Electronic phenomena studied in the nordic countries

Phonon contributions to high-temperature superconductivity in LSCO+O

Not scheduled

15m

Max IV Lund

Speaker

Mr Tim Tejsner (Copenhagen University and Institut Laue-Langevin)

Description

The compound La_2-xSr_xCuO₄ (LSCO) is superconducting for 0.05 < x < 0.3 and serves as an interesting model-system due to its relatively simple crystalline structure with only one Cu-O layer per unit cell, which is believed to contain the fundamental aspects of superconductivity. Furthermore, LSCO can be doped with interstitial oxygen to form La_2-xSr_xCuO_4+y (LSCO+O). The extra oxygen serves to stabilize the superconducting phase at T_c = 40K, as well as a stripe-like antiferromagnetic phase with coinciding T_N =40K, regardless of strontium content [1,2]. While conventional superconductivity is mediated by phonons, it is generally not believed to be the case for high temperature superconductors (HTSC) such as LSCO. However, studies have shown evidence of strong electron-phonon coupling in superconducting cuprates [3] and in light of these findings, we are currently investigating the phonon contributions to HTSC in LSCO+O with inelastic neutron scattering as the primary experimental probe. Recently, an anomaly (softening) in the longitudinal Cu-O bond-stretching phonon halfway to the zone boundary was found in optimally doped (x=0.15) LSCO. The anomaly disappears for underdoped and overdoped LSCO, suggesting a connection to HTSC [4]. We repeated the experiment for superconducting, but underdoped in terms of strontium, LSCO+O and found evidence for the same anomaly. These findings strengthen the hypothesis for a phonon contribution to HTSC and thus warrants further investigation. [1] L. Udby et al., Phys. Rev. Lett, 111, 227001 (2013). [2] H. E. Mohottala et al., Nature Materials, 5, 377 (2006). [3] L. Pintschovius, Phys. Stat. Sol. (B), 242, 30 (2005). [4] S.R. Park, Phys. Rev. B, 89, 020506 (2014).