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We report the results of a combined muon spin rotation and neutron scattering study on La2−xSrxCuO4 (LSCO) in the vicinity of the so-called 1/8-anomaly. Application of a magnetic field drives the system towards a magnetically ordered spin-density-wave state, which is fully developed at 1/8 doping. The results are discussed in terms of competition between antiferromagnetic and superconducting order parameters.

We report that in La2−xSrxCuO4 (La214) and Bi2Sr2CaCu2O 8+δ (Bi2212), the resistivity ρ and magnetic susceptibility χ decrease more sharply below T * , corresponding to the onset temperature of a spin gap (pseudogap) at least for Bi2212. In each system, T * scales with the superconducting gap magnitude 2Δ0 (T * ∼ 2Δ0/4.3kB), which increases as the doping level is lowered. We also report that 2Δ0 is almost proportional to Tmax (>T * ), where Tmax is the temperature exhibiting a broad peak of χ and kBTmax has been considered to give the measure of effective exchange energy J eff ; the α factors in 2Δ0 ∼ αkBTmax ∼ αJ eff are nearly one and two for La214 and Bi2212, respectively.KEYWORDS: cuprate superconductor, superconducting gap, pseudogap, STM/STS 2622 in-plane resistivity below T * , according to the spin gap formation. 13, 14) Temperature T * , much higher than T c at low p, decreases with increasing p and seems to connect smoothly to the T c -p curve at an overdoping level. Interestingly, it has recently been demonstrated by scanning tunneling spectroscopy (STS) that in Bi2212 the p dependence of T * correlates with that of the superconducting gap magnitude 2Δ 0 at T T c . 14) In this paper, we report that in La214 and Bi2212, the resistivity and magnetic susceptibility decrease more sharply below T * , comparable to the onset temperature of a spin gap (pseudogap) at least for Bi2212, and T * scales with 2Δ 0 examined by STS and break junction tunneling spectroscopy (BJTS) at T T c . Interestingly, T * ∼ 2Δ 0 /4.3k B in the entire p-range examined, corresponding to the transition temperature T MF c expected in the mean-field theory for d-wave superconductors or the onset temperature T RVB of spin-channel singlet pairing expected in the mean-field RVB theory. This fact suggests that the crossover behavior around T * will be superconducting in origin. We will also report that 2Δ 0 is almost proportional to k B T max (∼J eff ); the α factors in 2Δ 0 ∼ αk B T max ∼ αJ eff are nearly one and two for the typical monolayer and bilayer cuprates, La214 and Bi2212, respectively.Most of the present experiments were performed using single crystals, except for break junction tunneling, resistivity and magnetic susceptibility measurements of La214, which were mainly done using ceramic samples. The doping level p of La214 was defined as the Sr content x. On the other hand, the p value of Bi2212, controlled by annealing in oxygen or by substituting Dy 3+ for Ca 2+ , 14) was estimated from the T c value so that it falls on the T c -p curve determined by Gröen et al. 15) In STM/STS experiments, we used an Olympus LTSTM-300 system with a Pt-Ir alloy tip. The tip was approached along the direction perpendicular to the CuIt has been broadly thought in high-T c cuprates that the electronic properties exhibit two crossover behaviors around T max (or T 0 ) and T * (T max > T * > T c ), which are both reduced by increasing the carrier-doping level p of the Cu-O plane. Around T max , the uniform magnetic susceptibility χ, following th...

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We report the observation of a bulk charge modulation in La1.88Sr0.12CuO4 (LSCO) with a characteristic in-plane wave-vector of (0.236, ±δ), with δ=0.011 r.l.u. The transverse shift of the ordering wave-vector indicates the presence of rotated charge-stripe ordering, demonstrating that the charge ordering is not pinned to the Cu-O bond direction. On cooling through the superconducting transition, we find an abrupt change in the growth of the charge correlations and a suppression of the charge order parameter indicating competition between the two orderings. Orthorhombic LSCO thus helps bridge the apparent disparities between the behavior previously observed in the tetragonal "214" cuprates and the orthorhombic yttrium and bismuth-based cuprates and thus lends strong support to the idea that there is a common motif to charge order in all cuprate families.

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The properties of cuprate high-temperature superconductors are largely shaped by competing phases whose nature is often a mystery. Chiefly among them is the pseudogap phase, which sets in at a doping p* that is material-dependent. What determines p* is currently an open question. Here we show that the pseudogap cannot open on an electron-like Fermi surface, and can only exist below the doping p FS at which the large Fermi surface goes from hole-like to electron-like, so that p* ≤ p FS. We derive this result from high-magnetic-field transport measurements in La1.6−xNd0.4SrxCuO4 under pressure, which reveal a large and unexpected shift of p* with pressure, driven by a corresponding shift in p FS. This necessary condition for pseudogap formation, imposed by details of the Fermi surface, is a strong constraint for theories of the pseudogap phase. Our finding that p* can be tuned with a modest pressure opens a new route for experimental studies of the pseudogap.

We report the ultrafast optical response of quasiparticles (QPs) in both the pseudogap (PG) and superconducting (SC) states of an underdoped Bi2Sr2CaCu2O8 + y (Bi2212) single crystal measured with the time-resolved pump-probe technique. At a probe energy variant planck's over omegapr = 1.55 eV, it is found that the reflectivity change DeltaR/R changes its sign at exactly Tc, which allows the direct separation of the charge dynamics of PG and SC QPs. Further systematic investigations indicate that the transient signals associated with PG and SC QPs depend on the probe beam energy and polarization. By tuning them below Tc, two distinct components can be detected simultaneously, providing evidence for the coexistence of PG and SC QPs.