Strong electron-phonon coupling and enhanced phonon Gr\"uneisen parameters in valence-fluctuating metal EuPd$_2$Si$_2$

We study the valence crossover and strong electron-phonon coupling of EuPd 2 Si 2 by polarization-resolved Raman spectroscopy. The fully-symmetric phonon mode shows strongly asymmetric lineshape at low temperature, indicating Fano-type interaction between this mode and a continuum of electron-hole excitations. Moreover, the frequency and linewidth of the phonon modes exhibit anomalies across the valence-crossover temperature, suggesting the coupling between valence fluctuations and lattice vibration. In particular, two phonon modes show significantly enhanced Gr¨uneisen parameter, suggesting proximity to a critical elasticity regime. The relative contribution of the structural change and valence change to the phonon anomalies is evaluated by density functional theory calculations.

[1]  Yi Yao,et al.  An electronic nematic liquid in BaNi2As2 , 2022, Nature Communications.

[2]  Marco Peters,et al.  Influence of the Pd–Si Ratio on the Valence Transition in EuPd2Si2 Single Crystals , 2022, Crystal Growth & Design.

[3]  Minsung Kim,et al.  Lattice dynamics of the excitonic insulator Ta2Ni(Se1−xSx)5 , 2021, Physical Review B.

[4]  A. Sefat,et al.  Coupling of fully symmetric As phonon to magnetism in Ba(Fe1−xAux)2As2 , 2020 .

[5]  N. Brookes,et al.  Similar temperature scale for valence changes in Kondo lattices with different Kondo temperatures , 2016, Nature Communications.

[6]  K. Shimizu,et al.  Divalent, trivalent, and heavy fermion states in Eu compounds , 2017 .

[7]  J. Schlueter,et al.  Breakdown of Hooke’s law of elasticity at the Mott critical endpoint in an organic conductor , 2016, Science Advances.

[8]  A. Sefat,et al.  Stress-induced nematicity in EuFe 2 As 2 studied by Raman spectroscopy , 2016, 1604.01144.

[9]  E. Kaestner.  Mixed Valence Compounds , 2016 .

[10]  K. Binnemans.  Interpretation of europium(III) spectra , 2015 .

[11]  M. Garst,et al.  Mott metal-insulator transition on compressible lattices. , 2012, Physical Review Letters.

[12]  A. Georges,et al.  Proximity to valence transition in heavy fermion superconductor CeCu2Si2 under pressure , 2012 .

[13]  M. Mizumaki,et al.  Oxidation state sensitivity of Eu Lγ4 emission and its applications to oxidation state selective EXAFS spectroscopy of EuPd2Si2 , 2011 .

[14]  T. Uozumi,et al.  Temperature-Induced Valence Transition of EuPd2Si2 Studied by Hard X-ray Photoelectron Spectroscopy , 2011 .

[15]  M. Cazayous,et al.  Doping dependence of the lattice dynamics in Ba(Fe1-xCox)2As2 studied by Raman spectroscopy , 2009, 0906.4569.

[16]  Z. Fisk,et al.  Superconductivity and quantum criticality in the heavy-fermion system |[beta]|-YbAlB4 , 2008 .

[17]  G. Sheldrick.  A short history of SHELX. , 2008, Acta crystallographica. Section A, Foundations of crystallography.

[18]  A. Mitsuda,et al.  Temperature dependence of Eu 4f states in EuPd2Si2: bulk-sensitive high-resolution photoemission study , 2004 .

[19]  C. Geibel,et al.  Observation of Two Distinct Superconducting Phases in CeCu2Si2 , 2003, Science.

[20]  A. Mitsuda,et al.  Specific heat anomaly due to valence transition in Eu(Pd1−xPtx)2Si2 , 2001 .

[21]  K. Bohnen,et al.  LINEAR RESPONSE IN A DENSITY-FUNCTIONAL MIXED-BASIS APPROACH , 1999 .

[22]  Burke,et al.  Generalized Gradient Approximation Made Simple. , 1996, Physical review letters.

[23]  G. Güntherodt,et al.  Light scattering in rare earth and actinide intermetallic compounds , 1991 .

[24]  Klein,et al.  Raman scattering study of the electronic and vibrational excitations in CeCu2Si2. , 1986, Physical review. B, Condensed matter.

[25]  G. Güntherodt,et al.  Spectroscopic observation of intra- and inter-configurational excitations in the intermediate valence compound EuCu2Si2 , 1986 .

[26]  D. Vanderbilt,et al.  Optimally smooth norm-conserving pseudopotentials. , 1985, Physical review. B, Condensed matter.

[27]  G. Kaindl,et al.  Combined Mössbauer and LIII-edge X-ray absorption study of mixed- valent EuPd2Si2 and EuNi2P2 , 1985 .

[28]  L. C. Gupta,et al.  Mössbauer effects and LIII absorption measurements on EuPd2Si2 , 1985 .

[29]  Croft,et al.  Direct observation of intraionic and interconfigurational excitations in an intermediate-valence compound by Raman spectroscopy. , 1985, Physical review letters.

[30]  Sauér,et al.  Anomaly of the quadrupole interaction in mixed-valence EuPd2Si2. , 1985, Physical review. B, Condensed matter.

[31]  B. Wherrett.  Light Scattering in Solids IV , 1984 .

[32]  L. C. Gupta,et al.  Highly Resolved Surface Shifts in a Mixed-Valent System - Eupd2si2 , 1982 .

[33]  E. Müller-Hartmann.  Mixed Valence Compounds of the Rare Earths: A Fascinating Class of Solids , 1982 .

[34]  R. G. Pillay,et al.  A new and unique Eu-based mixed valence system: EuPd2Si2 , 1981 .

[35]  C. Raman,et al.  Scattering of Light in Crystals , 1945, Nature.