【作者】 徐象繁；

【导师】 许祝安；

【作者基本信息】 浙江大学 ， 凝聚态物理， 2008， 博士

【摘要】 层状Ruddlesden-Popper(RP)系列钌氧化合物Srn+1RunO3n+1随着RuO正八面体层数n的增加,其物理性质表现出了奇特的演变。n=1的化合物Sr2RuO4是目前为止发现的唯一一个自旋三态、奇宇称的p波超导体,其超导机理及正常态的奇异性质还有待进一步的理解,是当前凝聚态物理领域的热点问题；n=2的化合物Sr3Ru2O7在低温下出现变磁量子相变及量子涨落；n=3的化合物Sr4Ru3O10表现出很强的磁性各向异性；无穷层(n=∞)化合物SrRuO3表现出各向同性的巡游铁磁性(Tc=160K)。可以看出,随着RuO八面体层数的减少,磁性逐渐减弱直到超导出现。因此,对这一系列化合物的研究,有望揭示Sr2Ru04超导机理,甚至可为高温超导机制的理解提供旁证。另外,该系列化合物为研究晶体结构与物性之间的关系提供了一个很好的平台。我们首次报道了超导体Sr2Ru04正常态的Nernst效应。发现在100K以下Nernst信号逐渐增大,直到20K的附近出现一个负的最大值,随后随温度线性趋于零。我们提出增大的Nernst信号可能来自于多带的特性以及磁涨落的贡献。同时,我们结合其它谱学测量结果,提出在20K温度以下该体系进入了一种相干电子态,这种相干态可能是发生在γ带(dxy轨道)上。对热电势、Hall效应和比热的测量进一步支持了我们的这个图像。这种轨道相关的相干态可能是轨道相关的超导态的前驱,与其超导机理之间有着直接的联系。我们通过磁热电势、磁阻和磁化率测量研究了Sr4Ru3O10样品在沿平面内磁场(H//ab)导致的电子态变化。在T<35K时,磁阻、磁化率和磁热电势随磁场在1-2T出现跳变并伴随着磁回滞现象,证实了不同磁场下存在着两个不同的电子态。而且,在H//ab时磁化率在大约T=35K处出现一个宽的峰值。我们认为在T<35K时体系中存在“硬的铁磁磁畴”,磁性与晶体结构之间存在强的耦合,在强磁场H//ab下“硬的铁磁磁畴”转向到同一个方向而导致晶体结构的微小变化,从而导致电子态的变化。这个高磁场(H//ab)下出现的电子态还具有更大的热电势的特点,这是以前从来没有发现过的。通过研究Sr4Ru3O10样品在H//c时热电势的磁场关系,我们发现T<35K时,磁热电势与H／T有标度关系,证明了自旋熵对该体系的热电势有贡献,这是第一次在钌氧化合物中发现自旋熵的热电势贡献。同时,在T=35-110K的温区,我们观测到了热电势随温度变化的线性行为,且S/T与电子比热γ系数之间存在标度关系,我们提出强关联效应引起的电子质量重整化主导了这个温度区域的热电势。热电势的这两个标度行为,说明Sr4Ru3O10体系中的热电势存在两种不同的贡献项。论文的最后研究了钴氧化合物La1-xSrCoO3 (0≤x≤0.5)中Co的自旋态与热电势的关系。通过磁化率测量得到Co的有效磁矩,并由此计算出体系中Co4+处于高自旋态或者高自旋态和低自旋态的混合态。根据修正过的Heikes公式,我们用300K时的热电势数据计算出Co4+的自旋态与磁化率得到的吻合。我们提出电子的简并度和强关联共同决定了体系的热电势。更多还原

【Abstract】 Compounds in the Ruddlesden-Popper homologous series Srn+1RunO3n+1, with n=1 to infinity, exhibit a wide range of physical novel phenomena. The n=1 member of the series, Sr2RuO4, is a spin-triplet, odd-parity superconductor; the n=2 member, Sr3Ru207, possesses a quantum metamagentic phase transition; Sr4Ru3O10, the n=3 compound, is anisotropic quasi two-dimensional ferromagnet; SrRuO3 is a three-dimensional itinerant ferromagnet with Tc=160K. The magnetic order is suppressed with decreasing number of RuO6 layers in Srn+1RunO3n+1 and superconductivity emerges in the most two-dimensional compound Sr2RuO4. It is possible, therefore to investigate the mechanism of superconductivity in Sr2RuO4 by studying Srn+1RunO3n+1. This series also provides opportunities to investigate a central theme in the relationship between physical properties and structure of transitional metal oxides.We performed the first Nernst effect measurement in the normal state of Sr2RuO4. The Nernst signal starts to increase below 100K, after showing a negative maximum, it decreases linearly to zero with decreasing temperature. We propose that the enhanced Nernst signal may result from magnetic fluctuation and the multi-band nature of the normal state of Sr2RuO4. We propose that a band-dependent coherent state emerges in the y band (dxy) at low temperatures. The thermopower, Hall effect and specific heat measurements provide further support to this picture. The existence of band-dependent coherent state may provide fresh insight to the application of the mechanism of superconductivity in Sr2RuO4.We investigated the two electronic states of Sr4Ru3O10 induced by magnetic field applying along ab plane through the measurements of magnetothermopower, magnetoresistance and magnetization, which all showed a steep jump at Hc of 1-2T and a hysteresis between increasing and decreasing field below 35K. The ab-plane zero field cooling (ZFC) magnetization as a function of temperature was found to show a peak at 35K. We propose that, under a strong in-plane field, the "hard domains" reorient and form a single domain, which results in a change in the structure-related electronic state through the strong magnetic-elastic coupling. The electronic state at high field favors larger thermopower, which has never been observed previous in transitional metal oxides.We also investigated the magnetothermopower of Sr4Ru3O10 with a field applied along c axis. Below 35K, magnetothermopower shows a scaling behavior with H/T, where H is the field and T the temperature, which may be due to the spin entropy dominated thermopower. Meanwhile, at the temperature range between 35-110K, the zero field thermopower increases linearly with temperature and S/T shows a scaling behavior with electronic specific heat coefficient. Therefore, we propose two contribute to the thermopower of Sr4Ru3O10, one originate is from quasiparticles renormalized by electronic correlations and the other from the spin entropy.Finally we investigated the relationship between the spin state of Co ions and the thermopower in La1-xSrxCoO3 (0≤x≤0.5). Based on the effective momentum of Co ions determined from magnetism measurements, we found that most of Co4+ ions are in the high spin state. S (300 K) obeys the modified Heikes formula assuming Co3+ ions are in the IS state and Co4+ in the HS state or a mixed state of HS sate and LS state, respectively. These results imply that the large thermopower results from the degeneracy of charge carriers and the strong electron correlation in cobalt oxides.更多还原