Phonon Raman scattering in superconducting La1.85Sr

PHYSICAL REVIEW B VOLUME 35, NUMBER 16 phonon Raman scattering in superconducting JUNE 1987 Lat asSro t5Cu04 — „ Thompson, P. Killough, J. L. Smith, and Z. Fisk Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 23 March 1987) S. Blumenroeder, E. Zirngiebl, J. D. on the high- T, superconductor Raman experiments We have performed scattering Lais5Sro i5Cu04 — ~ (T, =37 K). We observe three of the four Raman-active optical-phonon modes and, on the basis of a comparison to the Raman spectra of K2MnF4, are able to tentatively No anomalous temperature dependence is observed for all three assign the mode symmetries. modes, indicative of the lack of strong electron-phonon coupling in these modes. in the system (La,X)2- Recently superconductivity with transition Cu04-~ (X=Ba, Sr) has been found' temperatures T, up to -40 K, which is almost 100% higher than the previously highest T, of about 23 K in Nb36e. In the conventional BCS theory of superconduc- tivity, a high T, results from strong electron-phonon cou- pling. Strong electron-phonon coupling is anticipated for (La, Sr)2Cu04 due to the nearly ideal nesting conditions of the Fermi surface that arise from the planar structure of this compound. Indeed, a Peierls transition in undoped La2Cu04 — ~ from tetragonal to orthorhombic crystal structure is found at 260'C. However, there also has that Cooper pairing in this new class been the suggestion of high-T, materials does not arise from phonon coupling but is primarily due to electronic coupling produced by valence bonds. This is appealing be- resonating Cu cause, as McMillan pointed out, soft phonons, which are cou- responsible in part for enhanced electron-phonon pling, also lead to lattice instabilities and can place an upper limit on T, . SubseqU. ent work by Allen and co- workers' showed that McMillan's constraints were un- necessarily severe, but that there may still be a limiting value for T, . In any case, microscopic information on the lattice dynamics of (La,Sr)zCu04 — ~ is highly desirable to arrive at a microscopic understanding of superconductivi- ty in this new class of high-T, superconductors. We have performed Raman scattering experiments on La 85Sro i 5Cu04 — ~ in an at tempt to reveal the basic mechanism for Cooper pairing in this compound. Our sample was made by grinding the oxides and firing the pressed mixture three times at 1100 C in an oxygen at- mosphere. The resulting powder was pressed into a pellet. transition temperature was deter- The superconducting mea- mined to be 37 K from dc magnetic-susceptibility surements' which also showed diamagnetism of 55% of 1/4z at 2 K, when the sample was cooled in zero applied field below T, . La~ 85Sro i5Cu04 ~ crystallizes in the tetragonal space group D4p, (14/mmm) having two formu- la units per unit cell. The different q =0 phonon modes of La~ 85Sr0~5Cu04 — y are listed in Table I, with the four Raman-active (RA) modes displayed' in Fig. 1. One 2 ~g and one Eg mode are due to oxygen vibrations and the other A~g and Eg modes result from movements of the lanthanum ion. In we show Raman of spectra Fig. Be- La~ 85Sr0~5Cu04 — y as a function of temperature. cause our sample was polycrystalline, we could not per- form a symmetry analysis of the observed peaks. Howev- er, from comparison with the symmetry analysis of the Raman spectrum of isostructural K2MnF4 (Ref. 14) or K2CuF4, ' we can tentatively assign the peaks in Fig. 2 to the following symmetries: The lanthanum Rig mode is at 100 cm ' scaled down from the value of the potassium A is mode in K2MnF4 (180 cm ') by the square root of (a) 0 X Y G E N B R AT I 0 N S (b) LANTHANUM C~. 0 0:o eg a1g TABLE I. Factor-group analysis of the q 0 lattice vibra- tions of Lal 85Sro»Cu04 crystallizing in the K2NiF4 structure (D4q or 14/mmm ). Raman active Infrared active Translational (acoustic modes) Others I C~ 0 Type V VIBRATIONS C~ 0 C~- O. La Cu Symmetry 2xA&z, 2xE~ 3xA2„, 4xE„, 1 x82„ 1xA2„, 1xE„ eg FIG. 1. Raman-active modes of (Lai. 85Sro i5)Cu04 — „with D4q. Filled circles denote Cu atoms, large open circles O-atoms, and small open circles La atoms (Sr atoms), respectively. 1987 The American Physical Society