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Register a new userThermodynamics and Charging of Interstellar Iron Nanoparticles
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Interstellar iron in the form of metallic iron nanoparticles may constitute a component of the interstellar dust. We compute the stability of iron nanoparticles to sublimation in the interstellar radiation field, finding that iron clusters can persist down to a radius of $simeq 4.5,$AA, and perhaps smaller. We employ laboratory data on small iron clusters to compute the photoelectric yields as a function of grain size and the resulting grain charge distribution in various interstellar environments, finding that iron nanoparticles can acquire negative charges particularly in regions with high gas temperatures and ionization fractions. If $gtrsim 10%$ of the interstellar iron is in the form of ultrasmall iron clusters, the photoelectric heating rate from dust may be increased by up to tens of percent relative to dust models with only carbonaceous and silicate grains.
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The local cosmic-ray (CR) spectra are calculated for typical characteristic regions of a cold dense molecular cloud, to investigate two so far neglected mechanisms of dust charging: collection of suprathermal CR electrons and protons by grains, and photoelectric emission from grains due to the UV radiation generated by CRs. The two mechanisms add to the conventional charging by ambient plasma, produced in the cloud by CRs. We show that the CR-induced photoemission can dramatically modify the charge distribution function for submicron grains. We demonstrate the importance of the obtained results for dust coagulation: While the charging by ambient plasma alone leads to a strong Coulomb repulsion between grains and inhibits their further coagulation, the combination with the photoemission provides optimum conditions for the growth of large dust aggregates in a certain region of the cloud, corresponding to the densities $n(mathrm{H_2})$ between $sim10^4$ cm$^{-3}$ and $sim10^6$ cm$^{-3}$. The charging effect of CR is of generic nature, and therefore is expected to operate not only in dense molecular clouds but also in the upper layers and the outer parts of protoplanetary discs.
The role of quantum tunneling effect in the electron accretion current onto a negatively charged grain immersed in isotropic plasma is analyzed, within the quasiclassic approximation, for different plasma electron distribution functions, plasma parameters, and grain sizes. It is shown that this contribution can be small (negligible) for relatively large (micron-sized) dust grains in plasmas with electron temperatures of the order of a few eV, but becomes important for nano-sized dust grains (tens to hundreds nm in diameter) in cold and ultracold plasmas (electron temperatures ~ tens to hundreds of Kelvin), especially in plasmas with depleted high-energy tails in the electron energy distribution.
A two dimensional magnetic particle in the presence of an external magnetic field is studied. Equilibrium thermodynamical properties are derived by evaluating analytically the partition function. When the external field is applied perpendicular to the anisotropy axis the system exhibits a second order phase transition with order parameter being the magnetization parallel to the field. In this case the system is isomorph to a mechanical system consisting in a particle moving without friction in a circle rotating about its vertical diameter. Contrary to a paramagnetic particle, equilibrium magnetization shows a maximum at finite temperature. We also show that uniaxial anisotropy in a system of noninteracting particles can be missinterpreted as a ferromagnetic or antiferromagnetic coupling among the magnetic particles depending on the angle between anisotropy axis and magnetic field.
This chapter is intended as a pedagogical introduction to the dynamics of optically levitated nanoparticles with a focus on the study of single particle thermodynamics. Much of the work studying thermodynamics with nano- and micro-particles has taken place in liquid, and this chapter will avoid reviewing this impressive body of work, focussing instead on studies of thermodynamics with nanoparticles levitated in a gas. For a recent literature review we refer the reader to Gieseler & Millen Entropy 20, 326 (2018). The authors will discuss extensions into the quantum regime where relevant throughout the chapter.
We report on self-assembled iron oxide nanoparticle films on silicon substrates. In addition to homogeneously assembled layers, we fabricated patterned trenches of 40-1000 nm width using electron beam lithography for the investigation of assisted self-assembly. The nanoparticles with a diameter of 20 nm +/- 7% were synthesized by thermal decomposition of iron oleate complexes in trioctylamine in presence of oleic acid. Samples with different track widths and nanoparticle concentration were characterized by scanning electron microscopy and by superconducting quantum interference device magnetometry.
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