Quantum Hall states, long celebrated for their robust, dissipationless edge modes that arise in two-dimensional electron systems under strong magnetic fields, have mystified scientists regarding the precise impact of electronic interactions along their boundaries. Although fractionalization and interaction effects have been theoretically anticipated, experimental access to the edge’s microscopic structure has remained tantalizingly out of reach, hindered by disorder and the lack of spatial resolution in traditional probes.

The present study targets this challenge head-on by utilizing STM — a technique renowned for atomic-scale imaging and spectroscopy — to directly observe electrostatically defined quantum Hall edges in high-quality graphene devices. The authors map out the spatial distribution and electronic structure of both integer and fractional quantum Hall states with exquisite resolution, revealing a rich tapestry of interaction effects that govern the physics at the one-dimensional chiral channels confined to the sample perimeter.

For the integer quantum Hall effect in the zeroth Landau level, the experiments reveal that electron correlations robustly renormalize the edge-mode velocity, altering the propagation speed from simplistic non-interacting models. More remarkably, the spatial profile of co-propagating edge modes is shown to be dictated by these interactions, producing a layering effect that departs significantly from textbook expectations of non-interacting electrons.

Perhaps the most striking revelation is the emergence of edge valley polarization — an electronic degree of freedom linked to graphene’s band structure — that is qualitatively different from the bulk material. This valley polarization not only signals subtle symmetry-breaking induced by many-body effects localized at the edge, but also challenges conventional mean-field theories, suggesting that fluctuations and inter-channel couplings are critically important and cannot be ignored.

In complementary segments of the study, the authors bravely push into the more delicate domain of fractional quantum Hall phases. Here, the STM spectra reveal interaction-driven signatures characteristic of chiral Luttinger liquid behavior, a hallmark of strongly correlated edge states where elementary excitations fractionalize and conventional quasiparticles dissolve into collective modes. These spectroscopic fingerprints provide some of the clearest experimental verification to date of the exotic physics predicted decades ago in theory.

The implications of this work are twofold: scientifically, it paves a new pathway to unravel complex strongly interacting topological edge modes in situ, bridging gaps between theory and experiment that have persisted for decades. Technologically, understanding and controlling these edge states with such precision offers an unprecedented route towards topological quantum devices that exploit their inherent robustness and exotic excitations.

Crucially, the experimental setup utilizes pristine graphene devices with ultra-clean edges, finely tuned by electrostatic gating to eliminate the disorder that has traditionally obscured microscopic phenomena. This cleanliness and control are vital for observing intrinsic interaction effects without the confounding influence of edge roughness or impurities, thereby ensuring the results reflect fundamental many-body physics.

The research also highlights how some classical approximations—specifically mean-field models—adequately explain certain phenomena such as edge velocity renormalization but falter in capturing the full richness of valley polarization and inter-channel interactions. This indicates the necessity of going beyond mean-field paradigms to fully comprehend the interplay of symmetry, fluctuations, and correlations at quantum edges.

Moreover, the scanning tunneling microscopy approach breaks new ground by enabling spatially resolved spectroscopy of fractional edge states—a feat that has deepened our appreciation of chiral Luttinger liquids and interaction-driven restructuring in topological phases. Such techniques could be adapted to emerging two-dimensional materials hosting fractional Chern insulators and other complex topological orders, expanding the frontier of quantum materials research.

This study marks a pivotal step toward harnessing topological phases not just in the bulk, but at their edges where quantum information processing and novel electronic devices might ultimately operate. By illuminating the intricate electronic landscapes sculpted by interactions, the work propels both fundamental physics and applications closer to reality.

As the field of condensed matter physics continues to grapple with the subtle influence of electronic correlations in topological systems, the ability to directly visualize these effects ushers in an era where theory, spectroscopy, and device engineering can synergize seamlessly. The study’s revelations about graphene’s quantum Hall edges underscore the fertile possibilities when advanced microscopy meets high-purity quantum materials.

Looking forward, these findings invite further exploration into how electron interactions modify other topological boundaries and interfaces, potentially impacting inside-outside physics in nanostructures and device geometries. The insights derived here could inform the design of new quantum platforms where edge modes serve as conduits for robust, low-dissipation current flow or exotic quasiparticle manipulation.

In sum, by charting the elusive restructuring of quantum Hall edge states at an unprecedented level of detail, Yu, Han, Wolinski, and colleagues open a captivating window into the soft, fluctuating, and profoundly correlated world of topological quantum matter’s edges. Their pioneering use of scanning tunneling microscopy as a microscope into the quantum boundary heralds broad new horizons in physics and technology.

Subject of Research: Quantum Hall edge states and interaction-driven modifications in graphene using scanning tunneling microscopy.

Article Title: Visualizing interaction-driven restructuring of quantum Hall edge states.

Article References:
Yu, J., Han, H., Wolinski, K.G. et al. Visualizing interaction-driven restructuring of quantum Hall edge states. Nature 648, 585–590 (2025). https://doi.org/10.1038/s41586-025-09858-3

Image Credits: AI Generated

DOI: 10.1038/s41586-025-09858-3

The quest to identify intrinsic topological superconductors has challenged physicists for decades. These unique materials exhibit exotic surface states that host Majorana fermions—quasi-particles theorized to encode quantum information in a manner inherently resistant to environmental disturbances. While many candidate materials have been proposed, none have conclusively satisfied all the strict criteria to be classified as intrinsic topological superconductors. Now, the team at UCC, leveraging an innovative experimental approach, has decisively evaluated one of the most promising candidates: Uranium ditelluride (UTe₂).

UTe₂, discovered in 2019, rapidly garnered scientific attention for its superconducting and possibly topological properties. Traditional experimental techniques were insufficient to unravel the complexities of this material’s electronic and quantum states. To address these limitations, Professor Séamus Davis of UCC, an expert in quantum physics, developed a novel “Andreev” scanning tunneling microscopy (STM) mode. This technology, available exclusively in three laboratories worldwide—including UCC, Oxford University, and Cornell University—enabled the team to peer beneath the electronic “noise” and directly observe the characteristics indicative of topological superconductivity.

Applying this espionage-like tool, PhD researcher Joe Carroll and Marie Curie postdoctoral fellow Kuanysh Zhussupbekov spearheaded the investigations into UTe₂’s surface states. Unlike conventional STM, which uses metallic probes that inevitably mix signals from trivial surface electrons, the Andreev STM employs a superconducting tip. This methodology filters out the normal electron contributions, isolating the signature of Majorana fermions. The precision of this technique permitted an unprecedented, unequivocal assessment of whether UTe₂ embodies intrinsic topological superconductivity.

The findings revealed that UTe₂ indeed operates as an intrinsic topological superconductor. Yet, intriguingly, it does not conform precisely to the archetype physicists had long sought. Instead, the material’s superconducting and topological wave functions exhibit a more nuanced symmetry and complexity, expanding the conceptual landscape of what qualifies as an intrinsic topological superconductor. These insights deepen the understanding of superconductivity’s quantum mechanical foundations and open new avenues for material synthesis aimed at optimizing quantum information applications.

Professor Davis emphasized the novelty of the approach by explaining that previous techniques relied heavily on metallic probes that contributed extraneous electron signatures, complicating data interpretation. By contrast, this superconductor-based STM eradicates the confounding background electrons, enabling a “pure” visualization of the zero-energy surface states where Majorana modes reside. This purity is critical because these Majorana fermions are theorized to enable quantum bits (qubits) to maintain coherence far longer than conventional systems, a holy grail for quantum computing.

The implications for the quantum computing industry are profound. As global efforts intensify to build scalable quantum processors, the challenges of qubit decoherence and error correction remain formidable obstacles. Synthetic topological superconductors, composed of engineered stacks of conventional materials, have shown promise but introduce complexity and scalability concerns. The UCC team’s demonstration that a single, intrinsic material like UTe₂ can host the requisite topological states hints at the potential to simplify quantum processor architectures significantly.

This breakthrough aligns with initiatives such as Microsoft’s Majorana 1 chip, the world’s first quantum processing unit driven by a topological core using synthetic superconductors. The Davis Group’s methodology offers an alternative strategy—one that could replace engineered heterostructures with straightforward single-crystal materials, reducing fabrication complexity and enhancing qubit density on chips. The ability to incorporate more qubits without proportionally increasing error rates is vital for the realization of practical quantum algorithms capable of tackling classically intractable problems.

Moreover, the use of Andreev STM as a diagnostic instrument transcends the immediate focus on UTe₂. It equips researchers with a universal probe capable of validating other candidate superconductors’ topological nature. This technological leap empowers the scientific community to systematically filter and identify materials with intrinsic topological superconductivity from a vast chemical universe, accelerating discovery cycles and guiding theoretical modeling.

In addition to its technical contributions, this research exemplifies international collaboration’s role in advancing quantum materials science. Input from distinguished theoretical physicist Prof. Dung-Hai Lee (UC Berkeley) and materials synthesis experts from Washington University and the University of Maryland enriched the study’s multidisciplinary depth, ensuring a comprehensive approach bridging theory and experiment.

While UTe₂’s exact topological superconductor class may differ from classic expectations, its confirmation as an intrinsic platform hosting Majorana modes represents an essential puzzle piece in the quantum computing saga. The work sets a new standard for experimental rigor and innovation, marking a hopeful trajectory toward building quantum processors that transcend today’s limitations on coherence time, qubit number, and error rates.

In sum, the Davis Group’s pioneering use of Andreev STM to visualize zero-energy surface states in UTe₂ is more than a scientific milestone—it heralds a new era in material exploration tailored for quantum technologies. Their findings cast light on the subtle symmetries governing superconductivity’s quantum phases and unlock practical pathways toward scalable, fault-tolerant quantum computing hardware. As the quantum race accelerates, such breakthroughs bridge the gaps between fundamental physics, material engineering, and the computational revolutions of tomorrow.

Subject of Research: Quantum materials; intrinsic topological superconductivity; Majorana fermions; Uranium ditelluride (UTe₂); scanning tunneling microscopy (STM) techniques.

Article Title: Pair wave function symmetry in UTe₂ from zero-energy surface-state visualization

News Publication Date: 29-May-2025

Web References: http://dx.doi.org/10.1126/science.adk7219

Image Credits: Clare Keogh (University College Cork)

Vibrational sum-frequency generation (VSFG) spectroscopy has long stood as a cornerstone technique for probing molecular structures at interfaces, owing to its intrinsic surface specificity and sensitivity to molecular vibrations. By simultaneously illuminating a sample with pulsed infrared and near-infrared lasers, VSFG generates a signal at the sum frequency, which encodes molecular vibrational fingerprints unique to species at surfaces or interfaces. These signals provide invaluable insights into molecular conformation, bonding, and orientation, all critical parameters across fields from catalysis to materials science.

However, traditional VSFG spectroscopy operates within the constraints of far-field diffraction limits, where spatial resolution struggles to break below sub-micrometer scales. This fundamental limitation means that the acquired spectra often represent averaged information from ensembles composed of millions or more molecules, obscuring vital nanoscale heterogeneities and localized behaviors. The inability to isolate molecular signals from nanometer-sized domains has posed a formidable challenge, particularly when studying complex, inhomogeneous, or dynamic interfaces where molecular-scale resolution is essential.

By integrating VSFG with scanning tunneling microscopy, the team led by Atsunori Sakurai and collaborators from the Institute for Molecular Science, NINS, have ingeniously circumvented this hurdle. The STM tip acts as a nanometric antenna, concentrating femtosecond pulsed mid-IR and near-IR laser radiation within the sub-nanometer gap between the tip and the gold substrate. This configuration generates an intense, highly confined optical near-field, which enhances the VSFG signal emanating exclusively from the few molecules residing inside the nanogap region.

Experimental data compellingly demonstrate the efficacy of this tip-enhanced system. When the STM tip is positioned at a distance exceeding 50 nanometers from the substrate, the VSFG signal becomes negligible. As the tip approaches into the tunneling regime—distances on the order of a nanometer—the VSFG intensity surges dramatically, unequivocally indicating the near-field enhancement effect. This spatial confinement was precisely controlled, revealing that detectable signals vanish when the tip-sample gap surpasses roughly one nanometer. Such sharp distance dependence affirms that the observed response originates strictly from the nanoscale junction.

Delving deeper into the spectral characteristics, the team meticulously tuned the mid-infrared excitation wavelength and recorded multiple VSFG spectra revealing distinct vibrational modes of the terminal methyl groups in the adsorbed molecules. These include symmetric stretching vibrations, a Fermi resonance involving bending overtone and symmetric stretching, and asymmetric stretching modes. Crucially, interferometric phase analysis of the spectral data allowed for extraction of the imaginary component of the vibrational resonant second-order susceptibility, Im[χ_R^(2)(ω_IR)], providing direct insights into molecular orientation.

This phase-sensitive detection disclosed negative values of Im[χ_R^(2)(ω_IR)], signifying that the terminal methyl groups orient their hydrogen atoms pointing away from the gold substrate surface. This finding underscores the technique’s remarkable capability to not only detect vibrational signatures from minute molecular populations but also to determine their absolute orientation with nanoscale spatial resolution. Such precise orientation mapping is vital in understanding surface reactivity, molecular self-assembly, and interfacial phenomena that dictate chemical function.

The profound near-field enhancement enabling this sensitivity emerges from two synergistic mechanisms. First, the antenna effect of the STM metallic tip effectively funnels and concentrates the mid-infrared electromagnetic fields onto the tip apex. Second, plasmonic enhancement arises within the nanogap formed between the metallic tip and substrate, boosting the efficiency of sum-frequency signal emission in the visible spectrum. These combined effects amplify electromagnetic fields to unprecedented strengths, dramatically elevating the signal-to-noise ratio and enabling the interrogation of sparse molecular populations.

Such nanoscale confinement and enhancement of the VSFG process represent a transformative milestone. The ability to detect signals from molecules restricted within less than one nanometer across a region consisting of only a few tens to hundreds of molecules opens the prospect of pushing this approach towards single-molecule detection. Moreover, ultrafast femtosecond pulse excitation incorporated in the setup positions this technique as a powerful tool to track ultrafast nuclear and electronic dynamics at molecular interfaces in real time.

From an application standpoint, the ramifications are far-reaching. Surface chemical reactions, heterogeneous catalysis, molecular electronics, and energy conversion processes often hinge on localized molecular behaviors inaccessible by ensemble-averaged measurements. TE-VSFG spectroscopy offers a window into these nanoscale processes, furnishing chemical, structural, and dynamical information with molecular specificity. Insights gained may facilitate the tailored design of catalysts with enhanced activity and selectivity or advance molecular-scale devices with refined control over interfacial properties.

Beyond practical applications, this research represents a significant conceptual leap bridging nonlinear optical spectroscopy and nanoscale surface science. It establishes a versatile platform where vibrational nanospectroscopy is performed under ambient conditions without requiring ultrahigh vacuum or cryogenic setups, broadening accessibility to diverse experimental environments. Combining precise tip positioning with phase-sensitive vibrational spectroscopy within an ultrafast pulsed laser framework heralds new frontiers in molecular-scale characterization.

In summary, the development of tip-enhanced vibrational sum-frequency generation spectroscopy grounded in scanning tunneling microscopy embodies a groundbreaking advance in nanoscale molecular spectroscopy. By harnessing intense near-field enhancements within sub-nanometer junctions, this technique captures molecular vibrational fingerprints from an unprecedentedly small number of molecules and reveals their orientation at surfaces with exquisite spatial precision. Expected to catalyze transformative progress in chemical physics, surface science, and nanotechnology, TE-VSFG offers a powerful lens into the hidden nanoscale world of molecules.

As this approach continues to mature, future directions may include extending spectral range, integrating complementary scanning probe methods, and pushing sensitivities towards the single-molecule threshold. This exciting pathway opens vast prospects for unraveling molecular transformations and dynamics at interfaces with chemical and spatial resolution previously unimaginable. The synergy of nonlinear optics, plasmonics, and scanning probe microscopy embodied here stands poised to rewrite the rules of vibrational nanospectroscopy and molecular imaging.

Subject of Research: Not applicable

Article Title: Tip-Enhanced Sum Frequency Generation for Molecular Vibrational Nanospectroscopy

News Publication Date: 10-Apr-2025

Web References: DOI: 10.1021/acs.nanolett.4c06065

Image Credits: Atsunori Sakurai

Keywords

Tip-enhanced spectroscopy, vibrational sum-frequency generation, scanning tunneling microscopy, nanospectroscopy, molecular orientation, plasmonic enhancement, near-field optics, ultrafast spectroscopy, surface chemistry, molecular vibrations, nonlinear optics, nanoscale characterization