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تسجيل مستخدم جديدStudy of diamagnetic fluctuations in YBa_2Cu_3O_{7-x} and Bi_2Sr_2CaCu_2O_{8+x}: Possible observation of phase correlations persisting 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.
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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.
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 report on a contrasting behavior of the in-plane and out-of-plane magnetoresistance (MR) in heavily underdoped antiferromagnetic (AF) YBa_2Cu_3O_{6+x} (x<0.37). The out-of-plane MR (I//c) is positive over most of the temperature range and shows a
sharp increase, by about two orders of magnitude, upon cooling through the Neel temperature T_N. A contribution associated with the AF correlations is found to dominate the out-of-plane MR behavior for H//c from far above T_N, pointing to the key role of spin fluctuations in the out-of-plane transport. In contrast, the transverse in-plane MR (I//a(b);H//c) appears to be small and smooth through T_N, implying that the development of the AF order has little effect on the in-plane resistivity.
We use angle resolved photoemission spectroscopy to probe the electronic excitations of the non-superconducting state that exists between the antiferromagnetic Mott insulator at zero doping and the superconducting state at larger dopings in Bi_2Sr_2C
aCu_2O_{8+delta}. We find that this state is a nodal liquid whose excitation gap becomes zero only at points in momentum space. Despite exhibiting a resistivity characteristic of an insulator and the absence of coherent quasiparticle peaks, this material has the same gap structure as the d-wave superconductor. We observe a smooth evolution of the spectrum across the insulator-to-superconductor transition, which suggests that high temperature superconductivity emerges when quantum phase coherence is established in a non-superconducting nodal liquid.
We present here high precision magnetisation measurements in polycrystalline $YBa_2Cu_3O_{x}$ samples, with oxygen content ranging from $x=6.19$ to $x=7.00$. By analysing the temperature derivative of the susceptibility, we found in the underdoped su
perconducting samples a singular point at a temperature corresponding to $T_{mag}$, the temperature below which polarised neutrons experiments have evidenced a symmetry breaking. We believe that this is a thermodynamic indication for the existence of a phase transition in the pseudogap state of underdoped $YBa_2Cu_3O_{x}$.
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