Synthesis from separate oxide targets of high quality La_2−xSr_xCuO₄thin films and dependence with doping of their superconducting transition width

Abstract

A series of superconducting La 2−x Sr x CuO 4 thin films, with 0.09 x 0.22, is grown over (100)SrTiO 3 substrates by means of a novel pulsed laser deposition method devised to increase the homogeneity and control of doping. We employ two separate parent oxide targets that receive ablation shots at arbitrary computer-controlled relative rates, instead of the conventional procedure that uses a single target whose doping determines the one of the film. We characterize the films both through conventional techniques (XRD, SEM, AFM and EDX) and by measuring their superconducting transition with a high-sensitivity SQUID magnetometer. The latter allows one to determine not only their average critical temperatures Tc (x) but also their dispersions due to inhomogeneities, T c (x). For Tc (x) we obtain the conventional parabolic law centered at x = 0.16, plus a Gaussian depression near x = 1/8 with a Tc -height of about 5 K and x-width about 0.03. For T c (x) we obtain, for all the dopings, values among the lowest reported up to now for La 2−x Sr x CuO 4 . The T c (x) dependence can be explained in terms of the unavoidable randomness of the positioning of the Sr ions (the so-called intrinsic chemical inhomogeneity) and a separate residual T c -inhomogeneity contribution of the order of 0.5 K, this last associated with the samples' structural inhomogeneities and films' substrate.

Calculations of Some Doping Nanostructurations and Patterns Improving the Functionality of High-Temperature Superconductors for Bolometer Device Applications

“…Also, for our R(T) calculations we shall use two alternative calculations, so as to be confident about the validity of the results: Finite-element computations first, and then effective-medium formulae (both paths have been successful in other studies of structured HTS [30][31][32][33][34][35], and we shall also include some confirming example comparisons with real data). Let us provide the details of all such procedures in the following sections.…”

“…The critical fluctuations around the transition play an important role in HTS and have been studied in detail, e.g., in [32,[41][42][43][44][45][46][47][48]. The effects of critical fluctuations in the resistance curves are commonly summarized via the so-called paraconductivity, ∆σ, defined as the additional contribution to the electrical conductivity due to fluctuations: In particular, the total conductivity σ(T) near the transition becomes…”

“…As it has been explicitly demonstrated in various relatively recent experimental and theoretical works [32,46,47,54,55], an additional (and crucial for some doping levels) ingredient to understand the phenomenology of the resistive transition in HTS is to take into account the random T c -inhomogeneities associated with the intrinsic disorder of the doping level. This intrinsic structuration is due to the fact that HTS compounds are non-stoichiometric, and therefore each dopant ion has at its disposal various lattice positions to occupy.…”

“…For concreteness, we focus our present article in the case of the YBa 2 Cu 3 O δ superconductor with oxygen as a dopant ion. Experimental measurement indicates that a typical size of each local inhomogeneity is about (30 nm) 2 for HTS [32,54,55]. This produces, therefore, a certain randomness in the doping at the local scale, unavoidably present in even the more carefully grown HTS samples.…”

Current-Voltage Characteristics and Vortex Dynamics in Highly Underdoped La2−x Sr x CuO4

“…For example, the penetration depth measurements in bulk YBa 2 Cu 3 O 7−x found no signatures Dragana Popović dragana@magnet.fsu.edu 1 National High Magnetic Field Laboratory, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, FL 32310, USA of the BKT physics in thick films [6] or crystals [7], while dc transport measurements suggested BKT-like behavior in bulk samples of several cuprates (see, e.g., Refs. [8][9][10][11]). A recent study of the paraconductivity and the I -V characteristics in highly underdoped La 2−x Sr x CuO 4 (x = 0.07 and 0.08) thick films (150 CuO 2 layers) in zero magnetic field [5] showed that the effective dimensionality of the samples is 2D and that the thermally-driven transition to the superconducting state is of the BKT type with a large vortex-core energy μ ≈ 1.4μ XY (μ XY = π 2 J s /2 is the conventional value that it assumes in the XY model; J s is the superfluid stiffness).…”