Quasi One-dimensional Organic Conductors (tmtcf) Abnormal Transport Properties <sub> 2 </ Sub> X And Ruthenium Oxide Research

The quasi-one-dimensional conductors (TMTCF)₂X have attracted much interest in recent year, since the discovery of the superconductivity within this system. The selenium salts or Bechgaard salts exhibit a fascinating variety of electronic and structure behavior, including spin-Peierls transition, spin-density-wave and charge-density-wave, metallic/superconductivity, magnetic-field-induced spin-density-waves(FISDW), anion order-disorder transition, quantized Hall resistance reminiscent of the quantum Hall effect. The replacement of selenium by sulphur or the use of a different anion can be regarded as a means of tuning the low-temperature properties. By changing pressure, magnetic fields or cooling speed, the ground state will change accordingly.In this thesis, the non-Fermi-liquid behavior, in particular, magnon-drag and spin-charge separation were probed through transport properties of (TMTSF)₂ClO₄ and (TMTSF)₂PF₆ salts over the temperature range from 6 K to 300 K. At low temperatures, by changing anions or cooling speed, the thermal transport and dynamic properties of different ground states were investigated. Unusual transport properties were confirmed in this system.Another work, discussed in this thesis, is related to the Ruddelson-Popper-type perovskite strontium ruthenates Sr_n+1Ru_nO_3n+1 system. As for the Sr₃Ru₂O₇ (n=2), its magnetic ground state remain controversial, while moderate applied fields induce a metamagnetic transition in this material, with T<16 K. There is considerable experimental evidence that the characteristic temperature of this end point is close to zero, indicating the presence of quantum critical point (QCP). This finding has generated a great deal of interest and provides unique opportunity for theoretical and experimental research. The electronic and thermal transport properties of Sr₃Ru₂O₇ single crystal were reported and the variation of magnetic correlations driven by structural transition was confirmed.The whole thesis consists of five chapters.Chapter 1: it consists of two parts. In part one; we give an introduction to thermopower by inducing the simple idea that the thermopower is a measure of the heat per carrier over temperature or the entropy per carrier. Most of the theories for thermopower which we used in this thesis were discussed in this part. In part two; we give a general survey of the history and the present research situation in quasi-one-dimensional organic conductor Bechgaard salts (TMTCF)₂X. Some related properties, such as the structural, electronic and phase diagram are introduced. Furthermore, the high temperature normal state properties, such as transport properties, magnetic properties and optical properties are also introduced. Especially, Fermi liquid and non-Fermi liquid theory, particularly, Luttinger liquid theory which predicts spin-charge separation, are explained. In the end, we sum up some of the existent issues.Chapter 2: In order to find the answer to the abnormal high temperature normal properties of organic conductor TMTSF salts and explore the possible existence of spin-excitation-drag and spin-charge separation, high quality single crystals of (TMTSF)₂ClO₄ and (TMTSF)₂PF₆ are grown and their thermopower are measured along the a axis. Moreover, by applying specially designed sample sticks, high quality data are obtained. We find that the temperature dependence of S_a can be understood quantitatively by the sum of a 1D tight-binding band and an antiferromagnetic-excitation-drag contributions of a Hubbard model with J_ex≈1400 K for both salts above 80 K. Below 40 K, the sign reversals in both thermopower and charge carriers imply a change of filling in a narrow band. Such a change of transport mechanism with decreasing temperature can be explained after taking into account the ZE and FE modes found in the optical conductivity. Our data are supportive of a spin-charge separation in both salts not only because there is a real dimerization gap in spin part, not in the charge part for (TMTSF)₂PF₆, but also because there is a crossover from 1D tight-binding band to a narrow band with a gap feature during cooling, while the spin degrees of freedom remain uniform. Non-Fermi-liquid behavior is confirmed at least at high temperatures. We caution however, a Luttinger liquid description within the temperature region studied, since no large negative contribution directly from spin excitations expected in LL theory is observed at high temperatures.Chapter 3: The temperature dependent thermopower S_a(T) of the Bechgaard salt (TMTSF)₂PF₆ was measured along the highly conducting a axis. Surprisingly, an abrupt change of gap value was found at 8 K. The gap values revealed by the thermopower measurements within the SDW phase are much smaller than that revealed in the resistivity measurement, implying the existence of small magnetic originated polaron hopping below T_SDW.Chapter 4: We report for the first time the temperature dependent thermopower of the Bechgaard salts (TMTSF)₂ClO₄ with various cooling rates going through the anion order-disorder transition temperature (T_AO=24 K). It seems that the melting temperature of the frozen configuration of anions increases with decreasing the cooling speed while the magnitude of a positive term is falling as well. It is surprising to recover the exactly the same anomaly specific heat peak from the result of different cooling speed. However, the estimated entropy under the anomaly is lower than the ideal R ln(2) value, expected for the two possible configurations of the ClO₄ tetrahedron in the disordered state. Considering the results of thermopower and the difference between the specific heat background of the two processes, it indicates that the partially melting/freezing processes of the anion configuration exists far above T_AO. And the influence of the anion disorder to thermopower at low temperature can be attributed to the deterring the transverse coupling from doping to narrow band and postpone the temperature where the sign of charge carriers changes.Chapter 5: In this chapter, we report hysteresis behaviors in thermal transport properties of Sr₃Ru₂O₇ single crystal in both ab plane and c direction. Three cooling speed dependent separate structural, electronic and magnetic transitions are identified. A change of magnetic correlation from AFM to FM at 241 K, a structural transition near 210 K and an electronic specific heat anomaly at 232 K is suggested by our results. Above 241 K, a constant thermopower implies a narrow band behavior. Comparing with the similar narrow band behavior of SrRuO₃ at high temperatures, we find a charge only term of Heikes formula for the thermopower of Sr₃Ru₂O₇.