Phonon bottleneck

Slow Magnetic Relaxation and Phonon Bottleneck Effects... 

As already observed for some isotropic polynuclear clusters [30 - 32], slow relaxation of the magnetization in an external magnetic field can occur because of the inefficient transfer of energy to the environment, for example, the helium bath, and consequent reabsorption of the emitted phonon by the spin system. The phenomenon, also known as phonon bottleneck (PB), was first introduced by Van Vleck [33]. It is characteristic of low temperatures, where relaxation is dominated by the direct process between closely spaced levels, and results from the low density of phonons with such a long wavelength to match the small energy separation... 

Figure 20 shows rLrs obtained from the data in Figs. 16 and 17. Taking the example of the resonance at v =14 GHz, B0 = 0.491 T, we obtain TLrs 0.2 (Fig. 20). As mentioned above, rs 1 s is rather large due to the phonon bottleneck effect. This allows to obtain sufficiently large values of rLr to observe significant radiation absorption even if the rate TL = 0.2. v is relatively small. Using expression (3) yield Bac 1 p.T, and similar values were obtained for all measured frequencies.

A.L. Efros, V.A. Kharchenko, M. Rosen, Breaking the phonon bottleneck in nanometer quantum dots Role of Auger-like processes, Solid State Commun. 93 (1995) 281. 

Chen Chen, Mitra Dutta, and Michael A. Stroscio, Surface-optical phonon assisted transitions in quantum dots. Journal of Applied Physics, 96, 2049-2054 (2004).31. Amit Raichura, Mitra Dutta, and Michael A. Stroscio, Acoustic Phonons and Phonon Bottleneck in Single Wall Nanotubes, J. of Computational Electronics, 4, 91-95, 2005. 

Figure 3.12 Hot phonon bottleneck to slow hot-electron cooling in QWs. At high light intensity, hot electrons produce hot phonons which can reheat electrons via phonon absorption to keep them hot. Pioss is power loss per electron.

QWs, the phonons are confined in the well and they exhibit slab modes (Campos et al, 1992), which enhance the hot phonon bottleneck effect. 

Experimental determination of cooling dynamics of phonon bottlenecks in QDs... 

In contradiction to the results showing slowed cooling in QDs, many other investigations are reported in the literature in which a phonon bottleneck was apparently not observed. These results have been reported for both self-organised SK QDs and II-VI colloidal QDs Nozik (2001b) gives a detailed reference list. However, in several cases (Heitz et al., 1991 Heitz et al, 1998 Sosnowski et al, 1998), hot-electron relaxation was found to be slowed, but not sufficientiy for the authors to conclude that this was evidence of a phonon bottieneck. 

Mukai K., Ohtsuka N., Shoji H. and Sugawara M. (1996), Emission from discrete levels in self-formed InGaAs/GaAs quantum dots by electric carrier injection inflnence of phonon bottleneck , Appl. Phys. Lett. 68, 3013-3015. 

Mukai K. and Sngawara M. (1999), The phonon bottleneck effect in qnantnm dots , in Sngawara M., ed., Self-Assembled InGaAs/GaAs Quantum Dots, Academic Press, San Diego, p. 209. 

Schaller R. D., Pietryga 1. M., Goupalov S. V., Petruska M. A., Ivanov S. A. and Klimov V. I. (2005c), Breaking the phonon bottleneck in semiconductor nanocrystals via multiphonon emission induced by intrinsic nonadiabatic interactions , Phys. Rev. Lett. 95, 196401. 

Sugawara M., Mukai K. and Shoji H. (1997), Effect of phonon bottleneck on quantum-dot laser performance , Appl. Phys. Lett. 71,2791-2793. 

One of the most successful explanations of the observed nanocrystal properties is called the electron-hole auger coupling mechanismand can be explained as follows. Most QDs, specifically, II-VI and III-V nanocrystals have sparse CB states and dense VB states. Though phonon assisted relaxation is unfeasible in the CB, there is no analogous phonon bottleneck for hole relaxation. It is thus possible for an excited electron to transfer energy to a hole in the VB that can subsequently relax by sequential phonon emission. This mechanism has been confirmed by several ultrafast studies that have observed rapid exciton cooling in QDs with timescales as short as a few hundred femtoseconds. The rates of exciton cooling... 

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