Cu nuclear magnetic resonance study of charge and spin stripe order in ${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125}{\mathrm{CuO}}_4$

We present a Cu nuclear magnetic/quadrupole resonance study of the charge stripe ordered phase of LBCO, with detection of previously unobserved (“wiped-out”) signal. We show that spin-spin and spin-lattice relaxation rates are strongly enhanced in the charge ordered phase, explaining the apparent signal decrease in earlier investigations. The enhancement is caused by magnetic, rather than charge fluctuations, conclusively confirming the long-suspected assumption that spin fluctuations are responsible for the wipeout effect. Observation of the full Cu signal enables insight into the spin and charge dynamics of the stripe-ordered phase, and measurements in external magnetic fields provide information on the nature and suppression of spin fluctuations associated with charge order. We find glassy spin dynamics, in agreement with previous work, and incommensurate static charge order with charge modulation amplitude similar to other cuprate compounds, suggesting that the amplitude of charge stripes is universal in the cuprates.

Figure 1

Spectra of pure Cu NQR (without applied magnetic field) of LBCO in dependence on temperature, measured with an echo time of 2.3 $\mu \mathrm{s}$, normalized to their total areas and shifted vertically for clarity. Two sets of lines are observed, referred to as A and B lines. Both appear twice for the two isotopes ${}^{63}\mathrm{Cu}$ and ${}^{63}\mathrm{Cu}$. Charge order sets in below $\sim 53$ K, where a progressive broadening of the lines is seen.

Figure 2

Width of the ${}^{63}\mathrm{Cu}$ NQR B line in dependence on temperature in the charge ordered phase, compared to a resonant x-ray measurement of the charge order peak intensity [30]. The NQR linewidth is seen to correspond exactly to the charge order parameter, as one should expect for incommensurable stripes. The LTO-LTT structural transition at $T_{LTT}\approx 57$ K and the spin-ordering temperature $T_{SO}\approx 40$ K are from a neutron diffraction study [1]. Inset shows the integrated B line signal intensity corrected for temperature and spin-spin relaxation, demonstrating that no significant decrease occurs below $T_{CO}$.

Figure 3

(a) Spin-spin relaxation curves measured at the center of the B line at two representative temperatures. Both curves are normalized to $M_0\left(56\text{K}\right)$ and extrapolate to 1, demonstrating that all Cu spins are observed in the charge-ordered phase. The relaxation has a significant Gaussian component at high temperatures, and changes to purely exponential below $T_{CO}$, similar to previous results on other cuprates. Data have been corrected for the usual Boltzmann factor and linewidth change. The constant offset in the 42 K curve is the noise floor. (b) Spin-lattice relaxation curves for the Cu A and B lines at 45 K, isotope ${}^{63}\mathrm{Cu}$ (squares) and ${}^{63}\mathrm{Cu}$ (circles). The echo time used in the detection sequence is 2.8 $\mu \mathrm{s}$, and the A line data are shifted for clarity. No significant stretching of the exponential relaxation is observed. The inset shows the A line magnetization recovery curve with a logarithmic time scale.

Figure 4

Spin-lattice relaxation rates for A and B lines in dependence on temperature, normalized to the respective high-temperature values. A strong increase of the relaxation rate is observed in the charge-ordered phase. Solid line is a fit to the Vogel-Fulcher-Tamman relation used for glassy relaxation (see text). Inset: raw $T_1$ data for the A and B lines.

Figure 5

Cu spin-spin relaxation times at 50 K in dependence of an applied external field in the copper-oxygen plane. The fluctuations responsible for the fast spin-spin relaxation at $B=0$ T are all but absent above $\sim 2.5$ T. Inset: lines are the six transitions of a ${}^{63}\mathrm{Cu}$ nucleus (spin $3/2$) calculated numerically for an external field perpendicular to the EFG symmetry axis (see text); points are actual frequencies used in the experiment to obtain the results in the main figure.