Quantum Critical superconductors at high pressures

当二级相变被抑制到零温度时，有序和无序状态之间的波动不再是热激发，而是量子性质。在金属中，这引起了全新的现象，包括偏离标准的朗道-费米液体行为。此外，在量子临界点附近经常观察到非常规的超导性。因此，量子临界性仍然是各种材料的热门和及时的研究领域。在铜氧化物和硫属化物超导体中，超导性与量子临界性的联系仍然存在争议。在铜氧化物超导体中，理论建议量子临界性来自于赝能隙相、电荷密度波序或超导性本身的抑制。在硫族化物材料中，量子临界性和涌现的超导性与电荷密度波序、向列性或自旋密度波序的抑制有关。与此同时，替代理论提出在铜酸盐中完全不存在相干电子态。该项目将使用电子输运测量和量子振荡研究作为决定性探针来检测量子临界性和超导性的关系，并将探索当材料在量子临界点上调谐时相干电子态是否持续存在。高压研究将用于以系统方法研究清洁样品。Sven Friedemann开发了一系列新测量的高压技术。这包括SQUID磁力测量以及在国际高磁场设施进行的量子振荡和电输运测量，两者都超过10 GPa。该学生将几乎完全基于布里斯托的项目的前半部分，并将在高压方法的培训。在此期间，他将对NbSe 2和TiSe 2硫属化物超导体进行实验室测量。为了准备下半年的项目，他将生长HgBa 2CuO 4晶体。下半年将包括在Radboud大学和国际设施的几个月的时间。该项目的这一部分将侧重于HgBa 2CuO 4和FeSeS中量子振荡和电子输运的设施研究。

When a second order phase transition is suppressed to zero temperature, fluctuations between the ordered and non-ordered state are no longer thermal excitations but rather quantum in nature. In metals, this gives rise to completely new phenomena including deviations from the standard Landau-Fermi liquid behaviour. In addition, unconventional superconductivity is often observed around quantum critical points. Thus, quantum criticality remains a topical and timely research area for a wide variety of materials. In cuprate and chalcogenide superconductors, the link of superconductivity to quantum criticality remains controversial. In cuprate superconductors theoretical proposals suggest quantum criticality from the suppression of the pseudogap phase, charge density wave order, or superconductivity itself. In chalcogenide materials quantum criticality and emergent superconductivity is associated with the suppression of charge density wave order, nematicity, or spin-density wave order. At the same time, alternative theories propose a complete absence of coherent electronic states in cuprates. This project will use electronic transport measurements and quantum oscillation studies as decisive probes to detect the relation of quantum criticality and superconductivity and will probe whether coherent electronic states persist as the materials are tuned across the quantum critical point. High pressure studies will be used to study clean samples in a systematic approach. Sven Friedemann has developed high-pressure techniques for a host of new measurements. This includes SQUID magnetometry as well as quantum oscillation and electrical transport measurements at international high-magnetic-field facilities, both beyond 10 GPa . The student will be based almost entirely in Bristol for the first half of the project and will be trained in high-pressure methods. During this time, he will conduct lab-based measurement on NbSe2 and TiSe2 chalcogenide superconductors. In preparation for the 2nd half of the project, he will grow crystals of HgBa2CuO4. The 2nd half will include periods of several months based at Radboud University and at international facilities. This part of the project will focus on facility-based studies of quantum oscillations and electronic transport in HgBa2CuO4 and FeSeS.