Mechanically transferred large-area Ga2_2O3_3 passivates graphene and suppresses interfacial phonon scattering

We demonstrate a large-area passivation layer for graphene by mechanical transfer of ultrathin amorphous Ga$_2$O$_3$ synthesized on liquid Ga metal. A comparison of temperature-dependent electrical measurements of millimetre-scale passivated and bare graphene on SiO$_2$/Si indicate that the passivated graphene maintains its high field effect mobility desirable for applications. Surprisingly, the temperature-dependent resistivity is reduced in passivated graphene over a range of temperatures below 220 K, due to the interplay of screening of the surface optical phonon modes of the SiO$_2$ by high-dielectric-constant Ga$_2$O$_3$, and the relatively high characteristic phonon frequencies of Ga$_2$O$_3$. Raman spectroscopy and electrical measurements indicate that Ga$_2$O$_3$ passivation also protects graphene from further processing such as plasma-enhanced atomic layer deposition of Al$_2$O$_3$.Comment: Journal article, 10 pages, 4 figure

Non-Drude THz conductivity of graphene due to structural distortions

The remarkable electrical, optical and mechanical properties of graphene make it a desirable material for electronics, optoelectronics and quantum applications. A fundamental understanding of the electrical conductivity of graphene across a wide frequency range is required for the development of such technologies. In this study, we use terahertz (THz) time-domain spectroscopy to measure the complex dynamic conductivity of electrostatically gated graphene, in a broad $\sim$0.1 - 7 THz frequency range. The conductivity of doped graphene follows the conventional Drude model, and is predominantly governed by intraband processes. In contrast, undoped charge-neutral graphene exhibits a THz conductivity that significantly deviates from Drude-type models. Via quantum kinetic equations and density matrix theory, we show that this discrepancy can be explained by additional interband processes, that can be exacerbated by electron backscattering. We propose a mechanism where such backscattering -- which involves flipping of the electron pseudo-spin -- is mediated by the substantial vector scattering potentials that are associated with structural deformations of graphene. Our findings highlight the significant impact that structural distortions and resulting electrostatic vector scattering potentials can have on the THz conductivity of charge-neutral graphene. Our results emphasise the importance of the planar morphology of graphene for its broadband THz electronic response.Comment: 74 pages, 21 figure

Nanomechanical sensing using spins in diamond

Nanomechanical sensors and quantum nanosensors are two rapidly developing technologies that have diverse interdisciplinary applications in biological and chemical analysis and microscopy. For example, nanomechanical sensors based upon nanoelectromechanical systems (NEMS) have demonstrated chip-scale mass spectrometry capable of detecting single macromolecules, such as proteins. Quantum nanosensors based upon electron spins of negatively-charged nitrogen-vacancy (NV) centers in diamond have demonstrated diverse modes of nanometrology, including single molecule magnetic resonance spectroscopy. Here, we report the first step towards combining these two complementary technologies in the form of diamond nanomechanical structures containing NV centers. We establish the principles for nanomechanical sensing using such nano-spin-mechanical sensors (NSMS) and assess their potential for mass spectrometry and force microscopy. We predict that NSMS are able to provide unprecedented AC force images of cellular biomechanics and to, not only detect the mass of a single macromolecule, but also image its distribution. When combined with the other nanometrology modes of the NV center, NSMS potentially offer unparalleled analytical power at the nanoscale.Comment: Errors in the stress susceptibility parameters present in the original arXiv version have been correcte

Structure of Chimpanzee Gut Microbiomes across Tropical Africa

Understanding variation in host-associated microbial communities is important given the relevance of microbiomes to host physiology and health. Using 560 fecal samples collected from wild chimpanzees (Pan troglodytes) across their range, we assessed how geography, genetics, climate, vegetation, and diet relate to gut microbial community structure (prokaryotes, eukaryotic parasites) at multiple spatial scales. We observed a high degree of regional specificity in the microbiome composition, which was associated with host genetics, available plant foods, and potentially with cultural differences in tool use, which affect diet. Genetic differences drove community composition at large scales, while vegetation and potentially tool use drove within-region differences, likely due to their influence on diet. Unlike industrialized human populations in the United States, where regional differences in the gut microbiome are undetectable, chimpanzee gut microbiomes are far more variable across space, suggesting that technological developments have decoupled humans from their local environments, obscuring regional differences that could have been important during human evolution.Additional co-authors: Heather Cohen, Charlotte Coupland, Tobias Deschner, Villard Ebot Egbe, Annemarie Goedmakers, Anne-Céline Granjon, Cyril C. Grueter, Josephine Head, R. Adriana Hernandez-Aguilar, Sorrel Jones, Parag Kadam, Michael Kaiser, Juan Lapuente, Bradley Larson, Sergio Marrocoli, David Morgan, Badru Mugerwa, Felix Mulindahabi, Emily Neil, Protais Niyigaba, Liliana Pacheco, Alex K. Piel, Martha M. Robbins, Aaron Rundus, Crickette M. Sanz, Lilah Sciaky, Douglas Sheil, Volker Sommer, Fiona A. Stewart, Els Ton, Joost van Schijndel, Virginie Vergnes, Erin G. Wessling, Roman M. Wittig, Yisa Ginath Yuh, Kyle Yurkiw, Klaus Zuberbühler, Jan F. Gogarten, Anna Heintz-Buschart, Alexandra N. Muellner-Riehl, Christophe Boesch, Hjalmar S. Kühl, Noah Fierer, Mimi Arandjelovic, Robert R. Dun

Modeling fuel treatment impacts on fire suppression cost savings: A review

High up-front costs and uncertain return on investment make it difficult for land managers to economically justify large-scale fuel treatments, which remove trees and other vegetation to improve conditions for fire control, reduce the likelihood of ignition, or reduce potential damage from wildland fire if it occurs. In the short-term, revenue from harvested forest products can offset treatment costs and broaden opportunities for treatment implementation. Increasingly, financial analysis of fuel treatments is also incorporating long-term savings through reduced fire suppression costs, which can be difficult to quantify. This paper reviews the findings and lessons from recent modeling work evaluating the potential relationship between fuel treatments and avoided fire suppression costs. Across studies, treatments are generally predicted to reduce future fire suppression costs, although the magnitude of savings is unlikely to fully offset fuel treatment costs. This funding gap highlights the importance of forest product revenues in facilitating landscape-scale treatment. Factors influencing the effects of fuel treatment investments on fire suppression costs include the causal pathway linking treatment inputs to suppression cost outcomes; the spatiotemporal uncertainty of wildfire-treatment interactions; and the scale of fuel treatment programs

Dirac Fermions at Interfaces

Investigating the electronic states of 2D Dirac surfaces, and their interaction at interfaces with other materials (magnetic, dielectric). These surfaces exist in materials like graphene, the layered material that makes up a graphite pencil, and a new class of materials called topological insulators that are like metallic wrapping of a chooclate block. By interfacing these materials with other layers, new enhanced phases of electronic behavior can emerge. I demonstrate that adding layers to graphene adds "good vibrations", and can reduce overall vibrational resistance

A global survey of mycobacterial diversity in soil

Mycobacteria are a diverse bacterial group ubiquitous in many soil and aquatic environments. Members of this group have been associated with human and other animal diseases, including the nontuberculous mycobacteria (NTM), which are of growing relevance to public health worldwide. Although soils are often considered an important source of environmentally acquired NTM infections, the biodiversity and ecological preferences of soil mycobacteria remain largely unexplored across contrasting climates and ecosystem types. Using a culture-independent approach by combining 16S rRNA marker gene sequencing with mycobacterium-specific hsp65 gene sequencing, we analyzed the diversity, distributions, and environmental preferences of soil-dwelling mycobacteria in 143 soil samples collected from a broad range of ecosystem types. The surveyed soils harbored highly diverse mycobacterial communities that span the full extent of the known mycobacterial phylogeny, with most soil mycobacteria (97% of mycobacterial clades) belonging to previously undescribed lineages. While mycobacteria tended to have higher relative abundances in cool, wet, and acidic soil environments, several individual mycobacterial clades had contrasting environmental preferences. We identified the environmental preferences of many mycobacterial clades, including the clinically relevant Mycobacterium avium complex that was more commonly detected in wet and acidic soils. However, most of the soil mycobacteria detected were not closely related to known pathogens, calling into question previous assumptions about the general importance of soil as a source of NTM infections. Together, this work provides novel insights into the diversity, distributions, and ecological preferences of soil mycobacteria and lays the foundation for future efforts to link mycobacterial phenotypes to their distributions

Economics of Wildfire ManagementThe Development and Application of Suppression Expenditure Models /

IX, 71 p. 11 illus.online resource

Passivating Graphene and Suppressing Interfacial Phonon Scattering with Mechanically Transferred Large-Area Ga₂O₃

We demonstrate a large-area passivation layer for graphene by mechanical transfer of ultrathin amorphous Ga2O3 synthesized on liquid Ga metal. A comparison of temperature-dependent electrical measurements of millimeter-scale passivated and bare graphene on SiO2/Si indicates that the passivated graphene maintains its high field effect mobility desirable for applications. Surprisingly, the temperature-dependent resistivity is reduced in passivated graphene over a range of temperatures below 220 K, due to the interplay of screening of the surface optical phonon modes of the SiO2 by high-dielectric-constant Ga2O3 and the relatively high characteristic phonon frequencies of Ga2O3. Raman spectroscopy and electrical measurements indicate that Ga2O3 passivation also protects graphene from further processing such as plasma-enhanced atomic layer deposition of Al2O3