Regensburg 1998 – wissenschaftliches Programm

Bereiche | Tage | Auswahl | Suche | Downloads | Hilfe

TT: Tiefe Temperaturen

TT 11: Elektronenstruktur und Phononen in HTSL

TT 11.8: Vortrag

Dienstag, 24. März 1998, 17:15–17:30, H 20

Ultrasonic investigations of the elastic properties of La₂CuO_4+δ — •Anja Geerkens¹, Siegfried Ewert¹, and Reinhard K. Kremer² — ¹Lehrstuhl für Experimentalphysik, BTU Cottbus — ²Max-Planck-Institut für Festkörperforschung, Stuttgart

The elastic properties of doped La₂CuO_4+δ-samples (0<δ <0.035) were investigated by very sensitive ultrasonic attenuation and sound velocity measurements at frequencies up to 30 MHz. In excess oxygen doped samples (δ = 0.01, δ = 0.035) a very remarkably sharp sudden drop of the sound attenuation is observed below 53 K. The decrease in attenuation amounts up to 1.5 dB/cm at 30 MHz. We attribute this sharp feature to a so far unknown structural phase transition, which is induced by the presence of excess oxygen atoms. Corresponding anomalies are observed in the sound velocity, which however are almost masked by an additional very broad and strong (up to 3 %) decrease of the sound velocity, starting already at around 100 K and marking a significant softening of the lattice due to the excess oxygen atoms. Centered at 255 K La₂CuO_4.01 exhibits a broad peak in the sound attenuation with a shoulder at the low temperature side. This peak can be understood as due to the presence of the miscibility gap between δ = 0.01 and δ = 0.05 in the phase diagram of La₂CuO_4+δ and the comcommitand chemical phase separation below ≈ 270 K. Several weak anomalies appear below 50 K, which partly are associated to the superconducting transition.