this work explores the concept of fractal dimension, clarifying the benefits of both full and partial recursions in QHARS devices. Three different partial recursion cases are visited for a near-1 G Ω QHARS device. These partial recursions, analyzed in the context of their fractal dimensions, offer increased flexibility in accessing desired resistances within a specific neighborhood of values compared to full recursion methods, though at the cost of the number of required elements.

This work introduces a pseudofractal analysis for optimizing high-resistance graphene-based quantized Hall array resistance standards (QHARS). The development of resistance standard device designs through star–mesh transformations is detailed, aimed at minimizing element count. Building on a recent mathematical framework, the approach presented herein refines QHARS device concepts by considering designs incorporating pseudofractals (which may be expressed as star–mesh transformations). To understand how future QHARS pseudofractal designs enable varying sizes of neighborhoods of available quantized resistance, Minkowski–Bouligand algorithms are used to analyze fractal dimensions of the device design topologies. Three distinct partial recursion cases are explored in addition to the original full recursion design, and expressions for their total element counts are derived. These partial recursions, assessed through their fractal dimensions, offer enhanced flexibility in achieving specific resistance values within a desired neighborhood compared to full recursion methods, albeit with an increased number of required elements. The formalisms presented are material-independent, making them broadly applicable to other quantum Hall systems and artifact standards.

The quantum Hall effect (QHE) has enhanced accessibility to measure and disseminate electrical units, owed in part to the recently redefined International System of Units in 2019. Graphene remains one of the preferred options to realize the ohm despite the limitations of high magnetic fields to produce a robust QHE. Topological insulators, on the other hand, show promise in providing quantized resistance via the quantum anomalous Hall effect, a phenomenon that removes the need for magnetic fields during operation. To optimize future devices for metrological applications, it is important to gain a better understanding of magnetically doped topological insulators like Cr-doped bismuth antimony telluride. The application of differential conductivity space analyses offers a more sensitive way to analyze the data and distinguish between 2D and 3D transport behaviors. This is particularly important in thin films, where the transition between 2D and 3D behavior can be subtle. The ability to confidently determine the dimensionality of the transport is crucial for selecting appropriate theoretical models for future device optimization. Furthermore, this work identifies variable range hopping as the dominant transport mechanism in the 2D regime using a rigorous statistical analysis (via the Bayes factor). These elements assist in the understanding of microscopic processes that govern charge transport in these materials.

A mathematical approach is adopted for optimizing the number of total device elements required for obtaining high effective quantized resistances in graphene-based quantum Hall array devices. This work explores an analytical extension to the use of star–mesh transformations such that fractal-like, or recursive, device designs can yield high enough resistances (like 1 EΩ, arguably the highest resistance with meaningful applicability) while still being feasible to build with modern fabrication techniques. Epitaxial graphene elements are tested, whose quantized Hall resistance at the ν=2 plateau (RH≈ 12 906.4 Ω) becomes the building block for larger effective, quantized resistances. It is demonstrated that, mathematically, one would not need more than 200 elements to achieve the highest pertinent resistances.

Extremely low temperatures have limited the availability and accuracy of experimental thermophysical property measurements for cryogens, particularly transport properties. Traditional scaling techniques, such as the theory of corresponding states, have long been known to be inaccurate for fluids with strong quantum effects. To address this need, this paper investigates how quantum effects impact thermodynamics and momentum transfer (shear viscosity) in the fluid phases of hydrogen, deuterium, and neon. We utilize experimental viscosity measurements and reference empirical equations of state to show that conventional entropy scaling is inadequate for quantum-dominated systems. We then provide a simple empirical correction to entropy scaling based on the ratio of quantum to packing length scale that accounts for the deviations.

Advances in the development of graphene-based technology have enabled improvements in DC resistance metrology. Devices made from epitaxially grown graphene have replaced the GaAs-based counterparts, leading to an easier and more accessible realization of the ohm. By optimizing the scale of the growth, it has become possible to fabricate quantized Hall array resistance standards (QHARS) with nominal values between 1 kΩ and 1.29 MΩ. One of these QHARS device designs accommodates a value of about 1.01 MΩ, which made it an ideal candidate to pursue a proof-of-concept that graphene-based QHARS devices are suitable for forming wye-delta resistance networks. In this work, the 1.01 MΩ array output nearly 20.6 MΩ due to the wye-delta transformation, which itself is a special case of star-mesh transformations. These mathematical equivalence principles allow one to extend the QHR to the 100 MΩ and 10 GΩ resistance levels with fewer array elements than would be necessary for a single array with many more elements in series. The 1.01 MΩ device shows promise that the wye-delta transformation can shorten the calibration chain, and, more importantly, provide a chain with a more direct line to the quantum SI.

We have demonstrated the fabrication of both armchair and zigzag epitaxial graphene nanoribbon (GNR) devices on 4H–SiC using a polymer-assisted sublimation growth method. The phenomenon of terrace step formation has traditionally introduced the risk of GNR deformation along sidewalls, but a polymer-assisted sublimation method helps mitigate this risk. Each type of 50 nm wide GNR is examined electrically and optically (armchair and zigzag), with the latter method being a check on the quality of the GNR devices and the former using alternating current to investigate resistance attenuation from frequencies above 100 Hz. Rates of attenuation are determined for each type of GNR device, revealing subtle suggested differences between armchair and zigzag GNRs.

This work reports the dynamics of transient hole doping in epitaxial graphene devices by using nitric acid as an adsorbent. The timescales associated with corresponding desorption processes are extracted from the data. The understanding of reversible hole doping without gating is of crucial importance to those fabricating devices with a particular functionality. Measurements of the electrical and optical properties of several devices postexposure were performed with transport temperatures between 300 and 1.5 K. Ambient conditions are applied to nontransport measurements to replicate the most likely laboratory conditions for handling devices using this doping method. The relevant timescales from transport measurements are compared with results from Raman spectroscopy measurements.