Author: José María De Teresa

Science@FELs 2022
This conference will be held in Hamburg, from 19-23 September 2022. Webpage: www.desy.de/scienceatfels2022

CMD-IOP online workshops
http://cmd29.iopconfs.org/onlineseries

Semiconductors and Insulators Section: Call for candidates
Call for candidates to take in charge the chair of the Semiconductors and Insulators Section. Following the resignation of Massimo Rontani (CNR, Italy) as per 31 December 2021, the EPS Condensed Matter Division (EPS CMD) Board is looking for an enthusiastic colleague specialized in Semiconductors and Insulators research to join the Board. In addition, the chosen candidate will chair the Semiconductors and Insulators section. If you are interested, please contact the EPS CMD Board chair (José María De Teresa, eps.cmd.chair@gmail.com) and the secretary of the Semiconductors and Insulators section (Erich Runge, erich.runge@tu-ilmenau.de). In order to support your candidature, please, submit a letter of interest and your CV. Deadline: 1st November 2021. The CMD Board warmly thanks Massimo Rontani for his contributions in these last years!

Author: Kees van der Beek

Following the advent of the COVID-19 pandemic in March, the EPS Condensed Matter Division, the Condensed Matter Group GEFES of the Spanish Royal Physical Society RSEF and the organising committee took the unprecedented decision to hold the biyearly CMD general conference wholly online.

Thus, it was in an entirely remote form that CMD28 took place from August 31^st till September 4^th, under the name “CMD2020GEFES ONLINE”. The conference attracted 2000 registered participants, and nearly 1000 contributed and invited papers. It involved up to 17 parallel oral morning sessions, followed by afternoon plenary- and semi-plenary talks, and a host of afternoon special- and poster sessions. These included events organised by EPS Young Minds, as well as a very well attended session on Diversity and Inclusiveness in Physics Research (50 participants). All in all, a genuine conference atmosphere was recreated, with participation remaining strong during the entire week.

The plenary talks were scheduled in the first half of the afternoon (CEST), allowing the participation of delegates from the Americas and Asia. Thus, the opening session and the first plenary talk by Prof. Pablo Jarillo-Herrero of the Massachusetts Institute of Technology was attended by more than 600 people. Gratuitous registration allowed the participation of physicists of all ages, and from all continents. Indeed, this edition of CMD has seen the largest number of participants bar the editions organized together with the German Physical Society DPG, it has also seen an unprecedented participation of 86 delegates from Latin America, 173 from Asia and the Middle East, and 29 from Africa.

CMD2020GEFES ONLINE hosted three Prize Award sessions. The Europhysics prize was awarded to Prof Jörg Wrachtrup of Stuttgart University on September 2, “for his pioneering studies on quantum coherence in solid-state systems, and their applications for sensing, and, in particular, for major breakthroughs in the study of the optical and spin properties of nitrogen vacancy centers in diamond”; the Olli V. Lounasmaa memorial prize for low temperature physics was awarded to Prof J.C. Seamus Davis of the University College Cork and Oxford University for his pioneering investigations and applications of exquisite scanning probe techniques for visualization of electronic quantum matter at the atomic scale on September 4^th, and the Spanish GEFES prizes for young scientists were awarded on September 3^rd.

In the weeks to come, the EPS Condensed Matter Division is planning to report on its experience with this novel conference format, its opportunities, its caveats, and lessons learned.

Figure Caption : Screenshot of the online CMD Europhysics Prize Award session during the CMD2020GEFES ONLINE  conference on Wednesday, September 2, 2020. (Top row) From left to right:  Prof. Dr; Jörg Wrachtrup, 2020 CMD Europhysics Prize laureate ; Kees van der Beek, chair of the EPS Condensed Matter Division; Petra Rudolf, EPS president ; Bart van Tiggelen, Editor-in-chief of EPL. (Bottom row) From left to right: Hermann Suderow, chair of the local organising committee; Serghei Klimin (delegate).

The prize is awarded every 2 years by the Condensed Matter Division of the European Physical Society for outstanding achievement in condensed matter physics.

The European Physical Society Condensed Matter Division is proud to announce that the 2016 EPS CMD Europhysics Prize is awarded to

• Peter Böni
• Aleksandr N. Bogdanov
• Christian Pfleiderer
• Achim Rosch
• Ashvin Vishwanath

“for the theoretical prediction, the experimental discovery and the theoretical analysis of a magnetic skyrmion phase in MnSi, a new state of matter.”

Discovery of a Skyrmion Phase in MnSi

Initially proposed as a model for hadrons in a mesonic field theory in particle physics, skyrmions have recently been discovered in magnetic systems thus establishing the existence of a new state of matter. Such structures exhibit a topological Hall effect and can be moved by currents much smaller than those required to displace domain walls thus opening the door to applications in data storage.

This nomination is in support of an exemplary collaboration of both theoretical and experimental groups on a most intriguing magnetic state of topological nature: a magnetic skyrmion crystal. In the remarkable pioneering studies by the nominees, this state was first proposed theoretically and subsequently discovered by neutron scattering in MnSi and its features have been impressively analyzed theoretically.

Magnetic skyrmions are spin textures on length scales of tens of nanometers that behave like particles. Similar to a vortex in a superconductor, a skyrmion cannot be unwound without creating discontinuities. This topological stability is reflected by a nonzero topological index also known as the skyrmion number.

Prior to the theoretical work by A.N. Bogdanov and co-workers [1, 2], conventional wisdom stated that skyrmions cannot spontaneously form as ground states in magnetic materials. It thus came as a considerable surprise that these authors were able to demonstrate that skyrmion lattices may spontaneously form in noncentrosymmetric magnetic systems if longitudinal fluctuations of the magnetization are facilitated. MnSi was proposed as one of the suitable candidates for such a scenario as it exhibited strong longitudinal magnetization fluctuations and the absence of a centre of inversion may result in a Dzyaloshinskii-Moriya contribution to the Hamiltonian.

Independently of these authors, A. Vishwanath and co-workers [3] theoretically proposed the existence of a “helical spin crystal” in MnSi, motivated by its enigmatic high-pressure phase. These authors also showed that the resulting skyrmion spin crystal can be interpreted as a multimode superposition of helical spin spirals. In subsequent work, Vishwanath and co-author also predicted the existence of a topological Hall effect resulting from a fixed phase relationship of these spin spirals, a phenomenon which has subsequently been verified in Ref. [4].

These theoretical proposals were taken up by a team around C. Pfleiderer, P. Böni, and A. Rosch, building on their respective expertise in transport properties of unconventional phases in MnSi, neutron scattering on noncollinearly ordered magnetic systems, and the provision of visionary theoretical support. Resulting from this combined expertise, the team was able to identify and theoretically explain the novel skyrmion phase in MnSi. The first experimental evidence for the existence of a skyrmion crystal in MnSi was provided by small angle neutron scattering (SANS) [5]. The SANS diffraction pattern was consistent with a hexagonal skyrmion lattice which was interpreted as a triple wavevector state. While prior theoretical work suggested the exclusive stability of a conical helical state in a field, the authors demonstrated within a sophisticated theoretical analysis in Ref. [5] that the thermodynamic fluctuations around the skyrmion crystal were fundamental in stabilizing the observed skyrmion crystal.

While highly suggestive of a skyrmion crystal, the SANS results did not yet unambiguously prove a fixed phase relationship between the spin spirals. To settle this issue, C. Pfleiderer, P. Böni, A. Rosch and co- workers investigated the Hall effect in Ref [4]. The observed Hall resistivity showed an additional contribution, in quantitative agreement with a Berry-phase induced emergent magnetic field originating from the nonvanishing skyrmion number of the topologically nontrivial skyrmion lattice.

Finally, C. Pfleiderer, P. Böni, A. Rosch and co-workers demonstrated that skyrmions can be manipulated similarly to other magnetic topological defects such as domain walls. Applying currents that were more than five orders of magnitude weaker than those used in similar experiments on domain walls, the authors observed a distinct rotation of the skyrmion lattice. This demonstrates the extraordinarily weak pinning of the skyrmion lattice and opens the door to manipulation of skyrmions in related systems.

This unique effort led by this group of theorists and experimentalists has resulted in the identification of a new state of matter that has literally created a “skyrmion-boom” in the condensed matter community. Not only have skyrmions also been identified in a larger class of systems, but also explicit proposals emerged to use skyrmions instead of domain walls for racetrack-type memory technologies. It is testament to the excellent work by P. Böni, A.N. Bogdanov, C. Pfleiderer, A. Rosch, A. Vishwanath that skyrmions have become such an exciting field of current research and they highly deserve the EPS CMD Europhysics Prize 2016.

Publications relevant for this nomination

[1] U.K. Rössler, A.N. Bogdanov, C. Pfleiderer, Spontaneous Skyrmion Ground States in Magnetic Metals, Nature 442, 797 (2006).

[2] A.N. Bogdanov , D.A. Yablonskii, Thermodynamically Stable “Vortices” in Magnetically Ordered Crystals – The Mixed State of Magnets, Sov. Phys. JETP 68, 101 (1989).

[3] B. Binz, A. Vishwanath, V. Aji, Theory of the Helical Spin Crystal: A Candidate for the Partially Ordered State of MnSi, Phys. Rev. Lett. 96, 207202 (2006).

[4] A.Neubauer, C. Pfleiderer, B. Binz, A. Rosch, R. Ritz, P.G. Niklowitz, P. Böni, Topological Hall Effect in the A-Phase of MnSi, Phys. Rev. Lett. 102, 186602 (2009).

[5] S. Mühlbauer, B. Binz, F. Jonietz, C. Pfleiderer, A. Rosch, A. Neubauer, R. Georgii, P. Böni, Skyrmion Lattice in a Chiral Magnet, Science 323, 915 (2009).

[6] F. Jonietz, S. Mühlbauer, C. Pfleiderer, A. Neubauer, W. Münzer, A. Bauer, T. Adams, R. Georgii, P. Böni, R.A. Duine, K. Everschor, M. Garst, A. Rosch, Spin Transfer Torques in MnSi at Ultralow Current Densities, Science 330, 1648 (2010).