Observation of phonon softening at the superconducting transition inBi2Sr2

Abstract

Resonant neutron absorption spectroscopy has been used to determine the temperature dependence of the average kinetic energy <£) of the Cu in Bi2Sr2CaCu2C>8 for the directions along and perpendicular to the c axis. (E) is directly related to the Cu phonon density of states and shows essentially normal behavior as a function of temperature for the c-axis direction. However, a sudden drop in (E) is observed at the superconducting transition for the motions in the Cu-O planes.

Recent neutron-scattering results on high-temperature superconductors

“…Earlier measurements using neutron resonance absorption spectroscopy (NRAS) had shown that the Cu a-b plane modes displayed an anomalous softening at a temperature slightly higher than the superconducting transition for Bi2Sr2CaCU208. [7] The There is a region centered around 25 meV where large parasitic scattering from the (0,0,6) reflection makes measurements impossible with the crystal orientation used.…”

Absence of discontinuities in ion-channeling parameters forYBa2Cu3Hecker1, Haakenaasen2, Golovchenko3 et al. 1997Phys. Rev. B3010View full textAdd to dashboardCite

“…Furthermore, neutron resonance absorption spectroscopy measurements showed an anomalous decrease in the momentum of the Cu atoms in the a-b plane of Bi 2 Sr 2 CaCu 2 O 8 , 16 but there was no evidence for such anomalies in YBCO. 17 We have therefore used high quality crystalline c-oriented thin films for a more detailed investigation of this controversy.…”

“…By using a beam energy of 3.1 MeV, which is slightly higher than the 3.045 MeV 16 O͑␣,␣͒ 16 O resonance energy, we could excite the 40 keV wide resonance for oxygen atoms in the middle region of the film. It was important to keep the beam energy constant and avoid ice buildup on the surface in order to probe the same depth region at each temperature.…”

“…This is not possible with bulk single crystal YBCO, since the signals from the different elements overlap. In our films the signal from O falls on top of the larger substrate signal, making analysis of the O yield difficult, but we took advantage of a 16 O͑␣,␣͒ 16 O resonance 19 to increase the O yield. An additional advantage of using thin films is that the superconducting phase transition can easily be monitored by resistance measurements.…”

The Polaron Scenario for High-TcSuperconductivity

“…The onset of superconductivity seems to freeze the dynamics of the lattice into a coherent state. Ion channeling experiments [7] and resonant neutron scattering [8] show that upon reducing the temperature below T, the atomic displacements become correlated and the thermal vibrational amplitude of the individual atoms decreases abruptly at T,. EXAFS measurements show strong dynamical fluctuations of certain intramolecular distances in the vicinity of T, [9,10].…”