Critical exponents at the ferromagnetic transition in tetrakis(diethylamino)ethylene-C60_{60} (TDAE-C60_{60})

Critical exponents at the ferromagnetic transition were measured for the first time in an organic ferromagnetic material tetrakis(dimethylamino)ethylene fullerene[60] (TDAE-C$_{60}$). From a complete magnetization-temperature-field data set near $T_{c}=16.1\pm 0.05,$ we determine the susceptibility and magnetization critical exponents $\gamma =1.22\pm 0.02$ and $\beta =0.75 \pm 0.03$ respectively, and the field vs. magnetization exponent at $T_{c}$ of $\delta =2.28\pm 0.14$. Hyperscaling is found to be violated by $\Omega \equiv d^{\prime}-d \approx -1/4$, suggesting that the onset of ferromagnetism can be related to percolation of a particular contact configuration of C$_{60}$ molecular orientations.Comment: 5 pages, including 3 figures; to appear in Phys. Rev. Let

Structural differences in two polymorphs of tetra-kis-(dimethylamino)-ethylene-C-60: An x-ray diffraction study

A type of low-temperature structure for ferromagnetic alpha-tetra-kis (dimethylamino) -ethylene (TDAE)-C-60 is proposed on the basis of low-temperature x-ray analysis. We observed that intense superlattice reflections with odd indices successively appeared below T-s = 170 K. The space group symmetry of the low-temperature phase is determined to be P2(1)/n. Two inequivalent C-60 sites exist in the low-temperature phase, which are indispensable to the orbital ordering model Of C-60. The contact configuration for the neighboring C(60)s along the stacking c direction is uniquely determined. The double bond between the hexagons faces the neighboring pentagon. We found that the surrounding TDAE molecules shift along the c axis (similar to 0.07 angstrom) and that these shifts correlate perfectly to the alignment of C-60. This result indicates that the steric effect betwee n C-60 and TDAE molecules plays an important role in the orientational ordering Of C-60, On the other hand, in the alpha' phase, no structural phase transition was observed below 30 K. This indicates that all the C(60)s are crystallographically equivalent. Structural differences separate the magnetic peculiarities of the two polymorphs in TDAE-C-60.</p

Cyclotron effective masses in layered metals

Many layered metals such as quasi-two-dimensional organic molecular crystals show properties consistent with a Fermi liquid description at low temperatures. The effective masses extracted from the temperature dependence of the magnetic oscillations observed in these materials are in the range, m^*_c/m_e \sim 1-7, suggesting that these systems are strongly correlated. However, the ratio m^*_c/m_e contains both the renormalization due to the electron-electron interaction and the periodic potential of the lattice. We show that for any quasi-two-dimensional band structure, the cyclotron mass is proportional to the density of states at the Fermi energy. Due to Luttinger's theorem, this result is also valid in the presence of interactions. We then evaluate m_c for several model band structures for the \beta, \kappa, and \theta families of (BEDT-TTF)_2X, where BEDT-TTF is bis-(ethylenedithia-tetrathiafulvalene) and X is an anion. We find that for \kappa-(BEDT-TTF)_2X, the cyclotron mass of the \beta-orbit, m^{*\beta}_c, is close to 2 m^{*\alpha}_c, where m^{*\alpha}_c is the effective mass of the \alpha- orbit. This result is fairly insensitive to the band structure details. For a wide range of materials we compare values of the cyclotron mass deduced from band structure calculations to values deduced from measurements of magnetic oscillations and the specific heat coefficient.Comment: 12 pages, 3 eps figure

Theory of the Shubnikov-de Haas effect in quasi-two-dimensional metals

The Shubnikov - de Haas effect in quasi-two-dimensional normal metals is studied. The interlayer conductivity is calculated using the Kubo formula. The electron scattering on short-range is considered in the self-consistent Born approximation. The result obtained differs from that derived from the Boltzmann transport equation. This difference is shown to be a general feature of conductivity in magnetic field. A detailed description of the two new qualitative effects -- the field-dependent phase shift of beats and of the slow oscillations of conductivity is provided. The results obtained are applicable to strongly anisotropic organic metals and to other quasi-two-dimensional compounds.Comment: 10 page

The polymer phase of the TDAE-C60_{60} organic ferromagnet

The high-pressure Electron Spin Resonance (ESR) measurements were preformed on TDAE-C$_{60}$ single crystals and stability of the polymeric phase was established in the $P - T$ parameter space. At 7 kbar the system undergoes a ferromagnetic to paramagnetic phase transition due to the pressure-induced polymerization. The polymeric phase remains stable after the pressure release. The depolymerization of the pressure-induced phase was observed at the temperature of 520 K. Below room temperature, the polymeric phase behaves as a simple Curie-type insulator with one unpaired electron spin per chemical formula. The TDAE$^+$ donor-related unpaired electron spins, formerly ESR-silent, become active above the temperature of 320 K and the Curie-Weiss behavior is re-established.Comment: Submitted to Phys. Rev.

INCREASE IN SURFACE AIR TEMPERATURE AS A RESULT OF CONVECTIVE AEROSOL REMOVEMENT TO DESERT TERRITORIES

&lt;p&gt;&lt;strong&gt;Abstract&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;We modified the mathematical model of the influence of convective aerosol removal (CR) from desert surfaces on near-surface air temperature, which we previously developed for "ideal" conditions (the absence of vegetation cover and the presence of wind loads - calm weather conditions) [1, 30].&lt;/p&gt;&lt;p&gt;During field experiments to validate the model, a significant influence of wind and vegetation cover on the thermal effect of convective aerosol removal (CR) was revealed, causing significant discrepancies between model and experimental data. Based on the obtained empirical dependences of the thermal impact of CR F on wind speed and the general projective cover of vegetation, as well as changes in some parameters of the model of convective aerosol removal (MCR), the MCR-2 version was developed taking into account real operating conditions.&lt;/p&gt;&lt;p&gt;According to the results of the implementation of MCR -2 for the territory of the Southern Aral Sea region in the warm period of the year (May-October) in desert zones, an increase in temperature in the near-surface layer is noted, on average, by 3.5 ° C, as a result of convective removal of aerosol. This value is very significant, which indicates the need to take into account the convective removal of aerosol in prognostic and climate models of surface air temperature in desert areas.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References:&lt;/strong&gt;&lt;/p&gt;&lt;ol&gt;&lt;li&gt;Arushanov M. L., Tleumuratova B. S., Narimbetov B. ZH. 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Conversion of polycyclic aromatic hydrocarbons to graphite and diamond at high pressures

International audienceThe polycyclic aromatic hydrocarbons (PAH): naphthalene, anthracene, pentacene, perylene, and coronene were submitted to temperatures up to 1500 °C at 8 GPa. To avoid catalytic action of metals on thermal conversion, graphite was used as container material. Moreover, graphite is very permeable to the gaseous products of thermal decomposition of PAH. The resulting thermal transformations and their evolution were studied by X-ray diffraction, Raman spectroscopy and scanning electron microscopy as a function of temperature for 60-s treatments. The nature of the initial compounds clearly affects the products of the different stages of carbonization and the first steps of graphitization. This becomes hardly discernible in the final stages of graphitization above 1000 °C. Above 1200 °C, graphite with high rystallinity forms in all cases. The temperature of the beginning of diamond formation does not seem to be influenced by the nature of the initial PAH and is equal to -1280 °C for all investigated compounds. Diamonds formed from the PAH are high-quality 5–40 pm single crystals. The p, T values of diamond formation here obtained are significantly lower than those previously known for direct graphite–diamond transformatio