MuSR and multi-probe studies of spin-charge interplays in emergent quantum phenomena

Non-technical abstract: Superconductivity is a prominent macroscopic manifestation of quantum phenomena, which can be applied to quantum computing devices, loss-free power-transmission, high-field magnets and high-speed trains. Superconducting mechanism for simple metals, such as Sn and Al, was explained by the theory of Bardeen, Cooper and Schrieffer (BCS) in 1957. However, since the discovery of cupper-oxide systems in 1986, many "unconventional" superconductors (USC) were found to exhibit signatures unexpected in BCS theory. Many of them are in the proximity to materials showing Mott metal-insulator transitions (MIT). Strong interplay between electric (charge) and magnetic (spin) interactions seems to be crucial for the USC and MIT phenomena. By performing muon spin relaxation, neutron scattering and scanning-tunneling microscopy measurements on selected systems, and by interpreting the results using Uemura-plot phenomenology developed by the PI since 1989, the present project aims to elucidate charge-spin interplays and unveil essential mechanisms of USC and MIT. The project also aims to clarify the role of dimensionality and topology on phase transitions of these systems and selected novel magnetic systems. Involving students and young researchers, these projects help develop human resources in international collaborations at major accelerator-based facilities. Technical abstract:Emergent quantum materials, such as unconventional superconductor (SC), Mott transition, Skyrmion and topological systems, exhibit novel quantum phenomena based on intertwining interplays of spin, charge, geometry and topology. Muon spin relaxation (MuSR) is a strong probe for studying these systems, with unique sensitivity to superfluid density in SC's and dynamic spin fluctuations, static order parameter, volume fraction and local spin correlations in magnetic transitions. By performing MuSR studies in combination with complementary measurements by neutron scattering and Scanning Tunneling Microscopy (STM), the PI aims to test and clarify: (1) a hypothesis of highly entangled spin-charge gap, analogous to the magnetic resonance mode, existing in non-superconducting Mott transition systems Ba(Co,Ni)S2 and Ni(S,Se)2; (2) magnetic-field-induced change in the normal state of Hg1201 high-Tc cuprates; (3) effect of topology in dynamic critical behavior of MnSi and other Skyrimon systems; and (4) spin fluctuations in topological Weyl semimetal Co3(Sn,In)2S2 with Kagome lattice. The PI also integrate these experimental efforts with his phenomenology on unconventional SC's towards the ultimate goal to advance comprehensive understandings of unconventional superconductivity. In addition, Frontiers of Condensed Matter Physics (FCMP) lectures and associated workshops organized by the PI facilitate motivated graduate students / researchers from over the world to study forefront CMP researches on superconductivity and magnetism with leading contributors.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.