The anelastic relaxation measurements (elastic energy loss and moduli as a function of temperature) yield information on any dynamical process which is coupled to strain and has a characteristic frequency of the order of the measuring frequency (0.1-100 kHz in the present case). Typical processes of this type are the reorientation of defects, the diffusion of atoms and polarons, the movement of domain walls. In the semiconductors for applications in electronics, the formation of complexes of interstitial H with dopants or defects and the H dynamics are a subject of great interest, which can be studied by anelastic experiments. Measurements on Si doped with P or B and charged with H or D agree with the theoretical predictions on the geometry of the dopant-H complex and yield a precise determination of the reorientation dynamics of H around B. First principle calculations show that, for some combinations of host and dopant atoms, interstitial H should occupy slightly off-center positions with respect to the so-called bond-center site between dopant and nearest neighbor host atom. In these cases, tunneling of H among the off-centre positions is expected, and should appear in the anelastic spectra. Indeed, in GaAs:Zn charged with H or D, a process is found with a relaxation rate which is several orders of magnitude faster than any other known process in hydrogenated semiconductors, and could arise from tunneling of H near Zn. Anelasticity experiments can provide useful information also on the high-Tc superconductors. The non-stoichiometric O in these cuprates controls the hole doping and can be mobile even below room temperature, giving rise to a variety of phenomena which affect the superconducting properties. From the anelastic spectra of YBa2Cu3O6+x and La2CuO4+? one obtains selective and precise information on all the possible types of jumps of interstitial O, namely the diffusive jumps of isolated or aggregated O atoms in the various structural phases, and the jumps between the off-centre positions in the Cu-O chains of YBa2Cu3O6+x. In La2CuO4+? the anelastic spectrum also contains two very intense contributions from relaxation-like dynamics of the sublattice of the O octahedra. One of them, also observed by NQR relaxation, has been identified with the propagation of soliton-like walls between domains with different tilt patterns of the octahedra. The other process, occurring below 30 K and strongly dependent on doping, indicates an instability of the lattice or can even be a signature that the orthorhombic phase of La2CuO4+? is a dynamical Jahn-Teller pahse. These phenomena are directly connected with the anharmonic potential felt by the octahedral units and with their local ordering. They must also be connected or even identified with the lattice stripes and charge phase separation.
Dynamics of atoms, polarons and defects in semiconductors and superconductors from anelastic relaxation
F Cordero;
1999
Abstract
The anelastic relaxation measurements (elastic energy loss and moduli as a function of temperature) yield information on any dynamical process which is coupled to strain and has a characteristic frequency of the order of the measuring frequency (0.1-100 kHz in the present case). Typical processes of this type are the reorientation of defects, the diffusion of atoms and polarons, the movement of domain walls. In the semiconductors for applications in electronics, the formation of complexes of interstitial H with dopants or defects and the H dynamics are a subject of great interest, which can be studied by anelastic experiments. Measurements on Si doped with P or B and charged with H or D agree with the theoretical predictions on the geometry of the dopant-H complex and yield a precise determination of the reorientation dynamics of H around B. First principle calculations show that, for some combinations of host and dopant atoms, interstitial H should occupy slightly off-center positions with respect to the so-called bond-center site between dopant and nearest neighbor host atom. In these cases, tunneling of H among the off-centre positions is expected, and should appear in the anelastic spectra. Indeed, in GaAs:Zn charged with H or D, a process is found with a relaxation rate which is several orders of magnitude faster than any other known process in hydrogenated semiconductors, and could arise from tunneling of H near Zn. Anelasticity experiments can provide useful information also on the high-Tc superconductors. The non-stoichiometric O in these cuprates controls the hole doping and can be mobile even below room temperature, giving rise to a variety of phenomena which affect the superconducting properties. From the anelastic spectra of YBa2Cu3O6+x and La2CuO4+? one obtains selective and precise information on all the possible types of jumps of interstitial O, namely the diffusive jumps of isolated or aggregated O atoms in the various structural phases, and the jumps between the off-centre positions in the Cu-O chains of YBa2Cu3O6+x. In La2CuO4+? the anelastic spectrum also contains two very intense contributions from relaxation-like dynamics of the sublattice of the O octahedra. One of them, also observed by NQR relaxation, has been identified with the propagation of soliton-like walls between domains with different tilt patterns of the octahedra. The other process, occurring below 30 K and strongly dependent on doping, indicates an instability of the lattice or can even be a signature that the orthorhombic phase of La2CuO4+? is a dynamical Jahn-Teller pahse. These phenomena are directly connected with the anharmonic potential felt by the octahedral units and with their local ordering. They must also be connected or even identified with the lattice stripes and charge phase separation.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
