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.

Role of Disorder and Anharmonicity in the Thermal Conductivity of Silicon-Germanium Alloys: A First-Principles Study

Abstract: The thermal conductivity of disordered silicon-germanium alloys is computed from density-functional perturbation theory and with relaxation times that include both harmonic and anharmonic scattering terms. We show that this approach yields an excellent agreement at all compositions with experimental results and provides clear design rules for the engineering of nanostructured thermoelectrics. For Si(x)Ge(1-x), more than 50% of the heat is carried at room temperature by phonons of mean free path greater than 1 mu m, and an addition of as little as 12% Ge is sufficient to reduce the thermal conductivity to the minimum value achievable through alloying. Intriguingly, mass disorder is found to increase the anharmonic scattering of phonons through a modification of their vibration eigenmodes, resulting in an increase of 15% in thermal resistivity.

Interpretation of the Thermal Conductivity of Glasses

Abstract: The thermal conductivity of glasses decreases with decreasing temperature, while the conductivity of crystalline substances increases with decreasing temperature. The behavior of glasses is interpreted in terms of an approximately constant free path for the lattice phonons, so that the conductivity decreases roughly with the specific heat. The value of the phonon mean free path at room temperature is of the order of magnitude of the scale of the disorder in the structure of glasses as determined from x-ray evidence---that is, of the order of 7A. This is about the size of the unit cell of the crystalline forms of silica.

Dicalcium nitride as a two-dimensional electride with an anionic electron layer

Abstract: The ionic crystal Ca2N is shown to be an electride in terms of [Ca2N]+·e−, with diffusive two-dimensional transport in dense electron layers. The physical properties of electrides — ionic crystals in which electrons behave as anions — significantly depend on the topology of the confining cavity for anionic electrons. Thus, an essential step towards practical electride applications is to discover new confinement spaces with unique topologies. Confined two-dimensional electron layers have previously been achieved by artificially fabricating hetero-interface structures usually of semiconducting materials. Here the authors extend the range of materials demonstrating such behaviour to an electride, dicalcium nitride (Ca2N). This compound has ideal properties for electron confinement: a layered structure with appropriate interlayer spacing and a chemistry that allows for loosely bound electron layers without electron trapping. By providing a new material image for electrides, this work should lead to a series of two-dimensional electrides with unique physical properties. Recent studies suggest that electrides—ionic crystals in which electrons serve as anions—are not exceptional materials but rather a generalized form, particularly under high pressure1,2,3. The topology of the cavities confining anionic electrons determines their physical properties4. At present, reported confining sites consist only of zero-dimensional cavities or weakly linked channels4. Here we report a layered-structure electride of dicalcium nitride, Ca2N, which possesses two-dimensionally confined anionic electrons whose concentration agrees well with that for the chemical formula of [Ca2N]+·e−. Two-dimensional transport characteristics are demonstrated by a high electron mobility (520 cm2 V−1 s−1) and long mean scattering time (0.6 picoseconds) with a mean free path of 0.12 micrometres. The quadratic temperature dependence of the resistivity up to 120 Kelvin indicates the presence of an electron–electron interaction. A striking anisotropic magnetoresistance behaviour with respect to the direction of magnetic field (negative for the field perpendicular to the conducting plane and positive for the field parallel to it) is observed, confirming diffusive two-dimensional transport in dense electron layers. Additionally, band calculations support confinement of anionic electrons within the interlayer space, and photoemission measurements confirm anisotropic low work functions of 3.5 and 2.6 electronvolts, revealing the loosely bound nature of the anionic electrons. We conclude that Ca2N is a two-dimensional electride in terms of [Ca2N]+·e−.

Magnetoresistance oscillations in a two-dimensional electron gas induced by a submicrometer periodic potential

Abstract: A new type of magnetoresistance oscillation periodic in 1/B is observed when the carrier density Ns of a two-dimensional electron gas is weakly modulated with a period smaller than the mean free path of the electrons. Experiments with high mobility AlGaAs-GaAs heterojunctions where Ns is modulated by holographic illumination at T ≤ 4.2 K show that the period of the additional quantum oscillation is determined by the separation a of the interference fringes. This period corresponds to Shubnikov-de Haas oscillations where only the electrons within the first reduced Brillouin zone with |k| < π/a contribute.

Molecular-dynamics simulation of thermal conductivity of silicon crystals

Abstract: We investigate the thermal conductivity of bulk silicon crystals based on molecular-dynamics (MD) simulations. If it is taken that the system size must be larger than the phonon mean free path, several hundreds of millions of atoms must be computed for crystals with large thermal conductivity values such as Si. We demonstrate in this work that the thermal conductivity of Si crystals can be simulated by MD techniques using several thousands of atoms with periodic boundary conditions. We identify that the key issues generating size artifacts in small molecular-dynamics systems are the frequency cutoff imposed by the simulation domain length and the correlation artifacts caused by the periodic boundary conditions. Our method relies on the spectral Green-Kubo formulation combined with a model-based extrapolation. The obtained thermal conductivity results are in good agreement with the reference data. Both the Green-Kubo formulation and the Boltzmann transport equation lead to the prediction that the thermal conductivities of bulk crystals depend on the frequency of the thermal disturbance. This result has important implications for high-frequency electronic devices.