Epitaxial thin films of charge-ordered Pr 0.625 Ca 0.375 MnO 3 have been studied using variable temperature Scanning tunneling microscopy and spectroscopy (STM/STS). The as grown films were found to be granular while the annealed films show atomic terraces at all temperatures and are found to be electronically homogeneous in 78-300K temperature range. At high temperatures (T>T CO ≈ 230 K) the local tunnel spectra of the annealed films show a depression in the density of states (DOS) near Fermi energy implying a pseudogap with a significant DOS at E F . The gap feature becomes more robust with cooling with a sharp jump in DOS at E F at T CO and with a gap value of ∼0.3 eV at 78K. At low temperatures we find a small but finite DOS at E F indicative of some delocalized carriers in the CO phase together with an energy gap. This is consistent with bulk transport, which shows weakening of the activation gap with cooling below 200K, and indicates the presence of two types of carriers at low temperatures.
1The narrow bandwidth charge-ordered (CO) manganites have been of significant research interest for their intriguing physics [1] and application potential in memory applications as the insulating CO phase can be melted into metallic phase by various perturbations such as magnetic field, electric field and electromagnetic radiation [2,3,4]. The manganites, in general, are best understood by Zener double-exchange mechanism [5] with modifications arising from electron-lattice coupling due to Jahn-Teller (JT) interaction [6,7]. The relative strength of the localizing JT interaction as compared to the hopping energy is an important parameter determining the electron mobility and bandwidth [7]. The ferromagnetic ordering promotes hopping of carriers due to double exchange while an antiferromagnetic ordering is expected to suppress it [7]. In narrow bandwidth (BW) manganites, like Pr 1−x Ca x MnO 3 (PCMO), the JT interactions dominate over hopping leading to the carrier trapping into localized JT polarons. These polarons hop with an activation energy as seen in the bulk transport at high temperatures [1, 8]. Due to these strong JT interactions, PCMO does not show the insulator-metal transition under normal conditions. In particular, for the doping range 0.30≤x≤0.65, PCMO shows a CO transition in 170-240K temperature range with cooling [9].In recent studies of PCMO manganites, the CO transition, structural transition and long range antiferromagnetic ordering have been investigated extensively by various experiments such as X-ray scattering, neutron scattering, optical conductivity and angle resolved photoemission spectroscopy (ARPES). The resonant X-ray scattering and neutron diffraction experiments reveal simultaneous structural and CO transitions [10,11,12,13,14]. A long range CE-type antiferromagnetic ordering has also been found in bulk PCMO below T N =170 K which is slightly below its T CO [11,13,14]. Thus the ground state of PCMO is CO with CE-type antiferromagnetic ordering [11,13]. In photoemission and optical measureme...