Time Resolved Probing of Unconventional Orders in Novel Kagome Metals

Massachusetts Institute Of Technology, Cambridge MA

Investigators

Abstract

Nontechnical Abstract: Despite tremendous advances in the recent decades, conventional electronics based on silicon will soon hit a limit. One way to circumvent this is to use novel materials with unconventional properties. In this respect, there has been a tremendous interest in a new class of metals in which the atoms are arranged in periodic patterns of corner sharing triangles known as the Kagome lattice. The electrons in these systems behave in a highly unusual manner displaying strong repulsion, geometric frustration and topological properties. When these materials are cooled, first they go into a state in which the electron density is periodically modulated (charge density wave) and upon further cooling, they become superconductors. A key challenge is to understand the mechanism of these phases and the relationship between the two. The goal of this project is to use different types of optical and electron based spectroscopies to investigate the charge density wave state of these novel Kagome metals. A central part of this project is the training of next generation of students both in advanced spectroscopies as well as in novel quantum materials. This program will also contribute to K-12 science education and improve the physics education at the freshman level. Technical Abstract: Recently, a new class of superconducting transition-metal Kagome compounds were discovered which display a complex interplay between multiple order parameters. Above the superconducting phase transition, these materials host a charge density wave order and a putative short-ranged chiral flux phase in which both inversion and time-reversal symmetry are broken. This project aims to use an array of ultrafast optical and electron probes to investigate the highly unconventional phase diagram of these systems. With static- and time-resolved Kerr rotation microscopy, the time-reversal symmetry-breaking suggested by recent muon spin relaxation measurements is studied at a length scale inaccessible to other probes. By aligning domains of opposite chirality with circularly polarized light, the chiral charge density wave order proposed by scanning tunneling measurements is investigated with second harmonic generation. Finally, the competition or cooperation between different charge density waves will be probed by a combination of ultrafast electron diffraction and time- and angle-resolved photoemission spectroscopy which will directly yield dynamics of electronic and lattice structure. These experiments will provide a comprehensive picture of the microscopic interactions in these materials and will shed light on to the unconventional orders. 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.