[Paper Review] Pseudogap and local pairs in high-Tc cuprate superconductors

A. L. Solovjov, M. A. Tkachenko|arXiv (Cornell University)|Dec 16, 2011

Physics of Superconductivity and Magnetism1 references3 citations

TL;DR

This paper proposes that the pseudogap in high-Tc cuprates arises from local pairs—intermediate bound states between Bose-Einstein condensates and conventional Cooper pairs—using resistivity and ARPES data from Bi2201 single crystals. The model explains the temperature dependence of spectral weight loss and excess conductivity, showing good agreement between calculated pseudogap and ARPES-measured spectral weight, supporting local pairs as a key mechanism for pseudogap formation.

ABSTRACT

Analysis of the resistivity data recently reported by Kondo et al. [1] for (Bi,Pb)2(Sr,La)2CuO{6-delta} (Bi2201) single-crystals has been performed within our model developed to study pseudogap (PG) in high-Tc superconductors (HTS's). The model is based on an assumption of the existence of local pairs in HTS's at temperatures well above Tc. Comparative analysis of our results and results of ARPES experiments reported by Kondo et al. suggests the local pairs to be one of the possible reason of the PG formation.

Motivation & Objective

• To explain the origin of the pseudogap in underdoped high-Tc cuprates, particularly in Bi2201 single crystals.
• To test the hypothesis that local pairs—intermediate bound fermion states—exist above Tc and drive pseudogap behavior.
• To reconcile resistivity measurements with ARPES data on spectral weight loss to validate the local pair model.
• To clarify the relationship between the pseudogap, local pair formation, and superconducting fluctuations.

Proposed method

• Analyzed resistivity data from Kondo et al. on OP35K Bi2201 single crystals to extract excess conductivity σ′(T).
• Applied a local pair model based on the transition from pre-formed bosonic pairs (SBB) to fluctuating Cooper pairs above Tc.
• Used the equation σ′(T) = [ρN(T) − ρ(T)] / [ρ(T)ρN(T)] to calculate excess conductivity, with ρN(T) = αT + b extrapolated from normal state.
• Fitted the pseudogap Δ*(T) using parameters including Tc = 35 K, T* = 160 K, ξ(0) ≈ 1.5 Å, and A4 = 25 to match experimental σ′(T).
• Compared the derived Δ*(T) with ARPES-measured spectral weight W(EF)(T) and spectral gap SG(T) from the same sample.
• Assessed the consistency of the model by checking the temperature dependence of W(EF)(T) and its correlation with local pair states.

Experimental results

Research questions

• RQ1 Does the pseudogap in Bi2201 arise from the formation of local pairs above Tc?
• RQ2 Is there a direct correlation between the temperature dependence of spectral weight loss W(EF)(T) and the pseudogap Δ*(T)?
• RQ3 Can the local pair model explain both the resistivity and ARPES data simultaneously?
• RQ4 Why is the spectral gap SG(T) shifted to lower temperatures compared to the pseudogap maximum Tpair?
• RQ5 What is the physical significance of the temperature Tpair ≈ 100 K in relation to local pair states?

Key findings

• The calculated pseudogap Δ*(T) shows good agreement with the ARPES-measured spectral weight loss W(EF)(T), particularly around Tmax ≈ 100 K, which marks the transition from SBB to fluctuating Cooper pairs.
• The value of 2Δ*(Tc)/kB Tc ≈ 4.57 is consistent with typical high-Tc superconductors, validating the model's physical plausibility.
• The maximum of Δ*(T)/Δ*max at T ≈ 100 K coincides with Tpair, indicating a phase transition from SBB to fluctuating Cooper pairs.
• The spectral weight W(EF)(T) exhibits linear dependence on T above Tpair, attributed to the presence of SBB, which disappears above T*.
• Below Tpair, the loss of spectral weight correlates with the formation of fluctuating Cooper pairs, consistent with superconducting fluctuations.
• The discrepancy in absolute values between SG(T) (up to 40 meV) and Δ*(T) (max 16.5 meV) suggests that SG and PG are not directly correlated, challenging a simple identification.