Searcharxiv⌕ Search
Home/Archive/arXiv

arXiv · 2609.37418

Nonlinear optical responses of semiconducting graphene nanoribbons: Strong constraint from a hidden symmetry

Jiawei Ruan·Weichen Tang·Yang-Hao Chan·Chen Hu·Xiaoxun Gong·Steven G. Louie
View original source ↗Read source document ↗Export metadata ↓

Abstract

Nonlinear optical (NLO) responses are powerful tools for probing material structures, as they are highly sensitive to spatial symmetries. For instance, it is well known that having inversion symmetry forbids even-order NLO effects. Here, we show that nonspatial symmetries can also significantly constrain NLO responses. In graphene nanoribbons (GNRs) with bipartite-lattice structures, we identify the approximate bipartite-lattice particle-hole (BLPH) symmetry strongly suppresses even-order NLO effects, even for systems with no inversion symmetry. Importantly, we demonstrate that this approximate symmetry enables the optical generation of nearly pure spin shift currents in ferromagnetic GNRs, with the charge shift currents strongly suppressed. The spin orientation of these photo-induced DC spin currents can be easily tuned by an external magnetic field, providing distinct advantages over previous schemes for generating pure spin shift currents in antiferromagnetic systems. We illustrate our findings using a recently synthesized ferromagnetic Janus GNR. Our results underscore the significance of an overlooked symmetry aspect of NLO responses and highlight magnetic GNRs as promising candidates for spintronic devices.

Explore related subjects

cond-mat.mes-hallcond-mat.mtrl-sci

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

Jiawei Ruan, Weichen Tang, Yang-Hao Chan, Chen Hu, Xiaoxun Gong, Steven G. Louie. 2026-09-29. Nonlinear optical responses of semiconducting graphene nanoribbons: Strong constraint from a hidden symmetry. https://arxiv.org/abs/2609.37418

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related papers

arXiv2026

Density-matrix quantum kinetics of spin-mode crossover and ac Edelstein response in spin--orbit-coupled chiral metals

To clarify spin relaxation and precession across the weak-to-strong SOC crossover in chiral conductors, we formulate a density-matrix quantum kinetic theory for a three-dimensional isotropic chiral metal with hedgehog SOC and nonmagnetic impurity scattering. The formulation retains interband coherence, and its collision integral conserves charge, energy, and spin during impurity scattering. We identify three spin modes that evolve continuously from a long-lived D'yakonov--Perel' relaxation mode and two strongly damped precessional modes at weak SOC to one relaxational and two coherent precessional modes at strong SOC. A closed multipole decomposition maps the spin dynamics onto an effective Bloch equation, providing a unified interpretation of relaxation and precession across the crossover. We further derive the ac Edelstein susceptibility and the reciprocal current response to a time-dependent Zeeman field, show that their poles coincide with the spin modes, and verify Onsager reciprocity. The theory thus provides a unified analytic description of spin relaxation, precession, and spin--charge conversion across the weak-to-strong SOC crossover.

cond-mat.mes-hall↗
arXiv2026

Incipient superconductivity and tunable Chern insulators in twisted bilayer-trilayer graphene

Moiré superlattices assembled by twisting Bernal and rhombohedral multilayer graphene host a rich set of interaction-driven magnetic and topological states, yet superconductivity has not been observed in these systems except in proximity to a transition-metal dichalcogenide. Here we report incipient superconductivity and tunable Chern insulators in twisted bilayer-trilayer graphene encapsulated by hexagonal boron nitride. Across twist angles from $θ= 1.05^\circ$ to $1.50^\circ$, Chern insulators form at integer and fractional moiré fillings for electron doping, with Chern numbers up to |C| = 3 set by twist angle and tuned by doping. At $θ= 1.18^\circ$, a symmetry-broken metallic region forms for hole doping and hosts a trivial insulator at band filling $ν= -2$. Displacement field alone drives this insulator into a pocket of incipient superconductivity with a sharp transition, a well-defined critical current, and a critical temperature that peaks near the insulating boundary, although the resistance does not fall to zero. In-plane magnetic field expands the pocket, which persists to more than four times the weak-coupling Pauli limit, and stabilizes a second pocket in which Fraunhofer-like modulation of the critical current signals phase-coherent pairing. Twisted bilayer-trilayer graphene thus offers a single gate-tunable system in which pairing can be interfaced with Chern insulators whose topology is itself an adjustable parameter.

cond-mat.mes-hall↗
arXiv2026

Electrically switchable one-dimensional quadrupolar excitons in lateral double heterojunctions

Two neighboring lateral interfaces provide a spatial degree of freedom for controlling one-dimensional charge-transfer excitons within a single semiconductor monolayer. We investigate a type-II WS2-MoS2-WS2 double heterojunction using an effective-mass two-particle Hamiltonian with a screened Coulomb interaction. For equivalent left and right interfaces at zero electric field,inter-interface coupling produces energetically split even- and odd-parity exciton states, each with zero permanent dipole. An electric field perpendicular to the interfaces continuously converts the lower state from a quadrupolar superposition with a quadratic Stark shift into a predominantly single-interface dipolar exciton with an approximately linear shift. The spatially resolved calculation gives binding energies of approximately 104 and 100 meV, a doublet splitting of 4.6 meV, and a crossover field of 0.87 V/um for a representative 1.5-nm MoS2 strip. Projection tests show that the lowest doublet controls the response near this crossover. Strip width tunes the coupling much more strongly than the binding energy, providing geometric control of the low-field Stark sensitivity. These results establish a continuum-model route to electrically reconfigurable one-dimensional quadrupolar excitons.

cond-mat.mes-hall↗