Experimental determination of momentum- resolved electron-phonon coupling, M. Rossi (a), R. Arpaia (a,b), R. Fumagalli (a), M. Moretti Sala (a), D. Betto (c), K. Kummer (c), G.M. De Luca (d,e), J. van den Brink (f,g), M. Salluzzo (e), N.B. Brookes (c), L. Braicovich (a,c) and G. Ghiringhelli (a,h), Phys. Rev. Lett. 123, 027001 (2019); https://doi.org/10.1103/ PhysRevLett.123.027001.
Determining the electron-phonon coupling in superconducting cuprates by resonant inelastic X-ray scattering: Methods and results on Nd1+xBa2−xCu3O7−δ, L. Braicovich (a,c), M. Rossi (a), R. Fumagalli (a), Y. Peng (a), Y. Wang (i), R. Arpaia (a,b), D. Betto (c), G.M. De Luca (d,e), D. Di Castro (j), K. Kummer (c), M. Moretti Sala (a), M. Pagetti (a), G. Balestrino (j), N.B. Brookes (c), M. Salluzzo (e), S. Johnston (i,f), J. van den Brink (f,g) and G. Ghiringhelli (a,h), Phys. Rev. Research 2,
023231 (2020); https://doi.org/10.1103/ PhysRevResearch.2.023231. (a) Politecnico di Milano (Italy) (b) Chalmers University of Technology, Göteborg (Sweden) (c) ESRF (d) Università di Napoli Federico II (Italy) (e) CNR-SPIN, Napoli (Italy) (f) IFW Dresden (Germany) (g) Technical University Dresden (Germany) (h) CNR-SPIN, Milano (Italy) (i) University of Tennessee, Knoxville (USA) (j) CNR-SPIN, Roma (Italy)
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PRINCIPAL PUBLICATION AND AUTHORS
where the biphonon is well seen. To expand the application, another approach independent of I2 is needed. This is done by exploiting the time scale of the intermediate state, which can be effectively decreased by detuning the incident energy below resonance . Thus, the intensity I1 as a function of detuning energy (the detuning profile) comes from the competition between the positive contribution by the EPC and the negative one from detuning. The EPC is determined by fitting the experimental detuning profile with the model of , in which the EPC enters as a parameter. The principal publication considers the best conditions for using I2/I1 or I1 alone.
This work presents the application to NdBa2Cu3O6 (NBCO), which is a typical parent compound of superconducting cuprates (YBCO family). The exceptional performances of beamline ID32 and of the ERIXS spectrometer were exploited. Figures 91a and 91b report the incident energy-dependence of Cu L3 RIXS spectra of NBCO. The bond-stretching (breathing) phonon mode is detected at 70 meV. By fitting the experimental detuning profile (Figure 91c), the EPC is determined at the
specific momentum transfer. The behaviour of the EPC in the accessible Brillouin zone can then be reconstructed by tracking the phonon intensity as a function of momentum (Figure 91d). The momentum dependence of the breathing phonon is sinusoidal as expected . The relatively large value of the EPC is found to be consistent with recent literature in favour of an increasing phonon role in high-temperature superconductors. The detuning procedure presented here paves the way to an extensive use of RIXS in phonon physics.
Fig. 91: a) Cu L3 X-ray absorption spectrum of NBCO. b) RIXS spectra of NBCO measured at various incident energies (see arrows in (a)). c) Detuning profile of the breathing phonon intensity (markers) and calculated intensity
scaling according to the model of  (solid line). d) Momentum dependence of the EPC (markers)
and expected sinusoidal trend (dashed line).