Analysis Of Fluctuation Conductivity Research Articles

Evolution of the pseudogap and excess conductivity of YBa2Cu3O7–δ single crystals in the course of long-term aging

The temperature dependences of both fluctuation conductivity (FLC) σ′(T) and pseudogap (PG) Δ*(T) derived from measurements of resistivity ρ(T) of an optimally doped YBa2Cu3O7−δ single crystal subjected to long-term storage have been studied. The as-grown sample S1 exhibits characteristics typical of optimally doped YBa2Cu3O7−δ single crystals containing twins and twin boundaries. Analysis of both FLC and PG showed an unexpected improvement in all characteristics of the sample after 6 years of storage (sample S2), indicating that the effect of twin boundaries is somehow limited. After 17 years of storage, all characteristics of the sample changed dramatically, which indicates a strong influence of internal defects formed during the aging process. For the first time, the temperature dependences of both FLC and PG were obtained after 17 years of storage.

Analysis of fluctuation conductivity in Y-=SUB=-0.5-=/SUB=-Cd-=SUB=-0.5-=/SUB=-Ba-=SUB=-2-=/SUB=-Cu-=SUB=-3-=/SUB=-O-=SUB=-7-delta-=/SUB=-

A study was made of the influence of substitution of up to 50% of yttrium for cadmium in YBa2Cu3O7-delta polycrystals on the mechanism of formation of excess conductivity. It has been established that such a substitution led to a significant increase in the resistivity of Y0.5Cd0.5Ba2Cu3O7-delta, and the value of the critical transition temperature Tc to the superconducting state decreases. The mechanism of formation of fluctuation conductivity (T) near Tc is considered within the framework of the Aslamazov-Larkin theory. The Ginzburg temperature, the critical temperature in the mean field approximation, and the 3D-2D crossover temperature were determined. It is shown that the doping of YBa YBa2Cu3O7-delta with cadmium leads to an increase in the coherence length along the c axis by a factor of 3.2. An analysis of the excess conductivity of the Y0.5Cd0.5Ba2Cu3O7-delta sample within the framework of the local pair model made it possible to determine the temperature dependences of the pseudogap and its maximum value. Keywords: superconductivity, fluctuation conductivity, pseudo-gap, coherence length.

Analysis of fluctuation conductivity in Y1−хCdxBa2Cu3O7−δ (x = 0−0.4)

The effect that the partial substitution of Cd for Y has on the mechanism of excess conductivity formation in polycrystalline Y1−хCdxBa2Cu3O7−δ with x = 0 (Y1), 0.1 (Y2), 0.3 (Y3), and 0.4 (Y4) is investigated. The resistivity ρ of the samples increases markedly with increasing x, and the critical temperature of the superconducting (SC) state transition, Tc, decreases. The mechanism responsible for the formation of fluctuation conductivity, σ'(T), is considered within the framework of the Aslamazov–Larkin theory near Tc. The Ginzburg temperature (TG), the critical temperature in the mean-field approximation (Tcmf), the temperature of the 3D–2D crossover (T0), and T01, which limits the region of the SC fluctuations from above, are determined. It is shown that doping with Cd at x = 0−0.4 increases the coherence length along the c axis, ξc(0), by 2.7 times, and the distance between the CuO2 planes, d01, by 2.2 times. The temperature dependences of the pseudogap (PG), Δ*(T), are determined by analyzing the excess conductivity within the framework of the local pair model. It is found that with an increase in substitution, the maximum value of the PG Δ*(Tpair) decreases from 250.2 to 215.7 K, while the real value of the PG, measured at TG,Δ*(TG), increases from 217.4 to 224.2 K.

Fluctuation conductivity analysis in Zn-doped YBa2Cu3O7 and related systems

Fluctuation conductivity σ' (ϵ) above Tc as a function of the reduced temperature ϵ was measured in polycrystalline YBa2Cu3O7 and LaBaCaCu3O7 with Zn doping. The results for σ' (ϵ) are analyzed in terms of the Aslamazov-Larkin theory for a layered superconductor in which effect of d-wave pairing as well as that of short-wavelength cutoff is taken into account. The d-wave character appears in the σ'(ϵ) formula as renormalization effects on the fluctuation amplitude and the reduced temperature. Good agreement is attained between theory and experiment even in a high e region, which allows us to discuss the doping dependence of the fluctuation amplitude and the out-of-plane coherence length. The d-wave nature could be responsible for the doping dependence of the fluctuation amplitude.

Fluctuation conductivity analysis for Bi2Sr2(Ca1−xYx)Cu2O8+δ single crystals in both overdoped and underdoped regimes

In-plane fluctuation conductivity σab′ is studied in Bi2Sr2(Ca1−xYx)Cu2O8+δ single crystals with the doping level x from the overdoped to underdoped regime. For the σab′ analysis, the Lawrence–Doniach (LD) theory is employed by taking account of a total-energy cutoff in the fluctuation spectrum, and a remarkable agreement is attained between theory and experiment over the whole temperature range investigated. From the theoretical fittings, it is found that not only the LD anisotropic parameter but also the cutoff parameter show specimen-dependent natures. The behavior of these parameters upon doping allows us to conclude that starting from the optimal doping level, the anisotropy of our Bi2212 system is highly enhanced in the underdoped region while also enhanced moderately as the system enters into the overdoped region. The present results are discussed in comparison to other studies of fluctuation effects in related systems.

Fluctuation conductivity analysis on the Bi‐based superconductors processed under same conditions

AbstractWe report electrical conductivity fluctuation measurements on different Bismuth‐based granular samples synthesized under same processing conditions.Using the fluctuation conductivity Δσ as a function of the reduced temperature ϵ in the range –7 < ln ϵ < 1, we identified Gaussian and critical fluctuation conductivity in (Bi,Pb)‐2223 and Bi‐2212 samples. Within the mean field region, samples show predominately two dimensional (2D) behaviour with exponent λ = –1. Closer to critical temperature T C, we have observed a crossover of λ from –2/3 to –1/3 in the critical region. The obtained exponents are consistent with 3D‐XY model predictions. The regime with the critical exponent –2/3 is dominate in the case of (Bi,Pb)‐2223 sample. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Kinetics and Boltzmann kinetic equation for fluctuation Cooper pairs

The Boltzmann equation for excess Cooper pairs above T_c is derived in the framework of the time-dependent Ginzburg-Landau (TDGL) theory using Langevin's approach of the stochastic differential equation. The Newton dynamic equation for the momentum dependent drift velocity is obtained and the effective drag force is determined by the energy dependent life time of the metastable Cooper pairs. The Newton equation gives just the Drude mobility for the fixed momentum of Cooper pairs. It is shown that the comparison with the well-known result for Aslamazov-Larkin paraconductivity and BCS treatment of the excess Hall effect can give the final determination of all the coefficients of TDGL theory. As a result the intuitive arguments used for an interpretation of the experimental data for fluctuation kinetics are successively introduced. The presented simple picture of the degenerated Bose gas in tau-approximation near to Bose-Einstein condensation temperature can be used for analysis of fluctuation conductivity for the cases of high frequency and external magnetic field for layered and bulk superconductors. The work of the Boltzmann equation is illustrated by frequency dependent Aslamazov-Larkin conductivity in nanowires, two dimensional case and the case of strong electric field where the TDGL equation is solved directly. There are also derived explicit formulas for the current in the case of arbitrary time dependence of electric field up to THz range, the distribution of fluctuation Cooper pairs for non-parabolic dispersion, the influence of the energy cut-off and the self consistent equation for the reduced temperature. The general theory is illustrated by formulae for fluctuation conductivity in nanowires and nanostructured superconductors.

Analysis of fluctuation conductivity in melt-textured DyBa 2Cu 3O y superconductors

We evaluated the superconducting properties of DyBa 2Cu 3O x (Dy123) with different amounts of Dy 2BaCuO 5 (Dy211) phase. The samples were melt-textured DyBaCuO superconductors, which were made from the mixtures in molar ratios of Dy123:Dy211=1: x ( x=0.03, 0.05, 0.08 and 0.1). The temperature dependence of resistance of DyBaCuO superconductors on different amounts of 211 phase was measured. The data of fluctuation amplitude and the dimension of conduction were obtained from analyzing the fluctuation. The results show that coherence length obtained from this analysis becomes larger as amounts of 211 phase increase. The evidence is found that the pinning force in the sample is well correlated with the characteristic length of superconductivity.

Effect of Pr doping on the superconductivity and interlayer coupling of the Bi2Sr2−xPrxCa1Cu2Oy system

We investigate the effect of Pr substitution on the superconductivity and interlayer coupling of the Bi2Sr2Ca1Cu2Oy system. Magnetic and transport measurements were performed for the purposes of characterization. The superconducting transition temperature Tc first increases and then decreases until it becomes zero at x = 0.6. The effective superconducting volume also decreases due to Pr substitution. From the fluctuation conductivity analysis, it is found that the interlayer coupling constant J decreases monotonically with the increase of the Pr content. This result shows that the Pr doping weakens the CuO2 interlayer coupling of the Bi2212 system due to the loss of local superconductivity in the CuO2 layers.

Fluctuation conductivity in NdBa/sub 2/Cu/sub 3/O/sub y/ superconductors with the different amounts of 422 phase

Fluctuation conductivity analysis was carried out to characterize NdBa/sub 2/Cu/sub 3/O/sub y/ superconductors with the different amounts of Nd/sub 4/Ba/sub 2/Cu/sub 2/O/sub y/ (Nd422) phase. Samples were prepared by melt-textured process from the mixtures in molar ratios of Nd123: Nd422 = 1:x (x = 0.2, 0.25, 0.3). The conduction dimensionality and the fluctuation amplitude were obtained by analyzing the fluctuation conductivity. The dimension of conduction on excess conductivity showed little variation by the addition of 422 phase. The coherence length and the correlation length obtained by this analysis became larger with increasing amount of 422 phase. The clear evidence was found that the pinning force in the sample was well correlated with the characteristic length of superconductivity.

Relation between Sintering Time and Conduction Behavior in Bi-2223 Superconductor.

It is known that critical current density Jc in Bi-Based superconductors increases with the length of sintering time, and further increases with cold isostatic pressing (CIP) process. Analysis of excess conductivity due to the thermal fluctuation above Tc provides information about fluctuation amplitude and the dimensionality of conduction, and makes it possible to evaluate the superconducting correlation length. In this study, the mechanism of how Jc of Bi-based copper oxide improves by increasing sintering time is discussed on the basis of fluctuation-conductivity analysis. The temperature dependence of the resistance of Bi-based superconductors with various sintering time was measured. It was found that correlation length and effective cross-sectional area obtained from analysis systematically decrease with the sintering time, and that these parameters correlate well with Jc of the samples. It is widely accepted that the increase in Jc of the samples is due to the improvement in the crystallinity, (i.e., the increase in crystal density and better alignment of the crystal grains) which brings about decrease in correlation length and effective cross-sectional area. Therefore, the effect of the improvement in the density and alignment of the crystals, an important factor for increasing critical current density, was evaluated from the fluctuation conductivity analysis.

Analysis of fluctuation conductivity in melt-textured YBa 2Cu 3O 7− y superconductors with Ag-doping

The difference in the properties between Ag-doped and undoped YBa 2Cu 3O 7− y (YBCO) melt-textured samples was investigated by fluctuation-conductivity analysis. The temperature dependence of the resistance in those samples was measured. Data about the dimensionality and amplitude of the fluctuation were obtained by analyzing the fluctuation conductivity on the basis of the Aslamazov–Larkin theory. The fluctuation conductivity of the undoped sample shows three-dimensional behavior in the mean-field region (MFR) with a one-dimensional (1D) character at further higher temperature. The effect of a small amount of Ag addition is manifested on a fractal conduction behavior in the 1D region of the undoped sample. Upon further Ag-doping, the conduction behavior is two-dimensional in the MFR with evidence for a rapid suppression of the short-wavelength fluctuation. The conduction behavior is discussed on the relevance to structural features of melt-textured YBCO samples with and without Ag-doping.

Fluctuation conductivity analysis for Bi-2223 superconductors with different sintering time

In a Bi-based superconductor, the critical current J c increases in accordance with the length of the sintering time and further increase is attained by cold isostatic pressing (CIP). The temperature dependence of the resistivity in polycrystalline Bi-2223 superconductors with different sintering time ( t s) of 10–200 h was measured. In order to investigate the mechanism to improve J c, the excess conductivity caused by the thermal fluctuation above T c was analyzed using the Aslamazov–Larkin theory. It was found that the samples show a two-dimensional fluctuation in conductivity irrespective of t s, and that the amplitude of fluctuation increases with t s. The analysis indicates that the correlation length, which is inversely proportional to the amplitude, decreases with the length of t s. Therefore, it can be deduced that J c of the samples improves as the crystals become more uniformly aligned with increased t s, resulting in a shorter correlation length as confirmed by the SEM study. A similar result has been obtained for the samples with the CIP process.

Analysis of fluctuation conductivity of YBCO-PrBCO superlattices

The fluctuation-enhanced conductivity measured on YBCO-PrBCO superlattices (SL) is analysed using various theoretical models describing weak fluctuations in layered superconductors and considering both Aslamazov-Larkin(AL) and Maki-Thompson(MT) terms. From the analysis ξ c (0)=0.16±0.01 nm and τφ(100K) ≅3∗10 −? s in clean limit are derived for the intralayer coherence length and temperature dependent phase-relaxation time respectively. Large τφ values and AL-MT crossover revealed for all samples studied are attributed to evident 2-dimentional (2D) nature of electronic state of SL above the crossover temperature. The resonating-valence-bond (RVB) model is used to discuss intralayer interaction in 2D fluctuation region.

Superconducting transition, fluctuation, and vortex motion in a two-dimensional single-crystal Nb film.

We report a detailed study on the transport properties of an ultrathin single-crystal Nb film, whose thickness (d\ensuremath{\approxeq}20 \AA{}) is less than all other relevant length scales except the lattice constant. The two-dimensional (2D) nature of the sample in the superconducting state is reflected in the characteristic distinction between its parallel and perpendicular critical fields, as well as in the temperature dependence of the fluctuation conductivity. Because of the long mean free path of this single-crystal film, we find it necessary to include both Maki-Thompson and Aslamazov-Larkin terms in the fluctuation-conductivity analysis. The logarithmic temperature dependence of the resistance, signature of the 2D electron-electron interaction effect, has been observed when the external magnetic fields exceed the upper critical field, indicating that the sample is also two dimensional in its normal state. In addition, dissipation below ${\mathit{T}}_{\mathit{c}}$ under small applied current is attributed to vortex motion; activation energy as a function of magnetic field has been obtained via the application of the Anderson-Kim model extended to two dimensions. The characteristics of the Kosterlitz-Thouless transition, which has been observed only in high-resistance 2D superconducting films, have been seen in our relatively low-resistance Nb film, although they are greatly affected by the presence of strong pinning. This work provides insight into the non- equilibrium properties of clean 2D superconducting films.