Raman scattering in single crystal La2CuO4

Abstract The results of a Raman scattering study of single crystal La 2 CuO 4 are reported. Three Ag symmetry modes are observed, rather than the two predicted by group theory. Mode assignments are discussed.

At present it is of great interest whether the mechanisms of superconductivity differ essentially for YBa2Cu3Ov_ 6 and (La,_xSrx)2Cu04, or whether it is one mechanism with various parameters. Ramen scattering (RS) is a powerful technique for investigations of phonon spectrum and electron-phonon coupling. Though the superconductivity in (La~_×Srx)2Cu04 was discovered earlier [1] than the 90 K superconductivity in YBa2Cu3Ov.~ [2], the latter compound has been studied more ~horoughly by Raman spectroscopy [3][4][5] than (La1_×Srx)2CuO 4. for which the data, mainly obtained on ceramic samples, are inconsistent [6][7][8][9][10]. The investigation of single crystals makes it possible to define unambiguously the symmetry of Raman-active phonon modes and to classify them, taking into account the masses of vibrating atoms and the spacing between them.
The RS spectra were measured for different faces of La2C~04_ ~ crystals in back scattering configuration at T = 300 K using a spectrometer with a microattachment.
Laser radiation (R = 4880 ~ and 5145 ~) was focussed to a spot with a diameter ~3 ~. To avoid the excited region overheating, the laser radiation power on the sample was less than 0.5 raW.
The RS experiments were performed using La2C~04~ crystal plates (~3 x 4 x 0.5 mm 3) with well-developed basal plane. The crystals were grown from a PbO-based flux. The superconducting transition for these crystals was found to be T c ~ 14 K. The single crystals had the orthorhombic syr~etry (point group D2h) with a = 5.29 + 0.02 ~, b = 5.34 + 0.02 ~, c = 12.96 ± 0.02 A and contained twins-with [110] twinning axis.
At high temperatures (T ~ 530 K) La2Cu04 crystals belong to the tetragonal crystal structure of K2NiF4 type (D~ v, 14/mmm) with alternating C~-0(2) and La-O(1) layers. A primitive cell contains 7 atoms (Fig. 1). The phonon spectrum includes 18 optic and 3 acoustic modes. The expansion in the types of symmetry at the F-polnt has the form: 2A~ + ~_g + 4A2u + B2u + 5Eu. The atoms of La anaoxygen 0(1), located on the long axis of the distorted octahedra surrounding Cu atoms, have equal position symmetry and participate in (A1g + Eg + A2u + Eu) vibrations. The atoms of oxygen 0(2) lying in the plane of C~ atoms take part in (A2 + B2u + 2Eu) vibrations, whereas Cu atoms with the position summary coinciding with that of an elementary cell give rise to an acoustic branch of (A2u + Eu) vibrations. Since only even vibrations are Raman-active, one may expect that tetragonal crystals of La2Cu04 will display two AI~ symmetry modes corresponding to axial vibrations of La and 0(1) atoms, and two doubly degenerate Eg vibrations of the same atoms.
At room temperature the La2Cu04 crystals are in the orthorhombic phase. In this case A~g modes transfer to Ag ones, and Eg splits into B2g and Bog. The experimental RS spectra in (zz) and (zx) polarizations are represented in Fig. 2a and 2b. All the three observed lines 220, 427 and 527 cm -~ correspond to Ag symmetry mode s , as can be concluded from their polarization properties.
It was found that under excitation normal to the basal plane the intensities of these modes are much smaller. We have not succeeded in recording B2g and B3g compounds because of their low RS efficiency.
Therefore, in the Raman spectra o£ single La2C~04_ 6 crystals we observe three Ag symmetry vibrations, polarized along the c-axls. This fact is somewhat surprising, since the grouptheory analysis suggests that only two modes of the Ag symmetry should be expected. These are connected with the vibrations of La and 0(1) atoms along the axis, binding them with Cu atoms.
It would be natural to associate the low frequency 220 cm "=~ mode with the vibrations of the heavy La atoms, the frequency of the vibration being in good agreement with the calculated value [10]. The high-frequency 427 and 527 cm -~ modes seem to be related to vibrations of oxygen. This is also confirmed by the fact that at high-densltles of the excitation power, which causes severe overheating of the crystal and the subsequent loss of oxygen, one may observe the broadening of the highfrequency l~ modes and their slight softening by 0.3 cm -~ (Fig. 2c).  An additional Ag symmetry oxygen mode, polarized along the c-axis, in the RS spectra may be due to a local breaking of symmetry in the basal plane. Such distortions can result from either spin correlations or the ordering of the oxygen vacancies, and they may lead to cell doubling in the (ab)-plane of the crystal.
In this case the number of Ag symmetry vibrations should increase from 2 to-6.
The number of axial vibrations related with La and 0(I) atoms will be doubled and, besides, there should appear two planar vibrations of oxygen 0(2). The frequency splitting of the axial mode of 0(1) atoms is due to the motion of the two neighboring (ab) in-plane atoms (translationally non-equivalent) with respect to their Cu atoms which may occur now in antiphase. Such vibrations are believed to have higher frequencies than those with cophased motion of neighboring 0(1) atoms.
It would be desirable to observe the Ag symmetry planar vibrations to confirm the hypothesis o£ the doubling of the lattice period in the basal plane. However, no new bends of noticeable intensity appeared under excitation normal to the (ab)-plane. Therefore, we can now only speculate that the RS efficiency of planar modes is lower than that of axial modes.