arXiv subjects

Miguel Sánchez Sánchez

Publications and source records attributed to Miguel Sánchez Sánchez.

9 recordsLinked to original sources

arXiv2026

In-plane magnetic response and Maki parameter of alternating-twist multilayers

We analytically study the orbital response of alternating-twist multilayer graphene to an in-plane magnetic field using the unitary transformation introduced by Khalaf et al. [Phys. Rev. B 100, 085109 (2019)]. This transformation maps an alternating-twist $N$-layer system onto $N/2$ decoupled twisted bilayer graphene (TBG) systems with distinct effective twist angles, together with a single decoupled layer for odd $N$, thereby generating a hierarchy of effective magic angles. For systems with an odd number of layers, we find that the orbital in-plane magnetic response is negligibly small. For even systems, we express the in-plane orbital susceptibility in terms of the corresponding TBG responses in the flat-band regime, which are large compared to the spin susceptibility and even diverge in the clean limit at charge neutrality near the magic angle. For the finite even-layer systems considered in this work, the in-plane magnetic response strongly depends on the effective magic angle within the hierarchy: the larger the twist angle, the smaller the total response. Moreover, we find a general relation between the outermost interlayer and total susceptibilities of the system when the corresponding effective TBG subsystem is in the flat-band regime. We finally introduce the in-plane Maki parameter as the ratio of the difference in orbital susceptibility between the normal and superconducting states to the paramagnetic Pauli susceptibility. For TBG, we find values up to 2 near the magic angle. Our analysis shows that for certain magic angles, the interpretation of Pauli-limit violation in alternating-twist multilayers requires taking into account the orbital contribution to the in-plane magnetic response.

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arXiv2026

Fermi velocity, interlayer couplings, and magic angle renormalization in twisted bilayer graphene

Through extensive self-consistent Hartree-Fock calculations in a tight-binding model of twisted bilayer graphene (TBG), we show that many-body effects lead to a considerable increase of the bandwidth of the flat bands and, concomitantly, to a shift of the magic angle (defined by the condition of minimum bandwidth). Specifically, we predict a shift from the $\textit{ab initio}$ magic angle of $0.99^\circ$ to a renormalized value of $0.88^\circ$ for a TBG sample suspended between metallic gates with a gate-to-gate distance of $10 \text{ nm}$. We derive analytical expressions for the renormalized Fermi velocity and interlayer couplings, finding good agreement with the numerical results, and investigate the convergence toward the numerical solutions with respect to the number of renormalized couplings of a generalized Bistritzer-MacDonald (BM) model. Using the derived analytical formulas, we demonstrate the possibility of tuning the flat bands via different dielectric environments and gate geometries in the experiments. Furthermore, we predict a significant enhancement of the flat-band Fermi velocity at intermediate twist angles relative to the bare value, and propose measurements in this range as a probe of the effective couplings of TBG. Our results imply a change of paradigm whereby the maximum $T_c$ for superconductivity would correspond to a condition of small but not minimum bandwidth.

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arXiv2026

Plasmon dynamics in graphene

Plasmon are collective oscillations of mobile electrons with dynamics controlled by their charge stiffness("Drude weight"). Using terahertz spacetime metrology, we probe Plasmon dynamics of mono- and bi-layer graphene. In both systems, the experimentally measured Drude weight systematically exceeds the prediction based on non-interacting electronic system. The relative enhancement increases as the carrier density decreases. We attribute the observed deviation to the interplay of interactions and wave function structure of the Dirac fermions in multi-layer graphene. Our results establish that pseudospin structure of the single-particle electronic wave function can directly influence collective excitations, with implications that extend beyond graphene to a broad class of quantum materials.

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arXiv2026

Flat-band projected versus fully atomistic twisted bilayer graphene

We benchmark the recently proposed projection method [Phys. Rev. B 111, 205133 (2025)] for magic-angle twisted bilayer graphene (MATBG) across various symmetry-breaking phases at charge neutrality. The flat-band projected solutions agree well with the full tight-binding, with band structures and total energies differing by only a few meV. The projection to the flat bands is justified, owing to the increased gap to the remote bands in the normal state. Moreover, we employ a novel set of order parameters that allow us to visualize the wave functions locally in real space and quantify the breaking of various symmetries in the correlated phases. These order parameters are suitable for characterizing MATBG and generic honeycomb systems.

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arXiv2025

Nonflat bands and chiral symmetry in magic-angle twisted bilayer graphene

In this work, we study an interacting tight-binding model of magic-angle twisted bilayer graphene (MATBG), with a twist angle of $1.05^\circ$. We derive effective theories based on a mean-field normal state at charge neutrality, thereby including the renormalizations coming from integrating out high-energy modes. In these theories, the flat bands display a sizable increase of the bandwidth, suggesting the renormalization of the magic angle. Additionally, the corresponding wavefunctions flow towards the limit of perfect particle-hole symmetry and sublattice polarization (the 'chiral' limit). We further represent the flat bands in the 'vortex Chern' basis and discuss the implications on the dynamics, regarding the 'flat' and 'chiral' symmetries of MATBG, as manifested in the symmetry-broken states at neutrality.

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arXiv2025

Roadmap on Nonlocality in Photonic Materials and Metamaterials

Photonic technologies continue to drive the quest for new optical materials with unprecedented responses. A major frontier in this field is the exploration of nonlocal (spatially dispersive) materials, going beyond the local, wavevector-independent assumption traditionally made in optical material modeling. On one end, the growing interest in plasmonic, polaritonic and quantum materials has revealed naturally occurring nonlocalities, emphasizing the need for more accurate models to predict and design their optical responses. This has major implications also for topological, nonreciprocal, and time-varying systems based on these material platforms. Beyond natural materials, artificially structured materials--metamaterials and metasurfaces--can provide even stronger and engineered nonlocal effects, emerging from long-range interactions or multipolar effects. This is a rapidly expanding area in the field of photonic metamaterials, with open frontiers yet to be explored. In the case of metasurfaces, in particular, nonlocality engineering has become a powerful tool for designing strongly wavevector-dependent responses, enabling enhanced wavefront control, spatial compression, multifunctional devices, and wave-based computing. Furthermore, nonlocality and related concepts play a critical role in defining the ultimate limits of what is possible in optics, photonics, and wave physics. This Roadmap aims to survey the most exciting developments in nonlocal photonic materials, highlight new opportunities and open challenges, and chart new pathways that will drive this emerging field forward--toward new scientific discoveries and technological advancements.

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arXiv2024

Nematic versus Kekulé phases in twisted bilayer graphene under hydrostatic pressure

We address the precise determination of the phase diagram of magic angle twisted bilayer graphene under hydrostatic pressure within a self-consistent Hartree-Fock method in real space, including all the remote bands of the system. We further present a novel algorithm that maps the full real-space density matrix to a $4\times4$ density matrix based on a $SU(4)$ symmetry of sublattice and valley degrees of freedom. We find a quantum critical point between a nematic and a Kekulé phase, and show also that our microscopic approach displays a strong particle-hole asymmetry in the weak coupling regime. We arrive then at the prediction that the superconductivity should be Ising-like in the hole-doped nematic regime, with spin-valley locking, and spin-triplet in the electron-doped regime.

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arXiv2024

Correlated phases and topological phase transition in twisted bilayer graphene at one quantum of magnetic flux

When the perpendicular magnetic flux per unit cell in a crystal is equal to the quantum of magnetic flux, $Φ_0=h/e$, we enter the 'Hofstadter regime'. The large unit cell of moiré materials like magic-angle twisted bilayer graphene (MATBG) allows the experimental study of this regime at feasible values of the field around $20$ to $30$ T. In this work, we report numerical analysis of a tight-binding model for MATBG at one quantum of external magnetic flux, including the long-range Coulomb and on-site Hubbard interaction. We study the correlated states for dopings of $-2,0$ and $2$ electrons per unit cell at the mean-field level. We find competing insulators with Chern numbers $2$ and $0$ at positive doping, the stability of which is determined by the dielectric screening, which opens up the possibility of observing a topological phase transition in this system.

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arXiv2023

The correlated insulators of magic angle twisted bilayer graphene at zero and one quantum of magnetic flux: a tight-binding study

Magic angle twisted bilayer graphene (MATBG) has become one of the prominent topics in Condensed Matter during the last few years, however, fully atomistic studies of the interacting physics are missing. In this work, we study the correlated insulator states of MATBG in the setting of a tight-binding model, under a perpendicular magnetic field of $0$ and $26.5$ T, corresponding to zero and one quantum of magnetic flux per unit cell. At zero field and for dopings of two holes ($ν=-2$) or two electrons ($ν=+2$) per unit cell, the Kramers intervalley coherent (KIVC) order is the ground state at the Hartree-Fock level, although it is stabilized by a different mechanism to that in continuum model. At charge neutrality, the spin polarized state is competitive with the KIVC due to the on-site Hubbard energy. We obtain a strongly electron-hole asymmetric phase diagram with robust insulators for electron filling and metals for negative filling. In the presence of magnetic flux, we predict an insulator with Chern number $-2$ for $ν=-2$, a spin polarized state at charge neutrality and competing insulators with Chern numbers $+2$ and $0$ at $ν=+2$. The stability of the $ν=+2$ insulators is determined by the screening environment, allowing for the possibility of observing a topological phase transition.

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