Mean free path

About: Mean free path is a research topic. Over the lifetime, 4412 publications have been published within this topic receiving 114418 citations.

Lattice Boltzmann Modeling of Subcontinuum Energy Transport in Crystalline and Amorphous Microelectronic Devices

Abstract: Numerical simulations of time-dependent energy transport in semiconductor thin films are performed using the lattice Boltzmann method applied to phonon transport. The discrete lattice Boltzmann method is derived from the continuous Boltzmann transport equation assuming first gray dispersion and then nonlinear, frequency-dependent phonon dispersion for acoustic and optical phonons. Results indicate that a transition from diffusive to ballistic energy transport is found as the characteristic length of the system becomes comparable to the phonon mean free path. The methodology is used in representative microelectronics applications covering both crystalline and amorphous materials including silicon thin films and nanoporous silica dielectrics. Size-dependent thermal conductivity values are also computed based on steady-state temperature distributions obtained from the numerical models. For each case, reducing feature size into the subcontinuum regime decreases the thermal conductivity when compared to bulk values. Overall, simulations that consider phonon dispersion yield results more consistent with experimental correlations.

Universal influence of disorder on MgB2 wires

Abstract: The influence of disorder on the superconducting properties of MgB2 wires was investigated. Disorder was introduced in three different ways: by the addition of SiC, by neutron irradiation or by a low processing temperature. We find a nearly identical influence of these three methods on the normal state resistivity, on the upper critical field and on the critical currents in all three cases. The residual resistivity turns out to be a useful parameter for disorder, if normalized appropriately. We extract the mean free path of the charge carriers in the σ band from the Gor'kov–Goodman relation. The wires investigated in this study fall in the range from the moderately clean to the dirty limit. The most important change in view of possible applications is the increase of the upper critical field, leading to higher critical currents in high magnetic fields.

The electromagnetic ion beam instability and energy loss of fast alpha particles

Abstract: A high-speed ion beam directed along a uniform magnetic field can excite an electromagnetic ion beam instability. The linear and second-order theory of this instability is considered. If the beam is sufficiently dense, and if the saturation level is sufficiently high, the anomalous energy loss distance is much shorter than the classical mean free path. A possible application to alpha-particle energy loss in a linear multiple mirror theta pinch is discussed.

Phonon Fluorescence in Superconductors and the Propagation Characteristics of High-Frequency Phonons in Ge: Sb and Al 2 O 3 : V 3+

Abstract: We present a detailed account of phonon generation by superconducting tunnel junctions (Sn-I-Sn) and by Sn and Pb 0.5 Tl 0.5 films pumped by a heat pulse. The spectrum and propagation characteristics of the generated phonons is studied through the resonance absorption by Sb-donor levels in uniaxially compressed Ge. At all values of generator power (up to a few watts) the emitted phonons show a large density at a value of energy equal to the superconducting energy gap (2Δ). In contrast, the spectrum emitted by a constantan heater is shown to be in quantitative agreement with the blackbody-radiation model. The experiments show that the mean free path of longitudinal and transverse phonons in the superconductor changes discontinuously when hω=2Δ. Theoretical calculations show that significant reabsorption of phonons of energy 2Δ results in a nonlinear buildup of the intensity of the 2Δ phonons. The propagation characteristics of these phonons in Ge: Sb show a frequency and polarization dependence in excellent agreement with the Griffin-Carruthers theory of resonance-fluorescence phonon scattering by donor levels. The magnetic field tunability of the generated 2Δ phonons in Sn is utilized to study the ground state of V 3+ in Al 2 O 3 . Transverse phonons of energy 1.02 meV propagating along the c axis are resonantly absorbed as expected according to theory.