arXiv subjects

Neil R. Wilson

Publications and source records attributed to Neil R. Wilson.

14 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↗

arXiv2025

The two conduction bands of monolayer CrSBr on Au

We report the electronic structure of monolayer CrSBr exfoliated onto mica template-stripped gold substrates. Angle-resolved photoemission spectroscopy reveals charge transfer from the substrate, populating the conduction band of monolayer CrSBr, accompanied by a pronounced reduction in the quasiparticle band gap. Furthermore, we observe two separate conduction bands that exhibit a splitting at the X point. This indicates a breaking of glide-mirror symmetry, which in the bulk or in a free-standing monolayer protects the band degeneracies at the Brillouin zone boundary. Our results demonstrate that ultraflat gold substrates do more than modify carrier densities and screening: they can lift symmetry-protected degeneracies and thus fundamentally reshape the band topology of 2D materials.

cond-mat.str-el↗

arXiv2025

Electronic structure of the interface between Au and WSe2

Understanding the interface between metals and two-dimensional materials is critical for their application in electronics and for the development of metal-mediated exfoliation of large area monolayers. Studying the intricate interactions at the interface requires model systems that enable control of the roughness, purity, and crystallinity of the metal surface. Here, we investigate the layer-dependent electronic structure of WSe_2 on template-stripped gold substrates fabricated using both silicon and mica templates, giving crystallographically disordered and Au(111) ordered surfaces, respectively, and contrast these findings with ab initio predictions. We observe strong hybridization around the Brillouin zone centre at $\overlineΓ$, indicating a covalent admixture in the gold-\WSe~interaction, and band shifts that suggest charge rearrangement at the Au(111) / WSe_2 interface. Core-level spectroscopy shows a single chemical environment for the interfacial WSe_2 layer on the template-stripped gold, distinct from the subsequent layers. These results reveal a mixture of van der Waals and covalent interactions, best described as a covalent-like quasi-bonding with intermediate interaction strength.

cond-mat.mtrl-sci↗

arXiv2024

Giant exchange splitting in the electronic structure of A-type 2D antiferromagnet CrSBr

We present the evolution of the electronic structure of CrSBr from its antiferromagnetic ground state to the paramagnetic phase above T_N=132 K, in both experiment and theory. Low temperature angle-resolved photoemission spectroscopy (ARPES) results are obtained using a novel method to overcome sample charging issues, revealing quasi-2D valence bands in the ground state. The results are very well reproduced by our QSGŴ calculations, which further identify certain bands at the X points to be exchange-split pairs of states with mainly Br and S character. By tracing band positions as a function of temperature, we show the splitting disappears above T_N. The energy splitting is interpreted as an effective exchange splitting in individual layers in which the Cr moments all align, within the so-called A-type antiferromagnetic arrangement. Our results lay firm foundations for the interpretation of the many other intriguing physical and optical properties of CrSBr.

cond-mat.str-el↗

arXiv2023

Revealing the conduction band and pseudovector potential in 2D moiré semiconductors

Stacking monolayer semiconductors results in moiré patterns that host many correlated and topological electronic phenomena, but measurements of the basic electronic structure underpinning these phenomena are scarce. Here, we investigate the properties of the conduction band in moiré heterobilayers using submicron angle-resolved photoemission spectroscopy with electrostatic gating, focusing on the example of WS2/WSe2. We find that at all twist angles the conduction band edge is the K-point valley of the WS2, with a band gap of 1.58 +- 0.03 eV. By resolving the conduction band dispersion, we observe an unexpectedly small effective mass of 0.15 +- 0.02 m_e. In addition, we observe replicas of the conduction band displaced by reciprocal lattice vectors of the moiré superlattice. We present arguments and evidence that the replicas are due to modification of the conduction band states by the moiré potential rather than to final-state diffraction. Interestingly, the replicas display an intensity pattern with reduced, 3-fold symmetry, which we show implicates the pseudo vector potential associated with in-plane strain in moiré band formation.

cond-mat.mes-hall↗

arXiv2021

Strong In-plane Anisotropy in the Electronic Properties of Doped Transition Metal Dichalcogenides exhibited in W1-xNbxS2

In this work, we study the electronic properties of monolayer transition metal dichalcogenide materials subjected to aliovalent doping, using Nb-doped WS2 as an exemplar. Scanning transmission electron microscopy imaging of the as-grown samples reveals an anisotropic Nb dopant distribution, prompting our investigation of anisotropy in electronic properties. Through electronic structure calculations on supercells representative of observed structures, we confirm that local Nb-atom distributions are consistent with energetic considerations, although kinetic processes occurring during sample growth must be invoked to explain the overall symmetry-breaking. We perform effective bandstructure and conductivity calculations on realistic models of the material that demonstrate that a high level of anisotropy can be expected in electronic properties including conductivity and mobility.

cond-mat.mtrl-sci↗

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 and electronic structure of two-dimensional Mo(1-x)WxS2 alloys

Alloying enables engineering of the electronic structure of semiconductors for optoelectronic applications. Due to their similar lattice parameters, the two-dimensional semiconducting transition metal dichalcogenides of the MoWSeS group (MX2 where M= Mo or W and X=S or Se) can be grown as high-quality materials with low defect concentrations. Here we investigate the atomic and electronic structure of Mo(1-x)WxS2 alloys using a combination of high-resolution experimental techniques and simulations. Analysis of the Mo and W atomic positions in these alloys, grown by chemical vapour transport, shows that they are randomly distributed, consistent with Monte Carlo simulations that use interaction energies determined from first-principles calculations. Electronic structure parameters are directly determined from angle resolved photoemission spectroscopy measurements. These show that the spin-orbit splitting at the valence band edge increases linearly with W content from MoS2 to WS2, in agreement with linear-scaling density functional theory (LS-DFT) predictions. The spin-orbit splitting at the conduction band edge is predicted to reduce to zero at intermediate compositions. Despite this, polarisation-resolved photoluminescence spectra on monolayer Mo0.5W0.5S2 show significant circular dichroism, indicating that spin-valley locking is retained. These results demonstrate that alloying is an important tool for controlling the electronic structure of MX2 for spintronic and valleytronic applications.

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↗

arXiv2019

Visualizing electrostatic gating effects in two-dimensional heterostructures

The ability to directly observe electronic band structure in modern nanoscale field-effect devices could transform understanding of their physics and function. One could, for example, visualize local changes in the electrical and chemical potentials as a gate voltage is applied. One could also study intriguing physical phenomena such as electrically induced topological transitions and many-body spectral reconstructions. Here we show that submicron angle-resolved photoemission (micro-ARPES) applied to two-dimensional (2D) van der Waals heterostructures affords this ability. In graphene devices, we observe a shift of the chemical potential by 0.6 eV across the Dirac point as a gate voltage is applied. In several 2D semiconductors we see the conduction band edge appear as electrons accumulate, establishing its energy and momentum, and observe significant band-gap renormalization at low densities. We also show that micro-ARPES and optical spectroscopy can be applied to a single device, allowing rigorous study of the relationship between gate-controlled electronic and excitonic properties.

cond-mat.mes-hall↗

arXiv2019

Indirect to direct gap crossover in two-dimensional InSe revealed by ARPES

Atomically thin films of III-VI post-transition metal chalcogenides (InSe and GaSe) form an interesting class of two-dimensional semiconductor that feature strong variations of their band gap as a function of the number of layers in the crystal [1-4] and, specifically for InSe, an earlier predicted crossover from a direct gap in the bulk [5,6] to a weakly indirect band gap in monolayers and bilayers [7-11]. Here, we apply angle resolved photoemission spectroscopy with submicrometer spatial resolution ($μ$ARPES) to visualise the layer-dependent valence band structure of mechanically exfoliated crystals of InSe. We show that for 1 layer and 2 layer InSe the valence band maxima are away from the $\mathbfΓ$-point, forming an indirect gap, with the conduction band edge known to be at the $\mathbfΓ$-point. In contrast, for six or more layers the bandgap becomes direct, in good agreement with theoretical predictions. The high-quality monolayer and bilayer samples enables us to resolve, in the photoluminescence spectra, the band-edge exciton (A) from the exciton (B) involving holes in a pair of deeper valence bands, degenerate at $\mathbfΓ$, with the splitting that agrees with both $μ$ARPES data and the results of DFT modelling. Due to the difference in symmetry between these two valence bands, light emitted by the A-exciton should be predominantly polarised perpendicular to the plane of the two-dimensional crystal, which we have verified for few-layer InSe crystals.

cond-mat.mtrl-sci↗

arXiv2016

Band parameters and hybridization in 2D semiconductor heterostructures from photoemission spectroscopy

Combining monolayers of different two-dimensional (2D) semiconductors into heterostructures opens up a wealth of possibilities for novel electronic and optical functionalities. Exploiting them hinges on accurate measurements of the band parameters and orbital hybridization in separate and stacked monolayers, many of which are only available as small samples. The recently introduced technique of angle-resolved photoemission spectroscopy with submicron spatial resolution (μ-ARPES) offers the capability to measure small samples, but the energy resolution obtained for such exfoliated samples to date (~0.5 eV) has been inadequate. Here, we show that by suitable heterostructure sample design the full potential of μ-ARPES can be realized. We focus on MoSe2/WSe2 van der Waals heterostructures, which are 2D analogs of 3D semiconductor heterostructures. We find that in a MoSe2/WSe2 heterobilayer the bands in the K valleys are weakly hybridized, with the conduction and valence band edges originating in the MoSe2 and WSe2 respectively. There is stronger hybridization at the Γ point, but the valence band edge remains at the K points. This is consistent with the recent observation of interlayer excitons where the electron and hole are valley polarized but in opposite layers. We determine the valence band offset to be 300 meV, which combined with photoluminescence measurements implies that the binding energy of interlayer excitons is at least 200 meV, comparable with that of intralayer excitons.

cond-mat.mes-hall↗

arXiv2013

Friction force microscopy: a simple technique for identifying graphene on rough substrates and mapping the orientation of graphene grains on copper

At a single atom thick, it is challenging to distinguish graphene from its substrate using conventional techniques. In this paper we show that friction force microscopy (FFM) is a simple and quick technique for identifying graphene on a range of samples, from growth substrates to rough insulators. We show that FFM is particularly effective for characterising graphene grown on copper where it can correlate the graphene growth to the three-dimensional surface topography and map the crystallographic orientation of the graphene nondestructively, reproducibly and at high resolution. We expect FFM to be similarly effective for studying graphene growth on other metal/locally crystalline substrates, including SiC, and for studying growth of other two-dimensional materials such as molybdenum disulphide and hexagonal boron nitride.

cond-mat.mtrl-sci↗