Revival of Strain Susceptibilities: Magnetostrictive Coefficient and Thermal-Expansion Coefficient

In thermodynamics, volume is an essential extensive variable. Strain-line, area, or volume change-therefore offers a direct window into correlated quantum matter: tiny length changes {\Delta}L track how the lattice responds when state variables such as magnetic field H and/or temperature T are varied, revealing phases, transitions, and dynamics. Direct, high-precision strain measurements are already difficult; their susceptibilities are harder still. Very recently, several direct techniques have made vital progress on two key quantities: the magnetostrictive coefficient d{\lambda}/dH (often denoted qijk or dij in the magnetostriction literatures), and the linear thermal-expansion coefficient {\alpha}= d{\lambda}/dT. Considering these two strain susceptibilities together-they are fundamental and complementary-clarifies why these thermodynamic properties merit renewed attention.