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1.
The low-voltage-level current-voltage characteristics of Bi2Sr2Ca2Cu3O10+x
tapes and bulk samples with preferential crystallite orientation were measured in applied magnetic fields both parallel and perpendicular to the samples' surfaces. The data were interpreted within the framework of the collective pinning model. The collective pinning exponent changes from 1 to 0.5 (close toT
c), while in the case of unoriented samples it is constant (and equal to the theoretical value 1/2) as shown in an earlier paper. We suggest that this temperature dependence of is due to the change of the dimensionality of the intragranular vortices. 相似文献
2.
Using the self-template technique, c-axis-oriented epitaxial YBa2Cu3O7 – thin films have been prepared in situ on LaAlO3 substrate by the d.c. magnetron sputtering method. The properties of thin film dependence on the deposition conditions of the two-step self-template method have been systematically investigated. By optimizing the parameters, high-quality YBCO thin films with T
c0 90 K, T
c 1 K, R
s (77 K, 10 GHz)500 were reproducibly obtained. The best sample grown under optimal conditions gave a low R
s of 330 at 77 K, 10 GHz, which can be used in a microwave field. 相似文献
3.
H. A. Blackstead D. B. Pulling P. J. McGinn W. H. Chen 《Journal of Superconductivity》1991,4(4):263-269
Using a microwave (13 GHz) technique which is sensitive to field-induced changes in the surface resistance, we have observed twin-boundary pinning in zone-melt-textured crystals of YBa2Cu3O7-. With the rotation of a static 1.9-T field applied in thea-b plane, the surface resistanceR
s exhibits minima at 90° intervals. The twin-boundary minima are shown to be connected to the orientation of the crystalline axes of the sample. The resistance minima are observed superimposed on a flux-flow response over the temperature range ofT
c to the lowest temperature attempted. Substantial flux mobility is observed to the lowest temperatures. The angular dependence of the flux-flow response at low temperatures is consistent with the brick model for local intergranular conductivity, which provides for an in-plane isotropic component. 相似文献
4.
Yuji Yamaguchi Dong Han Ha Fumitoshi Iga Yoshikazu Nishihara 《Journal of Superconductivity》1994,7(2):291-293
The ac susceptibility under a biased dc field near the irreversibility field (H
irr) of a Bi2Sr2CaCu2O8 single crystal has been measured. The frequency dependence, the ac-power dependence, and the nearly lossless character of the vs.H
dc curve forHa-axis have been roughly explained from a reversible (elastic) fluxoid motion, while those forH c-axis have been explained from a thermally assisted flux-flow (TAFF) model. The obtained parameters are discussed in relation to anisotropic flux-pinning mechanisms in the layered structure of this compound. 相似文献
5.
N. Klein U. D?hne U. Poppe N. Tellmann K. Urban S. Orbach S. Hensen G. Müller H. Piel 《Journal of Superconductivity》1992,5(2):195-201
We used a dielectric resonator technique for highly sensitive measurements of the temperature dependence of the microwave surface resistanceR
s
of 1×1 cm2 superconducting films at 18.7 GHz. It consists of a sapphire disc positioned on the film under investigation within a copper cavity which is acting as a radiation shield. In the TE01 oscillation mode the highly reproducible quality factor of about 105 results in a sensitivity of ±50 forR
s
measurements. The temperature dependence ofR
s
can be measured up to values as high as 1 . We have investigated several YBa2Cu3O7 thin films prepared by high oxygen pressure d.c. sputtering on LaAlO3 and NdGaO3. Our best films exhibit a pronounced nonlinear behavior of the d.c. resistivity(T) with(300K)/(100K) values of about 3.7. Those films show, besides the initial fall-off just belowT
c
, a further strong decrease ofR
s
at low temperatures. This was observed both at 18.7 GHz and 87 GHz, as measured by a conventional cavity end plate replacement technique. ForTTc/2 these films exhibit an exp (–T
c/T) dependence ofR
s
with-values around 0.4. These observations may be explained by a superconducting energy gap with 2/kT
c0.8 for charge carriers localized in the CuO chains for YBa2Cu3O7. 相似文献
6.
C. C. Homes S. V. Dordevic D. A. Bonn R. Liang W. N. Hardy 《Journal of Superconductivity》2004,17(1):93-96
The optical conductivity sum rule is used to examine the evolution of the spectral weight N() in both the normal and superconducting states of optimally and underdoped YBa2Cu3O6+x
along the a axis. Differences in N() above and below T
c allow the strength of the superconducting condensate s to be determined. In the optimally-doped material, s is fully formed at energies comparable to the full superconducting gap maximum (0.1 eV), while in the underdoped material the energy scale for convergence is considerably higher (0.6 eV). This difference is discussed in terms of normal-state properties. 相似文献
7.
Takashi Noji Yoji Koike Hideo Iwasaki Masatsune Kato Norio Kobayashi Yoshitami Saito 《Journal of Superconductivity》1996,9(1):65-71
Anisotropie properties of the single crystal Pb2Sr2Ho0.5Ca0.5Cu3O8 have been investigated by measuring the electrical resistivity in theab-plane
ab
(H, ,T), which depends on the angle between theab-plane and the magnetic-field direction, in various constant fieldsH perpendicular to the current direction. All the angle-dependent values of
ab
(H, ,T) at a constant temperature are scaled to be on one curve as a function of reduced field. The anisotropic parameter (m
c
*
/m
ab
*
)1/2 is estimated as 12–13, which is larger than that of YBa2Cu3O7 and much smaller than that of Bi2Sr2CaCu2O8. It has been concluded that the anisotropy does not always depend on the thickness of the blocking layer but seems to depend on the overlap of the electronic wave functions along thec-axis. Anisotropy in the pinning potential has also been discussed from the resistive tail in the temperature dependence of
ab
(H,,T). 相似文献
8.
P. Tarasewicz 《Journal of Superconductivity》2004,17(3):431-437
A gas of spin 1/2 fermions with an interaction V+W = -||-1 k,k
g
k,k(k)(k)
b
k
b
-k - 2 k(k)n
k+
n
k-, where n
k± =
a
k±, b
k = a
k+
a
k- and a
k,
satisfy Fermi anticommutation relations, is investigated in the
-pairing case. W+V4 is nonzero only within a thin layer of one-fermion energies around the chemical potential , and (k) denotes the characteristic function of the corresponding range of momenta. Two cases are studied: 10 = 0, 20 = 0.10025 eV. In the first case the system exhibits a first order transition, in the second, the transition is second order. Temperature dependence of the system's thermodynamic functions is examined and compared with that of the s-pairing case. 相似文献
9.
A base composition of Fe-15Cr-15Ni was melted in a hot-isostatic-pressure furnace under nitrogen pressures from 0.1–150 MPa (1–1500 atm) to produce materials with nitrogen concentrations from 0.04–3.6 wt%. Microstructural characterization of the as-cast materials was completed using optical microscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy techniques. The phases present in the materials were austenite, nitrides (Cr2N and CrN), and martensite. All of the materials solidified with primary austenite dendrites. Dendritic solidification of CrN occurred in interdendritic regions of the austenite in alloys containing between 0.6 and 1.9 wt % N. Cellular precipitation of Cr2N occurred in alloys with intermediate nitrogen concentrations (0.6 and 1.0 wt %). The orientation relationship between the Cr2N and austenite, expressed as {1 1 1}y{0 0 0 1}Cr2N and 1 1 0 –1 1 0 0Cr2N, was confirmed. Precipitates of CrN with both lamellar and disc morphologies formed in the austenite. Both had a cube-cube orientation relationship with the austenite. Only about 10% of the 3.6 wt% N material solidified as primary austenite. The remainder of the material solidified as a eutectic according to the reaction L +CrN. All of the austenite in the eutectic region subsequently transformed to martensite. Increasing volume fractions of martensite formed with increasing bulk nitrogen concentrations in the materials due to alloy depletion of austenite caused by nitride formation. 相似文献
10.
We observed, for the first time, adsorption of weakly bonded oxygen at low temperature (<-250°C) by a Y5Ba6Cu11O
y
sample, using thermogravimetric analysis. The resulting oxygen enriched phase in the surface layers may be attributed to the observation of a superconductivity-like transition at above 200 K. 相似文献
11.
LaBa2Cu3–y
Fe
y
O
x
ceramic samples with y = 0.00–1.50 are synthesized by the solid-state reaction technique. Rietveld analysis for X-ray diffraction is performed on these iron-doped samples. A BaCuO2 impurity phase and a ceramic cuprate phase coexist in each sample. An orthorhombic-to-tetragonal (OT) phase transition occurs in the doping range of 0.03y0.06, and a tetragonal-to-orthorhombic (TO) one occurs in the doping level of 0.10y0.25. There is a jump in the structural parameters due to the iron doping. The occupancy of oxygen at the O(4) site, which is in the La plane at z = 1/2, increases with increase in iron content. These results may relate to the iron preferential occupancy for the Cu(1) site at the lower doping level, and for Cu(2) sites at the higher doping level. 相似文献
12.
S.-L. Drechsler H. Rosner J. M. An W. E. Pickett V. D. P. Servedio T. Mishonov E. Forzani K. Winzer 《Journal of Low Temperature Physics》2003,131(5-6):1175-1180
We compare full potential LDA band calculations of the Fermi surfaces areas and band masses of MgB2 and ZrB2 previously reported and new dHvA data. Discrepancies in areas in MgB2 can be removed by a small shift of bands relative to bands. Comparison of effective masses lead to orbit averaged el-ph coupling constants =1.3 and =0.5, whereas for ZrB2 only weak el-ph coupling with <0.3 is found. The ARPES data can be also well described by the LDA showing the presence of surface states. 相似文献
13.
S. N. Heavens 《Journal of Materials Science》1984,19(7):2223-2232
Changes in the phase content, microstructure and lattice parameter are observed in stabilized/ alumina specimens following extended sintering and annealing treatments. The resulting state is dependent on composition of the starting powder and on temperature and duration of heat treatment; the kinetics of transformation between and alumina are generally slow and certain/ ceramics remain in a metastable state even after a prolonged high temperature anneal. Following post-sinter heat treatment, splitting of X-ray diffraction peaks reveals a segregation of the phase into two components of differing lattice parameter. With sintering schedules of a long duration the splitting may even be present in the as-fired condition as recently reported by Harbach [1]. The splitting is attributed to a structural change resulting from the expulsion of Na2O from supersaturated grains. 相似文献
14.
The field dependence of real () and imaginary () component of ac susceptibility of superconductors within the critical state model can conveniently be used for evaluating the critical current when field amplitude is larger than the penetration field. A method to analyze the real () and imaginary () component of fundamental ac susceptibility with the objective of extracting the temperature dependence of critical current density J
c(T) is reported. The procedure makes use of the ac susceptibility data of two polycrystalline (Bi-Pb)-2223 samples measured with different excitation amplitudes below and close to the critical temperature. The temperature dependence of J
c is extracted using the isothermal scan over and data. Results obtained from this procedure are found to be in fair agreement with J
c(T) calculated from traditional loss-maximum data. 相似文献
15.
K. A. Moler D. J. Baar Ruixing Liang W. N. Hardy A. Kapitulnik 《Journal of Superconductivity》1995,8(5):571-574
The total specific heat of YBa2Cu3O6.95 single crystals includes contributions from phonons and spin-1/2 particles, as well as electronic contributions. The electronic specific heat is described by a quadratic term T2 in zero field and a linear term [(0)+(H)]T which is increased when a magnetic field H is applied perpendicular to the CuO2 planes. In agreement with d-wave superconductivity, we find that n/Tc and (H)n(H/Hc2)1/2, where n is the coefficient of the normal-state linear term. The H1/2 dependence of the density of states at the Fermi level was predicted by G. Volovik for lines of nodes in the gap: the quasiparticles which contribute to this density of states are close to the nodes in momentum space and are located outside the vortex core. 相似文献
16.
We present data on the copper isotope effect (63Cu-65Cu), Cu =-nTc/nmCu, for two isotopic pairs of oxygen-deficient YBa2Cu3O7–, where varies between 0.06 and 0.52. Cu is below 0.01 at =0.06 (fully oxygenated), it takes values between –0.14 and –0.34 in the 60 K plateau. Larger negative values of Cu are observed away from the plateau. The dependence of Cu is similar to that of the pressure effect dnTc/dP. 相似文献
17.
The thermally activated flux motion in transport-current-carrying epitaxial Bi2Sr2Ca2Cu3O10+ thin films was investigated by conventional resistive measurements, with the magnetic fieldB applied parallel to thec-axis and ranging between 10 mT and 3 T. It was found that the magnetic field and temperature dependence of the activation energy, as well as the form of the irreversibility line in the thermally assisted flux flow regime, change significantly at a field valueB
cr0.1 T. This behavior is similar to that reported by us in the case of Bi2Sr2Ca2Cu3O10+ thin films, and was interpreted in terms of the occurrence of a dimensional crossover in the vortex system, due to the increase of the tilt modulus at low fields. We determined an anisotropy factor 140 for our Bi2Sr2Ca2Cu3O10+ films, which is lower than that observed for the Bi2Sr2CaCu2O8– films. 相似文献
18.
The erosion properties of brittle materials depend upon plastic deformation and crack generation at an impact or indented site. Vickers indentations have been used to investigate the plastic processes and crack systems in chemical vapour deposited zinc sulphide of different grain sizes. The hardness,H, and the local fracture toughnessK
c, are dependent upon the grain size of the material. For small grain size material (<50 m) the Vickers hardness was found to increase with decreasing grain size in accord with the Petch mechanism, i.e.H=H
0 +kd
–1/2 wherek andH
0 are constants andd is the grain diameter. A maximum hardness of ca. 4 GPa has been observed for material with an average 0.5 m grain diameter. In large grain size material, hardness anisotropy within the grains causes significant experimental scatter in the hardness measurements because the plastic impression formed by the indenter (load 10 N and 100 N) is smaller than the grain diameter. The values ofK
c obtained using an indentation technique show that for grain sizes less than 8 mK
c decreases with decreasing grain size. For materials with a grain size in the range 500 m to 8 m, well developed median cracks were not observed, however, the radius of the fracture zone was measured in order to estimate an effectiveK
c. The effectiveK
c was found to increase approximately linearly with the reciprocal root of the grain size. Consideration of the models for elastic/plastic impact and micromechanics of crack nucleation in conjuction with the variation ofK
c andH, indicate that zinc sulphide with a mean grain size of 8 m will give the optimum solid particle and rain erosion resistance. 相似文献
19.
The manner in which oxygen is incorporated into YBa2Cu3O
x
(YBCO) at 800°C for values ofx close to 6 is shown to be in the form of neutral oxygen interstitials, O
i
x
. The experimental data on which this conclusion is based are obtained from measurements of oxygen partial pressure,P(O2), as a function of compositionx and temperatureT (5.99x 6.35, 825T1120 K). The data are obtained by a solid-state electrochemical method. Other conclusions of this study include: (a) O
i
x
are noninteracting forx 6. (b) The stoichiometric composition of YBCO isx 6.0. (c) The reaction enthalpy of oxidation is 179 kJ/mol O2. (d) The Fermi level changes by –0.2 eV asx increases from 6.05 to 6.35. 相似文献
20.
J. Halbritter 《Journal of Superconductivity》1992,5(2):171-180
Surface impedance measurements in the normal and superconducting state are an excellent method to study the conduction electron dynamics in metals. This holds especially in the relaxation range, i.e., for distances traveled in one r.f. periods=
F/(f is the Fermi velocity) being smaller or of the order of the penetration depth and mean free pathl. For materials with
F<-107 cm/sec the relaxation range is easily accessible forf>0.1 THz. Then, in the normal state, relaxation defines the surface impedance with a penetration depth approaching the London penetration depth L, andR
0l/2 as surface resistance allowing a measure of L and relaxation time (T, ). In the superconducting state the photon interaction scales with
F/L=l/ (
f is the dimension of Cooper pairs for l) and causes at low frequencies an absorption rate growing with, which is decreasing with
F/l. The rate increase proportional to turns to a decrease above 0.1 THz, being accompanied by a decrease of with frequency which is stronger for large and small
F/l. These characteristic dependences allow a measurement of material parameters, anisotropy, and dynamics of electrons. To evaluate the consequences of theâ, b, and-direction anisotropy, the integral kernels for normal and superconducting surface impedances in its nonintegrated, angle-dependent form are presented, analyzed, and compared with impedance measurements above 0.1 THz of YBa2Cu3Ox. 相似文献