We present measurements of the electric resistivity, the Hall effect, the Nernst effect and the thermopower in the mixed state of Bi 1.76 Pb 0.24 Sr 2 Ca 2 Cu 3 O δ and of Tl 2 Ba 2 Ca 2 Cu 3 O δ . We find a large broadening of the resistive transition independent of the current direction with respect to the magnetic field, thermally activated behavior of the resistivity ϱ at low temperatures and a strongly non-linear dependence of ϱ on the magnetic field. The Hall coefficient R H is strongly temperature-dependent and shows a pronounced sign change in small magnetic fields. We find a large Nernst effect, from which the entropy/heat transported by the vortices is estimated. The thermopower is larger by a factor of 1000 compared to what is expected from the conventional theory of flux motion. It is remarkably similar to the resistivity.

Resistivity, Hall effect, Nernst effect and thermopower in the mixed state of Bi- and Tl-based high-Tc superconductors

Data on the magnetothermopower and specific heat of three compounds belonging to ``1111'' oxypnictides family are reported. One specimen $({\text{SmAsFeO}}_{0.8}{\text{F}}_{0.2})$ is a superconductor with ${T}_{c}=53\text{ }\text{K}$, while two others (SmAsFeO and NdAsFeO) are nonsuperconducting parent compounds. Our results confirm that spin-density-wave (SDW) order is present in SmAsFeO and NdAsFeO. In these two samples a strict connection between the thermoelectric power and electronic specific heat is found in the vicinity of SDW transition, which indicates that the chemical potential of charge carriers strongly depends on temperature in this region. Low-temperature data suggest presence of significant contribution magnon drag to the thermoelectric power.

Evidence of spin-density-wave order in R FeAsO 1 − x F x from measurements of thermoelectric power

We report on measurements of the Seebeck-effect, the Nernst-effect, and the magnetoresistance in the mixed state of ac-axis oriented expitaxial film of Y−Ba−Cu−O. In contrast to conventional superconductors we find a large Seebeck-coefficientS, which is comparable in magnitude to the Nernst-effect. The broadening of the super-conducting transitions of magnetoresistance and Seebeck-effect are rather similar with respect to (1) the temperature dependence, (2) the dependence on the direction between magnetic field and crystal axis and (3) the dependence on the direction between magnetic field and driving forces. The large Seebeck-effect has to be attributed to dissipation due to “normal” quasiparticle-excitations, since the vortex-contribution to the Seebeck-effect is by far too small to account for the observed magnitude ofS. It is argued that such a quasiparticle contribution to the dissipation is large in the high-Tc superconductors because of the small coherence lengths and thus the small vortex cores. Another possibility is that granularity leads to dissipation proportional to the normal state transport properties. The Seebeck-voltage depends on all dissipative processes other than vortex motion, whereas the Nernst-effect depends only on the vortex motion. Therefore by measurements of thermomagnetic effects the various dissipative properties may be separated.

Seebeck-effect in the mixed state of Y−Ba−Cu−O

We show that LaFe4Sb12 is a ferromagnetic quantum critical point system and that CeFe4Sb12 is a moderate heavy fermion compound. This is supported by our thermoelectric and thermodynamic experiments under magnetic field on both compounds, and a comparison with other physical properties of these compounds. For LaFe4Sb12, the quenching of the ferromagnetic spin fluctuations explains both the negative magnetothermopower and the decrease in the electronic heat capacity under magnetic field. We propose that the negative magnetothermopower is a generic property of compounds with ferromagnetic spin fluctuations when the diffusion term by spin fluctuations dominates at low temperature. On the other hand these critical ferromagnetic fluctuations are smeared out in CeFe4Sb12 due to the antiferromagnetic coupling between the d electrons of Fe and the 4f electron of Ce. As a result, CeFe4Sb12 is a moderate heavy fermion compound with Kondo temperature TK of about 80 K, which is consistent with the fact that cerium is almost trivalent in this material, and the partially screened magnetic moment of the cerium ions at TTk is 0.3B. Finally, in both compounds, the power laws observed at low temperature in the lattice thermal conductivity l could be explained by electron-phonon scattering.

Thermoelectric properties of Ce(La)Fe4Sb12 skutterudites under a magnetic field

The thermoelectric power of superconducting ${\mathrm{MgB}}_{2}$ single crystals was measured in the temperature range 4\char21{}300 K and in a magnetic field up to 13 T. Both the in-plane ${(S}_{\mathrm{ab}})$ and out-of-plane ${(S}_{c})$ thermopowers was found to be positive, with similar, metal-like temperature dependences and an anisotropy ratio ${S}_{\mathrm{ab}}{/S}_{c}$ of the order of 3\char21{}4 in the whole temperature range. Unexpectedly low values of ${S}_{c}$ were qualitatively explained by the properties of the Fermi surface topology. The analysis of the low temperature $(Tl40\mathrm{K}),$ normal-state thermopower measured in the magnetic field indicated a domination of the phonon-drag thermopower over the diffusive contribution and gave evidence of a strong (by a factor of $7\ifmmode\pm\else\textpm\fi{}1)$ anisotropy of the electron-phonon coupling. The positive magnetothermopower found above ${T}_{c}$ for both directions as well as a negative feature in ${S}_{c}(T)$ at $T\ensuremath{\sim}27\mathrm{K}$ for $Bg2T$ were tentatively attributed to phonon-drag effects.

F.J. Blatt

Papers

Phonon drag thermopower of noble metals in a high magnetic field