Starts 15 Sep 2017 15:30
Ends 15 Sep 2017 16:45
Central European Time
ICTP
Leonardo Building - Luigi Stasi Seminar Room
Abstract:

Finding new states of matter is one main goal of research in condensed matter which often gives rise to novel concepts and sometimes to remarkable technological innovations. In the field of magnetism, the frustration of the magnetic interactions in well-chosen lattice geometries is the key ingredient to promote truly original ground states. This field of research has grown tremendously during the last 15 years with the emergence of new concepts such as the spin ices or the spin liquids and the corresponding original excitations, magnetic monopoles and spinons. Spin liquids for instance can be viewed as resonant singlets (antiferromagnetic spin pairs), a model quantum ground state first advocated for the interpretation of high Tc superconductivity [1].

After a long and basic introduction of this field of research and the challenges it has opened, I’ll focus on the emblematic case of the kagome geometry. Indeed the low connectivity of the kagome network together with the quantum fluctuations enhanced for low spin 1⁄2 destabilize any ordered state in favor of a fluctuating ground state at T=0, coined a spin liquid. The recent discovery of model compounds for this physics, namely with a true kagome lattice decorated by quantum S=1/2 spins (Cu2+ or V4+), has boosted both the theoretical and the experimental research in this field. The spin liquid state is for instance realized in the archetypal ZnCu3(OH)6Cl2 “herbertsmithite” compound. The latter shows no sign of frozen on-site magnetism, ie no spontaneous breaking of symmetry, down to mK temperatures while the magnetic interaction is of the order of hundreds of Kelvin [2,3]! The precise nature of the ground state and of its elementary excitations, the phase transitions that can be induced by various parameters are at the heart of the current debates and a seed for new concepts in the field of frustrated magnetism. I will review the major and recent advances in the field and will underline the power of two local techniques, NMR and μSR to study such a field of research [4].

References

[1] P. W. Anderson, Science 235, 1196 (1987)
[2] P. Mendels et al., Phys. Rev. Lett. 98, 0772014 (2007); J. S. Helton et al. , Phys. Rev. Lett. 98 , 07204 (2007).
[3] P.A. Lee, Science, Perspectives 321, 1306 (2008).
[4] For a review, see P. Mendels and F. Bert, Special Topics Section on "Novel States of Matter Induced by Frustration", J. Phys. Soc. Jpn 1, 011001 (2010); J. Phys. Conf. Series 320 , 012004 (2011). M.R. Norman, Rev. Mod. Phys. 88, 041002 (2016). P. Mendels and F. Bert, Comptes Rendus Physique, 17, 455 (2016).