Phase-controlled quasi-bound states in the continuum and thermoelectric enhancement in Majorana-quantum-dot nanostructures
We investigate how the interplay between Majorana zero modes (MZMs) and bound states in the continuum (BICs) governs the electronic thermoelectric response of a crossbar-shaped quantum dot (QD) coupled to two topological-superconductor nanowires. Using the Green-function formalism, exact linear-response energy integrals, and their low-temperature Sommerfeld expansion, we analyze the spectral and thermoelectric properties of the system. We show that symmetry breaking converts BICs into quasi-BICs, allowing them to contribute to electrical and thermal transport and thereby generate a finite thermoelectric response. While unequal nanowire lengths, reflected in different intra-Majorana coupling strengths, produce only a modest enhancement of $ZT_{el}$, detuning the QD level increases $ZT_{el}$ by approximately one order of magnitude. Superconducting-phase control produces a much stronger enhancement, reaching $ZT_{el} \simeq 0.75$ through a quadratic transmission zero and a pronounced violation of the Wiedemann-Franz law. The low-temperature values $ZT^{max}_{el} \simeq 0.755$ and $\mathscr{L} /\mathscr{L}_{0} = 21/5$ are universal consequences of this quadratic antiresonance. Our results establish phase-tunable thermoelectric signatures of the Majorana-coupled interference structure and identify superconducting-phase control as an efficient means of engineering the electronic response of topological hybrid nanostructures.