Physique de basses énergies des sytèmes magnétiques géométriquement frustrés – HFM

Our scientific project sets in the general context of the search for new electronic states of matter, which turns out to be spectacularly fruitful in oxides due to the strong electronic correlations at play in these materials. For magnetic materials, the challenge is to overcome the paradigm of ferro- and antiferro-magnetism in order to stabilize new states of magnetic matter such as the recently discovered spin ice states, collective paramagnets or spin liquids.. As early as in 1973, PW Anderson pointed out that the Néel state may not be the only possible ground state for an antiferromagnet, in particular when strong geometrical frustration of the magnetic interactions is at play as in triangular lattices. In this latter case, the geometry of the magnetic lattice, itself, precludes the crystallisation of a simple Néel state. As an alternative to an ordered ground state, he proposed that antiferromagnetically coupled S=1/2 spins for which quantum fluctuations are enhanced, at the vertices of a triangular lattice, may realise his famous resonating valence bond state (RVB); a novel macroscopic quantum state based on the resonance of all the possible coverages of the lattice by spin singlets. In the 90's, it was theoretically evidenced that the triangular lattice most likely orders in a 3 sub-lattice scheme at T=0. However on more loosely connected lattices such as kagome lattice based on the corner-sharing triangles, the ground state is not ordered, although the microscopic description of this liquid state is still not settled yet. Specifically to the S=1/2 kagome case, the excitation spectrum shows a continuum of non magnetic excitations and a surprisingly small spin gap in the triplet sector which may or may not vanish at the limit of a real macroscopic system. Experiences have been lagging well behind theories for long because of the absence of a suitable S=1/2 kagome compound to confront the theoretical predictions. Only recently,, in 2005, a structurally perfect S=1/2 mineral, Herbertsmithite, could be first synthesized at the MIT. Through muSR we could prove that Herberstmithite indeed remains in a liquid phase down to at least 50 mK. The Herbertsmithite enables us for the first time to tackle the question of the nature of the ground state of the S=1/2 kagome model in close comparison to theoretical predictions. In return the existence of a real compound to compare with has obviously revived the theoretical interest for this difficult question and new theoretical proposals are now emerging. Furthermore, from 17O NMR local probe investigations, we could accurately and uniquely measure the intrinsic kagome plane susceptibility which is hidden in macroscopic measurements by a dominant defect contribution that we could also identify. At low temperatures (T~J/100), both the local susceptibility and relaxation measurements point at the absence of a gap. This finding revives the longstanding debate on the existence of spin-triplet gap for the Heisenberg Hamiltonian on the kagomé lattice and demands further understanding of the precise Hamiltonian describing Herbertsmithite. Recently, we could measure the leading perturbation, a magnetic anisotropy of the Dzyaloshinsky-Moriya (DM) type by ESR measurements. This perturbation had not been studied in the quantum case. Recent calculations by O. Cépas open a completely new and unexpected field of investigation, involving quantum criticality physics. These preliminary results certainly makes Herberthsmite a fascinating case and demand further studies to still lower temperatures and as a function of controlled perturbation such as non magnetic impurities or the external magnetic field or even pressure to modify the DM interaction and investigate the newly evidenced phase diagram. In the proposed 3 year experimental project we aim at unveiling the ground state of the quantum kagome antiferromagnet and its low energy excitations by a combination of local probe and thermodynamic measurements of real materials. This program demands experimental developments towards low temperatures in the subkelvin range, for NMR experiments as well as heat capacity measurements, a thermodynamic quantity sensitive to the magnetic and non magnetic excitation of the system. In a 'perturb to reveal' strategy we also aim at studying the response of the kagome network to non magnetic perturbative defects in the quantum case. Finally in a more prospective manner, we also aim at exploring new material families of kagome compounds. This project sets in a context of growing interest at the international scale for spin liquids and more generally highly frustrated magnetism, especially revived after the discovery of Herbertsmithite. In a context of intense international competition, our expertise of local probe techniques (17O NMR and 'SR) has already allowed us to publish important contributions to the field and will certainly serve the success of the present scientific program.