arXiv subjects

Tobias Stauber

Publications and source records attributed to Tobias Stauber.

At least 19 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.

cond-mat.mes-hall↗

arXiv2026

Prediction of the Hubbard U parameter from scanning tunneling microscopy images of moire systems using image recognition

The atomistic Hubbard interaction U, representing the on-site Coulomb repulsion, serves as a pivotal parameter in theoretical models describing correlated systems, yet its precise experimental determination, especially in moire systems, remains challenging. Scanning Tunneling Microscopy (STM) provides real-space images of the local density of states (LDOS), offering rich data sets that reflect the unique electronic structure of the material. Here, we introduce a systematic methodology for extracting the Hubbard U parameter directly from these LDOS images through the application of machine learning (ML) in the case of twisted bilayer graphene in the flat-band regime. Accurate regression of U is achieved despite the extremely high visual similarity between FT-LDOS images corresponding to different interaction strengths. Subsequent data analysis further suggests a gradual interaction-dependent redistribution of spectral weight, with a possible crossover scale of order U/t ~ 1. To assess robustness beyond idealized simulations, we introduce physically motivated perturbations that emulate experimental STM imperfections and demonstrate that augmentation-based training substantially improves generalization to noisy and symmetry-broken FT-LDOS images.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

arXiv2026

High-temperature superconductivity in flat-band sheared bilayer graphene

We propose a new route to induce flat bands with a strong superconducting instability in graphene bilayers with heteroshear, where the 1D character of the moiré leads to stronger correlations than in twisted bilayer graphene. We adopt an exact diagonalization approach, on top of a real-space self-consistent Hartree-Fock approximation, to show how the valley polarization of the flat band of a sheared bilayer drives the condensation of Cooper pairs. A unique feature of the 1D moiré is that single-particle states with reverse sign of the valley polarization have complementary charge distributions in the moiré supercell. This leads to many-body states where the Coulomb repulsion in a Cooper pair is greatly reduced by placing electrons with opposite spin in different valleys. At small hole-doping of the flat band, the many-body ground states are formed by recursive addition of single-hole states, which allows us to reconstruct a quasi-1D Fermi line in the originally flat band. We show that even (odd) numbers of holes lead consistently to ground states with lower (higher) values of the compressibility. This provides the signature of the condensation of Cooper pairs with emergent quasiparticles above a large energy gap, unveiling a strong-coupling route to high-temperature superconductivity in topological flat-band systems.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

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.

cond-mat.str-el↗

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.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

arXiv2024

Optical response of alternating twisted trilayer graphene

We study the optical response of the alternating twisted trilayer graphene by making use of a unitary transformation for the trilayer Hamiltonian and the Kubo formulation of linear response theory. The layer-resolved optical conductivities are expressed in terms of contributions from effective twisted bilayer and single-layer systems along with their coupling. We show that the in-plane magnetic response is proportional to this coupling between the twisted bilayer and single-layer systems; and, due to the different energy scales, the in-plane magnetic response is negligibly small. We also formulate a local electro-magnetic response that involves the vertical gradients of the magnetic field and moment.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

arXiv2024

Chirality probe of twisted bilayer graphene in the linear transport regime

We propose minimal transport experiments in the coherent regime that can probe the chirality of twisted moiré structures. We show that only with a third contact and in the presence of an in-plane magnetic field (or other time-reversal symmetry breaking effect), a chiral system may display non-reciprocal transport in the linear regime. We then propose to use the third lead as a voltage probe and show that opposite enantiomers give rise to different voltage drops on the third lead. Additionally, in the scenario of layer-discriminating contacts, the third lead can serve as a current probe, capable of detecting different handedness even in the absence of a magnetic field. In a complementary configuration, applying opposite voltages on the two layers of the third leads gives rise to a chiral (super)current in the absence of a source-drain voltage whose direction is determined by its chirality.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

arXiv2023

Addressing the spin-valley flavors in moir'e mini-bands of MoS2

The physics of moir'e superlattices and the resulting formation of mini-bands in van der Waals materials have opened up an exciting new field in condensed matter physics. These systems exhibit a rich phase diagram of novel physical phenomena and exotic correlated phases that emerge in the low-dispersing bands. Transition metal dichalcogenides, in particular, molybdenum disulfide (MoS2), are potential candidates to extend the studies on moir'e electronics beyond graphene. Our transport spectroscopy measurements and analysis reveal a correlation-driven phase transition and the emergence of discrete mini-bands in MoS2 moir'e superlattices that remained elusive so far. We resolve these mini-bands arising from quantum mechanical tunneling through Schottky barriers between the MoS2 and its metallic leads. Energy scales deduced from a first approach exhibit an astounding agreement with our experimental observations. The behavior under thermal activation suggests a Lifshitz phase transition at low temperatures that is driven by a complete spin-valley symmetry breaking. These intriguing observations bring out the potential of twisted MoS2 to explore correlated electron states and associated physics.

cond-mat.mes-hall↗

arXiv2022

In-plane optical phonon modes of current-carrying graphene

In this work, we study the in-plane optical phonon modes of current-carrying single-layer graphene whose coupling to the $π$ electron gas is strong. Such modes are expected to undergo a frequency shift compared to the non-current-carrying state due to the non-equilibrium occupation of the Dirac cone electronic eigen-states with the flowing $π$ electron gas. Large electron-phonon coupling (EPC) can be identified by an abrupt change in the slope of the phonon mode dispersion known as the Kohn anomaly, which mainly occurs for (i) the in-plane longitudinal/transverse optical (LO/TO) modes at the Brillouin zone (BZ) center ($Γ$ point), and (ii) the TO modes at the BZ corners ($K$ points). We show that the breaking of the rotational symmetry by the DC current results in different frequency shifts to the $Γ$-TO and $Γ$-LO modes. More specifically, the DC current breaks the TO-LO mode degeneracy at the $Γ$ point which ideally would be manifested as the splitting of the Raman G peak.

cond-mat.mes-hall↗

arXiv2021

Theory of plasmonic edge states in chiral bilayer systems

We analytically describe the plasmonic edge modes for an interface that involves the twisted bilayer graphene (TBG) or other similar Moire van der Waals heterostructure. For this purpose, we employ a spatially homogeneous, isotropic and frequency-dependent tensor conductivity which in principle accounts for electronic and electrostatic interlayer couplings. We predict that the edge mode dispersion relation explicitly depends on the chiral response even in the nonretarded limit, in contrast to the collective bulk plasmonic excitations in the TBG. We obtain a universal function for the dispersion of the optical edge plasmon in the paramagnetic regime. This implies a correspondence of the chiral-TBG optical plasmon to a magnetoplasmon of a single sheet, and chirality is interpreted as an effective magnetic field. The chirality also opens up the possibility of nearly undamped acoustic modes in the paramagnetic regime. Our results may guide future near-field nanoscopy for van der Waals heterostructures. In our analysis, we retain the long-range electrostatic interaction, and apply the Wiener-Hopf method to a system of integral equations for the scalar potentials of the two layers.

cond-mat.mes-hall↗

arXiv2021

Hyperbolic enhancement of photocurrent patterns in minimally twisted bilayer graphene

Quasi-periodic moiré patterns and their effect on electronic properties of twisted bilayer graphene (TBG) have been intensely studied. At small twist angle $θ$, due to atomic reconstruction, the moiré superlattice morphs into a network of narrow domain walls separating micron-scale AB and BA stacking regions. We use scanning probe photocurrent imaging to resolve nanoscale variations of the Seebeck coefficient occurring at these domain walls. The observed features become enhanced in a range of mid-infrared frequencies where the hexagonal boron nitride (hBN), which we use as a TBG substrate, is optically hyperbolic. Our results illustrate new capabilities of nano-photocurrent technique for probing nanoscale electronic inhomogeneities in two-dimensional materials.

cond-mat.mes-hall↗

arXiv2020

Chiral plasmons with twisted atomic bilayers

Van der Waals heterostructures of atomically thin layers with rotational misalignments, such as twisted bilayer graphene, feature interesting structural moiré superlattices. Due to the quantum coupling between the twisted atomic layers, light-matter interaction is inherently chiral; as such, they provide a promising platform for chiral plasmons in the extreme nanoscale. However, while the interlayer quantum coupling can be significant, its influence on chiral plasmons still remains elusive. Here we present the general solutions from full Maxwell equations of chiral plasmons in twisted atomic bilayers, with the consideration of interlayer quantum coupling. We find twisted atomic bilayers have a direct correspondence to the chiral metasurface, which simultaneously possesses chiral and magnetic surface conductivities, besides the common electric surface conductivity. In other words, the interlayer quantum coupling in twisted van der Waals heterostructures may facilitate the construction of various (e.g., bi-anisotropic) atomically-thin metasurfaces. Moreover, the chiral surface conductivity, determined by the interlayer quantum coupling, determines the existence of chiral plasmons and leads to a unique phase relationship (i.e., +/-π/2 phase difference) between their TE and TM wave components. Importantly, such a unique phase relationship for chiral plasmons can be exploited to construct the missing longitudinal spin of plasmons, besides the common transverse spin of plasmons.

physics.optics↗