Quantum Superlattices Demonstrate 41 Tesla Superconductivity and Evidence for Two-Band Mechanisms

Superconductivity, the ability of a material to conduct electricity with zero resistance, remains a central challenge in modern physics, and researchers continually seek to enhance its properties and understand its underlying mechanisms. Gaetano Campi, Andrea Alimenti, Sang-Eon Lee, and colleagues demonstrate significant progress in this field by investigating artificial high-temperature superconductors, meticulously engineered structures composed of alternating layers of materials. Their work focuses on how these layered structures respond to extremely strong magnetic fields, up to 41 Tesla, revealing a consistent behaviour across the entire range of superconducting temperatures. These results provide compelling evidence supporting the theory that superconductivity in these materials arises from multiple contributing factors, and crucially, demonstrate that atomic-scale engineering can control both the temperature at which superconductivity occurs and the fundamental size of the superconducting pairs, opening new avenues for advanced device development and a deeper understanding of unconventional superconductivity.

Multigap Superconductivity and Upper Critical Fields

This body of work explores the complex phenomenon of superconductivity, particularly in materials exhibiting multiple superconducting energy gaps, known as multigap superconductivity. Researchers investigate materials like cuprates and iron-based superconductors, focusing on the upper critical field (Hc2), the magnetic field strength that destroys superconductivity, which is crucial for applications like high-field magnets. The studies highlight the intricate relationship between material structure and superconducting properties. Investigations center on high-temperature superconductors, materials that exhibit superconductivity at relatively high, though still sub-ambient, temperatures.

Researchers note the anisotropic nature of superconductivity in these materials, meaning properties vary depending on the direction within the crystal structure. Recent work explores twistronics, stacking 2D materials with a slight twist to create novel electronic properties and potentially enhance superconductivity. The research demonstrates the importance of multigap superconductivity as a common feature in many high-temperature superconductors, playing a crucial role in their unique properties. Material engineering, particularly through techniques like twistronics, appears to be a promising avenue for enhancing superconducting properties. The development of these materials with high critical fields and coherence lengths is crucial for building quantum technologies, such as superconducting qubits.

Superlattice Growth and Strain Engineering

Scientists engineered artificial high-temperature superconducting structures, known as AHTS, to investigate the fundamental mechanisms governing superconductivity. These structures consist of alternating layers of La1. 55Sr0. 45CuO4 and La2CuO4, meticulously grown using molecular beam epitaxy, a technique allowing precise control over layer thickness and composition at the atomic scale. The growth process, conducted on LaSrAlO4 substrates, introduced a controlled compressive strain in the LCO layers, influencing the electronic properties of the resulting superlattice.

Real-time monitoring using reflection high-energy electron diffraction ensured layer-by-layer assembly with exceptional precision. Following growth, samples underwent a carefully controlled cooling sequence to ensure stoichiometric composition and remove any residual interstitial oxygen. Structural integrity and the periodicity of the superlattice were rigorously verified using synchrotron x-ray diffraction, confirming the quality of the fabricated structures. Electrical resistance and magneto transport measurements, conducted at high magnetic fields and low temperatures, allowed scientists to map the superconducting dome and determine the upper critical field.

Two-Band Superconductivity in Artificial Superlattices

Scientists have achieved a comprehensive understanding of superconductivity in artificially layered materials, known as artificial high-temperature superlattices (AHTS). These structures exhibit a superconducting dome, and the team systematically investigated this phenomenon across the entire dome using high-field magneto transport measurements. Results demonstrate universal upward-concave behavior in the temperature dependence of the upper critical magnetic field, providing compelling evidence for two-band superconductivity. The research confirms predictions from multigap theory and reveals that atomic-scale engineering directly controls not only the critical temperature but also the intrinsic size of the superconducting pairs.

Measurements of the superconducting coherence length demonstrate this precise control, indicating that the geometric ratio of layer thicknesses directly influences the critical temperature and pair size. The team meticulously tuned this ratio, extending measurements above and below an optimal value, to map the evolution of multiband superconductivity across the dome. Experiments revealed an enhancement of the critical temperature at the optimal ratio, coinciding with theoretical predictions based on a model incorporating quantum shape resonance between condensates with distinct energy gaps. This work validates the universality of multigap superconductivity in oxide superlattices and demonstrates the tunability of superconducting properties through atomic-scale design.

Multigap Superconductivity and Magnetic Field Tuning

High-field measurements reveal that multigap superconductivity consistently appears throughout artificial high-temperature superconducting superlattices, even as their composition varies. The team observed a pronounced upward curvature in the temperature-dependent upper critical magnetic field, confirming the coexistence of multiple superconducting condensates with differing Fermi velocities. A key finding is the divergence between the temperature at which superconductivity emerges and the upper critical magnetic field, demonstrating that the magnetic critical field is particularly sensitive to multiband pairing and the tuning of the intrinsic size of Cooper pairs. This work establishes that precise control over superlattice architecture allows for predictive control over both critical temperature and Cooper pair size, opening new avenues for designing superconducting materials optimized for high magnetic field applications.