Lattice Instability in High-Temperature Superconductors

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High-temperature superconductor La_<2-x>Sr_xCuO_4 (LSCO) exhibits a cascade of structural phase transitions (THT-OMT-TLT) with temperature. The transitions are characterized by two independent order parameters, which correspond to TO phonons at the Brillouin-zone boundary X point and constitute the star. We have investigated the nature of these transitions by neutron diffraction and obtained the following results.I-1.New insights into the structural transitions1.The neutron experiments were performed on single crystals (x=0.14) grown at Hiroshima University.2.Measurements of the acoustic phonon dispersion along the (xixi0) direction revealed that the LA and TA (Z) phonons at the zone center GAMMA point are in good agreement with the corresponding elasticity C_<66>+ (C_<11>+C_<12>) /2 and C_<44>, measured by ultrasound.3.The dispersion of the soft mode X^+_4 near the X point indicates that the softening is in progress already at room temperature.4.Temperature dependence of the soft-mode … More frequency was obtained by energy scans on the superlattice reflection (1/2 1/2 4). Moreover, the temperature dependence of the order parameter was determined from the peak intensity.The presence of twin boundaries in the OMT phase may affect the above measurements. Investigation is underway to clarify this issue by measurements on mono-domain crystals.I-2.Interplay between structural transitions and high-temperature superconductivityA phonon mode corresponding to one of the two order parameters quenches at the THT-OMT structural transition. It is conceived that the superconducting transition temperature is enhances in part by the presence of the soft mode. We were not able to observe this soft mode in the OMT phase. We plan to continue the search by using mono-domain crystals.II.Phonon anomaly at the Brillouin-zone boundary Z pointWe performed an energy scan at the wave number (008), but could not obtain evidence for the anomalous phonon behavior at the Z point. This is probably because the structural instability competing with superconductivity is not strong enough to drive the structural transition. Therefore, we will pursue this issue with Nd-substituted LSCO,which actually exhibits the OMT-TLT transition. Less