Raman spectroscopy study of the interface structure in (CaCuO2)n/(SrTiO3)m superlattices

Raman spectra of CaCuO2/SrTiO3 superlattices show clear spectroscopic marker of two structures formed in CaCuO2 at the interface with SrTiO3. For non-superconducting superlattices, grown in low oxidizing atmosphere, the 425 cm-1 frequency of oxygen vibration in CuO2 planes is the same as for CCO films with infinite layer structure (planar Cu-O coordination). For superconducting superlattices grown in highly oxidizing atmosphere, a 60 cm-1 frequency shift to lower energy occurs. This is ascribed to a change from planar to pyramidal Cu-O coordination because of oxygen incorporation at the interface. Raman spectroscopy proves to be a powerful tool for interface structure investigation

Pressure induced magnetic phase separation in La0.75_{0.75}Ca0.25_{0.25}MnO3_{3} manganite

The pressure dependence of the Curie temperature T$_{C}(P)$ in La$_{0.75}$Ca$_{0.25}$MnO$_{3}$ was determined by neutron diffraction up to 8 GPa, and compared with the metallization temperature T$_{IM}(P)$ \cite{irprl}. The behavior of the two temperatures appears similar over the whole pressure range suggesting a key role of magnetic double exchange also in the pressure regime where the superexchange interaction is dominant. Coexistence of antiferromagnetic and ferromagnetic peaks at high pressure and low temperature indicates a phase separated regime which is well reproduced with a dynamical mean-field calculation for a simplified model. A new P-T phase diagram has been proposed on the basis of the whole set of experimental data.Comment: 5 pages, 4 figure

Optical study of the vibrational and dielectric properties of BiMnO3

BiMnO3 (BMO), ferromagnetic (FM) below Tc = 100 K, was believed to be also ferroelectric (FE) due to a non-centro-symmetric C2 structure, until diffraction data indicated that its space group is the centro-symmetric C2/c. Here we present infrared phonon spectra of BMO, taken on a mosaic of single crystals, which are consistent with C2/c at any T > 10 K, as well as room-temperature Raman data which strongly support this conclusion. We also find that the infrared intensity of several phonons increases steadily for decreasing T, causing the relative permittivity of BMO to vary from 18.5 at 300 K to 45 at 10 K. At variance with FE materials of displacive type, no appreciable softening has been found in the infrared phonons. Both their frequencies and intensities, moreover, appear insensitive to the FM transition at Tc

Effect of Al doping on the optical phonon spectrum in Mg(1-x)Al(x)B(2)

Raman and infrared absorption spectra of Mg(1-x)Al(x)B(2) 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

Coexistence of pressure-induced structural phases in bulk black phosphorus: a combined x-ray diffraction and Raman study up to 18 GPa

We report a study of the structural phase transitions induced by pressure in bulk black phosphorus by using both synchrotron x-ray diffraction for pressures up to 12.2 GPa and Raman spectroscopy up to 18.2 GPa. Very recently black phosphorus attracted large attention because of the unique properties of fewlayers samples (phosphorene), but some basic questions are still open in the case of the bulk system. As concerning the presence of a Raman spectrum above 10 GPa, which should not be observed in an elemental simple cubic system, we propose a new explanation by attributing a key role to the non-hydrostatic conditions occurring in Raman experiments. Finally, a combined analysis of Raman and XRD data allowed us to obtain quantitative information on presence and extent of coexistences between different structural phases from ~5 up to ~15 GPa. This information can have an important role in theoretical studies on pressure-induced structural and electronic phase transitions in black phosphorus

High pressure Raman study of La1-xCaxMnO3-δ manganites

We report a high-pressure Raman study on two members of the La1-xCaxMnO3-δ manganite family (x = 0.20, δ = 0 and δ = 0.08). The results obtained for the δ = 0 sample show a different behavior in the low and high pressure regime which is ascribed to the onset of a new pressure-activated interaction previously invoked in other manganite compounds. The comparison of our results with literature data gives further support to the identification of the Jahn-Teller sensitive stretching mode and shows that pressure-induced octahedral symmetrization is more effective in systems exhibiting a lower metallic character. On the contrary the new interaction sets in at a pressure which decreases on increasing the metallic character of the system indicating an important role of the Mn-Mn hopping integral in its activatio

Pressure-tuning of the electron-phonon coupling: the insulator to metal transition in manganites

A comprehensive understanding of the physical origin of the unique magnetic and transport properties of A_(1-x)A'^xMnO_3 manganites (A = trivalent rare-earth and A' = divalent alkali-earth metal) is still far from being achieved. The complexity of these systems arises from the interplay among several competing interactions of comparable strength. Recently the electron-phonon coupling, triggered by a Jahn-Teller distortion of the MnO_6 octahedra, has been recognised to play an essential role in the insulator to metal transition and in the closely related colossal magneto-resistance. The pressure tuning of the octahedral distortion gives a unique possibility to separate the basic interactions and, at least in principle, to follow the progressive transformation of a manganite from an intermediate towards a weak electron-phonon coupling regime. Using a diamond anvil cell, temperature and pressure-dependent infrared absorption spectra of La_(0.75)Ca_(0.25)MnO_3 have been collected and, from the spectral weight analysis, the pressure dependence of the insulator to metal transition temperature T_IM has been determined for the first time up to 11.2 GPa. The T_IM(P) curve we proposed to model the present data revealed a universality character in accounting for the whole class of intermediate coupling compounds. This property can be exploited to distinguish the intermediate from the weak coupling compounds pointing out the fundamental differences between the two coupling regimes.Comment: 8 pages, 4 figure

High pressure Raman study of La1−x_{1-x}Cax_xMnO3−δ_{3-\delta} manganites

We report on a high-pressure Raman study on two members of the La$_{1-x}$Ca$_x$MnO$_{3-\delta}$ manganite family ($x=0.20$, $\delta=0$ and $\delta=0.08$). The results obtained for the $\delta=0$ sample show a different behavior in the low and high pressure regimes ascribed to the onset of a new pressure-activated interaction previously invoked in other manganite compounds. The comparison of our results with literature data gives further support to the identification of the Jahn-Teller active stretching mode and shows that pressure-induced octahedral symmetrization is more effective in systems exhibiting a lower metallic character. On the contrary the new interaction sets in at pressure which decreases on increasing the metallic character of the system indicating a relevant role of the Mn-Mn hopping integral in its activation.Comment: 4 pages, 3 figure, submitted to Phys. Rev.

Tuning topological disorder in MgB2_{2}

We carried out Raman measurements on neutron-irradiated and Al-doped MgB$_2$ samples. The irradiation-induced topological disorder causes an unexpected appearance of high frequency spectral structures, similar to those observed in lightly Al-doped samples. Our results show that disorder-induced violations of the selection rules are responsible for the modification of the Raman spectrum in both irradiated and Al-doped samples. Theoretical calculations of the phonon density of states support this hypothesis, and demonstrate that the high frequency structures arise mostly from contributions at ${\bf q}\not=0$ of the E$_{2g}$ phonon mode.Comment: 4 pages, 4 figure