Séminaire de l'IQ : Quentin Barthélémy

Quentin Barthélémy Laboratoire de Physique des Solides Université Paris-Saclay Orsay, France Candidat pour une bourse postdoctorale Invité par Jeffrey Quilliam Titre : New quantum spin states on model frustrated lattices: herbertsmithite and beyond Résumé : The kagome Heisenberg antiferromagnet decorated with quantum spins is a fascinating model to look for exotic quantum states including the elusive spin liquids. Despite the apparent simplicity of this model, there is still no consensus on the exact nature of the ground state and the excitation spectrum. Nevertheless, the current availability of quantum kagome materials, each with their own deviation to the pure Heisenberg model, allows to confront a fair amount of experimental results to theoretical predictions and study the effect of perturbations inherent to real materials. In the first part of this talk, I will focus on herbertsmithite ZnCu3(OH)6Cl2. This is the emblematic compound in this family of materials because it is the closest realization to the pure Heisenberg model so far, with a nonmagnetic dynamical ground state. I will describe some selected developments of our recent comprehensive 17O NMR study on high quality single crystals, where we measured the low temperature static spin susceptibility and spin dynamics [1]. It allows us to show unambiguously that the excitation spectrum is gapless, restoring some convergence with a Dirac cone model now advocated for in most of numerical works. I will also present our recent high field specific heat study, which provides another light on the low energy density of states and the nature of the excitations [2]. In the second part, I will introduce two new closely related materials that implement interesting variants of the simple model realized in herbertsmithite: YCu3(OH)6OxCl3-x, with compositions x = 0 and x = 1/3. In these two compounds, the ground state properties are determined using μSR, and are quite unexpected [3]. The x = 0 compound has a perfect kagome geometry but yet displays an ordered ground state, likely because of a large Dzyaloshinskii–Moriya anisotropy [4]. The x = 1/3 compound has a slightly distorted kagome geometry with fully anisotropic interactions but exhibits features of spin liquid physics. Ongoing experiments on recently available single crystals may help to further understand this material. [1] P. Khuntia, M. Velazquez, Q. Barthélemy et al., Nature Physics (2020) [2] Q. Barthélemy et al., in preparation [3] Q. Barthélemy et al., Phys. Rev. Materials (2019) [4] T. Arh et al., arXiv:1912.09047 (2019)