Superconducting Transition Region Research Articles

Spontaneous conductance fluctuations and percolation in YBa/sub 2/Cu/sub 3/O/sub 7/ thin films

Experimental results of the conductivity noise in the superconducting transition region of YBCO thin films prepared by co-evaporation and sputtering are presented. In the case of ex-situ fabricated samples made by co-evaporation Cooper-pair number fluctuations (induced by electron trapping) have been identified in the high temperature part of the transition. Classical percolation noise was found in the low temperature part of the transition. In the case of in-situ fabricated samples made by co-evaporation, electron mobility fluctuations, shunted by the conductance of Cooper-pairs, were identified in the upper temperature range. In the low temperature part of the transition of the in-situ fabricated samples, a new type of fluctuation has been discovered; the fluctuation of the volume concentration of the superconducting phase. This implies new universal noise exponents very different from the exponents of classical percolation models. This effect is a consequence of intergrain critical current fluctuations. >

Evidence for the spectral dependence of nonequilibrium picosecond photoresponse of YBaCuO thin films

The transient voltage photoresponse of current biased YBaCuO thin films to 20 ps laser pulses of 0.63 and 1.54 μm wavelengths is measured for temperatures around the superconducting transition region. The fast picosecond decay of the response is followed by a slow nanosecond relaxation which is associated with the bolometric effect. The magnitude of the fast component of the response varies in proportion to the square root of wavelength that plausibly reflects multiplication processes of photoexcited electrons via electron–electron scattering and interaction with high energy phonons.

Nonlinear electrodynamics of YBa 2Cu 3O 7 thin films in the microwave region

We report on investigations of the nonlinear behavior of thin films of YBa 2Cu 3O 7 by the method of phase-resolved microwave harmonic mixing both with and without an applied static magnetic field at low microwave power levels. Temperature dependent measurements on high quality films show that nonlinearities appear only in the superconducting transition region. The dependence of the nonlinear signal on the microwave field strength E ω exhibits an E ω 2.2-power law. No dependence on the strength of a magnetic field up to 300 G was found. The mixing results are compared to those of harmonic generation experiments done by other groups. The possible mechanisms for the nonlinear behavior are discussed.

Conductance noise and percolation in YBa2Cu3O7 thin films

Experimental results of the conductivity noise in the superconducting transition region of YBCO thin films prepared by co-evaporation are presented. In the case of ex situ fabricated samples, Cooper-pair number fluctuations (induced by electron trapping) have been identified in the high-temperature part of the transition. Classical percolation noise was found in the low-temperature part of the transition. In the case of samples made by the in situ method, the noise is smaller by several orders of magnitude in the upper part of the transition. This indicates a much more ordered microstructure in these samples. Electron mobility fluctuations, shunted by the conductance of Cooper-pairs, were identified in this temperature range. In the low-temperature part of the transition, a new type of fluctuation has been discovered; the fluctuation of the volume fraction of the superconducting phase. This implies new scaling exponents very different from the exponents of classical percolation models. This effect is a consequence of intergrain critical current fluctuations and can be caused by, for instance, magnetic flux motion, defect motion or trapping of electrons in the barriers between grains. Moreover, in the in situ fabricated samples, a dimensional crossover, 3D→2D, of the percolating network has been observed. From this effect, the length scale of the microscopic disorder can be estimated.

1/f noise in Bi2Sr2Ca2Cu3–O and its impact on bolometer performance

The magnitude 1/f noise in the high Tc superconducting transition region of Bi2Sr2Ca2Cu3–O materials was investigated. The results are similar to those previously reported for Y1Ba2Cu3–O. The results indicate that thermal fluctuations are the source of excess 1/f noise detected in the lower temperature region of the transition. In the higher temperature region, however, thermal fluctuation 1/f noise decreases, and is overpowered by an undetermined source of 1/f noise. The implications on bolometer performance are discussed.

Anomalous 1/ φ noise spectrum of potassium-doped bulk C 60 and C 70 in both the normal and superconducting transition region

We have prepared potassium-doped bulk K xC 60,70 and measured noise spectrum power as a function of temperature. We discovered that in the normal state, the magnitude of the normalized noise spectral power density is greater by about eleven orders of magnitude than the value of a normal metal predicted by Hooge's formula. Another unusual findings is the observation of a markedly enhanced peak near the superconducting transition, which cannot be explained by presently available theories.

Anisotropic 1/f noise and motion of magnetic vortices in YBa2Cu3O7- delta.

Measurements of the 1/f noise have been performed on ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ single crystals for currents in the a-b plane and along the c axis. In the normal state, the normalized noise spectral density ${\mathit{S}}_{\mathit{v}}$/${\mathit{V}}^{2}$ in the a-b plane is comparable with that along the c axis. In the superconducting transition region, a large enhancement of ${\mathit{S}}_{\mathit{v}}$/${\mathit{V}}^{2}$ is observed along the c axis while no such enhancement is observed in the a-b plane. Our experimental data suggest that while the 1/f noise in the normal state may be due to structural defects, a completely different noise source exists in the superconductig transition region. We discuss the 1/f noise behavior in the superconducting transition region in terms of the anisotropic nature of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ and the motion of magnetic vortices.

Superconducting transition in κ-(BEDT-TTF)2Cu[N(CN)2]Br under magnetic field

Abstract Interlayer and intralayer coherence lengths of κ-(BEDT-TTF)2Cu[N(CN)2]Br are derived as ξ b (0)=6±2 A , ξ ac (0)=24±5 A , respectively, by magnetic measurement. The renormalized theory of the order parameter fluctuation near the superconducting transition region was employed for the analysis. The broad resistive transitions under magnetic field are discussed on the same footing.

Noise measurements in YBaCuO thin films

Noise power in YBaCuO thin films has been measured. Measured noise power wa several orders of magnitude larger than that of conventional metals. We observed two noise peaks, a broad peak in normal state and another in superconducting transition region. Increasing the current and magnetic field, the noise peak in transition region grows and shifts toward lower temperature side

Hall Effect of an Oriented Superconducting YBa2Cu3Oy Film

A superconducting YBa2Cu3Oy thin film, which is well oriented at (001) direction and in which no grain boundary is observed in the resolution limit of scanning electron microscope micrograph, is selected for the Hall effect measurement from 80 to 220 K under a magnetic field of 3 T. A very narrow and negative peak of the Hall voltage (VH) is found in the superconducting transition region. A linear dependence of the carrier concentration (nH) on temperature is found in the normal state. The appearance of such a negative peak is attributed to the intrinsic anisotropy of the Hall coefficient (RH) related to the direction of the applied field and current.

1/f noise in Tl2Ba2Ca1Cu2O8 thin films: Influence of crystal structure

1/f noise measurements performed on Tl2Ba2Ca1Cu2O8 thin films show that the amplitude of the noise spectral density (Sv/V2) in the normal state may be comparable with that in conventional metals, in contrast with the much larger Sv/V2 observed in other copper oxide materials. In the superconducting transition region the 1/f noise is greatly reduced in some films that are highly aligned along the c axis.

Anomalous Hall effect in C-oriented high-T c phase BiPbSrCaCuO thin film

The Hall effect of superconducting high- T c phase BiPbSrCaCuO thin film was measured in the temperature region of 50 K to 250 K in magnetic fields up to 6 T. The Hall voltage is linear with magnetic fields down to a few degrees K above the Ginsburg-Landau transition temperature. The Hall coefficient is positive in the normal state with the usual 1/ T dependence but with a less pronounced slope, showing a maximum at 106 K obtained from the linear part of a high field followed by a drastic change to negative value near the zero resistivity temperature at low fields, and then reaches zero in the superconducting state. We concentrate on the careful measurement of this anomalous Hall effect in the superconducting transition region. The anomalous effect is discussed with the flux motion antiparallel to the transport current, considering the KT phase transition near the critical temperature.

1/f NOISE IN Y1Ba2Cu3O7−δ SINGLE CRYSTALS

1/f noise experiments performed on Y 1 Ba 2 Cu 3 O 7−δ single crystals indicate that (1) the normalized noise spectral density Sυ/V2 is much larger than that of metals; (2) the temperature dependence of Sυ/V2 differs from that of the resistivity; and (3) in the superconducting transition region, the behavior of 1/f noise is distinct from that observed in bulk and thin film copper oxide systems. Possible explanations for these observations including the important role played by the crystal morphology are discussed.

1/f noise and morphology of YBa2Cu3O7- delta single crystals.

1/f noise measurements have been performed on ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ single crystals with distinct resistivity characteristics. The noise spectral density is found to be several orders of magnitude larger than that of metals, with a temperature dependence that is independent of the resistivity. In the superconducting-transition region, we do not observe anomalously enhanced 1/f noise. We discuss the important role played by the crystal morphology and we present a tunneling model which explains the experimental data quantitatively.

1⧸ƒ noise power measurements on Tl 2Ba 2Ca n−1 Cu nO 4+2 n ( n = 2 and 3)

We have performed systematic 1⧸ƒ noise power measurements on bulk Tl 2Ba 2Ca n−1 Cu n O 4+2 n ( n = 2 and 3) materials normal and superconducting states. The 1⧸ƒ noise power is generally found to decrease with temperature until the superconducting transition region is approached. We observe no correlation between the magnitude of the 1⧸ƒ noise power in the normal state and the sample quality. In contrast to bulk Y 1Ba 2Cu 3 O 7− δ samples, the enhancement of the 1⧸ƒ noise in the superconducting transiti on region is reduced or even totally disappears in some of our good quality samples. We discuss a model calculation based on potential fluctuations in the grain boundary regions modeled as metal-insulator-metal junctions. We demonstrate that if direct knowledge of the barrier characteristics of the junctions is available, it is possible to explain our experimental data quantitstively.

Resistance drift noise in high T c superconducting transition region bolometers

Measurements of current induced noise at points of constant resistance anywhere within the transition region of thin film YBa 2Cu 3O 7 are known to reveal a noise voltage spectral density that varies as 1 ƒ with frequency. In this letter we report transition region noise measurements that were taken while the sample resistance was allowed to change. The resulting spectrum varied with frequency as 1 ƒ α , α ≈ 2, was high in magnitude, and could affect the sensitivity of superconducting transition region bolometers.

Observation of resistivity anisotropy on oriented YBa 2Cu 3O 7-δ films

The difference between the longitudinal and transverse d.c. resistivity in oriented YBa 2Cu 3O 7-δ films has been observed by special design of electrical contacts. It is attributed to the anisotropy of the in-plane resistivity, which at least indicates that some untwinned regions exist in the films. A sign change of the off-diagonal element of the resistivity tensor is also observed in the superconducting transition region for one of the films. Its possible origin is discussed.

Spontaneous conductivity fluctuations in YBaCuO thin films: Scaling of fluctuations, experimental evidence of percolation at the superconducting transition

Spontaneous conductivity fluctuations in high- T c superconducting films on sapphire and SrTiO 3 substrates have been measured through the superconducting transition. It is found that the normalized spectrum for the sapphire substrate film is two orders of magnitude larger than for the SrTiO 3 substrate film, due to a stronger disorder. At sufficiently low temperatures, in the superconducting transition region, the normalized spectrum scales with the conductivity. This is experimental evidence of a temperature controlled percolating superconducting network in high- T c superconductors.

119Sn Mössbauer study of high-temperature superconductor YBa2Cu3O7−y doped with Sn

All samples were prepared at the same condition: prefired at 900°C for 10 hours in oxygen and sintered at 1000°C for 30 hour in oxygen. The zero resistance temperature decreased with increasing Sn content. 119Sn Mossbauer spectra of YBa2Cu2.8Sn0.2O7−y superconductor from 80 K to room temperature were measured. The sites occupied by Sn were indicated. The variation of the isomer shifts and quadrupole splittings were observed in the superconducting transition region. The observation of the resonant line intensity shown the change of the lattice rigidity.

Anomalous Hall coefficient in ceramic materials YBa2Cu3?xSnxO7+?

The relations of a.c. susceptibility, a.c. resistance, remanent resistance (under a magnetic field) versus temperature and Hall coefficient (RH) in transition region have been investigated for 123 phase YBa2Cu3−xSnxO7+δ system. In normal state,RH is positive for the single phase samples with x=0.05, 0.1, 0.2 and 0.3, but negative for that with x=0.4 which contains impyrity phases. In superconducting transition region, a negativeRH peak is found for each of the single phase samples. Several situations are considered in the discussion of the origin of such anomalousRH, and it is suspected to be closely related to the anisotropy of the grain and itsRH.