arXiv subjects

Randolph E. Elmquist

Publications and source records attributed to Randolph E. Elmquist.

18 recordsLinked to original sources

arXiv2025

A unified realization of electrical quantities from the quantum International System of Units

In the revised International System of Units (SI), the ohm and the volt are realized from the von Klitzing constant and the Josephson constant, and a practical realization of the ampere is possible by applying Ohm's law directly to the quantum Hall and Josephson effects. As a result, it is possible to create an instrument capable of realizing all three primary electrical units, but the development of such a system remains challenging. Here we report a unified realization of the volt, ohm, and ampere by integrating a quantum anomalous Hall resistor (QAHR) and a programmable Josephson voltage standard (PJVS) in a single cryostat. Our system has a quantum voltage output that ranges from 0.24 mV to 6.5 mV with combined relative uncertainties down to 3 $μ$V/V. The QAHR provides a realization of the ohm at zero magnetic field with uncertainties near 1 $μΩ$/$Ω$. We use the QAHR to convert a longitudinal current to a quantized Hall voltage and then directly compare that against the PJVS to realize the ampere. We determine currents in the range of 9.33 nA to 252 nA, and our lowest uncertainty is 4.3 $μ$A/A at 83.9 nA. For other current values, a systematic error that ranges from -10 $μ$A/A to -30 $μ$A/A is present due to the imperfect isolation of the PJVS microwave bias.

cond-mat.mes-hall↗

arXiv2024

Constructing 100 MΩ and 1 GΩ Resistance Standards via Star-Mesh Transformations

A recent mathematical framework for optimizing resistor networks to achieve values in the MΩ through GΩ levels was employed for two specific cases. Objectives here include proof of concept and identification of possible apparatus limitations for future experiments involving graphene-based quantum Hall array resistance standards. Using fractal-like, or recursive, features of the framework allows one to calculate and implement network designs with substantially lower-valued resistors. The cases of 100 MΩ and 1 GΩ demonstrate that, theoretically, one would not need more than 100 quantum Hall elements to achieve these high resistances.

cond-mat.mes-hall↗

arXiv2023

Fractal-like star-mesh transformations using graphene quantum Hall arrays

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 nu=2 plateau (R_H = 12906.4 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

cond-mat.mes-hall↗

arXiv2023

Star-Mesh Quantized Hall Array Resistance Devices

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.

cond-mat.mes-hall↗

arXiv2022

Fabrication of uniformly doped graphene quantum Hall arrays with multiple quantized resistance outputs

In this work, limiting factors for developing metrologically useful arrays from epitaxial graphene on SiC are lifted with a combination of centimeter-scale, high-quality material growth and the implementation of superconducting contacts. Standard devices for metrology have been restricted to having a single quantized value output based on the $ν$ = 2 Landau level. With the demonstrations herein of devices having multiple outputs of quantized values available simultaneously, these versatile devices can be used to disseminate the ohm globally. Such devices are designed to give access to quantized resistance values over the range of three orders of magnitude, starting as low as the standard value of approximately 12.9 k$Ω$ and reaching as high as 1.29 M$Ω$. Several experimental methods are used to assess the quality and versatility of the devices, including standard lock-in techniques and Raman spectroscopy.

cond-mat.mes-hall↗

arXiv2022

Chromium-Doped Bismuth Antimony Telluride for Future Quantum Hall Resistance Standards

Since 2017, epitaxial graphene has been the base material for the US national standard for resistance. A future avenue of research within electrical metrology is to remove the need for strong magnetic fields, as is currently the case for devices exhibiting the quantum Hall effect. The quantum Hall effect is just one of many research endeavours that revolve around recent quantum physical phenomena like composite fermions, charge density waves, and topological properties [1-2]. New materials, like magnetically doped topological insulators (MTIs), offer access to the quantum anomalous Hall effect, which in its ideal form, could become a future resistance standard needing only a small permanent magnet to activate a quantized resistance value [3-5]. Furthermore, these devices could operate at zero-field for measurements, making the dissemination of the ohm more economical and portable. Here we present results on precision measurements of the h/e2 quantized plateau of Cr-Doped (BixSb1-x)2Te3 and give them context by comparing them to modern graphene-based resistance standards. Ultimately, MTI-based devices could be combined in a single system with magnetic-field-averse Josephson voltage standards to obtain an alternative quantum current standard.

cond-mat.mes-hall↗

arXiv2022

Timescales for Nitric Acid Desorption in Epitaxial Graphene Devices

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 post-exposure were performed with transport temperatures between 300 K and 1.5 K. Ambient conditions are applied to non-transport 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.

cond-mat.mes-hall↗

arXiv2022

Spectroscopic assessment of short-term nitric acid doping of epitaxial graphene

This work reports information on the transience of hole doping in epitaxial graphene devices when nitric acid is used as an adsorbent. Under vacuum conditions, desorption processes are monitored by electrical and spectroscopic means to extract the relevant timescales from the corresponding data. It is of vital importance to understand the reversible nature of hole doping because such device processing can be a suitable alternative to large-scale, metallic gating. Most measurements are performed post-exposure at room temperature, and, for some electrical transport measurements, at 1.5 K. Vacuum conditions are applied to many measurements to replicate the laboratory conditions under which devices using this doping method would be measured. The relevant timescales from transport measurements are compared with results from X-ray photoelecton spectroscopy and Fourier transform infrared spectroscopy measurements, with the latter performed at ambient conditions and accompanied by calculations of the spectra in the Reststrahlen band.

cond-mat.mes-hall↗

arXiv2022

Fabrication of quantum Hall p-n junction checkerboards

Measurements of fractional multiples of the ν=2 plateau quantized Hall resistance (R_H {\approx} 12906 Ω) were enabled by the utilization of multiple current terminals on millimetre-scale graphene p-n junction devices fabricated with interfaces along both lateral directions. These quantum Hall resistance checkerboard devices have been demonstrated to match quantized resistance outputs numerically calculated with the LTspice circuit simulator. From the devices' functionality, more complex embodiments of the quantum Hall resistance checkerboard were simulated to highlight the parameter space within which these devices could operate. Moreover, these measurements suggest that the scalability of p-n junction fabrication on millimetre or centimetre scales is feasible with regards to graphene device manufacturing by using the far more efficient process of standard ultraviolet lithography.

cond-mat.mes-hall↗

arXiv2022

Algorithm for constructing customized quantized resistances in graphene $p-n$ junctions

An algorithm is introduced for predicting quantized resistances in graphene p-n junction devices that utilize more than a single entry and exit point for electron flow. Depending on the configuration of an arbitrary number of terminals, electrical measurements yield fractional multiples of the typical quantized Hall resistance at the $ν=2$ plateau $R_H \approx 12906 Ω$ and take the form: $\frac{a}{b}R_H$. This theoretical formulation is independent of material, and applications to other material systems that exhibit quantum Hall behaviors are to be expected. Furthermore, this formulation is supported with experimental data from graphene-based devices with multiple source and drain terminals.

cond-mat.mes-hall↗

arXiv2022

Designs for programmable quantum resistance standards based on epitaxial graphene p-n junctions

We report the fabrication and measurement of top gated epitaxial graphene p-n junctions where exfoliated hexagonal boron nitride (h-BN) is used as the gate dielectric. The four-terminal longitudinal resistance across a single junction is well quantized at the von Klitzing constant R_K with a relative uncertainty of 10-7. After the exploration of numerous parameter spaces, we summarize the conditions upon which these devices could function as potential resistance standards. Furthermore, we offer designs of programmable electrical resistance standards over six orders of magnitude by using external gating.

cond-mat.mes-hall↗

arXiv2021

Crystalline Formations of NbN/4H-SiC Heterostructure Interfaces

Given the importance of incorporating various superconducting materials to device fabrication or substrate development, studying the interface for possible interactions is warranted. In this work, NbN films sputter-deposited on 4H-SiC were heat-treated at 1400 C and 1870 C and were examined with transmission electron microscopy to assess whether the interfacial interactions undergo temperature-dependent behavior. We report the diffusion of NbN into the SiC substrate and the formation of NbN nanocrystallites therein during the 1400 C treatment. After the 1870 C treatment, tiered porosity and the formation of voids are observed, likely due to catalytic reactions between the two materials and accelerated by the stresses induced by the differences in the materials' coefficients of thermal expansion. Lastly, Raman spectroscopy is employed to gain an understanding of the interface lattices' optical responses.

physics.app-ph↗

arXiv2018

Confocal laser scanning microscopy: A tool for rapid optical characterization of 2D materials

Confocal laser scanning microscopy (CLSM) is a non-destructive, highly-efficient optical characterization method for large-area analysis of graphene on different substrates, which can be applied in ambient air, does not require additional sample preparation, and is insusceptible to surface charging and surface contamination. CLSM leverages optical properties of graphene and provides greatly enhanced optical contrast and mapping of thickness down to a single layer. We demonstrate the effectiveness of CLSM by measuring mechanically exfoliated and chemical vapor deposition graphene on Si/SiO2, and epitaxial graphene on SiC. In the case of graphene on Si/SiO2, both CLSM intensity and height mapping is powerful for analysis of 1-5 layers of graphene. For epitaxial graphene on SiC substrates, the CLSM intensity allows us to distinguish features such as dense, parallel 150 nm wide ribbons of graphene (associated with the early stages of the growth process) and large regions covered by the interfacial layer and 1-3 layers of graphene. In both cases, CLSM data shows excellent correlation with conventional optical microscopy, atomic force microscopy, Kelvin probe force microscopy, conductive atomic force microscopy, scanning electron microscopy and Raman mapping, with a greatly reduced acquisition time. We demonstrate that CLSM is an indispensable tool for rapid analysis of mass-produced graphene and is equally relevant to other 2D materials.

physics.app-ph↗

arXiv2017

Rapid characterization of wafer-scale 2D material: Epitaxial graphene and graphene nanoribbons on SiC

We demonstrate that the confocal laser scanning microscopy (CLSM) provides a non-destructive, highly-efficient characterization method for large-area epitaxial graphene and graphene nanostructures on SiC substrates, which can be applied in ambient air without sample preparation and is insusceptible to surface charging or surface contamination. Based on the variation of reflected intensity from regions covered by interfacial layer, single layer, bilayer, or few layer graphene, and through the correlation to the results from Raman spectroscopy and SPM, CLSM images with a high resolution (around 150 nm) reveal that the intensity contrast has distinct feature for undergrown graphene (mixing of dense, parallel graphene nanoribbons and interfacial layer), continuous graphene, and overgrown graphene. Moreover, CLSM has a real acquisition time hundreds of times faster per unit area than the supplementary characterization methods. We believe that the confocal laser scanning microscope will be an indispensable tool for mass-produced epitaxial graphene or applicable 2D materials.

physics.app-ph↗

arXiv2017

Part-per-million quantization and current-induced breakdown of the quantum anomalous Hall effect

In the quantum anomalous Hall effect, quantized Hall resistance and vanishing longitudinal resistivity are predicted to result from the presence of dissipationless, chiral edge states and an insulating 2D bulk, without requiring an external magnetic field. Here, we explore the potential of this effect in magnetic topological insulator thin films for metrological applications. Using a cryogenic current comparator system, we measure quantization of the Hall resistance to within one part per million and longitudinal resistivity under 10 m$Ω$ per square at zero magnetic field. Increasing the current density past a critical value leads to a breakdown of the quantized, low-dissipation state, which we attribute to electron heating in bulk current flow. We further investigate the pre-breakdown regime by measuring transport dependence on temperature, current, and geometry, and find evidence for bulk dissipation, including thermal activation and possible variable-range hopping.

cond-mat.mes-hall↗

arXiv2016

Epitaxial graphene homogeneity and quantum Hall effect in millimeter-scale devices

Quantized magnetotransport is observed in 5.6 x 5.6 mm^2 epitaxial graphene devices, grown using highly constrained sublimation on the Si-face of SiC(0001) at high temperature (1900 °C). The precise quantized Hall resistance of Rxy = h/2e^2 is maintained up to record level of critical current Ixx = 0.72 mA at T = 3.1 K and 9 T in a device where Raman microscopy reveals low and homogeneous strain. Adsorption-induced molecular doping in a second device reduced the carrier concentration close to the Dirac point(n ~ 1E10 (1/cm^2)), where mobility of 43700 cm^2/Vs is measured over an area of 10 mm^2. Atomic force, confocal optical, and Raman microscopies are used to characterize the large-scale devices, and reveal improved SiC terrace topography and the structure of the graphene layer. Our results show that the structural uniformity of epitaxial graphene produced by face-to-graphite processing contributes to millimeter-scale transport homogeneity, and will prove useful for scientific and commercial applications.

cond-mat.mtrl-sci↗

arXiv2016

Floating up of the zero-energy Landau level in monolayer epitaxial graphene

We report on magneto-transport measurements on low-density, large-area monolayer epitaxial graphene devices grown on SiC. We show that the zero-energy Landau level (LL) in monolayer graphene, which is predicted to be magnetic field ($B$)-independent, can float up above the Fermi energy at low $B$. This is supported by the temperature ($T$)-driven flow diagram approximated by the semi-circle law as well as the $T$-independent point in the Hall conductivity $σ_{xy}$ near $e^2/h$. Our experimental data are in sharp contrast to conventional understanding of the zeroth LL and metallic-like behavior in pristine graphene prepared by mechanical exfoliation at low $T$. This surprising result can be ascribed to substrate-induced sublattice symmetry breaking which splits the degeneracy of the zeroth Landau level. Our finding provides a unified picture regarding the metallic behavior in pristine graphene prepared by mechanical exfoliation, and the insulating behavior and the insulator-quantum Hall transition in monolayer epitaxial graphene.

cond-mat.dis-nn↗

arXiv2014

Low Carrier Density Epitaxial Graphene Devices On SiC

Monolayer epitaxial graphene (EG) grown on hexagonal Si-terminated SiC substrates is intrinsically electron-doped (carrier density is about 10^13 cm^(-2)). We demonstrate a clean device fabrication process using a precious-metal protective layer, and show that etching with aqua regia results in p-type (hole) molecular doping of our un-gated, contamination-free EG. Devices fabricated by this simple process can reach a carrier density in the range of 10^10 cm^(-2) to 10^11 cm^(-2) with mobility about 8000 cm^2/V/s or higher. In a moderately doped device with a carrier density n = 2.4 x 10^11 cm^(-2) and mobility = 5200 cm^2/V/s, we observe highly developed quantized Hall resistance plateaus with filing factor of 2 at magnetic field strengths of less than 4 T. Doping concentrations can be restored to higher levels by heat treatment in Ar, while devices with both p-type and n-type majority carriers tend to drift toward lower carrier concentrations in ambient air.

cond-mat.mes-hall↗