Temperature Dependent Mean Free Path Spectra of Thermal Phonons Along the c-axis of Graphite

Heat conduction in graphite has been studied for decades because of its exceptionally large thermal anisotropy. While the bulk thermal conductivities along the in-plane and cross-plane directions are well known, less understood are the microscopic properties of the thermal phonons responsible for heat conduction. In particular, recent experimental and computational works indicate that the average phonon mean free path (MFP) along the c-axis is considerably larger than that estimated by kinetic theory, but the distribution of MFPs remains unknown. Here, we report the first quantitative measurements of c-axis phonon MFP spectra in graphite at a variety of temperatures using time-domain thermoreflectance measurements of graphite flakes with variable thickness. Our results indicate that c-axis phonon MFPs have values of a few hundred nanometers at room temperature and a much narrower distribution than in isotropic crystals. At low temperatures, phonon scattering is dominated by grain boundaries separating crystalline regions of different rotational orientation. Our study provides important new insights into heat transport and phonon scattering mechanisms in graphite and other anisotropic van der Waals solids

The mirn23a and mirn23b microrna clusters are necessary for proper hematopoietic progenitor cell production and differentiation

Mice deficient for microRNA (miRNA) cluster mirn23a exhibit increased B lymphopoiesis at the expense of myelopoiesis, whereas hematopoietic stem and progenitor cell (HSPC) populations are unchanged. Mammals possess a paralogous mirn23b gene that can give rise to three mature miRNAs (miR-23b, miR-24-1, and miR-27b) that have identical seed/mRNA-targeting sequences to their mirn23a counterparts. To assess whether compound deletion of mirn23a and mirn23b exacerbates the hematopoietic phenotype observed in mirn23a−/− mice, we generated a compound mirn23a−/−mirn23bfl/fl:Mx1-Cre conditional knockout mouse and assayed hematopoietic development after excision of mirn23b. Loss of both genes in adult bone marrow further skewed HSPC differentiation toward B cells at the expense of myeloid cells, demonstrating a dosage-dependent effect on regulating cell differentiation. Strikingly, double-knockout (DKO) mice had decreased bone marrow cellularity with significantly decreased hematopoietic stem cell and HSPC populations, a phenotype not observed in mice deficient for mirn23a alone. Competitive transplantation assays showed decreased contribution of mirn23a−/−mirn23b−/− HSPCs to hematopoietic lineages at 6 and 12 weeks after transplantation. Defects in the proliferation of mirn23a−/−b−/− HSPCs was not observed; however, DKO cells were more apoptotic compared with both wild-type and mirn23a−/− cells. Together, our data show that complete loss of mirn23a/mirn23b miRNAs results in decreased blood production and affects lineage output in a concentration-dependent manner

Coherent control of thermal phonon transport in van der Waals superlattices

van der Waals (vdW) heterostructures are a central focus of materials science and condensed matter physics due to the novel physical phenomena and properties obtained by precisely stacking heterogeneous atomically thin layers. vdW heterostructures are expected to allow for the coherent manipulation of THz lattice vibrations and hence heat conduction due to the ability to precisely control chemical composition at the atomic scale, but little work has focused on thermal transport in these materials. Here, we report an ab initio study of thermal transport in vdW superlattices consisting of alternating transition metal dichalcogenide atomic layers. Our calculations show that the lattice vibrational spectrum and scattering rates can be precisely manipulated by the choice of each atomically thin layer, resulting in materials with novel properties such as large thermal anisotropies approaching 200 and ultralow cross-plane thermal conductivities comparable to those of amorphous materials. Our work demonstrates how coherent manipulation of phonons in vdW superlattices can expand the property space beyond that occupied by natural materials and suggests an experimental route to realize these properties

Elastic and thermal properties of free-standing molybdenum disulfide membranes measured using ultrafast transient grating spectroscopy

Molybdenum disulfide (MoS_2), a member of transition-metal dichalcogenide family, is of intense interest due to its unique electronic and thermoelectric properties. However, reports of its in-plane thermal conductivity vary due to the difficulty of in-plane thermal conductivity measurements on thin films, and an experimental measurement of the in-plane sound velocity has not been reported. Here, we use time-resolved transient grating spectroscopy to simultaneously measure the in-plane elastic and thermal properties of free-standing MoS_2 membranes at room temperature. We obtain a longitudinal acoustic phonon velocity of 7000 ± 40 m s^(−1) and an in-plane thermal conductivity of 74 ± 21 W m^(−1)K^(−1). Our measurements provide useful insights into the elastic and thermal properties of MoS_2 and demonstrate the capability of transient grating spectroscopy to investigate the in-plane vibrational properties of van der Waals materials that are challenging to characterize with conventional methods

High frequency atomic tunneling yields ultralow and glass-like thermal conductivity in chalcogenide single crystals

Crystalline solids exhibiting glass-like thermal conductivity have attracted substantial attention both for fundamental interest and applications such as thermoelectrics. In most crystals, the competition of phonon scattering by anharmonic interactions and crystalline imperfections leads to a non-monotonic trend of thermal conductivity with temperature. Defect-free crystals that exhibit the glassy trend of low thermal conductivity with a monotonic increase with temperature are desirable because they are intrinsically thermally insulating while retaining useful properties of perfect crystals. However, this behavior is rare, and its microscopic origin remains unclear. Here, we report the observation of ultralow and glass-like thermal conductivity in a hexagonal perovskite chalcogenide single crystal, BaTiS₃, despite its highly symmetric and simple primitive cell. Elastic and inelastic scattering measurements reveal the quantum mechanical origin of this unusual trend. A two-level atomic tunneling system exists in a shallow double-well potential of the Ti atom and is of sufficiently high frequency to scatter heat-carrying phonons up to room temperature. While atomic tunneling has been invoked to explain the low-temperature thermal conductivity of solids for decades, our study establishes the presence of sub-THz frequency tunneling systems even in high-quality, electrically insulating single crystals, leading to anomalous transport properties well above cryogenic temperatures

Revisiting the Historic Distribution and Habitats of the Whooping Crane

Understanding the historic range and habitats of an endangered species can assist in conservation and reintroduction efforts for that species. Individuals reintroduced into a species’ historic core range have a higher survival rate compared to individuals introduced near the periphery or outside the historic range (Falk and Olwell, 1992; Griffith et al., 1989). Individuals on the periphery of a species’ range tend to occupy less favorable habitats and have lower and more variable densities than those near the core of their range (Brown, 1984; Brown et al., 1995, 1996). Such conclusions, however, presume that historic habitats have not changed since a species was extirpated from core areas – a difficult assumption for many areas, and particularly for wetland habitat (Prince, 1997). Many endangered species persist only on the periphery of their historic range because of habitat loss or modification in their core range (Channell and Lomolino, 2000), which can bias our understanding of the species’ habitat preferences. Further, habitat models based on locations where species persist necessarily emphasize local conditions rather than historical conditions (Kuemmerle et al., 2011). For example, habitat models for the European bison (Bison bonasus) suggested it was a woodland species, but assessment of the bison’s historic range indicated it preferred mosaictype landscapes and had a more eastern and northern distribution than previously reported (Kuemmerle et al., 2011, 2012). Hence, accurate determination of the historic range and habitat conditions for endangered species can improve our understanding of their ecology and assist in conservation and reintroduction efforts. Examining the historic range from an ecological perspective can also help identify where appropriate habitat still exists that could sustain a population

Hydrographic changes in the eastern subpolar North Atlantic during the last deglaciation

Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Quaternary Science Reviews 29 (2010): 3336-3345, doi:10.1016/j.quascirev.2010.08.013.Millennial-scale climate fluctuations of the last deglaciation have been tied to abrupt changes in the Atlantic Meridional Overturning Circulation (MOC). A key to understanding mechanisms of MOC collapse and recovery is the documentation of upper ocean hydrographic changes in the vicinity of North Atlantic deep convection sites. Here we present new high-resolution ocean temperature and δ18Osw records spanning the last deglaciation from an eastern subpolar North Atlantic site that lies along the flow path of the North Atlantic Current, approaching deep convection sites in the Labrador and Greenland-Iceland-Norwegian (GIN) Seas. High-resolution temperature and δ18Osw records from subpolar Site 980 help track the movement of the subpolar/subtropical front associated with temperature and Atlantic MOC changes throughout the last deglaciation. Distinct δ18Osw minima during Heinrich-1 (H1) and the Younger Dryas (YD) correspond with peaks in ice-rafted debris and periods of reduced Atlantic MOC, indicating the presence of melt water in this region that could have contributed to MOC reductions during these intervals. Increased tropical and subtropical δ18Osw during these periods of apparent freshening in the subpolar North Atlantic suggest a buildup of salt at low latitudes that served as a negative feedback on reduced Atlantic MOC.Support for this research was provided by the U.S. National Science Foundation (JFM and DWO) and a postdoctoral scholarship funded in part by the Gary Comer Science and Education Foundation (HB)

Global Law as Intercontextuality and as Interlegality

Since the 1990s the effects of globalization on law and legal developments has been a central topic of scholarly debate. To date, the debate is however marked by three substantial deficiencies which this chapter seeks to remedy through a reconceptualization of global law as a law of inter-contextuality expressed through inter-legality and materialized through a particular body of legal norms which can be characterized as connectivity norms. The first deficiency is a historical and empirical one. Both critics as well as advocates of ‘non-state law’ share the assumption that ‘law beyond the state’ and related legal norms have gained in centrality when compared with previous historical times. While global law, including both public and private global governance law as well as regional occurrences such as EU law, has undergone profound transformations since the structural transformations which followed the de-colonialization processes of the mid-twentieth century, we do not have more global law relatively to other types of law today than in previous historical times. The second deficiency is a methodological one. The vast majority of scholarship on global law is either of an analytical nature, drawing on insights from philosophy, or empirically observing the existence of global law and the degree of compliance with global legal norms at a given moment in time. While both approaches bring something to the table they remain static approaches incapable of explaining and evaluating the transformation of global law over time. The third deficiency is a conceptual-theoretical one. In most instances, global law is understood as a unitary law producing singular legal norms with a planetary reach, or, alternatively, a radical pluralist perspective is adopted dismissing the existence of singular global norms. Both of these approaches however misapprehend the structural characteristics, function and societal effects of global law. Instead a third positon between unitary and radical pluralist perspectives can be adopted through an understanding of global law and its related legal norms as a de-centred kind of inter-contextual law characterised by inter-legality

Phenotypic variability in synthetic biology applications:Dealing with noise in microbial gene expression

The stochasticity due to the infrequent collisions among low copy-number molecules within the crowded cellular compartment is a feature of living systems. Single cell variability in gene expression within an isogenic population (i.e. biological noise) is usually described as the sum of two independent components: intrinsic and extrinsic stochasticity. Intrinsic stochasticity arises from the random occurrence of events inherent to the gene expression process (e.g. the burst-like synthesis of mRNA and protein molecules). Extrinsic fluctuations reflect the state of the biological system and its interaction with the intra and extracellular environments (e.g. concentration of available polymerases, ribosomes, metabolites, and micro-environmental conditions). A better understanding of cellular noise would help synthetic biologists design gene circuits with well-defined functional properties. In silico modelling has already revealed several aspects of the network topology’s impact on noise properties; this information could drive the selection of biological parts and the design of reliably-engineered pathways. Importantly, while optimizing artificial gene circuitry for industrial applications, synthetic biology could also elucidate the natural mechanisms underlying natural phenotypic variability. In this review, we briefly summarize the functional roles of noise in unicellular organisms and address their relevance to synthetic network design. We will also consider how noise might influence the selection of network topologies supporting reliable functions, and how the variability of cellular events might be exploited when designing innovative biotechnology applications