Thermoelectric Effect In Three Typical Low-Dimension Electronic Systems

Transport properties measurement is a powerful method to probe various intrinsic scattering mechanism of materials. The transport properties are closely related to electronic structure and low-energy excitation, and are affected by defects and impurities. Therefore, transport measurement has been widely used in the research of superconductors, heavy fermion systems, quantum phase transition and critical behaviors. In particular, Nernst ef-fect emerged recently as a very sensitive probe of electronic structures and phase transition attracts more and more attentions.In this dissertation, we carried out the measurements of transport properties, includ-ing thermomagnetic effect, on three kinds of typical low-dimension electronic systems: (1)Transport measurement including Nernst effect on the recently discovered 1111 phase of iron-based superconductor LaFeAsOo.9Fo.1; (2)transport properties Measurement on semi-metal graphite in low temperature up to 12T magnetic field; (3)Two types of transport measurements(Seebeck effect and resistivity) on heavy fermions superconductor URu2Si2. We carried out the first Nernst effect measurement on LaFeAsO0.9F0.1 and graphite. All the results are reported for the first time. The main conclusions of the dissertation are high-lighted as following:(1) For 1111 system LaFeAsOo.9Fo.1(Tc=26 K), we first report its Nernst effect mea-surement. The Nernst signal in the normal state is negative and shows weak dependence on temperature. Then the signal deviates from the normal state background as T< 50 K, and increases drastically around Tc. By measuring other transport properties such as re-sistivity, Hall coefficient and thermopower, the off-diagonal Peltier coefficient is obtained. Meanwhile, the off-diagonal Peltier is suppressed at the same temperature around 50 K. We argue that the suppressed spin fluctuation could responsible for this Nernst signal anomaly around 50 K. We also observed that a positive Nernst coefficient peak caused by vortex motion below Tc. Since the 1111 system have 2D character, the flux flowing regime is quite large. This is analogous to the case of High-Tc cuprate superconductor.(2) Nernst effect is expected to be very small in normal metals because of Sond-heimer Cancellation. Therefore, the investigations on Nernst signal is out of fashion for several decades. Recently Nernst effect has been proved to be a very sensitive probe, yet is poorly understood theoretically. Nernst measurement on 3D graphite, a macroscopic stack of graphene layers has been carried out in sub-Kelvin and in the presence of high field up to 12 T. In sharp contrast with single-layer 2D graphene, where according to the recent reports, Nernst signal vanishes when a Landau level passes the Fermi level, Nernst sig-nal in 3D graphite sharply peaks. This points to the degrees of freedom provided by finite interlayer coupling as a source of enhanced thermoelectric response in the vicinity of the quantum limit.Furthermore, by comparing the differences between these two Fermi surfaces, we found that Landau quantization slices a three-dimensional Fermi surface, each intersection of a Landau level with the Fermi level modifies the Fermi surface topology. In other words, the even quantization slices move towards each other when magnetic field increase. Fermi surface topology transition happens when the even slices merge as odd number. We argue that the peaks in Nernst effect are due much to the Fermi surface topology transition. The experiment establishes a fundamental link between dimensionality of an electronic system and its Nernst response. According to our results, the most prominent signature of such a topological phase transition emerges in the transverse thermoelectric response.(3) Heavy Fermions superconductor is a unconventional superconductor which has even longer history than High-Tc cuprate. But it is still a fascinating systems. We check two principal oriented URu2Si2 samples with two transport measurements(resistivity and See-beck effect) extended down to sub-Kelvin temperature range uncovering a unique case of anisotropy. Avoiding being overwhelmed by large transverse magnetoresistance, we mea-sured resistivity of both samples with magnetic field parallel to the current. In contrast to almost all anisotropic superconductors such as high-Tc cuprates, organic superconductor etc, the upper critical field of URu2Si2 is larger in the less conducting direction a-axis. Moreover, when the current is injected along the c-axis, URu2Si2 behaves as a low-density Fermi liquid, i.e., the resistivity is T2 dependent and the thermopower is T-linear, no mat-ter that magnetic field is applied or not. But when current flows along the a-axis, even in presence of a large field, resistivity remains T-linear down to Tc and the Seebeck shows a strange behavior and undergoes a sign change at very low temperature and 12 T. We con-clude that the characteristic energy scale is anisotropic and vanishingly small in the basal plane a-axis. This may be the key to explain the anomalous anisotropy of the upper critical field of URu2Si2.