J Low Temp Phys DOI 10.1007/s10909-016-1728-0 Pressure Effect on the Einstein-Like Phonon Mode in Superconducting YB 6 Mat. Orendᡠc 1,2 · S. Gabáni 1 · E. Gažo 1 · G. Pristáš 1 · K. Flachbart 1 · T. Mori 3 · X. Wang 4 · K. Kamenev 4 Received: 15 July 2016 / Accepted: 8 December 2016 © Springer Science+Business Media New York 2016 Abstract YB 6 is known as a conventional type-II BCS superconductor in which the strong coupling superconductivity with 2/ k B T c ≈ 4.1 is mediated by the Einstein-like phonon mode of Y atoms located at ¯ h ω E ≈ 8meV. We have inves- tigated the pressure effect on ω E by Raman scattering up to 14GPa. The received linear pressure increase in this mode d ω E /dp ≈ 0.21 meV/GPa yields an isother- mal Grüneisen coefficient γ T =-∂ lnω E /∂ lnV = 3.85. Moreover, the pressure effect on the electron–phonon interaction λ calculated from the McMillan–Allen– Dynes expression for the superconducting transition temperature was determined to be ∂ lnλ/∂ lnV ∼ = 7.2. Keywords Superconductivity · High pressures · Raman scattering 1 Introduction Yttrium hexaboride YB 6 has the second highest superconducting transition tempera- ture (T c < 8.4 K) among boron compounds. One of the explanations of this rather high T c is the strong coupling of electrons with the dominant Einstein-like acoustic mode of Y ions at ¯ h ω E ≈ 8 meV [1–3]. The observed fast initial decrease in T c with pressure, B S. Gabáni gabani@saske.sk 1 Institute of Experimental Physics, SAS, Watsonova 47, 04001 Kosice, Slovakia 2 Institute of Physics, Faculty of Science, P.J. Safarik University, Park Angelinum 9, 04154 Košice, Slovakia 3 MANA, National Institute for Materials Science, Namiki 1-1, Tsukuba 305 0044, Japan 4 Centre for Science at Extreme Conditions, University of Edinburgh, Mayfield road, Edinburgh EH9 3JZ, UK 123