Vortex Fluctuations in High-TcFilms: Flux Noise Spectrum and Complex Impedance

Abstract

The flux noise spectrum and complex impedance for a 500 Å thick YBCO film are measured and compared with predictions for two-dimensional vortex fluctuations. It is verified that the complex impedance and the flux noise spectra are proportional to each other, that the logarithm of the flux noise spectra for different temperatures has a common tangent with slope ഠ21, and that the amplitude of the noise decreases as d 23 , where d is the height above the film at which the magnetic flux is measured. A crossover from normal to anomalous vortex diffusion is indicated by the measurements and is discussed in terms of a two-dimensional decoupling. PACS numbers: 74.40. + k, 74.25.Nf, 74.76.Bz Spontaneously created two-dimensional (2D) vortices drive the phase transition of Kosterlitz-Thouless (KT) type between the superconducting and the normal state for thin film superconductors [1]. This means that the physics of the vortices is responsible for the features in a region around the transition [1]. More recently it has been found that high-T c superconductors also show behavior characteristic of 2D vortex fluctuations [1][2][3][4][5]. This is expected for films thin enough to be in the "quasi" 2D regime, while the phase transition for thicker films is related to the 3D bulk transition. However, also for such thick films 2D vortex fluctuations have been observed just above the transition [4,5], which has been ascribed to a decoupling of the superconducting planes associated with the CuO 2 layers [6][7][8]. The evidence in case of BSCCO, which has a fairly broad resistive transition, is better established than for YBCO which has a very narrow transition [4,5]. One may argue that for BSCCO, since it has a much larger anisotropy than YBCO, the interlayer coupling should be much weaker, and as a consequence the quasi 2D character much stronger [1]. From this perspective the possibility and the significance of 2D vortex fluctuations remains to be clarified for YBCO. In the present Letter, we address this question by focusing on the dynamical features of the vortices reflected in the flux noise spectrum and the complex impedance for a 500 Å thick YBCO film; a strategy which was also used in Ref.[3] for a quasi-2D BSCCO film. To resolve the narrow resistive region for our YBCO sample, we use a high resolution temperature control in the experimental setup.The sample is a square shaped (5 3 5 mm 2 ) 500 Å thick YBCO-123 film grown epitaxially on a LaAlO 3 substrate by pulsed laser deposition. According to x-ray u-2u and f scans the film is highly c-axis oriented and only [100] YBCOk͓100͔ LaAlO 3 in-plane orientation is observed. The experimental setup is a SQUID based system designed for complex impedance and flux noise measurements. The sample space is magnetically shielded by m-metal and niobium cans, resulting in a residual dc field of approximately 1 mOe. The pickup coil having a diameter of 1.2 mm is a first order gradiometer, each section containing 2 layers (8 1 7 turns) of 0.05 mm NbTi wire. The amplitude of the field in t...

Fluctuation-dissipation theorem and flux noise in overdamped Josephson-junction arrays

“…(69) is reduced to This means that with increase of h the amplitude of the noise has first to change very slowly and then to decay proportionally to 1/h 2 . The experimental results of Festin et al [4], obtained on thin YBCO film at h/r ∼ 1, are compatible with 1/h 2 dependence even better than with 1/h 3 dependence predicted by Kim and Minnhagen [17]. In the opposite limit of strong screening (Λ ≪ h) L ✷ (ω) is negligibly small in comparison with L s (ω) and S(ω) depends only on R ✷ (ω), but not on L ✷ (ω).…”

“…The two basic experimental methods used for contactless investigation of finite frequency properties of two-dimensional superconducting systems (such as thin films [1][2][3][4][5], Josephson junction arrays [6][7][8][9] and wire networks [10,11]) are two-coil mutual-inductance technique [1][2][3][4][9][10][11] and flux noise power spectrum analysis [3][4][5][6][7][8].…”

Extremely High Frequency Dependence of Two-Dimensional Superfluid Onset

“…The KT transition is now a general concept for superconducting films, 3) Josephson-junction arrays, 4) and the Bose-Einstein condensation of a 2D atomic Bose gas.…”

Nonequilibrium steady states of driven magnetic flux lines in disordered type-II superconductors

“…These results are similar to the experimental data reported in Refs. [58,59], where the voltage noise spectrum decays at high frequency like 1/ω at elevated temperature and like 1/ω 3/2 at lower temperature. The exponent α = 1.5 also emerges from a functional renormalization group calculation for point defects to one-loop order in three dimensions [60].…”