Pseudogap and quantum-transition phenomenology in HTS cuprates

The low-energy excitation spectrum of HTS cuprates is examined in the light of thermodynamic, transport, quasiparticle and spin properties. Changes in the thermodynamic spectrum associated with the normal-state pseudogap disappear abruptly at a critical doping state, pcrit = 0.19 holes per Cu. Moreover, ARPES data at 100K show that heavily damped quasiparticles (QP) near (π,0) suddenly recover long lifetimes at pcrit, reflecting an abrupt loss of scattering from AF spin fluctuations. This picture is confirmed by μSR zero-field relaxation measurements which indicate the presence of a novel quantum glass transition at pcrit. Consistent with this picture resistivity studies on thin films of Y0.7Ca0.3Ba2Cu3O7−δ reveal linear behavior confined to a V-shaped domain focussed on pcrit at T=0. The generic phase behavior of the cuprates may be governed by quantum critical fluctuations above pcrit and the pseudogap appears to be caused by short-range AF correlations. PACS numbers: 74.25.Bt, 74.25.Fy, 74.25.Ha, 74.72.Jt.