Angle Resolved Photoemission Spectroscopy of Delafossite Metals by Veronika Sunko

Author:Veronika Sunko
Language: eng
Format: epub, pdf
ISBN: 9783030310875
Publisher: Springer International Publishing

Fig. 4.9The dispersion along the direction measured in PtCoO at a and b c Full width at half maximum of Lorentzian fits to MDCs extracted at the binding energy of as a function of temperature from measurements on the same sample as dispersions in a and b (sample1, circles), and a different sample (sample2, triangles). The line is a prediction of the Debye model, with

4.3.3 Electron-Phonon Scattering

The measured dispersions exhibit no obvious signs of electron-phonon coupling, discussed in Sect. 2.​5.​3: there are no resolvable kinks in the spectral function, nor does the binding energy dependent linewidth show a step at a phonon energy. In Fig. 4.8 I simulate the spectral function using the same bare-band parabolic dispersion as above, with a self-energy describing the electron-phonon interaction within the Debye model. The Debye temperature is set to , as deduced from heat capacity measurements [11]. The electron-phonon coupling constants of and are used to calculate the spectral functions in Fig. 4.8a, b, respectively; no kinks are obvious in either of them. In Fig. 4.8c I compare the experimentally extracted linewidth broadening with that expected from the Debye model with (solid line) and (dashed line). For the step in linewidth at the Debye energy is smaller than the scatter in our data, so we would probably not be able to resolve the coupling of that strength. On the other hand, we would be able to resolve the electron-phonon coupling with . Therefore, although we observe no evidence of electron-phonon coupling, we can use our data to set an upper limit on the electron-phonon coupling strength to . For comparison, the bulk value in copper is reported to be 0.15, while the electron-phonon coupling strength of the states localised on copper surface is found to be 0.14 in photoemission measurements [12].

As the number of occupied phonon modes increases with temperature, so does the self-energy due to electron-phonon coupling. Another way to assess the strength of the coupling is to perform measurements at higher temperatures. We have therefore measured the dispersion along the direction in PtCoO as a function of temperature up to . As seen in Fig. 4.9a, b, no broadening is obvious in this temperature range. The experiment was later repeated on a different sample, for temperatures up to . The linewidth at the binding energy of , which is larger than the Debye energy of , is shown as a function of temperature for both samples in Fig. 4.9c. It is constant within our resolution, and the scatter in the data is larger than the change of linewidth predicted by the Debye model with . We would therefore not be able to resolve the temperature dependence of the linewidth even for the largest possible coupling of 0.2.

It is interesting to note that the scatter in the extracted linewidth is larger as a function of temperature than as a function of binding energy. Again, this is a consequence of surface inhomogeneity. As we vary the temperature the whole manipulator holding the sample changes length, and the light shines on a slightly different spot on the sample.

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