Femtosecond optical studies of cuprates

Abstract

Femtosecond optical reflectivity measurements of La 2-x Sr x CuO 4 , La 2 CuO 4+y , Bi 2 Sr 2 CuO 6+z and Bi 2 Sr 2 CaCu 2 O 8+δ thin films and single crystal samples indicate qualitative changes with fluence. At the lowest fluencies, there is a power law divergence in the relaxation time. The divergence has an onset temperature of 55±15K, independent of whether the sample is in the superconducting or normal states. At slightly higher fluencies, still perturbative, the additional response does not exhibit this power law divergence. At quite high fluencies-no longer perturbative-the metallic samples exhibit oscillations in the reflectivity amplitude. The period of these oscillations varies with the probe wavelength but not with the pump wavelength. The oscillations exhibit a decay time as long as 10 nsec.

Time-resolved optical response of all-oxide YBa2Cu3O7/LaParlato1, Arpaia2, Lisio3 et al. 2013Phys. Rev. BSelf Cite

“…This is consistent with other pump-probe studies performed under low-fluence excitations. 13,14 Contrary to some earlier observations, 15,16 our data are fitted by a simple linear recombination model, and weak, damped oscillations on top of the exponential decay might, tentatively, be ascribed to displaced excitation of coherent acoustic phonons. 15,17 Overall, our observations for the pure YBCO film are in agreement with reported data 14,[18][19][20] and corroborate current interpretation that the slow dynamics below T c (several tens of picoseconds) cannot be ascribed to the acoustic phonon bottleneck, in contrast with the case of low-T c superconductors.…”

“…According to earlier studies on high-T c materials, 13 the amplitude of the DR/R signal grows below T c , while, simultaneously, the recovery becomes progressively slower. This is consistent with other pump-probe studies performed under low-fluence excitations.…”

Theory of the electron phonon relaxation time in cuprates: Reproducing the observed temperature behaviour

“…But the leading terms are the same which are found in the case of metals in the above reference. The electron-phonon relaxation time follows T −3 law for cuprates which agrees well with the experimental results [2,3].…”

“…For that, it has been shown that the relaxation time follows the T −3 law [1] (T is phonon temperature) at low temperature (T e , T << T 0 ). The same evidence for relaxation time has also been reported experimentally for cuprates in low temperature region and it varies as T −(3±0.5) [2,3].…”

“…In this section, we compare our results for electron-phonon relaxation time (τ ep ) with experiment [2,3]. First let us discuss the behavior of energy transferred per unit volume per second from electrons to phonons ( Ė) at low and high temperature regimes for cuprates.…”