Термофорез одноатомных частиц в открытых нанотрубках

Using the method of molecular dynamics, it is shown that thermophoresis of particles (atoms) inside single-walled carbon nanotubes (CNTs) is highly efficient. Placing a particle inside the CNT involved in heat transfer causes it to move in the direction of the heat flow at a constant speed, the value of which weakly depends on the length of the nanotube. The heat flow along the CNT leads to the formation of a constant thermophoresis force for the particles inside. The direction of this force coincides with the direction of heat transfer. The monatomic nature of the particle allowed us to numerically calculate this force and to determine the contribution to this force of interaction with each thermal phonon of the nanotube. It is shown that the magnitude of the force is almost completely determined by the interaction of the particle with long-wave bending phonons of the nanotube, which have a long free run path. Therefore, the speed of the particle movement and the value of the thermophoresis force depend weakly on the length of the nanotube, but are determined by the temperature difference at its ends. Because of this, the mode of thermophoresis of particles inside nanotubes is ballistic, not diffusive.

Radiative and non-radiative transitions of excited Ti3+ cations in sapphire

We have measured the fluorescence quantum efficiency in Ti3+:sapphire single crystals between 150 K and 550 K. Using literature-given effective fluorescence lifetime temperature dependence, we show that the zero temperature radiative lifetime is (4.44 ± 0.04) μs, compared to the 3.85 μs of the fluorescence lifetime. Fluorescence lifetime thermal shortening resolves into two parallel effects: radiative lifetime shortening, and non-radiative transition rate enhancement. The first is due to thermally enhanced occupation of a ΔE = 1,700 cm−1 higher (top) electronic state of the upper multiplet, exhibiting a transition oscillator strength of f = 0.62, compared to only 0.013 of the bottom electronic state of the same multiplet. The non-radiative rate relates to multi-phonon decay transitions stimulated by the thermal phonon occupation. Thermal enhancement of the configuration potential anharmonicity is also observed. An empiric expression for the figure-of-anharmonicity temperature dependence is given as $$\hat{{\bf{H}}}$$Hˆ (T) = $$\hat{{\bf{H}}}$$Hˆ (0)(1 + β exp(−ℏωco /kBT )), where $$\hat{{\bf{H}}}$$Hˆ (0) = 0.276, β = 5.2, ℏωco = 908 cm−1, and kB is the Boltzmann constant.

Coherent control of thermal phonon transport in van der Waals superlattices

Coherent manipulation of thermal phonon transport in vdW superlattices can expand the property space beyond that occupied by natural materials.