Abstract:
A major mystery in condensed matter systems over the last four decades is the microscopic origin of the strange metal state from which unconventional superconductivity directly emerges by lowering temperatures. This state has been widely observed in various unconventional (including high-Tc) superconductors, including: cuprate, iron pnictides and chalcogenides, nickelates, heavy-fermion compounds, and twisted bi-layer graphene. It is characterized by the incoherent charge transport with linear-in-temperature resistivity and logarithmic-in-temperature specific heat coefficient over a wide range in temperatures. Whether these strange metal phenomena observed in different materials share a common origin is an outstanding open problem. In these strange metals, electrons lose their individual identities, acting collectively in a “soup”, in which all particles are connected through quantum entanglement. The most intriguing class of strange metal is the “Planckian metal phase”, showing quantum critical ħω/kBT scaling in AC scattering rate and universal linear-in-temperature DC scattering rate: 1/τ = αP kB T / ħ with a universal constant prefactor αP ~ 1 as well as logarithmic-in-temperature singular specific heat coefficient. It has been observed in various high-Tc cuprate superconductors over a finite range in doping near optimal doping. Revealing the mystery of the Planckian metal state is believed to be the key to understanding the mechanism for high-Tc superconductivity in cuprates. In this talk, I will review these phenomena and my recently proposed generic microscopic mechanism for this state, based on quantum-critical local bosonic charge Kondo fluctuations coupled to both spinon and a heavy conduction-electron Fermi surfaces within the heavy-fermion formulated t-J model. See the full abstract on the website.