Out-of-plane Bond Order Phase, Superconductivity, and Their Competition in the t-J∥-J⊥ Model for Pressurized Nickelates: A Large-N Approximation Study

Core Concepts

The study reveals that the out-of-plane magnetic exchange interaction (J⊥) is crucial for high-temperature superconductivity in pressurized bilayer nickelates, while also inducing a competing out-of-plane bond order phase (z-BOP) that shapes the superconducting dome as a function of doping and pressure.

Abstract

Bejas, M., Wu, X., Chakraborty, D., Schnyder, A. P., & Greco, A. (2024). Out-of-plane bond order phase, superconductivity, and their competition in the 𝑡−𝐽∥−𝐽⊥ model for pressurized nickelates. arXiv preprint arXiv:2411.00269v1.

This study investigates the role of the out-of-plane magnetic exchange interaction (J⊥) in the emergence of high-temperature superconductivity in pressurized bilayer nickelates, using the t-J∥-J⊥ model as a minimal model.

Key Insights Distilled From

Out-of-plane bond order phase, superconductivity, and their competition in the $t$-$J_\parallel$-$J_\perp$ model for pressurized nickelates

by Matí... at arxiv.org 11-04-2024

https://arxiv.org/pdf/2411.00269.pdf

Deeper Inquiries

How does the presence of the z-BOP potentially impact other physical properties of nickelate superconductors, such as their transport properties or magnetic susceptibility?

The presence of the out-of-plane bond order phase (z-BOP) in nickelate superconductors can significantly impact their physical properties due to the restructuring of the electronic and magnetic structure it induces. Here's how it could affect transport properties and magnetic susceptibility:
Impact on Transport Properties:

Resistivity: The z-BOP can lead to an increase in resistivity. While the Fermi surface is not fully gapped in the z-BOP state, the formation of splitted pockets due to the out-of-plane bond ordering can reduce the number of available charge carriers and decrease their mobility, leading to enhanced scattering and increased resistivity. This effect might be particularly pronounced at temperatures below the z-BOP transition temperature (Tz-BOP).
Hall Coefficient: The Hall coefficient, which provides information about the type and density of charge carriers, could show anomalies or changes in sign across the z-BOP transition. This is because the z-BOP can alter the effective carrier concentration and their effective mass.
Anisotropic Transport: Given the out-of-plane nature of the z-BOP, anisotropic transport properties are expected. The resistivity might show different temperature dependencies depending on whether the current is applied parallel or perpendicular to the nickelate planes.
Impact on Magnetic Susceptibility:

Suppression of Magnetic Ordering: The z-BOP, being a charge order, can compete with magnetic ordering tendencies. The formation of the z-BOP might suppress or modify the magnetic susceptibility compared to a system without z-BOP. This suppression could be observed as a decrease in the magnetic susceptibility below Tz-BOP.
Anisotropy in Susceptibility: Similar to transport properties, the magnetic susceptibility might also exhibit anisotropy due to the z-BOP. The response to an external magnetic field could differ depending on the field's orientation relative to the nickelate planes.
Experimental Detection:
Experimentally, these effects could be probed through:

Temperature-dependent resistivity and Hall coefficient measurements: These can reveal anomalies associated with the z-BOP transition.
Angle-resolved photoemission spectroscopy (ARPES): This technique can directly probe the Fermi surface reconstruction and the formation of splitted pockets due to the z-BOP.
Magnetic susceptibility measurements: These can show the suppression or modification of magnetic ordering tendencies due to the z-BOP.

Could alternative theoretical frameworks, beyond the large-N approximation, provide further insights into the interplay between superconductivity and the z-BOP in nickelates?

Yes, alternative theoretical frameworks beyond the large-N approximation can provide more refined and potentially different insights into the interplay between superconductivity and the z-BOP in nickelates. Here are some approaches:

Dynamical Mean Field Theory (DMFT): DMFT is a powerful method that can treat local electronic correlations in a more sophisticated way than the large-N approximation. It can capture the effects of strong correlations on the electronic structure and provide insights into the competition between different phases, including superconductivity and z-BOP.
Variational Monte Carlo (VMC) and Density Matrix Renormalization Group (DMRG): These are numerical methods that can treat strongly correlated systems with high accuracy, albeit for limited system sizes. They can provide valuable insights into the ground state properties and the competition between different phases.
Quantum Monte Carlo (QMC) Methods: QMC methods can simulate the finite-temperature properties of strongly correlated systems. They can be used to study the phase diagram, critical exponents, and other thermodynamic properties of nickelates, shedding light on the interplay between superconductivity and z-BOP.
Field-Theoretical Approaches Beyond Mean-Field: Going beyond mean-field theory in field-theoretical approaches, such as including fluctuations or using renormalization group methods, can provide a more accurate description of the phase transitions and critical behavior.
These alternative frameworks can address the limitations of the large-N approximation, such as its mean-field nature and the neglect of certain fluctuations. They can provide a more realistic picture of the electronic structure, the competition between different phases, and the role of quantum fluctuations in nickelate superconductors.

What are the broader implications of understanding the role of interlayer coupling in layered superconductors for designing materials with even higher superconducting transition temperatures?

Understanding the role of interlayer coupling in layered superconductors is crucial for designing materials with higher superconducting transition temperatures (Tc) because it can significantly influence the superconducting pairing mechanism and the overall electronic structure. Here are some broader implications:

Enhancing Existing Pairing Mechanisms: In some cases, interlayer coupling can enhance existing pairing mechanisms. For example, in nickelates, the out-of-plane exchange interaction (J⊥) can lead to out-of-plane s-wave superconductivity, which can coexist or compete with the in-plane d-wave superconductivity arising from in-plane interactions. This interplay can potentially lead to higher Tc values.
Inducing New Pairing Mechanisms: Interlayer coupling can also introduce new pairing mechanisms that are absent in purely two-dimensional systems. For instance, it can mediate interactions between electrons in different layers, leading to unconventional pairing symmetries or enhancing electron-phonon coupling, which can result in higher Tc.
Tuning the Electronic Structure: By controlling the interlayer distance, the type of atoms in the intervening layers, or applying pressure, one can tune the electronic structure of layered superconductors. This tuning can influence the Fermi surface topology, the density of states at the Fermi level, and the strength of electronic correlations, all of which can impact Tc.
Controlling Competing Orders: Interlayer coupling can influence the competition between superconductivity and other electronic orders, such as charge density waves or spin density waves. By understanding this interplay, one can potentially suppress competing orders and enhance superconductivity.
Design Principles:
These insights suggest some design principles for high-Tc superconductors:

Strong Interlayer Coupling: Materials with strong interlayer coupling, either through direct orbital overlap or through mediating atoms or layers, are promising candidates for high-Tc superconductivity.
Tunable Electronic Structure: Layered materials with tunable electronic structures, where the interlayer coupling can be controlled, offer a platform for exploring different pairing mechanisms and optimizing Tc.
Suppression of Competing Orders: Strategies to suppress competing electronic orders, such as chemical doping or applying external pressure, can enhance superconductivity in layered materials.
By systematically exploring these design principles, researchers can develop a deeper understanding of how to manipulate interlayer coupling to engineer layered superconductors with enhanced superconducting properties.