arXiv subjects

Jiayi Gong

Publications and source records attributed to Jiayi Gong.

3 recordsLinked to original sources

arXiv2026

Net and Hidden Spin-Valley Locking Enable Ultrahigh Hole Mobility in Covalent Bulk WN$_2$

High carrier mobility at room temperature underpins high-performance electronics, yet high hole mobility remains rare in bulk semiconductors. Spin-valley locking can suppress intervalley scattering and enhance mobility, but it is limited to materials with broken inversion symmetry. Hidden spin polarization offers a possible route beyond this constraint, although whether its compensated spin textures could protect charge transport remains unclear. Using ab initio electron-phonon and transport calculations, we show that the two hexagonal phases of bulk WN$_2$ realize net and hidden spin-valley locking and exhibit ultrahigh room-temperature hole mobilities. In non-centrosymmetric $α$-WN$_2$, a large valley spin splitting produces net spin-valley locking that nearly eliminates phonon-mediated intervalley scattering. In centrosymmetric $β$-WN$_2$, hidden Zeeman-type spin polarization yields a compensated, sector-resolved spin texture that reverses between valleys and suppresses intervalley scattering as effectively as the net locking does. The stiff W-N/N-N covalent network further keeps the remaining intravalley scattering weak. Our results establish hidden spin polarization as an effective transport-protection mechanism and extend spin-valley engineering to centrosymmetric bulk semiconductors.

cond-mat.mtrl-sci↗

arXiv2025

Free-carrier screening unlocks high electron mobility in ultrawide bandgap semiconductor CaSnO$_3$

Alkaline earth stannates have emerged as promising transparent conducting oxides due to their wide band gaps and high room-temperature electron mobilities. Among them, CaSnO$_3$ possesses the widest band gap, yet reported mobilities vary widely and are highly sample-dependent, leaving its intrinsic limit unclear. Here, we present ab initio calculations of electron mobility in CaSnO$_3$ across a range of temperatures and doping levels, using state-of-the-art methods that explicitly account for free-carrier screening in electron-phonon interactions. We identify the dominant limiting mechanism to be the long-range longitudinal optical phonon scattering, which is significantly suppressed at high doping due to free-carrier screening, leading to enhanced phonon-limited mobility. While ionized impurity scattering emerges as a competing mechanism at carrier concentrations up to ~10$^{20}$ cm$^{-3}$, the phonon scattering reduction dominates, yielding a net mobility increase with predicted room-temperature values reaching about twice the highest experimental report. Our work highlights the substantial untapped conductivity in CaSnO$_3$, establishing it as a compelling ultrawide bandgap semiconductor for transparent and high-power electronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Phonons Drive the Topological Phase Transition in Quasi-One-Dimensional Bi$_4$I$_4$

Quasi-one-dimensional bismuth halides offer an exceptional platform for exploring diverse topological phases, yet the nature of the room-temperature topological phase transition in Bi$_4$I$_4$ remains unresolved. While theory predicts the high-temperature $β$-phase to be a strong topological insulator (TI), experiments observe a weak TI. Here we resolve this discrepancy by revealing the critical but previously overlooked role of electron-phonon coupling in driving the topological phase transition. Using our newly developed ab initio framework for phonon-induced band renormalization, we show that thermal phonons alone drive $β$-Bi$_4$I$_4$ from the strong TI predicted by static-lattice calculations to a weak TI above ~180 K. At temperatures where $β$-Bi$_4$I$_4$ is stable, it is a weak TI with calculated surface states closely match experimental results, thereby reconciling theory with experiment. Our work establishes electron-phonon renormalization as essential for determining topological phases and provides a broadly applicable approach for predicting topological materials at finite temperatures.

cond-mat.mtrl-sci↗