Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Abstract: The pursuit of understanding parity anomaly in condensed matter systems has led to significant advancements in both theoretical and experimental research in recent years. In this study, we explore the parity anomaly of massless Dirac fermions in a semimagnetic topological insulator (TI) thin film subjected to a finite magnetic field. Our findings reveal an anomalous half-quantized Hall conductance arising from the occupied electronic states far below the Fermi level, which is directly associated with the parity anomaly. This observation demonstrates a crossover from one-half quantized Hall conductance in a metallic phase at zero field to one or zero quantized Hall conductance in the insulating phase at a strong field in the presence of disorders, serving as a key indicator for confirming parity anomaly. Our work provides valuable insights into the intricate relationship between band topology in condensed matter systems and quantum anomaly in quantum field theory.

Abstract: Recent breakthroughs in the transport spectroscopy of 2-D material quantum-dot platforms have engendered a fervent interest in spin-valley qubits. In this context, Pauli blockades in double quantum dot structures form an important basis for multi-qubit initialization and manipulation. Focusing on double quantum dot structures, and the experimental results, we first build theoretical models to capture the intricate interplay between externally fed gate voltages and the physical properties of the 2-D system in such an architecture, allowing us to effectively simulate Pauli blockades. Employing the master equations for transport and considering extrinsic factors such as electron-photon interactions, we thoroughly investigate all potential occurrences of Pauli blockades. Notably, our research reveals two remarkable phenomena: (i) the existence of multiple resonances within a bias triangle, and (ii) the occurrence of multiple Pauli blockades. Leveraging our model to train a machine learning algorithm, we successfully develop an automated method for real-time detection of multiple Pauli blockade regimes. Through numerical predictions and validations against test data, we identify where and how many Pauli blockades are likely to occur. We propose that our model can effectively detect the generic class of Pauli blockades in practical experimental setups and hence serves as the foundation for future experiments on qubits that utilize 2-D material platforms.

Abstract: The interactions between charges and excitons involve complex many-body interactions at high densities. The exciton-polaron model has been adopted to understand the Fermi sea screening of charged excitons in monolayer transition metal dichalcogenides. The results provide good agreement with absorption measurements, which are dominated by dilute bright exciton responses. Here we investigate the Fermi sea dressing of spin-forbidden dark excitons in monolayer WSe2. With a Zeeman field, the valley-polarized dark excitons show distinct p-doping dependence in photoluminescence when the carriers reach a critical density. This density can be interpreted as the onset of strongly modified Fermi sea interactions and shifts with increasing exciton density. Through valley-selective excitation and dynamics measurements, we also infer an intervalley coupling between the dark trions and exciton-polarons mediated by the many-body interactions. Our results reveal the evolution of Fermi sea screening with increasing exciton density and the impacts of polaron-polaron interactions, which lay the foundation for understanding electronic correlations and many-body interactions in 2D systems.

Abstract: We develop here a theory of the electronic properties of a finite number of valence holes in gated WSe$_2$ quantum dots, considering the influence of spin, valley, electronic orbitals, and many-body interactions. The single-particle wave functions are constructed by combining the spin-up and down states of the highest valence bulk bands employing a multi-million atom ab-initio based tight-binding model solved in the wave-vector space, allowing to study up to 100 nm radius quantum dots atomistically. The effects of the many-body interactions are determined using the configuration interaction (CI) technique, applied up to $N = 6$ holes occupying up to 6 electronic shells with 42 orbitals. Our results show that N=2 holes are in valley and spin anti-ferromagnetic ground state, independent of the interaction strength and the quantum dot size. However, we predict that higher number of holes can undergo a transition to spontaneously broken symmetry valley and spin polarized ferromagnetic phases, highlighting the interplay between the many-body effects and the quantum dot lateral size and confining potential depth.

Abstract: Anyonic Fabry-P\'erot and Mach-Zehnder interferometers have been proposed theoretically and implemented experimentally as tools to probe electric charges and statistics of anyons. The experimentally observed visibility of Aharonov-Bohm oscillations is maximal at a high transmission through an interferometer but simple theoretical expressions for the electric currents and noises are only available at low visibility. We consider an alternative version of a Mach-Zehnder interferometer, in which anyons tunnel between co-propagating chiral channels on the edges of quantum Hall liquids at the filling factors $n/(2n+1)$. We find simple exact solutions for any transmission. The solutions allow a straight-forward interpretation in terms of fractional charges and statistics.