Competition of Superconductivity and Charge Density Waves in Cuprates: Recent Evidence and Interpretation

摘要

Explicit and implicit experimental evidence for charge density wave (CDW) presence in high-T_c superconducting oxides is analyzed. The theory of CDW superconductors is presented. It is shown that the observed pseudogaps and dip-hump structures in tunnel and photoemission spectra are manifestations of the same CDW gapping of the quasiparticle density of states. Huge pseudogaps are transformed into modest dip-hump structures at low temperatures, T, when the electron spectrum superconducting gapping dominates. Heat capacity jumps at the superconducting critical temperature and the paramagnetic limit are calculated for CDW superconductors. For a certain range of parameters, the CDW state in a d-wave superconductor becomes reentrant with T, the main control quantity being a portion of dielectrcally gapped Fermi surface. It is shown that in the weak-coupling approximation, the ratio between the superconducting gap at zero temperature Δ( T = 0) and T_c has the Bardeen-Cooper-Schrieffer value for s-wave Cooper pairing and exceeds the corresponding value for d-wave pairing of CDW superconductors. Thus, large experimentally found values 2Δ( T = 0)/T_c≈5 ÷ 8 are easily reproduced with reasonable input parameter values of the model. The conclusion is made that CDWs play a significant role in cuprate superconductivity.