Binding between endohedral Na atoms in Si clathrate I; a first principles study

We investigate the binding nature of the endohedral sodium atoms with the ensity functional theory methods, presuming that the clathrate I consists of a sheaf of one-dimensional connections of Na@Si$_{24}$ cages interleaved in three perpendicular directions. Each sodium atom loses 30% of the 3s$^1$ charge to the frame, forming an ionic bond with the cage atoms; the rest of the electron contributes to the covalent bond between the nearest Na atoms. The presumption is proved to be valid; the configuration of the two Na atoms in the nearest Si$_{24}$ cages is more stable by 0.189 eV than that in the Si$_{20}$ and Si$_{24}$ cages. The energy of the beads of the two distorted Na atoms is more stable by 0.104 eV than that of the two infinitely separated Na atoms. The covalent bond explains both the preferential occupancies in the Si$_{24}$ cages and the low anisotropic displacement parameters of the endohedral atoms in the Si$_{24}$ cages in the [100] directions of the clathrate I.Comment: First page: Affiliation added to PDF and PS versio

A 6-year global cloud climatology from the Atmospheric InfraRed Sounder AIRS and a statistical analysis in synergy with CALIPSO and CloudSat

We present a six-year global climatology of cloud properties, obtained from observations of the Atmospheric Infrared Sounder (AIRS) onboard the NASA Aqua satellite. Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) combined with CloudSat observations, both missions launched as part of the A-Train in 2006, provide a unique opportunity to evaluate the retrieved AIRS cloud properties such as cloud amount and height. In addition, they permit to explore the vertical structure of different cloud types. AIRS-LMD cloud detection agrees with CALIPSO about 85% over ocean and about 75% over land. Global cloud amount has been estimated from 66% to 74%, depending on the weighting of not cloudy AIRS footprints by partial cloud cover from 0 to 0.3. 42% of all clouds are high clouds, and about 42% of all clouds are single layer low-level clouds. The "radiative" cloud height determined by the AIRS-LMD retrieval corresponds well to the height of the maximum backscatter signal and of the "apparent middle" of the cloud. Whereas the real cloud thickness of high opaque clouds often fills the whole troposphere, their "apparent" cloud thickness (at which optical depth reaches about 5) is on average only 2.5 km. The real geometrical thickness of optically thin cirrus as identified by AIRS-LMD is identical to the "apparent" cloud thickness with an average of about 2.5 km in the tropics and midlatitudes. High clouds in the tropics have slightly more diffusive cloud tops than at higher latitudes. In general, the depth of the maximum backscatter signal increases nearly linearly with increasing "apparent" cloud thickness. For the same "apparent" cloud thickness optically thin cirrus show a maximum backscatter about 10% deeper inside the cloud than optically thicker clouds. We also show that only the geometrically thickest opaque clouds and (the probably surrounding anvil) cirrus penetrate the stratosphere in the tropics

Coupling efficiency for phase locking of a spin transfer oscillator to a microwave current

The phase locking behavior of spin transfer nano-oscillators (STNOs) to an external microwave signal is experimentally studied as a function of the STNO intrinsic parameters. We extract the coupling strength from our data using the derived phase dynamics of a forced STNO. The predicted trends on the coupling strength for phase locking as a function of intrinsic features of the oscillators i.e. power, linewidth, agility in current, are central to optimize the emitted power in arrays of mutually coupled STNOs

Dynamics of two coupled vortices in a spin valve nanopillar excited by spin transfer torque

We investigate the dynamics of two coupled vortices driven by spin transfer. We are able to independently control with current and perpendicular field, and to detect, the respective chiralities and polarities of the two vortices. For current densities above $J=5.7*10^7 A/cm^2$, a highly coherent signal (linewidth down to 46 kHz) can be observed, with a strong dependence on the relative polarities of the vortices. It demonstrates the interest of using coupled dynamics in order to increase the coherence of the microwave signal. Emissions exhibit a linear frequency evolution with perpendicular field, with coherence conserved even at zero magnetic field

Phase diagram of silicon from atomistic simulations

In this letter we present a calculation of the temperature-pressure phase diagram of Si in a range of pressures covering from -5 to 20 GPa and temperatures up to the melting point. The phase boundaries and triple points between the diamond, liquid, $\beta$-Sn and ${Si}_{34}$ clathrate phases are reported. We have employed efficient simulation techniques to calculate free energies and to numerically integrate the Clausius-Clapeyron equation, combined with a tight binding model capable of an accuracy comparable to that of first-principles methods. The resulting phase diagram agrees well with the available experimental data.Comment: 5 pages, 1 figure, accepted in PR

Domain wall displacement by remote spin-current injection

We demonstrate numerically the ability to displace a magnetic domain wall by a remote spin current injection. We consider a long and narrow magnetic nanostripe with a single domain wall (DW). The spin-polarized current is injected perpendicularly to the plane of the film (CPP) through a small nanocontact which is located at certain distance from the domain wall initial position. We show theoretically that the DW motion can be initiated not only by conventional spin-transfer torque but also by indirect spin-torque, created by a remote spin-current injection and then transferred to the DW by the exchange-spring mechanism. An analytical description of this effect is proposed. This finding may lead to a solution of bottleneck problems of DW motion-based spintronic and neuromorphic devices with perpendicular spin-current injection.Comment: 6 pages, 4 figure

Parallel pumping of magnetic vortex gyrations in spin-torque nano-oscillators

We experimentally demonstrate that large magnetic vortex oscillations can be parametrically excited in a magnetic tunnel junction by the injection of radio-frequency (rf) currents at twice the natural frequency of the gyrotropic vortex core motion. The mechanism of excitation is based on the parallel pumping of vortex motion by the rf orthoradial field generated by the injected current. Theoretical analysis shows that experimental results can be interpreted as the manifestation of parametric amplification when rf current is small, and of parametric instability when rf current is above a certain threshold. By taking into account the energy nonlinearities, we succeed to describe the amplitude saturation of vortex oscillations as well as the coexistence of stable regimes.Comment: Submitted to Phys. Rev. Let