Observation of electronic bound states in charge-ordered YBa$_2$Cu$_3$O$_y$

TL;DR

This study reports the unexpected detection of electronic bound states in YBa₂Cu₃O₆₊y when a magnetic field induces charge-density-wave order, revealing new insights into disorder effects and charge ordering in cuprate superconductors.

Contribution

It demonstrates the formation of bound states in a charge-ordered cuprate, a phenomenon not previously observed in such systems, linking disorder to charge order effects.

Findings

Bound states observed in charge-ordered YBa₂Cu₃O₆₊y

Asymmetric NMR line profiles indicate local density of states inhomogeneity

Disorder-induced bound states provide insight into charge ordering mechanisms

Abstract

Observing how electronic states in solids react to a local symmetry breaking provides insight into their microscopic nature. A striking example is the formation of bound states when quasiparticles are scattered off defects. This is known to occur, under specific circumstances, in some metals and superconductors but not, in general, in the charge-density-wave (CDW) state. Here, we report the unforeseen observation of bound states when a magnetic field quenches superconductivity and induces long-range CDW order in YBa$_2$Cu$_3$O$_y$. Bound states indeed produce an inhomogeneous pattern of the local density of states $N(E_F)$ that leads to a skewed distribution of Knight shifts which is detected here through an asymmetric profile of $^{17}$O NMR lines. We argue that the effect arises most likely from scattering off defects in the CDW state, which provides a novel case of disorder-induced bound states in a condensed-matter system and an insightful window into charge ordering in the cuprates.