Many materials store information about what has happened to them in a sort of material memory, like wrinkles on a once crumpled piece of paper. Now, a team led by Penn State physicists has uncovered how, under specific conditions, some materials seemingly violate underlying mathematics to store memories about the sequence of previous deformations. According to the researchers, the method, described in a paper appearing today (Jan. 29) in the journal Science Advances, could inspire new ways to store information in mechanical systems, from combination locks to computing.
One way some materials form memories is called return-point memory, which operates much like a single dial combination lock, according to Nathan Keim, associate professor of physics in the Penn State Eberly College of Science and leader of the research team. With a lock, rotating the dial clockwise and counterclockwise in a particular sequence yields a result - the lock opening - that depends on how the dial was moved. Likewise, for materials with return-point memory, alternating between positive and negative deformations can leave a memory of the sequence that researchers can read or erase.
"The same underlying mechanism or mathematics of this memory formation can describe systems from the magnetization of computer hard drives to damage in solid rock," Keim said, noting that his research group recently showed that the same math also describes memories stored in disordered solids, in which the arrangement of particles seems random but actually contains details about past deformations.
Return-point memory relies on the alternating of direction of the external force, or "driving," such as the alternating of positive or negative magnetic field or pulling on a material from one side and then the other. However, materials should not be able to form return-point memory when the force only occurs in one direction. For example, Keim said, a bridge might sag slightly as cars drive over it, but it doesn't curve upwards once the cars are gone.
"The mathematical theorems for return-point memory say that we can't store a sequence if we only have this 'asymmetrical' driving in one direction," Keim said. "If the combination lock dial can't go past zero when turning counterclockwise, it only stores one number in the combination. But we found a special case when this kind of asymmetrical driving can, in fact, encode a sequence."
