In the copper oxide parent compounds of the high-transition-temperature superconductors the valence electrons are localized--one per copper site--by strong intra-atomic Coulomb repulsion. A symptom of this localization is antiferromagnetism, where the spins of localized electrons alternate between up and down. Superconductivity appears when mobile 'holes' are doped into this insulating state, and it coexists with antiferromagnetic fluctuations. In one approach to describing the coexistence, the holes are believed to self-organize into 'stripes' that alternate with antiferromagnetic (insulating) regions within copper oxide planes, which would necessitate an unconventional mechanism of superconductivity. There is an apparent problem with this picture, however: measurements of magnetic excitations in superconducting YBa2Cu3O6+x near optimum doping are incompatible with the naive expectations for a material with stripes. Here we report neutron scattering measurements on stripe-ordered La1.875Ba0.125CuO4. We show that the measured excitations are, surprisingly, quite similar to those in YBa2Cu3O6+x (refs 9, 10) (that is, the predicted spectrum of magnetic excitations is wrong). We find instead that the observed spectrum can be understood within a stripe model by taking account of quantum excitations. Our results support the concept that stripe correlations are essential to high-transition-temperature superconductivity.

We present a neutron scattering study of stripe correlations measured on a single crystal of La1.875Ba0.125CuO4. Within the low-temperature-tetragonal (LTT) phase, superlattice peaks indicative of spin and charge stripe order are observed below 50 K. For excitation energieshω ≤ 12 meV, we have characterized the magnetic excitations that emerge from the incommensurate magnetic superlattice peaks. In the ordered state, these excitations are similar to spin waves. Following these excitations as a function of temperature, we find that there is relatively little change in the Q-integrated dynamical spin susceptibility forhω ∼ 10 meV as stripe order disappears and then as the structure transforms from LTT to the low-temperature-orthorhombic (LTO) phase. The Qintegrated signal at lower energies changes more dramatically through these transitions, as it must in a transformation from an ordered to a disordered state. We argue that the continuous evolution through the transitions provides direct evidence that the incommensurate spin excitations in the disordered state are an indicator of dynamical charge stripes. An interesting feature of the thermal evolution is a variation in the incommensurability of the magnetic scattering. Similar behavior is observed in measurements on a single crystal of La1.875Ba0.075Sr0.050CuO4; maps of the scattered intensity in a region centered on the antiferromagnetic wave vector and measured athω = 4 meV are well reproduced by a model of disordered stripes with a temperature-dependent mixture of stripe spacings. We discuss the relevance of our results to understanding the magnetic excitations in cuprate superconductors.

The attempt to understand cuprate superconductors is complicated by the presence of multiple strong interactions. While many believe that antiferromagnetism is important for the superconductivity, there has been revived interest in the role of electron-lattice coupling. 1,2,3,4The recently studied conventional superconductor MgB 2 has a very strong electron-lattice coupling, involving a particular vibrational mode (phonon), that was predicted by standard theory and confirmed quantitatively by experiment. 5Here we present inelastic scattering measurements that show a similarly strong anomaly in the Cu-O bondstretching phonon in the cuprate superconductors La 2-x Sr x CuO 4 (with x=0.07, 0.15). This is in contrast to conventional theory, which does not predict such

We have performed a series of elastic neutron scattering measurements on 1/8-hole doped La1.875Ba0.125−xSrxCuO4 single crystals with x = 0.05, 0.06, 0.075 and 0.085. Both charge-densitywave (CDW) and spin-density-wave (SDW) orders are found to develop simultaneously below the structural transition temperature between the low-temperature orthorhombic (LTO) and lowtemperature tetragonal (LTT) or low-temperature less-orthorhombic (LTLO) phases. In the ground state the CDW order is observed only in the LTT/LTLO phase and drastically degrades towards the LTO boundary. The x-dependence of Tc strongly suggests a direct effect of the CDW order on the suppression of superconductivity. Results are discussed in comparison with those from the La1.6−xNd0.4SrxCuO4 system within the framework of the stripe model. PACS numbers: 74.72. Dn, 71.45.Lr, 75.30.Fv, 74.25.Dw The interplay between magnetism and superconductivity is a central issue in high-T c superconductivity [1]. Neutron scattering measurements on the superconducting La 2−x Sr x CuO 4 (LSCO) system have shown dynamical incommensurate (IC) magnetic correlation [2][3][4]. The linear doping dependence of incommensurability with the superconducting transition temperature (T c ) in the under-doped region suggests a relationship between magnetic correlation and superconductivity [5].On the other hand, it is well known that superconductivity in the La 2−x M x CuO 4 (M=Ba, Sr) system is anomalously suppressed at the specific hole concentration of x ∼1/8. For the La 2−x Ba x CuO 4 (LBCO) system, this 1/8-anomaly is accompanied by an occurrence of the lowtemperature tetragonal (LTT) phase with P4 2 /ncm symmetry [6][7][8]. Superconductivity in the low-temperature orthorhombic (LTO) phase of LSCO with Bmab symmetry is weakly suppressed in comparison with the LBCO system [9,10]. Maeno et al. initiated systematic studies on La 1.875 Ba 0.125−x Sr x CuO 4 (LBSCO) using polycrystalline samples to clarify the relation between T c and crystal structure [11]. We comprehensively studied the same system using single crystals and revealed a drastic change in x-dependence of T c [12,13]. In the present study we demonstrate a close relation between T c and crystal structure as shown in Fig.4.Recently, the IC spin-density-wave (SDW) and the charge-density-wave (CDW) orders were discovered in La 1.6−x Nd 0.4 Sr x CuO 4 (LNSCO) with x ∼ 1/8. These provide important clues for understanding the mechanism of the 1/8-anomaly based on the stripe model [14,15]. In the framework of the stripe model it is recognized that the orders are a manifestation of dynamical spin/charge correlation and that stabilization of the orders competitively induces instability of superconductivity [16]. In the LTT phase stripe-shaped orders parallel or perpendicular to Cu-O bonding are favorably stabilized by the corrugated pattern of the in-plane lattice potential. Hence, T c is suppressed to a greater extent in the LTT phase than in the LTO phase. Whether visible signs, such as the anomalous suppression of T c , app...

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