Fermiology of High Temperature Superconductors

尽管高温铜超导体（HTSC）在二十多年前就已被发现，但其不同寻常的性质仍远未被充分了解。当然，最重要的问题是，为什么超导转变温度如此之高，以及如何进一步提高它？与此密切相关的一个问题是，这些材料的正常（非超导）状态的本质是什么？随着化学掺杂改变传导电子的数量密度，这种状态是如何演变的？显然，要完全理解前者，就需要了解后者。直到最近，我们对正常状态的主要实验探测是宏观输运测量（电阻率，霍尔效应等）和角度分辨光发射光谱（ARPES）。最近的发现，量子振荡也可以在一些铜超导体在非常高的磁场（bbb40特斯拉）中观察到，这对该学科产生了非常大的影响。最重要的是，它揭示了轻微掺杂铜酸盐的费米表面的真实性质与先前通过ARPES测量得出的结果有很大不同。量子振荡技术的主要特点是，首先，测量在费米能级探测电子结构，比ARPES具有更高的能量和动量分辨率，其次，它们探测的是大部分样品，因此不受表面缺陷或重建的影响。研究费米表面的第二个相关技术是角相关磁电阻（ADMR）。这最近被证明是一项非常成功的技术，特别是用于研究更多重掺杂的HTSC，并且非常详细地揭示了一个特定铜族的完整三维费米表面拓扑结构。目前，ARPES无法获得此类三维信息。与传统的量子振荡技术相比，ADMR还有一个额外的优势，即它还可以揭示输运散射率的温度和动量依赖性，而输运散射率是在很大程度上决定HTSC在相图上异常输运性质演变的关键参数。由于最近能够产生非常高的磁场（高达100特斯拉）的设备的发展以及这些设备的信噪比的最新改进，量子振荡和ADMR现在已经成为HTSC研究中的强大技术。本提案的目标是充分利用这些及时的发展，并研究各种不同HTSC材料在非常高磁场下的量子振荡和ADMR。通过这样做，我们的目标是更深入地了解高温铜超导体的正常状态电子结构如何随着电子密度的变化而变化。我们还将尝试将我们的费米表面研究与它们在极限低温（高磁场）状态下的电阻率和霍尔效应的测量相结合。

Despite having been discovered more than twenty years ago, the unusual properties of high temperature cuprate superconductors (HTSC) are still far from being well understood. The most important question is of course, why is the superconducting transition temperature so high and how might it be raised even further? A strongly linked question to this is, what is the nature of the normal (non-superconducting) state of these materials and how does this evolve as the number density of conduction electrons is varied by chemical doping? Clearly, a complete understanding of the former requires knowledge of the latter. Until recently our main experimental probes of the normal state were macroscopic transport measurements (electrical resistivity, Hall effect etc.) and angle resolved photoemission spectroscopy (ARPES). The recent discovery that quantum oscillations can also be observed in some cuprate superconductors at very high magnetic field (> 40 Tesla) has made a very big impact on the subject. Most importantly, it reveals that the true nature of the Fermi surface of the slightly doped cuprates is significantly different to that suggested previously by ARPES measurements. The main features of the quantum oscillation technique are first that the measurements probe the electronic structure right at the Fermi level with much higher energy and momentum resolution than ARPES, and second that they probe the bulk of the sample and so are not influenced by surface defects or reconstructions. A second related technique for investigating the Fermi surface is angle dependent magnetoresistance (ADMR). This has recently proven a very successful technique particularly for investigating more heavily doped HTSC and has revealed in great detail the full three-dimensional Fermi surface topology on one particular cuprate family. Such three-dimensional information is currently inaccessible to ARPES. ADMR has an additional advantage over conventional quantum oscillation techniques in that it can also reveal the temperature and momentum dependence of the transport scattering rate, a key parameter that largely determines the evolution of the anomalous transport properties of HTSC across the phase diagram.Both the quantum oscillations and ADMR have now become powerful techniques in the study of HTSC because of the recent development of facilities which are able to produce very high magnetic fields (up to 100 Tesla) and recent improvements in the signal-to-noise ratios at these facilities. The goal of this proposal is to make best use of these timely developments and to study quantum oscillations and ADMR in a variety of different HTSC materials at very high magnetic fields. In so doing we aim to gain a deeper understanding of how the normal state electronic structure of high temperature cuprate superconductors evolves as a function of the electron density. We will also attempt to couple our Fermi surface studies to measurements of their resistivity and Hall effect in the limiting low temperature (high magnetic field) regime.