Nonequilibrium phonon mean free paths in anharmonic chains

Harnessing the power of low-dimensional materials in thermal applications calls for a solid understanding of the anomalous thermal properties of such systems. We analyze thermal conduction in one-dimensional systems by determining the frequency-dependent phonon mean free paths (MFPs) for an anharmonic chain, delivering insight into the diverging thermal conductivity observed in computer simulations. In our approach, the MFPs are extracted from the length-dependence of the spectral heat current obtained from nonequilibrium molecular dynamics simulations. At low frequencies, the results reveal a power-law dependence of the MFPs on frequency, in agreement with the diverging conductivity and the recently determined equilibrium MFPs. At higher frequencies, however, the nonequilibrium MFPs consistently exceed the equilibrium MFPs, highlighting the differences between the two quantities. Exerting pressure on the chain is shown to suppress the mean free paths and to generate a weaker divergence of MFPs at low frequencies. The results deliver important insight into anomalous thermal conduction in low-dimensional systems and also reveal differences between the MFPs obtained from equilibrium and nonequilibrium simulations.Comment: 8 pages, 7 figures, minor changes to v

Role of anharmonic phonon scattering in the spectrally decomposed thermal conductance at planar interfaces

Detailed understanding of vibrational heat transfer mechanisms between solids is essential for the efficient thermal engineering and control of nanomaterials. We investigate the frequency dependence of anharmonic scattering and interfacial thermal conduction between two acoustically mismatched solids in planar contact by calculating the spectral decomposition of the heat current flowing through an interface between two materials. The calculations are based on analyzing the correlations of atomic vibrations using the data extracted from non-equilibrium molecular dynamics simulations. Inelastic effects arising from anharmonic interactions are shown to significantly facilitate heat transfer between two mass-mismatched face-centered cubic lattices even at frequencies exceeding the cut-off frequency of the heavier material due to (i) enhanced dissipation of evanescent vibrational modes and (ii) frequency-doubling and frequency-halving three-phonon energy transfer processes at the interface. The results provide substantial insight into interfacial energy transfer mechanisms especially at high temperatures, where inelastic effects become important and other computational methods are ineffective.Comment: minor changes to v

Influence of the initial- and final-state configuration interaction on the anisotropy of the resonant Auger decay of Kr 3d⁻¹5p and Xe 4d⁻¹6p states

The anisotropy of resonant Auger decay of photo-excited Kr 3d−13/2,5/25p and Xe 4d−13/2,5/26p states has been studied by multiconfiguration Dirac-Fock method. The calculations account for the configuration interaction both in the initial and in the final states of the Auger decay. For the nonresolved nd−1(n+2)p(J=1) resonances (n=3 for Kr and n=4 for Xe) the average intensities and anisotropies of Auger lines were calculated by weighing each partial rate by the pertinent Dirac-Fock photoexcitation probabilities. Our results show that, in addition to the initial- and final-state correlation, both the relaxation and the exchange interaction have a substantial effect on the anisotropy of these Auger spectra. For most Auger lines there is good agreement between our calculated β parameters and experimental values for the Kr and Xe nd−15/2(n+2)p photoexcitation resonances; there is also satisfactory agreement for the Kr 3d−13/25p and Xe 4d−13/26p resonant Auger spectra. However, the remaining notable discrepancies between theory and experiment indicate that important correlation effects are still omitted in our calculations. Discrepancies between different experimental results stress the need for further improvements on the experimental side

Shake-Modified Resonant Autoionization In Magnesium

Anomalous features in the resonantly excited 2p autoionization spectrum of Mg are attributed to overlapping shake transitions of the excited bound (spectator) electron. Universal features of the shake spectrum are displayed and related to the post-collision interaction

Coherence and correlation in the anisotropy of Ne KL-LLL satellite Auger decay

The energies, intensities, and angular anisotropies of the Ne KL-LLL satellite Auger lines have been studied by the multiconfiguration Dirac-Fock method. In addition to the initial- and final-state correlation effects we have studied the influence of the quantum beat effect on this Auger spectrum. Since the energy splitting of the Ne 1s−12p−13P multiplet is much smaller than the lifetime broadening, the coherent excitation of these initial states by the in time and space localized electromagnetic pulse of the projectile has a drastic effect on the angular distribution of Auger electrons. To analyze this coherence effect we have generalized the theory of the angular distribution of Auger electrons to the case of coherent excitation of partially overlapping initial states. The results of our calculations are in good overall agreement with experiment. However, for a quantitative study of the influence of the coherence and the initial spin state on the anisotropy of these Auger lines new measurements with lower error limits are necessary

Cooling of radiative quantum-dot excitons by terahertz radiation: A spin-resolved Monte Carlo carrier dynamics model

We have developed a theoretical model to analyze the anomalous cooling of radiative quantum dot (QD) excitons by THz radiation reported by Yusa et al [Proc. 24th ICPS, 1083 (1998)]. We have made three-dimensional (3D) modeling of the strain and the piezoelectric field and calculated the 3D density of states of strain induced quantum dots. On the basis of this analysis we have developed a spin dependent Monte Carlo model, which describes the carrier dynamics in QD's when the intraband relaxation is modulated by THz radiation. We show that THz radiation causes resonance transfer of holes from dark to radiative states in strain-induced QD's. The transition includes a spatial transfer of holes from the piezoelectric potential mimima to the deformation potential minimum. This phenomenon strongly enhances the QD ground state luminescence at the expense of the luminescence from higher states. Our model also reproduces the delayed flash of QD ground state luminescence, activated by THz radiation even $\sim1$ s after the carrier generation. Our simulations suggest a more general possibility to cool the radiative exciton subsystem in optoelectronic devices.Comment: 18 pages, 1 table, 8 figures, submitted to Physical Review B v2: major conceptual changes. The article was extended considerably to suit Physical Review B (instead of Physical Review Letters

Meaningful change: defining the interpretability of changes in endpoints derived from interactive and mHealth technologies in healthcare and clinical research

Immersive, interactive and mHealth technologies are increasingly being used in clinical research, healthcare and rehabilitation solutions. Leveraging technology solutions to derive new and novel clinical outcome measures is important to the ongoing assessment of clinical interventions. While demonstrating statistically significant changes is an important element of intervention assessment, understanding whether changes detected reflect changes of a magnitude that are considered meaningful to patients is equally important. We describe methodologies used to determine meaningful change and recommend that these techniques are routinely included in the development and testing of clinical assessment and rehabilitation technology solutions

Temperature dependence of carrier relaxation in strain-induced quantum dots

We report experimental observation and theoretical interpretation of temperature-dependent, time-resolved luminescence from strain-induced quantum dots. The experimental results are well described by a master equation model for the electrons. The intraband relaxation in the conduction band and the radiative recombination rate are governed by the hole populations resulting in prominent temperature dependence of the relaxation process. Even when only a few electrons and holes are confined in a single quantum dot the Auger-like process provides a rapid intraband relaxation channel for electrons that can replace the phonon scattering as the dominant relaxation mechanism.Peer reviewe

Flame-vortex interactions - Effects of buoyancy from microgravity imaging studies

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76483/1/AIAA-1997-669-671.pd