High-Tc Superconductivity in La NiO: Symmetry Analysis Enables New Material Design

The pursuit of unconventional superconductivity has gained momentum with the discovery of high-temperature superconductivity in lanthanum nickelate materials, but a fundamental question remains regarding the nature of the superconducting state. Zhan Wang, Yuxin Wang, and Kun Jiang, alongside Jiangping Hu and Fu-Chun Zhang, investigated the superconducting order in lanthanum nickelate, focusing on the crucial issue of pairing symmetry. The team modelled the material’s superconducting behaviour, considering the established structure of its electrons and exploring two leading possibilities for how electrons bind together. By predicting the characteristics of several experimentally measurable quantities, including how the material responds to tunnelling, contact with a probe, and light, the researchers provide a clear pathway for identifying the pairing symmetry in lanthanum nickelate and, ultimately, understanding the microscopic origins of superconductivity in these novel materials.

Lanthanum Nickelate Pairing Symmetry via Bogoliubov-de Gennes

This research pioneers a comprehensive theoretical framework to identify the pairing symmetry within the novel high-temperature superconductor, lanthanum nickelate. The study employs a multi-orbital Bogoliubov-de Gennes (BdG) approach, constructing a Hamiltonian that accurately represents the material’s electronic structure and incorporates both kinetic and pairing terms. This allows scientists to model the behavior of superconducting electrons and predict observable consequences of different pairing symmetries. The team meticulously constructed the Hamiltonian, accounting for interactions between electrons in various orbitals and considering pairing between electrons on the same and neighboring atoms.

To connect theory with experiment, the researchers calculated several key physical quantities, including the tunneling density of states, point contact spectroscopy, superfluid density, and Raman spectra, for both conventional and unconventional pairing scenarios. These calculations involved solving the BdG equations to determine the energy and momentum of quasiparticles, which are the excited states of the superconducting system. The team then used these quasiparticle properties to predict the expected signals in each experimental probe, establishing a direct link between theoretical predictions and measurable quantities. The approach leverages the Kubo formula to calculate the current response and, subsequently, the superfluid density, a crucial parameter for understanding superconductivity.

The calculation of Raman spectra involved determining the Raman response function, which describes how the material responds to incoming and scattered light. This required calculating the correlation function of the Raman density operator, a complex process that accounts for the quantum mechanical interactions between electrons and photons. The team employed Gorkov Green’s functions, a powerful tool for describing the behavior of electrons in superconducting materials, to evaluate this correlation function. By carefully analyzing the predicted signals in each experimental probe for different pairing symmetries, the study establishes a concrete and experimentally testable route for identifying the microscopic nature of nickelate superconductivity.,.

Lanthanum Nickelate Pairing Symmetry Determined by Calculations

This work presents a detailed theoretical investigation into the superconducting order of lanthanum nickelate, a material exhibiting high-temperature superconductivity, with the goal of determining the symmetry of the electron pairing responsible for this phenomenon. Scientists modeled the superconducting order using established characteristics of the material’s electron structure, combined with theoretical pairing functions representing both s± and d-wave symmetries, considered the leading possibilities in current research. The team calculated several experimentally measurable quantities, tunneling density of states, point contact spectroscopy, superfluid density, and Raman spectroscopy, each of which exhibits unique characteristics depending on the pairing symmetry. Calculations of normalized conductance, simulating a tunneling process, revealed distinct behaviors for the two pairing symmetries.

For nodal s± symmetry, coherent peaks were observed at specific bias voltages, originating from the superconducting gap, while the d-wave case showed a V-shaped gap with a strong zero-bias peak. These directional dependencies in conductance provide a potential diagnostic tool for distinguishing between the pairing symmetries. Further analysis focused on superfluid density, a measure of the system’s ability to sustain long-range phase coherence, revealing that the decay of superfluid density follows a square root of temperature for the nodal s± symmetry. In contrast, the d-wave symmetry exhibited a linear decay with temperature, providing a clear signature for identifying the pairing symmetry. The team also investigated Raman spectroscopy, a technique sensitive to the vibrational modes of the material, and found that it provides another means of probing the superconducting pairing symmetry. Measurements of superfluid density and Raman spectroscopy, alongside tunneling experiments, offer complementary pathways to determine the pairing symmetry in lanthanum nickelate and advance understanding of unconventional superconductivity.,.

Nickelate Superconductivity, Pairing Symmetry Identification Methods

Scientists have developed a theoretical framework to distinguish between different types of superconductivity in a recently discovered class of nickelate materials. The research focuses on understanding how electrons pair up within these materials, a crucial factor determining their superconducting properties. By modeling the electronic structure of lanthanum nickelate, the team investigated how various pairing symmetries, specifically s±-wave and d-wave, affect experimentally measurable quantities. The study demonstrates that distinct signatures in tunneling density of states, point contact spectroscopy, superfluid density, and Raman spectroscopy can differentiate between these pairing symmetries.

These calculations provide a concrete pathway for experimental verification of the superconducting order in nickelates, clarifying the underlying mechanisms driving superconductivity in this novel material system. The researchers constructed a tight-binding model representing the material’s electronic structure, allowing them to simulate the impact of different pairing scenarios on observable properties. This approach successfully predicts unique spectroscopic signatures for each symmetry, offering a means to experimentally determine the pairing symmetry within lanthanum nickelate. The authors acknowledge that their model relies on phenomenological pairing functions, meaning the microscopic origin of pairing is not addressed within this work.

Future research could focus on incorporating the microscopic details of pairing interactions to refine the model and explore the potential for unconventional pairing mechanisms in these nickelate materials., The pursuit of unconventional superconductivity has gained momentum with the discovery of high-temperature superconductivity in lanthanum nickelate materials, but a fundamental question remains regarding the nature of the superconducting state. The team modelled the material’s superconducting behaviour, considering the established structure of its electrons and exploring two leading possibilities for how electrons bind together. By predicting the characteristics of several experimentally measurable quantities, including how the material responds to tunnelling, contact with a probe, and light, the researchers provide a clear pathway for identifying the pairing symmetry in lanthanum nickelate and, ultimately, understanding the microscopic origins of superconductivity in these novel materials.

The discovery of high-Tc superconductivity in Ruddlesden-Popper nickelate materials, exemplified by La3Ni2O7, has opened new avenues in the search for unconventional superconductivity. A central unresolved issue concerns the pairing symmetry of the superconducting order. This paper models the superconducting order of La3Ni2O7, utilising the established Fermi surface structure together with phenomenological pairing functions belonging to the s± and d-wave symmetry classes, which represent the leading possibilities in current debate. The team computes several experimentally accessible observables, including the tunneling density of states.