Anisotropy of conductivity due to warm holes inp-type germanium andp-type silicon

The dependence of the electric conductivity on a d.c. electric field of medium intensity is measured for 3.5 ohm-cmp-type silicon and 0.9 ohm-cmp-type germanium. From these data and the symmetry properties of the cubic crystal lattice the dependence of the conductivity on crystal orientation is determined. Also, an analytical treatment is made for silicon by assuming a model of constant energy surfaces near momentum?k=0 which consists of spheroids of rotational symmetry penetrating each other. The usually accepted model of warped surfaces does not seem to be tractable. The ratio of effective masses parallel and perpendicular to the axis of rotation enters as a parameter which is determined by the experimental value of the conductivity anisotropy. This mass ratio for silicon at 275 °K is 3.25, at 193 °K 4.64 and at 77 °K 37. With germanium the observed anisotropy is stronger than the anisotropy calculated with any finite mass ratio which means that the model is not adequate.     

Persistent photoconductivity and dangling bonds in amorphous germanium

It was shown experimentally that persistent photoconductivity occurs in an amorphous germanium film, submitted to laser irradiation. It is demonstrated that a strong electron-lattice coupling around the dangling bonds present in the material may explain this phenomenom qualitatively.     

Theory of the photon-drag effect inp-type germanium with a parabolic and anisotropic band-structure approximation

This paper shows that many features of the photon-drag voltage generated by illumination ofp-germanium with 10.6 μm radiation can be understood on the basis of a parabolic but anisotropic band-structure model. The group velocities of the heavy and light holes participating in the optical transitions are obtained by an analysis of the conservation laws. Since the nonequilibrium hole densities in both bands depend directly on the momentum relaxation times in the determinant equations of continuity, these times are the parameters with the strongest influence on the numerical results. The photon-drag voltage consists of components longitudinal and transverse with respect to the axis of illumination. For the longitudinal components, good agreement between our theoretical results and experimental data from the literature is achieved if the crystal is illuminated along a [100] or [111] direction. Though a transition probability independent of light polarisation has been assumed, the transverse effect (illumination along [111]) can be computed for special measuring conditions. The calculated effects of anisotropy are higher than known from experiments possibly due to the approximations used for the valence bands and the transition probability. An erratum to this article is available at .     

Relaxation of the shallow acceptor center in germanium

Polarized negative muons were used to study relaxation mechanisms of shallow acceptors in germanium. In p-type germanium at low temperatures relaxation of the muon spin was observed, indicating that the muonic atom (gallium-like acceptor center) formed via capture of the negative muon by a host atom is in the paramagnetic state and its magnetic moment is relaxing. The relaxation rate of the muon spin was found to depend on temperature and on concentration of gallium impurity. We conclude that to the relaxation of the magnetic moment of the Ga acceptor in Ge there contribute both scattering of phonons and quadrupole interaction between the acceptors. We estimate, for the first time, the hyperfine interaction constant for the gallium acceptor in germanium as 0.11 MHz.     

Muon state dynamics in germanium and silicon

We have carried out implanted positive muon studies on the molecular metal system d_n- (DMe-DCNQI)₂ Cu in order to understand better its novel magnetic properties. Examples of these salts at different levels of deuteration were studied. The fully deuterated ( d₈) salt shows a metal–insulator (MI) transition around 80 K and a magnetic transition around 7 K. The muon spin relaxation rate is enhanced below the MI transition, reflecting the localisation of spins along the Cu columns, however, the increase in muon spin relaxation rate occurs well above the metal–insulator (MI) transition and suggests a slowdown of the spin fluctuations around 120 K. At temperatures below 7 K a zero field precession signal was observed as a result of the 3D magnetic ordering of the Cu spins. For a muon site associated with the ring of the DCNQI molecule, the local field distribution was found to be consistent with the previously proposed magnetic structure. A sharp nuclear quadrupolar level crossing resonance (QLCR) was observed at 50 G which was assigned to resonance with the imine nitrogen on the DCNQI molecule. This revised version was published online in July 2006 with corrections to the Cover Date.     

Temporal dynamics of impurity photoconductivity in n-GaAs and n-InP

The dynamics of impurity photoconductivity in n-GaAs and n-InP under photoexcitation with a short light pulse has been calculated. It has been shown that the photoconductivity dynamics in a nanosecond time range is determined by cooling of electrons, while the role of cascade capture of electrons by impurity is insignificant in this range. A nonmonotonic time dependence of photoconductivity caused by the competition between different relaxation mechanisms of the electron pulse has been predicted.     

Transient photoconductivity associated with the formation of electron-hole droplets in optically pumped germanium

Transient decay properties of photoconductivity of highly excited germanium are investigated at 1.6–4.2 K by use of a pulsed laser with the width of 5 nsec. An anomalous profile is observed in the decay curve of the photocurrent, which is interpreted in terms of the electron-hole droplets model.     

Relaxation dynamics of semiflexible polymers

We study the relaxation dynamics of a semiflexible chain by introducing a time-dependent tension. The chain has one of its ends attached to a large bead, and the other end is fixed. We focus on the initial relaxation of the chain that is initially strongly stretched. Using a tension that is self-consistently determined, we obtain the evolution of the end-to-end distance with no free parameters. Our results are in good agreement with single molecule experiments on double stranded DNA.     

Magneto-Raman scattering inp-type indium antimonide

We report for the first time stimulated magneto-Raman scattering inp-type InSb. Two different Raman scattering processes were observed. The first one has a Raman shift of about 2cm⁻¹/kG and is observed at magnetic fields up to 30kG. The other one is observable only at high magnetic fields above 30kG and shows Raman shifts between 1.2cm⁻¹ and 3.0cm⁻¹ with a tuning rate of about 0.2cm⁻¹/kG. The first process can be interpreted either as spin-flip Raman scattering by photo-excited electrons in the conduction band or as Raman scattering by holes in the valence band involving transitions from heavy to light hole states. The other Raman shift observed seems to occur on account of transitions between the heavy hole ladders.     

Molecular dynamics simulations of EXAFS in germanium

Classical molecular dynamics simulations have been performed for crystalline germanium with the aim to estimate the thermal effects within the first three coordination shells and their influence on the single-scattering and multiple-scattering contributions to the Ge K-edge extended x-ray absorption fine structure (EXAFS).     

Relaxation dynamics in quantum electron glasses

It is experimentally shown that, depending on the carrier concentration of the system n, the dynamics of electron glasses either slows down with increasing temperature or it is independent of it. This also correlates with the dependence of a typical relaxation time (or "viscosity") on n. These linked features are argued to be consistent with a model for dissipative tunneling. The slow relaxation of the electron glass may emerge then as a manifestation of friction in a many-body quantum system. Our considerations may also explain why strongly localized granular metals are likely to show electron-glass effects while semiconductors are not.     

Relaxation dynamics of a crumpled globule

The relaxation dynamics of elastic networks of crumpled (fractal) globules obtained by computer simulation of the collapse of a polymer chain in different modes is studied. It is shown that, in their dynamic properties, folded globules are similar to proteins, molecular machines.     

Relaxation dynamics of charged gravitational collapse

We study analytically the relaxation dynamics of charged test fields left outside a newly born charged black hole. In particular, we obtain a simple analytic expression for the fundamental quasinormal resonances of near-extremal Reissner-Nordström black holes. The formula is expressed in terms of the black-hole physical parameters: , where TBH and Φ are the temperature and electric potential of the black hole, and q is the charge of the field.     

Molecular dynamics simulations of nanoindentation of monocrystalline germanium

Three-dimensional molecular dynamics simulations using the Tersoff potential are conducted to investigate the nanoindentation process of monocrystalline germanium (Ge). It is found that a phase transformation from fourfold-coordinated diamond cubic phase (Ge-I) to sixfold-coordinated β-tin phase (Ge-II) occurs during the nanoindentation process. The simulation results suggest that a pressure-induced phase transformation instead of dislocation-assisted plasticity is the dominant deformation mechanism of monocrystalline Ge thin films during the nanoindentation process.