Raman light scattering study on phonon anomalies in unconventional superconductors

This work presents a series of detailed temperature-dependent Raman light scattering studies on optical phonons in di erent unconventional hightemperature superconductors. It divides into three main projects. The rst project presents a study on two molecular-beam-epitaxygrown lms of the electron-doped superconductor La2−xCexCuO4 (LCCO) close to optimal doping (x ∼ 0.08, Tc = 29 K and x ∼ 0.1, Tc = 27 K), that had been the basis of prior transport measurements. The main focus of this project was a detailed characterization and microstructural analysis of the lms. Based on micro-Raman spectroscopy in combination with X-ray di raction, energy-dispersive X-ray analysis, and scanning electron microscopy, some of the observed phonon modes can be attributed to micron-sized inclusions of Cu2O. In the slightly underdoped lm (x ∼ 0.08), both the Cu2O modes and others that can be assigned to the LCCO matrix show pronounced softening and narrowing upon cooling below T ∼ Tc. Control measurements on commercial Cu2O powders and a comparison to prior Raman scattering studies of other high-temperature superconductors motivate the speculation that proximity e ects at the LCCO/Cu2O interface may be responsible for these anomalies. Experiments on the slightly overdoped LCCO lm (x ∼ 0.1) did not reveal comparable phonon anomalies. (The main results of this project are published under Rahlenbeck et al., Eur. Phys. J. B, 75, 461 (2010). ) The second project presents a study on superconducting and nonsuperconducting single crystals of the iron pnictides BaFe2As2, SrFe2As2, and R1−xKxFe2As2 with R = Ba, Sr (hole-doped) and BaFe2−xCoxAs2 (electron-doped) with di erent doping levels, respectively. In the undoped and hole-doped samples a pronounced continuous narrowing of the Ramanactive Fe and As vibrations upon cooling below the spin-density wave (SDW) transition Ts is observed. This e ect is attributed to the opening of a SDW gap. The electron-phonon linewidths inferred from these data greatly exceed the predictions of ab initio density-functional calculations without spin polarization, which may imply that local magnetic moments survive well above Ts. A rst-order structural transition accompanying the SDW transition induces discontinuous jumps in the phonon frequencies and anomalously large splittings of the in-plane modes. These anomalies are continuously suppressed for increasing doping concentrations. The corresponding phonon anomalies in the electron-doped samples are in agreement with a splitting of the structural and SDW transitions in temperature. At the superconducting transition temperature Tc subtle phonon anomalies are observed with a behavior qualitatively similar to that in the cuprate superconductors. (The main results of this project are published under Rahlenbeck et al., Phys. Rev. B, 80, 064509 (2009). ) The third project presents a study on superconducting and non-superconducting single crystals of the iron chalcogenide systems Fe1+yTe and Fe1+ySexTe1−x with di erent concentrations of Se doping and Fe excess. In undoped Fe1.08Te no phonon anomaly is observed at the transition temperature to long-range antiferromagnetic order TN that is accompanied with a structural phase transition. On the other hand, Se-doped Fe1.05Se0. 42Te0.58 and Fe1.08Se0. 28Te0.72 show a pronounced softening in the phonon frequency of the Fe vibration at the onset temperature of shortrange incommensurate spinuctuations TIC upon cooling, which largely exceeds the amplitude of the phonon anomalies at the structural and SDW phase transitions in the iron pnictides. The pronounced phonon softening in the iron chalcogenides may indicate a strong spin-phonon coupling, induced by the short-range incommensurate spinuctuations. Additional contributions due to the presence of Fe excess cannot be excluded.