Heat capacity and phonon mean free path of wurtzite GaN

We report on lattice specific heat of bulk hexagonal GaN measured by the heat flow method in the temperature range 20-300 K and by the adiabatic method in the range 5-70 K. We fit the experimental data using two temperatures model. The best fit with the accuracy of 3 % was obtained for the temperature independent Debye's temperature $\theta_{\rm D}=365$ {\rm K} and Einstein's temperature $\theta_{\rm E}=880$ {\rm K}. We relate these temperatures to the function of density of states. Using our results for heat conduction coefficient, we established in temperature range 10-100 K the explicit dependence of the phonon mean free path on temperature $\it{l}_{\rm ph}\propto T^{-2}$. Above 100 K, there is the evidence of contribution of the Umklapp processes which limit phonon free path at high temepratures. For phonons with energy $k_{\rm B}\times 300$ {\rm K} the mean free path is of the order 100 {\rm nm}Comment: 5 pages, 4 figure

Analysis of Experimental Conditions for Simultaneous Measurements of Transport and Magnetotransport Coefficients of High Temperature Superconductors

Experimental conditions for simultaneous measurements of transport coefficients of high temperature superconductors in zero and non-zero magnetic fields are analysed. Test measurements of the thermal conductivity, the thermoelectric power and the Nernst - Ettingshausen effect of a textured Bi2212 sample are reported in an external magnetic field of 2T. Errors related to parameters of the thermocouple used and to the spurious heat flows are discussed for a new experimental set-up built based on a closed cycle helium refrigerator. Possible optimising of experimental conditions is suggested.Comment: 9 pages + 4 figures. accepted for publication in Cryogenic

Genome biology of the paleotetraploid perennial biomass crop Miscanthus

Miscanthus is a perennial wild grass that is of global importance for paper production, roofing, horticultural plantings, and an emerging highly productive temperate biomass crop. We report a chromosome-scale assembly of the paleotetraploid M. sinensis genome, providing a resource for Miscanthus that links its chromosomes to the related diploid Sorghum and complex polyploid sugarcanes. The asymmetric distribution of transposons across the two homoeologous subgenomes proves Miscanthus paleo-allotetraploidy and identifies several balanced reciprocal homoeologous exchanges. Analysis of M. sinensis and M. sacchariflorus populations demonstrates extensive interspecific admixture and hybridization, and documents the origin of the highly productive triploid bioenergy crop M. x giganteus. Transcriptional profiling of leaves, stem, and rhizomes over growing seasons provides insight into rhizome development and nutrient recycling, processes critical for sustainable biomass accumulation in a perennial temperate grass. The Miscanthus genome expands the power of comparative genomics to understand traits of importance to Andropogoneae grasses

Plant resources of Triarrhena and Miscanthus species in China and its meaning for Europe

The giant grasses, some speeies in Triarrhena and Miscanthus, have high potential of biomass productivity and cou'd be used as a source of renewab'e raw materia' and energy. China is one of the most important distribution areas of these plants. An overall investigation on the high potential speeies and their geographical distribution in China is undertaken and the resultsare presented. T.lutarioriparia, ever named Miscanthus saccharif/orus, is an endemie speeies in China. It can grow to 6-7 m high and is being used as an important paper-making materia!. This speeies distributes in China in warm temperate regions from 28°36’ North latitude to 34°49’ North latitude; and from 110°41’ East longitude to 121°29’ East longitude. A cultivation trial shows that this speeies can grow over 3.5 m and over-winter safely in Braunschweig, Germany. The product quality, botanical and agronomie properties of this speeies are somewhat different to another hybrid speeies, Miscanthus × giganteus, the well-known speeies being cultivated and investigated in Europe. It is very interesting to integrate the newly introduced plant, T. lutarioriparia into the start materiais of Miscanthus breeding work. There are more information about other related plants given in this paper

Proportional correlation between heat capacity and thermal expansion of atomic, molecular crystals and carbon nanostructures

Correlation between thermal expansions β(T) and heat capacity C(T) of atomic and molecular crystals, amorphous materials with a structural disorder, carbon nanomaterials (fullerite C_60, bundles SWCNTs of single-walled carbon nanotubes) was analyzed. The influence of the contribution to the coefficient of linear thermal expansion α_Xe(T) of Xe atoms adsorbed on the SWCNTs bundles is considered. The proportional correlation was found between the contribution to the coefficient of linear thermal expansion α_Xe(T) and the normalized to the gas constant heat capacity C Xe(T)/R of Xe atoms adsorbed on the SWCNTs bundles. The proportional correlation (β/β*) ∼ (CV/R) with the parameter β* for the bulk thermal expansion coefficient for cryocrystals is proposed. In the case of atomic crystals such as Xe and Ar, the proportional correlation (β/β*) ∼ (C_V/R) is observed in the temperature range from the lowest experimental to temperatures where C_V/R ≈ 2.3. The correlation is not observed in the temperatures where 2.3 < C_V/R < 3 (classical Dulong-Petit law). It was found that the universal proportional correlation is also observed for molecular crystals with linear symmetry, such as CO_2, CO, and N_2O if the normalized heat capacity below the values C_V/R ≈ 3 ÷ 3.5. It indicates that the proportional correlation between thermal expansions (β/β*) and heat capacity (C_V/R) is related not only to the translational, but also to the rotational degrees of freedom of the molecule in the crystal. In the case of the C_0, molecular crystal with translational and rotational degrees of freedom and intramolecular vibrations, the discussed above correlation occurs below the values of normalized heat capacity C_V/R ≈ 7.5. In strongly anisotropic systems, such as systems of compacted bundles of single-walled carbon nanotubes and SWCNTs bundles with adsorbed Xe atoms, this universal dependence appears in a limited temperature range that does not include the lowest temperatures. A qualitative explanation of the observed correlation is proposed

Low-temperature anomalies in the magnetic and thermal properties of molecular cryocrystals doped with oxygen impurity

The magnetic properties of oxygen pair clusters are investigated theoretically for different cluster geometries which can be realized by doping molecular cryomatrices with oxygen. Anomalous temperature and pressure behavior of the magnetic susceptibility, heat capacity, and entropy is predicted. It is proposed to use these anomalies for studying the parameters characterizing the oxygen clusters and the parameters of the host matrix: the effective spin-figure interaction constant D for the molecule in the matrix, the exchange parameter J, and the number of pair clusters Np, which can deviate markedly from the purely random value Np=6Nc² (N is Avogadro’s number, and c is the molar concentration of the impurity). The data on the magnetic susceptibility may be used to analyze the character of the positional and orientational short-range order in the solid solution. The value of D contains information about the orientational order parameter; the distance and angular dependence of the exchange interaction parameter are still subject to discussion in the literature. The temperature dependence of Np contains information about diffusion and clusterization processes in the system

Thermoactivated heat transfer mechanism in molecular crystals: Thermal conductivity of benzophenone single crystals

Thermal conductivities of two benzophenone single crystals have been measured at temperatures from 4.7 to 270 K. The experimental data for both are consistent for temperatures above 15 K. The thermal conductivity of benzophenone can be represented as a sum of two contributions: κ1 + κTA where κ1 is due to the standard phonon mechanisms accepted for ordered crystals and κTA takes into account the heat flow due to intermolecular hopping of thermally activated intramolecular vibrational modes. The thermal activation contribution in unsubstituted benzophenone is substantially smaller when compared to that in any of the two para-bromobenzophenone polymorphs studied previously. Unlike in the 4-bromobenzophenone crystals, the microscopic agent responsible for intramolecular excitation(s) was not determined. The characteristic intramolecular excitation energy was evaluated to be 220 K, about three times less compared to 4-bromobenzophenone