Understanding the many-body behaviors of interacting electron systems remains one of the central topics in condensed matter physics. Novel correlated phases coupled to lattice symmetry, topological orders and hidden geometrical degrees of freedom could be induced and modulated by external electric or magnetic fields. Extensive attention have been drawn to these research directions which are of significant interests for both fundamental understanding and practical applications of many-body electron systems. In this dissertation I report optical spectroscopic studies on the Coulomb coupling, phase interplay and geometric fluctuations of interacting two-dimensional electron systems. The research provides a key approach to engineering many-body ground states and offers critical insights into their underlying nature. Electric potential or magnetic field modulations are applied to the electrons hosted in semiconductor quantum wells. Through lateral superlattice nanopatterning, we fabricate semiconductor artificial graphene where resonant inelastic light scattering is employed to characterize the engineered band structures. Flat bands hosting van Hove singularities are directly observed by optical emission. Coulomb coupling between electrons with diverging density of states are found to have significant impacts on the energies and line-shapes of the optical spectra. The results demonstrate a novel and tunable platform to explore intriguing many-body physics. External magnetic fields have been known to trigger a rich phase diagram in interacting two-dimensional electron systems, encompassing phenomena such as the fractional quantum Hall effect. The phase interplay gives rise to domain textures in the bulk of electron systems and affects the dispersion of collective excitations. We probe impacts of domain textures on low-lying neutral excitations through doubly resonant inelastic light scattering. We demonstrate that large domains of quantum fluids can support well-defined long-wavelength modes which could be interpreted by theories for uniform phases. Equipped with ultra-high mobility quantum wells and circularly polarized light scattering techniques, we resolve the spin of long-wavelength magnetoroton modes and provide characteristic evidence of the chiral graviton at Landau level filling factor $\nu= ⅓ fractional quantum Hall state. The results offer the first experimental evidence of geometrical degrees of freedom in the fractional quantum Hall effect.

In narrow electron bands in which the Coulomb interaction energy becomes comparable to the bandwidth, interactions can drive new quantum phases. In this dissertation, we achieve narrow bands by twisting two atomically thin layers of the semiconducting van der Waals material WSe₂. The resulting moiré potential from the twist angle modulates the electronic bands, yielding minibands of tens of meV on the valence band. We perform transport measurements at cryogenic temperatures and observe signatures of collective phases over twist angles that range from 4 to 5.1°. At half-band filling, a correlated insulator appeared that is tunable with both twist angle and displacement field. Near the boundary between ordered and disordered quantum phases, several experiments have demonstrated metallic behaviour that defies the Landau Fermi paradigm. We find that the metal-insulator transition as a function of both density and displacement field is continuous. At the metal–insulator boundary, the resistivity displays strange metal behaviour at low temperatures, with dissipation comparable to that at the Planckian limit. Further into the metallic phase, Fermi liquid behaviour is recovered at low temperature, and this evolves into a quantum critical fan at intermediate temperatures, before eventually reaching an anomalous saturated regime near room temperature. An analysis of the residual resistivity indicates the presence of strong quantum fluctuations in the insulating phase. We further show via magnetotransport measurements that new correlated electronic phases can exist independent of moiré commensurability, and are instead driven by weak interactions in twisted WSe₂. The first of these phases is an antiferromagnetic metal that is driven by proximity to the van Hove singularity (vHS), which trails a range of incommensurate dopings. The temperature, magnetic field and density dependence of the Hall effect carry signatures of the reconstructed Fermi surface due to itinerant magnetic ordering. The second is an excitonic metal-insulator phase that exists at high external magnetic field in the vicinity of half-filling of the moiré superlattice. For a 4.2° sample, magnetic field dependence of the longitudinal resistance shows metallic behavior at fields above 5 T, but transitions to an insulating state above ∼ 24 T. A detailed analysis of of the Landau fans and the high field 𝝆_𝜘𝛾 near the gap rules out the possibility of a trivial insulator. We propose an Ising excitonic insulator as the most likely scenario. Moreover, in the electron-imbalanced excitonic metal, a set of correlated Landau levels emerge. The observation of tunable collective phases in a simple band, which hosts only two holes per unit cell at full filling, establishes twisted bilayer transition metal dichalcogenides as an ideal platform to study correlated physics in two dimensions on a triangular lattice.

Halo nuclei are among the strangest nuclear structures.<p>They are viewed as a core containing most of the nucleons<p>surrounded by one or two loosely bound nucleons. <p>These have a high probability of presence at a large distance<p>from the core.<p>Therefore, they constitute a sort of halo surrounding the other nucleons.<p>The core, remaining almost unperturbed by the presence<p>of the halo is seen as a usual nucleus.<p><p><P><p><p>The Coulomb breakup reaction is one of the most useful<p>tools to study these nuclei. It corresponds to the<p>dissociation of the halo from the core during a collision<p>with a heavy (high <I>Z</I>) target.<p>In order to correctly extract information about the structure of<p>these nuclei from experimental cross sections, an accurate<p>theoretical description of this mechanism is necessary.<p><p><P><p><p>In this work, we present a theoretical method<p>for studying the Coulomb breakup of one-nucleon halo nuclei.<p>This method is based on a semiclassical approximation<p>in which the projectile is assumed to follow a classical trajectory.<p>In this approximation, the projectile is seen as evolving<p>in a time-varying potential simulating its interaction with the target.<p>This leads to the resolution of a time-dependent Schrödinger<p>equation for the projectile wave function.<p><p><P><p><p>In our method, the halo nucleus is described<p>with a two-body structure: a pointlike nucleon linked to a<p>pointlike core.<p>In the present state of our model, the interaction between<p>the two clusters is modelled by a local potential.<p><p><P><p><p>The main idea of our method is to expand the projectile wave function<p>on a three-dimensional spherical mesh.<p>With this mesh, the representation of the time-dependent potential<p>is fully diagonal.<p>Furthermore, it leads to a simple<p>representation of the Hamiltonian modelling the halo nucleus.<p>This expansion is used to derive an accurate evolution algorithm.<p><p><P><p><p>With this method, we study the Coulomb breakup<p>of three nuclei: ¹¹Be, ¹⁵C and ⁸B.<p>¹¹Be is the best known one-neutron halo nucleus.<p>Its Coulomb breakup has been extensively studied both experimentally<p>and theoretically.<p>Nevertheless, some uncertainty remains about its structure.<p>The good agreement between our calculations and recent<p>experimental data suggests that it can be seen as a<p><I>s1/2</I> neutron loosely bound to a ¹⁰Be core in its<p>0⁺ ground state.<p>However, the extraction of the corresponding spectroscopic factor<p>have to wait for the publication of these data.<p><p><P><p><p>¹⁵C is a candidate one-neutron halo nucleus<p>whose Coulomb breakup has just been studied experimentally.<p>The results of our model are in good agreement with<p>the preliminary experimental data. It seems therefore that<p>¹⁵C can be seen as a ¹⁴C core in its 0⁺<p>ground state surrounded by a <I>s1/2</I> neutron.<p>Our analysis suggests that the spectroscopic factor<p>corresponding to this configuration should be slightly lower<p>than unity.<p><p><P><p><p>We have also used our method to study the Coulomb breakup<p>of the candidate one-proton halo nucleus ⁸B.<p>Unfortunately, no quantitative agreement could be obtained<p>between our results and the experimental data.<p>This is mainly due to an inaccuracy in the treatment<p>of the results of our calculations.<p>Accordingly, no conclusion can be drawn about the pertinence<p>of the two-body model of ⁸B before an accurate reanalysis of these<p>results.<p><p><P><p><p>In the future, we plan to improve our method in two ways.<p>The first concerns the modelling of the halo nuclei.<p>It would be indeed of particular interest to test<p>other models of halo nuclei than the simple two-body structure<p>used up to now.<p>The second is the extension of this semiclassical model to<p>two-neutron halo nuclei.<p>However, this cannot be achieved<p>without improving significantly the time-evolution algorithm so as to<p>reach affordable computational times. / Doctorat en sciences appliquées / info:eu-repo/semantics/nonPublished