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Freeman, Matthew

Publications and source records attributed to Freeman, Matthew.

3 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Topological kink plasmons on magnetic-domain boundaries

Two-dimensional topological materials bearing time reversal-breaking magnetic fields support protected one-way edge modes. Normally, these edge modes adhere to physical edges where material properties change abruptly. However, even in homogeneous materials, topology still permits a unique form of edge modes - kink modes - residing at the domain boundaries of magnetic fields within the materials. This scenario, despite being predicted in theory, has rarely been demonstrated experimentally. Here, we report our observation of topologically-protected high-frequency kink modes - kink magnetoplasmons (KMPs) - in a GaAs/AlGaAs two-dimensional electron gas (2DEG) system. These KMPs arise at a domain boundary projected from an externally-patterned magnetic field onto a uniform 2DEG. They propagate unidirectionally along the boundary, protected by a difference of gap Chern numbers in the two domains. They exhibit large tunability under an applied magnetic field or gate voltage, and clear signatures of nonreciprocity even under weak-coupling to evanescent photons.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Crenulation-­1 Flash Report

We briefly report on the first of PHELIX driven Crenulation experiments diagnosed with proton radiography. PHELIX is a 300 kJ capacitor bank located at the LANL LANSCE pRad facility. It is capable of delivering a 4 MA, 10 us current pulse to a low inductance cylindrical load. Using a magnetically driven cylindrical liner, we have shocked a concentric tin cylindrical shell to melt-on-release and compared to theory and calculations. The inner surface of the tin cylinder has three sectors of single-mode perturbations, which are subject to Richtmyer-Meshkov Instability (RMI). Recent theoretical work on the EOS of tin has modified both the Hugoniot and the isentropes for release into various states in tabular data. The new multiphase EOS for tin, SESAME 2161, includes the beta and gamma solid phases as well as a liquid phase. It predicts a lower pressure boundary for release to pure solid (~20 GPa) and a higher pressure boundary for release to pure liquid (~35 GPa) than the existing SESAME 2160 table.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Extended carbon line emission in the galaxy: Searching for dark molecular gas along the G328 sightline

We present spectral data cubes of the [CI] 809 GHz, {sup 12}CO 115 GHz, {sup 13}CO 110 GHz, and HI 1.4 GHz line emission from a ∼1 square degree region along the l=328{sup ∘} (G328) sightline in the Galactic Plane. Emission arises principally from gas in three spiral arm crossings along the sightline. The distribution of emission in the CO and [CI] lines is found to be similar, with the [CI] slightly more extended, and both are enveloped in extensive HI. Spectral line ratios per voxel in the data cubes are found to be similar across the entire extent of the Galaxy. However, toward the edges of the molecular clouds the [CI]/{sup 13}CO and {sup 12}CO/{sup 13}CO line ratios rise by ∼50%, and the [CI]/HI ratio falls by ∼10%. We attribute this to these sightlines passing predominantly through the surfaces of photodissociation regions (PDRs), where the carbon is found mainly as C or C{sup +}, while the H{sub 2} is mostly molecular, and the proportion of atomic gas also increases. We undertake modeling of the PDR emission from low density molecular clouds excited by average interstellar radiation fields and cosmic-ray ionization to quantify this comparison, finding that depletion of sulfur and reduced PAH abundance is needed to match line fluxes and ratios. Roughly one-third of the molecular gas along the sightline is found to be associated with this surface region, where the carbon is largely not to be found in CO. Approximately 10% of the atomic hydrogen along the sightline is cold gas within PDRs.

79 ASTRONOMY AND ASTROPHYSICS↗