Semi-classical spin dynamics of the antiferromagnetic Heisenberg model on the kagome lattice

We investigate the dynamical properties of the classical antiferromagnetic Heisenberg model on the kagome lattice using a combination of Monte Carlo and molecular dynamics simulations. We find that frustration induces a distribution of timescales in the cooperative paramagnetic regime (i.e. far above the onset of coplanarity), as recently reported experimentally in deuterium jarosite. At lower temperature, when the coplanar correlations are well established, we show that the weath- ervane loop fluctuations control the system relaxation : the time distribution observed at higher temperatures splits into two distinct timescales associated with fluctuations in the plane and out of the plane of coplanarity. The temperature and wave vector dependences of these two components are qualitatively consistent with loops diffusing in the entropically dominated free energy landscape. Numerical results are discussed and compared with the $O(N)$ model and recent experiments for both classical and quantum realizations of the kagome magnets.Comment: 18 pages, 14 figure

Hall Effect in spinor condensates

We consider a neutral spinor condensate moving in a periodic magnetic field. The spatially dependent magnetic field induces an effective spin dependent Lorentz force which in turn gives rise to a spin dependent Hall effect. Simulations of the Gross-Pitaevskii equation quantify the Hall effect. We discuss possible experimental realizations

X-ray Absorption Near-Edge Structure calculations with pseudopotentials. Application to K-edge in diamond and alpha-quartz

We present a reciprocal-space pseudopotential scheme for calculating X-ray absorption near-edge structure (XANES) spectra. The scheme incorporates a recursive method to compute absorption cross section as a continued fraction. The continued fraction formulation of absorption is advantageous in that it permits the treatment of core-hole interaction through large supercells (hundreds of atoms). The method is compared with recently developed Bethe-Salpeter approach. The method is applied to the carbon K-edge in diamond and to the silicon and oxygen K-edges in alpha-quartz for which polarized XANES spectra were measured. Core-hole effects are investigated by varying the size of the supercell, thus leading to information similar to that obtained from cluster size analysis usually performed within multiple scattering calculations.Comment: 11 pages, 4 figure

CP2K: An electronic structure and molecular dynamics software package - Quickstep: Efficient and accurate electronic structure calculations

CP2K is an open source electronic structure and molecular dynamics software package to perform atomistic simulations of solid-state, liquid, molecular, and biological systems. It is especially aimed at massively parallel and linear-scaling electronic structure methods and state-of-the-art ab initio molecular dynamics simulations. Excellent performance for electronic structure calculations is achieved using novel algorithms implemented for modern high-performance computing systems. This review revisits the main capabilities of CP2K to perform efficient and accurate electronic structure simulations. The emphasis is put on density functional theory and multiple post–Hartree–Fock methods using the Gaussian and plane wave approach and its augmented all-electron extension

Quelques mécanismes non conventionnels de l'effet Hall anormal

This thesis is divided in three distinct parts. In the first andsecond parts, we speak about unconventional mechanisms of theAnomalous Hall Effect (AHE). The first part gives a short review ofthe conventional and unconventional mechanisms of the AHE (chapter~1).chapter~2 is devoted to geometrical phases. The second part of this thesis answer to some questions about the AHE.The third chapter treats the role of impurities in intrinsic mechanismof the AHE. By calculating of the off-diagonal conductivity in thelinear response theory for a model of 2DEG in presence of spin-orbitcoupling, we obtained two contributions related to the diffusion ofelectrons by impurities of defaults but independent of theconcentration of impurities. The second one is the Karplus andLuttinger term, which depends on the topology of the energy band. Inthe chapter~4, we propose an experimental setup based on 2DEG inpresence of inhomogeneous magnetic field created by magneticnanostructure. In this system, the Hall conductivity has discontinuousvariations related to the topology of the magnetic field, which can bemodified by applying an additional constant magnetic field. In thechapter~5, we study the spin chirality mechanism of the AHE on thekagome lattice and discuss the results of recent experiments on the pyrochlore compound Nd$_2$Mo$_2$O$_7$. The last part of this thesis is devoted to electron dynamic ininhomogeneous magnetic field. The chapters~6 and~7 give a introductionof electrons dynamic in two dimensional periodic magnetic field. Thelast chapter is a study of electron dynamic in 2D periodic magneticfield. We find that it exists persistent currents related tolocalization of electrons around the regions where the magnetic fieldis canceling.Ce manuscrit est divisé en trois parties distinctes. les deuxpremières parties sont axées sur l'étude de quelques mécanismesintrinsèques de l'effet Hall anormal. Nous montrons notamment que ladiffusion des porteurs de charge par des impuretés ou des défautsdonne une contribution intrinsèque non négligeable à l'effet Hallanormal. Nous proposons ensuite une expérience, basée sur un réseau denanocylindres magnétiques, placé au dessus d'un gaz d'électronsbidimensionnel, dans laquelle les contributions intrinsèques del'effet Hall anormal sont clairement identifiables et contrôlables.Enfin, nous abordons le problème du mécanisme de chiralité de spinproposé pour expliquer l'effet Hall dans le composé pyrochloreNd$_2$Mo$_2$O$_7$. En utilisant un modèle de gaz sur réseau ainsiqu'une configuration magnétique non colinéaire (chirale), nousmontrons que la conductivité transverse a une dépendance complexe parrapport à la chiralité de spin.La dernière partie de ce manuscrit est dédiée à l'étude de quelquespropriétés des gaz d'électrons soumis à un champ magnétiqueinhomogène. Après un bref rappel sur la dynamique d'électron enprésence d'un gradient de champ magnétique constant, nous abordons leproblème d'un champ magnétique périodique spatialement. En calculantquelques états de Bloch aux points de haute symétrie, nous montrons qu'ilexiste des états pour lesquels les électrons sont localisés auvoisinage des lignes de champ nul. Le calcul des courants deprobabilité montre que ces états sont porteurs de courants permanentsdont l'origine est liée aux inhomogénéités du champ magnétique auvoisinages des lignes de champ nul

Quelques mécanismes non conventionnels de l'effet Hall anormal

Ce manuscrit est divisé en trois parties distinctes. les deux premières parties sont axées sur l'étude de quelques mécanismes intrinsèques de l'effet Hall anormal. Nous montrons notamment que la diffusion des porteurs de charge par des impuretés ou des défauts donne une contribution intrinsèque non négligeable à l'effet Hall anormal. Nous proposons ensuite une expérience, basée sur un réseau de nanocylindres magnétiques, placé au dessus d'un gaz d'électrons bidimensionnel, dans laquelle les contributions intrinsèques de l'effet Hall anormal sont clairement identifiables et contrôlables. Enfin, nous abordons le problème du mécanisme de chiralité de spin proposé pour expliquer l'effet Hall dans le composé pyrochlore Nd2Mo2O7. En utilisant un modèle de gaz sur réseau ainsi qu'une configuration magnétique non colinéaire (chirale), nous montrons que la conductivité transverse a une dépendance complexe par rapport à la chiralité de spin. La dernière partie de ce manuscrit est dédiée à l'étude de quelques propriétés des gaz d'électrons soumis à un champ magnétique inhomogène. Après un bref rappel sur la dynamique d'électron en présence d'un gradient de champ magnétique constant, nous abordons le problème d'un champ magnétique périodique spatialement. En calculant quelques états de Bloch aux points de haute symétrie, nous montrons qu'il existe des états pour lesquels les électrons sont localisés au voisinage des lignes de champ nul. Le calcul des courants de probabilité montre que ces états sont porteurs de courants permanents dont l'origine est liée aux inhomogénéités du champ magnétique au voisinages des lignes de champ nul.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

XANES calculation with an efficient ``Non Muffin-Tin'' method: Application to the angular dependence of the AlK-edge in corundum

We present a reciprocal-space ``non mufin-tin'' scheme for calculating X-ray Absorption Near-Edge Structure (XANES). The method uses pseudopotentials and reconstructs all electron wave functions within the Projector Augmented Wave framework. The method incorporates a recursive method to compute absorption cross section as a continued fraction. The continued fraction formulation of absorption is advantageous in that it permits the treatment of core-hole-electron interaction through large supercells ( hundreds of atoms). This opens new fields of applications like surfaces, molecules, small aggregates or amorphous materials, for which large supercells are required. The method is applied to the natural linear dichroism at the Al K-edge in corundum (alpha-Al2O3). Details about the convergence process of the calculation are given. The influence of the core-hole effects is emphasized and comparison with ``muffin-tin'' multiple scattering calculation is made

Competing Spin Liquids and Hidden Spin-Nematic Order in Spin Ice with Frustrated Transverse Exchange

Frustration in magnetic interactions can give rise to disordered ground states with subtle and beautiful properties. The spin ices Ho2Ti2O7 and Dy2Ti2O7 exemplify this phenomenon, displaying a classical spin-liquid state, with fractionalized magnetic-monopole excitations. Recently, there has been great interest in closely related “quantum spin-ice” materials, following the realization that anisotropic exchange interactions could convert spin ice into a massively entangled, quantum spin liquid, where magnetic monopoles become the charges of an emergent quantum electrodynamics. Here we show that even the simplest model of a quantum spin ice, the XXZ model on the pyrochlore lattice, can realize a still-richer scenario. Using a combination of classical Monte Carlo simulation, semiclassical molecular-dynamics simulation, and analytic field theory, we explore the properties of this model for frustrated transverse exchange. We find not one, but three competing forms of spin liquid, as well as a phase with hidden, spin-nematic order. We explore the experimental signatures of each of these different states, making explicit predictions for inelastic neutron scattering. These results show an intriguing similarity to experiments on a range of pyrochlore oxides