An alternative theory on relaxation rates in cuprate superconductors

Transport properties of high transition temperature (high-Tc) superconductors apparently demonstrate two distinct relaxation rates in the normal state. We propose that this superficial inconsistence can be resolved with an effective carrier (quasiparticle) density n almost linear in temperature T. Experimental evidence both for and against this explanation is analyzed and we conclude that this offers a clear yet promising scenario. Band structure calculation was utilized to determine the Fermi surface topology of the cuprate superconductor versus doping. The results demonstrate that an electron-like portion of the Fermi surface exists in a wide range of doping levels even for a p-type superconductor, exemplified by La2-xSrxCuO4-δ (LSCO). Such electron-like segments have also been confirmed in recent photoemission electron spectroscopy. The Coulomb interaction between electron-like and hole-like quasiparticles then forms a bound state, similar to that of an exciton. As a result the number of charge carriers upon cooling temperature is decreased. A quantum mechanical calculation of scattering cross section demonstrates that a T2 relaxation rate is born out of an electron-hole collision process. Above the pseudogap temperature T* the normal state of high-Tc cuprates is close to a two-component Fermi liquid. It, however, assumes non-Fermi-liquid behavior below T*.