Recombination and propagation of quasiparticles in cuprate superconductors

Thesis/Dissertation · Sat May 01 00:00:00 EDT 2004

DOI:https://doi.org/10.2172/842568· OSTI ID:842568

Gedik, Nuh ^[1]

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1. Univ. of California, Berkeley, CA (United States)

Rapid developments in time-resolved optical spectroscopy have led to renewed interest in the nonequilibrium state of superconductors and other highly correlated electron materials. In these experiments, the nonequilibrium state is prepared by the absorption of short (less than 100 fs) laser pulses, typically in the near-infrared, that perturb the density and energy distribution of quasiparticles. The evolution of the nonequilibrium state is probed by time resolving the changes in the optical response functions of the medium that take place after photoexcitation. Ultimately, the goal of such experiments is to understand not only the nonequilibrium state, but to shed light on the still poorly understood equilibrium properties of these materials. We report nonequilibrium experiments that have revealed aspects of the cup rates that have been inaccessible by other techniques. Namely, the diffusion and recombination coefficients of quasiparticles have been measured in both YBa₂Cu₃O_6.5 and Bi₂Sr₂CaCu₂O_8+x using time-resolved optical spectroscopy. Dependence of these measurements on doping, temperature and laser intensity is also obtained. To study the recombination of quasiparticles, we measure the change in reflectivity ΔR which is directly proportional to the nonequilibrium quasiparticle density created by the laser. From the intensity dependence, we estimate β, the inelastic scattering coefficient and γ_th thermal equilibrium quasiparticle decay rate. We also present the dependence of recombination measurements on doping in Bi₂Sr₂CaCu₂O_8+x. Going from underdoped to overdoped regime, the sign of ΔR changes from positive to negative right at the optimal doping. This is accompanied by a change in dynamics. The decay of ΔR stops being intensity dependent exactly at the optimal doping. We provide possible interpretations of these two observations. To study the propagation of quasiparticles, we interfered two laser pulses to introduce a spatially periodic density of quasiparticles. Probing the evolution of the initial density through space and time yielded the quasiparticle diffusion coefficient, and both inelastic and elastic scattering rates. Measured diffusion coefficient suggests that the quasiparticles induced by the laser occupy primarily states near the antinodal regions of the Brillouin zone.

• Research Organization: Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• DOE Contract Number: AC03-76SF00098
• OSTI ID: 842568
• Report Number(s): LBNL-55855; R&D Project: 504601; TRN: US0503620
• Resource Relation: Other Information: doctoral thesis, Ph.D, University of California, Berkeley, Berkeley, CA
• Country of Publication: United States
• Language: English

Related Subjects

75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
ABSORPTION
BRILLOUIN ZONES
CUPRATES
DECAY
DIFFUSION
ELASTIC SCATTERING
ELECTRONS
ENERGY SPECTRA
INELASTIC SCATTERING
LASERS
RECOMBINATION
REFLECTIVITY
RESPONSE FUNCTIONS
SPECTROSCOPY
SUPERCONDUCTORS
THERMAL EQUILIBRIUM
superconductivity cuprates quasiparticles