Periodic Reporting for period 1 - PairNoise (Electron pairs without superconductivity)
Période du rapport: 2023-02-01 au 2025-07-31
Superconductivity emerges when electrons form pairs that condense into a phase-coherent quantum state with zero electrical resistance. In conventional superconductors, pairing and condensation occur simultaneously at the critical temperature (Tc). However, a long-standing hypothesis suggests that in certain quantum materials, electron pairs might exist without superconductivity, particularly above Tc in high-temperature and disordered superconductors.
Despite decades of research and tantalizing indirect evidence, the field lacks a direct, unambiguous method to detect and quantify electron pairing without superconductivity. This hinders our understanding of fundamental phases in quantum materials, including the mysterious pseudogap state in cuprates and the nature of the superconductor-insulator transition in disordered films.
PairNoise addresses this challenge by developing a revolutionary electron pair microscope (EPM) that combines scanning tunneling microscopy (STM) with shot-noise spectroscopy. This instrument can unambiguously detect electron pairing through the change in shot-noise from an effective charge of 1e to 2e, providing direct, quantitative, and spatially resolved measurements of pairing in quantum materials.
Building on a proof-of-concept instrument developed in our group, PairNoise aims to create two complementary microscopes: EPM1, a tank-based design with wide temperature range and high sensitivity, and EPM2, a smart-tip based design with radically extended bandwidth and sensitivity up to two orders of magnitude higher than current technology. This dramatic improvement is essential for studying d-wave superconductors, where the mixed presence of nodal quasiparticles makes detecting pairing significantly more challenging.
The project objectives include: (1) establishing whether and in what form electron pairs exist above Tc in high-temperature and disordered superconductors, (2) determining the role of pairing in the formation of the pseudogap, (3) characterizing the states in vortex cores of unconventional and topological superconductors, and (4) identifying what limits superconductivity at higher temperatures.
The impact of PairNoise extends beyond addressing these specific questions. It opens the new field of electron pair microscopy with applications in detecting fractional charges and Majorana modes, investigating exotic proposals about 4e superconductivity in twisted bilayer graphene, and studying Kondo physics and dynamical processes in quantum materials. By providing unprecedented insight into quantum matter, PairNoise promises to transform our understanding of unconventional superconductivity and potentially guide future efforts to achieve higher transition temperatures.
Despite decades of research and tantalizing indirect evidence, the field lacks a direct, unambiguous method to detect and quantify electron pairing without superconductivity. This hinders our understanding of fundamental phases in quantum materials, including the mysterious pseudogap state in cuprates and the nature of the superconductor-insulator transition in disordered films.
PairNoise addresses this challenge by developing a revolutionary electron pair microscope (EPM) that combines scanning tunneling microscopy (STM) with shot-noise spectroscopy. This instrument can unambiguously detect electron pairing through the change in shot-noise from an effective charge of 1e to 2e, providing direct, quantitative, and spatially resolved measurements of pairing in quantum materials.
Building on a proof-of-concept instrument developed in our group, PairNoise aims to create two complementary microscopes: EPM1, a tank-based design with wide temperature range and high sensitivity, and EPM2, a smart-tip based design with radically extended bandwidth and sensitivity up to two orders of magnitude higher than current technology. This dramatic improvement is essential for studying d-wave superconductors, where the mixed presence of nodal quasiparticles makes detecting pairing significantly more challenging.
The project objectives include: (1) establishing whether and in what form electron pairs exist above Tc in high-temperature and disordered superconductors, (2) determining the role of pairing in the formation of the pseudogap, (3) characterizing the states in vortex cores of unconventional and topological superconductors, and (4) identifying what limits superconductivity at higher temperatures.
The impact of PairNoise extends beyond addressing these specific questions. It opens the new field of electron pair microscopy with applications in detecting fractional charges and Majorana modes, investigating exotic proposals about 4e superconductivity in twisted bilayer graphene, and studying Kondo physics and dynamical processes in quantum materials. By providing unprecedented insight into quantum matter, PairNoise promises to transform our understanding of unconventional superconductivity and potentially guide future efforts to achieve higher transition temperatures.
The PairNoise project has made progress in developing novel instrumentation and applying it to address fundamental questions about electron pairing in quantum materials. Here a summary of the most important areas of progress.
Development of Electron Pair Microscopes (WP1)
A central focus of our work has been developing the two complementary electron pair microscopes (EPMs) that form the technological foundation of the project. These instruments combine scanning tunneling microscopy with shot-noise spectroscopy to enable direct detection of electron pairing with atomic-scale resolution.
We have successfully completed EPM1, our tank-based design with high sensitivity. This instrument has exceeded our initial performance specifications, demonstrating improved sensitivity and stability compared to our original design. EPM1 utilizes a resonance circuit with niobium titanium tanks and improved shielding to increase the Q-factor, resulting in much, much enhanced sensitivity.
Simultaneously, we have made progress on EPM2, our more innovative smart-tip based design. This instrument aims to achieve even higher sensitivity through broadband impedance matching using nanofabricated circuits. While the development of EPM2 has been more challenging than EPM1, we are making progress.
Direct Evidence for Pairing in Cuprate Superconductors (WP2)
Using EPM1, we have conducted comprehensive measurements on underdoped Bi2Sr2CaCu2O₈₊ᵟ samples, achieving a major scientific breakthrough. Our most significant finding, detailed in our manuscript "Equivalence of pseudogap and pairing energy in a cuprate high-temperature superconductor" (currently under review), provides the first direct evidence that the pseudogap energy in cuprates coincides with the onset of electron pairing.
Through our noise-STM measurements, we have demonstrated that electron pairing occurs up to energy scales reaching 70 meV, with significant spatial heterogeneity across the sample. This result addresses a decades-old debate about the nature of the pseudogap in high-temperature superconductors, showing conclusively that it is related to pairing rather than competing orders.
These findings represent a significant advance in our understanding of high-temperature superconductivity. By directly observing that electrons remain paired at temperatures well above Tc and at energies corresponding to the pseudogap, we have provided new insight.
Understanding Transparency Effects in Shot Noise Measurements
A particularly important methodological advance came from our investigation into why shot noise measurements in mesoscopic junctions often fail to detect electron pairing despite the presence of superconductivity. This work, published in Physical Review Letters (Niu et al., 2024, "Why Shot Noise Does Not Generally Detect Pairing in Mesoscopic Superconducting Tunnel Junctions"), identified that in typical mesoscopic junctions, the large number of channels with very low transparency obscures the pairing effect in noise measurements.
We discovered that the transparency of tunnel junctions is the key parameter controlling whether shot noise can detect pairing. This understanding explains seemingly contradictory results in the literature and provides crucial guidance for designing junctions that allow the detection of electron pairing.
Initial Studies on Disordered Superconductors (WP4)
We have recently begun systematic investigations of pairing in weakly disordered superconductors as part of WP4a. These experiments build upon our previous demonstration of pairing above T₇ in titanium nitride and are providing new insights into the relationship between disorder, pairing, and superconductivity.
Our measurements on TiN, NbN, and InO samples aim to determine whether a gas or liquid of preformed pairs exists in these systems. While this work is still in its early stages, it has the potential to challenge current models of the state above T₇ in disordered superconductors and to provide a more complete picture of how disorder affects electron pairing and phase coherence.
Development of Electron Pair Microscopes (WP1)
A central focus of our work has been developing the two complementary electron pair microscopes (EPMs) that form the technological foundation of the project. These instruments combine scanning tunneling microscopy with shot-noise spectroscopy to enable direct detection of electron pairing with atomic-scale resolution.
We have successfully completed EPM1, our tank-based design with high sensitivity. This instrument has exceeded our initial performance specifications, demonstrating improved sensitivity and stability compared to our original design. EPM1 utilizes a resonance circuit with niobium titanium tanks and improved shielding to increase the Q-factor, resulting in much, much enhanced sensitivity.
Simultaneously, we have made progress on EPM2, our more innovative smart-tip based design. This instrument aims to achieve even higher sensitivity through broadband impedance matching using nanofabricated circuits. While the development of EPM2 has been more challenging than EPM1, we are making progress.
Direct Evidence for Pairing in Cuprate Superconductors (WP2)
Using EPM1, we have conducted comprehensive measurements on underdoped Bi2Sr2CaCu2O₈₊ᵟ samples, achieving a major scientific breakthrough. Our most significant finding, detailed in our manuscript "Equivalence of pseudogap and pairing energy in a cuprate high-temperature superconductor" (currently under review), provides the first direct evidence that the pseudogap energy in cuprates coincides with the onset of electron pairing.
Through our noise-STM measurements, we have demonstrated that electron pairing occurs up to energy scales reaching 70 meV, with significant spatial heterogeneity across the sample. This result addresses a decades-old debate about the nature of the pseudogap in high-temperature superconductors, showing conclusively that it is related to pairing rather than competing orders.
These findings represent a significant advance in our understanding of high-temperature superconductivity. By directly observing that electrons remain paired at temperatures well above Tc and at energies corresponding to the pseudogap, we have provided new insight.
Understanding Transparency Effects in Shot Noise Measurements
A particularly important methodological advance came from our investigation into why shot noise measurements in mesoscopic junctions often fail to detect electron pairing despite the presence of superconductivity. This work, published in Physical Review Letters (Niu et al., 2024, "Why Shot Noise Does Not Generally Detect Pairing in Mesoscopic Superconducting Tunnel Junctions"), identified that in typical mesoscopic junctions, the large number of channels with very low transparency obscures the pairing effect in noise measurements.
We discovered that the transparency of tunnel junctions is the key parameter controlling whether shot noise can detect pairing. This understanding explains seemingly contradictory results in the literature and provides crucial guidance for designing junctions that allow the detection of electron pairing.
Initial Studies on Disordered Superconductors (WP4)
We have recently begun systematic investigations of pairing in weakly disordered superconductors as part of WP4a. These experiments build upon our previous demonstration of pairing above T₇ in titanium nitride and are providing new insights into the relationship between disorder, pairing, and superconductivity.
Our measurements on TiN, NbN, and InO samples aim to determine whether a gas or liquid of preformed pairs exists in these systems. While this work is still in its early stages, it has the potential to challenge current models of the state above T₇ in disordered superconductors and to provide a more complete picture of how disorder affects electron pairing and phase coherence.
AI-Based Methods for Enhancing Experimental Data
In addition to our primary experimental work, we have collaborated on a novel self-supervised learning techniques for denoising quasiparticle interference (QPI) data. Published in Physical Review B (Kuijf et al., 2025), this methodological advance significantly improves the signal-to-noise ratio of our measurements while preserving fine details.
We believe that this development represents an nice intersection of machine learning and experimental condensed matter physics.
In addition to our primary experimental work, we have collaborated on a novel self-supervised learning techniques for denoising quasiparticle interference (QPI) data. Published in Physical Review B (Kuijf et al., 2025), this methodological advance significantly improves the signal-to-noise ratio of our measurements while preserving fine details.
We believe that this development represents an nice intersection of machine learning and experimental condensed matter physics.