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1.
We present DC and low frequency AC magnetization measurements on various RBa2Cu3O7 superconductors. We identify features intrinsic to these compounds, and establish the features originating from intergranular
links in sintered pellets. The isothermal magnetization curves, and the temperature dependence of magnetization in field-cooled
and zero field-cooled states are shown to be consistent with the calculations done following a recent extension of Bean’s
model. Low field anomalies predicted within this model are observed, and yieldH
c1 values of a few Oe. These values are shown to be consistent with the temperature variation of magnetization. A comparison
is made with the other existing data and it is demonstrated that earlier quoted values ofH
c1 are gross overestimates. 相似文献
2.
We have studied the hysteresis loops of RBa2Cu3O7 (R=Gd, Ho and Y) and detected anomalies in some of them. The observed anomalies support a recent prediction by Ravi Kumar
and Chaddah based on an extension of Bean’s model. The anomalies indicate lowH
c1 values and we have confirmed this by studying the onset of low-field hysteresis in less than 10 Oe at 77 K for these highT
c superconductors. 相似文献
3.
R Srinivasan V Sankaranarayanan N P Raju S Natarajan U V Varadaraju G V Subba Rao 《Pramana》1987,29(2):L225-L230
The absolute thermopower of single phase YBa2Cu3O7 and Y0.8Er0.2Ba2Cu3O7 has been measured in the range 250 K to the superconducting transition temperature. It is found that these compounds show
a large enhancement of thermopower in the range 150 K down toT
c. This enhancement shows a steep exponential drop as the temperature increases from the transition temperature. The temperature
variation of the enhancement is too steep to be accounted for by electron-phonon or electron-local structural excitation mechanisms. 相似文献
4.
We present a calculation of the isothermal magnetization hysteresis curves appropriate to highT
csuperconductors. We discuss the nature of the low field anomaly as one goes from this strong pinning case to the weak pinning
case. We show that the shape of the equilibrium (thermodynamic) magnetization curve is recovered in the limit ofJ
capproaching zero. 相似文献
5.
A theoretical framework for treating the effects of magnetic fieldH on the pairing theory of superconductivity is considered, where the field is taken in an arbitrary direction with respect
to crystal axes. This is applicable to closed, as well as open normal state Fermi surface (FS), including simple layered metals.
The orbital effects of the magnetic field are treated semiclassically while retaining the full anisotropic paramagnetic contribution.
Explicit calculations are presented in the limits |H| → |H
c2(T)|,T ∼ 0 andT →T
c(|H|), |H| ∼ 0. Effects of weak nonmagnetic impurity scattering, without vertex corrections, have also been taken into account in a
phenomenological way. The final results for the case of open FS and layered materials are found to differ considerably from
those of the closed FS. For example, an important parameter,h(T=0)=|Hc2(0)|/[-Tδ|H
c2
T|δT]T{s0} for the case of a FS open ink
z-direction with thek
z-bandwidth, 4t
3, very small compared to the Fermi energy,E
F, is close to 0.5906, compared to 0.7273 for the closed FS, in the clean limit. Analytical results are given for the magnetic
field dependence ofT
c and the temperature dependence of H
c2 for a model of layered superconductors with widely open FS. For a set of band structure parameters for YBa2Cu3O7 used elsewhere, we find reasonable values for the upper critical fieldH
c2(0), the slope (dH
c2/dT)T
c0, anisotropic coherence lengths ζi(T=0),i=x, y, z, and (dT
c/d|H|)|H| → 0. 相似文献
6.
Measurements of the coefficient of linear thermal expansion α have been carried out for the orthorhombic and tetragonal phases
of RE1Ba2Cu3O
y
(RE=Y, Gd, Dy) compounds using a high resolution capacitance dilatometer in the temperature range 77–300 K. All the superconducting
samples exhibit a jump Δα at their respective transition temperatures,T
c. Evidences of, sample-to-sample variation in α values and dependence of Δα on the sample preparation conditions, have been
obtained. The non-superconducting samples, in general, exhibit lower values of α possibly because of lowering of oxygen content. 相似文献
7.
K. I. Kugel L. G. Mamsurova K. S. Pigalskiy A. L. Rakhmanov 《Physica C: Superconductivity and its Applications》1998,300(3-4):270-280
The nature of hysteretic behavior of the flux line lattice (FLL) contribution to ac magnetic permeability (μv) is analyzed for the case of YBa2Cu3Ox single crystal (at applied magnetic field Hc axis). It is shown that hysteresis loops μv(H) corresponding to different temperatures (T=70–84 K) are scaled to a universal curve in normalized coordinates. Such a behavior is interpreted in terms of the FLL interaction with the crystal surface. The explicit relationship between μv and magnetic induction B is found for the near-surface region of the superconductor. It is shown that the μv(H) loops are closely related to the hysteresis of B at cycling of applied magnetic field. The latter hysteresis stems from the Bean–Livingston surface barrier. The estimates demonstrate strong suppression of the surface barrier in YBa2Cu3Ox crystal in comparison to that expected for the ideal surface. As a result, the lower branch of the hysteresis loop corresponding to the increasing field is very close to the equilibrium μv(H) curve and the surface barrier appreciably affects only the upper branch when magnetic flux leaves the sample. The comparison of theoretical predictions and experimental data provides an opportunity to refine the actual range of stability Hmax(B)–Hmin(B) for the FLL at fixed B for YBa2Cu3Ox crystal in the case of Hc. 相似文献
8.
The effects of second-neighbor spin coupling interactions and a magnetic field are investigated on the free energies of a finite-size 1-D Ising model. For both ferromagnetic of nearest neighbor (NN) and next-nearest neighbor (NNN) spin coupling interactions, the finite-size free energy first increases and then approaches a constant value for any size of the spin chain. In contrast, when NNN and NN spin coupling interactions are antiferromagnetic and ferromagnetic, respectively, the finite-size free energy gradually decreases by increasing the competition factor and eventually vanishes for large values of it. When a magnetic field is applied, the finite-size free energy decreases with respect to the case of zero magnetic fields for both ferromagnetic and antiferromagnetic spin coupling interactions. Deviation of free energy per size for finite-size systems relative to the infinite system increases when the spin coupling interactions as well as the f parameter (the ratio of the magnetic field to NN spin coupling interaction) increase. 相似文献