Electron-phonon Processes Research Articles

Femtosecond investigation of electron thermalization in gold.

Electron-electron and electron-phonon thermalization processes are investigated in gold films using a high-sensitivity multiple-wavelength femtosecond pump-probe technique. A nonequilibrium electron distribution is excited by free-carrier absorption of an infrared femtosecond pulse and its relaxation dynamics followed by measuring the transient reflectivity and transmissivity with a visible probe pulse. Measurements performed in a thin gold film in the very low perturbation limit (\ensuremath{\Delta}${\mathit{T}}_{\mathit{e}}$\ensuremath{\sim}20 K) yield evidence for the existence of a non-Fermi electron distribution with an electron thermalization time of \ensuremath{\sim}500 fs and an electron-lattice interaction time of 1 ps. The apparent thermalization dynamics of the electron gas is much faster in optically thick samples and is shown to be dominated by transport effects.

Quantum transport of buried single-crystalline CoSi2 layers in (111)Si and (100)Si substrates.

Magnetoresistance data for clean crystalline ${\mathrm{CoSi}}_{2}$ layers were analyzed in terms of weak localization, Coulomb interactions, and superconducting fluctuations. The ${\mathrm{CoSi}}_{2}$ layers with thicknesses of 11.5 nm in (111)Si and 23 nm in (100)Si were fabricated by high-dose ion implantation and subsequent annealing in a rapid thermal annealer (known as ion-beam synthesis or mesotaxy). The magnetic-field dependence of the resistance is interpreted in terms of two-dimensional weak localization with strong spin-orbit interaction and an addtional classical contribution proportional to ${\mathrm{H}}^{2}$. No indication of magnetic scattering was found, which is a sign of the ``cleanness'' of the samples. Long phase-coherence lengths of ${\mathit{l}}_{\mathrm{\ensuremath{\varphi}}}$\ensuremath{\approxeq}0.75 \ensuremath{\mu}m in (111)Si and ${\mathit{l}}_{\mathrm{\ensuremath{\varphi}}}$\ensuremath{\approxeq}2.3 \ensuremath{\mu}m in (100)Si at 4.2 K were determined by fitting the magnetoresistance data. The inferred inelastic-scattering time is interpreted as a sum of a clean-limit electron-electron process (dominant at temperatures below \ensuremath{\approxeq}6 K) and an electron-phonon process dominant at higher temperatures. We further observed a general orientation dependence of the electrical transport properties of mesotaxial ${\mathrm{CoSi}}_{2}$ layers, such as anisotropy in the residual resistance, Hall coefficient, and the prefactor for the classical ${\mathrm{H}}^{2}$ dependence of the magnetoresistance. This is probably related to multiple-band effects in ${\mathrm{CoSi}}_{2}$.

Theory of Auger upconversion in quantum wells in a quantizing magnetic field

Scattering rates of Auger and electron-phonon processes between discrete Landau levels in a quasi-two-dimensional electron system are calculated. Using these rates the authors provide an understanding of the upconversion observed in the magneto-luminescence of one-side modulation doped GaAs/AlGaAs quantum wells; in particular, of its dependence on excitation power and magnetic field.

The interaction between electron and phonon localized in a double-barrier resonant tunneling diode

The electron-phonon interaction (the transition probability from the initial electronic state to the final state via the electron-phonon interaction) is analyzed when electrons and phonons are either extended or localized in a quantum system. Compared with the situation of the bulk material where all wave functions are extended, it is generally found that the electron-phonon interaction is reduced when one state is localized while it will be enhanced when more states involved in the interaction process become localized. It has been shown that the electron-phonon interaction is inversely proportional to the well width when all the states involved are localized. Special attention is then focused on the double-barrier resonant tunneling diode. A simple Monte Carlo scheme is developed to include the electron-phonon interaction process in the quantum well between two barriers. The numerically calculated I-V characteristics agrees much better with the experimental spectra if the electron-phonon interaction has been taken into account.

Equilibrium solutions of the non-linear Boltzmann equation for an electron gas in a semiconductor

We study the collision operator of the non-linear Boltzmann equation describing the electron flow in a semiconductor under the assumption of electron-phonon interaction processes. We establish anH-theorem and prove the existence of an infinite number of equilibrium functions in addition to the Fermi-Dirac distribution. The meaning of the corresponding collision invariants is shown to derive from the nature of the electron-phonon scattering.

Influence of Umklapp processes on the sign of phonon-drag thermopower in semiconductor superlattices

The phonon-drag thermopower, Sgp, is calculated for semiconductor superlattices, with the temperature gradient parallel to the superlattice axis, taking account of electron-phonon Umklapp processes. Sgp changes sign when the Fermi energy reaches the top of the miniband. The anomalous behaviour is attributed to the inclusion of U-processes.

Diffraction models, dynamical structure tensors and electron-phonon interaction

A coherent exposition of the theory of scattering from a dynamic lattice, which applies to the scattering of Bloch-wave electrons as well as to the scattering of plane waves, is presented. This is achieved by developing a generalised diffraction model, along the lines conceived by Van Hove, appropriate for the scattering of Bloch waves. The resulting diffraction model provides a basis for treating processes controlled by electron-phonon interaction in general and subsumes the standard diffraction model results. The diffraction model origins of the standard approximate expressions used to describe electron-phonon interaction-controlled processes are also revealed.

Electron-phonon relaxation in pure metals and superconductors at very low temperatures.

The electron-phonon relaxation processes in pure metals and superconductors are considered at low temperatures when electrons interact with both longitudinal and transverse phonons. The screening of the electron-ion interaction is treated in a gauge-invariant form, so that both longitudinal and transverse electromagnetic fields are taken into consideration. The results obtained are applied to the analysis of the heating effect of the electron system. The interaction of electrons with transverse phonons increases the electron-phonon relaxation rate by a factor of {congruent}15 in comparison with the case when only longitudinal phonons are taken into consideration. The peculiarity of the screening effects in a superconductor leads to the conclusion that at low temperatures {ital T}{much lt}{Delta} transverse phonons do not participate in the electron-phonon relaxation processes.

WEAK LOCALIZATION EFFECT IN METALLIC GLASS Zr78Co22

Measurements of magnetoresistance of upper critical magnetic field have been made on superconducting metallic glass Zr78Co22. The results correspond qualitatively with the weak localization theory including spin-orbit scattering for three demensional system. Quantitatively, there exists an obvious deviation, which originates mainly from superconducting fluctuations. In the measuring temperature range, the value of upper critical field Mc2 varies linearly with temperature, and we obtain the inelastic scattering rate τi-1=1.3×1O10T2, which can be atributed to the electron-phonon processes in disordered metals This value is slightly lower than that predicted by Bergmann theory.

Electron transport in multiple-quantum-well structures

The authors consider electronic transport in a multiple-quantum-well structure (MQW). The transport is described in terms of a superposition of various transport paths each associated with a given length (coherence length as determined by energy relaxation). Their starting point is the simple double-barrier situation, and the sequential tunnelling approximation. They estimate the elastic and inelastic transfer rates including electron-phonon, electron-electron, and impurity-assisted processes. They find that in most cases one should expect defects in the barrier and impurities to be the reason for the absence of marked non-linearities in the I-V characteristics.

Ab-Initio Calculation of the Electron-Phonon Interaction in Simple Metallic Systems

Within the framework of the rigid muffin-tin approximation, it is shown that Hartree-Fock total energy cluster calculations can be combined with traditional band structure results to calculate truly ab-initio the mass enhancement parameter λ. The average phonon force constant m<ω2> is calculated ab-initio through the cluster results, whereas the electronic parameter η is calculated as usual from the band structure calculation. This procedure, which can be enhanced with non-rigid muffin-tin corrections, emphasizes both the banding and the bonding characteristics of the electron-phonon interaction process. Besides the first few elemental metals for which an excellent agreement with experiment exists, the method has been recently applied to various bonding configurations of the high Tc La-Cu-O superconductors. In this last case, the results are only preliminary.

Superconducting fluctuations, weak anti-localization and interaction effects in Nb0.53Ti0.47-Ge multilayers

Abstract The effect of weak anti-localization, electron interactions and superconducting fluctuations on the transport properties of disorder Nb 0.53 Ti 0.47 -Ge multilayers were studied. The temperature dependence of the inelastic scattering time was found to be τ in ∼ T −1±0.25 and τ in ∼ T −1.75±0.25 for temperatures lower and higher than ∼ 6K, repectively for a sample with thick Ge layers. The effect of the Ge thickness on the prefacter A in the expression R □ ∼ ln T may arise from an interlayer electron-phonon process.

Transient photocurrents in a-Si: H and weak electron-phonon coupling processes

Abstract The multiple trapping (MT) model is modified by introducing weak-coupling multiphonon trapping processes. This model is applied to the primary and secondary transient photocurrents occurring in a-Si:H. Previously reported experimental results can be explained with the aid of this model, and difficulties previously encountered in using the conventional MT models are overcome.

Localization and electron-interaction effects in metallic glasses.

Magnetoresistance measurements have been made on amorphous Lu/sub 75/Pd/sub 25/ and Lu/sub 75/Ni/sub 25/ alloys. In the absence of contributions from spin-splitting interaction (because of the high spin-orbit scattering rate 1/tau/sub so/approx. =10/sup 14/ s/sup -1/) and superconducting fluctuations, direct comparison of experimental data with three-dimensional weak localization theory can be made. Our results follow the trend predicted by theory, but are about 20% larger. Interaction effects with a negative screening parameter can explain both the discrepancy and the order of magnitude of resistivity rho(T). We obtain the inelastic scattering time tau/sub i/(T)approx. =2 x 10/sup -10/T/sup -2/ s which can be attributed to electron-phonon processes in disordered metals, according to the theory of Bergmann and Takayama. Prior results obtained for amorphous Zr/sub 40/Cu/sub 60/ are discussed only qualitatively as this alloy does exhibit superconducting fluctuations above 0.5 K.

Laser-induced electron–phonon processes on metal surfaces

Electronic charge transfer from bulk to surface states in metals induced by laser radiation is examined. Due to the position of the Fermi energy, it is necessary to also include phonon excitations. Cross sections for this second-order process are presented. Comparison with the surface excitation in semiconductors is made. It is seen that for metals the required laser frequencies are larger and the cross sections substantially less than for semiconductors.

Inelastic electron scattering mechanisms in clean aluminum films

Magnetoresistance data for clean aluminum films (${R}_{\ensuremath{\square}}=0.2\ensuremath{-}4 \ensuremath{\Omega}$) are analyzed in terms of localization and superconducting fluctuations. The inferred inelastic-scattering rate ${\ensuremath{\tau}}_{i}^{\ensuremath{-}1}$ is interpreted as the sum of electron-phonon and dirty-limit electron-electron processes. Extrapolation of these results for ${\ensuremath{\tau}}_{i}^{\ensuremath{-}1}$ shows good agreement with results of superconducting nonequilibrium studies of Chi and Clarke for ${R}_{\ensuremath{\square}}\ensuremath{\gtrsim}1 \ensuremath{\Omega}$. For lower ${R}_{\ensuremath{\square}}$, another inelastic mechanism is evident, possibly clean-limit electron-electron scattering.

Importance of electron-phonon processes in the inelastic lifetime of conduction electrons

The temperature of conduction electrons can be raised above the lattice temperature by a high electric current density. The achieved temperature difference is proportional to the electron-phonon relaxation time. Weak localization yields information about the electron temperature and allows the conclusion that electron-phonon processes are an essential determinant for the inelastic lifetime of the conduction electrons.

New approach to the calculation of ground-state properties in solids

A novel application of the minimum property of the ground-state energy, within the density-functional approach, enables us to obtain a simple and accurate expression for total energy differences. The crucial role here is played by the difference between one-electron energies, under a condition. This way we can explain the energy cancellations responsible for the success of the $d$-bond model in transition metals and their compounds. In an expression obtained for shear and phonons, two important energy terms are present. One is essentially ionic and describes a renormalized bare phonon. The other is essentially covalent and expressed again by differences between one-electron energies under the frozen-potential condition. This expression justifies the use of approximations based on a rigidly displaced potential for electron-phonon coupling calculations, which yielded good results for transition metals in the past. Their success is shown to be based on the fact that the frozen-potential condition accounts well for the many-body effects associated with the electron-phonon coupling process, because under this condition self-consistency and screening effects are mutually canceled.

The hall coefficient for isotropic electron scattering

The Hall coefficient, R H, of the ternary alloys Cu>  AuGe and Cu  NiGe is found to be independent of temperature from room temperature down to (at least) 5 K. Above about 100 K the values of R H are in close agreement with those of pure copper. From these two observations we deduce that, in accordance with predictions from de Haas van Alphen measurements, the scattering for both electron-phonon and electron-impurity processes is isotropic.

Effects of inelastic electron-electron scattering on branch imbalance relaxation

The rate for branch-imbalance relaxation in a superconductor associated with inelastic electron-electron (Coulomb) scattering is derived, valid at arbitrary temperatures. The calculation is performed for clean superconductors, and shown to extend to cases where impurity scattering is important, using a technique of Schmid. As expected, for temperatures in the vicinity of ${T}_{c}$ the form for the relaxation rate is identical with that arising from inelastic electron-phonon processes, if one replaces the inelastic electron-phonon scattering time with the inelastic electron-electron scattering time.