Unconventional magnetic properties of cuprates

Abstract

Recently experiments on high critical temperature superconductors have shown that the doping levels and the superconducting gap are usually not uniform properties but strongly dependent on their positions inside a given sample. We show here that the large diamagnetic signal above the critical temperature T c and the unusual temperature dependence of the upper critical field H c2 with the temperature can be explained taking the inomogeneities and a distribution of different local critical temperatures into account.There are increasing evidences that high critical temperature superconductors (HTSC) are intrinsic inhomogeneous materials. This is probably the cause of several unconventional behavior. In particular, recent magnetic imaging through a scanning superconducting quantum device (SQUID) microscopy has displayed a static Meissner effect at temperatures as large as three times the T c of an underdoped LSCO film [1]. Following up SQUID magnetization measurements on powder oriented YBCO and LSCO single crystals [2,3] have shown a rather high magnetic response which, due to its large signal and structure, cannot be attributed solely to the Ginzburg-Landau (GL) theory of fluctuating superconducting magnetization [4,8]. On the other hand the H-T phase diagram of the HTSC possess, in certain cases, positive curvature for H c2 (T ), with no evidence of saturation at low temperatures [5]. These lack of saturation at low temperatures may minimize the importance of strong fluctuations of the order parameter.In this paper we develop a unified view for all these anomalous properties. The basic ideas are [9,10]: the charge distribution inside a HSCT is highly inhomogeneous and may be divided in two types. A hole-poor branch which represents the AF domains and a hole-rich which characterizes the metallic regions. The width of the metallic distribution decreases with the average doping since usually, the samples becomes more homogeneous as the average doping level or average charge density increases. Due to the spatially varying local charge density, it is also expected that the T c , instead of being a single value as in usual metallic superconductors, becomes locally dependent. Therefore a given HTSC compound with an average charge density n m possess a distribution of charge density n(r), zero temperature superconducting gap ∆ 0 (r) and superconducting critical temperature T c (r) where the symbol (r) means a point inside the sample. In this scenario we identify the largest T c (r) with the the pseudogap temperature T * of the compound [11]. Metallic domains with low (high) doping level have high (low) T c (r). Upon cooling below T * the superconducting regions develop at isolated regions as droplets of rain in the air and, eventually they percolates at the superconducting critical temperature T c of the compound at which superconducting long range order is established. The Model for the MagnetizationIn order to estimate the M (B) we follow the ideas and the procedures of the Critical State Model (CSM) to each supercondu...

Fibonacci oscillators in the Landau diamagnetism problem

“…The results obtained for the deformed magnetization are very interesting, because we can compare it with experimental results obtained for superconducting materials (which are perfect diamagnetic materials) as a function of temperature variation [42], in order to strength the understanding of the q-deformation as a factor of impurity. In these references [42] one was found that the minimum of magnetization deepens as temperature or pressure decreases.…”