Charge and lattice dynamics of ordered state in La1/2Ca1/2MnO3: infrared reflection spectroscopy study

Abstract

We report an infrared reflection spectroscopy study of La 1/2 Ca 1/2 MnO3 over a broad frequency range and temperature interval which covers the transitions from the high temperature paramagnetic to ferromagnetic and, upon further cooling, to antiferromagnetic phase. The structural phase transition, accompanied by a ferromagnetic ordering at TC =234 K, leads to enrichment of the phonon spectrum. A charge ordered antiferromagnetic insulating ground state develops below the Néel transition temperature TN =163 K. This is evidenced by the formation of charge density waves and opening of a gap with the magnitude of 2∆0 = (320 ± 15) cm −1 in the excitation spectrum. Several of the infrared active phonons are found to exhibit anomalous frequency softening. The experimental data suggest coexistence of ferromagnetic and antiferromangetic phases at low temperatures. PACS numbers:Manganite perovskites R 1−x A x MnO 3 (where R is a trivalent rare earth and A is a divalent alkaline rare earth) exhibit rich phase diagram and a variety of intriguing properties due the delicate interplay of spin, charge, lattice, and orbital degrees of freedom (Ref. [1,2,3] and references cited therein). Well defined anomalies of physical properties at commensurate carrier concentrations of x=N/8 (where N=1, 3, 4, 5, and 7) and x = L/3 (where L=1,2) were unambiguously established. Of particular interest is the phenomenon of charge and orbital ordering, most clearly pronounced for x=1/2 [4,5,6,7,8,9,10].In this communication we report the results of an infrared reflection study of La 1/2 Ca 1/2 MnO 3 over broad frequency range and at temperatures, covering the transitions from the high temperature paramagnetic to ferromagnetic and, upon further cooling, to antiferromagnetic phase. By making use of Kramers-Kronig analysis we obtained the spectral dependence of conductivity. Its analysis yields information on the evolution of phonons and electronic excitations as a function of temperature. We found that additional phonon modes appear in the spectra in the ferromagnetic phase, a fact which implies the occurrence of a structural phase transition. Upon further cooling the conductivity spectrum shows development of a gap, which signals the formation of a charge ordered state at low temperatures. At the same time a Drude-like component of the conductivity does not vanish completely and suggests the coexistence of a metallic and insulating phases.The measurements were performed on a dense ceramic pellet of La 1/2 Ca 1/2 MnO 3 , mechanically polished to optical quality. The material preparation technique is described in Ref. [3,9,11]. The samples were intensively characterized by the X-ray scattering, neutron diffraction, magnetization measurements, Raman scattering as well as Mössbauer spectroscopy (doped with 1% Sn or Fe). These measurements unambiguously identified the transition from the paramagnetic to ferromagnetic phase at T C =234 K, and further to antiferromagnetic phase at T c N =163 K (cooling cycle) or T h N =196 K (heating cycle), see The re...

Infrared absorption study of charge ordered $La{}_{0.5}Ca{}_{0.5-x}Sr_{x}MnO_{3}$ $(0.1\leq x\leq0.5$

“…Another study on La 0.5 Ca 0.5 M nO 3 reports the gap to be 320cm −1 [32]. However, in the La 5/8−y P r y Ca 3/8 M nO 3 system, where the charge ordering is known to be of La 0.5 Ca 0.5 M nO 3 type, the insulating gap is ∼ 0.38eV (3065cm −1 ) [33].…”