Electronic Raman scattering on under-doped Hg-1223 high-Tc superconductors:investigations on the symmetry of the order parameter

In order to obtain high quality, reliable electronic Raman spectra of a high-Tc superconductor compound, we have studied strongly under-doped HgBa2Ca2Cu3O8+d. This choice was made such as to i)minimize oxygen disorder in the Hg-plane generated by oxygen doping ii) avoid the need of phonon background subtraction from the raw data iii)eliminate traces of parasitic phases identified and monitored on the crystal surface. Under these experimental conditions we are able to present the pure electronic Raman response function in the B2g, B1g, A1g+B2g and A1g+B1g channels. The B2g spectrum exhibits a linear frequency dependence at low energy whereas the B1g one shows a cubic-like dependence. The B2g and B1g spectra display two well defined maxima at 5.6kBTc and 9kBTc respectively. In mixed A1g channels an intense electronic peak centered around 6.4 kBTc is observed. The low energy parts of the spectra correspond to the electronic response expected for a pure dx2-y2 gap symmetry and can be fitted up to the gap energy for the B1g channel. However, in the upper parts, the relative position of the B1g and B2g peaks needs expanding the B2g Raman vertex to second order Fermi surface harmonics to fit the data with the dx2-y2 model. The sharper and more intense A1g peak appears to challenge the Coulomb screening efficiency present for this channel. As compared to previous data on more optimally doped, less stoichiometric Hg-1223 compounds, this work reconciles the electronic Raman spectra of under- doped Hg-1223 crystals with the dx2-y2 model, provided that the oxygen doping is not too strong. This apparent extreme sensitivity of the electronic Raman spectra to the low lying excitations induced by oxygen doping in the superconducting state is emphasized here and remains an open question.Comment: 12 pages, 5 figure

Role of pair-breaking and phase fluctuations in c-axis tunneling in underdoped Bi2_{2}Sr2_{2}CaCu2_{2}O8+δ_{8+\delta}

The Josephson Plasma Resonance is used to study the c-axis supercurrent in the superconducting state of underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ with varying degrees of controlled point-like disorder, introduced by high-energy electron irradiation. As disorder is increased, the Josephson Plasma frequency decreases proportionally to the critical temperature. The temperature dependence of the plasma frequency does not depend on the irradiation dose, and is in quantitative agreement with a model for quantum fluctuations of the superconducting phase in the CuO$_{2}$ layers.Comment: 2 pages, submitted to the Proceedings of M2S-HTSC VIII Dresde

Evidence for two distinct energy scales in the Raman spectra of YBa2(CuNi)O6.95

We report low energy electronic Raman scattering from Ni-substituted YBa2Cu3O6.95 single crystals with Tc ranging from 92.5 K to 78 K. The fully symmetrical A1g channel and the B1g channel which is sensitive to the dx2-y2 gap maximum have been explored. The energy of the B1g pair-breaking peak remains constant under Ni doping while the energy of the A1g peak scales with Tc (EA1g/Tc=5). Our data show that the A1g peak tracks the magnetic resonance peak observed in inelastic neutron scattering yielding a key explanation to the long-standing problem of the origin the A1g peak.Comment: 10 pages, 4 figures and 1 tabl

Total suppression of superconductivity by high magnetic fields in YBa2 Cu3O6.6

We have studied in fields up to 60T the variation of the transverse magnetoresistance (MR) of underdoped YBCO6.6 crystals either pure or with Tc reduced down to 3.5K by electron irradiation. We evidence that the normal state MR is restored above a threshold field H'c(T), which is found to vanish at T'c>>Tc. In the pure YBCO6.6 sample a 50 Tesla field is already required to completely suppress the superconducting fluctuations at Tc. While disorder does not depress the pseudogap temperature, it reduces drastically the phase coherence established at Tc and weakly H'c(0), T'c and the onset Tnu of the Nernst signal which are more characteristic of the 2D local pairing.Comment: 4 pages, 4 figure

Coupling between quasiparticles and a bosonic mode in the normal state of HgBa2_2CuO4+δ_{4+\delta}

We report a doping dependent study of the quasiparticles dynamics in HgBa$_2$CuO$_{4+\delta}$ via Electronic Raman Scattering. A well-defined energy scale is found in the normal state dynamics of the quasiparticles over a broad doping range. It is interpreted as evidence for coupling between the quasiparticles and a collective bosonic mode whose energy scale depend only weakly with doping. We contrast this behavior with that of the superconducting gap whose amplitude near the node continuously decreases towards the underdoped regime. We discuss the implications of our findings on the nature of the collective mode and argue that electron-phonon coupling is the most natural explanation.Comment: 5 pages, 4 figure

Nernst effect and disorder in the normal state of high-T_{c} cuprates

We have studied the influence of disorder induced by electron irradiation on the Nernst effect in optimally and underdoped YBa2Cu3O(7-d) single crystals. The fluctuation regime above T_{c} expands significantly with disorder, indicating that the T_{c} decrease is partly due to the induced loss of phase coherence. In pure crystals the temperature extension of the Nernst signal is found to be narrow whatever the hole doping, contrary to data reported in the low-T_{c} cuprates families. Our results show that the presence of "intrinsic" disorder can explain the enhanced range of Nernst signal found in the pseudogap phase of the latter compounds.Comment: revised version. to be published in Physical Review Letter