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Raman and infrared absorption spectra of Mg1−xAlxB2 have been collected for 0 ≤ x ≤ 0.5 in the spectral range of optical phonons. The x-dependence of the peak frequency, the width and the intensity of the observed Raman lines has been carefully analized. A peculiar x-dependence of the optical modes is pointed out for two different Al doping ranges. In particular the onset of the high-doping structural phase previously observed in diffraction measurements is marked by the appearence of new spectral components at high frequencies. A connection between the whole of our results and the observed suppression of superconductivity in the high doping region is established.The recent discovery 1 of superconductivity below 39 K in MgB 2 has stimulated a great deal of effort among the scientific community and a large number of theoretical and experimental papers have been published within few months. The debate on the origin of this unexpected superconductivity is still open, although both experimental 2-4 and theoretical 5-7 works indicate that MgB 2 is a BCS-like system. In this framework, the obvious relevant interaction in the superconducting transition is the electron-phonon (e-ph) coupling. Owing to the simple hexagonal structure (space group P 6 mmm), four zone-center optical modes are predicted for MgB 2 : a silent B 1g mode, the E 2g Raman mode, and the infrared active E 2u and A 2u modes. While the doubly-degenerate E 2u and E 2g modes are ascribed to in-plane stretching modes of the boron atoms, both non-degenerate A 2u and B 1g modes involve vibrations along the perpendicular direction (c axis). It is quite a general statement that the E 2g mode is expected to allow for the strongest e-ph coupling 5-7 and then to play a relevant role in superconductivity. Raman experiments 8-12 carried out on MgB 2 have shown that the spectrum is dominated by a quite large and asymmetric band around 600 cm −1 , ascribed to the E 2g mode. The anomalous width of this phonon peak has been interpreted as a signature of the e-ph coupling.Up to now, no other isostructural boride (XB 2 ) has shown the peculiar high temperature superconductivity of MgB 2 . In particular, MgAl 2 is not superconducting. Indeed, several studies on the Mg 1−x Al x B 2 compounds have shown that superconductivity is progressively suppressed for increasing x and vanishes for x>0.5. [13][14][15] In order to achieve a deeper understanding of the effects of Al doping, we have studied the evolution of the phonon spectrum of Mg 1−x Al x B 2 in the 0 ≤ x ≤ 0.5 range by means of both Raman and infrared spectroscopy.Pure MgB 2 and Al doped polycrystalline samples have been synthesized at high temperature by direct reaction of the elements in a tantalum crucible under argon atmosphere. The samples, which show an average grain dimension around 1-2µm, have been characterized by xray diffraction and by resistivity measurements, in order to determine, in particular, the x-dependence of the superconductivity transition temperature T c . 14,16 .The Raman spectra were measure...

An extended temperature and pressure-dependent investigation is carried out on a La0.75Ca0.25MnO3 sample exploiting the infrared absorption technique coupled to a diamond anvil cell. The pressure dependence of the insulator to metal transition temperature T(IM) is determined for the first time up to 11.2 GPa. The T(IM)(P) curve we propose to model the present data has an exponential-like behavior with an associated characteristic pressure P* playing the role of a decay constant. It is found that the equivalence between an external and an internal (chemical) pressure holds over a limited range of pressure, namely, P< or =2P*. Moreover, a certain universality character is associated with the proposed model curve in its ability to account for a large class of low-disorder manganites characterized by intermediate electron-phonon coupling.

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.

Organo-lead halide perovskites are nowadays considered to be an emerging photovoltaic material. It is clear that the peculiar hybrid nature of this class of materials is central for their outstanding optical and transport properties. However, the role of the organic cation and its interplay with the inorganic framework remains elusive. To get insight into the interactions at play, high-pressure Raman, infrared, and X-ray absorption spectroscopy measurements were performed on MAPbBr3 (MA = CH3NH3 +). Since lattice compression allows for a fine-tuning of the organic/inorganic interaction, we were able to follow the pressure evolution of the MA dynamics within the PbBr6 cage and identify different phases. From a MA dynamical disordered configuration, the system enters at first a cation ordered phase and, at higher pressure, a static disordered MA phase. Data analysis points at H-bonding as the driving force for molecular reorientation. Since the MA dynamics directly influence the formation of polarons in hybrid perovskites and their ferroelectric properties, the present results provide the basis for the understanding of the transport mechanisms at the core of the outstanding properties of this class of materials.