Complete elastic constants and giant softening ofc66in superconductingLa1.86

Abstract

A novel resonant ultrasound technique is used to measure all elastic constants and their temperature dependence in single-crystal Lai 86Sro 14CUO4. The in-plane shear modulus C66 drops by nearly half upon cooling to 227 K, an effect attributed to order-parameter fluctuations (of 2D Gaussian character) near the tetragonal-orthorhombic phase boundary. Implications for current models of the structural phase transition are discussed, as well as the effect of domains in the low-temperature orthorhombic state.PACS numbers: 74.70.Vy, 43.35.+d, 62.65.+k The rich assortment of phases found in La2-*M X -CUO4 includes superconducting, magnetic, and structurally distorted materials. The relation between magnetic and superconducting phases is a critical issue that continues to attract attention. 1 A relation between the tetragonal-orthorhombic (TO) structural phase transition (SPT) and high-7 c superconductivity seemed less compelling until the recent work by Axe et al. 2 showed that a rearrangement of the structural distortion can destroy superconductivity. Here we report a surprisingly large elastic anomaly at the same SPT.Studies on polycrystals 3,4 indicate that all the elastic moduli and their temperature dependences are important for modeling both the superconducting transition at T c and the SPT at T JO in La 2 Cu0 4 . Bhattacharya et al. 4 found a break in slope of the shear-wave speed at T c in sintered powders of Lai.ssSro.isCuO^ Complicating the issue was the observation of similar behavior 5 at the same temperature in nonsuperconducting single-crystal La2CuC>4 and features in the thermal expansion of nonsuperconducting polycrystal. 6 At higher temperatures, both shear and longitudinal velocities of the sintered superconductor exhibited an extremely large softening 7 (-15% near 200 K), in contrast to the weak temperature dependence of the insulator. This unusual softening could not be attributed unambiguously to the development of the tetragonal-orthorhombic SPT. 8 This Letter resolves some of these issues by reporting high-resolution measurements of all c /y in a single-crystal sample of Lai.86Sro.uCu04. The measurements cover the temperature range 220-300 K, including 7>o (about 220 K in our sample). The most interesting feature observed is the monotonic and strong decrease of c^. At 227 K, c^ has less than 60% of its room-temperature value, while the other c,/s show only small changes (C44 increases less than 0.8% over the same temperature range). We will show that the unusual elastic behavior can be understood from a Landau theory of the SPT.The sample was prepared from a Cu-O flux using a traveling solvent floating-zone technique. 9 The crystal was aligned along the principal axes of the tetragonal cell and polished so the faces were parallel to within 1°, leaving a finished sample with dimensions 2.890x2.816 x2.405 mm 3 all ±0.01 mm, the shortest dimension corresponding to the tetragonal c axis. The density was measured to be 6.882 g/cm 3 , consistent with the nominal strontium concentration. Although we...

A survey of the Van Hove scenario for high-tc superconductivity with special emphasis on pseudogaps and striped phases

“…There is a substantial Curie-Weiss-like softening of c 66 as the HTT-LTO transition is approached from above, and the quality factor Q is also substantially reduced. This was initially interpreted in terms of critical fluctuations [564]. However, the softening persists to very high temperatures, 80K above the HTT-LTO transition, much higher than a theoretical estimate for fluctuations.…”

“…Moreover, throughout the LTO phase the elastic properties remain anomalous; c 66 remains soft, and Q is reduced by a factor of ∼ 100 [564,566]. This is assumed to be caused by martensitic-like domain wall motion.…”

Unconventional pairing symmetry of layered superconductors caused by acoustic phonons

“…16 The small ratio of two interactions, J / V ph Ϸ t / U 1 or J / V ph Ϸ V c / U 1, and the giant sound-speed anisotropy 17,18 favor conventional EPI as the origin of the unconventional pairing both in underdoped cuprates, where the pairing is individual, 14 and in overdoped samples apparently with Cooper pairs. 3 Nowadays, compelling evidence for a strong EPI has arrived from isotope effects, 22 more recent high resolution angle resolved photoemission spectroscopies, 23 and a number of earlier optical, [24][25][26][27] neutron-scattering, 28 and recent inelastic scattering measurements 29 in cuprates.…”

Ultrasonic velocity variations in La 2– x Sr x CuO 4 single crystals

“…However, in the case of La 2-x Sr x CuO 4 (LSCO) some experiments have been realized. Migliori et al [1] reported the measurement of all the elastic constants in compounds with x=0.1 and 0.14. They noticed a giant softening of the c 66 coefficient at the tetragonal to orthorhombic transition.…”