Local noncentrosymmetric structure of Bi2Sr2CaCu2O8+y by X-ray magnetic circular dichroism at Cu K-edge XANES

TL;DR

This study provides evidence for local noncentrosymmetric structure at Cu sites in Bi2Sr2CaCu2O8+y, challenging the common assumption of centrosymmetry in its CuO2 layers, using advanced X-ray techniques.

Contribution

The paper presents experimental evidence of local noncentrosymmetric Cu site structure in Bi2212 using polarized XANES and XMCD, supported by numerical simulations.

Findings

XMCD signal indicates noncentrosymmetric local structure

Agreement with noncentrosymmetric Bb2b space group in simulations

Challenges the assumption of centrosymmetry in CuO2 layers

Abstract

The two-dimensional Bi2Sr2CaCu2O8+y (Bi2212), the most studied prototype cuprate superconductor, is a lamellar system made of a stack of two-dimensional corrugated CuO2 bilayers separated by Bi2O2+ySr2O2 layers. While the large majority of theories, proposed to interpret unconventional high Tc superconductivity in Bi2Sr2CaCu2O8+y, assume a centrosymmetric tetragonal CuO2 lattice for the [CuO2]Ca[CuO2] bilayer here we report new compelling results providing evidence for local noncentrosymmetric structure at the Cu site. We have measured polarized Cu K-edge XANES (x-ray absorption near edge structure) and the K-edge X-ray magnetic circular dichroism (XMCD) of a Bi2212 single crystal near optimum doping. The Cu K edge XMCD signal was measured at ID12 beamline of ESRF with the k-vector of x-ray beam parallel to c-axis i.e. with the electric field of x-ray beam E//ab, using a 17 T magnetic field parallel to the c-axis of a Bi2212 single crystal. Numerical simulations of the XMCD signal of Bi2212 by multiple scattering theory have shown agreement with the experimental XMCD signal only for the local structure with noncentrosymmetric Bb2b space group of Bi2Sr2CaCu2O8+y.