L. Börjesson

Temperature dependence of phonon Raman scattering in Y2Ba4Cu7O15-δ

Abstract The temperature dependence of A g symmetry phonons in a Y 2 Ba 4 Cu 7 O 15−δ (δ≈0.18, T c =93 K) superconductor have been investigated between 10 and 300 K by means of Raman scattering. The observed changes in frequency and line-width between T c and 10 K for the CuO 2− and BaO-plane oxygen modes are consistent with the opening of a superconducting gap, 2δ, betwee 340 and 430 cm −1 . The magnitudes of these phonon anomalies are similar to what has been reported for the respective modes in YBa 2 Cu 3 O 7−δ ( δ =0.19). The behaviours of the Ba mode and the chain-oxygen mode below T c , i.e. softening and hardening respectively, are approximately the same as those we observe in a YBa 2 Cu 4 O 8 ( T c =82 K) sample. Evidence of phonon frequency anomalies at temperatures above T c is discussed in relation to differences in doping-level between YBa 2 Cu 4 O 8 and Y 2 Ba 4 Cu 7 O 15−δ .

Anomalous behaviour of the 147 cm−1 Cu(2) Raman mode in YBa2Cu4O8 under high pressure. Signature of change in the electronic state of the CuO2 plane

Abstract The effect of pressure on Raman modes of ceramic superconducting samples of YBa 2 Cu 4 O 8 has been investigated up to 16 GPa at room temperature. While frequencies of the modes at 101, 250, 310, 339, 500 and 605 cm −1 increase almost linearly with increasing pressure, the mode at 147 cm −1 corresponding to the z -motion of Cu(2) atoms in the CuO 2 plane exhibited a nonlinearity at pressure exceeding 6 GPa. A correlation is observed between the pressure-induced changes in T c and the change in the frequency of the Cu(2) mode. We propose that the anomalous nonlinear behaviour of the Cu(2) mode in the high pressure regime is a signature of changes in the electronic state of the CuO 2 plane resulting from a charge carrier redistribution between Cu(2) and O(2/3) in the conducting CuO 2 plane.

A neutron and x-ray diffraction investigation of intermediate range structure in silver borate glasses

The ion diffusion mechanism in ion conducting glasses has in many reports been suggested to be linked to the structure. However, in most cases the structures of the glasses are not very well known. We have investigated the structure of the glass system (AgI)x (Ag2O-2B2O3)1−x, with x=0, 0.6 by neutron and x-ray diffraction experiments. AgI doping produces an intense peak in the neutron data at an anomalously low Q-value (0.8 Å−1) that indicates increased intermediate range ordering. Only a weak peak is observed at a similar Q-value in the x-ray structure factor. By comparing the different neutron and x-ray weighting factors the origin of the anomalous peak is unambiguously attributed to correlations within the B-O network. The experimental results have been used to model the glasses by Reverse Monte Carlo simulations to obtain more detailed views of the structures.

Is there a correlation between Tc and the features of the B1g Raman continuum in YBa2Cu3O7−gd?

Abstract The B 1g -symmetry Raman spectra of lightly Co-substituted YBa 2 (Cu 3−x Co x )O 6.95 (x=0−0.05, T c =91.4K−86K) thin films have been investigated. We find that the low temperature peak in the B 1g continuum, at −500 cm −1 , shifts to higher frequencies with increasing Co-substitution, corresponding to a reduction of T c in the slightly underdoped region. Comparing these findings with published results on slightly overdoped YBa 2 Cu 3 O 7 we conclude that the B 1g Raman continuum peak frequency scales inversely with hole doping within the T c ∼90K doping plateau of the Y123 system. The lack of a correlation between T c and the frequency of the B 1g continuum peak suggests that this peak is not due to scattering across a superconducting gap.

Influence of Zn-doping on the electronic raman scattering of YBa2Cu3O7 − δ

Abstract We present temperature dependent Raman measurements on single crystal YBa 2 ( Cu 1 − x Zn x ) 3 O 7 − δ with x = 0, 1.5%, and 3.5%. The superconductivity induced redistribution of the electronic background in the B 1 g symmetry scattering configuration was found to disappear already for the smallest Zn concentration. In contrast, for A 1 g symmetry the redistribution is observed for all samples even though the magnitude decreases with increasing doping. The peak position in A 1 g symmetry decreases with increasing Zn doping and scales approximately with T c , indicating a superconducting gap energy of 2Δ k B T c ≈ 5.0 .

Substitution of Pr for Y in YBa2Cu4O8 and YBa2Cu3.5O7.5−δ superconductors : phonon modes and charge transfer effects

Abstract We report on a Raman scattering study of ( Y x Pr 1 − x ) 2 Ba 4 Cu 6 + n O 14 + n − δ ( n = 1, 2) superconductors with 0 ≤ x ≤ 1 in the temperature range 10–300 K. The observed changes of the phonon frequencies, including the double chain modes, with Pr-content x in general follow from the variations of the bond distances. The apex-oxygen mode is, however, found to exhibit stronger variation with x in single ( n = 0) then in double-chain ( n = 1, 2) materials, which may indicate charge redistribution effects between chains and planes.

Symmetry-dependent phonon interactions in YBa2Cu4O8 superconductors: a Raman and infrared spectroscopic study

Abstract Superconductivity induced renormalization of Raman- and infrared-active optical phonons is studied for undoped and T c -reduced Pr- and Zn-substituted double-chain YBa 2 Cu 4 O 8 materials. The magnitude of the phonon frequency and the linewidth variation upon cooling below T c is found to be dependent on the mode symmetry; odd- end even-parity phonons respond qualitatively different to the reduction of T c . Assuming phonon coupling to the excitations participating in the formation of the superconducting gap, these experimental facts provide evidence for the gap anisotropy of YBa 2 Cu 4 O 8 .