Interaction spin-orbite

vignette|Structures fines et hyperfines dans l'hydrogène. Le couplage des différents moments cinétiques conduit à la division du niveau d'énergie. Non dessiné à l'échelle. Le moment cinétique de spin électronique, S est couplé au moment cinétique orbital électronique, L, pour former le moment angulaire électronique total , J. Celui-ci est ensuite couplé au moment cinétique de spin nucléaire, I, pour former le moment cinétique total, F. Le terme symbole prend la forme 2S+1L avec les valeurs de L représentées par des lettres (S,P,D ,F ,G,H,... = 0,1,2,3,4,5,...) de sorte que, par exemple, un terme 2P représente un état avec S=1/2 et L=1. L'électron unique dans une sous-couche 1s donne naissance au terme 2S. L = 0 et S = 1/2 ne peuvent se combiner que pour donner J = 1/2. Cela peut à son tour se combiner avec le spin nucléaire, I = 1/2, pour donner le moment cinétique total F = 0, 1. L'électron unique dans une sous-couche 2p donne naissance au terme 2P. L = 1 et S = 1/2 peuvent se combine

Charge transport properties of spin-orbit coupled materials in different correlation regimes

Soudabeh Sheikholharam Mashhadi

From recent advances in solid state physics, a novel material classification scheme has evolved which is based on the concept of topology and provides an understanding of different phenomena ranging from quantum transport to unusual flavors of superconductivity. Spin-orbit coupling is a major term in defining the topology of materials, and its interplay with electron electron correlations yields novel, intriguing phenomena. In the weak to intermediate correlation regime, spin-orbital entanglement leads to the emergence of topological insulators, which constitute Dirac materials with 2D spin-polarized helical edge or surface states and an insulating bulk. In the specific case of topological insulators, theory predicts that their unique band structure leads to a high thermoelectric performance, which is of practical relevance for energy conversion applications.In the strong correlation regime, the spin-orbit coupling removes the orbital degeneracy, thereby enhancing quantum fluctuations of the spin-orbit entangled states. This in turn can lead to the emergence of novel phases such as quantum spin liquids and unconventional superconductivity. The Kitaev quantum spin liquid (QSL) is a topological state of matter that exhibits fractionalized excitations in the form of Majorana fermions. The main aim of this thesis was to study these novel states of matter in the form of two different materials that fall in the weak and strong correlation regime, respectively. On this basis, it should furthermore be explored whether benefit can be taken of their properties by combining them with other quantum materials into heterostructures.In the first part of this thesis, nanoplatelets of bismuth chalcogenide-based 3D topological insulators were grown by chemical vapor deposition, and characterized by magnetotransport experiments. Having established charge transport characteristics of the single components, in the aim to exploit their high thermoelectric efficiency, the lateral heteroctructures of them are fabricated. Their investigation by scanning photocurrent microscopy revealed strong photocurrent generation at the p-n junction. The obtained results demonstrate that such type of hybrid heterostructure is a promising route to enhance the thermoelectric performance in nanostructured devices. In the second part of this thesis, the Kitaev spin liquid candidate, RuCl3 was studied. Raman spectroscopy on exfoliated RuCl3 revealed a broad magnetic continuumat low energies which according to its particular deviation from bosonic character is assigned to the fractionalized excitations. Furthermore, in complementary charge transport experiments, it was found that themagnetic fluctuations and structural changes in thismaterial are highly entangled, and influence the emergence of the fractionalized excitations. Finally, it was investigated whether the magnetic insulator character of ®-RuCl3 can be exploited to alter the graphene band structure through a proximity effect in vertical graphene/ RuCl3 heterostructures. Measurements of their low-temperature, nonlocal resistance confirmed the possibility to induce pure spin currents in this manner. At the same time, the charge transport through the graphene served as a probe of the magnetic ordering in RuCl3, although the complex magnetic behavior in this material necessitates more studies.

EPFL2018

Dégénérescence orbitale et ordre magnétique sur le réseau triangulaire

François Vernay

Compounds made of transition metal ions on a triangular lattice may have orbitally degenerate eg states. This is the case for LiNiO2 and NaNiO2 : the Ni3+ ions are in a low-spin state, leaving the last electron of the d-shell in the eg orbitals. In order to understand the magnetic behavior of such systems, we investigate a spin-orbital effective hamiltonian. Using exact diagonalization algorithms of finite clusters and variational approaches we have investigated the phase diagram as a function of the hopping integrals between eg orbitals and of the magnitude of the Hund's rule coupling. We show the existence of intermediate phases lying between a ferromagnetic state and the SU(4)-symmetric point. In particular we show that different types of spin arrangements can be realized ranging from dimer coverings to antiferromagnetic chains or 2D structures. In order to have a better characterization of the dimer-covering phase corresponding to LiNiO2, we derived an effective quantum dimer model adapted to the Kugel-Khomskii model we started with. The relevant terms for this quantum dimer hamiltonian are kinetic ones. A systematic investigation of this model has been carried out with exact diagonalizations and quantum Monte-Carlo. We show that a competition between two kinetic terms can lead to a resonating valence bond (RVB) state for a finite range of parameters. On the basis of the available experiments we argue that this RVB phase gives a correct description of LiNiO2. Another phase that lies nearby in parameter space, ferro-orbitally ordered, has been investigated by spin-wave calculations. The magnetization curves of NaNi02 showed the caracteristic feature of a spin-flop transition. We show that we can reproduce such a behavior in the frame of a spin-orbital model.

EPFL2005

Charge transport properties of spin-orbit coupled materials in different correlation regimes

Soudabeh Sheikholharam Mashhadi

EPFL2018

Charge transport properties of spin-orbit coupled materials in different correlation regimes

Dégénérescence orbitale et ordre magnétique sur le réseau triangulaire

Spin-Orbit Coupling with the Auxiliary Many-Electron Wavefunction

Charge transport properties of spin-orbit coupled materials in different correlation regimes

Dégénérescence orbitale et ordre magnétique sur le réseau triangulaire

Spin-Orbit Coupling with the Auxiliary Many-Electron Wavefunction