A Balancing Act Between Phonons and Spins
Abstract: Most materials expand when heated since atomic vibrations are enhanced by temperature. However, in 1895 C.E. Guillaume combined Fe and Ni to discover a material with near-zero thermal expansion, named invar. This discovery was awarded the 1920 Physics Nobel Prize and sparked thousands of scientific investigations. Since the anomalous invar effect is associated with magnetism, nearly all studies have focused on the electronic and spin structure of Fe-Ni. However, without including phonons, the proposed models have not been successful in describing the invar behavior. In this talk, I will present an experimental method that combines different spectroscopy techniques to isolate the effects of phonons and spins on the thermal expansion of invar. Since the thermal energy of materials is related to entropy (through a Maxwell relation), we indirectly probe the thermal expansion by measuring the vibrational and magnetic entropies as a function of pressure. The phonon spectrum was probed by nuclear resonant inelastic X-ray scattering (NRIXS) and the magnetization by Mössbauer spectroscopy, with samples in diamond-anvil cells. We show that the invar behavior stems from a competition between phonons and spins that precisely balance each other, resulting in the anomalous invariant volume of invar. Lastly, with the support of calculations, we identify a spin-phonon coupling that improves the precision of this cancellation.
About the QSQM: The EFRC-QSQM center aims to develop and apply nontrivial quantum sensing to measure and correlate local and nonlocal quantum observables in exotic superconductors, topological crystalline insulators, and strange metals. The center is led by the University of Illinois at Urbana-Champaign in partnership with the University of Illinois at Chicago and the SLAC National Accelerator Laboratory.