Higher Order Van Hove Singularities and Precision Engineering Propel Quantum Technology Forward

Insider Brief:

• Quantum technology advancements depend on multiple factors, including further understanding of quantum materials with properties like high-temperature superconductivity, quantum phase transitions, and topological traits, driven by their highly correlated electrons.

• A recent study from Loughborough University and St. Andrews identifies higher-order Van Hove singularities as relevant features that amplify electron interactions, enabling the tuning of material properties for applications such as superconductivity and quantum devices.

• Advanced tools like density functional theory, angle-resolved photoemission spectroscopy, and scanning tunneling microscopy reveal how subtle structural changes, such as RuO₆ octahedral rotation, can stabilize HOVHS and induce unique electronic behavior.

• HOVHS offer opportunities to design quantum materials with tailored properties for next-generation technologies, though precise control and scalability remain significant challenges for practical applications.

The advancement of quantum technology relies on more than just the state of qubits, quantum algorithms, or reliable logical operations; it is the culmination of numerous interconnected components, including the study of quantum materials. These materials, characterized by their highly correlated electrons, exhibit properties such as high-temperature superconductivity, quantum phase transitions (transitions driven by quantum fluctuations rather than thermal energy, as seen in classical phase transitions), and topological traits.

While their potential applications in quantum technologies are promising, a deeper understanding of their underlying mechanisms remains a priority. The quest to understand and manipulate quantum materials often circles back to their electronic structures, as they are heavily influenced by the behavior of their electrons, particularly near features called Van Hove singularities. In a recent Nature Communications study, researchers from Loughborough University and St. Andrews demonstrate how these VHS, particularly their “higher-order” forms, may provide unique opportunities to engineer and enhance the properties of quantum materials for use in quantum technology.

Quantum Materials and Van Hove Singularities

In classical materials, the density of electronic states—essentially the number of ways electrons can occupy energy levels—changes predictably as energy approaches the Fermi level. The Fermi level is the highest energy that electrons occupy at absolute zero, serving as a reference point for how electrons behave in a material. In quantum materials, however, Van Hove singularities disrupt this smooth transition, creating sharp changes in the density of states that amplify interactions between electrons. This amplification is what gives rise to unusual and sometimes exotic phases of matter.

Higher-order Van Hove singularities introduce even greater complexity. These occur when not only the slope but also the curvature of the electronic dispersion vanishes, leading to extreme changes in electronic behavior. Such singularities play a role in phenomena like superconductivity, where materials can conduct electricity without resistance, and could enable fine-tuning of material properties for quantum technologies​.

The material Sr₂RuO₄, studied extensively in this research, is used to demonstrate the impact of these singularities. The material’s surface hosts a two-dimensional electronic system influenced by rotations in the atomic structure, specifically the RuO₆ octahedra. When these octahedra rotate by about 9°, the system undergoes a Lifshitz transition—a shift in the topology of the energy landscape electrons can occupy—ultimately leading to HOVHS.

Tools for Tuning Quantum States

To explore these phenomena, the researchers used advanced experimental and theoretical tools. Techniques like density functional theory calculations, which simulate electronic structures from first principles, and angle-resolved photoemission spectroscopy, which maps how electrons occupy energy levels in materials, provided a precise picture of the material’s behavior. These were complemented by scanning tunneling microscopy, which visualizes surface-level electronic properties at very fine resolutions.

The study showed that subtle changes to structural symmetry, such as tweaking the rotation angle of the RuO₆ octahedra, can notably impact the behavior of electrons. For instance, at specific angles, the density of states transitions from a logarithmic divergence to a power-law divergence with an exponent of -1/4, representing unique electronic behavior that could be key to stabilizing new quantum phases.

Implications for Quantum Technology

Beyond academic curiosity, HOVHS could unlock new possibilities for material design in quantum technologies. These singularities impact electron interactions, potentially stabilizing phases that are relevant for superconductivity or can improve quantum device performance. The study outlines how external parameters like strain or applied fields can be used to control these properties, providing a roadmap for tailoring materials to specific applications​.

For example, materials like Sr₂RuO₄ could serve as platforms for exploring exotic superconducting states or developing devices that lean on their unique electronic properties. The ability to fine-tune these systems makes them promising candidates for next-generation quantum technologies.

Challenges and Future Directions

While promising, the engineering of HOVHS is not without challenges. The sensitivity of these states to competing electronic effects means precise control is essential, and scalability for practical applications remains an open question. Nonetheless, the methodologies developed in this research—combining theoretical and experimental insights—provide a framework for studying and manipulating HOVHS in other materials, such as twisted bilayer graphene or kagome metals​.

Overall, the engineering of higher-order Van Hove singularities represents the intersection between theory and experiment, providing new ways to explore and control the behaviors of quantum materials.

Contributing authors on the study include Anirudh Chandrasekaran, Luke C. Rhodes, Edgar Abarca Morales, Carolina A. Marques, Phil D. C. King, Peter Wahl, and Joseph J. Betouras.