Revisiting $^{63}$Cu NMR evidence for charge order in superconducting La$_{1.885}$Sr$_{0.115}$CuO$_4$

TL;DR

This study uses advanced 63Cu NMR techniques to investigate charge order in La$_{1.885}$Sr$_{0.115}$CuO$_4$, revealing time-scale dependent anomalies and inhomogeneous spin correlations associated with charge ordering.

Contribution

It demonstrates that 63Cu NMR signals exhibit unprecedented dependence on measurement time scale, uncovering new insights into charge order dynamics in LSCO.

Findings

Emergence of broad wing-like NMR signals below T(charge) for short measurement times

Strong dependence of linewidth and relaxation rates on measurement time below T(charge)

Charge order induces inhomogeneous Cu spin-spin correlations

Abstract

The presence of charge and spin stripe order in the La2CuO4-based family of superconductors continues to lead to new insight on the unusual ground state properties of high Tc cuprates. Soon after the discovery of charge stripe order at T(charge)=65K in Nd3+ co-doped LSCO ($T_{c}\simeq6$~K) [Tranquada et al., Nature {\bf 375} (1995) 561], Hunt et al. demonstrated that La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$ and superconducting LSCO with x~1/8 (Tc ~ 30K) share nearly identical NMR anomalies near $T_{charge}$ of the former [Phys. Rev. Lett. {\bf 82} (1999) 4300]. Their inevitable conclusion that LSCO also undergoes charge order at a comparable temperature became controversial, because diffraction measurements at the time were unable to detect Bragg peaks associated with charge order. Recent advances in x-ray diffraction techniques finally led to definitive confirmations of the charge order Bragg peaks in LSCO with an onset at as high as T(charge)=80K. Meanwhile, improved instrumental technology has enabled routine NMR measurements that were not feasible two decades ago. Motivated by these new developments, we revisit the charge order transition of a LSCO single crystal based on 63Cu NMR techniques. We demonstrate that 63Cu NMR properties of the nuclear spin $I_{z}$ = -1/2 to +1/2 central transition below T(charge) exhibit unprecedentedly strong dependence on the measurement time scale set by the NMR pulse separation time $\tau$; a new kind of anomalous, very broad wing-like 63Cu NMR signals gradually emerge below T(charge) only for extremely short $\tau \lesssim 4~\mu$s, while the spectral weight of the normal NMR signals is progressively wiped out. The NMR linewidth and relaxation rates depend strongly on $\tau$ below T(charge), and their enhancement in the charge ordered state indicates that charge order turns on strong but inhomogeneous growth of Cu spin-spin correlations.