We show that important anomalous features of the normal-state thermoelectric power S of high-Tc materials can be understood as being caused by doping dependent short-range antiferromagnetic correlations. The theory is based on the fluctuation-exchange approximation applied to Hubbard model in the framework of the Kubo formalism. Firstly, the characteristic maximum of S as function of temperature can be explained by the anomalous momentum dependence of the single-particle scattering rate. Secondly, we discuss the role of the actual Fermi surface shape for the occurrence of a sign change of S as a function of temperature and doping.PACS numbers: 71.27.+a, 74.25.Fy, The thermoelectric power (TEP), S, of the high-T c materials in the normal state exhibits anomalous features [1][2][3][4] that are not well understood at present. For example, its temperature dependence shows a characteristic maximum that can be found in all optimally and underdoped cuprates, which is in contrast to the conventional linear-T behavior of a weakly correlated Fermi liquid. Furthermore, a better understanding of the doping dependence, which has been shown to be universal for a large class of high-T c materials [1], is needed.Experimentally, the TEP is positive for the underdoped cuprates. It increases rapidly as a function of temperature, reaches a maximum S ⋆ at a temperature T ⋆ and falls off almost linearly with temperature. The size of the maximum S ⋆ and the value of T ⋆ decrease rapidly with increasing doping, while the slope for the fall-off at T > T ⋆ is almost independent of the doping concentration. The TEP of nearly optimally doped materials shows a similar behavior below room temperature, but the over-all size of the TEP is reduced. Furthermore, the TEP for most of the optimally doped samples changes sign at approximately room temperature [1]. For overdoped samples, the behavior of the TEP is very different: It is negative and decreases linearly with increasing temperature. At present there are only two classes of high-T c materials that do not follow this generic trend, namely La 2−x Sr x CuO 4 (LSCO) and YBa 2 Cu 3 O 7−δ (YBCO). For LSCO the TEP remains positive in the overdoped regime, where it exhibits a maximum of decreasing height for increasing hole concentration [4], whereas YBCO shows a negative TEP in the overdoped regime but with a positive slope at temperatures T > T ⋆ [1,2]. Recently Bernhard and Tallon [2] presented strong experimental evidence that the chain contribution is responsible for this non-generic TEP of YBCO while the contribution of the CuO 2 planes follows the generic behavior.Theoretically, important open problems concerning the characteristic behavior of the TEP remain, although there have been several attempts to explain this behavior, ranging from the van Hove scenario [5,6] to phonon drag effects [7]. The most important open questions are the physical origin of the characteristic temperature scale T ⋆ and of the doping dependence of the sign of S.In this paper, we demonstrate that short-range anti...

We investigate the interplane magnetic coupling of the multilattice compound Y2Ba4Cu7O15by means of a bilayer Hubbard model with inequivalent planes. We evaluate the spin response, effective interaction and the intra-and interplane spin-spin relaxation times within the fluctuation exchange approximation. We show that strong in-plane antiferromagnetic fluctuations are responsible for a magnetic coupling between the planes, which in turns leads to a tendency of the fluctuation in the two planes to equalize. This equalization effect grows whit increasing in-plane antiferromagnetic fluctuations, i. e., with decreasing temperature and decreasing doping, while it is completely absent when the in-layer correlation length becomes of the order of one lattice spacing. Our results provide a good qualitative description of 71.27.+a, to appear in Phys. Rev. B (RC) Jan. 99Although many models for high-Tc cuprates are restricted to a single layer, it has become clear that both superconducting and magnetic properties of these materials are affected by the coupling between two or more layers. A rather strong coupling between the layers has been observed principally by inelastic neutron scattering 1 (INS) and nuclear magnetic resonance 2-5 (NMR). Furthermore, the observation of a qualitatively different behavior of the odd and even channel in INS 6 and of a bilayer splitting of the Fermi surface found in angular resolved photoemission experiments (ARPES) 7,8 demonstrate that low energy excitations of cuprates are affected by the presence of more than one layer per unit cell. An exciting perspective on the nature of the coupling between CuO 2 -layers was offered by NMR experiments by Stern et al. on Y 2 Ba 4 Cu 7 O 15 (247). This material has a variety of structural similarities to the extensively studied YBa 2 Cu 3 O 7 (123) and YBa 2 Cu 4 O 8 (124) systems. The compound 247 can be considered as a natural multilattice, whose bilayers are build up of one CuO 2 layer which belongs to the 123 block and one layer to the 124 block. Based on the analysis of the NQR spectra it turned out that the charge carrier content in these nonequivalent adjacent layers is very close to that of the related parent compounds of the two blocks, 123 and 247. Interestingly, the highest transition temperature (T c = 95 K) occurs in the 247 compound, in comparison with the 92 K of 123 and 82 K of the 124 system.In this paper, we want to provide a theoretical understanding in terms of a microscopic model of some striking experimental observations of Refs. 2,3, namely: (i) the spin-spin relaxation rates T −1 2G of the two layers in Y 2 Ba 4 Cu 7 O 15 , measured in a spin-echo double resonance experiment, behave very similarly as a function of temperature, despite the different doping of the layers; (ii) the spin-spin relaxation rate in the 124 (247) layer of Y 2 Ba 4 Cu 7 O 15 is reduced (enhanced) with respect to one of the constituent compound at low temperatures; (iii) the interplane transverse relaxation rate, increases for decreasing temperature faster t...

We analyze single-particle electronic and two-particle magnetic properties of the Hubbard model in the underdoped and optimally-doped regime of YBa2Cu3O 7−δ by means of a modified version of the fluctuation-exchange approximation, which only includes particle-hole fluctuations. Comparison of our results with Quantum-Monte Carlo (QMC) calculations at relatively high temperatures (T ∼ 1000K) suggests to introduce a temperature renormalization in order to improve the agreement between the two methods at intermediate and large values of the interaction U . We evaluate the temperature dependence of the spin-lattice relaxation time T1 and of the spin-echo decay time T2G and compare it with the results of NMR measurements on an underdoped and an optimally doped YBa2Cu3O 7−δ sample. For U/t = 4.5 it is possible to consistently adjust the parameters of the Hubbard model in order to have a good semi-quantitative description of this temperature dependence for temperatures larger than the spin gap as obtained from NMR measurements. We also discuss the case U/t ∼ 8, which is more appropriate to describe magnetic and single-particle properties close to half-filling. However, for this larger value of U/t the agreement with QMC as well as with experiments at finite doping is less satisfactory.

We investigate, within the fluctuation-exchange approximation, a correlated-electron model for Y2Ba4Cu7O15 represented by two inequivalent Hubbard layers coupled by an interlayer hopping t ⊥ . An energy offset δ is introduced in order to produce a different charge carrier concentration in the two layers. We compare several single-particle and magnetic excitations, namely, the single particle scattering rate, the spectral function and the spin lattice as well as spin-spin relaxation times in the two layers as a function of δ. We show that the induced interlayer magnetic coupling produces a tendency to "equalization" of the magnetic properties in the two layers whereby antiferromagnetic fluctuations are suppressed in the less doped layer and enhanced in the heavily doped one. The strong antiferromagnetic bilayer coupling causes the charge carriers in the plane with larger doping concentration to behave similar to those of the underdoped layer, they are coupled to. This effect grows for decreasing temperature. For high temperatures or if both layers are optimally or overdoped, i.e. when the antiferromagnetic correlation length becomes of the order or smaller than one lattice site the charge carrier and magnetic dynamics of the two layers is disconnected and the equalization effect disappears. These results are in good agreement with NMR experiments on Y2Ba4Cu7O15 by Stern et al. Phys. Rev B 51, 15478 (1995). We also compare the results with calculations on bilayer systems with equivalent layers as models for the constituent compounds YBa2Cu3O7 and YBa2Cu4O8.