Ordered States in Disordered Magnets and Superconductors

The interplay order and disorder in condensed matter has intrigued scientists for decades and continues to be an active area of research today. Recently, a new approach to synthesizing materials with extreme amounts of point-defect disorder has been developed and successfully applied to both metal alloys and ceramics. This process is known as ‘entropy-stabilized’ or ‘entropy-enhanced’ alloying and works by placing 4-5 elements on a lattice site that typically only hosts 1 element. This increases the number of possible microstates and configurational entropy of the material, which in turn drives the free energy negative and stabilizes homogeneously-mixed, single-phase materials with large disorder.

In this talk, I will describe two recent studies where my group has used entropy-enhanced alloying to study the effects of disorder localized to specific lattice sites on the stability of ordered phases like magnetism and superconductivity. In the first study, we entropy-alloyed different sites in the perovskite (ABO₃) crystal structure and measured the system’s ability to form long-range magnetic structures. Alloying on the perovskite B-site causes the magnetic structure to fragment into a mixed phase microstructure, whereas alloying on the A-site resulted in only minor perturbations to the expected long-range magnetic structure. In the second study, we applied entropy-alloying to the high-temperature cuprate superconductor, YBa₂Cu₃O_7-x. Remarkably, our results show almost no suppression of the electron pairing interaction strength as measured by the transition temperature, despite large amounts of spin disorder added to the system. Taken together, these studies highlight how electronic and magnetic order in materials can be sensitive to some types of disorder while robust to others, and demonstrate how entropy-alloyed materials provide a highly versatile platform for re-examining order-disorder phenomena in condensed matter.