Research Unit FOR538
Doping Dependence of Phase Transitions and Ordering Phenomena in CopperOxygen Superconductors



P9 Raman Study of Competing Ordering Phenomena in the Cuprates
Inelastic scattering of light (Raman effect, Fig. 1+Fig. 2) is complementary to neutron and optical spectroscopy. By photons, phononic, magnetic and charge excitations can be probed. With visible light the momentum transfer q is close to 0. However, by using polarized light excitations with different symmetries can be projected out separately. In the case of phonons, vibrations with specific eigenvectors can be observed for a given combination of light polarizations.
For spin and charge excitations symmetryrelated form factors become effective. This translates into a sensitivity in momentum space. For the cuprates, the diagonals and the principle axes of the Brillouin zone can be accessed independently by crossed polarizations at 0 or 45°, respectively, in the coordinate system of the CuO_{2} planes (Fig. 2).
We are mainly interested in charge excitations. Then the Raman response is similar to a conductivity with momentum resolution, i.e. different parts of the Fermi surface can be projected out as indicated in Fig. 3. In this way the momentum dependence of the electron dynamics both in the superconducting and in the normal state could be mapped out. AboveT_{c}, the B_{1g} spectra are strongly suppressed with decreasing carrier concentration indicating incipient localization along the principle axes below p = 0.22 holes/CuO_{2}. It became clear that the energy gap below T_{c} is best compatible with dwave symmetry (B_{1g}, see Fig. 3). Recently, fluctuating charge order best described in terms of an incipient charge density wave (CDW) was observed. The effect sets in around optimal doping (maximal T_{c}) in La_{2x}Sr_{x}CuO_{4} and manifests itself as a peak at very low but finite energy in B_{1g} symmetry. In the compounds with high T_{c} such as YBa_{2}Cu_{3}O_{6+x} the additional response is found only below the onset point of superconductivity at approximately p = 0.05. Here, similarly as in La_{2x}Sr_{x}CuO_{4} at low doping, the lowenergy mode appears in B_{2g} symmetry indicating the charge density to be modulated along the diagonal of the CuO_{2} plane Various discontinuous changes in the spin, charge, and vibrational spectra (Fig. 4) occurring at p = 0.05 are driven by the reorientation of the superstructure.
In the project we plan to extend the experiments to new samples in particular to the electron doped side of the phase diagram. The main focus will be placed on charge ordering at low doping and its relationship to the antiferromagnetism and superconductivity. The purpose is to better understand the origin of the chargeordering instability and its connection to other phases. In this context, we also wish to clarify whether or not charge ordering leads to time reversal symmetry breaking. At the transition to the superconducting state the spectra are modified over energy ranges much larger than the gap. In spite of the absence of a "Raman sum rule" the renormalization of the spectra can be studied now with an accuracy better than 1%. Hence, the temperature dependence of the different symmetries will be investigated up to energies of approximately 1.5 eV. In general, all results be compared with those from the other experiments on a quantitative basis. This is now much more precise than in earlier attempts since the very same set of samples will be used. The purpose is to compare the differences between singleparticle and twoparticle methods and to study the renormalization effects due to the interaction between the electrons. This can finally lead to a better understanding of the high transition temperatures in the cuprates.
Selected Papers: Observation of an Unconventional MetalInsulator Transition in Overdoped CuO_{2} Compounds
Phys. Rev. Lett.
89 ,
107003
(2002)
Raman scattering versus infrared conductivity: Evidence for onedimensional conduction
in La_{2x}Sr_{x}CuO_{4}
Phys. Rev. B
66 ,
060502
(2002)
Light Scattering from Charge and Spin Excitations in Cuprate Systems
The Gap Symmetry and Fluctuations in HighT_{c} Superconductors, NATO Science Series: B (Springer)
p. 249290
(2002)
Electronic Raman scattering in highT_{c} superconductors: A probe of d_{x}2_{y}2 pairing
Phys. Rev. Lett.
72 ,
396399
(1994)

