High-temperature superconductivity in copper oxides, with critical temperature well above what was anticipated by the BCS theory, remains a major unsolved physics problem. The problem is fascinating because it is simultaneously simple - being a single band and ½ spin system, yet extremely rich - boasting d-wave superconductivity, pseudogap, spin and charge orders, and strange metal phenomenology. For this reason, cuprates symbolize the intellectual challenge for correlated electrons – stimulating conversations on the physics of Hubbard model, quantum critical point, Planckian metal, and influencing the discussions from cold atoms to twist-stacked 2D materials.
Central to this debate is whether the Hubbard model, which is the natural starting point for the undoped magnetic insulator, contains the essential ingredients for key physics in cuprates. In this talk, I will discuss photoemission evidence for a multifaceted answer to this question [1-3], leading to the conclusion that the Hubbard physics is necessary but not sufficient to describe the rich physics. First, we show results that naturally points to the importance of Coulomb and magnetic interactions, including d-wave superconducting gap structure , exchange energy (J) control of bandwidth in single-hole dynamics . Second, we evidence effects beyond the Hubbard model, including band dispersion anomalies at known phonon frequencies [6, 7], polaronic spectral lineshape and the emergence of quasiparticle with doping . Third, we show properties likely of hybrid electronic and phononic origin, including two energy scales for pseudogap (defined as gaps above Tc), and the almost vertical phase boundary near the critical 19% doping [9-12]. Fourth, we illustrate examples of small q phononic coupling that cooperates with d-wave superconductivity [13-15]. The additional phononic involvement is consistent with the recent experimental advance in synthesizing and investigating doped one-dimensional (1D) cuprates , where a more reliable 1D calculation enables a robust comparison. Finally, we discuss the physics of strong phase fluctuations in cuprates [17-18]. These observations allow us to connect the dots of the above classes of experimental observables and provide a holistic picture of the essential microscopic ingredients for cuprates physics, including the elusive d-wave superconductivity in 2D Hubbard model.
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Biography: Zhi-Xun Shen is a member of the US National Academy of Sciences, the American Academy of Arts and Sciences, and the Chinese Academy of Sciences. His primary interest is novel quantum phenomena in materials. His work has been recognized by the E.O. Lawrence Award, the Oliver E. Buckley Prize, the H. Kamerlingh Onnes Prize, the Einstein Professorship Award of CAS, and the Tage Erlander professorship of the Swedish Research Council. He has been Chief Scientist of SLAC and director of the Institute for Material and Energy Sciences and of the Geballe Laboratory for Advanced Materials at Stanford University. He has mentored close to one hundred graduate students and post-docs and he is a co-inventor of several patents.