Efthymios Liarokapis - Academia.edu (original) (raw)
Papers by Efthymios Liarokapis
Acta Physica Polonica A, 2009
Recently, a precession X-ray study has discovered that ferroelectric BaTiO3 single crystals have ... more Recently, a precession X-ray study has discovered that ferroelectric BaTiO3 single crystals have a coherent hybrid structure that consists of tetragonal and monoclinic lattices that share the (101) face of the tetragonal [11, 12]. Microscopic observations attest that the single crystal has two kinds of stripes (dark and white regions). Raman spectra of all sides of the cubic crystal have been collected at several scattering configurations. Two kinds of domains have been observed by the Raman measurements within the stripes, which are identified as light and dark spots under the optical microscope. The spectra structures and line widths are strongly affected when the laser direction is parallel to the b-axes and the polarization parallel to the a-or c-axes of the 90 • domain (ac-side). It appears that there is a symmetry reduction and disorder, which affects the Raman active phonons. The results are in agreement with the coexistence of the two (tetragonal and monoclinic) lattices.
Physical Review B, 1999
ABSTRACT A comparative Raman study of Re-free and Re-doped HgBa2Can-1CunO2n+2+δ with n=1,3,4,5 is... more ABSTRACT A comparative Raman study of Re-free and Re-doped HgBa2Can-1CunO2n+2+δ with n=1,3,4,5 is presented in an attempt to further clarify the structural and phononic modifications brought about by Re substitution. A number of extra high-frequency phonon peaks show up in the spectra of the Re-doped samples and are attributed to the oxygen modes of a strongly bound, almost decoupled ReO6 octahedron. As regards the apex oxygen in the Hg site, a clear transfer of spectral weight from the 590 to the 570 cm-1 apex phonon band is observed upon Re substitution. Such a change may well be accounted for assuming increased excess oxygen content for the Re-doped samples. Another interesting result is the enhancement upon Re doping of a narrow peak probably attributed to c-axis vibrations of Ba whose frequency shows a distinctive change with the number n of the CuO2 layers, providing an easy way to identify the various phases in a sample.
Acta Crystallographica Section A Foundations of Crystallography, 2009
Ab initio electronic structure theory is known as a useful tool for prediction of materials prope... more Ab initio electronic structure theory is known as a useful tool for prediction of materials properties and for their understanding. However, majority of simulations still deal with calculations of total energies at zero temperature. In this talk we show that for large number of problems it is necessary to take proper experimental conditions into account to obtain reliable theoretical results. In particular, we show that the problem of lattice stability, the energy difference between different crystal structures of a material, can be consistently described by theory and experiment if the former is done at elevated rather than at zero temperature [1]. A decisive role of lattice vibrations will be illustrated by theoretical search for metastable fccrelated SiN phases [2], which are the tissue phases of (Ti-Si)N thin film nanocomposites, as well as in first-principles study of stability of newly synthesized ternary perovskite Sc 3 AlN and a family of its isoelectronic compounds Sc 3 EN (E=B, Ga, In) [3].
International Journal of Biomaterials, 2009
Studies of the average crystal structure properties of human dental apatite as a function of age ... more Studies of the average crystal structure properties of human dental apatite as a function of age in the range of 5–87 years are reported. The crystallinity of the dental hydroxyapatite decreases with the age. The a-lattice constant that is associated with the carbonate content in carbonate apatite decreases with age in a systematic way, whereas the c-lattice constant does not change significantly. Thermogravimetric measurements demonstrate an increase of the carbonate content with the age. FTIR spectroscopy reveals both B and A-type carbonate substitutions with the B-type greater than the A-type substitution by a factor up to ~5. An increase of the carbonate content as a function of age can be deduced from the ratio of the to the IR modes.
Physical Review B, 2003
We report site-selective oxygen isotope (16 O/ 18 O) effect (OIE) measurements on the in-plane pe... more We report site-selective oxygen isotope (16 O/ 18 O) effect (OIE) measurements on the in-plane penetration depth λ ab in underdoped Y0.6Pr0.4Ba2Cu3O 7−δ , using the muon-spin rotation (µSR) technique. A pronounced OIE on the transition temperature Tc as well as on λ −2 ab (0) was observed, which mainly arises from the oxygen sites within the superconducting CuO2 planes (100 % within error bar). The values of the corresponding relative isotope shifts were found to be Tc/Tc = −3.7(4) % and ∆λ −2 ab (0)/λ −2 ab (0) = −6.2(1.0)%. Our results imply that in this compound the phonon modes involving the movement of planar oxygen are dominantly coupled to the electrons.
Physical Review B, 2003
... The strains induced by the two different substrates do not seem to affect the Raman spectra, ... more ... The strains induced by the two different substrates do not seem to affect the Raman spectra, probably due to their relaxation towards the surface of the films used in our studies. ... Rev. B 59, 1701 1999 . 23 ST Aruna, M. Muthuraman, and KC Patil, Solid State Ionics 120, 275 1999 . ...
Physica B: Condensed Matter, 2006
ABSTRACT A systematic Raman study has been carried out on single crystals of the heavy fermion UR... more ABSTRACT A systematic Raman study has been carried out on single crystals of the heavy fermion URu2Si2 compound at low temperatures (down to ) and high hydrostatic pressures (up to ). At ambient pressures and below 20 K, small modifications in the Si A1g mode have been observed. With increasing pressure, this mode shifts to higher energies at a high increment rate at room temperature, while the pressure dependence of its linewidth shows an anomalous behavior decreasing suddenly above . Hydrostatic pressure measurements at and low temperatures have revealed a broad weak peak at , which is absent at ambient pressures and disappears above . These effects can be related with a pressure-dependent phase separation.
Physical Review B, 2003
We study the formation and evolution of phases in the La substituted Y 1Ϫx La x Ba 2 Cu 3 O y ͓͑Y... more We study the formation and evolution of phases in the La substituted Y 1Ϫx La x Ba 2 Cu 3 O y ͓͑Y-La͒123͔ superconductors from XRD and Raman measurements. We find characteristic modifications in the interatomic distances, bond valence sum ͑BVS͒ values and crystallite microstrains, which correlate with corresponding features of the micro-Raman spectra at nominal La compositions xϭ0.2, 0.5, and 0.8. Based on the Raman spectra we conclude that two phases appear, an intermediate of chemical formula Y 1Ϫx La x Ba 2 Cu 3 O y with x values less than nominal, and another phase of pure La123 with partial substitution of La for Ba and reduced T c. The crystallographic changes and the reduction of T c can be interpreted based on the formation of this pure La123 subphase upon La doping.
Physical Review B, 1999
Raman spectra were measured for the full range of hole doping of the orthomanganites of the form ... more Raman spectra were measured for the full range of hole doping of the orthomanganites of the form La 1Ϫx Ca x MnO 3 (0рxр1) from ambient temperature to 75 K. Two high-frequency phonons that we assign to stretching modes of the oxygen atoms in the MnO 6 octahedra soften below the transition temperature to ferromagnetic insulator and disappear at a temperature that depends on the Ca concentration. The correlation of their properties with the lattice distortions is examined. At room temperature, and for xϳ0.35 and xϳ0.70, phonons attributed to the relative rotations of the octahedra show a strong correlation with the octahedral distortion and the reported low-temperature modifications of the magnetic properties of these compounds.
Physical Review B, 2009
Combined synchrotron angle-dispersive powder diffraction and micro-Raman spectroscopy are used to... more Combined synchrotron angle-dispersive powder diffraction and micro-Raman spectroscopy are used to investigate pressure-induced lattice instabilities that are accompanied by superconducting Tc anomalies in YBa2Cu4O8 and optimally doped YBa2Cu3O 7−δ , in comparison with the nonsuperconducting PrBa2Cu3O6.92. In the first two superconducting systems there is a clear anomaly and hysteresis in the evolution of the lattice parameters and increasing lattice disorder with pressure, which starts at ≈ 3.7 GPa. On the contrary, in the Pr-compound the lattice parameters follow very well the expected equation of state (EOS) up to 7 GPa. The micro-Raman data of the superconducting compounds show that the energy and width of the Ag phonons exhibit anomalies over the same pressure range where the lattice parameters deviate from the EOS and the average Cu2-O pl bond length exhibits a strong contraction that correlates with the non-linear pressure dependence of Tc. The anomalous Raman behavior is not observed for the non superconducting Pr compound, clearly indicating a connection with the charge carriers. It appears that the cuprates close to optimal doping are at the edge of lattice instability.
Physical Review B, 2009
We report the boron isotope effect on the E 2g phonon mode by micro-Raman spectroscopy on the ter... more We report the boron isotope effect on the E 2g phonon mode by micro-Raman spectroscopy on the ternary Mg 1-x Al x B 2 system, synthesized with pure isotopes 10 B and 11 B. The isotope coefficient on the phonon frequency is near 0.5 in the full range decreasing near x = 0. The intraband electron-phonon (e-ph) coupling, for the electrons in the sigma band, has been extracted from the E 2g line-width and frequency softening. Tuning the Fermi energy near the electronic topological transition (ETT), where the sigma Fermi surface changes from 2D to 3D topology the E 2g mode, shows the known Kohn anomaly on the 2D side of the ETT and a splitting of the E 2g phonon frequency into a hard and soft component from x = 0 to x = 0.28. The results suggest a minor role of the intraband phonon mediated pairing in the control of the high critical temperature in Mg 1-x Al x B 2. The common physical features of diborides with the novel multigap FeAs-based superconductors and cuprates is discussed.
Physical Review B, 2006
The doping dependence of the micro-Raman spectra of high quality La2−xSrxCuO4 microcrystals with ... more The doping dependence of the micro-Raman spectra of high quality La2−xSrxCuO4 microcrystals with x=0.0-0.45 has been investigated in the region 10-300 K. The phonon at ∼100 cm −1 of the orthorhombic phase shows a classical soft mode behavior up to x=0.17, supporting its correlation with the LTO to HTT transition. In the xx/yy polarization spectra of the superconducting concentrations new modes at ∼150 and 370 cm −1 related to the symmetry breaking and a broad band at ∼280 cm −1 are attributed to the charge ordering at temperatures well above Tc. At all temperatures studied a correlation has been found between the doping dependence of the transition temperature and the intensity of the bands at ∼150, ∼280, ∼370 cm −1 , the intensity of the La/Sr mode, and the asymmetry of the apex phonon.
Physical Review B, 2005
A micro-Raman spectroscopic study has been carried out on fully oxygenated Y1-yCayBa2Cu3Ox (YBCO)... more A micro-Raman spectroscopic study has been carried out on fully oxygenated Y1-yCayBa2Cu3Ox (YBCO) compounds with y=0.0-0.2 and oxygen concentration varying with Ca in the range ( xtilde 6.960 and 6.995). The spectral profiles of the 4Ag and 1B1g -like symmetry phonons show minor modifications with Ca, but two modes that appear close in energy to the in-phase and the out-of-phase phonons gain intensity with the Ca substitution. The changes in the intensity of these two satellite modes with the amount of Ca are correlated with modifications in the dimpling of the CuO2 planes, as detected by extended x-ray absorption fine structure, and can be attributed to the formation of superstructures and to phase separation phenomena. The evolution of two bands in the in-phase phonon, associated in pure YBCO with local structural distortions and the coexistence of optimal and oxygen overdoped phases, remains completely independent of the Ca concentration. It appears that the Ca atoms have the tendency to be surrounded by Y atoms forming an independent nanophase of the type Y0.5Ca0.5Ba2Cu3Ox with the additional carriers from Ca not contributing to an excess doping of the system.
physica status solidi (b), 2009
... Vasyl Shenderovskii*, 1, Valentyn Baidakov1, Sergii Budsuliak1, Andrii Gorin1, Valerii Ermako... more ... Vasyl Shenderovskii*, 1, Valentyn Baidakov1, Sergii Budsuliak1, Andrii Gorin1, Valerii Ermakov1,Volodymyr Kolomoets1, Efthymios Liarokapis2, Galina Gromova3, B. Kazbekova3, and L ... [5] SI Budsulyak, AE Gorin, VM Ermakov, VV Kolo-moets, EF Venger, P. Verma, M ...
physica status solidi (b), 2009
... A Merrill–Bassett type diamond anvil cell (DAC) [45], equipped with type one diamonds suitabl... more ... A Merrill–Bassett type diamond anvil cell (DAC) [45], equipped with type one diamonds suitable for optical measurements, was employed for the high-pressure meas-urements, which has allowed the Raman studies to be car-ried out in a back scattering ... [29] P. Calvani, G. De ...
physica status solidi (b), 2009
... Volodymyr Kolomoets*, 1, Valentyn Baidakov1, Anatolii Fedosov1, Andrii Gorin1, Valerii Ermako... more ... Volodymyr Kolomoets*, 1, Valentyn Baidakov1, Anatolii Fedosov1, Andrii Gorin1, Valerii Ermakov1, Efthymios Liarokapis2, Galina Gromova3, B. Kazbekova3, L. Taimuratova3, and B. Orasgulyev3 ... [6] VM Ermakov, SI Budsuljak, EF Venger, VV Kolo-moets et al., Tensoresistivity ...
physica status solidi (b), 2007
ABSTRACT The phonon dynamics of the strontium vanadium oxide bronze (β-Sr0.33V2O5) is studied usi... more ABSTRACT The phonon dynamics of the strontium vanadium oxide bronze (β-Sr0.33V2O5) is studied using Raman spectroscopy at different temperature under high pressure up to 5.7 GPa. Interesting features of the spectra, such as the narrowing of some modes that harden under pressure (250–300 cm–1) or the softening of a low-frequency mode, are presented and discussed. Our low temperature measurements confirm that under high hydrostatic pressure the charge ordered phase is suppressed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (b), 2009
ABSTRACT We report a systematic study of Pr1–xCax MnO3 (x = 0.4 and 0.6) thin films grown on LaAl... more ABSTRACT We report a systematic study of Pr1–xCax MnO3 (x = 0.4 and 0.6) thin films grown on LaAlO3 (LAO) substrate (PCMO/LAO). X-ray diffraction measurements have shown that the PCMO/LAO film is [101]-oriented and under compressive strain due to the lattice mismatch. According to Raman spectroscopy the Jahn–Teller modes are suppressed under the application of pressure in the PCMO (x = 0.4) thin film. In particular, it was found that a pressure of 2 GPa suppresses the charge ordered state. Further high pressure measurements have confirmed that resistivity decreases linearly up to 2.4 GPa corresponding to a total reduction of about 35% in resistivity leading to a more conductive state on both concentrations. The temperature dependence of the resistivity has shown that the samples under pressure retain their semiconducting behavior. At ambient pressure the PCMO (x = 0.6) film shows a moderate magnetoresistance in the range 0–7 T and no metal to insulator transition (MIT). Raman spectroscopy confirmed that the application of a magnetic field of 1 T does not induce any remarkable changes on the electron–phonon interaction. Although, the hydrostatic pressure is not enough to induce a macroscopic metallic state, its effect is much greater than the one of the magnetic field. In addition, the effect of the 7 T magnetic field appears the same at ambient pressure and 2.5 GPa. Therefore the two effects of hydrostatic pressure and magnetic field do not seem to be coupled, at least up to the magnitudes studied. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Physica C: Superconductivity, 2007
We investigate the Y 1Àx Pr x Ba 2 Cu 3 O 7Àd (0 6 x 6 1) and YBa 2Ày Pr y Cu 3 O 7Àd (y = 0.1 an... more We investigate the Y 1Àx Pr x Ba 2 Cu 3 O 7Àd (0 6 x 6 1) and YBa 2Ày Pr y Cu 3 O 7Àd (y = 0.1 and 0.2) polycrystalline compounds via X-ray diffraction and Raman measurements. The oxygen concentration measurements show all samples to be optimally doped or overdoped. The magnetic measurements M(T) and v ac (T) verify that the increase of the Pr content leads to the expected strong systematic decrease of the critical temperature T c and the suppression of superconductivity for x > 0.55. Systematic micro-Raman studies of the observed vibrational modes reveal a coexistence of locally different rare-earth seemingly nanoscopic environments in the mixed rare-earth Y123 systems that has been overlooked to a great extent up to now. The comparison of the Raman spectra from the two sets of compounds proves that an increased amount of Ba is occupied by Pr in Y 1Àx Pr x Ba 2 Cu 3 O 7Àd and indicates that loss of superconductivity is mainly due to hole filling by this occupancy.
Physica C: Superconductivity, 2009
The doping dependence of the Raman spectra of high quality La 2Àx Sr x Cu 16,18 O 4 polycrystalli... more The doping dependence of the Raman spectra of high quality La 2Àx Sr x Cu 16,18 O 4 polycrystalline compounds has been investigated at low temperatures. It is shown that symmetry forbidden bands peaked at 150cmAˋ1,150 cm À1 , 150cmAˋ1,280 cm À1 , and $370 cm À1 are activated in the (xx/yy) polarization Raman spectra due to the local breaking of the inversion symmetry mainly at low temperatures and for doping concentrations for which the compound is superconducting. The apparent A 1-character of the activated modes in the symmetry reduced phase indicates a reduction from the D 2h to C 2v or D 2 crystal symmetries, which associates the observed modes to specific IR-active phonons with eigenvectors mainly along the c-axis. The temperature and doping dependence of this inversion symmetry breaking and the superconducting transition temperature are very similar, though the symmetry reduction occurs at significantly higher temperatures.
Acta Physica Polonica A, 2009
Recently, a precession X-ray study has discovered that ferroelectric BaTiO3 single crystals have ... more Recently, a precession X-ray study has discovered that ferroelectric BaTiO3 single crystals have a coherent hybrid structure that consists of tetragonal and monoclinic lattices that share the (101) face of the tetragonal [11, 12]. Microscopic observations attest that the single crystal has two kinds of stripes (dark and white regions). Raman spectra of all sides of the cubic crystal have been collected at several scattering configurations. Two kinds of domains have been observed by the Raman measurements within the stripes, which are identified as light and dark spots under the optical microscope. The spectra structures and line widths are strongly affected when the laser direction is parallel to the b-axes and the polarization parallel to the a-or c-axes of the 90 • domain (ac-side). It appears that there is a symmetry reduction and disorder, which affects the Raman active phonons. The results are in agreement with the coexistence of the two (tetragonal and monoclinic) lattices.
Physical Review B, 1999
ABSTRACT A comparative Raman study of Re-free and Re-doped HgBa2Can-1CunO2n+2+δ with n=1,3,4,5 is... more ABSTRACT A comparative Raman study of Re-free and Re-doped HgBa2Can-1CunO2n+2+δ with n=1,3,4,5 is presented in an attempt to further clarify the structural and phononic modifications brought about by Re substitution. A number of extra high-frequency phonon peaks show up in the spectra of the Re-doped samples and are attributed to the oxygen modes of a strongly bound, almost decoupled ReO6 octahedron. As regards the apex oxygen in the Hg site, a clear transfer of spectral weight from the 590 to the 570 cm-1 apex phonon band is observed upon Re substitution. Such a change may well be accounted for assuming increased excess oxygen content for the Re-doped samples. Another interesting result is the enhancement upon Re doping of a narrow peak probably attributed to c-axis vibrations of Ba whose frequency shows a distinctive change with the number n of the CuO2 layers, providing an easy way to identify the various phases in a sample.
Acta Crystallographica Section A Foundations of Crystallography, 2009
Ab initio electronic structure theory is known as a useful tool for prediction of materials prope... more Ab initio electronic structure theory is known as a useful tool for prediction of materials properties and for their understanding. However, majority of simulations still deal with calculations of total energies at zero temperature. In this talk we show that for large number of problems it is necessary to take proper experimental conditions into account to obtain reliable theoretical results. In particular, we show that the problem of lattice stability, the energy difference between different crystal structures of a material, can be consistently described by theory and experiment if the former is done at elevated rather than at zero temperature [1]. A decisive role of lattice vibrations will be illustrated by theoretical search for metastable fccrelated SiN phases [2], which are the tissue phases of (Ti-Si)N thin film nanocomposites, as well as in first-principles study of stability of newly synthesized ternary perovskite Sc 3 AlN and a family of its isoelectronic compounds Sc 3 EN (E=B, Ga, In) [3].
International Journal of Biomaterials, 2009
Studies of the average crystal structure properties of human dental apatite as a function of age ... more Studies of the average crystal structure properties of human dental apatite as a function of age in the range of 5–87 years are reported. The crystallinity of the dental hydroxyapatite decreases with the age. The a-lattice constant that is associated with the carbonate content in carbonate apatite decreases with age in a systematic way, whereas the c-lattice constant does not change significantly. Thermogravimetric measurements demonstrate an increase of the carbonate content with the age. FTIR spectroscopy reveals both B and A-type carbonate substitutions with the B-type greater than the A-type substitution by a factor up to ~5. An increase of the carbonate content as a function of age can be deduced from the ratio of the to the IR modes.
Physical Review B, 2003
We report site-selective oxygen isotope (16 O/ 18 O) effect (OIE) measurements on the in-plane pe... more We report site-selective oxygen isotope (16 O/ 18 O) effect (OIE) measurements on the in-plane penetration depth λ ab in underdoped Y0.6Pr0.4Ba2Cu3O 7−δ , using the muon-spin rotation (µSR) technique. A pronounced OIE on the transition temperature Tc as well as on λ −2 ab (0) was observed, which mainly arises from the oxygen sites within the superconducting CuO2 planes (100 % within error bar). The values of the corresponding relative isotope shifts were found to be Tc/Tc = −3.7(4) % and ∆λ −2 ab (0)/λ −2 ab (0) = −6.2(1.0)%. Our results imply that in this compound the phonon modes involving the movement of planar oxygen are dominantly coupled to the electrons.
Physical Review B, 2003
... The strains induced by the two different substrates do not seem to affect the Raman spectra, ... more ... The strains induced by the two different substrates do not seem to affect the Raman spectra, probably due to their relaxation towards the surface of the films used in our studies. ... Rev. B 59, 1701 1999 . 23 ST Aruna, M. Muthuraman, and KC Patil, Solid State Ionics 120, 275 1999 . ...
Physica B: Condensed Matter, 2006
ABSTRACT A systematic Raman study has been carried out on single crystals of the heavy fermion UR... more ABSTRACT A systematic Raman study has been carried out on single crystals of the heavy fermion URu2Si2 compound at low temperatures (down to ) and high hydrostatic pressures (up to ). At ambient pressures and below 20 K, small modifications in the Si A1g mode have been observed. With increasing pressure, this mode shifts to higher energies at a high increment rate at room temperature, while the pressure dependence of its linewidth shows an anomalous behavior decreasing suddenly above . Hydrostatic pressure measurements at and low temperatures have revealed a broad weak peak at , which is absent at ambient pressures and disappears above . These effects can be related with a pressure-dependent phase separation.
Physical Review B, 2003
We study the formation and evolution of phases in the La substituted Y 1Ϫx La x Ba 2 Cu 3 O y ͓͑Y... more We study the formation and evolution of phases in the La substituted Y 1Ϫx La x Ba 2 Cu 3 O y ͓͑Y-La͒123͔ superconductors from XRD and Raman measurements. We find characteristic modifications in the interatomic distances, bond valence sum ͑BVS͒ values and crystallite microstrains, which correlate with corresponding features of the micro-Raman spectra at nominal La compositions xϭ0.2, 0.5, and 0.8. Based on the Raman spectra we conclude that two phases appear, an intermediate of chemical formula Y 1Ϫx La x Ba 2 Cu 3 O y with x values less than nominal, and another phase of pure La123 with partial substitution of La for Ba and reduced T c. The crystallographic changes and the reduction of T c can be interpreted based on the formation of this pure La123 subphase upon La doping.
Physical Review B, 1999
Raman spectra were measured for the full range of hole doping of the orthomanganites of the form ... more Raman spectra were measured for the full range of hole doping of the orthomanganites of the form La 1Ϫx Ca x MnO 3 (0рxр1) from ambient temperature to 75 K. Two high-frequency phonons that we assign to stretching modes of the oxygen atoms in the MnO 6 octahedra soften below the transition temperature to ferromagnetic insulator and disappear at a temperature that depends on the Ca concentration. The correlation of their properties with the lattice distortions is examined. At room temperature, and for xϳ0.35 and xϳ0.70, phonons attributed to the relative rotations of the octahedra show a strong correlation with the octahedral distortion and the reported low-temperature modifications of the magnetic properties of these compounds.
Physical Review B, 2009
Combined synchrotron angle-dispersive powder diffraction and micro-Raman spectroscopy are used to... more Combined synchrotron angle-dispersive powder diffraction and micro-Raman spectroscopy are used to investigate pressure-induced lattice instabilities that are accompanied by superconducting Tc anomalies in YBa2Cu4O8 and optimally doped YBa2Cu3O 7−δ , in comparison with the nonsuperconducting PrBa2Cu3O6.92. In the first two superconducting systems there is a clear anomaly and hysteresis in the evolution of the lattice parameters and increasing lattice disorder with pressure, which starts at ≈ 3.7 GPa. On the contrary, in the Pr-compound the lattice parameters follow very well the expected equation of state (EOS) up to 7 GPa. The micro-Raman data of the superconducting compounds show that the energy and width of the Ag phonons exhibit anomalies over the same pressure range where the lattice parameters deviate from the EOS and the average Cu2-O pl bond length exhibits a strong contraction that correlates with the non-linear pressure dependence of Tc. The anomalous Raman behavior is not observed for the non superconducting Pr compound, clearly indicating a connection with the charge carriers. It appears that the cuprates close to optimal doping are at the edge of lattice instability.
Physical Review B, 2009
We report the boron isotope effect on the E 2g phonon mode by micro-Raman spectroscopy on the ter... more We report the boron isotope effect on the E 2g phonon mode by micro-Raman spectroscopy on the ternary Mg 1-x Al x B 2 system, synthesized with pure isotopes 10 B and 11 B. The isotope coefficient on the phonon frequency is near 0.5 in the full range decreasing near x = 0. The intraband electron-phonon (e-ph) coupling, for the electrons in the sigma band, has been extracted from the E 2g line-width and frequency softening. Tuning the Fermi energy near the electronic topological transition (ETT), where the sigma Fermi surface changes from 2D to 3D topology the E 2g mode, shows the known Kohn anomaly on the 2D side of the ETT and a splitting of the E 2g phonon frequency into a hard and soft component from x = 0 to x = 0.28. The results suggest a minor role of the intraband phonon mediated pairing in the control of the high critical temperature in Mg 1-x Al x B 2. The common physical features of diborides with the novel multigap FeAs-based superconductors and cuprates is discussed.
Physical Review B, 2006
The doping dependence of the micro-Raman spectra of high quality La2−xSrxCuO4 microcrystals with ... more The doping dependence of the micro-Raman spectra of high quality La2−xSrxCuO4 microcrystals with x=0.0-0.45 has been investigated in the region 10-300 K. The phonon at ∼100 cm −1 of the orthorhombic phase shows a classical soft mode behavior up to x=0.17, supporting its correlation with the LTO to HTT transition. In the xx/yy polarization spectra of the superconducting concentrations new modes at ∼150 and 370 cm −1 related to the symmetry breaking and a broad band at ∼280 cm −1 are attributed to the charge ordering at temperatures well above Tc. At all temperatures studied a correlation has been found between the doping dependence of the transition temperature and the intensity of the bands at ∼150, ∼280, ∼370 cm −1 , the intensity of the La/Sr mode, and the asymmetry of the apex phonon.
Physical Review B, 2005
A micro-Raman spectroscopic study has been carried out on fully oxygenated Y1-yCayBa2Cu3Ox (YBCO)... more A micro-Raman spectroscopic study has been carried out on fully oxygenated Y1-yCayBa2Cu3Ox (YBCO) compounds with y=0.0-0.2 and oxygen concentration varying with Ca in the range ( xtilde 6.960 and 6.995). The spectral profiles of the 4Ag and 1B1g -like symmetry phonons show minor modifications with Ca, but two modes that appear close in energy to the in-phase and the out-of-phase phonons gain intensity with the Ca substitution. The changes in the intensity of these two satellite modes with the amount of Ca are correlated with modifications in the dimpling of the CuO2 planes, as detected by extended x-ray absorption fine structure, and can be attributed to the formation of superstructures and to phase separation phenomena. The evolution of two bands in the in-phase phonon, associated in pure YBCO with local structural distortions and the coexistence of optimal and oxygen overdoped phases, remains completely independent of the Ca concentration. It appears that the Ca atoms have the tendency to be surrounded by Y atoms forming an independent nanophase of the type Y0.5Ca0.5Ba2Cu3Ox with the additional carriers from Ca not contributing to an excess doping of the system.
physica status solidi (b), 2009
... Vasyl Shenderovskii*, 1, Valentyn Baidakov1, Sergii Budsuliak1, Andrii Gorin1, Valerii Ermako... more ... Vasyl Shenderovskii*, 1, Valentyn Baidakov1, Sergii Budsuliak1, Andrii Gorin1, Valerii Ermakov1,Volodymyr Kolomoets1, Efthymios Liarokapis2, Galina Gromova3, B. Kazbekova3, and L ... [5] SI Budsulyak, AE Gorin, VM Ermakov, VV Kolo-moets, EF Venger, P. Verma, M ...
physica status solidi (b), 2009
... A Merrill–Bassett type diamond anvil cell (DAC) [45], equipped with type one diamonds suitabl... more ... A Merrill–Bassett type diamond anvil cell (DAC) [45], equipped with type one diamonds suitable for optical measurements, was employed for the high-pressure meas-urements, which has allowed the Raman studies to be car-ried out in a back scattering ... [29] P. Calvani, G. De ...
physica status solidi (b), 2009
... Volodymyr Kolomoets*, 1, Valentyn Baidakov1, Anatolii Fedosov1, Andrii Gorin1, Valerii Ermako... more ... Volodymyr Kolomoets*, 1, Valentyn Baidakov1, Anatolii Fedosov1, Andrii Gorin1, Valerii Ermakov1, Efthymios Liarokapis2, Galina Gromova3, B. Kazbekova3, L. Taimuratova3, and B. Orasgulyev3 ... [6] VM Ermakov, SI Budsuljak, EF Venger, VV Kolo-moets et al., Tensoresistivity ...
physica status solidi (b), 2007
ABSTRACT The phonon dynamics of the strontium vanadium oxide bronze (β-Sr0.33V2O5) is studied usi... more ABSTRACT The phonon dynamics of the strontium vanadium oxide bronze (β-Sr0.33V2O5) is studied using Raman spectroscopy at different temperature under high pressure up to 5.7 GPa. Interesting features of the spectra, such as the narrowing of some modes that harden under pressure (250–300 cm–1) or the softening of a low-frequency mode, are presented and discussed. Our low temperature measurements confirm that under high hydrostatic pressure the charge ordered phase is suppressed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (b), 2009
ABSTRACT We report a systematic study of Pr1–xCax MnO3 (x = 0.4 and 0.6) thin films grown on LaAl... more ABSTRACT We report a systematic study of Pr1–xCax MnO3 (x = 0.4 and 0.6) thin films grown on LaAlO3 (LAO) substrate (PCMO/LAO). X-ray diffraction measurements have shown that the PCMO/LAO film is [101]-oriented and under compressive strain due to the lattice mismatch. According to Raman spectroscopy the Jahn–Teller modes are suppressed under the application of pressure in the PCMO (x = 0.4) thin film. In particular, it was found that a pressure of 2 GPa suppresses the charge ordered state. Further high pressure measurements have confirmed that resistivity decreases linearly up to 2.4 GPa corresponding to a total reduction of about 35% in resistivity leading to a more conductive state on both concentrations. The temperature dependence of the resistivity has shown that the samples under pressure retain their semiconducting behavior. At ambient pressure the PCMO (x = 0.6) film shows a moderate magnetoresistance in the range 0–7 T and no metal to insulator transition (MIT). Raman spectroscopy confirmed that the application of a magnetic field of 1 T does not induce any remarkable changes on the electron–phonon interaction. Although, the hydrostatic pressure is not enough to induce a macroscopic metallic state, its effect is much greater than the one of the magnetic field. In addition, the effect of the 7 T magnetic field appears the same at ambient pressure and 2.5 GPa. Therefore the two effects of hydrostatic pressure and magnetic field do not seem to be coupled, at least up to the magnitudes studied. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Physica C: Superconductivity, 2007
We investigate the Y 1Àx Pr x Ba 2 Cu 3 O 7Àd (0 6 x 6 1) and YBa 2Ày Pr y Cu 3 O 7Àd (y = 0.1 an... more We investigate the Y 1Àx Pr x Ba 2 Cu 3 O 7Àd (0 6 x 6 1) and YBa 2Ày Pr y Cu 3 O 7Àd (y = 0.1 and 0.2) polycrystalline compounds via X-ray diffraction and Raman measurements. The oxygen concentration measurements show all samples to be optimally doped or overdoped. The magnetic measurements M(T) and v ac (T) verify that the increase of the Pr content leads to the expected strong systematic decrease of the critical temperature T c and the suppression of superconductivity for x > 0.55. Systematic micro-Raman studies of the observed vibrational modes reveal a coexistence of locally different rare-earth seemingly nanoscopic environments in the mixed rare-earth Y123 systems that has been overlooked to a great extent up to now. The comparison of the Raman spectra from the two sets of compounds proves that an increased amount of Ba is occupied by Pr in Y 1Àx Pr x Ba 2 Cu 3 O 7Àd and indicates that loss of superconductivity is mainly due to hole filling by this occupancy.
Physica C: Superconductivity, 2009
The doping dependence of the Raman spectra of high quality La 2Àx Sr x Cu 16,18 O 4 polycrystalli... more The doping dependence of the Raman spectra of high quality La 2Àx Sr x Cu 16,18 O 4 polycrystalline compounds has been investigated at low temperatures. It is shown that symmetry forbidden bands peaked at 150cmAˋ1,150 cm À1 , 150cmAˋ1,280 cm À1 , and $370 cm À1 are activated in the (xx/yy) polarization Raman spectra due to the local breaking of the inversion symmetry mainly at low temperatures and for doping concentrations for which the compound is superconducting. The apparent A 1-character of the activated modes in the symmetry reduced phase indicates a reduction from the D 2h to C 2v or D 2 crystal symmetries, which associates the observed modes to specific IR-active phonons with eigenvectors mainly along the c-axis. The temperature and doping dependence of this inversion symmetry breaking and the superconducting transition temperature are very similar, though the symmetry reduction occurs at significantly higher temperatures.