V.G. Hadjiev

Raman spectra of superconducting and impurity phases in Bi-Ca-Sr-Cu-O ceramics

Abstract Micro-Raman technique in conjunction with electron microanalysis were employed for identification of Raman spectra of superconducting and impurity phases in Bi-Ca-Sr-Cu-O ceramics obtained by three different technologies. Two of the main impurity phases are (Ca0.9Sr0.1)2CuO3 and (Ca0.6Sr0.4)Cu1.75O3 and their Raman spectra are very similar in the region below 400 cm−1. Another impurity phase is Bi2Ca1Sr3Ox (with slight variations of Bi-Ca-Sr composition) which displays strong Raman scattering at 615–622 cm−1 and a sharp peak at 90 cm−1. The Raman spectrum of the superconducting Bi2Ca1Sr2Cu2Ox phase is characterized by a strong peak at 635 cm−1 and a weak feature near 460 cm−1. The Raman spectra from microcrystals of Bi-Ca-Sr-Cu-O composition close to 3233, 3223, 2142 and 4123 have common features with the one of Bi2Ca1Sr2Cu2Ox. Some of them, e.g. 2142, exhibit almost continuous scattering below 620 cm−1, which may be caused by strong disorder thus displaying the one-phonon density of states.

Polarized Raman scattering from Bi-based HTS single crystals and its relation to the lattice dynamics of Y-123 compounds

Abstract The polarized Raman spectra of Bi-HTS microcrystals with defined orientation of the c -axis and of different Bi-Ca-Sr-Cu composition (including close to 2122 and 2223) were measured at room temperature by means of the Raman microprobe technique. The phonon lines at 635 cm -1 (A 1g ), 460 cm -1 (B 1g , near 300 cm -1 (A 1g ) and at 115 cm -1 (A 1g ) appear for alll microcrystals, whereas the position of the relatively strong (A 1g ) line in the low frequency region depends on the crystal. The Raman lines are assigned to certain lattice vibrations on the basis of comparison of the Raman spectra of Y-123 and Bi-HTS and of the structural data for Y-123 and Bi-2122. Earlier assignments of the Raman lines in the orthorhombic and tetragonal Y-123 are revised.

Raman and Mössbauer study of the pseudo-orthorhombic-to-tetragonal phase transition in YBa2(Cu1-xFex)3O7-δ(0.02≤x≤0.15)

Abstract The polarized Raman spectra from microcrystals of YBa 2 (Cu 1− x Fe x ) 3 O 7− δ (0.02⩽ x ⩽0.15) were studied in various scattering configurations allowing one to follow the variations with x of both diagonal (A g ) and non-diagonal (B 2g and B 3g ) Raman modes. It was found that the splitting of the strongest in intensity B 2g , B 3g Raman pair at 210 and 300 cm −1 associated with O(4) vibrations along a and b , respectively, decreases slightly with x , thus indicating that in a microscopic scale the structure remains orthorhombic over the whole substitutional range. The Mossbauer spectra for x =0.05, 0.10, and 0.15 showed a superlinear increase of the number of five-fold oxygen-coordinated Fe-atoms at the Cu(1)-sites. This is consistent with the assumption that Fe-clusters are formed along the microtwin boundaries at higher x . In this sense YBa 2 (Cu 1− x Fe x ) 3 O 7− δ could be considered as a two-phase system. The observed splitting of the A g Raman mode of Ba at x ⩾0.07 supports such an assumption. The Fe substitution increases the local disorder thus including additional Raman scattering of one-phonon density-of-states origin with a maximum at 580 cm −1 .

Raman study of hydrogenated RBa2Cu3O7−δ (R = Y, Gd)

Abstract Single crystal Raman spectra of various scattering geometries were obtained from ceramic H1.0YBa2Cu3O6.7 and H1.0GdBa2Cu3Oy using the microprobe technique. Apart from the five prominent Ag Raman lines being characteristic for YBa2Cu3O7, three more lines at 210 cm−1, 248 cm−1 and 610 cm−1 with exact Ag symmetry are observed in H1.0YBa2Cu3O6.7. The lowest frequency extra Raman lines are unambiguously attributed to IR vibrations ofY and Cu(1), respectively. The line at 610 cm−1 arises from oxygen vibrations but can not be conclusively assigned. Hydrogen occupies the interstitial sites around Cu(1) and Cu(2) (within the Y-plane) and perturbs the vibrations of its nearest-neighbours. The experimental results imply that the modes involving otherwise only odd parity vibrations become Raman active under hydrogen doping. The Raman scattering evidence is given that the hydrogen doping decreases the free holes concentration in YBa2Cu3O7−δ.

Structural study, XPS and Raman spectra of 107 K and 114 K Tl based superconductors

Abstract TlBaCaCuO superconducting ceramics with zero resistance at 107 and 114 K were obtained using nitrates. Two different superconducting phases are identified: Tl2Ba2Ca1Cu2Ox with a=b=3.859 A (5.475 A), c=29.352 A and the slight orthorhombic Tl2Ba2Ca2Cu3Ox with a=b=3.853 A (5.448 A), c=35.680 A ( a=3.81 A , b=3.85 A and c=35.58 A are determined by TEM diffraction). The XPS data for Tl2Ba2Ca1Cu2Ox show no evidence for a Cu3+ state. The two superconducting phases contained in the multiphase samples are clearly distinguished by their Raman spectra provided micro-Raman technique is used. The characteristic Raman lines appear at 519, 148 and 102 cm−1 for the Tl2Ba2Ca1Cu2Ox (tentatively assigned) and 595, 495, 410, 230, 132 and 96 cm−1 for the Tl2Ba2Ca2Cu3Ox phases. Raman superconducting phases often exist in small grains. TEM diffraction patterns also support this conclusion.

A Raman study of the structural properties of YBa2(Cu1 − xFex)3Oy

Abstract The effects of Fe-substitution of YBa 2 Cu 3 O y have been investigated by means of Raman scattering, X-ray diffraction, resistivity and susceptibility measurements. A series of samples of YBa 2 (Cu 1 − x Fe x ) 3 O y with different dopant concentration (0 ⩽ x ⩽ 0.15) has been prepared in two batches, the second set having undergone twice the heat and mechanical treatment used to produce the first batch. Considerable improvement in the superconducting transition temperature, T c , is obtained upon reprocessing. A phase transformation from orthorhombic to tetragonal symmetry is observed for x =0.05 from the X-ray measurements in agreement with previous work. Using a micro-Raman technique, all five A g vibrational modes have been measured and their dependence on Fe-concentration is analyzed. There are indications that iron substitutes for copper at both sites and that the structure is a mixture of orthorhombic and tetragonal microdomains for all x .

Polarised Raman scattering from orthorhombic and tetragonal GdBa2Cu3O7−δ single crystals: New evidence of chains-to-planes charge redistribution

Abstract Raman scattering spectra of various polarisation configurations have been obtained from as-grown single crystals of Gd-Ba 2 Cu 3 O 7−δ in both the orthorhombic (δ≈0.1) and tetragonal (0.7 g phonons at 118, 149, 331, 433 and 506 cm -1 , two more Raman lines at 205 and 302 cm -1 have been observed in cross-polarisation spectra of the orthorhombic phase exhibiting B 2g and B 3g symmetries respectively. There was no observation of respective nondiagonal E g symmetry modes in the tetragonal phase. The changes in the lineshapes and intensities for the same scattering configurations of the orthorhombic and tetragonal phases provide evidence for a strong influence of the free carriers on the scattering process. The experimental results are compatible with the concept of chains-to-planes charge transfer.

Polarized Raman spectra of superconducting and semiconducting YBa2Cu3Ox single crystals

Abstract The polarization properties of Raman scattering from single crystals of YBa2Cu3Ox are studied at room temperature for both the superconducting (x≊7) and semiconducting (x≊6.25) phases. The assignment of the phonon lines at 502, 435, 339, 148, 118 and 59 cm−1 (for the orthorhombic phase) and at 482, 450, 341, 208?, and 140 cm−1 (for the tetragonal phase) to certain lattice vibrations is discussed.

Normal state charge redistribution as determined by the Raman scattering from orthorhombic and tetragonal single crystals of YBa2Cu3O7-δ

A polarised Raman study of YBa2Cu3O7- delta single crystals revealed significant changes of light scattering anisotropy with reducing oxygen content. Strongly (y,y) and (x,x) polarised scattering from free carriers and its coupling with the phonon system (Fano interference) are characteristic for the orthorhombic crystal ( delta approximately=0). In contrast, in the tetragonal crystal ( delta approximately=0.8) enhanced continuum scattering and asymmetry lines were observed only in the tetragonal (z,z) polarisation. These findings are attributed to the reduction of free carriers in the xy CuO2 planes due to chains-to-planes charge redistribution initiated by the decreased oxygen concentration.

Transformations of the locat structure of YBa2Cu3O7−δ due to laser annealing

Abstract Raman spectroscopy has been applied to probe the local oxygen arrangements before, during, and after laser irradiation. During annealing the structure can be described as the high-temperature T′ phase inlaid with ordered domains. The phase separation reported earier for the case δ ∼ 0.5 is found to take place over the whole 0 δ n +1 O(1) n ( n = 1, 2,3.) chain fragments.