Resonant interaction of acceptor states and optical phonons in silicon

The resonance broadening of line 2 in the excitation spectrum of gallium acceptors in silicon due to near coincidence of its energy with that of the zone center optical phonons, $\text{h}\text{?}$ω₀, has been confirmed under significantly improved experimental conditions. An additional feature labeled X and line 2 are interpreted as mixed excitations of the bound-hole and the optical phonon. Under uniaxial stress, the stress induced components of line 2 which approach $\text{h}\text{?}$ω₀ become more phonon-like and get “pinned” while the components of X become bound-hole-like as they recede from $\text{h}\text{?}$ω₀, and exhibit a striking increase in intensity.     

Hall effect in dilute magnetic alloys

We review the theory and the experimental results on the Hall effect in noble metals containing magnetic impurities of transition metals. In order to illustrate the various types of observed effects, we focus succesively on selected systems: $\text{Cu}$Mn, with only enhancement of the ordinary Hall effect due to the existance of different spin-up and spin-down currents; $\text{Au}$Fe and $\text{Au}$Cr, with skew scattering by magnetic impurities; $\text{Cu}$MnT ternary alloys (where T is a non-magnetic impurity), with skew scattering effects due to combined spin—orbit scattering by non-magnetic impurities and spin scattering by Mn impurities. The skew scattering in $\text{Au}$Fe and $\text{Au}$Cr can be ascribed to the orbital character of the impurity moments and accounted for in an orbitally degenerate virtual bound state model. However, the anomalous temperature dependence of the skew scattering in Kondo alloys at low temperature is not well understood. We also present some magnetoresistance data in order to describe the links between the Hall effect and the magnetoresistance in magnetic alloys. In particular, we relate the skew scattering and the magnetoresistance anisotropy observed in $\text{Au}$Cr alloys.     

Prediction of strong magnetic quantum oscillations of the nuclear spin relaxation in a quasi-two-dimensional metal

The nuclear spin lattice relaxation rate in a quasi-two-dimensional (2-D) metal under strong magnetic fields is studied in a special case where the electronic cyclotron mass is small compared with the free electron mass. In the pure limit (ω_cτ ? 1) and for sufficiently low temperatures ($\text{h}\text{?}\text{ω}{}_{\mathrm{c}}\text{> 2π}{}^2\text{k}{}_{\mathrm{B}}\text{T}$) we find remarkable quantum oscillations of the relaxation rate as a function of the magnetic field. The period of these oscillations is identical to that of the de Haas-van Alphen oscillations and their amplitude grows linearly with the magnetic field. The possibility of observing such oscillations experimentally in the quasi-2-D mercury chain compound Hg_3?δAsF₆ is discussed.     

Free carrier absorption in quantizing magnetic fields: Nonpolar optical phonon scattering

A quantum theory of free carrier absorption in nondegenerate semiconductors and in strong magnetic fields which was previously developed to treat the case when acoustic phonon scattering dominates the free carrier absorption process [1] is extended to treat the case when nonpolar optical scattering is important. When the electromagnetic radiation field is polarized parallel to the direction of the applied magnetic field, results are obtained which are similar to those when acoustic phonon scattering is dominant. The free carrier absorption is an oscillatory function of the magnetic field which on the average increases in magnitude with the magnetic field. However, more structure in the free carrier absorption occurs when nonpolar optical phonon scattering dominates. This is due to the fact that there are two periods in the oscillatory magnetic field dependence associated with the emission or the absorption of optical phonons during the intraband transitions. When the cyclotron frequency exceeds the sum of the photon and optical phonon frequencies, i.e. ω_c > θ + ω_o, the free carrier absorption is predicted to increase linearly with magnetic field when ?ω_c? k_BT. The magnetic field dependence of the free carrier absorption can be explained in terms of phonon-assisted transitions between the various Landau levels in a band involving the emission and absorption of optical phonons.     

Electron-phonon interaction in superconducting Nb

Acoustic phonon damping in Nb due to superconductivity is accurately measured by the inelastic neutron scattering technique. The phonon line widths can be characterized by the reduced parameters $\text{h}\text{?}\text{ω}{}_{\mathrm{p}}\text{/2Δ(0)}$ and T/T_c, and are in good qualitative agreement with the calculation by Bobetic based on the BCS theory.     

Repeated zones and Mahan cones in the angular dependence of photoemission

The intensity of photoemission from the Ta-derived d states in the layer compound 1T-TaS₂ has been measured as a function of both polar and azimuthal angle of emission at the photon energy $\text{h}\text{?}\text{ω=21.2}\text{eV}$. The light source was a high intensity He resonance lamp employing an aluminum window in place of the more conventional differentially pumped capillary system. The azimuthal dependence of the d emission displays three kinds of lobes which we refer to as “heads, chins and ears”. The heads and chins are oriented along directions close to the bifurcated lobes observed previously at $\text{h}\text{?}\text{ω=10.2}\text{eV}$. The ear lobes, however, are oriented quite differently and appear only at high polar angles, corresponding to larger relative values of the parallel wavevector. The results can be understood in terms of a repeated zone scheme in which the ears and chins (not necessarily at the same polar angle) correspond to different Mahan cones of the same optical transitions.     

Mode propagation in superconductors near Tc

The linearized Eliashberg theory for dirty superconductors in the vicinity of T_c is used to generate the dynamic Ginzburg-Landau (GL) equation for the order parameter and the coupled Boltzmann equation for the quasi-particle distribution function, with and without magnetic impurities and inelastic phonon scattering. The Schmid and Schön (SS) separation of the order parameter into longitudinal (real) and transverse (imaginary) components is shown to be exact to linear order. The former does not couple with the wave vector (k) and frequency (ω) dependent external field, while the latter does. Explicit expressions are exhibited for the current and charge densities to leading order [$\text{Δ}\text{T}$ in the gap regime, in which Δτ_s > 1 where τ_s is the conduction electron spin flip lifetime; and $\text{Δ}{}^2\text{T(ω + iDk}{}^2\text{)}$ in the gapless regime, Δτ_s < 1. D is the diffusion constant]. The continuity equation is shown to be obeyed to this order.Use of the Poisson equation in relation to the transverse component leads to a propagating mode, which, in the absence of magnetic impurities, reduces to the SS result. We find that the mode propagates only in the gap regime, whereas it is purely diffusive in the gapless regime and for gapless superconductors. In the presence of magnetic impurities, the solution reduces to the SS limit, but with reduced mode velocity and damping. The charge density appropriate to the propagating mode is shown to be zero to order $\text{ω}\text{ω}{}_{\mathrm{p}}{}^2\text{τ}{}_1$, where τ₁ is the impurity scattering time and θ_p the plasma frequency. The characteristic penetration length of an electric field into a superconductor is calculated, taking into account the phonon inelastic scattering time. The result reproduces the Waldram and SS expressions. Careful examination is made of the conditions under which these results pertain. The characteristic penetration length is also obtained in the gapless regime and for gapless superconductors for which T_cτ_s < 1.Finally, the longitudinal and transverse pair susceptibilities are calculated with and without magnetic impurities and in the gap and gapless regimes.     

Raman scattering in magnetic US3

A Raman scattering investigation of magnetic US₃ has been made from 7 to 300 K. Comparison of room temperature spectra with those of the non magnetic isostructural sulfide HfS₃ allowed the assignment of most of the lines to $\textcolor{red}{}\text{=}\text{0}$ optical phonons. Drastic changes take place in the 10?150 cm^?1 range when lowering the temperature down to 7 K : four equally spaced lines appear at 54, 72.5, 91 and 109.5 cm^?1. Three of them broaden significantly with increasing temperature and disappear near 50 K, at which previous measurements indicate a maximum in the magnetic susceptibility and suggest a magnetic phase transition. The stronger fourth line is still observed at 100 K and merges into a phonon line at higher temperature. These four lines are attributed to electronic transitions within the 5f² configuration of U⁴⁺. Their temperature dependences appear to involve a spin-dependent scattering mechanism and are consistent with antiferromagnetic ordering.     

Level splitting in semimagnetic semiconductors under spin-magnetophonon resonance conditions

The splitting of electronic levels in quantum wells of semimagnetic semiconductors typically characterized by large effective g factors is analyzed theoretically. They are found to be capable of supporting resonance, provided the Zeeman spin-level splitting is equal to the energy of the longitudinal optical phonon ?ω_‖. The resonance condition can be written as ?ω_‖ = gμ_B B. This condition can be satisfied by choosing the magnetic field Bsuch that the sum of the energies of the lowest spin level and the optical phonon coincides with the energy of the highest level. It is shown that these two degenerate energy levels should experience mutual repulsion. The magnitude of the corresponding splitting depends on both the electron-phonon and spin-orbit interactions in semiconductors; moreover, it turns out substantially lower than the Zeeman energy gμ_B B. Resonant passage of light through and its reflection from a quantum well are considered as one of possible ways to observe this energy level splitting.     

Electron scattering with spin flip in indium antimonide and indium arsenide

The longitudinal magnetoresistance has been investigated at temperatures in the range from 2.8 to 200 K in a magnetic field of up to 200 kOe with the aim of determining the temperature range and the magnetic field strength at which charge carrier scattering with spin flip occurs in n-type indium arsenide and n-type indium antimonide. It is established that quantum oscillations of the longitudinal magnetoresistance of indium arsenide exhibit weak zero maxima due to electron scattering with spin flip at temperatures in the range from 4 to 35 K in a magnetic field of 146 kOe. For the longitudinal magnetoresistance of indium antimonide, zero maxima caused by electron scattering with spin flip are revealed in the temperature range from 60 to 80 K in a magnetic field of 132 kOe.     

Possible beat effect of specific heat due to impurities in semi-metal in external magnetic field

The simple model of a single-band semi-metal or degenerate semiconductor with impurities in an external quantizing magnetic field is discussed. The susceptibility and specific heat are studied in their dependence on the field-induced quasi-local levels, due to the impurities. These levels are shown to cause additional oscillations in the thermodynamical quantities as functions of the magnetic field. Their contribution to the susceptibility and specific heat is comparable to the values for the undoped system if the impurity concentration c_i fulfils the condition $\text{c}{}_{\mathrm{i}}\text{= (c}{}_{\mathrm{e}}\text{/4√π)(}\text{h}\text{?}\text{ω/ε}{}_{\mathrm{F}}\text{)}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$, with c_e the electron concentration.     

Hall effect in GaTe single crystals

Hall effect measurements have been carried out over the temperature range 77–500K on p-type GaTe single crystals, grown from the melt. The results indicate that scattering by homopolar optical phonons polarized normally to the layers is largely dominant in GaTe. From the analysis of Hall mobility data, carried out according to the Schmid's model, a phonon energy $\text{h}\text{?}\text{ω = 9.8 meV}$ and the product between the coupling constant and the hole conduction mass along the layers g²m_{h⊥ = v.205me} has been found Finally, an acceptor center, lying at 138 meV from the valence band, displays an hydrogen-like behaviour, with its energy dependent from concentration according to the theory.     

Spin waves in an Invar alloy (Fe0.65Ni0.35)

The spin wave dispersion relation in an Invar alloy Fe_0.65Ni_0.35 has been measured at 4.2 K in the [111] direction by neutron inelastic scattering.Well defined magnon groups have been observed up to an energy transfer of about 80 meV. The spin wave dispersion is well described by ?ω=Dq²(1?βq²) with $\text{D=143}\text{meV}\text{A}\text{?}$ and $\text{β=0.12}\text{A}\text{?}{}^2$. The value of D is in accord with the value extrapolated from other neutron scattering results at higher contents of Ni and disagrees with spin wave resonance results.No trace of γ-iron type antiferromagnetic order could be detected at 4.2 K in this alloy by elastic neutron scattering measurements.     

Phonon heating by hot electrons in n-InSb in quantizing magnetic fields

The effect of the phonon “narrow throat” was experimentally found in n-InSb in crossed electrical and quantizing magnetic fields at temperatures 1.6—4.2°K. The phenomenon of energy relaxation by hot electrons on phonons was detected with $\text{T}\text{S}\text{?}\text{h}\text{?}\text{λ}{}^{\mathrm{?1}}$ in the case of absence of a phonon thermal tank (S is sound velocity, λ is magnetic length, T is temperature). The value of a critical electric field (E_cr) on the S-type current-voltage characteristic (CVC) was measured as a function of temperature and the magnetic field.     

Evidence of an induced polariton branch in CuCl by hyper-Raman scattering

We study the renormalization of the dispersion of excitonic polaritons, induced by a strong light source at frequency ω_l. Since biexcitons with energy E_Bi show a giant oscillator strength, this renormalization effect is located around frequencies Ω such as $\text{h}\text{?}\text{Ω +}\text{h}\text{?}\text{ω}{}_{\mathrm{l}}\text{= E}{}_{\mathrm{Bi}}$. It gives rise to an induced polariton branch, the dispersion of which depends on the frequency of the exciting light source. It manifests itself through additional emission lines in hyper-Raman scattering processes.     

Influence of the surface on the quantum P.E.M. effect in InP

The photoelectromagnetic effect of InP is studied in quantizing magnetic fields at 4·2 K in an energy range 1·4–1·5 eV for linearly polarized light. Depending on the sample surface condition two types of spectral oscillations may appear, those associated with interband transitions between Landau levels or the LO phonon type usually seen in photoconductivity. An analysis of the spectral oscillations gives: E₀ = 1·423±0·001 eV; Δ₀ = 0·102±0·006 eV; hω_L = 0·036 eV.     

Dispersion of empty surface states on Ni(1 1 0)

We have used k-resolved bremsstrahlung isochromat spectroscopy at $\text{h}\text{?}\text{ω=9.5eV}$ to investigate Ni(1 1 0) along the ΓKL bulk mirror plane. Empty surface states of both the crystal-induced and image-potential-induced type have been detected besides two bulk direct transitions. We studied their different behaviour against oxygen contamination and mapped their energy dispersion E(k_∥) along the ΓY direction of the surface Brillouin zone.     

Quantum oscillations in screening in a two-dimensional electron gas

The static dielectric constant of a two-dimensional electron gas is studied as a function of the strength of a dc magnetic field applied normal to the plane of the electron gas. At high temperatures $\text{(kT ?}\text{h}\text{?}\text{ω}{}_{\mathrm{c}}\text{)}$ the static dielectric function is independent of magnetic field, and for long wavelengths is given by ? ? ?₀ + 2n_vme²/q, where ?₀ is the background dielectric constant and n_v is the valley degeneracy. At low-temperatures, quantum oscillations become important and dramatically modify the screening.     

Photoconductivity and photovoltaic excitation spectra and their wavelength derivative in Cu2O

Photoconductivity and photovoltaic excitation spectra, both direct and wavelength modulated, have been performed on monocrystalline Cu₂O from 6 to 300K. At low temperatures both types of direct spectra exhibit the excitonic structures already observed by other techniques such as absorption and reflectivity. The wavelength-modulated PC spectra reveal additional structures and in particular well-defined oscillations in the green fundamental continuum, related to photocarrier LO phonon interactions. The analysis of these results shows that the 149cm^?1 (18.5 meV) LO phonon is responsible for the observed periodic structures. The values found for the energy of the collector level (2.305 ±0.001 eV) and for the effective mass ratio $\text{(}\text{m}{}_{\mathrm{h}}\text{m}{}_{\mathrm{e}}\text{bsime; 12)}$, indicate that the oscillations are produced by the electrons excited from the heavy hole valence band Γ₈⁺ to the Γ₁⁺ conduction band. A study of the position and of the contrast of the phonon oscillations as functions of temperature illustrates the behaviour of the gap displacement with changes in temperature and throws some light onto the mechanisms responsible for the existence of such oscillations.     

Neutron scattering from FeSi

The inelastic neutron scattering from the unusual paramagnet FeSi has been measured at temperatures of up to 700 K in order to investigate the possible existence of a local magnetic moment at high temperatures. The estimated intensities of the central peaks around $\text{h}\text{?}\text{ω = 0}$ and the nearly constant backgrounds ($\text{h}\text{?}\text{ω ? 16 meV}$) indicate that the paramagnetic state above 600 K in FeSi cannot be interpreted in terms of a simple localized spin model with $\text{S ～}\text{1}\text{2}$.