摘要:

AbstractWe present measurements of the Peltier‐effect in the mixed state of Bi1.76Pb0.24Sr2Ca2Cu3Oδ. The Peltier‐coefficient broadens in a magnetic field quite similar to the resistivity and the thermopower. Comparison with the thermopower shows that Onsagers relation holds well. The occurrence of the Peltier‐heat in the mixed state well belowTcimplies that the electric current is accompanied by a large heat current. We show that the vortex contribution to the Peltier‐heat is negligibly small. Therefore the heat current has to be attributed to normal quasiparticle excitations. Our results indicate that this quasiparticle contribution to the heat current remains large even at temperatures far

ISSN:0003-3804    

DOI:10.1002/andp.19945060402

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractIn this paper we present a microscopic model that allows us to study the effects of charge fluctuations on the phonon dispersion of the high‐temperature superconductor YBa2Cu3O7and its insulating counterpart, YBa2Cu3O6. An ab‐initio rigid‐ion model with pair potentials calculated by the Gordon‐Kim method from the free‐ion charge densities is used as a reference system. Starting from this reference system, charge fluctuations at the copper‐ and oxygen ions are introduced into the model. The charge fluctuations are treated as adiabatic degrees of freedom in a non‐phenomenological way. The parameters entering the model are estimated consistently with the reference system from first principles rather than refering to the experimentally determined phonon dispersion. In addition to the metallic behavior (appropriate to YBa2Cu3O7) obtained in this way, insulating behavior (appropriate to YBa2Cu3O6) is simulated by requiring the polarizability function to fulfill a corresponding long. wavelength sum rule. Screened site‐potential changes are defined that (besides the charge fluctuations) constitute a qualitative measure of the electron‐phonon‐interaction potential. Furthermore we investigate the long‐wavelength limiting behavior of the most important quantities occurring in our formalism. We derive formulae that allow us to calculate the contribution of the charge fluctuations to the macroscopic dielectric constant and the transverse effective charges in

ISSN:0003-3804    

DOI:10.1002/andp.19945060403

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractUsing the model presented in the preceding paper we investigate the effects of charge fluctuations (CF) on the phonon dispersion of YBa2Cu3O6(O6) and YBa2Cu3O7(O7). Starting from an ab‐initio rigid‐ion model as a reference system, CF are allowed for at the copper‐ and oxygen ions. The CF are treated as adiabatic electronic degrees of freedom. Within the rigid‐ion model (RIM) the structural parameters are calculated by minimization of the energy. The results agree reasonably well with the experiment, indicating the suitability of the ionic model as a starting point and the importance of ionic forces for the properties of the high‐temperature superconductors (HTSC) in general.Next, the phonon dispersion is calculated in the RIM as well as including CF additionally and the renormalization of the individual modes is discussed. By restricting the CF optionally to the planes, effects arising specifically from CF in the planes on the one hand and from CF in the chain as well as at the axial bridging oxygens (O4) on the other hand can be separated. We find the oxygen axial modes at the γ‐ andZpoint (A1g/A2μin O6, Ag/B1μin O7) particularly interesting. Most of these modes show considerable renormalizations. Moreover, the γ/Z‐axial modes are characterized by the possibility of having CF of the same sign in the whole CuO planes what distinguishes them from the modes at other symmetry points. In particular, theZ‐point axial modes are singular in having CF of alternating sign in consecutive structural units incdirection. Such a “c‐direction‐charge‐transfer” has been shown previously to be an effective screening mechanism in La2CuO4. Indeed, we find a drastic renormalization of the plane‐oxygen Agmode at theZpoint (Ag(O23;Z)) in O7(oxygen ions in neighboring planes vibrating in‐phase), at least in the adiabatic approximation used here. In the insulating phase this mode exhibits, on the other hand, very large changes of the potential at the ion sites, whereas its renormalization is moderate only. The reason for this behaviour is that in the insulating phase in case of a two‐dimensional electronic structure the charge transfer (screening) is restricted locally in the structural unit and long‐range charge transfer is not possible as in the metal. However, a strong suppression of screening for this mode can also be expected for themetallic phasein O7in casenon‐adiabaticelectron‐phonon coupling would be important. The Ag(O23;Z)‐mode thus seems to be by far the most interesting mode in O7. These features are directly related to the layered structure of the HTSC compounds considered here. The O4‐axial‐breathing modes show significant renormalizations too, and are characterized by plane‐chain charge transfer. Moreover, besides the O23‐ and O4‐modes, the yttrium modes appear to be important too. In addition to the phonon‐dispersion curves, we present values for the CF amplitudes and screened site‐potential changes at the copper‐and oxygen ions. Finally, we give transverse effective charges and dielectric constants for t

ISSN:0003-3804    

DOI:10.1002/andp.19945060404

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractTo account for the non‐equilibrium character of quark‐gluon plasma formation in relativistic heavy‐ion collisions, it is proposed to use the product of energy density ϵ and interaction time τint(rather than ϵ) as the critical quantity for plasma formation. In a geometrical model, τ is approximated by the transit time, and ϵ by the maximum energy density calculated in the center‐of‐mass frame. For central collisions, an analytical expression for ϵ·τ is given. As an example, the systems16O +16O and208Pb +208Pb are investigated as functions of c.m. energy, and their respective suitability for plasma forma

ISSN:0003-3804    

DOI:10.1002/andp.19945060405

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractThe incorporation of two‐ and three‐dimensional Δ‐function perturbations into the path integral formalism is discussed. Contrary to the one‐dimensional case, a regularization prescription is needed due to the divergence of the Green function corresponding to a potentialV,G(v)(x,y;E) (x, y ϵ ℝ2, ℝ3) forx→y. The known procedure to define proper self‐adjoint extensions for Hamiltonians with point‐interactions can be exploited to define the incorporation of δ‐function perturbations in the path integral. Several examples il

ISSN:0003-3804    

DOI:10.1002/andp.19945060406

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

ISSN:0003-3804    

DOI:10.1002/andp.19945060401

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY