We investigated the thermoelectric transport properties of EuNi2P2 and EuIr2Si2 in order to evaluate the relevance of Kondo interaction and valence fluctuations in these materials. While the thermal conductivities behave conventionally, the thermopower curves exhibit large values with pronounced maxima as typically observed in Ce- and Yb-based heavy-fermion materials. However, neither the positions of these maxima nor the absolute thermopower values at low temperature are in line with the heavy-fermion scenario and the moderately enhanced effective charge carrier masses. Instead, we may relate the thermopower in our materials to the temperature-dependent Eu valence by taking into account changes in the chemical potential. Our analysis confirms that valence fluctuations play an important role in EuNi2P2 and EuIr2Si2.
We present thermopower measurements on Yb(Rh$_{1-x}$Co$_x$)$_2$Si$_2$. Upon Co substitution the Kondo temperature is decreasing and the single large thermopower minimum observed for YbRh$_2$Si$_2$ splits into two minima. Simultaneously, the absolute thermopower values are strongly reduced due to a weaker exchange coupling between the $4f$ and the conduction electron states with increasing $x$. Pure YbCo$_2$Si$_2$ is considered a stable, trivalent system. Nevertheless, we still observe two minima in the thermopower indicative of weak residual Kondo scattering. This is in line with results from …
Heavy Fermion superconductor CeCu$_2$Si$_2$ under high pressure: multiprobing the valence crossover
第一个重费米子超导体CeCu $_2$ Si $_2$还没有完全揭开它的神秘面纱。在高压下，超导性被认为是由价态涨落介导的，而在环境压力下，自旋涨落最有可能充当配对粘合剂。我们对高质量的CeCu $_2$ Si $_2$单晶进行了多探针（电输运、热电势、交流比热、霍尔和能斯特效应）实验，直至$7 \text{GPa}$。可靠的电阻率数据揭示了第一次的标度行为接近假定的价态转变，并允许定位在$4.5\pm0.2 \text{GPa}$和一个稍微负的温度的临界终点。在相同的压力区域，显着的功能也已被检测到的其他物理性质，作为进一步的签名的Ce价交叉和相关的临界波动。
Certain classes of strongly correlated systems promise high thermopower efficiency, but a full understanding of correlation effects on the Seebeck coefficient is lacking. This is partly due to limitations of Boltzmann-type approaches. One needs a formula for the thermopower that allows separate investigations of the kinetic and potential energy contributions to the evolution with temperature and doping of the thermopower. Here we address this issue by deriving for Hubbard-like interactions a formula for the thermopower that separates the potential from the kinetic energy contribution and facil…
The thermoelectric effect in a two-dimensional (2D) massless Dirac fermion (DF) system at the quantum limit is discussed to verify the prediction of high-performance thermopower in an organic conductor α-(BEDT-TTF)2I3. Because of relatively large Zeeman splitting in α-(BEDT-TTF)2I3, the boundless increase of thermopower at high magnetic fields, predicted without the Zeeman effect, is hardly expected, whereas there appears to be a broad local maximum. This is characteristic of 2D DF systems with Zeeman splitting and is recognized in the previous experiment. In contrast to 3D Dirac/Weyl semimeta…
We consider non-Fermi liquids in which the inelastic scattering rate has an intrinsic particle-hole asymmetry and obeys $ω/T$ scaling. We show that, in contrast to Fermi liquids, this asymmetry influences the low-temperature behaviour of the thermopower even in the presence of impurity scattering. Implications for the unconventional sign and temperature dependence of the thermopower in cuprates in the strange metal (Planckian) regime are emphasized.
The iron antimonide FeSb$_2$ possesses an extraordinarily high thermoelectric power factor at low temperature, making it a leading candidate for cryogenic thermoelectric cooling devices. However, the origin of this unusual behavior is controversial, having been variously attributed to electronic correlations as well as the phonon-drag effect. The optical properties of a material provide information on both the electronic and vibrational properties. The optical conductivity reveals an anisotropic response at room temperature; the low-frequency optical conductivity decreases rapidly with tempera…
Nernst Effect as a Probe of Local Kondo Scattering in Heavy Fermions
CeCu $_2$ Si $_2$和Ce $_{0.8}$ La $_{0.2}$ Cu $_2$ Si $_2$在$T$ = 2 K和300 K之间观察到大的、强烈温度依赖性的能斯特系数$ν$。增强的$ν(T)$由现场近藤（传导电子$-4f$电子）散射速率的不对称性决定。考虑到测得的霍尔迁移率，$μ_H$，这些系统的高度不寻常的热电势，$S$，可以半定量地描述为$S(T)$ $=$ $-$ $ν(T)/μ_H(T)$，这明确表明，热电势起源于本地近藤散射过程在很宽的温度范围内从远高于低于相干温度（$\approx$ 20 K的CeCu $_2$ Si $_2$）。我们的结果表明，能斯特效应可以作为一个适当的探针的本地电荷载流子散射。这对探索适合于潜在应用的相关材料中的非常规热电势增强具有重要意义。
The thermoelectric effect is the generation of an electrical voltage from a temperature gradient in a solid material due to the diffusion of free charge carriers from hot to cold. Identifying materials with large thermoelectric response is crucial for the development of novel electric generators and coolers. In this paper we consider theoretically the thermopower of Dirac/Weyl semimetals subjected to a quantizing magnetic field. We contrast their thermoelectric properties with those of traditional heavily-doped semiconductors and we show that, under a sufficiently large magnetic field, the the…
We investigated the effect of pressure on the magnetic and thermoelectric properties of Sr$_{3.1}$Y$_{0.9}$Co$_{4}$O$_{10+δ}$. The magnetization is reduced with the application of pressure, reflecting the spin-state modification of the Co$^{3+}$ ions into the nonmagnetic low-spin state. Accordingly, with increasing pressure, the Seebeck coefficient is enhanced, especially at low temperatures, at which the effect of pressure on the spin state becomes significant. These results indicate that the spin-orbital entropy is a key valuable for the thermoelectric properties of the strongly correlated c…
We report thermopower measurements under hydrostatic pressure on the cuprate superconductor La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$ (Nd-LSCO), at low-temperature in the normal state accessed by suppressing superconductivity with a magnetic field up to $H = 31$ T. Using a newly developed AC thermopower measurement technique suitable for high pressure and high field, we track the pressure evolution of the Seebeck coefficient $S$. At ambient pressure and low temperature, $S/T$ was recently found to suddenly increase in Nd-LSCO at the pseudogap critical doping $p^{\star} = 0.23$, consistent with a dro…
The effect of scattering of conduction electrons by dynamical spin fluctuations on the thermopower in metals near a thermal phase transition into an antiferromagnetic phase is considered. We are interested in a transition at room temperature, as has been studied in a heterostructure involving layers of IrMn. We show that the electrical resistivity exhibits a narrow but low peak at the transition, which may be difficult to detect on top of the main contributions induced by phonons and impurities. By contrast, the thermopower is found to exhibit a prominent peak both as a function of temperature…
Na-site substitution effects of the thermoelectric properties of NaCo_2O_4
测量并分析了Na $_{1+x}$ Co $_2$ O $_4$和Na $_{1.1-x}$ Ca $_x$ Co $_2$ O $_4$的电阻率和热电势。在Na $_{1+x}$ Co $_2$ O $_4$中，电阻率随$x$增加，而热电势几乎与$x$无关。这表明过量的Na不太可能提供载流子，并且减少了样品中的有效导电路径。在Na $_{1.1-x}$ Ca $_x$ Co $_2$ O $_4$中，电阻率和热电势随着$x$而增加，并且Ca $^{2+}$取代Na $^+$减少了NaCo $_2$ O $_4$中的多数载流子。这意味着它们是孔，这与热电势的正号一致。该化合物的电阻率随温度变化的特性证明了该化合物具有很强的相关性。
Recent experiments have measured the signatures of the Kondo effect in the zero-field thermopower of strongly correlated quantum dots [Svilans {\em et al.,} Phys. Rev. Lett. {\bf 121}, 206801 (2018); Dutta {\em et al.,} Nano Lett. {\bf 19}, 506 (2019)]. They confirm the predicted Kondo-induced sign change in the thermopower, upon increasing the temperature through a gate-voltage dependent value $T_{1}\gtrsim T_{\rm K}$, where $T_{\rm K}$ is the Kondo temperature. Here, we use the numerical renormalization group (NRG) method to investigate the effect of a finite magnetic field $B$ on the thermo…
