Lattice Instability in High-Temperature Superconductors

高温超导体La_&lt；2-x&gt；Sr_xCuO_4(LSCO)随温度呈现一系列结构相变(THT-OMT-TLT)。跃迁由两个独立的序参数来表征，它们对应于布里渊区边界X点处的声子，构成了恒星。我们用中子衍射法研究了这些相变的性质，得到了以下结果：1.对结构相变的新认识1.在广岛大学生长的单晶(x=0.14)上进行了中子实验。2.声子色散的测量表明，区中心伽马点的LA和TA(Z)声子与相应的弹性C_&lt；66&gt；+(C_&lt；11&gt；+C_&lt；12&gt；)/2和C_&lt；44&gt；3.软模X^+_4在X点附近的色散表明软模…在室温下已经开始软化通过对超晶格反射(1/2 1/2 4)进行能量扫描，获得了更多的频率。此外，由峰强度确定了有序参数与温度的关系，OMT相中孪晶界的存在可能会影响上述测量结果。通过对单畴晶体的测量来阐明这一问题的研究正在进行中。I-2.结构转变与高温超导的相互作用对应于两个序参数之一的声子模在THT-OMT结构转变处猝灭。认为软模的存在部分地提高了超导转变温度。在OMT阶段，我们没有观察到这种软模式。我们计划继续使用单域晶体进行搜索。II.布里渊区边界Z点的声子异常我们在波数(008)处进行了能量扫描，但无法获得Z点处的异常声子行为的证据。这可能是因为与超导竞争的结构不稳定性不足以驱动结构转变。因此，我们将在实际呈现OMT到TLT转变的ND取代LSCO中探讨这一问题。较少

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