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• 19 February
  [Séminaires] Ashutosh Singh
  19 February

  [Séminaires] Ashutosh Singh

  Location : D3-2038 Description : Ashutosh Singh (McMaster University) Titre: Optical response in Dirac and Weyl semi-metals Résumé: This talk is divided into two parts. In the first part I will focus on the nonlinear and anisotropic optical response in Graphene. Optical response, in general, can be quantified in terms of optical susceptibility or optical conductivity which reveal the system’s behaviour to an external field in terms of Polarization or Current density respectively. In the presence of a continuous wave (cw) optical beam, the system attains quasi-equilibrium steady-state owing to the balance between continuous excitation of carriers and relaxation due to damping. This regime is well described by the steady-state solution of the Optical Bloch Equation with phenomenological damping rates. Using the steady-state solution, we study wave-mixing and other nonlinear effects in Graphene. In contrast to Graphene, Doped Weyl-semimetal optical conductivity varies linearly with respect to photon frequency, given that the frequency is greater that twice as that of doping level. Tilting the Weyl cones changes this feature dramatically. Tilting gives rise to regions in the spectrum where the optical conductivity is neither linear nor zero. We have calculated the absorptive part of the dynamic conductivity for both right and left handed circular polarized light in the Kubo formalism. Due to finite contribution from the imaginary part of Hall conductivity, we see dichroism in certain frequency regime. Including the effect of chiral pumping, which transfers charge from one node to the other, difference is created in the doping level of the two nodes. This leads to opening of new frequency regions where dichroism remains finite. Although the kinks in the spectrum at the boundaries of different frequency regimes are not expected to survive at finite temperature cases, still in some regions dichroism survives. (invité de R. Côté)

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• 20 February
  [Institut Quantique] Séminaire de l’IQ : Quentin Barthélémy
  20 February

  [Institut Quantique] Séminaire de l’IQ : Quentin Barthélémy

  Location : D3-2041 Description : 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)

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• 25 February
  [Institut Quantique] Horizon IQ : Atelier sur les ressources d’aide et la résolution de conflits
  25 February

  [Institut Quantique] Horizon IQ : Atelier sur les ressources d’aide et la résolution de conflits

  Location : Description : Inscription / Registrationhttps://forms.gle/cDfYrhdpZJFu5j2V7  Salle / Room :DescriptionCette activité débutera avec un panel de 30 minutes suivi d’un atelier interactif en petits groupes. Au cours du panel, des membres des organismes de l’Université de Sherbrooke suivants présenteront leur rôle: Service Respect, prévention et soutien en matière de conflits et de harcèlementService de psychologie et d’orientationOmbudsman des étudiantes et des étudiantsCoordinatrice de l’Équité, de la Diversité et de l’Inclusion (ÉDI) à la Faculté des SciencesChaire pour les femmes en sciences et en génie au QuébecPar la suite, des mises en situation inspirées de faits réels seront présentées aux participantes et aux participants. Cet atelier interactif sous forme de table ronde permettra aux équipes d’explorer différentes façons de résoudre des conflits pouvant se produire dans un groupe de recherche et dans un environnement d’études. Les panélistes interagiront avec les équipes afin de les aider dans leurs réflexions.***This activity will begin with a 30 minute panel followed by an interactive workshop in small groups. During the panel, members of the following Université de Sherbrooke organizations will present their role:Respect, prevention and support for conflicts and harassment Psychology and orientation serviceStudent OmbudsmanEquity, Diversity and Inclusion coordinator at the Faculty of SciencesChair for women in sciences and engineering in QuébecAfterwards, scenarios inspired by true events will be presented to the participants. This workshop will let the small teams explore different ways of resolving conflicts that can occur in a research group and in a study environment. The panelists will interact with the teams in order to help them with their reflections. 

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