Electron dynamics in metallic and spin-glass cuprates

Abstract

Using femtosecond pump-probe measurements, we present transient optical reflectivity measurements of metallic, superconducting and spin-glass La 2−x Sr x CuO 4 cuprates. The rise time is temperature-independent and resolution-limited ͑Ͻ50 fs͒ for metallic sample, and is much faster than spin-glass samples ͑ϳ200 fs͒ at all temperatures. A divergence in the electron-lattice relaxation ͑ R ͒ time with temperature is observed for the metallic samples, but not for spin-glass samples. PACS number(s): 61.66. Ϫf, 61.72.Ff Femtosecond optical pump-probe measurements provide insight on ultrafast carrier dynamics occurring on femtosecond and picosecond time scales. 1-9 Recent ARPES and SR reports 10-12 on La 2−x Sr x CuO 4 provide a better understanding of the electronic structure and transport properties that aid in interpreting femtosecond pump-probe measurements. Specifically, for ͑0.02-0.03͒ Ͻ x Ͻ 0.06, the samples are in a spin-glass phase; the band of carriers in the ͗0,͘ direction of the Brillouin zone exhibits a binding energy of ϳ0.2 eV. However, Ref. 11 reports an important and unexpected result: in the nodal ͑͗ , ͒͘ direction of the Brillouin zone, there is a quasiparticle state with some coherent spectral intensity that disperses through the chemical potential. By contrast, x ജ 0.06 samples have a van Hove singularity, that is, a saddle point in the band dispersion which leads to a divergence in the density of states, near the chemical potential. There is general agreement that the van Hove singularity is near the chemical potential for all metallic cuprates. 11 We measured the dynamics when the carriers near the chemical potential are in the nodal direction ͑0.02Ͻ x Ͻ 0.06͒, and, separately, when most carriers near the chemical potential are in the van Hove singularity ͑x = 0.06-0.30͒. Further, since x = 0.06 samples are superconducting and x = 0.30 samples are not, we can distinguish the effects of the superconducting phase transition. This is, to our knowledge, the first time femtosecond pump-probe methods have been explicitly used to obtain information about carriers in specific parts of the Brillouin zone.Three different sample stoichiometries of 100-nm-thick La 2−x Sr x CuO 4 films were measured, x = 0.02-0.03, x = 0.06, and x = 0.30. The films were epitaxially grown with pulsed laser deposition on SrTiO 3 with a 100 nm SrLaAlO 4 buffer layer between the substrate and the La 2−x Sr x CuO 4 film. 13 All of the samples were cooled, after deposition, in molecular oxygen. A range of 100 nm thin film samples with 0.00ഛ x ഛ 0.30 were grown with the SrLaAlO 4 buffer layer on SrTiO 3 substrates. The T C values for the samples were determined by susceptibility measurements. The superconducting samples exhibited a 10-90% width of less than 2 K. It was found that the samples exhibit a phase diagram similar to that of the bulk material with slightly higher T C values in the films due to the induced strain from the buffer layer and substrate. Further details of sample preparation and characterization are provided in...

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