Quantum kagome frustrated antiferromagnets: One route to quantum spin liquids

AbstractAfter introducing the field of Highly Frustrated Magnetism through the quest for a quantum spin liquid in dimension higher than one, we focus on the emblematic case of the kagome network. From a theoretical point of view, the simple Heisenberg case for an antiferromagnetic kagome lattice decorated with quantum spins has been a long-standing problem, not solved yet. Experimental realizations have remained scarce for long until the discovery of herbertsmithite ZnCu3(OH)6Cl2 in 2005. This is one of the very few quantum kagome spin liquid candidates that triggered a burst of activity both on theory and experiment sides. We give a survey of theory outcomes on the “kagome” problem, review the experimental properties of that model candidate and shortly discuss them with respect to recent theoretical results

Possible quadrupolar nematic phase in the frustrated spin chain LiCuSbO4_4: an NMR investigation

The frustrated one-dimensional (1D) quantum magnet LiCuSbO$_4$ is one rare realization of the $J_1-J_2$ spin chain model with an easily accessible saturation field, formerly estimated to 12~T. Exotic multipolar nematic phases were theoretically predicted in such compounds just below the saturation field, but without unambiguous experimental observation so far. In this paper we present extensive experimental research of the compound in the wide temperature (30mK$-$300K) and field (0$-$13.3T) range by muon spin rotation ($\mu$SR), $^7$Li nuclear magnetic resonance (NMR) and magnetic susceptibility (SQUID). $\mu$SR experiments in zero magnetic field demonstrate the absence of long range 3D ordering down to 30mK. Together with former heat capacity data [S.E. Dutton \emph{et al}, Phys. Rev. Lett. 108, 187206 (2012)], magnetic susceptibility measurements suggest short range correlated vector chiral phase in the field range $0-4$T. In the intermediate field values (5$-$12T), the system enters in a 3D ordered spin density wave phase with 0.75$\mu_B$ per copper site at lowest temperatures (125mK), estimated by NMR. At still higher field, the magnetization is found to be saturated above 13T where the spin lattice $T_1^{-1}$ relaxation reveals a spin gap estimated to 3.2(2)K. We narrow down the possibility of observing a multipolar nematic phase to the range 12.5$-$13T.Comment: 12 pages, 15 figure

Interplay of thermal and quantum spin fluctuations on the kagome lattice

We present a Raman spectroscopic investigation of the Herbertsmithite ZnCu3(OH)6Cl2, the first realization of a Heisenberg s=1/2 antiferromagnet on a perfect kagome lattice. The magnetic excitation spectrum of this compound is dominated by two components, a high temperature quasi elastic signal and a low temperature, broad maximum. The latter has a linear low energy slope and extends to high energy. We have investigated the temperature dependence and symmetry properties of both signals. Our data agree with previous calculations and point to a spin liquid ground state.Comment: 5 figure

Matching pursuit-based shape representation and recognition using scale-space

In this paper, we propose an analytical low-level representation of images, obtained by a decomposition process, namely the matching pursuit (MP) algorithm, as a new way of describing objects through a general continuous description using an affine invariant dictionary of basis function (BFs). This description is used to recognize multiple objects in images. In the learning phase, a template object is decomposed, and the extracted subset of BFs, called meta-atom, gives the description of the object. This description is then naturally extended into the linear scale-space using the definition of our BFs, and thus providing a more general representation of the object. We use this enhanced description as a predefined dictionary of the object to conduct an MP-based shape recognition task into the linear scale-space. The introduction of the scale-space approach improves the robustness of our method: we avoid local minima issues encountered when minimizing a nonconvex energy function. We show results for the detection of complex synthetic shapes, as well as real world (aerial and medical) images. © 2007 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 16, 162-180, 200

Calibration and evaluation of optical systems for full-field strain measurement

The design and testing of a reference material for the calibration of optical systems for strain measurement is described, together with the design and testing of a standardized test material that allows the evaluation and assessment of fitness for purpose of the most sophisticated optical system for strain measurement. A classification system for the steps in the measurement process is also proposed and allows the development of a unified approach to diagnostic testing of components or sub-systems in an optical system for strain measurement based on any optical technique. The results described arise from a European study known as SPOTS whose objectives were to begin to fill the gap caused by a lack of standards

Quantum Kagome antiferromagnet ZnCu3(OH)6Cl2

The frustration of antiferromagnetic interactions on the loosely connected kagome lattice associated to the enhancement of quantum fluctuations for S=1/2 spins was acknowledged long ago as a keypoint to stabilize novel ground states of magnetic matter. Only very recently, the model compound Herbersmithite, ZnCu3(OH)6Cl2, a structurally perfect kagome antiferromagnet, could be synthesized and enables a close comparison to theories. We review and classify various experimental results obtained over the past years and underline some of the pending issues.Comment: 23 pages, 16 figures, invited paper in J. Phys. Soc. Jpn, special topics issue on "Novel States of Matter Induced by Frustration", to be published in Jan. 201

Etude, par RMN du 55Mn, de l'aimantation locale dans les verres de spin RKKY

SIGLET 55681 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

ETUDE PAR RMN DE LA SUSCEPTIBILITE ET DES EFFETS DE DILUTION DANS LE COMPOSE MAGNETIQUE GEOMETRIQUEMENT FRUSTRE SRCR9PGA12-9PO19

ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Antiferromagnétisme frustré à deux dimensions (de la géométrie triangulaire dans NaCrO2 à la géométrie kagomé dans Cu3Zn(OH)6Cl2)

Cette thèse est consacrée à l'étude expérimentale des composés antiferromagnétiques géométriquement frustrés à deux dimensions. Dans ces systèmes, la structure magnétique formée de triangles interdit de satisfaire simultanément toutes les interactions antiferromagnétiques et peut générer des états fondamentaux originaux. Nous présentons l'étude par RMN et muSR du composé triangulaire à côtés partagés NaCrO2, un système Heisenberg modèle de spin 3/2. Nous mettons ainsi en évidence un régime de fluctuations très étendu en dessous d'un maximum de chaleur spécifique, signature d'excitations originales spécifiques à cette classe de systèmes. Nous présentons ensuite l'étude de l'Herbertsmithite Cu3Zn(OH)6Cl2, première réalisation expérimentale d'une structure kagomé parfaite de spin 1/2. Nous montrons par muSR que dans la famille plus large des Paratacamites Cu(4-x)Znx(OH)6Cl2, lorsque x passe de 0 à1, le système évolue à basse température d'un état ordonné à longue distance vers un état fluctuant, type liquide de spin, pour x>0.66. Les mesures de RMN de l'oxygène 17 dans l'Herbertsmithite nous on permis d'isoler la contribution intrinsèque de celle des défauts issus des substitutions Cu/Zn. L'évolution de la susceptibilité, ainsi que les mesures de relaxation démontrent l'absence de gap dans le spectre des excitations magnétiques. Nos mesures suggèrent que ces excitations ont un poids constant sur toute la zone de Brillouin, comme prédit théoriquement. L'évolution de la raie des défauts est en accord qualitatif avec l'image théorique de la localisation des singulets non magnétiques autour de l'impureté.The object of this thesis is the experimental study of bi-dimensional geometrically frustrated antiferromagnetic compounds. In these systems, the magnetic structure composed of triangles prevents from satisfying simultaneously all the antiferromagnetic interactions and can generate original ground states. We present the muSR and NMR study of NaCrO2, an edge sharing triangular compound that appears as a model Heisenberg system of spin 3/2. We show that it presents a very wide fluctuating regime, much below a specific heat maximum, signature of original excitations typical for this class of systems. We also present the study of Herbertsmithite Cu3Zn(OH)6Cl2, the first experimental realisation of a perfect kagomé structure of spin 1/2. We show by muSR that in the larger Paratacamite family Cu(4-x)Znx(OH)6Cl2, when x increases from 0 to 1, the system evolves at low temperatures from a long range ordered state to a fluctuating, spin liquid one for x>0.66. NMR measurements using oxygen 17 allowed us to distinguish the intrinsic contribution from that of defects issued from Cu/Zn substitution. The evolution of the intrinsic susceptibility, as well as the dynamical measurements demonstrate the absence of a gap in the spectrum of magnetic excitations. Our measurements suggest that these axcitations have the same weight over the whole Brillouin zone, in agreement with theoretical results. The evolution of the defect contribution is in qualitative agreement with the theoretical image of localised singlets created around the impurity.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Etude par RMN et MuSR des composés antiferromagnétiques fortement frustrés à géométrie de bicouches kagomé

Dans les composés antiferromagnétiques Heisenberg à géométrie kagomé, la frustration des interactions est à l'origine d'un état liquide de spins à T=0. Un état RVB ("Resonating Valence Bond"), originalement proposé par Anderson dans les réseaux triangulaires puis dans les cuprates, décrit probablement ce fondamental, dont l'état singulet est dégénéré exponentiellement dans un faible gap singulet-triplet. Peu de composés expérimentaux se rapprochent aujourd'hui du système idéal et l'existence de perturbations ou d'anisotropie lève souvent la dégénérescence du fondamental lorsque T->0.Cette thèse présente l'étude, par RMN, MuSR et SQUID, des composés à géométrie de bicouches kagomé de spin 3/2, Ba2Sn2ZnGa10-7pCr7pO22 et SrCr9pGa12-9pO19, considérés comme les archétypes d'un hamiltonien de spins purement Heisenberg sur un réseau kagomé.Outre l'absence caractéristique de transition jusqu'à une température Tg bien plus basse que la température de Curie-Weiss, nous mesurons des propriétés physiques semblables dans ces deux systèmes malgré des défauts radicalement différent. Nous en déduisons les propriétés intrinsèques de cette géométrie, à savoir : - une décroissance de la susceptibilité en dessous de 45 K (RMN), compatible avec des calculs hautes températures ainsi qu'avec l'existence d'un gap de spin ; - l'existence de fluctuations quantiques jusqu'à 30 mK0.This work present the NMR, MuSR and SQUID study of the S=3/2 kagomé bilayers, Ba2Sn2ZnGa10-7pCr7pO22 and SrCr9pGa12-9pO19, considered as archetypes of Heisenberg 2D frustrated magnets. Because of the frustration of the interactions, a transition temperature Tg is measured only well below the Curie-Weiss temperature. We measure very close magnetic properties in both systems despite different kinds of defects, which underly the intrinsic behavior of the geometry :- the susceptibility decreases below 45 K (NMR), which is consistent with high temperature expansion approaches as well with a spin gap ;- spin fluctuations are measured down to 30 mK << Tg using MuSR and suggest a fluctuating state ;- this T independent fluctuating state only appears below the spin glass like transition.?A phenomenological approach of the muon relaxation is presented for the first time in theses systems and suggests the existence of a RVB state below Tg.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF