Bent Weber

Coulomb interactions among graphene's massless Dirac fermions are unusually strong, making graphene a unique testbed for correlated quantum phases and interacting field theories. Among the predicted phases is an excitonic insulator, in which strong interactions drive spontaneous electron-hole pairing and condensation into a superfluid ground state. This transition has evaded experimental observation because unscreened long-range Coulomb interactions renormalize and enhance the Dirac velocity, keeping graphene in a weakly coupled semimetallic regime. Here, we overcome this constraint by decorating graphene with nanoscale metals that screen long-range interactions and suppress velocity renormalization. Scanning tunnelling spectroscopy down to 150 mK reveals a hard spectral gap (2Δ ≈ 14 meV) that emerges strictly at the Dirac point and is strongly susceptible to doping, temperature, and magnetic field --- signatures of a many-body rather than single-particle gap. The gap closes at an anomalously low critical temperature (Δ/ k _B T _c ≥ 100), consistent with a Berezinskii--Kosterlitz--Thouless transition, and coincides with a charge-density wave at q = K' - K that reflects intervalley excitonic pairing. Together, these results provide strong evidence for spontaneous excitonic mass generation in graphene, a correlated insulating phase long predicted but never before observed.

Amorphous solids represent the extreme limit of structural disorder, in which broken translational symmetry challenges the Bloch description of electronic states. In strictly two dimensions (2D), lattice disorder should lead to complete localization electronic states. In stark contrast, here we show that 2D monolayer amorphous carbon (MAC)– a chemically uniform but maximally disordered random network of π-conjugated carbon retains a quantum-critical extended state at the band centre (E ∼ 0)despite the absence of crystalline order. This is confirmed by correlating lattice disorder with local wavefunction structure in atomic-resolution scanning tunnelling microscopy and spectroscopy. Our statistical analysis of the wavefunction probability density reveals a diverging correlation length and verifies the multifractal scaling relation η = −∆ ₂ – characteristic of Anderson criticality– in agreement with atomistic tight-binding calculations. We attribute the presence of this critical state to an emergent chiral symmetry of the continuous random network, described by a Wess-Zumino Witten (WZW) topological term that protects the band-centre. Our results thus identify topological disorder as stabilizing agent for quantum criticality in a strictly 2D amorphous system.

The quantum spin Hall (QSH) effect represents a prototypical 2D topological phase in which time‐reversal symmetry protects dissipationless, spin‐polarized helical edge states coexisting with an insulating bulk. Owing to its magnetic‐field‐free operation, intrinsic spin–momentum locking, and electrically tunable topology, the QSH state has long been regarded as a promising platform for ultra‐low‐power, ultra‐fast electronics, spintronics, and topological quantum computation. Despite this compelling vision, experimental realizations of QSH‐based functionalities remain confined to a limited number of material systems and device geometries, typically operating at cryogenic temperatures and over restricted length scales. In this Perspective, we critically reassess the field by synthesizing the central promises of the QSH effect and analyzing the intrinsic material limitations and extrinsic functional bottlenecks that have hindered its technological translation. We argue that future progress will depend not only on the discovery of novel QSH materials but also on the engineering of topological functionality through large‐gap systems, chemically robust platforms, and interaction‐ or proximity‐induced phases. Viewed from this perspective, the QSH effect emerges not as a stalled technology, but as a foundational framework for designing next‐generation electronic, spintronic, and quantum devices.

Atomically confined spins are emerging as active components in quantum optoelectronic devices such as quantum bits and sensors. However, interrogating single spins at atomic length scales remains a sizable challenge, limited by diffraction in conventional optics. Here, we show that the highly local excitation provided by injecting energetic charge carriers from the atomically sharp probe of a scanning tunneling microscope can trigger single-photon emission from individual atomic vacancy centers in a layered semiconductor. With an effective spatial resolution of <1 nm, we show that the captured light closely mirrors the orbital symmetry of the bound-state wave function of the vacancy center, while photon-correlation measurements confirm single-photon emission, as reflected in clear photon antibunching signatures. Our results constitute an important step toward the realization of an electrically addressable single-atom quantum light source and solid-state spin-photon interface, addressed at the atomic scale.