KSEF RDE: Colossal Negative Volume Thermal Expansion Coupled to Magnetic and Orbital Orders in Mott Insulators

Emerging technologies are increasingly predicated upon the discovery of novel functional materials that exhibit extraordinary structural and physical properties such as anomalous thermal expansion effects. Materials with negative thermal expansion (NTE) are clearly of fundamental interest and technological importance for applications such as metrology, chronometry, precision optics and cryogenic storages. It is not surprising that studies of NTE are often prominently published and cited in literature. Most materials expands upon heating as the inherent anharmonicity of lattice vibrations causes the average bond distances to expand with temperature, therefore there are only a handful of materials that exhibit negative or zero thermal expansion driven by transverse phonon modes or rigid unit modes, which usually occurs along one crystallographic direction with a modest NTE coefficient. Much fewer materials possess negative volume thermal expansion (NVTE). Almost all classic NTE materials are insulating and nonmagnetic; therefore possess no electric or magnetic functions. All these inherent constraints limit applications of NTE materials in electronics and spintronics, where electric current and magnetic moment play functional roles. The need for novel NTE materials having desirable physical properties is obvious and compelling. We have recently discovered that NVTE commonly exists in ruthenates, which are a class of novel Mott insulators. The observed NTE is extraordinary in that (1) the average magnitude of the NTE coefficient is often 100 times greater than that for most classic NTE materials, which constitutes colossal NTE, and (2) the observed colossal NTE closely tracks the orbital and magnetic orders, suggesting an electronically-driven mechanism, in sharp contrast to that driving classic NTE. This discovery is a breakthrough that opens an entirely new avenue to not only gain a quantum-mechanical insight into the new physics of NTE but also better harness the power of NTE materials in a more technically favorable fashion.