Electron-Phonon Interactions in Superconducting ${\mathrm{La}}_{1.84}{\mathrm{Sr}}_{0.16}{\mathrm{CuO}}_4$ Films

We have measured quasiparticle tunneling across a junction perpendicular to the superconducting copper oxide planes. The tunneling spectra show peaks in the density of states. There are 11 minima in the second derivative $d^{2}I/d{V}^2$, where $I$ is the current and $V$ the voltage, suggesting multiple boson-quasiparticle interactions. These minima match precisely with the published Raman scattering data, leading us to conclude that the relevant bosons in superconducting ${\mathrm{La}}_{1.84}{\mathrm{Sr}}_{0.16}{\mathrm{CuO}}_4$ films are phonons.

Figure 1

The resistance of the grain boundary junction and film plotted as a function of temperature. The inset shows current-voltage ($I\mathrm{\text{−}}V$) characteristics of the junction measured at 4.2 K.

Figure 2

Dynamic conductance of the ozone annealed junction as a function of bias voltage measured at various temperatures. The inset on the right shows the variation of conductance with bias voltage near zero voltage for three temperatures to illustrate the constant conductance around zero for the 4.2, 7.2, and 11.1 K data. The inset on the left shows the conductance of the junction above the $T_c$ of the film.

Figure 3

Experimental values of $2{\Delta }_s$, where ${\Delta }_s$ is the superconducting gap as a function of temperature and the BCS prediction. The BCS curve is given by the equation $\Delta (T)/\Delta (0)=\tanh \{(T_c/T)[\Delta (T)/\Delta (0)]\}$ [15].

Figure 4

The second derivative of the data shown in Fig. 2 and plotted as a function of $V-2{\Delta }_s(T)/e$. The minima in the second derivative, with the exception of the first minimum, are associated with peaks in the excitation spectra. The first minimum is associated with a lack of normalization of the data by the BCS density of states and the normal state density of states. It is not used for comparison with spin or phonon spectra. The curves have been shifted vertically by arbitrary amounts for purposes of viewing. However, the $y$ scale is unchanged.

Figure 5

The second derivative data (shown in Fig. 4 as the 4.2 K data) multiplied by ($-1$) are shown along with the published Raman scattering data in Ref. 11 measured at 30 K on single crystal LSCO samples with 20% Sr. The polarization of the incident and scattered light in the Raman data is parallel to the copper oxide planes of the crystal. The peak in the tunneling data at 21 meV is present in the Raman scattering data of Lampakis et al. [12].