Spin excitations in a single La$_2$CuO$_4$ layer

TL;DR

This study investigates spin excitations in a single layer of La$_2$CuO$_4$, revealing persistent magnons despite the absence of magnetic order and identifying a high-energy continuum unexplained by current theories.

Contribution

First direct measurement of spin dynamics in an isolated La$_2$CuO$_4$ layer, showing coherent magnons without magnetic order and highlighting unexplained high-energy magnetic continuum.

Findings

Coherent magnons persist in a single La$_2$CuO$_4$ layer despite lack of magnetic order.

High-energy magnetic continuum observed, not explained by existing theories.

Magnon excitations are well described by linear spin wave theory in this 2D system.

Abstract

The dynamics of S=1/2 quantum spins on a 2D square lattice lie at the heart of the mystery of the cuprates \cite{Hayden2004,Vignolle2007,Li2010,LeTacon2011,Coldea2001,Headings2010,Braicovich2010}. In bulk cuprates such as \LCO{}, the presence of a weak interlayer coupling stabilizes 3D N\'{e}el order up to high temperatures. In a truly 2D system however, thermal spin fluctuations melt long range order at any finite temperature \cite{Mermin1966}. Further, quantum spin fluctuations transfer magnetic spectral weight out of a well-defined magnon excitation into a magnetic continuum, the nature of which remains controversial \cite{Sandvik2001,Ho2001,Christensen2007,Headings2010}. Here, we measure the spin response of \emph{isolated one-unit-cell thick layers} of \LCO{}. We show that coherent magnons persist even in a single layer of \LCO{} despite the loss of magnetic order, with no evidence for resonating valence bond (RVB)-like spin correlations \cite{Anderson1987,Hsu1990,Christensen2007}. Thus these excitations are well described by linear spin wave theory (LSWT). We also observe a high-energy magnetic continuum in the isotropic magnetic response. This high-energy continuum is not well described by 2 magnon LSWT, or indeed any existing theories.