Repulsive Interactions Between Electrons Enable Superconductivity in Two-Dimensional Systems

Scientists are increasingly focused on elucidating the origins of superconductivity within strongly correlated systems, a pursuit with the potential to revolutionise materials science and technology. Humberto M. Silva from Universidad Nacional de Ingeniería, Francisco Dinola Neto, Griffith M. A. R., Minos A. Neto and Octavio D. R. Salmon, all from Universidade Federal do Amazonas, UFAM, present new insights into this phenomenon, working in collaboration with Mucio A. Continentino and Amos Troper from Centro Brasileiro de Pesquisas Físicas. Their research, detailed in a new paper, employs the Hubbard model with local repulsive interactions to explore a two-dimensional system and utilises the Green’s functions method within a Hubbard-I mean field approximation to address strong interaction limits. The team’s findings demonstrate the existence of a superconducting ground state even in the presence of repulsive interactions, mediated by kinetic electronic energy, and reveal a saturation of the critical temperature at higher temperatures, offering a novel perspective on the complex interplay of forces governing superconductivity.

Their work demonstrates that superconductivity can emerge even when electrons strongly repel each other, a scenario traditionally thought to suppress it. This discovery hinges on a detailed analysis of the Hubbard model, a cornerstone of condensed matter physics used to describe interacting electrons in materials. The team’s calculations reveal that kinetic energy, the energy of electron motion, can mediate the formation of superconducting ground states within this repulsive framework. The research establishes a superconducting ground state arising from kinetic electronic energy and characterised by a non-local order parameter when repulsive interactions exceed this Umin value. This saturation suggests a limiting value beyond which increasing the interaction strength no longer elevates the critical temperature, indicating a transition to a different physical regime. Calculations reveal that the non-local superconducting gap amplitude dominates for strong repulsive interactions, while the local pairing term diminishes; conversely, weak interactions favour the persistence of only the local pairing contribution. This interplay between local and non-local pairing mechanisms highlights the complex nature of superconductivity in strongly correlated systems and the importance of considering both contributions to the overall superconducting state. Employing the Green’s functions method within a Hubbard-I mean field approximation, the researchers solved a complex system of equations to map the zero-temperature phase diagram of the model. This approach allowed them to explore the behaviour of the system under extreme interaction strengths and focused on a two-dimensional system, providing insights relevant to materials like high-temperature cuprates where electron correlations play a dominant role. Self-consistent equations for the gap and number of electrons were derived to study the superconducting gap and chemical potential as functions of interaction strength; these equations are fundamental to understanding the system’s behaviour and predicting its properties under varying conditions. The findings also demonstrate a surprising parallel between strongly repulsive systems and those with strong attractive interactions, exhibiting a saturation of the critical temperature. This saturation suggests a fundamental limit to how high the superconducting transition temperature can be pushed in these materials and provides a new theoretical framework for understanding superconductivity in complex materials. The team’s approach, utilising Green’s functions and the Hubbard-I approximation, offers a powerful tool for exploring the intricate world of strongly correlated electron systems and opens avenues for designing novel superconducting materials with tailored properties. Scientists have long sought to understand the subtle mechanisms driving superconductivity, particularly in materials where strong interactions between electrons defy simple explanations. This work, employing a well-established Hubbard model, offers a nuanced perspective on how repulsive forces between electrons can, counterintuitively, foster the emergence of a superconducting state. The challenge has always been to move beyond simplified theoretical frameworks and account for the complex interplay of these interactions in two-dimensional systems. What distinguishes this research is the identification of kinetic energy as a key mediator of electron pairing in the repulsive regime, suggesting that superconductivity isn’t solely reliant on attractive forces, opening up new avenues for designing materials with enhanced properties. However, the reliance on a mean-field approximation introduces inherent limitations; real materials are far messier than these models, and the neglect of non-local effects may obscure crucial details. The prediction of phase separation, signalled by unstable quasiparticles, also demands further investigation; confirming this experimentally would be a significant step forward. Future work might focus on incorporating these missing complexities, perhaps through more sophisticated computational techniques or by exploring the role of disorder, ultimately aiming to bridge the gap between these theoretical insights and the creation of practical, room-temperature superconductors.