arXiv subjects

Felipe Pérez Riffo

Publications and source records attributed to Felipe Pérez Riffo.

3 recordsLinked to original sources

arXiv2026

Symmetry selection rule for the band-edge shift current in two dimensions

The shift current is the intrinsic bulk photovoltaic response of a crystal without an inversion center. In graphene multilayers, calculations report large band-edge shift currents that reverse sign under a gate, a behavior that neither the quantum metric nor the Berry curvature captures. We show that this follows from a symmetry principle. At the absorption edge of a gapped, inversion-broken two-dimensional Dirac-like system, an emergent rotational symmetry forbids the response, and the trigonal warping switches it on linearly in its strength, an angular selection rule that unifies multilayer graphene and the kagome lattice. The released current is governed by a signed, detuning-weighted three-point Bargmann invariant: the optical amplitude stays fixed while the sign alignment of the Bargmann triangles grows with the warping, invisible to any positive-definite figure of merit. In bilayer and trilayer graphene the gate alone reverses the sign, making the band-edge shift current a gate-switchable bulk photovoltaic response.

cond-mat.mes-hall↗

arXiv2025

Unveiling the Electronic Origin of Anomalous Contact Conductance in Twisted Bilayer Graphene

This study theoretically investigates the contact conductance in twisted bilayer graphene (TBG), providing a theoretical explanation for recent experimental observations from scanning tunneling microscopy (STM) and conductive atomic force microscopy (c-AFM). These experiments revealed a surprising non-monotonic current pattern as a function of the TBG rotation angle $θ$, with a peak at $θ\approx 5^\circ$, a finding that markedly departs from the well-known magic angle TBG behavior. To elucidate this phenomenon, we develop a comprehensive theoretical and computational framework. Our calculations, performed on both relaxed and rigid TBG structures, simulate contact conductance by analyzing the local density of states across a range of biases and rotational angles. Contrary to the current interpretation, our results demonstrate that the maximum conductance at $θ\approx 5^{\rm o}$ is not caused by structural relaxation or AA stacking zone changes. Instead, we attribute this peak to the evolution of the electronic band structure, specifically the shifting of van Hove singularities (vHs) to the Fermi level as the twist angle decreases. We further show that the precise location of this conductance maximum is dependent on the applied bias voltage. This interplay between twist angle, bias, and vHs energy provides a robust explanation for the experimental findings.

cond-mat.mes-hall↗

arXiv2025

Stoner ferromagnetism in low-angle twisted bilayer graphene at three-quarters filling

We present a theoretical investigation of the magnetic properties exhibited by twisted bilayer graphene (TBG) systems with small twist angles, where the appearance of flat minibands strongly enhances electron-electron interaction effects. We show that, at three-quarters filling of the conduction miniband, the Stoner mechanism induces a ferromagnetic polarization in the AA-stacking regions, which aligns with recent experimental observations. Our approach models the electronic properties by a tight-binding Hamiltonian combined with a Hubbard mean-field interaction term. We employ a real-space recursion technique to self-consistently calculate the system's local density of states and use our method to investigate the magnetic properties of small-angle TBG at three-quarters filling. The recursion method's $O({\cal N})$ efficiency makes it possible to address extremely large superlattices through a full real-space approach. We validate our procedure by comparing it with mean-field momentum-space calculations from the literature, which identify a magnetic phase in charge-neutral TBGs.

cond-mat.mes-hall↗