Session C02: Condensed Matter Physics

The Effect of RF Sputter Deposition on MgF2 Thin Films

Optical and physical properties of magnesium fluoride (MgF2) thin films were investigated. The films were grown on glass microscope slides using radio frequency magnetron sputter deposition. Thickness of the thin films were determined using data from x-ray reflectivity measurements. The stoichiometric ratios of the films were confirmed using measurements from energy dispersive x-ray spectroscopy. Values for the reflection and transmission coefficients were calculated from direct measurements of the intensities of a helium neon laser beam through the films and substrates. After preliminary measurements of physical properties, several films were annealed to various temperatures and the measurements were retaken. The relationship between thickness and optical properties was investigated, but the results were inconclusive due to the presence of absorption of light in the thin films, suggesting impurities in the film. These results will lead to a greater understanding of the effect of sputter deposition on optical properties of anti-reflective coatings, but further work will be necessary to minimize the impurities of the film and the light being absorbed.   

Analyzing Nanoscale Thermal Transport Using Time-resolved X-ray Diffraction

Classical models of thermal transport breakdown at lengthscales below a few microns in many materials. Time-resolved x-ray diffraction has been proposed as one method to investigate this regime of nanoscale thermal transport, especially inside semiconductor materials where other techniques can not penetrate or yield quantitative results. We benchmarked a new, portable, and fast open-source x-ray dynamical diffraction code (\emph{TRXD}) for strained crystals developed by DePaul University against an existing standard server-based closed-source calculation tool (\emph{GID\_SL}, Grazing Incidence Diffraction for Superlattices). \emph{TRXD} is also validated against experimental x-ray peak lineshapes by convolving the calculation results with an appropriate instrumentation resolution function. \emph{TRXD} is shown to properly predict the long time-scale classical thermal behavior of a cooling semiconductor, while revealing discrepancies at the short time-scale where new nanoscale thermal transport models are under development. A new high-resolution x-ray diffraction data set is compared to a previously published low-resolution data set, and found to give the same result for delayed thermal transport in ultrafast laser-excited 100 nm metal film on a Gallium Arsenide crystal substrate.   

First Demonstration of AI-assisted automation of single crystal neutron diffraction

Single-crystal neutron diffraction experiments can provide insight into a material's atomic structure and the origin of a material's properties. Current methods of analyzing data from these experiments rely on Bragg peak recognition, signal extraction and multiple codes' executions. This type of analysis is time-consuming and inefficient. Automated real-time analysis of the images and a common coding language would greatly increase the efficiency of single-crystal neutron diffraction experiments. We present the first demonstration of machine-learning-assisted automated single-crystal neutron diffraction experiments at Oak Ridge National Laboratory. Real-time analysis will optimize the use of neutron beam time and more precisely reduce the data.~We plan to integrate our demonstration into real-time analysis methods which will become the new analysis standard at the neutron-scattering user facility at Oak Ridge National Laboratory.   

A ruthenium oxide thermometer for dilution refrigerators operating down to 5 mK

At the lowest temperatures achieved in dilution refrigerators, ruthenium oxide resistance thermometers often saturate and therefore lose their sensitivity. In an effort to extend the range of such temperature sensors, we built a thermometer which maintains sensitivity to 5 mK. A key component of this thermometer is an in situ radio frequency filter which is based on a modern rf absorption material. We show that the use of such a filter is only effective when it is encased in the same rf-tight enclosure as the ruthenium oxide sensor. Our design delivers an attenuation level that is necessary to mitigate the effects of parasitic heating of a fraction of pW present in our circuit. Furthermore, we show that the likely origin of this parasitic heating is the black body radiation present within the experimental space of the refrigerator.   

Breakdown of the nu $=$ 1 integer quantum Hall state in a high mobility sample

The integer quantum Hall Wigner solid was recently observed in high mobility GaAs two-dimensional electron gas samples near filling factor nu $=$ 1. We performed large signal current-voltage characteristic measurements in the region of this phase. We observed well-defined breakdown behavior in the regions of both the integer quantum Hall Wigner solid and the Anderson insulator. To our surprise, we find that the critical current exhibits a monotonic dependence as the filling factor moves away from the center of the nu $=$ 1 plateau, even in the region of the Wigner solid. Therefore, it appears that the breakdown in the current-voltage characteristics measured along the nu $=$ 1 integer quantum Hall plateau does not differentiate the reentrant integer quantum Hall Wigner solid from the Anderson insulator. *The work at Purdue was supported by the US DOE, Office of Basic Energy Sciences under the award DE-SC0006671. Sample growth efforts at Princeton University were supported by the National Science Foundation MRSEC Grant DMR-1420541 and the Gordon and Betty Moore Foundation Grant GBMF 4420.