Although many years have passed since the discovery of high-critical-temperature (high-) superconducting materials, the underlying mechanism is still unknown. The B1g phonon anomaly in high-Tc cuprate superconductors has long been studied; however, the correlation between the B1g phonon anomaly and superconductivity has yet to be clarified. In the present study, we successfully reproduced the B1g phonon anomaly in YBaCuO (YBCO) using an ab initio molecular dynamics (AIMD) simulation and temperature-dependent effective potential (TDEP) method. The Ag phonon by Ba atoms shows a more severe anomaly than the B1g phonon at low temperatures. Our analysis of the phonon anomaly and the temperature-dependent phonon dispersion indicated that decoupling between thermal phenomena and electron transport at low temperatures leads to layer-by-layer thermal decoupling in YBCO. Electronically and thermally isolated Ba atoms in YBCO are responsible for the thermal decoupling. The analytic study of the thermal dcoupling revealed that Planckian dissipation expressing linear-T resistivity is another expression of the Fermi liquid of the CuO plane. The Uemura plot of the relationship between Tc and the Fermi temperature, as well as the superconducting dome in YBCO, is explained rigorously and quantitatively. Our findings not only present a new paradigm for understanding high-Tc superconductivity but also imply that the spontaneous formation of low-temperature layers in materials can lead to revolutionary changes in the thermal issues of industrial fields.

ArXiv, arXiv:2303.11600 (2023) Link

Article Link

Paper Link

The structural transformations of graphene defects have been extensively researched through aberration-corrected transmission electron microscopy (AC-TEM) and theoretical calculations. For a long time, a core concept in understanding the structural evolution of graphene defects has been the Stone-Thrower-Wales (STW)–type bond rotation. In this study, we show that undercoordinated atoms induce bond formation and breaking, with much lower energy barriers than the STW-type bond rotation. We refer to them as mediator atoms due to their mediating role in the breaking and forming of bonds. Here, we report the direct observation of mediator atoms in graphene defect structures using AC-TEM and annular dark-field scanning TEM (ADF-STEM) and explain their catalytic role by tight-binding molecular dynamics (TBMD) simulations and image simulations based on density functional theory (DFT) calculations. The study of mediator atoms will pave a new way for understanding not only defect transformation but also the growth mechanisms in two-dimensional materials. 

MORE

Science Advances, Vol 6, No 24, eaba4942 (2020)