arXiv subjects

Samuel Magorrian

Publications and source records attributed to Samuel Magorrian.

5 recordsLinked to original sources

arXiv2026

Momentum-space engineering in van der Waals heterostructures

Twenty years of graphene research have established Dirac fermions as a versatile platform for exploring quantum topological phenomena. Yet control over graphene's low-energy spectrum mostly follows the route of miniband engineering using real-space superlattices. Here we introduce a complementary approach based on momentum-space 'Umklapp' band engineering. This is realised by matching reciprocal space positions of band edges of two adjacent highly incommensurate 2D crystals to coincide upon a boost from their Bragg vectors, creating resonant interlayer hybridisation that qualitatively reconstructs their spectra. To demonstrate this approach experimentally, we use monolayer graphene on MoS$_2$, twisted such that MoS$_2$ Q-valley states are brought by Umklapp into energy-momentum resonance with graphene's Dirac cones. The ensuing hybridization results in the formation of a clear additional Dirac-like spectral feature, identified by a distinct Landau-level fan and Shubnikov-de Haas oscillations characteristic to Berry phase $π$ electrons. These new topological spectral features create a spin-valley locked Ising system which is robust against small twist-angle variations that can be further tuned by an external displacement field. More broadly, Umklapp band engineering offers a practical strategy for tailoring the electronic structure in van der Waals heterostructures, expanding the design space of quantum materials beyond moiré superlattices and weak proximity effects.

cond-mat.mes-hall↗

arXiv2021

Band energy landscapes in twisted homobilayers of transition metal dichalcogenides

Twistronic assembly of 2D materials employs the twist angle between adjacent layers as a tuning parameter for designing the electronic and optical properties of van der Waals heterostructures. Here, we study how interlayer hybridization, weak ferroelectric charge transfer between layers, and piezoelectric response to deformations set the valence and conduction band edges across the moir{é} supercell in twistronic homobilayers of MoS$_2$, MoSe$_2$, WS$_2$ and WSe$_2$. We show that, due to the lack of inversion symmetry in the monolayer crystals, bilayers with parallel (P) and anti-parallel (AP) unit cell orientations display contrasting behaviors. For P-bilayers at small twist angles we find band edges in the middle of triangular domains of preferential stacking. In AP-bilayers at marginal twist angles ($θ_{AP} < 1^\circ$) the band edges are located in small regions around the intersections of domain walls, giving highly localized quantum dot states.

cond-mat.mes-hall↗

arXiv2021

Ghost anti-crossings caused by interlayer umklapp hybridization of bands in 2D heterostructures

In two-dimensional heterostructures, crystalline atomic layers with differing lattice parameters can stack directly one on another. The resultant close proximity of atomic lattices with differing periodicity can lead to new phenomena. For umklapp processes, this opens the possibility for interlayer umklapp scattering, where interactions are mediated by the transfer of momenta to or from the lattice in the neighbouring layer. Using angle-resolved photoemission spectroscopy to study a graphene on InSe heterostructure, we present evidence that interlayer umklapp processes can cause hybridization between bands from neighbouring layers in regions of the Brillouin zone where bands from only one layer are expected, despite no evidence for moir/'e-induced replica bands. This phenomenon manifests itself as 'ghost' anti-crossings in the InSe electronic dispersion. Applied to a range of suitable 2DM pairs, this phenomenon of interlayer umklapp hybridization can be used to create strong mixing of their electronic states, giving a new tool for twist-controlled band structure engineering.

cond-mat.mes-hall↗

arXiv2020

Atomic reconstruction in twisted bilayers of transition metal dichalcogenides

Van der Waals heterostructures form a massive interdisciplinary research field, fueled by the rich material science opportunities presented by layer assembly of artificial solids with controlled composition, order and relative rotation of adjacent atomic planes. Here we use atomic resolution transmission electron microscopy and multiscale modeling to show that the lattice of MoS$_2$ and WS$_2$ bilayers twisted to a small angle, $θ<3^{\circ}$, reconstructs into energetically favorable stacking domains separated by a network of stacking faults. For crystal alignments close to 3R stacking, a tessellated pattern of mirror reflected triangular 3R domains emerges, separated by a network of partial dislocations which persist to the smallest twist angles. Scanning tunneling measurements show that the electronic properties of those 3R domains appear qualitatively different from 2H TMDs, featuring layer-polarized conduction band states caused by lack of both inversion and mirror symmetry. In contrast, for alignments close to 2H stacking, stable 2H domains dominate, with nuclei of an earlier unnoticed metastable phase limited to $\sim$ 5nm in size. This appears as a kagome-like pattern at $θ\sim 1^{\circ}$, transitioning at $θ\rightarrow 0$ to a hexagonal array of screw dislocations separating large-area 2H domains.

cond-mat.mes-hall↗

arXiv2019

Ultra-thin van der Waals crystals as semiconductor quantum wells

Control over the electronic spectrum at low energy is at the heart of the functioning of modern advanced electronics: high electron mobility transistors, semiconductor and Capasso terahertz lasers, and many others. Most of those devices rely on the meticulous engineering of the size quantization of electrons in quantum wells. This avenue, however, hasn't been explored in the case of 2D materials. Here we transfer this concept onto the van der Waals heterostructures which utilize few-layers films of InSe as quantum wells. The precise control over the energy of the subbands and their uniformity guarantees extremely high quality of the electronic transport in such systems. Using novel tunnelling and light emitting devices, for the first time we reveal the full subbands structure by studying resonance features in the tunnelling current, photoabsorption and light emission. In the future, these systems will allow development of elementary blocks for atomically thin infrared and THz light sources based on intersubband optical transitions in few-layer films of van der Waals materials.

cond-mat.mes-hall↗