Quasiparticle relaxation dynamics in superconductors with different gap structures: theory and experiments on YBa_{2}Cu_{3}O_{7-δ}
Abstract
Photoexcited quasiparticle relaxation dynamics are investigated in a
YB
a
2
C
u
3
O
7−δ
superconductor as a function of doping
δ
and temperature
T
using ultrafast time-resolved optical spectroscopy. A model calculation is presented which describes the temperature dependence of the photoinduced quasiparticle population
n
pe
, photoinduced transmission
ΔT/T
and relaxation time
τ
for three different superconducting gaps: (i) a temperature-dependent collective gap such that
Δ(T)→0
as
T→
T
c
, (ii) a temperature-independent gap, which might arise for the case of a superconductor with pre-formed pairs and (iii) an anisotropic (e.g. d-wave) gap with nodes. Comparison of the theory with data of photoinduced transmission
|ΔT/T|
, reflection
|DeltaR/R|
and quasiparticle recombination time
τ
in
YB
a
2
C
u
3
O
7−δ
over a very wide range of doping (
0.1<δ<0.48)
is found to give good quantitative agreement with a temperature-dependent BCS-like isotropic gap near optimum doping (
δ<0.1)
and a temperature-independent isotropic gap in underdoped
YB
a
2
C
u
3
O
7−δ
(
0.15<δ<0.48
). A pure d-wave gap was found to be inconsistent with the data.