[Paper Review] Coexisting pseudo-gap and superconducting gap in the high-Tc superconductor La2-xSrxCuO4

T. Yoshida, Walid Malaeb+13 more|arXiv (Cornell University)|Aug 14, 2012

Physics of Superconductivity and Magnetism3 citations

TL;DR

This study uses angle-resolved photoemission spectroscopy (ARPES) to demonstrate that in underdoped La2-xSrxCuO4 (x=0.10, 0.14), a pure d-wave superconducting gap coexists with a larger pseudogap in the anti-nodal region, with the superconducting peak appearing on top of the pseudogap. The d-wave gap parameter Δ₀ is nearly doping-independent (~12–14 meV), indicating distinct origins for the two gaps and supporting a two-gap scenario in high-Tc cuprates.

ABSTRACT

Relationship between the superconducting gap and the pseudogap has been the subject of controversies. In order to clarify this issue, we have studied the superconducting gap and pseudogap of the high-Tc superconductor La2-xSrxCuO4 (x=0.10, 0.14) by angle-resolved photoemission spectroscopy (ARPES). Through the analysis of the ARPES spectra above and below Tc, we have identified a superconducting coherence peak even in the anti-nodal region on top of the pseudogap of a larger energy scale. The superconducting peak energy nearly follows the pure d-wave form. The d-wave order parameter Δ_0 [defined by Δ(k)=Δ_0(cos(kxa)-cos(kya)) ] for x=0.10 and 0.14 are nearly the same, Δ_0 ~ 12-14 meV, leading to strong coupling 2Δ_0/kB Tc ~ 10. The present result indicates that the pseudogap and the superconducting gap are distinct phenomena and can be described by the "two-gap" scenario.

Motivation & Objective

• To clarify the relationship between the pseudogap and superconducting gap in high-Tc cuprates, particularly in La2-xSrxCuO4.
• To determine whether the pseudogap and superconducting gap are distinct phenomena or part of a single gap structure.
• To investigate the momentum and temperature dependence of the gaps using high-resolution ARPES.
• To test the validity of the two-gap scenario in single-layer cuprates versus bilayer systems like Bi2212.

Proposed method

• Performed angle-resolved photoemission spectroscopy (ARPES) on high-quality single crystals of La2-xSrxCuO4 (x=0.10, 0.14) at synchrotron (SSRL) and UV laser (ISSP) sources.
• Acquired spectra below (T < Tc) and above (T > Tc) the superconducting transition temperature to isolate superconducting features.
• Used Fermi-Dirac function convolution with energy resolution to normalize spectra and enhance spectral peak visibility.
• Analyzed spectral line shapes to identify the superconducting coherence peak, especially in the anti-nodal region.
• Compared the energy scale and temperature evolution of the pseudogap and superconducting gap across different doping levels.
• Applied scaling analysis using 2Δ = 4.3kB T to compare gap behavior with theoretical models and other cuprates.

Experimental results

Research questions

• RQ1 Do the pseudogap and superconducting gap coexist in the anti-nodal region of underdoped La2-xSrxCuO4?
• RQ2 Is the superconducting gap in the anti-node region consistent with a pure d-wave symmetry?
• RQ3 How does the superconducting gap parameter Δ₀ vary with doping in the underdoped regime?
• RQ4 Why are two distinct energy scales (pseudogap and superconducting gap) resolved in single-layer cuprates but not in bilayer cuprates like Bi2212?
• RQ5 What is the origin of the pseudogap, and how does it differ from the superconducting gap in terms of energy scale and temperature dependence?

Key findings

• A clear superconducting coherence peak was identified in the anti-nodal region of La2-xSrxCuO4 (x=0.10, 0.14) below Tc, even on top of a pre-existing pseudogap.
• The superconducting gap follows a nearly pure d-wave form, with the order parameter Δ₀ ≈ 12–14 meV for both x=0.10 and x=0.14, indicating doping independence in the underdoped regime.
• The pseudogap energy scale is larger than the superconducting gap and persists above Tc, indicating it is a distinct phenomenon from superconductivity.
• The two-gap scenario is supported by the observation that the superconducting gap opens below Tc within the pre-existing pseudogap, which is consistent with the YRZ phenomenological model.
• The absence of resolved two energy scales in Bi2212 is attributed to comparable Tc and T* (pseudogap onset temperature), unlike in single-layer cuprates where Tc ≪ T*.
• The pseudogap in the anti-nodal region is likely linked to a two-dimensional charge order or density-wave state, as suggested by STM and ARPES evidence of particle-hole asymmetry and Fermi arc suppression.