SURFACE LATTICE DYNAMICS OF SINGLE - LAYERED TRANSITION - METAL OXIDES : Sr

Transition-metal oxides (TMOs) have been investigated extensively because this class of materials exhibits a fascinating array of phenomena because of a strong coupling among lattice, charge, spin, and orbital degrees of freedom. Creating a surface by breaking the symmetry is a way to disturb the coupled system, thus providing a fresh approach for studying the spin-charge-lattice coupling in the complex materials. The couplings (i.e., electron-phonon, magnon-phonon, magneto-elastic, etc.) can be revealed in the phonon dispersion and lifetime. Consequently, studying the surface lattice dynamics is an important step for understanding the behavior of surface TMOs. In this work, the lattice dynamics of singlecrystal Sr2RuO4 and La0.5Sr1.5MnO4 surfaces cleaved in ultrahigh vacuum were studied. These two systems have the same type of lattice structure in the bulk, but completely different physical properties. Sr2RuO4 is a good metal and a p-wave superconductor below ~1.5 K while La0.5Sr1.5MnO4 is an insulator undergoing orbital and charge ordering at a temperature of about 220 K and is antiferromagnetically ordered below 110 K, respectively. The surface of Sr2RuO4 has a C(2 × 2) reconstruction while the surface of La0.5Sr1.5MnO4 keeps a (1 × 1) truncated structure. For both systems, three surface optical phonon features (ω1, ω2, and ω3) were clearly observed (Fig. 1). In Fig. 1, ω1 is an external mode due to vibration of the Sr/La atom along the c–axis. ω2 is the out-of-plane Ru(Mn)-O-Ru(Mn) bending mode, and ω3 is a stretching mode associated with the vibrations of the oxygen atoms against the Ru(Mn) ion. Linewidths of surface optical phonons of ω1, ω2, and ω3 are found to be 5.3, 2.5, and 7.5 meV for Sr2RuO4, and 1.8, 4.2, and 8.8 meV for La0.5Sr1.5MnO4, respectively. The large linewidth of ω1 for Sr2RuO4 shows that there are two phonons because of surface reconstruction. Surprisingly, the linewidth of ω1 for La0.5Sr1.5MnO4 is small, indicating this phonon mode is purely from a pure Sr atom because of Sr segregation at the surface, but without surface reconstruction. All surface modes for both systems are significantly higher in energies than the corresponding bulk modes, indicating that the surface properties of these systems are appreciably different than in the bulk, which will be true for TMOs in general.