Evidence for a charge collective mode associated with superconductivity in copper oxides from neutron and x-ray scattering measurements of La2−xSrxCuO

Abstract

In superconducting copper oxides some Cu-O bond-stretching phonons around 70meV show anomalous giant softening and broadening of electronic origin and electronic dispersions have large renormalization kinks near the same energy. These observations suggest that phonon broadening originates from quasiparticle excitations across the Fermi surface and the electronic dispersion kinks originate from coupling to anomalous phonons. We measured the phonon anomaly in underdoped (x=0.05) and overdoped (x=0.20,0.25) La2−xSrxCuO4 by inelastic neutron and x-ray scattering with high resolution. Combining these and previously published data, we found that doping-dependence of the magnitude of the giant phonon anomaly is very different from that of the ARPES kink, i.e. the two phenomena are not connected. We show that these results provide indirect evidence that the phonon anomaly originates from novel collective charge excitations as opposed to interactions with electron-hole pairs. Their amplitude follows the superconducting dome so these charge modes may be important for superconductivity.PACS numbers: 74.25.Kc, 63.20.kd, 74.20.Mn Lattice vibrations in metals can be damped and/or softened by either electronic quasiparticles or collective charge excitations (e.g. plasmons). Giant phonon softening and line broadening of electronic origin of the longitudinal Cu-O bond stretching phonons near half-way to the zone boundary (giant anomaly) was observed in copper oxide high temperature superconductors (HTSCs) 1-11 . It was previously found at superconducting compositions and was also absent in undoped and overdoped nonsuperconducting copper oxides [5][6][7] .First reports interpreted the phonon anomaly as a signature of unit cell doubling 3 followed by a different interpretation 6 in terms of coupling of the phonon to dynamic charge stripes. Subsequently, close kinematic relationship between the longitudinal Cu-O bond stretching mode dispersion and renormalization of electronic quasiparticles in Bi 2 Sr 1.6 La 0.4 CuO 6 (Bi2201) indicated that the phonon anomaly may originate from the coupling of phonons to electronic quasiparticles 11 . t − J modelbased calculations also predicted strong coupling of optical phonons to electron-hole excitations 12 .If phonons couple strongly to electronic quasipaticles, then a BCS-type mechanism of superconductivity may still be valid. Alternatively, the phonon anomaly would arise from a novel collective charge excitations at low energies (at least 70 meV). Then such collective mode may provide the pairing interaction 13-15 .We measured Cu-O bond-stretching phonons in La 2−x Sr x CuO 4 for undoped x=0.00, nonsuperconducting underdoped x=0.05, and superconducting overdoped x=0.20, 0.25. x=0.05 did not show characteristic signatures of the giant phonon anomaly. x=0.20 showed a very strong phonon anomaly, which was dramatically reduced already at x=0.25. On the other hand, the magnitude of the kink in the electronic dispersions gradually decreases from x=0.05 to x=0.30 16,17 . The new data, combine...

Emergent charge order from correlated electron-phonon physics in cuprates

“…This temperature-dependent softening was initially understood to indicate strong el-ph coupling, and indeed the size of the anomaly correlates with the superconducting T c 40 . However, trends with doping do not show any clear correlation with spin-charge stripes 39 , static charge density waves 42 , or kinks in the electronic dispersion 40 , which seems to rule out conventional el-ph coupling mechanisms of the type contempleted here. Rather, it is proposed that the softening reflects coupling to a collective charge mode of the electron liquid 40,43 .…”

“…There is, furthermore, a bond-stretching phonon that develops a Kohn-like anomaly as temperature is reduced [34][35][36][37][38][39][40][41] . This temperature-dependent softening was initially understood to indicate strong el-ph coupling, and indeed the size of the anomaly correlates with the superconducting T c 40 .…”

Charge density wave fluctuations inLa2−xSrxCuO4and their competition

“…It has been known for some time that the optic phonons in LSCO 23,24 and other cuprates 25 show such anomalies. In La 1.85 Sr 0.15 CuO 4 anomalies are observed 24 at q = (0.25, 0, 0), i.e.…”

“…Extended x-ray absorption fine structure (EXAFS) 21 and atomic pair distribution function (PDF) analysis of neutron scattering data 22 has provided evidence of ordering structural distortions. Finally, the LSCO system 23,24 together with other cuprates 25 show (Kohn) anomalies in the optic phonons. These are often associated with charge ordering.…”

Phase diagrams of a 2D Ising spin-pseudospin model

“…The studying of the interplay between magnetism and superconductivity in cuprates has a long history [1,2,3]. Over the last fifteen years, a wealth of experimental results has suggested the presence of the charge ordering [4,5,6,7,8,9,10,11,12,13,14] and the interplaying spin and charge orderings [15,16,17,18,19,20,21] in cuprates. Recently [22] we argued that an unique property of high-T c cuprates is related to the charge-transfer instability in the CuO 2 planes.…”