Observing Atomic Collapse Resonances in Artificial Nuclei on Graphene (1510.02890v1)

Abstract: Relativistic quantum mechanics predicts that when the charge of a superheavy atomic nucleus surpasses a certain threshold, the resulting strong Coulomb field causes an unusual atomic collapse state; this state exhibits an electron wave function component that falls toward the nucleus, as well as a positron component that escapes to infinity. In graphene, where charge carriers behave as massless relativistic particles, it has been predicted that highly charged impurities should exhibit resonances corresponding to these atomic collapse states. We have observed the formation of such resonances around artificial nuclei (clusters of charged calcium dimers) fabricated on gated graphene devices via atomic manipulation with a scanning tunneling microscope. The energy and spatial dependence of the atomic collapse state measured with scanning tunneling microscopy revealed unexpected behavior when occupied by electrons.

• The paper demonstrates that fabricating tunable artificial nuclei on graphene using Ca dimer clusters produces atomic collapse resonances as predicted by relativistic quantum mechanics.
• The study employs STM dI/dV mapping to reveal spatially extended, radially symmetric collapse states over 10 nm, closely mirroring theoretical models.
• The research shows that gate voltage modulation significantly influences resonance energy and intensity, highlighting critical electron-electron interaction effects in graphene.

Atomic Collapse Resonances in Graphene-Based Artificial Nuclei

The paper "Observing Atomic Collapse Resonances in Artificial Nuclei on Graphene" explores the phenomenon of atomic collapse states in graphene, a robust two-dimensional material where charge carriers exhibit properties analogous to massless relativistic particles. The paper provides experimental evidence for the formation of atomic collapse resonances, a phenomenon predicted by relativistic quantum mechanics, in controlled graphene-based environments.

Relativistic quantum mechanics predicts that super-heavy atomic nuclei, characterized by a nuclear charge $Z$ exceeding a critical threshold $Z_c$, would become unstable in their electronic states. At this supercritical regime, the electron wave function includes a component that spirals towards the nucleus and another that extends outward—a condition termed atomic collapse. Previous attempts to paper this phenomenon in traditional atomic systems required exceedingly high nuclear charges, unfeasible for laboratory synthesis, thus remaining largely theoretical until graphene-based systems emerged as a viable alternative.

This research exploits the unique electronic properties of graphene to overcome the limitations faced when studying atomic collapse in traditional settings. In graphene, the charge carriers (Dirac fermions) and their strong Coulombic interactions allow for the realization of supercritical charge phenomena at much lower values of $Z$.

Experimental Approach and Observations

The authors fabricated artificial nuclei on a graphene substrate using ionized calcium (Ca) dimers. Using a scanning tunneling microscope (STM), they manipulated these dimers to create tunable supercritical Coulomb centers. The STM was also employed to observe the electronic characteristics of graphene near these artificial nuclei, providing dI/dV spectroscopy measurements, which revealed an electron-like resonance indicating the presence of atomic collapse states.

Key observations were as follows:

1. Charge and Resonance: The researchers identified a quasi-bound state as the effective charge of the Ca clusters increased, supporting the transition from subcritical to supercritical regimes. This transition was marked by a distinct resonance in the local density of states (LDOS) near the Fermi level of graphene, observable as the number of dimers increased from three to five.
2. Spatial Distribution: The STM-based dI/dV mapping demonstrated a radial symmetry in the collapse state intensity around a 5-dimer cluster, extending over 10 nm from the cluster’s center. This spatial behavior is consistent with theoretical predictions for atomic collapse resonances.
3. Gate Voltage Influence: The paper revealed that the energy and visibility of the atomic collapse states varied with electric doping. Under p-doping, these states shifted in energy towards the Dirac point. Conversely, n-doping resulted in the states being populated by carriers, leading to a significant quenching in intensity, suggesting strong electron-electron interactions.

Implications and Theoretical Considerations

The experimental confirmation of atomic collapse in graphene opens up new avenues in exploring relativistic quantum phenomena in more accessible conditions. This research corroborates theoretical predictions using Dirac equation simulations, integrating effective charge calculations and density functional theory (DFT) to validate the observed phenomena.

The implications of this work are twofold. Firstly, it demonstrates the power of using graphene and similar two-dimensional materials to explore quantum behavior that traditionally requires extreme conditions. Secondly, the phenomena could be harnessed for novel electronic and optoelectronic applications, where supercritical charge dynamics are pivotal.

Moving forward, this paper suggests several potential directions for further exploration, including the effects of different substrate materials on the atomic collapse behavior and the exploration of multi-state spectra suggested by the underlying theory. Additionally, the complex interactions within the n-doping regime spotlight the need for deeper analyses into electron correlation effects in graphene.

In conclusion, this work represents a significant advancement in the experimental observation of atomic collapse states, leveraging the tunability of graphene-based systems to investigate fundamental questions in quantum electrodynamics in practical and scalable settings.