arXiv subjects

Yuncheng Mao

Publications and source records attributed to Yuncheng Mao.

6 recordsLinked to original sources

arXiv2026

Magnetic Bulk Photovoltaic Effect in Bernal Bilayer Graphene

We investigate the shift-current response of inversion-broken AB-stacked bilayer graphene under in-plane and perpendicular magnetic fields, from the perturbative regime to strong orbital quantization, in both two-dimensional bulk systems and finite nanoribbons. An in-plane field enters through opposite momentum shifts in the two layers and leaves the bulk band dispersion essentially unchanged, producing only a modest, frequency-dependent redistribution of the shift-current spectrum. A weak perpendicular field is treated using a gauge-covariant Peierls expansion. The resulting band corrections are concentrated near Berry-curvature hot spots, although the dominant shift-current transitions occur elsewhere, and the valley-summed response is even in the field with a leading quadratic correction. At strong perpendicular field, a rational-flux magnetic supercell reveals nearly flat Landau-level-like bulk bands and a greatly enhanced density of states, yet the bulk shift current is almost completely quenched because the relevant optical matrix elements and shift-vector contributions are suppressed or cancel. Finite ribbons retain optically active boundary channels: in zigzag ribbons, magnetic reconstruction turns edge-derived states from dark states into bright photovoltaic channels, with the associated peak scaling inversely with ribbon width. These results show that magnetic control of the nonlinear photovoltaic response is governed by wave-function reconstruction and quantum-geometric matrix elements rather than by the density of states alone.

cond-mat.mes-hall↗

arXiv2025

Shift current in 2D Janus Transition-Metal Dichalcogenides: the role of excitons

We investigate the shift current in two-dimensional (2D) Janus transition-metal dichalcogenides (TMDs). The shift current is evaluated using a real-time approach, where the coupling with an external field is described in terms of a dynamical Berry phase. This methodology incorporates electron-hole interactions and quasiparticle band structure renormalization through an effective Hamiltonian derived from many-body perturbation theory. We find that the shift current is strongly enhanced in correspondence with C excitons. An analysis in terms of the electron-hole pairs reveals that electron and hole are localized on different atoms, and thus, following an optical excitation, the center of the electron charge is displaced, giving rise to a significant photocurrent. Janus TMDs, with their intrinsic out-of-plane asymmetry and tunable electronic properties, are particularly appealing for next-generation optoelectronic and energy-harvesting technologies. These results highlight the role of excitons in the shift-current response of Janus TMDs and demonstrate their potential as promising building blocks for future photovoltaic devices.

cond-mat.mes-hall↗

arXiv2025

Moiré amplification of highly tunable shift current response in twisted trilayer graphene

In this work we analyze the shift current conductivity in helical twisted trilayer graphene. Without loss of generality, we show that the density of states and the twist angle set an upper bound for this response, which is inversely proportional to the square of the twist angle. For the case of ABA stacking and at the magic angle, the shift photoconductivity can reach values of order $10^4~\mathrm{μA \cdot nm \cdot V}^{-2}$ for frequencies below 50 meV, which can be attributed to the interband transitions between the two flattened middle bands close to the Fermi level. By tuning the twist angle, we demonstrate that the photoconductivity is shifted in the frequency range and it is further influenced by two additional factors: The magnitude of the shift vector and the energy separation between the bands. Furthermore, we propose a scenario in the AAA stacked configuration, where the photoconductivity can be of order $10^5~\mathrm{μA \cdot nm \cdot V}^{-2}$ in the THz regime, revealing a potential influence of the stacking in the optimization of the shift current conductivity. Therefore, a large density of states, a small twist angle and the layer stacking are ingredients that hold promising functionality for photovoltaic applications in moiré heterostructures.

cond-mat.mes-hall↗

arXiv2023

Nature of even and odd magic angles in helical twisted trilayer graphene

Helical twisted trilayer graphene exhibits zero-energy flat bands with large degeneracy in the chiral limit. The flat bands emerge at a discrete set of magic twist angles and feature properties intrinsically distinct from those realized in twisted bilayer graphene. Their degeneracy and the associated band Chern numbers depend on the parity of the magic angles. Two degenerate flat bands with Chern numbers $C_A=2$ and $C_B=-1$ arise at odd magic angles, whereas even magic angles display four flat bands, with Chern number $C_{A/B}=\pm1$, together with a Dirac cone crossing at zero energy. All bands are sublattice polarized. We demonstrate the structure behind these flat bands and obtain analytical expressions for the wavefunctions in all cases. Each magic angle is identified with the vanishing of a zero-mode wavefunction at high-symmetry position and momentum. The whole analytical structure results from whether the vanishing is linear or quadratic for the, respectively, odd and even magic angle. The $C_{3z}$ and $C_{2y}T$ symmetries are shown to play a key role in establishing the flat bands. In contrast, the particle-hole symmetry is not essential, except from gapping out the crossing Dirac cone at even magic angles.

cond-mat.mes-hall↗

arXiv2023

Chern mosaic and ideal flat bands in equal-twist trilayer graphene

We study trilayer graphene arranged in a staircase stacking configuration with equal consecutive twist angle. On top of the moiré cristalline pattern, a supermoiré long-wavelength modulation emerges that we treat adiabatically. For each valley, we find that the two central bands are topological with Chern numbers $C=\pm 1$ forming a Chern mosaic at the supermoiré scale. The Chern domains are centered around the high-symmetry stacking points ABA or BAB and they are separated by gapless lines connecting the AAA points, where the spectrum is fully connected. In the chiral limit and at a magic angle of $θ\sim 1.69^\circ$, we prove that the central bands are exactly flat with ideal quantum curvature at ABA and BAB. Furthermore, we decompose them analytically as a superposition of an intrinsic color-entangled state with $\pm 2$ and a Landau level state with Chern number $\mp 1$. To connect with experimental configurations, we also explore the non-chiral limit with finite corrugation and find that the topological Chern mosaic pattern is indeed robust and the central bands are still well separated from remote bands.

cond-mat.mes-hall↗

arXiv2023

Supermoiré low-energy effective theory of twisted trilayer graphene

Stacking three monolayers of graphene with a twist generally produces two moiré patterns. A moiré of moiré structure then emerges at larger distance where the three layers periodically realign. We devise here an effective low-energy theory to describe the spectrum at distances larger than the moiré lengthscale. In each valley of the underlying graphene, the theory comprises one Dirac cone at the ${\bf Γ}_M$ point of the moiré Brillouin zone and two weakly gapped points at ${\bf K}_M$ and ${\bf K}'_M$. The velocities and small gaps exhibit a spatial dependence in the moiré-of-moiré unit cell, entailing a non-abelian connection potential which ensures gauge invariance. The resulting model is numerically solved and a fully connected spectrum is obtained, which is protected by the combination of time-reversal and twofold-rotation symmetries.

cond-mat.mes-hall↗