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تسجيل مستخدم جديدThe average kinetic energy density of Cooper pairs above T_c in YBa_2Cu_3O_{7-x}}, Bi_2Sr_2CaCu_2O_{8+delta}, and Nb}
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We have obtained isofield curves for the square root of the average kinetic energy density of the superconducting state for three single crystals of underdoped YBa_2Cu_3O_{7-x}, an optimally doped single crystal of Bi_2Sr_2CaCu_2O_{8+delta}, and Nb. These curves, determined from isofield magnetization versus temperature measurements and the virial theorem of superconductivity, probe the order parameter amplitude near the upper critical field. The striking differences between the Nb and the high-T_c curves clearly indicate for the latter cases the presence of a unique superconducting condensate below and above T_c.
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We report on isofield curves of $sqrt{-M}$ vs. T, where M is the reversible magnetization, of YBa_2Cu_3O_{6.95}, YBa_2Cu_3O_{6.65}, and Bi_2Sr_2CaCu_2O_{8+x} with the magnetic field, H, applied parallel to the c-axis of the samples (and also parallel
to the ab- planes for YBaCuO). For temperatures close to the critical temperature, T_c, the quantity sqrt{-M} is proportional to the order parameter amplitude |psi|. Curves of sqrt{-M} vs. T allowed to study the asymptotic behavior of the form (T_a-T)^m of |psi| near T_c, as a function of field. Results for the studied samples produced values of T_a(H) lying above T_c, suggesting that the magnetic field gradually allows to probe a region of temperatures where phase correlations persist above T_c. The study performed here in YBaCuO samples allowed to study how phase correlations evolve with doping in the pseudo-gap region of YBaCuO. sqrt{-M} vs. T curves for all samples show a rather large amplitude fluctuation with no phase correlation extending well above T_a(H) which is interpreted in terms of a Gaussian Ginzburg-Landau approach with a {total-energy} cutoff in the fluctuation spectrum. Resulting values for the exponent m found for all samples, 0.5 < m < 0.7, are interpreted as due to phase fluctuations of the d-wave pairing symmetry of the order parameter in the ab- planes.
Microwave absorption measurements in magnetic fields from zero up to 16 T were used to determine the temperature range of superconducting fluctuations above the superconducting critical temperature T_c in YBa_2Cu_3O_{7-delta}. Measurements were perfo
rmed on deeply underdoped, slightly underdoped, and overdoped single crystals. The temperature range of the superconducting fluctuations above T_c is determined by an experimental method which is free from arbitrary assumptions about subtracting the nonsuperconducting contributions to the total measured signal, and/or theoretical models to extract the unknown parameters. The superconducting fluctuations are detected in the ab-plane, and c-axis conductivity, by identifying the onset temperature T. Within the sensitivity of the method, this fluctuation regime is found only within a fairly narrow region above T_c. Its width increases from 7 K in the overdoped sample (T_c = 89 K), to at most 23 K in the deeply underdoped sample (T_c = 57 K), so that T falls well below the pseudogap temperature T*. Implications of these findings are discussed in the context of other experimental probes of superconducting fluctuations in the cuprates.
We study sharp low-energy resonance peaks in the local density of states (LDOS) induced by Zn impurities or possible Cu vacancies in superconducting Bi_2Sr_2CaCu_2O_{8+delta}. The measured structure of these near-zero-bias resonances is quantitativel
y reproduced by an extended impurity potential without invoking internal impurity states or sophisticated tunneling models. The Zn potential extends at least to the nearest-neighbor Cu sites, and the range of order parameter suppression extends at least 8 AA away from the Zn site. We further show that the local spin susceptibilities near Zn impurities increase rather than decrease with decreasing temperature in the superconducting state due to the sharp increase of LDOS near the Fermi level.
We study the critical current I_c dependence on applied magnetic field H for multifacet YBa_2Cu_3O_{7-delta}-Au-Nb ramp-type zigzag Josephson junctions. For many experiments one would like to apply a homogeneous field in the junction plane. However,
even tiny misalignments can cause drastic deviations from homogeneity. We show this explicitly by measuring and analyzing I_c vs. H for an 8 facet junction, forming an array of 4times(0-pi)-segments. The ramp angle is theta_r=8^circ. The facet width is 10,mum. H is applied under different angles theta relative to the substrate plane and different angles phi relative to the in-plane orientation of the zigzags. We find that a homogeneous flux distribution is only achieved for an angle theta_happrox 1^circ - 2^circ and that even a small misalignment sim 0.1^circ relative to theta_h can cause a substantial inhomogeneity of the flux density inside the junction, drastically altering its I_c vs. H interference pattern. We also show, that there is a dead angle theta^*_d relative to theta_h of similar magnitude, where the average flux density completely vanishes.
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far above the second peak. The direct observation of Bragg peaks proves that the characteristics of a lattice are still present. No structural features related to a symmetry breaking, such as a liquid like or an amorphous state, can be observed. However, the associated scattered intensity is very low and is difficult to explain. We discuss the coexistence between two FLL states as a possible interpretation.
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