Observation of Kosterlitz‐Thouless Transition and Conductivity Fluctuation in Superconducting Ybco Thin Films

1989 ◽  
Vol 169 ◽  
Author(s):  
Q. Y. Ying ◽  
H. S. Kwok
Keyword(s):  
Thin Films ◽  
Transition Temperature ◽  
Power Law ◽  
Mean Field ◽  
Superconducting Thin Films ◽  
Ybco Thin Films ◽  
The Mean ◽  
Conductivity Fluctuation ◽  
Field Transition

AbstractKosterlitz‐Thouless properties of laser deposited high Tc superconducting thin films of YBa2Cu3O7‐x were examined. The exponent of the power law dependence of V on I showed a “universal jump” at Tc. ID like conductivity fluctuation was also found at temperatures slightly above the mean‐field transition temperature Tco, which may be the result of imperfections of the sample.

scholarly journals MEAN-FIELD TRANSITION TEMPERATURE OF STRONGLY DISORDERED SUPERCONDUCTORS

2000 ◽  
Vol 14 (06) ◽  
pp. 575-587 ◽  
Author(s):  
IGOR F. HERBUT
Keyword(s):  
Field Theory ◽  
Transition Temperature ◽  
Nearest Neighbor ◽  
Mean Field Theory ◽  
Mean Field ◽  
Localized States ◽  
Effective Theory ◽  
The Mean ◽  
Field Transition ◽  
Infinite Range

I discuss the mean-field theory of superconductivity in a strongly disordered system of fermions with short-range attraction. It is argued that in this limit the effective theory at low energies is equivalent to the disordered Bose–Hubbard model, and I consider both the infinite-range and the "nearest-neighbor" hopping of bosons between the localized states. In the infinite-range case the mean-field theory is exact, and the superconducting gap is uniform in space, while in the latter case, the gap becomes highly non-uniform in space, but, surprisingly, is finite everywhere at T < T MF . I find that the mean-field transition temperature T MF > 0 always, and argue that the superconductor–insulator transition at T = 0 in models with net attraction between fermions is in the universality class of "dirty-bosons".

Modification of charge density wave fluctuations by charge perturbations

2002 ◽  
Vol 12 (9) ◽  
pp. 77-78
Author(s):  
S. N. Artemenko
Keyword(s):  
Electric Field ◽  
Linear Response ◽  
Chemical Potential ◽  
Density Wave ◽  
Mean Field ◽  
Phase Fluctuations ◽  
Fluctuation Dissipation ◽  
Fluctuation Dissipation Theorem ◽  
The Mean ◽  
Field Transition

Spectral density of fluctuations of the CDW phase are calculated taking into account electric field induced by phase fluctuations. The approach based upon the fluctuation-dissipation theorem (FDT) combined with equations of linear response of the CDW conductor is used. Fluctuating electric field is found to suppress fluctuations of the phase, while fluctuations of the electric potential are sizeable. This suggests that transition from the CDW to the normal state (which is usually observed well below the mean-field transition temperature) may he provoked by fluctuations of the chemical potential, rather than by destruction of the CDW coherence between conducting chains due to phase fluctuations.

In-Situ Laser Processing and Microstructural Characteristics of YBa2Cu3O7−δ Thin Films on Si with TiN Buffer Layer

1992 ◽  
Vol 285 ◽  
Author(s):  
P. Tiwari ◽  
T. Zheleva ◽  
J. Narayan
Keyword(s):  
Electron Microscopy ◽  
Thin Films ◽  
Transition Temperature ◽  
Buffer Layer ◽  
Processing Parameters ◽  
Superconducting Transition ◽  
Epitaxial Relationship ◽  
Ybco Thin Films ◽  
Zero Resistance

ABSTRACTWe have prepared high-quality superconducting YBa2Cu3O7−δ (YBCO) thin films on Si(100) with TiN as a buffer layer using in-situ multitarget deposition system. Both TiN and YBCO thin films were deposited sequentially by KrF excitner laser ( λ = 248 nm ) at substrate temperature of 650°C. Thin films were characterized using X-ray diffraction (XRD), four-point-probe ac resistivity, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Rutherford backscattering (RBS). The TiN buffer layer was epitaxial and the epitaxial relationship was found to be cube on cube with <100> TiN // <100> Si. YBCO thin films on Si with TiN buffer layer showed the transition temperature of 90–92K with Tco (zero resistance temperature) of ∼84K. We have found that the quality of the buffer layer is very important in determining the superconducting transition temperature of the thin film. The effects of processing parameters and the correlation of microstructural features with superconducting properties are discussed indetail.

Low-frequency excess noise in YBCO thin films near the transition temperature

1993 ◽  
Author(s):  
Sisi Jiang ◽  
Peter Hallemeier ◽  
Charles Surya ◽  
Julia M. Phillips
Keyword(s):  
Thin Films ◽  
Transition Temperature ◽  
Low Frequency ◽  
Excess Noise ◽  
Ybco Thin Films

MICROPHASE SEPARATION IN POLYDISPERSE GRAFT COPOLYMERS

2013 ◽  
Vol 27 (32) ◽  
pp. 1350189
Author(s):  
M. A. ALIEV ◽  
N. Yu KUZMINYKH
Keyword(s):  
Transition Temperature ◽  
Phase Diagrams ◽  
Microphase Separation ◽  
Mean Field ◽  
Side Chains ◽  
Field Phase ◽  
Special Cases ◽  
Regions Of Stability ◽  
The Mean ◽  
Disorder Transition Temperature

The phase behavior of the polydisperse An+1Bn multigraft copolymer melt is considered in a framework of weak segregation theory. The phase diagrams of melt are constructed within first harmonics approximation for two special cases: (i) lengths of backbone subchains between successive grafting points in each macromolecule are distributed according to the Schultz–Zimm distribution whereas lengths of side chains are fixed and (ii) both lengths of backbone subchains and side chains are distributed according to the same Schultz–Zimm distribution. The regions of stability of LAM, HEX and BCC mesophases in phase diagrams deform considerably as compared with the case of monodisperse graft copolymer. For both the cases the mean field phase diagrams indicate that order–disorder transition temperature grows as polydispersity index increases.

FLUX DYNAMICS IN YBCO THIN FILMS

2000 ◽  
Vol 14 (27n28) ◽  
pp. 949-959
Author(s):  
S. K. HASANAIN ◽  
UZMA KHALIQUE
Keyword(s):  
Thin Films ◽  
Power Law ◽  
Magnetic Measurements ◽  
Sweep Rate ◽  
Flux Dynamics ◽  
Activation Barriers ◽  
Current Dependence ◽  
Ybco Thin Films ◽  
Relaxation Measurements ◽  
The Moment

Magnetic measurements are reported on YBCO thin films (d<500 nm ) in the perpendicular geometry, H‖c. By analyzing the sweep rate dependence of the magnetization for temperatures 35 ≤ T ≤ 77 K and H≤10 kOe , we find that the moment has a power law dependence on the sweep rate. This dependence is indicative of power law E(J) characteristics for these thin films and from this a logarithmic current dependence of the activation potential U(J) is inferred. We find that our magnetic relaxation measurements are also consistent with the logarithmic U(J) dependence obtained from the sweep rate measurements. Activation barriers for flux creep are determined for various temperatures and their field dependence is discussed.

INHOMOGENEITY INDUCED CONDUCTIVITY FLUCTUATION IN YBa2Cu3O7-δ/BaTiO3–CoFe2O4 COMPOSITE

2013 ◽  
Vol 27 (20) ◽  
pp. 1350099 ◽  
Author(s):  
MOUSUMIBALA SAHOO ◽  
DHRUBANANDA BEHERA
Keyword(s):  
Model Fitting ◽  
Mean Field ◽  
Xrd Analysis ◽  
Fluctuation Analysis ◽  
Superconducting Transition ◽  
Solid State Route ◽  
Zero Resistance ◽  
The Mean ◽  
2D And 3D ◽  
Conductivity Fluctuation

Polycrystalline (1-x) YBa 2 Cu 3 O 7-y + x BaTiO 3– CoFe 2 O 4(x = 0.0, 0.2, 0.4, 0.6 wt. %) superconductors were prepared by solid state route. XRD analysis reveals no significant change in "b" parameter and increase in "a" and "c" parameters. SEM micrographs show no change in grain size of the samples. With the increase of BaTiO 3– CoFe 2 O 4 (BTO–CFO) addition it has been analyzed that the superconducting transition temperatures (Tc) determined from standard four-probe method was decreased and dropped sharply with higher wt.% addition. Excess conductivity fluctuation analysis using Aslamazov–Larkin model fitting reveals transition of two dominant regions (2D and 3D) above Tc. The decrease in 2D–3D crossover temperature T LD (Lawerence–Doniach temperature) in the mean field region has been observed as a consequent dominance of 3D region to increase in wt.% in the composite. The increasing value of ρwl and ρ0 and the decreasing trend in the value of zero-resistance critical temperature (Tc0) indicates that the connectivity between grains decreases gradually with the addition of magneto–electric composite BTO–CFO.

DISORDERED SUPERCONDUCTORS: A SIMPLE MODEL MANIFESTING PSEUDOGAP AND BCS-BEC CROSSOVER

2009 ◽  
Vol 23 (20n21) ◽  
pp. 4245-4256 ◽  
Author(s):  
POULUMI DEY ◽  
SAURABH BASU
Keyword(s):  
Transition Temperature ◽  
Harmonic Approximation ◽  
Mean Field ◽  
S Wave ◽  
Phase Fluctuations ◽  
Low Dimensions ◽  
Kt Transition ◽  
Amplitude Fluctuations ◽  
Site Disorder ◽  
The Mean

The effect of random on-site disorder on s-wave (BCS) superconductors described by a two dimensional negative-U Hubbard model is studied using Bogoliubov-de Gennes (BdG) method. The mean field pairing amplitudes become spatially inhomogeneous at large values of disorder where the system breaks up into superconducting islands with large pairing amplitudes, separated by insulating strips. The amplitude fluctuations are correctly accounted for via BdG calculations, however it misses phase fluctuations which are inherent to low dimensions. The phase fluctuations affect superconducting properties strongly, and the effect is more pronounced in the limit of large disorder. We provide a close estimate of the actual transition temperature, Tc by incorporating phase fluctuations about the inhomogeneous BdG state. This is obtained by relating the jump in renormalized Ds (obtained from a self consistent Harmonic approximation on a phase-only Hamiltonian) at the Kosterlitz-Thouless (KT) transition and the KT transition temperature, TKT obtained from Ds(T) = (2/π)TKT as temperature tends to TKT from below. This yields opening of a large region sandwiched between [Formula: see text] (obtained by the vanishing of [Formula: see text]) and TKT where there is no phase coherence between the pairs, however amplitude correlations continue to exist, reminiscent of a pseudogap phase in high-Tc superconductors which is marked by short ranged preformed pairs without any definite phase relation between them. Further, the appearance of the superconducting islands from a homogeneous phase indicates the evolution of a system consisting of large and overlapping pairs to one that contains short and tightly bound pairs - a scenario termed as BCS-BEC crossover. We have investigated in details the crossover phenomenon as a function of disorder and confirmed it's existence at small values of electron concentration, however, absent at larger densities.

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