准一维有机导体（TMTCF）₂X和钌氧化物的异常输运性质研究

【作者】 柴一晟；

【导师】 曹烈兆； 杨宏顺；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2007， 博士

【摘要】 准一维有机导体（TMTCF）₂X自发现超导电性以来，就引起了广泛的关注。大量的深入研究结果表明，该有机导体体系有多种可能的基态：自旋Peierls态、自旋密度波态和电荷密度波态（SDW和CDW）、金属态／超导态、场感应自旋密度波态（FISDW）等等。当改变有机分子或无机阴离子团，各个化合物处于不同的基态。改变压力，降温速率和磁场，亦可改变其基态。本论文主要通过研究（TMTSF）₂ClO₄和（TMTSF）₂PF₆盐的热电输运性质，考察可能的非费米液体行为，特别是磁子曳引和电荷自旋分离的现象。低温下，通过改变阴离子基团或者降温速率，探索了各个不同基态下的热动力学和热电性质。证实了该体系中存在的一些异乎寻常的输运机制。另外一个工作是关于近年来刚刚发现的Ruddelson-Popper结构的Sr_n+1Ru_nO_3n+1体系，由于具有类似于高温超导体的钙钛矿结构而备受关注。特别是Sr₃Ru₂O₇（n=2）的基态磁性仍然存在争论，但是其在T＜16K和中等强度的场下存在变磁性（metamagnetic）行为，表明存在量子关键点（QCP），因此具有独特的理论和实验的价值，成为近年来研究的热点。作者通过大量的电学和热学性质，证实其高温存在与结构有关的磁关联的转变过程。为基态的磁性提供了新的信息。本论文的主要工作总结如下：第一章包括两个部分，第一部分从热电势的基本物理图像出发，以简洁的方式，叙述了热电势的基本理论，特别是讨论了本论文中所涉及到的大多数理论模型。第二部分全面介绍了准一维有机导体Bechgaard盐（TMTCF）₂X的研究现状和发展。其中首先回顾了它的各种性质，包括晶体结构，电子结构和相图以及正常态的各种输运性质、磁性质和光学性质的最新研究结果。特别讨论了费米液体理论和非费米液体理论，介绍了预言存在自旋电荷分离的Luttinger液体理论，最后总结了（TMTCF）₂X目前研究所最关注的一些期待解决的问题。第二章为了解答准一维有机导体TMTSF盐高温区的异常的一些正常态性质，并考察可能存在的自旋激发曳引和自旋电荷分离，通过合成（TMTSF）₂ClO₄和（TMTSF）₂PF₆的高品质单晶，研究了其α方向的输运性质。特别是设计了全新的测量平台，得到了高精度的数据。通过对热电势数据的拟合得到其随温度的依赖关系可以量化的表示为两部分的贡献。一维紧束缚能带产生的线性项贡献加上交换积分为J_ex≈1400K的Hubbard模型的反铁磁子曳引的贡献。当温度低于40 K，热电势和载流子的变号表明存在窄能带填充的变化。这种输运机制的改变在考虑到光电导观测的结果后，可以用零频率模式和有限频率模式随温度的变化来解释。通过对拟合结果物理意义的分析，作者发现了高温区存在自旋电荷分离的证据。这不仅是因为在高温区，（TMTSF）₂PF₆的自旋部分而不是电荷部分存在spin-Peierls能隙，而且是因为在降温过程中，两种盐参与输运的有效载流子浓度变为高温区的1％，而自旋自由度和参与输运的自旋数目没有明显的变化。这些证据都证明TMTSF盐的高温区不是费米液体行为。第三章研究了准一维有机导体（TMTSF）₂PF₆在自旋密度波（SDW）态附近沿α轴方向的输运性质的研究，在8 K附近观察到SDW态的能隙宽度发生了改变。令人吃惊的是，热电势观察到的热激活能比电阻率观察到的要小很多，作者首次提出有机导体在低温区存在磁起源的极化子跳跃行为。第四章作者首次报道了准一维有机导体（TMTSF）₂ClO₄以不同降温速率通过阴离子有序-无序相变（AO）温度T_AO=24 K的α轴方向的热电势数据，发现阴离子取向的解冻温度随降温速率的减小而增加，而热电势的附加贡献同时也在减小。比热的测量结果同时发现，比热的跃变峰在不同降温速率下完全一致，但不等于理论的Rln（2），且两者比热的声子背底存在差异，结合热电势的结果，表明远在T_AO之上已存在部分阴离子取向的冻结／解冻过程。考虑到电荷部分的窄能带模型，将热电势曲线随降温速度的变化用能带填充及载流子变号温度的延迟来解释。第五章通过对浮区熔融法生长的高品质Sr₃Ru₂O₇单晶的输运性质、热学性质和磁性质研究。观察到了显著的与升降温速度有关的热滞现象，分辨出三个不同的结构、电子和磁相变。且这些相变的发生与升温和降温的速度有直接关系。我们的结果表明在241K，Sr₃Ru₂O₇存在从铁磁到反铁磁关联的转变。而在210 K存在c方向的结构相变，低温下出现半周期的调制结构。而在升温过程中，232 K存在电子的比热跃变峰。特别是在241 K以上，热电势变为常数，出现窄能带行为。作者将其与SrRuO₃高温的热电势纯自旋贡献进行了比较，认为Sr₃Ru₂O₇的热电势行为与其不同，都来自于电荷项的贡献。更多还原

【Abstract】 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.更多还原