Phonon-drag

Theoretical interpretation is incomplete early measurements on La-Sr-Cu-O by Hundley et al. (76) were suggested to indicate a phonon drag to explain some features although more recently a narrow band Hubbard model has been developed by Fisher (77). 

Both the room-temperature thermoelectric power and the phonon-drag component at low temperatures increase with hydrostatic pressure due to a decrease in the volume fraction of strong-correlation fluctuations in an itinerant-electron matrix. 

In pure metals the electron-phonon interaction is inversely proportional to the number of thermal phonons Te-ph 7 [1]. This result is valid for pure limit 97- / 1 qr is thermal phonon wave vector, / is the electron mean free path) [2,3]. In dirty limit qt / 1) electrons mostly scatter from defects and impurities and the electron-phonon interaction demonstrates more complicated behavior. According to the theoretical analysis made by Thouless [4] and Reizer [3] the relaxation time is proportional to T Te.pf x T ) in the case of full phonon drag of scattering centers. 

In all of the superconducting transitions observed, the superconductivity is due to the pairing of holes, as evidenced by the abrupt decrease in the thermopower from a positive value. The thermopower decreases to zero at the lower transition as expected. However, there are numerous known cases where the thermopower has the opposite sign of the carrier charge. The noble metals are a classic example. The measured thermopower has both positive and negative contributions, from holes and electrons, respectively, each of which can be enhanced by such effects as phonon drag and... 

By taking into account second-order effects in the electron-phonon interaction, Nielsen and Taylor (1974) have suggested that the low-temperature deviation from linearity of the thermopower may be partially explained in terms of the diffusion component itself. The Nielsen-Taylor effect is sometimes referred to as phony phonon drag . 

Craig, P.P., W.I. Goldberg, T.A. Kitchens and J.I. Budnick, 1967, Phys. Rev. Lett. 19, 1334. Crisp, R.S., 1978, The Diffusion Thermopower on the Two-band Model Separation of Diffusion and Phonon Drag Components in Noble Metal Alloy Systems, in Proc. First Int. Conf. on Thermoelectric Properties of Metallic Conductors, Michigan, 1977, pp. 65-70. 

Three features of Fig. 15a are noteworthy (i) the p(T) curves, which are similar at ambient pressure to those reported by others [80, 81], have a temperature dependence typical for a Fermi liquid however, they are too high and pressure-sensitive for a conventional metal with a mean-free path of more than one lattice parameter (ii) the low-temperature phonon-drag enhancement, which has a maximum at T ax 70 K in the oxide perovskites, is largely suppressed but it is partially restored by pressure (iii) a d a(300 K) / dP > 0 (a is enhanced by 15% in 14 kbar pressure) indicates an anomalous increase in m with pressure. Features (ii) and (iii) were also found in CaV03 where they were shown (see Sect 1.2.1) to be a signature for the existence of strong-correlation fluctuations in a Fermi liquid. The features (i) and (iii) are also present in the metallic phase of the orthorhombic samples, see Fig. 15b, c ... 

The phonon-drag component is increasingly suppressed as x increases, but it is enhanced by the application of pressure. 

Figure 44 also shows that the character of the thermoelectric power a(T) changes dramatically between the underdoped composition x = 0.10 and the bulk superconductor x = 0.15. Figure 46 shows that a(T) for x = 0.15 is nearly temperature-independent above a critical temperature Tj. At low temperatures it exhibits an unusual enhancement with a maximum value near 140 K a phonon-drag enhancement would have its maximum near 70 K. We have shown that this unusual enhancement is a characteristic and unique feature of all the superconductive copper oxides [284-286]. 

Ho Hamiltonian of the band energy of Spb phonon-drag thermopower... 

Sph and are the contributions to the thermopower due to electron diffusion, phonon drag and magnon drag (or paramagnon drag), respectively. 

Several reasons have been discussed to explain the minimum (or maximum) in the thermopower at low temperatures. As is discussed in sect. 2.3, the phonon-drag effect... 

The U-shaped temperature dependence 5( T) appears only when the hopping contribution (usually in the inhomogeneous limit) becomes significant, as shown in Fig. 2.51b. The least squares fitting parameters for B and C [Eq. (26)] are listed in Table 2.8. Neither the phonon drag effect [190], which is usually suppressed by disorder, nor the positive contribution due to the electron-phonon... 

For a given material, this input takes the form of an activation enthalpy versus shear stress curve and phonon-drag coefficients calculated for each pressure under consideration. Here full activation enthalpy curves have been calculated at selected pressures in Ta, Mo, and V, and phonon drag has been studied as a function of pressure and temperature in the case of Ta. These results have been fitted and modeled in suitable analytic forms to interface smoothly with the DD simulation codes. Detailed DD simulations have then been carried out in Ta and Mo as a function of pressure, temperature, and strain rate. Our DD simulations have been performed in part with the pioneering lattice-based serial code developed for bcc metals [21,22] but even more extensively with the general node-based Parallel Dislocation Simulator (ParaDiS) code recently developed at the Lawrence Livermore National Laboratory [27-30]. 

---