Gap and subgap tunnelling in cuprates.pdf

a r X i v : c o n d - m a t / 0 0 0 5 3 1 5 v 3 [ c o n d - m a t .s u p r - c o n ] 2 5 M a y 2 0 0 0 Gap and subgap tunnelling in cuprates A.S. Alexandrov? and A.F. Andreev?? * Department of Physics, Loughborough University, Loughborough LE11 3TU, United Kingdom, **P.L. Kapitza Institute for Physical Problems, Kosygin st.2, 117973- Moscow, GSP-1, Russia We describe strongly attractive carriers in cuprates in the framework of a simple quasi-one dimensional Hamiltonian with a local attraction. In contrast with the conventional BCS theory there are two energy scales, a temperature independent incoherent gap ?p and a temperature dependent coherent gap ?c(T ) combining into one temperature dependent global gap ? = (?2p +? 2 c) 1/2. The temperature dependence of the gap and single particle (Giaver) tunnelling spectra in cuprates are quantitatively described. A framework for understanding of two distinct energy scales observed in Giaver tunnelling and electron-hole reflection experiments is provided. PACS numbers:74.20.-z,74.65.+n,74.60.Mj There is convincing experimental evidence that the pairing of carriers takes place well above Tc in under- doped cuprates (for a review see Ref. [1]). If carriers are paired their magnetic moments compensate each other so one could expect that the normal state uniform magne- tization should fall with decreasing temperature because more and more holes are bound into singlet pairs. This unexpected drop of the normal state magnetic suscepti- bility was experimentally observed [2,3] and explained in the framework of the bipolaron theory of cuprates [4,3]. There is also a gap in the tunnelling and photoemission, which is almost temperature independent below Tc [5] and exists well above Tc [6–8], so that some segments of a ‘large Fermi surface’ are actually missing [9,10]. Ki- netic [11] and thermodynamic [12] data suggest that the gap opens in both charge and spin channels and exists at any relevant temperature in a wide range of do