Angular momentum coupling | EPFL Graph Search

In quantum mechanics, the procedure of constructing eigenstates of total angular momentum out of eigenstates of separate angular momenta is called angular momentum coupling. For instance, the orbit and spin of a single particle can interact through spin–orbit interaction, in which case the complete physical picture must include spin–orbit coupling. Or two charged particles, each with a well-defined angular momentum, may interact by Coulomb forces, in which case coupling of the two one-particle angular momenta to a total angular momentum is a useful step in the solution of the two-particle Schrödinger equation. In both cases the separate angular momenta are no longer constants of motion, but the sum of the two angular momenta usually still is. Angular momentum coupling in atoms is of importance in atomic spectroscopy. Angular momentum coupling of electron spins is of importance in quantum chemistry. Also in the nuclear shell model angular momentum coupling is ubiquitous. In

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

Evolution of the Valley Position in Bulk Transition-Metal Chalcogenides and Their Monolayer Limit

Dumitru Dumcenco, Zahid Hussain, Gang Xu, Kai Yan, Yi Zhang, Bo Zhou

Layered transition metal chalcogenides with large spin orbit coupling have recently sparked much interest due to their potential applications for electronic, optoelectronic, spintronics, and valleytronics. However, most current understanding of the electronic structure near band valleys in momentum space is based on either theoretical investigations or optical measurements, leaving the detailed band structure elusive. For example, the exact position of the conduction band valley of bulk MoS2 remains controversial. Here, using angle resolved photoemission spectroscopy with submicron spatial resolution (micro-ARPES), we systematically imaged the conduction/valence band structure evolution across representative chalcogenides MoS2, WS2, and WSe2, well as the thickness dependent electronic structure from bulk to the monolayer limit. These results establish a solid basis to understand the underlying valley physics of these materials, and also provide a link between chalcogenide electronic band structure and their physical properties for potential valleytronics applications.

American Chemical Society2016

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

Soudabeh Sheikholharam Mashhadi

EPFL2018