Interfacial effects revealed by ultrafast relaxation dynamics in ${\mathrm{BiFeO}}_3/{\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ bilayers

The temperature dependence of the relaxation dynamics in the bilayer thin film heterostructure composed of multiferroic ${\mathrm{BiFeO}}_3$ (BFO) and superconducting ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ (YBCO) grown on a (001) ${\mathrm{SrTiO}}_3$ substrate is studied by a time-resolved pump-probe technique, and compared with that of pure YBCO thin film grown under the same growth conditions. The superconductivity of YBCO is found to be retained in the heterostructure. We observe a speeding up of the YBCO recombination dynamics in the superconducting state of the heterostructure, and attribute it to the presence of weak ferromagnetism at the BFO/YBCO interface as observed in magnetization data. An extension of the Rothwarf-Taylor model is used to fit the ultrafast dynamics of BFO/YBCO, that models an increased quasiparticle occupation of the ferromagnetic interfacial layer in the superconducting state of YBCO.

Figure 1

X-ray diffraction patterns of (a) the BFO/YBCO bilayer and corresponding (b) YBCO thin film.

Figure 2

(a) $5\times 5\mu {\mathrm{m}}^2$ topographic AFM image of BFO (130 nm) grown on YBCO/STO, and (b) corresponding piezoresponse phase image. Yellow (bright) and purple (dark) correspond to the original up and down domains, respectively.

Figure 3

Magnetization vs temperature measured on BFO/YBCO bilayer and corresponding YBCO thin films.

Figure 4

Magnetization hysteresis loops of the BFO/YBCO bilayer recorded (a) below and (b) above $T_c$.

Figure 5

Time dependence of the photoinduced change in reflectivity $\mathrm{\Delta }R/R$ of the YBCO thin film. The solid lines show single-exponential fits to the data.

Figure 6

Time dependence of the photoinduced change in reflectivity $\mathrm{\Delta }R/R$ of the BFO/YBCO thin film.

Figure 7

Comparison of the time dependence of the normalized differential reflectivity for the BFO/YBCO and YBCO thin films at (a) 35, (b) 50, (c) 80, and (d) 100 K.

Figure 8

(a) Temperature dependence of the relaxation amplitude $A\left(T\right)$ for the YBCO film. Solid line = fit using Eq. (1). (b) Temperature dependence of the relaxation time $\tau \left(T\right)$ obtained from a single-exponential fit (solid circles). Solid line = fit of $\tau \left(T\right)$ using the RT model in Eq. (3).

Figure 9

Schematic representation of the RT Model. Photoexcited QPs recombine to form Cooper pairs and HFPs. The bottleneck for QP relaxation dynamics due to HFP induced reexcitation is governed by the decay rate of HFPs into low-frequency phonons (LFPs).

Figure 10

Waterfall plots of $\mathrm{\Delta }R/R$ as a function of the pump-probe time delay of YBCO thin films at different temperatures. Solid lines are fits to the RT Model [Eq. (2)].

Figure 11

Temperature dependence of the HFP relaxation time ${\gamma }^{-1}\left(T\right)$ obtained from fits of the numerical solution of the RT model to the experimental $\mathrm{\Delta }R/R$ data of pure YBCO.

Figure 12

Temperature dependence of the QP recombination rate $\mathcal{R}\left(T\right)$ obtained from fits of the numerical solution of the RT model to the experimental $\mathrm{\Delta }R/R$ data of pure YBCO. Approaching $T_c$ from below, the recombination rate starts to increase, possibly related to the vanishing of the superconducting energy gap $\mathrm{\Delta }$.

Figure 13

Schematic of the extended RT Model, where an additional energy level $E_2$ is present due to the weak ferromagnetic interfacial layer.

Figure 14

The excitation and relaxation rates $\left({\alpha }_1,{\alpha }_2\right)$ between the gap edge state $E_1$ and the ferromagnetic interfacial layer $E_2$ show a peak close to $T_c\sim 84$ K. This indicates that more QPs are trapped in the interfacial state $E_2$ and this in turn causes a reduction of QP recombinations. The peak in $\alpha$ coincides with the closing of the energy gap (see Fig. 12).

Figure 15

Waterfall plots of $\mathrm{\Delta }R/R$ as a function of the pump-probe time delay of BFO/YBCO thin films at different temperatures. Solid lines are fits to the extended RT Model [Eq. (5)].