Disappearance of Antiferromagnetic Spin Excitations in Overdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$

Magnetic excitations for energies up to $\sim 100\mathrm{meV}$ are studied for overdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ with $x=0.25$ and 0.30, using time-of-flight neutron spectroscopy. Comparison of spectra integrated over the width of an antiferromagnetic Brillouin zone demonstrates that the magnetic scattering at intermediate energies, $20\lesssim \omega \lesssim 100\mathrm{meV}$, progressively decreases with overdoping. This strongly suggests that the magnetism is not related to Fermi surface nesting, but rather is associated with a decreasing volume fraction of (probably fluctuating) antiferromagnetic bubbles.

Figure 1

Contour plots of $\omega \frac{d\sigma }{d\Omega d\omega }$ for (a) LSCO $x=0.25$ and (b) LBCO $x=1/8$, all measured with $E_i=140\mathrm{meV}$ at 10 K. Data for LBCO $x=1/8$ are from Xu et al. [12]. The plotted quantities are integrated for $0.5\le H\le 1.5\mathrm{r}.\mathrm{l}.\mathrm{u}.$ and projected on the $K$ axis. (c) and (d) show contour plots of differential intensities derived by subtracting data of LSCO $x=0.30$ from those of LSCO $x=0.25$ and those of LBCO $x=1/8$, respectively. The white regions arising from $K\sim \pm 0.7$ correspond to areas without detector coverage.

Figure 2

Intensity profiles cut along the $K$ axis, with data integrated over $0.5\le H\le 1.5\mathrm{r}.\mathrm{l}.\mathrm{u}.$, corresponding to the $H$-width of the shaded area in the top figure, also integrated over an energy range of $\pm 5\mathrm{meV}$ about the excitation energy specified in each panel. All data are taken with $E_i=140\mathrm{meV}$. Circles, diamonds, and squares represent LBCO $1/8$, LSCO $x=0.25$ and 0.3, respectively. Solid lines are results of fits to a double-Gaussian function. The fits for $x=0.30$ at $\omega =35$ and 55 meV are set equal to background; in the bottom panels, background is shown by dashed lines.

Figure 3

$\mathbf{Q}$-integrated dynamic structure factor $S(\omega )$ which is derived from the wide-$H$ integrated profiles for LBCO $1/8$ (squares), LSCO $x=0.25$ (diamonds; filled for $E_i=140\mathrm{meV}$, open for $E_i=80\mathrm{meV}$), and $x=0.30$ (filled circles) plotted over $S(\omega )$ for LBCO $1/8$ (open circles) from 2. The solid lines following data of LSCO $x=0.25$ and 0.30 are guides to the eyes.