More chaos in the news

Our work on pattern generation and symbolic dynamics was recently relayed as a news item by the Institut de Physique and Institut de Chimie of CNRS

Path sampling for lifetimes of metastable magnetic skyrmions and direct comparison with Kramers’ method

Our recent study on the lifetime of magnetic skyrmions, led by Louise Desplat, has just appeared as a Rapid Communication in Physical Review B. We studied the issue of thermal stability theoretically using two distinct methods. The first involves a generalisation of Kramer’s framework to multidimensional phase spaces due to Langer, while the second is a path sampling method, called forward flux sampling, which is useful for simulating rare events.

Despite the different assumptions that underpin these two approaches, we find overall quantitative agreement between the two, as shown in the figure above in which we plot the Arrhenius prefactor, f0, and the average lifetime, tau, as a function of applied magnetic field. This result provides confirmation of the validity of Langer’s approach (and consequently, similar techniques like harmonic transition state theory), and represents the first application of path sampling methods for studying skyrmion lifetimes.

Pattern generation and symbolic dynamics in a nanocontact vortex oscillator

Top: Schematic illustration of a nanocontact vortex oscillator, showing gyration in the film plane and periodic core reversal. Bottom: Dynamics in the reconstructed phase space, with the different patterns generated shown in yellow, red, and blue. Reference: M.-W. Yoo et al., Nature Communications 11, 601 (2020).

Our latest work on chaotic dynamics in nanocontact vortex oscillators has just appeared in Nature Communications.

This represents the fruit of many years of labour, which began with a number of interesting discussions with Sébastien Petit-Watelot at the Institut Jean Lamour (CNRS/Univ Lorraine) in Nancy and Damien Rontani at the Laboratoire Matériaux Optiques, Photonique et Systèmes (CentraleSupélec/Univ Lorraine) in Metz. We had the idea to look at the possible waveform patterns that could be generated by the nanocontact vortex oscillator in its chaotic state. Myoung-Woo Yoo, whilst on his Marie Sklodowska-Curie fellowship with me on this topic, devised a clever pattern filtering algorithm to analyse the experimental time traces, which are quite noisy even at 77 K as a result of the relative low signal to noise ratios in these samples. This technique allowed us to really delve into the different patterns available, which turn out to be quite simple, as shown in the figure above.

Ultimately, we were able to show that the complexity and entropy of the signal can be controlled with the applied current, where the most interesting things occur in the chaotic regime. Basic benchmarks show that the oscillator can be a good random number generator with some interesting symbolic dynamics.

This work has been funded by the Agence Nationale de la Recherche as part of the CHIPMuNCS project.

Outstanding Referee of the American Physical Society

I’m humbled and honoured to be named an Outstanding Referee of the American Physical Society. The communique from APS reads:

Ridge, NY, 26 February 2019 — The American Physical Society (APS) has selected 143 Outstanding Referees for 2019 that have demonstrated exceptional work in the assessment of manuscripts published in the Physical Review journals. A full list of the Outstanding Referees is available online at

Instituted in 2008, the Outstanding Referee program annually recognizes approximately 150 of the currently active referees for their invaluable work. Comparable to Fellowship in the APS and other organizations, this is a lifetime award. The selection this year was made from 30 years of records on over 71,000 referees who have been called upon to review manuscripts, including more then 40,000 that were submitted in 2018. The basis for the Outstanding Referees selection takes into account the quality, number and timeliness of a referee’s reports, without regard for membership in the APS, country of origin, or field of research. Individuals with current or very recent direct connections to the journals, such as editors and editorial board members, were excluded.

The 2019 honorees come from 29 different countries, with large contingents from the U.S., Germany, U.K., Canada, and France. All recipients of this distinction have been notified, and sent a lapel pin and a certificate to commemorate their achievement. The selection for this achievement is always difficult and APS expresses its appreciation to all referees that help make the Physical Review collection some of the most cited physics journals in the world.

The efforts of these individuals not only keep the standards of the journals at a high level, but in many cases also help authors improve the quality and readability of their articles—even those that are not published by APS. The Outstanding Referees are to be congratulated and thanked for their outstanding service to the physics community.

I join Ulf Gennser in the cohort of recipients at C2N.