Workshop on "Avalanches in Functional Materials" (AFM)

The workshop will bring together scientists who work on 'avalanches and jerks' experimentally, in statistical mechanics, and by computer modelling.
Jerks exist in nano-devices where domain boundaries are excited by external fields such as in ferroelectric thin films which have information written on them by local electric fields. This information can then be moved to a reading device. The issue is the following: is the information corrupted by the shift or can a continuous shift of domain walls be achieved. Recent simulations have shown that very small and thin devices will suffer from 'jerky' movements and the formation of avalanches ( such as in snow avalanches where one event will trigger a multitude of secondary events or as in earth quakes where each shock can trigger an after-shock). There is tremendous experience in this subject distributed over many disciplines but we have not yet brought the various communities together. This will happen in the proposed workshop.

An important research aspect for jerks and avalanches is the role played by temperature. Theoretical work ( analytical and simulation work) has focused on the low temperature regime. Here the avalanches are 'a-thermal'. This means that jerks will not be thermally activated (while stress and strain conditions are very important for the nucleation of avalanches). Only in 2013 has it become clear that this result is misleading for real device materials: at higher temperatures the thermal activation becomes important and the avalanche behaviour changes dramatically. Between the two thermal regimes is a cross-over regime where kinetic rate laws with stretched exponential dominate. The crossover point for many materials seems to be around room temperature so that this effect is not a curiosity of a strange phenomenon but becomes important for many device applications.

While the understanding of 'jerks and avalanches' stems from a multitude of specialised research areas, we find that the experimental approaches are also different between the various communities and not much progress has been made so far to disseminate experimental approaches from one community to another. Typical are static and quasi-static approaches where an external state variable is changed adiabatically slowly and the dynamic change of the system is observed. Typically this gives rise to a jerk when a threshold value is overcome. A much better approach would be to perform truly dynamic measurements with a large number of observed avalanches (resonance methods). Such methods are developed in Cambridge in 2013 but nowhere else. The reason is that the effects happen on two very different time scales. Tuning times (e.g ramping up temperature or electric fields) have to be very slow (and take sometimes weeks) while the measurement of a jerk has to be very fast (a jerk has an intrinsic time scale related to the speed of sound propagation). Techniques are being developed for acoustic measurements and piezoelectric measurements where the resonance frequencies are in the MHz range while temperature is changed over milliK/sec.

We strongly believe that much progress in this field could be made if the various experimental approaches were better known within a wider community and potentially transferred between groups.

We expect 3 types of impact. First, we wish to better coordinate the research in the fields of avalanches and noise. To obtain better cooperation, the various groups first need to meet and discuss their particular scientific approaches. The systematics of the various research groups are very different even within the 'materials device' community. Groups working on martensites know little about multiferroics and vice versa. The reason is that the historic developments of these fields were very different with different applications in mind - while the simple fact that ferroelastic and martensitic phase transitions follow the same physical mechanisms got simply lost. The differences are even greater in research on avalanches: the language of renormalization group theory and that of ferroelectric microstructures have almost nothing in common. Nevertheless, noise pattern in nano-scale ferroelectrics will follow predictions of renormalization group theory even if this connection is not yet fully understood. The outcome could be the creation of an international network of cooperation, in particular between the UK, the US and China. This could be supported by joint EPSRC - NSF funding with some Chinese component. Another possibility is to apply to DOE - funding which has been provided in the past.

Second, we expect significant impact by transferring results from the field of the collapse of porous materials to multiferroics. We know that the fingerprint for the collapse is the power law probability of the emitted jerk spectra (and the observation that simple linear response theories such as in the Dissipation-Fluctuation theorem or, more applied, the Kramers-Kronig transformation are no longer valid). One particular aspect where applications are possible is the area of Omori law effects as precursors for major collapse events. Similar effects exist in ferroic materials but the analysis is much less developed. The workshop will help with the technology transfer between these fields. This relates to theory but also to identifying better ways to measure time correlations of moving domain walls.

The formal connection between jammed systems and collapsing porous materials is so far given by arguments of statistical mechanics but not by comparisons of the underlying mechanisms. However, the same kind of observations are made for porous and jammed microstructures whereby the jamming becomes more serious when the sample size reduces to the nanometer scale. The impact of our workshop will be that the most prominent members of the various communities, as described in this application, will be at the same workshop and talk with each other. We then hope that talking will lead to joined-up action, such as joint research programmes. The centre for some of this research is Europe and involves activities in the UK. We plan to discuss whether a European framework could be set up in this field. We had in the past support for the mathematical aspects but this funding has ended. We will explore follow-up funding or the possibility of bi-lateral funding UK-Spain. The US DoE has already planned some major initiatives in Health (avalanches and stimulating fields for brain repair) and ferroic device design. We have planned activities between Cambridge and Illinois (with Profs Kriven and Dahmen) and wish to structure this collaboration further.

The third type of 'impact' relates to the search for novel materials. A major step in this direction was the rediscovery of multiferroics. We have made some progress in optimizing multiferroic device materials such as BiFeO3 but we are nowhere near the performance of simple magnetic memory techniques such as used in nano-wires. The key question is how we can design ways for avalanche pathways to occur. This appears much easier for magnetic materials and we will explore how similar methods may be usefully applied to ferroelectrics.