WSe2: Insights Into Superconductivity and Twisted Layers

Tungsten diselenide (WSe₂) has gained attention for its unique electronic and optical behaviors, particularly in monolayer and bilayer forms. Recent research highlights how twisting layers of WSe₂ at specific angles can lead to emergent properties such as superconductivity, making it a key material in condensed matter physics.

Understanding the interplay between crystallographic structure, electronic interactions, and external influences like spin-orbit coupling provides valuable insights into these phenomena.

Crystallographic Features And Composition

WSe₂ crystallizes in a layered hexagonal structure, belonging to the transition metal dichalcogenide (TMD) family. Each monolayer consists of a tungsten (W) atomic plane sandwiched between two selenium (Se) layers, forming a trigonal prismatic coordination. This arrangement results in strong in-plane covalent bonding, while adjacent layers are held together by weak van der Waals forces, allowing for easy exfoliation into monolayers. The hexagonal symmetry of WSe₂ influences its anisotropic electronic and mechanical properties in both bulk and few-layer forms.

The stacking order of WSe₂ layers varies, with the 2H and 1T’ phases being the most common. The 2H phase, which is semiconducting, features a staggered stacking where each tungsten atom in one layer aligns with the center of a hexagon in the adjacent layer. In contrast, the 1T’ phase exhibits a distorted octahedral coordination, often associated with metallic or topologically nontrivial states. These phases can transition under external stimuli such as pressure or strain, adding to the material’s tunability.

Defects and doping further modify WSe₂’s crystallographic landscape. Selenium vacancies introduce localized states within the bandgap, altering charge transport. Intentional doping with elements like niobium or rhenium shifts the electronic structure, enabling precise control over carrier concentration. These modifications are particularly relevant for applications requiring tailored conductivity or enhanced catalytic activity.

Electronic And Optical Properties

The electronic structure of WSe₂ depends on its dimensionality. Monolayers exhibit a direct bandgap, while bulk counterparts display an indirect bandgap due to quantum confinement effects. In monolayers, the direct bandgap of approximately 1.65 eV enables strong light-matter interactions, making WSe₂ promising for optoelectronic applications. As thickness increases, interlayer coupling shifts the band extrema, suppressing photoluminescence efficiency.

Excitonic effects dominate WSe₂’s optical response due to reduced dielectric screening and strong Coulomb interactions. Tightly bound excitons, with binding energies exceeding 300 meV, result in pronounced absorption and emission features. The material also supports trions—charged excitonic complexes—whose formation depends on carrier density and temperature. These quasiparticles contribute to tunable optical properties, as external gating or doping can modulate their population.

Beyond neutral and charged excitons, WSe₂ exhibits higher-order excitonic states such as biexcitons and exciton-polaritons when coupled to optical cavities. These interactions are relevant for nonlinear optical applications, where exciton recombination dynamics influence quantum optics. Strong spin-orbit coupling introduces valley-dependent optical selection rules, allowing circularly polarized light to selectively excite carriers in distinct valleys of the Brillouin zone. This characteristic supports valleytronic applications, where information is encoded in the valley degree of freedom.

Twisted Bilayer Observations

When two monolayers of WSe₂ are stacked with a controlled rotational misalignment, new electronic and structural interactions emerge. These twisted bilayers form moiré superlattices, where periodic potential variations modulate charge distribution and band structure. The resulting moiré potential can create flat electronic bands at specific twist angles, significantly altering charge carrier dynamics.

At small twist angles, moiré patterns lead to minibands that modify the density of states and enhance electron correlation effects. This can result in insulating states at fractional fillings, where electron interactions dictate charge transport. Scanning tunneling microscopy has revealed localized states within the superlattice, demonstrating how twist-induced periodic potentials reshape the energy landscape. Angle-resolved photoemission spectroscopy (ARPES) confirms that certain twist angles induce band flattening, enhancing electron-electron interactions and enabling novel quantum phases.

Interlayer hybridization and moiré modulation also affect excitonic behavior. Twisted bilayer WSe₂ exhibits spatially confined interlayer excitons, where electron-hole pairs are separated across individual monolayers yet remain bound by Coulomb attraction. These excitons display extended lifetimes compared to monolayer counterparts due to reduced recombination rates, making them appealing for optoelectronic applications. Localized excitonic states within moiré potential wells introduce possibilities for single-photon emission, relevant for quantum information processing.

Superconducting Phases

Superconductivity in WSe₂ has drawn interest due to its unconventional characteristics and tunability. Unlike conventional BCS superconductors, where electron-phonon interactions drive Cooper pair formation, WSe₂ exhibits superconducting behavior influenced by strong spin-orbit coupling and electron correlations. This makes it a compelling platform for studying exotic pairing mechanisms, particularly in doped or twisted configurations where moiré engineering modifies the electronic environment.

Superconductivity in WSe₂ can be induced through electrostatic gating, chemical doping, or pressure application. Electrolyte gating effectively tunes carrier density, with studies showing superconductivity onset at critical temperatures around 2–4 K in gated monolayers. Unlike bulk WSe₂, which remains semiconducting under ambient conditions, surface-modified layers exhibit metallic behavior at high carrier concentrations, transitioning into a superconducting phase at low temperatures. This suggests a delicate balance between charge screening and coupling strength in stabilizing the superconducting state.

In twisted bilayer systems, superconductivity emerges similarly to magic-angle graphene, where flat bands enhance electron correlations. The presence of a moiré superlattice modifies the density of states, promoting strong pairing interactions that may lead to unconventional superconducting phases. Theoretical models suggest these states could host topological superconductivity, with implications for fault-tolerant quantum computing. While experimental verification is ongoing, transport measurements have revealed zero-resistance states at low temperatures, indicative of superconducting order.

Spin-Orbit Coupling And Valley Physics

Strong spin-orbit coupling in WSe₂ plays a defining role in its electronic and optical properties. Tungsten’s heavy atomic mass introduces significant spin splitting in the valence band, leading to spin-layer locking effects that influence charge transport and optical transitions. Unlike conventional semiconductors, WSe₂ exhibits pronounced spin splitting at the K and K’ points of the Brillouin zone, with energy differences exceeding 400 meV. This makes it a prime candidate for spintronic applications, where spin-polarized currents can be manipulated without external magnetic fields. Spin-orbit interaction also protects spin-valley polarization against rapid relaxation, enabling longer coherence times crucial for quantum information processing.

Beyond spin-orbit effects, the valley degree of freedom in WSe₂ introduces additional tunability. Electrons and holes in distinct valleys can be selectively excited using circularly polarized light, a feature leveraged for valleytronic devices. This optical addressability enables encoding information in valley states, offering an alternative to charge-based logic. Interlayer excitons in bilayer and twisted WSe₂ structures retain valley-dependent optical selection rules, leading to long-lived valley polarization due to spatial charge separation. Experimental demonstrations show valley coherence persisting over nanosecond timescales, suggesting potential for robust valley-based information storage. These combined effects of spin-orbit coupling and valley physics position WSe₂ at the forefront of emerging quantum technologies, where precise control over electronic states is paramount.