| The discovery of unconventional superconductivity in the vicinity of antiferromagnetic (AFM) fluctuations in layered iron pnictides and iron chalcogenides has ushered in a new age of high-temperature superconductivity. Superconductivity in these compounds share some similarities with high- Tc cuprates and is widely believed to be mediated by antiferromagnetic fluctuations. In these systems, iron chalcogenide Fe(Te1-xSex) is structurally the simplest, and exhibits unusual superconducting and magnetic properties. We have investigated the interplay between superconductivity and magnetism of this system via establishing the phase diagram. We found that two magnetic correlations with wave vectors (pi, pi) and (pi, 0) coexist in Fe(Te 1-xSex) , with their strength tuned by Se concentration. The competition of them leads to an unique phase diagram: an intermediate phase region with charge carrier weak localization and superconductivity suppression occurs between the long-rang AFM state (x 0.29).;Our follow-up studies on the coupling between (pi, 0) magnetism and superconductivity in Fe(Te1-xSex) further revealed the underlying physics behind this usual phase diagram. We found that the charge carrier localization and superconductivity suppression in underdoped samples originate from incoherent scattering by (pi, 0) magnetic fluctuations. With the suppression of (pi, 0) magnetic fluctuations by magnetic field, we observed surprising effect of a remarkable increase in the superconducting volume fraction under moderate magnetic fields for nearly optimally doped samples. The role of incoherent (pi, 0) magnetic fluctuations is also examined in samples with rich interstitial Fe, where we found that the region for charge carrier localization and superconductivity suppression extends to high Se content samples due to enhanced (pi, 0) fluctuations by interstitial Fe. Furthermore, we investigated the annealing effects of Fe(Te1-x Sex) system in various atmospheres. Our results revealed that annealing samples in oxidization argents leads to interstitial Fe deintercalation, which consequently suppresses(pi , 0) fluctuations and enhances superconductivity in underdoped samples.;In addition, in this thesis work we also studied the gap structure in optimally doped Fe(Te0.57Se0.43) sample using specific heat measurements. The electronic specific heat Ce was successfully separated from the phonon contribution using the specific heat of a non-superconducting Cu-doped sample as a reference. The normal-state Sommerfeld coefficient of the superconducting sample is found to be 26.6 mJ/mol K2, indicating intermediate electronic correlation. The temperature dependence of C e in the superconducting state can be best fitted using a double-gap model. The large gap magnitudes derived from fitting, as well as the large specific heat jump, indicate strong-coupling superconductivity. Furthermore, the magnetic field dependence of specific heat shows strong evidence for multiband superconductivity.;We also expanded our study to thermoelectric materials beta-K 2Bi8Se13. Our goal is to study the spin-orbit coupling effect in this material and seek for possible topological surface states. We found that the beta-K2Bi 8Se13 bulk single crystals exhibit three-dimensional weak antilocalization behavior induced by strong spin-orbit coupling, which can be quenched by magnetic Mn doping but is robust against non-magnetic Te doping. Such strong spin-orbit coupling may be associated with the high thermoelectric efficiency of this material. |
