The persistence of memory in ionic conduction probed by nonlinear optics

Mobile metal ions hop between distinct lattice sites

Predicting practical rates of transport in condensed phases enables the rational design of materials, devices and processes.  This is especially critical to developing low-carbon energy technologies such as rechargeable batteries.

For ionic conduction, the collective mechanisms, variation of conductivity with timescales and confinement, and ambiguity in the phononic origin of translation, call for a direct probe of the fundamental steps of ionic diffusion: ionic hops.  Such hops, however, are rare-event large-amplitude translations, and are challenging to excite and detect.  

In the paper 'The persistence of memory in ionic conduction probed by nonlinear optics' published in Nature, a team of researchers from this department, the SLAC National Accelerator Laboratory, Newcastle University and Arizona State University used single-crystal terahertz pumps to impulsively trigger ionic hopping in battery solid electrolytes.  This was visualised by an induced transient birefringence, which enabled direct probing of anisotrophy in ionic hopping on the picosecond timescale.  

The relaxation of the transient signal measured the decay of orientational memory, and the production of entropy in diffusion.  The authors extended their experimental results by using in silico transient birefringence to identify vibrational attempt frequencies for ion hopping.

Using non-linear optical methods, they were able to probe ion transport at its fastest limit, distinguish correlated conduction mechanisms from a true random walk at the atomic scale, and demonstrated the connection between activated transport and thermodynamics of information.