For the cuprate high temperature superconductivity (high-Tc) research over the past three decades, the biggest challenge is to identify the relevant low energy degrees of freedom that are critical to formulating the correct theoretical model for high-Tc superconductivity. The main difficulty lies in the closeness between various relevant energy scales. For low energy processes that are comparable to the superconducting gap energy ∆sc, there are the spin exchange energy J, the lattice vibration (phonon) energy Ωph, and the van Hove singularity energy E(π,0). However, anomalous isotope effects on Tc and superfluid density in the cuprates cannot be captured by traditional phonon-mediated superconductivity theories. Historically, a purely electronic Hamiltonian – the Hubbard model – was widely regarded to encapsulate all the core physics of the high-Tc phenomena.
In a recent paper published in Science, scientists from Stanford University and from Stanford Institute for Materials and Energy Sciences (SIMES), in collaboration with material scientists from Japan and theoreticians from Japan, the Netherlands, and Berkeley, reinstated the substantial role of the lattice vibration in the cuprate high-Tc superconductivity – however, in a subtle way that is highly intertwined with the electronic correlations. They finely straddled 18 differently hole-doped high-Tc compound Bi2Sr2CaCu2O8+δ within 8% change of hole carrier concentration, a doping range where Tc evolves from 47 K to 95 K through a putative quantum critical point, around which the electronic correlation effect experiences a sudden change. Then systematic experiments were carried out using the angle-resolved photoemission spectroscopy (ARPES) facility at SSRL Beam Line 5-4. Here, the high-resolution ARPES end station provided critical information of both the superconducting gap and the electron-lattice coupling.
Image: Intertwined growth of the superconductivity and the electron-phonon coupling tuned by the hole concentration. The red line is an illustration of the Tc in Bi-2212 (Tcmax = 95 K). The blue shade and line represent the single-layer Bi-2201 system, where the coupling to the B1g mode is weak and T max is only 38 K. The yellow ball represents the optimally doped tri-layer Bi-2223 where Tcmax is 108 K. The top-right inset shows the intertwined relation between the pseudogap and the EPC under strong electronic correlation. The Madelung potential and the lattice stacking along the c-axis are schematically depicted for the single- layer, bi-layer and tri-layer systems. The dark grey blocks represent the CuO2n- plane, and the light grey blocks represent the charge reservoir layers (Ca2+, SrO, BiO+). The orange dots mark the CuO2n- planes that experience to the first order a non-zero out-of-plane electric field.
Credit: Science, doi: 10.1126/science.aar3394