Project

unconventional superconductivity; vortex dynamics; quantum phase transition; vortex lattice; quantum criticality

Lead
Materialien mit starken Elektronenwechselwirkungen (Korrelationen) zeigen immer wieder aufs Neue faszinierende und unerwartete Eigenschaften und Ordnungsphänomene. Unser Verständnis der ihnen zu Grunde liegenden Prozesse und deren theoretischen Beschreibung stoßen regelmäßig an ihre Grenzen.Stark korrelierte Elektronensysteme stellen damit ein faszinierendes Forschungsfeld zum Erproben neuer theoretischer Erkärungsmodelle und mathematischer Konzepte dar.
Lay summary
Inhalt und Ziel des Forschungsprojekts:  Innerhalb dieses Forschungsprojekt wollen wir Eigenschaften der supraleitenden Phasen von CeCoIn5 detailliert untersuchen. Die starken Korrelationen der Elektronen in diesem Material führen zu einem supraleitenden Grundzustand, der als unkonventionell bezeichnet wird. Das heißt, er kann bezüglich seiner Symmetrie, sowie der Art der Wechselwirkung, die zur Supraleitung führt, nicht mit der konventionellen Theorie der Supraleitung (BCS-Theorie) beschrieben werden. Darüber hinaus findet sich in CeCoIn5 eine weitere supraleitende, magnetische Phase wobei beide Ordnungsphänomene sich gegenseitig zu stabilisieren scheinen, eine Tatsache die der konventionellen Theorie widerspricht. Ziel des Projektes ist es die Eigenschaften beider supraleitender Phasen sowie die Natur des Phasenübergangs zwischen beiden im Detail zu untersuchen und zu charakterisieren. Hierfür sollen Ultraschalluntersuchungen verwendet und weiterentwickelt werden.  Wissenschaftliche und gesellschaftliche Relevanz:  Das Projekt soll einen Beitrag zu unserem Verständnis von Systemen mit komplexen Ordnungsparametern leisten. Es soll neue theoretisch Modelle motivieren und ihnen als Referenzpunkt dienen.

Direct link to Lay Summary

Last update: 13.01.2017

Number	Title	Start	Funding scheme

Strong electronic fluctuations give rise to ground states that are governed by quantum fluctuations. In heavy-fermion systems, the strong coupling between magnetic moments and conduction electrons, and the competition between localization and itinerancy leads to novel magnetic or superconducting phases that occur in the vicinity of quantum critical points. Here we propose an in-depth study of the heavy-fermion system CeCoIn5. We have shown previously [45], that a new superconducting phase (Q-phase) emerges from an unconventional d-wave superconducting phase in a continuous quantum phase transition. We found evidence that this phase contains a spatially-modulated p-wave pair density wave (PDW). This order parameter linearly couples to d-wave superconductivity and antiferromagnetic spin-density wave (SDW) order. This is the first observation of an order parameter of this kind. We propose to further explore the topology of the SDW/PDW parameter and their coupling. This will be done utilizing ultrasound attenuation measurements. It has been shown [56] that ultrasound couples to the low-level excitation present in the zero gap nodes of the superconducting order parameter. This coupling depends strongly on the relative orientation between the sound waves and the nodal direction. Moreover, the coupling provides an additional attenuation channel for the sound waves and the temperature dependence of this attenuation reflects the topology of the nodal region. The PDW proposed in [45] breaks the fourfold symmetry of the d-wave order parameter and spatially modulates the d-wave nodes in one direction. The selection of the particular direction depends on its relative orientation to the applied magnetic. Therefore, we plan ultra-high precision angular-dependent ultrasound attenuation measurements to reveal this effect. Our work will provide greater insights and will help in the development of a microscopic theory for this novel quantum state.CeCoIn5 also constitutes a unique material to study the vortex dynamics in a d-wave superconductor. The vortex lattice of CeCoIn5 has been studied extensively in the past and a wealth of phases has been found [36]. Here, we propose to investigate the pinning mechanisms present in this material. We have performed preliminary studies of the vortex flow resistance to an applied electric current. Interestingly, our data indicate that an additional pinning mechanism facilitated by domain boundaries exists. As an initial step, we will extend these studies of the vortex flow resistance. We plan to explore a larger fraction of the phase-diagram. An improved setup enabling higher densities for the driving current will enable us to access even the low temperature and low magnetic field region deep inside the superconductors phase-diagram. The high cleanness of the crystals available for the project assures weak impurity pinning. The presence of vortex lattices with different symmetry will allow us to investigate the intrinsic pinning mechanism by domain walls in great detail. At a later stage, we plan to complement our flow resistance data with measurements of the dynamic response and the local pinning potential. The results obtained in this study should provide important data for the test of theoretical concepts on vortex dynamics in unconventional superconductors.