[Paper Review] Enhanced Superconductivity in Superlattices of High-$T_c$ Cuprates

Satoshi Okamoto, Thomas Maier|arXiv (Cornell University)|Jun 13, 2008

Physics of Superconductivity and Magnetism3 citations

TL;DR

This study proposes that superlattices of underdoped (UD) and overdoped (OD) high-Tc cuprates can enhance superconductivity in OD layers via proximity coupling to strong pairing correlations from UD layers. Using cluster dynamical mean-field theory and slave-boson mean-field calculations, it demonstrates that the superconducting order parameter in OD layers exceeds its maximum in uniform systems, and transition temperatures surpass the optimal Tc of bulk cuprates, suggesting a pathway to higher-Tc superconductivity in artificial heterostructures.

ABSTRACT

The electronic properties of multilayers of strongly correlated models for cuprate superconductors are investigated using cluster dynamical mean-field techniques. We focus on combinations of underdoped and overdoped layers and find that the superconducting order parameter in the overdoped layers is enhanced by the proximity effect of the strong pairing scale originating from the underdoped layers. The enhanced order parameter can even exceed the maximum value in uniform systems. This behavior is well reproduced in slave-boson mean-field calculations which also find higher transition temperatures than in the uniform system.

Motivation & Objective

• To investigate whether superlattices of underdoped and overdoped cuprates can enhance superconductivity beyond uniform bulk systems.
• To explore the role of proximity coupling between layers with strong pairing (UD) and high coherence (OD) in boosting superconducting order parameters and transition temperatures.
• To determine if artificial heterostructures can achieve higher Tc than optimal doping in homogeneous cuprates.
• To validate the enhancement mechanism using multiple theoretical approaches, including cluster dynamical mean-field theory and slave-boson mean-field theory.

Proposed method

• Employed layer-extended cellular dynamical mean-field theory (CDMFT) with exact diagonalization of 2×2 clusters to study the Hubbard and t-J models in superlattices.
• Used the dynamic cluster approximation (DCA) with noncrossing approximation (NCA) for finite-temperature calculations, mapping the bulk system onto independent effective clusters in momentum space.
• Applied slave-boson mean-field (SBMF) theory to analyze superconducting order parameters and transition temperatures in the superlattice geometry.
• Fixed the on-site potential profile and Fermi level in short-period superlattices to model charge transfer between layers without full self-consistency.
• Computed superconducting order parameter Ψ as a function of carrier density n, comparing uniform systems and superlattices with alternating UD and OD layers.
• Used U = 10t, tz = 0.5t, J = 0.3t, and Jz = 0.25J in the t-J model to simulate realistic cuprate parameters.

Experimental results

Research questions

• RQ1 Can the superconducting order parameter in overdoped cuprate layers be enhanced beyond its maximum value in uniform systems through proximity to underdoped layers?
• RQ2 Does the superlattice structure lead to a higher superconducting transition temperature Tc than the optimal Tc in homogeneous cuprates?
• RQ3 How do interlayer hopping (tz) and doping balance affect the enhancement of superconductivity in the superlattice?
• RQ4 To what extent do cluster dynamical mean-field and slave-boson mean-field methods agree on the superconducting response in these heterostructures?

Key findings

• The superconducting order parameter in overdoped layers of UD/OD superlattices exceeds the maximum value observed in uniform systems for certain doping combinations.
• At T = 0, CDMFT calculations show that the order parameter in the overdoped layer can surpass the peak value in the uniform system, particularly when the average doping is slightly below optimal.
• Slave-boson mean-field calculations predict a superconducting transition temperature Tc in the superlattice that exceeds the maximum Tc in the uniform system.
• Finite-temperature DCA-NCA results confirm enhanced order parameters in overdoped layers near Tc, indicating a potential increase in Tc relative to the uniform case.
• The enhancement is sensitive to interlayer hopping: reducing tz diminishes the effect, and it vanishes in the limit tz = 0, confirming the role of interlayer coupling.
• The optimal recipe involves combining layers with strong pairing (UD) and high coherence (OD), with average carrier density slightly below the optimal doping of the uniform system.