Influence of Spin and Charge Fluctuations on Magnetic-Field-Induced Insulator-to-Metal Transitions

在某些凝聚态物质中，电子、磁场和结构自由度的复杂相互作用导致了意想不到的和壮观的新物理现象。其中包括铜酸盐中的高温超导以及锰氧化物和磁性半导体中的所谓巨磁电阻(CMR)。显然，这些复杂材料的一个普遍特征是在纳米尺度上发生本征(即非化学)空间相分离，经常导致渗流电子相变和磁相变。有趣的是，这种非均匀状态的许多方面，特别是它们的动力学性质，到目前为止还没有被详细地研究过，因此也没有被很好地理解。事实证明，在不太复杂的稀土化合物中也会出现电子相分离和巨大的磁电阻。我们的计划是在一个相对简单的模型系统，即立方各向同性EuB6的基础上，研究磁和电子相分离的基本性质和动力学。在对取代/掺杂系列(Eu，Ca)B6和(Eu，Sm)B6的系统研究中，我们的目标是跟踪磁极化子的发展，了解CMR效应对等电子取代的强烈增强以及添加巡回载流子和增强局域反铁磁耦合的效果。为此，我们的目标是采用宏观、微观和时间分辨测量技术的独特组合，这非常适合我们在本提案中旨在回答的问题。具体地说，这些方法是时间分辨电子传输(电阻噪声光谱)和磁通量噪声光谱的组合，允许研究慢渗流动力学的电荷和自旋通道，以及原子分辨低温扫描隧道显微镜和光谱，具有使用自旋极化尖端的选项，以可视化并最终验证磁团簇的存在及其在进入磁场状态或在增强磁场中的渗流。从上述研究得出的极化子概念的普遍适用性将通过研究其他CMR材料，如HgCr2Se4和Eu5In2Sb6来仔细审查。所选择的方法是为了揭示支持或反对某些理论预测的行为类型的直接证据，并描绘有助于更好地了解磁化状态对电子性质的影响的重要系统参数。示范模型系统的预期结果为更好地理解和进一步开发更复杂的材料提供了见解。此外，鉴于最近在EuB6中提出了一个非平凡的拓扑结构(这在SmB6中已经讨论了一段时间)，我们对(Eu，Sm)B6取代系列的测量肯定会有助于这一讨论。

In certain condensed-matter materials, the complex interplay of electronic, magnetic and struc- tural degrees of freedom causes unexpected and spectacular new physical phenomena. Among these are high-temperature superconductivity in cuprates and the so-called colossal magnetoresistance (CMR) in manganites and magnetic semiconductors. A universal characteristic of these complex materials apparently is the occurrence of intrinsic (i.e. non-chemical) spatial phase separation at the nanoscale, often leading to percolative electronic and magnetic phase transitions. Interestingly, many aspects of such inhomogeneous states, in particular their dynamic properties, until now have not been investigated in much detail and, therefore, are not well understood. It turns out that electronic phase separation and colossal magnetoresistance also occur in less-complex rare-earth compounds. Our project aims to study the fundamental properties and the dynamics of magnetic and electronic phase separation on the basis of a comparatively simple model system, namely the cubic and magnetically-isotropic EuB6. In systematic investigations of the substitution/doping series (Eu,Ca)B6 and (Eu,Sm)B6 we aim to follow the development of magnetic polarons and understand the strong enhancement of the CMR effect upon isoelectronic substitution and the effect of adding itinerant carriers and enhancing a local antiferromagnetic coupling. To that end, we aim to employ a unique combination of macroscopic, microscopic and time-resolved measurement techniques, which ideally fits to the questions we aim to answer in this proposal. The methods are, in particular, a combination of time-resolved electronic transport (resistance noise spectroscopy) and magnetic-flux-noise spectroscopy allowing to study both the charge and spin channels of slow percolation dynamics, as well as atomically resolved low-temperature scanning tunneling microscopy and spectroscopy, with the option of employing spin-polarized tips, to visualize and ultimately verify the existence of magnetic clusters and their percolation upon entering the magnetic state or in increasing magnetic fields. The general applicability of the polaron concept as derived from the above investigations is going to be scrutinized by studying selected other CMR materials, such as HgCr2Se4 and Eu5In2Sb6. The chosen approach is set out to reveal direct evidence for or against certain theoretically predicted types of behavior and to delineate important system parameters that help to better understand the influence of the state of magnetization on the electronic properties. The expected results on an exemplary model system promise insights for a better understanding and further development of more complex materials. Moreover, in view of the very recent proposal of a non-trivial topology in EuB6 (which is discussed for SmB6 for some time) our measurements on the (Eu,Sm)B6 substitution series will most certainly contribute to this discussion.