HFM-Workshop ITFRUS 07

Mobile Fermions and Bosons on Frustrated Lattices

Organizers: P. Fulde, Z. Hiroi,
P. Lemmens, R. Moessner

10.-13.2.2007, Dresden, MPI-PKS, Germany

with joint support of the MPI-PKS and the ESF-HFM

Summary of the HFM-Workshop ITFRUS07:

Summary

The workshop was organised within the scientific program Highly Frustrated Magnetism (HFM) supported by the European Science Foundation (ESF) and the Max-Planck Institute for the Physics of Complex Systems (MPI-PKS) Dresden. The organizers were: P. Fulde (Dresden, Germany), Z. Hiroi (Tokyo, Japan), P. Lemmens (Braunschweig, Germany), and R. Moessner (Oxford, United Kingdom). The local organization of the workshop was performed by K. Lantsch from the MPI-PKS Dresden, Germany

The initial planning for the workshop and first announcement happened during the HFM steering committee and the first workshop held in La Londe les Maures in France, November 2005. The focus on a restricted set of topics allowed adding well-established experts in the field, both contacted directly by the organizers and from the numerous applications. A webpage for the workshop was set-up and updated until the workshop started: http://www.mpipks-dresden.mpg.de/~itfrus07/ with a brief report at http://www-public.tu-bs.de:8080/~plemmens/esf-hfm/hfm-workshop-2007.html

The topic 'frustrated systems with itinerant aspects' was divided into several subtopics that where represented by key speakers in addition to posters. These were: cobaltates, triangular organics, itinerant pyrochlores, spinels, frustration and ordering, quantum criticality, bosons and supersolids, fundamental mechanisms, checkerboard physics. Material science, modelling as well as theoretical aspects were discussed.

The workshop had 96 participants (36 oral, 63 poster contributions and one concluding remark). The organizers tried to bring together a mix of young scientists on the level of PhD students, and postdocs together with senior researchers. The interdisciplinary approaches of the workshop lead to the additional advantage of a mutual exchange about all material related aspects.

All scientific activities as well as lunch and dinner were organized in the central building of the MPI-PKS in Dresden. This led to close contact and intensive discussions between the participants, and optimised the use of the time available.

Description of the scientific content

Materials physics topics:

The topic cobaltate was represented using the experimental techniques neutron scattering (Keimer, Lee), photo emission (Hasan), NMR (Alloul, Yoshimura), preparation and thermodynamic investigations (Yoshimura) and theory (Maekawa, Eremin, Mochizuki). The compound Na_xCoO₂ is one of the novel systems that motivated this workshop due to its exceptional ground state and transport properties. Especially noteworthy is the very large thermopower that reaches application relevant magnitudes. The observation of unconventional superconductivity is an additional highlight and motivation. Strong activities exist in Japan, where the system has been established. In France important spectroscopic investigations have been performed. In Germany activities started with one of the organizers at the MPI-FKF in Stuttgart and recently spread out to the TU Braunschweig in the frame of a project supported by the German Science Foundation (DFG).

The case of triangular organics is similar with respect to the strong position of Japan and the prospects of the materials. In our workshop thermodynamic aspects and the phase diagram have been discussed by Lang and Kanoda. Theoretical modelling relevant for these materials is more widespread. All important aspects were discussed by Baskaran and McKenzie. In general the interest in organic materials with competing interactions is recently growing in Europe.

Pyrochlores and spinels are two classes or families of compounds characterized by wellknown and long-investigated crystallographic structures. These families of compounds show an enormous variety of compositions and physical properties. The interplay of magnetism (or other long range ordered states) and competing interactions is more recently in the centre of interest. Accordingly, the speakers touched different aspects ranging from unconventional ground states to more materials related or application based interest. The respective seminars touched electron-phonon coupling and superconductivity (Hiroi), multiferroicity (Loidl), metal-insulator transitions (Kremer, Takagi), Kondo-type physics (Nakatsuji) and different theoretical approaches (Valenti, Lacroix, Tsunetsugu) with an emphasis on unconventional transport properties. During the workshop it has been widely accepted that both the pyrochlores and the spinels will be of increasing importance in the field of frustrated magnetism.

Conceptual foundations:

Additional emphasis was placed on addressing, mainly from a theoretical vantage point, a range of fundamental issues raised in the context of itinerant frustration. These included questions as fundamental as the origin of magnetic behaviour, with a talk by Oshikawa demonstrating the presence of ferromagnetism by statistical transmutation, and one by Shastry on kinetic antiferromagnetism. A classification of topological insulators was discussed by Moore. Vojta's talk considered the possibility of dimensional reduction near a critical point in the framework of an RG treatment.

The nature of ordering in the presence of frustration was also extensively discussed. Emphasis was placed on the interplay of magnetic with 'ferroic' ordering behaviour by Ramirez, while Mila addressed field-driven crystallisation phenomena manifesting themselves in magnetisation plateaux.

A pair of talks (Becca, Poilblanc) was devoted to different aspects of dimer model physics. These models are used to describe the properties singlet-dominated magnetic phases. One aspect concerned the details of the ordering behaviour of the triangular dimer model, the phase diagram of which includes not only a topological liquid but also a complex plaquette RVB state. The other dealt with the problem of doping such models -- a topic of great interest due to their provenance from high-temperature superconductivity. The related topic of physics on the checkerboard lattice -- one of the most highly frustrated lattices in d=2 - was given much emphasis, including talks on connections with the lore of high-energy physics (Fulde), as well as on its peculiarities of ordering (Penc) and Fermionic quantum-dynamics (Shtengel). This was backed up with a talk on numerical studies on 1-d ladder-type systems (Brenig).

Finally, the workshop reached out to closely related topics of interest to the Helium and cold atom communities. Saunders discussed the properties of Helium films adsorbed on different substrates. On the topic of supersolid phases and Bosons in magnetic fields, lattice models and their ordering properties were discussed by Troyer, Melko and Auerbach.

Impact of the event

As discussed in the preceding paragraphs, the combination of competing interactions with itinerant aspects is a very strong and rapidly developing field which links fundamental conceptual, modelling as well as materials related questions. Given the well-focussed topic of the workshop, it was possible to accomodate almost all of the applications for participations (application to the workshop was open to everyone). This gave a large number of younger participants on the student and postdoc level the chance to interact with world-renowned experts in the field on the highest scientific level. The excellent local organization of the workshop by K. Lantsch and the team at the MPI-PKS helped to prepare a very productive environment. The importance of the meeting was also highlighted by the large number of steering committee members present (C. Lacroix, P. Lemmens, Ph. Mendels, F. Mila, R. Stern, and K. Penc), as well as by a strong participation from outside Europe, despite the short duration of the workshop.

A copy from the closing remarks at Dresden:

The next events within the European Science Network:

This is an information on the ESF Programm Highly Frustrated Magnetism.

p.lemmens at tu-bs.de, last change: 17.02.2007

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