Universal quantum phase transition from superconducting to insulating-like states in pressurized Bi2Sr2CaCu2O8+δ superconductors

Copper oxide superconductors have been continually fascinating the communities of condensed matter physics and material sciences because they host the highest ambient-pressure superconducting transition temperature ( T c ) and mysterious physics 1–3. Searching for the universal correlation between the superconducting state and its normal state or neighboring ground state is believed to be an effective way for finding clues to elucidate the underlying mechanism of the superconductivity. One of the common pictures for the copper oxide superconductors is that a well-behaved metallic phase will present after the superconductivity is entirely suppressed by chemical doping 4–8 or application of the magnetic field 9. Here, we report the first observation of universal quantum phase transition from superconducting state to insulating-like state under pressure in the under-, optimally- and over-doped Bi2Sr2CaCu2O8+δ (Bi2212) superconductors with two CuO2 planes in a unit cell. The same phenomenon has also been found in the Bi2Sr1.63La0.37CuO6+δ (Bi2201) superconductor with one CuO2 plane and the Bi2.1Sr1.9Ca2Cu3O10+δ (Bi2223) superconductor with three CuO2 planes in a unit cell. These results not only provide fresh information on the cuprate superconductors but also pose a new challenge for achieving unified understandings on the mechanism of the high-Tc superconductivity.

Universal scaling of the critical temperature and the strange-metal scattering rate in unconventional superconductors

Dramatic evolution of properties with minute change in the doping level is a hallmark of the complex chemistry which governs copper oxide superconductivity as manifested in the celebrated superconducting domes as well as quantum criticality taking place at precise compositions. The strange metal state, where the resistivity varies linearly with temperature, has emerged as a central feature in the normal state of copper oxide superconductors. The ubiquity of this behavior signals an intimate link between the scattering mechanism and superconductivity. However, a clear quantitative picture of the correlation has been lacking. Here, we report the observation of quantitative scaling laws between the superconducting transition temperature Tc and the scattering rate associated with the strange metal state in electron-doped copper oxide La2-xCexCuO4 (LCCO) as a precise function of the doping level (x). High-resolution characterization of epitaxial composition-spread films, which encompass the entire overdoped range of LCCO has allowed us to systematically map its structural and transport properties with unprecedented accuracy and increment of Δx = 0.0015. We have uncovered the relations Tc ~ (xc-x)0.5 ~ (A1)0.5, where xc is the critical doping where superconductivity disappears on the overdoped side and A1 is the scattering rate of perfect T-linear resistivity per CuO2 plane. We argue that the striking similarity of the Tc vs A1 relation among copper oxides, iron-based and organic superconductors is an indication of a common mechanism of the strange metal behavior and unconventional superconductivity in these systems.

First-Principles Calculation of Copper Oxide Superconductors That Supports the Kamimura-Suwa Model

In 1986 Bednorz and Műller discovered high temperature superconductivity in copper oxides by chemically doping holes into La2CuO4 (LCO), the antiferromagnetic insulator. Despite intense experimental and theoretical research during the past 34 years, no general consensus on the electronic-spin structures and the origin of pseudogap has been obtained. In this circumstance, we performed a first-principles calculation of underdoped cuprate superconductors La2-xSrxCuO4 (LSCO) within the meta-generalized gradient approximation of the density functional theory. Our calculations clarify first the important role of the anti Jahn-Teller (JT) effect, the backward deformation against the JT distortion in La2CuO4 by doping extra holes. The resulting electronic structure agrees with the two-component theory provided by the tight-binding model of Kamimura and Suwa (K-S), which has been also used to elucidate the d-wave superconductivity. Our first-principles calculation thus justifies the K-S model and demonstrates advanced understanding of cuprates. For example, the remarkable feature of our calculations is the appearance of the spin-polarized band with a nearly flat-band character, showing the peaky nature in the density of states at the Fermi level.