Spectroscopy of Exotic Orders- Higgs boson in Superconductors – SEO-HiggS2

Discovering and understanding the new states of matter and their fundamental excitations found in the rich phase diagrams of condensed matter systems represent some of the most challenging problems in quantum many body physics. While the quest for the Higgs boson in particle physics is reaching its goal, there is growing interest in the search for such fundamental excitation in quantum many body systems where the Higgs boson manifests itself as a fundamental collective mode. The first SEO-HiggS2 projects goal is to search for and understand the mechanism of the visibility of such Higgs excitation of a well-known quantum state, the superconducting phase. The second part of the project is to tackle a fundamental question of condensed matter physics, the nature of the exotic order of URu2Si2. To reach these two objectives, we will use the electronic Raman spectroscopy under high pressure and in the time domain to manipulate the delicate interaction between the various orders of the studied systems.

The first axis of our project deals with the question of the observation and the study of the amplitude mode of the superconducting state, a fundamental excitation analogous to the Higgs boson. The only known (and still controversial) cases of such modes in bulk superconductors has been reported in dichalcogenides 2H-NbSe2 and A15 compounds. We aim at evidencing the „Higgs type nature of these modes and understanding the mechanism for their visibility. In the latter compounds, we will study the interplay between the modes related to the martensitic transition and the superconducting signatures, both by varying the temperature and the pressure. In the former compound, the observation of such excitation by Raman scattering is claimed to be due to the coupling to the excitation of the adjacent charge density wave order. The SEO-HiggS2 project intends to study the dynamical behavior of both excitations and their interplay by time-resolved Raman scattering. Particularly, comparison of the pressure and time-dependent behaviors with theoretical prediction should help to definitively disentangle between different theoretical propositions. Moreover, we aim to discover and study new Higgs modes especially in the coexisting zone of charge density wave and superconducting orders reached under pressure in other dichalcogenides (2H and 1T systems). By exploring various compounds with different type of charge density wave order (commensurate/incommensurate) originating from different type of mechanism (purely electronic, strongly phononic), we expect to draw conclusions about the universality of the “Higgs” mode in superconductors.

The second topic is focused on the longstanding mystery of the hidden order of URu2Si2. Thanks to the study of the fundamental A2g excitations that we have recently shown to be the fingerprints of this mysterious quantum state, we plan to make an important step toward unravelling its nature. This mysterious order turns into a simple antiferromagnetic under pressure. There is high interest in following these Raman signatures under pressure up to the antiferromagnetic order to definitively associate them with the order parameter of the hidden order.

The Raman scattering technique under high pressure, already developed by the leader of the project, will be used. This clearly positions us as a very competitive and maybe unique team for this promising project. The project will support the research of a young CNRS member at LMPQ, Université Paris Diderot with the recruitment of a post-doc for 2 years. In addition to valorisation of the results through communications and publications, to collaborations with theoreticians (for ex. C. Varma, L. Benfatto, S. Burdin, D. Manske), the SEO-HiggS2 project will provide an excellent basis for the communication to large audience (“how to find a Higgs boson between two diamonds?”), able to wake up vocations and in general propagate our scientific approach.