Investigation of Quantum Materials Using Pump-Probe Raman Scattering

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Non-Technical Abstract In quantum materials such as superconductors and topological insulators quantum physics underlies most important properties. Unraveling the puzzles posed by these materials is one of the main scientific challenges of the 21st century, with direct relevance to new generations of electronic, energy, and medical technologies. This project is devoted to observing how these materials evolve in time after being hit by a short laser pulse. A weak laser pulse may kick some electrons out of their orbits. Observing how these hit atoms and other electrons react reveals interactions between electrons as well as between electrons and the atomic lattice. If the laser pulse is strong, the material can briefly go into a different state, e.g. a superconductor will become nonsuperconducting. Observing how it comes back to the original state elucidates otherwise hidden mechanisms. This knowledge is used to test fundamental theories and helps to harness quantum materials for practical applications. The experimental technique for making these observations is known as pump-probe Raman spectroscopy and allows making movies of how quantum materials evolve in time taking frames every 0.00000000000001 sec. Light travels a fraction of a millimeter during these time intervals, so very fast processes can be detected in slow motion. The work is carried out at the new ultrafast laser spectroscopy laboratory at the University of Colorado (CU)-Boulder built with the funds previously provided by NSF. It is a part of a Ph.D. thesis of two graduate students and contributes to undergraduate education. The project will also contribute to outreach efforts through the existing Partnership in Formal Science Education in the Community program. Technical Abstract This project focuses on groundbreaking measurements of materials where quantum physics underlies most important physical properties. The general goal is to understand how these materials behave away from thermal equilibrium: The investigated sample is driven out of equilibrium by a short laser pulse, and another time-delayed laser pulse will probe its relaxation by serving as the excitation laser for Raman spectroscopy. The work is carried out at the new ultrafast laser spectroscopy laboratory at the University of Colorado (CU)-Boulder built with the funds previously provided by NSF. The laser system at the heart of this laboratory is one of the most advanced of its kind in the world. It is utilized for pump-probe angle-resolved photoemission spectroscopy and pump-probe Raman spectroscopy. This project represents the Raman component of this effort. All Raman active excitations are tracked during relaxation. These include phonons, magnons and some electronic excitations across gaps of up to 5eV. Raman scattering directly probes the energies, lifetimes, and population of excited states (e.g. phonon occupation numbers). The time-resolved Raman scattering measurements are on charge density wave (CDW) systems, conventional and cuprate superconductors, and Mott insulators. These materials display diverse and often still enigmatic physical phenomena of great interest. The project is a part of a Ph.D. thesis of two graduate students and contributes to undergraduate education and will also contribute to outreach efforts through the existing Partnership in Formal Science Education in the Community program.