Infrared Absorption from Charge Density Waves in Magnetic Manganites

Abstract

The infrared absorption of charge density waves coupled to a magnetic background is first observed in two manganites La1−xCaxMnO3 with x = 0.5 and x = 0.67. In both cases a BCS-like gap 2∆(T ), which for x = 0.5 follows the hysteretic ferro-antiferromagnetic transition, fully opens at a finite T0 < T Neel , with 2∆(T0)/kBTc ≃ 5. These results may also explain the unusual coexistence of charge ordering and ferromagnetism in La0.5Ca0.5MnO3.The close interplay between transport properties and magnetic ordering in the colossal magnetoresistance (CMR) manganites La(Nd) 1−x Ca(Sr) x MnO 3 is presently explained in terms of magnetic double exchange promoted by polaronic carriers along the path Mn +3 -O −2 -Mn +4 .[1] Charge hopping promotes the alignment of Mn +3 and Mn +4 magnetic moments, and vice versa. The polaronic effects are due to the dynamic Jahn-Teller distortion of the oxygen octahedra around the Mn +3 ions. The above mechanism explains how, in manganites with 0.2 < x < 0.48, any increase in the magnetization enhances the dc conductivity, and vice versa. However, La 0.5 Ca 0.5 MnO 3 shows an unpredicted coexistence of ferromagnetism and incommensurate charge ordering (CO). This compound is paramagnetic at room temperature, becomes ferromagnetic (FM) at T c ≃ 225 K and, by further cooling (C), antiferromagnetic (AFM) at a Néel temperature T C N ≃ 155 K.[2] Upon heating the sample (H) the FM-AFM transition is instead observed at T H N ≃ 190 K .[3] The dc conductivity σ(0) of La 0.5 Ca 0.5 MnO 3 is quite insensitive to the PM-FM transition at T c . [3] Xray, neutron [4] and electron diffraction [5] show quasicommensurate charge and orbital ordering in the AFM phase with wavevector q = (2π/a)( 1 2 − ǫ, 0, 0). The incommensurability ǫ increases with temperature and follows the hysteretic behavior of the AFM-FM transition, until charge ordering disappears above the Curie point T c .[5] At higher Ca doping, for 0.5 < ∼ x < ∼ 0.75, a transition to a charge ordered phase [6] is observed in the paramagnetic phase at T CO . T CO is a maximum (265 K) for x = 0.67 ≃ 2 3 , where the charge ordering is commensurate with the lattice. Below T CO , the system enters at T N an antiferromagnetic phase. For x = 0.67, T N ≃ 140 K.

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

“…The different value of the gap compared to that of Ref. [13] could, at least in part, be due to different size of charge ordered domains in the material.…”

“…The different value of the gap compared to that of Ref. [13] could, at least in part, be due to different size of charge ordered domains in the material.In conclusion, the infrared reflection study of La 1/2 Ca 1/2 MnO 3 revealed the occurrence of a structural phase transition at T C , evidenced by appearance of additional phonon lines. The charge and orbital ordering below the antiferromagnetic transition temperature T N was found to drastically modify the carrier dynamics as charge density waves develop, leading to the creation of a gap in the excitation spectrum with magnitude of 2∆ 0 =(320 ± 15) cm −1 .…”

“…shows zero contribution to the dc conductivity. It signals opening of a gap, related to the formation of a charge density wave in charge ordered state due to realspace ordering of Mn 3+ and Mn 4+ ions [1,4,13].…”

“…The temperature, at which this gap opens, unambiguously identifies it as being due to the formation of a charge ordered state in La 1/2 Ca 1/2 MnO 3 because the antiferromagnetic transition temperature for the sample is T c N =163 K. The gap fully opens below 150 K, where it reaches the value of 2∆ 0 =(320 ± 15) cm −1 (Fig. 3).Earlier experimental study of the charge density waves in La 1/2 Ca 1/2 MnO 3 by optical transmission technique yielded the gap value of 710 cm −1 [13]. Even larger value of about 3600 cm −1 was obtained from a reflection studies by Kim et al[15].…”

“…shows zero contribution to the dc conductivity. It signals opening of a gap, related to the formation of a charge density wave in charge ordered state due to realspace ordering of Mn 3+ and Mn 4+ ions [1,4,13].Theoretical consideration of a charge ordered system yields the following frequency dependence of conductivity [14]:where 2∆ is the magnitude of the charge gap and α = 1/2. Using this prediction we fitted the conductivity spectra at frequencies above the highest energy phonon (in the range 750 to 2800 cm −1 ) using 2∆ and α as parameters.…”