We present triple-axis neutron scattering studies of low-energy magnetic fluctuations in strongly underdoped La2-xSrxCuO4 with x = 0.05, 0.06 and 0.07, providing quantitative evidence for a direct competition between these fluctuations and superconductivity. At dopings x = 0.06 and x = 0.07, three-dimensional superconductivity is found, while only a very weak signature of two-dimensional superconductivity residing in the CuO2 planes is detectable for x = 0.05. We find a surprising suppression of the low-energy fluctuations by an external magnetic field at all three dopings. This implies that the response of two-dimensional superconductivity to a magnetic field is similar to that of a bulk superconductor. Our results provide direct evidence of a very gradual onset of superconductivity in cuprates.

We present triple-axis neutron scattering studies of static and dynamic magnetic stripes in an optimally oxygen-doped cuprate superconductor, ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4+y}$, which exhibits a clean superconducting transition at ${T}_{c}=42\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Polarization analysis reveals that the magnetic stripe structure is equally represented along both of the tetragonal crystal axes and that the fluctuating stripes display significant weight for in-plane as well as out-of-plane spin components. Both static magnetic order as well as low-energy fluctuations are fully developed in zero applied magnetic field and the low-energy spin fluctuations at $\ensuremath{\hbar}\ensuremath{\omega}=0.3\text{--}10\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ intensify on cooling. We interpret this as an indication that superconductivity and low-energy spin fluctuations coexist microscopically in spatial regions which are separated from domains with static magnetic order.

Inelastic neutron scattering has been used to study the in-plane Cu-O bond-stretching mode in oxygen-doped ${\mathrm{La}}_{1.94}{\mathrm{Sr}}_{0.06}{\mathrm{CuO}}_{4.035}$ $({T}_{c}=38\phantom{\rule{0.28em}{0ex}}\text{K})$ and ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4+\ensuremath{\delta}}$ $({T}_{c}=43\phantom{\rule{0.28em}{0ex}}\text{K})$. Similar to results from optimally doped ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}{\mathrm{CuO}}_{4}$ $({T}_{c}=35\phantom{\rule{0.28em}{0ex}}\text{K})$, we observe anomalous features in the dispersion of this half-breathing mode in the form of a softening halfway through the Brillouin zone. Considering the differences in electronic structure and local environment between the oxygen- and strontium-doped compounds with similar ${T}_{\text{c}}$, we rule out a connection between the phonon anomaly and structural instabilities related to the specific dopant type. We interpret the phonon anomaly as a signature of correlated charge fluctuations ubiquitous in optimally doped superconductors.

Neutron diffraction has been a very prominent tool to investigate high-temperature superconductors, in particular through the discovery of an incommensurate magnetic signal known as stripes. We here report the findings of a neutron diffraction experiment on the superconductor (La,Sr)(2)CuO4, where a spurious signal appeared to be magnetic stripes. The signal strength was found to be strongly dependent on the neutron energy, peaking at E = 4.6 meV. We therefore attribute the origin of this signal to be a combination of multiple scattering and crystal twinning. A forward calculation of the scattering intensity including these two effects almost completely recovers our experimental observations. We emphasize the need for employing such analysis when searching for ways to avoid spurious scattering signals.

Inelastic neutron scattering has been used to study the in-plane Cu-O bond-stretching mode in oxygen doped La_1.94Sr_0.06CuO_4.035 (T_c = 38 K) and La_2CuO_4+δ (T_c = 43 K). Similar to results from optimally doped La_1.85Sr_0.15CuO_4 (T_c = 35 K), we observe anomalous features in the dispersion of this half-breathing mode in the form of a softening halfway through the Brillouin Zone. Considering the differences in electronic structure and local environment between the oxygen- and strontium-doped compounds with similar T_c, we rule out a connection between the phonon anomaly and structural instabilities related to the specific dopant type. We interpret the phonon anomaly as a signature of correlated charge fluctuations possibly connected to stripes.