We report muon spin rotation and magnetic susceptibility experiments on in-plane stress effects on the static spin-stripe order and superconductivity in the cuprate system ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$ with $x=0.115$. An extremely low uniaxial stress of $\sim 0.1\mathrm{GPa}$ induces a substantial decrease in the magnetic volume fraction and a dramatic rise in the onset of 3D superconductivity, from $\sim 10$ to 32 K; however, the onset of at-least-2D superconductivity is much less sensitive to stress. These results show not only that large-volume-fraction spin-stripe order is anticorrelated with 3D superconducting coherence but also that these states are energetically very finely balanced. Moreover, the onset temperatures of 3D superconductivity and spin-stripe order are very similar in the large stress regime. These results strongly suggest a similar pairing mechanism for spin-stripe order and the spatially modulated 2D and uniform 3D superconducting orders, imposing an important constraint on theoretical models.

We demonstrate the presence of ferromagnetic (FM) fluctuations in the superconducting and nonsuperconducting heavily overdoped regimes of high-temperature superconducting copper oxides, using $(\mathrm{Bi},\mathrm{Pb}{)}_2{\mathrm{Sr}}_2{\mathrm{CuO}}_{6+\delta }$ (Bi-2201) single crystals. Magnetization curves exhibit a tendency to be saturated in high magnetic fields at low temperatures in the heavily overdoped crystals, which is probably a precursor phenomenon of a FM transition at a lower temperature. Muon spin relaxation detects the enhancement of spin fluctuations at high temperatures below 200 K. Correspondingly, the $ab$-plane resistivity follows a $4/3$ power law in a wide temperature range, which is characteristic of metals with two-dimensional FM fluctuations due to itinerant electrons. As the Wilson ratio evidences the enhancement of spin fluctuations with hole doping in the heavily overdoped regime, it is concluded that two-dimensional FM fluctuations reside in the heavily overdoped Bi-2201 cuprates, which is probably related to the decrease in the superconducting transition temperature in the heavily overdoped cuprates.

We use resonant inelastic x-ray scattering to investigate charge-stripe correlations in ${\mathrm{La}}_{1.675}{\mathrm{Eu}}_{0.2}{\mathrm{Sr}}_{0.125}{\mathrm{CuO}}_4$. By differentiating elastic from inelastic scattering, it is demonstrated that charge-stripe correlations precede both the structural low-temperature tetragonal phase and the transport-defined pseudogap onset. The scattering peak amplitude from charge stripes decays approximately as $T^{-2}$ towards our detection limit. The in-plane integrated intensity, however, remains roughly temperature independent. Therefore, although the incommensurability shows a remarkably large increase at high temperature, our results are interpreted via a single scattering constituent. In fact, direct comparison to other stripe-ordered compounds (${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125}{\mathrm{CuO}}_4$, ${\mathrm{La}}_{1.475}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.125}{\mathrm{CuO}}_4$, and ${\mathrm{La}}_{1.875}{\mathrm{Sr}}_{0.125}{\mathrm{CuO}}_4$) suggests a roughly constant integrated scattering intensity across all these compounds. Our results therefore provide a unifying picture for the charge-stripe ordering in La-based cuprates. As charge correlations in ${\mathrm{La}}_{1.675}{\mathrm{Eu}}_{0.2}{\mathrm{Sr}}_{0.125}{\mathrm{CuO}}_4$ extend beyond the low-temperature tetragonal and pseudogap phase, their emergence heralds a spontaneous symmetry breaking in this compound.

In this review article, we show our recent results relating to the undoped (Ce-free) superconductivity in the electron-doped high- $T_{\mathrm{c}}$ cuprates with the so-called T’ structure. For an introduction, we briefly mention the characteristics of the electron-doped T’-cuprates, including the reduction annealing, conventional phase diagram and undoped superconductivity. Then, our transport and magnetic results and results relating to the superconducting pairing symmetry of the undoped and underdoped T’-cuprates are shown. Collaborating spectroscopic and nuclear magnetic resonance results are also shown briefly. It has been found that, through the reduction annealing, a strongly localized state of carriers accompanied by an antiferromagnetic pseudogap in the as-grown samples changes to a metallic and superconducting state with a short-range magnetic order in the reduced superconducting samples. The formation of the short-range magnetic order due to a very small amount of excess oxygen in the reduced superconducting samples suggests that the T’-cuprates exhibiting the undoped superconductivity in the parent compounds are regarded as strongly correlated electron systems, as well as the hole-doped high- $T_{\mathrm{c}}$ cuprates. We show our proposed electronic structure model to understand the undoped superconductivity. Finally, unsolved future issues of the T’-cuprates are discussed.

In order to investigate the inhomogeneity of the superconducting (SC) state in the overdoped high-$T_{\rm c}$ cuprates, we have measured the magnetic susceptibility, $\chi$, of La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) single crystals in the overdoped regime in magnetic fields parallel to the c-axis up to 7 T on warming after zero-field cooling. It has been found for $x$ = 0.198 and 0.219 that the temperature dependence of $\chi$ in 1 T shows a plateau, that is, $\chi$ is almost independent of temperature in a moderate temperature-range in the SC state. Moreover, a so-called second peak in the magnetization curve has markedly appeared in these crystals. These results indicate an anomalous enhancement of the vortex pinning and strongly suggest the occurrence of a $microscopic$ phase separation into SC and normal-state regions in the overdoped high-$T_{\rm c}$ cuprates.Comment: 4 pages, 4 figure

The ability to efficiently control charge and spin in the cuprate high-temperature superconductors is crucial for fundamental research and underpins technological development. Here, we explore the tunability of magnetism, superconductivity, and crystal structure in the stripe phase of the cuprate La 2 − x Ba x CuO 4 , with x = 0.115 and 0.135, by employing temperature-dependent (down to 400 mK) muon-spin rotation and AC susceptibility, as well as X-ray scattering experiments under compressive uniaxial stress in the CuO 2 plane. A sixfold increase of the three-dimensional (3D) superconducting critical temperature T c and a full recovery of the 3D phase coherence is observed in both samples with the application of extremely low uniaxial stress of ∼ 0.1 GPa. This finding demonstrates the removal of the well-known 1/8-anomaly of cuprates by uniaxial stress. On the other hand, the spin-stripe order temperature as well as the magnetic fraction at 400 mK show only a modest decrease under stress. Moreover, the onset temperatures of 3D superconductivity and spin-stripe order are very similar in the large stress regime. However, strain produces an inhomogeneous suppression of the spin-stripe order at elevated temperatures. Namely, a substantial decrease of the magnetic volume fraction and a full suppression of the low-temperature tetragonal structure is found under stress, which is a necessary condition for the development of the 3D superconducting phase with optimal T c . Our results evidence a remarkable cooperation between the long-range static spin-stripe order and the underlying crystalline order with the three-dimensional fully coherent superconductivity. Overall, these results suggest that the stripe- and the SC order may have a common physical mechanism.