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1.
本文研究了C-15结构化合物(Zr0.5Hf0.5-xTax)V2(x≤0.2)和(Hf0.5Zr0.5-xNbx)V2(x≤0.2)的超导转变温度Tc及其压力效应?Tc/?P。报道了实验方法与结果。与(Hf1-xTax)V2和(Zr1-xTax)V2的情况不同,在(Zr0.5Hf0.5-xTax)V2中Ta的引入使常压下的Tc下降,然而(?)Tc/(?)P却大为提高。因此高压下Zr0.5Hf0.45Ta0.05V2的Tc反而比Hf0.5Zr0.5V2更高。导出了一个基于角动量分波表象能带论方法的描述压力效应的新关系式。它指出导带电子波函数中的高角动量成分变化对Tc的压力效应影响比较重要。这个公式有助于理解上述复杂的实验现象,并能合理地解释某些元素(如Cs,Ba,La等)的Tc随压力剧增的事实。
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2.
INTERNAL FRICTION AND ULTRASONIC ATTENUATION RELATED TO CARRIERS IN HIGH Tc SUPERCONDUCTORS
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For both BiSrCaCuO and TlBaCaCuO samples, internal frictions (Q-1) in the kHz range reveal a plateau (Q-1p) above Tc and a rapid drop below Tc with the turning points located just at Tc for various samples with different Tc. This anomaly cannot be observed in non-superconducting samples. The ultrasonic attenuation (α) in TlBaCaCuO displays similar results to the internal friction. Moreover, it is discovered that the Q-1p is nearly proportional to the carrier density for Y(Pr)BaCuO and Gd(Pr)BaCuO with different Pr contents. These results show that the drop of Q-1 and a below Tc is closely related to superconducting condensation. The Q-1p and αp can be explained using coupling-model of carriers with local dynamic distortion because high Tc superconductors are strong-correlation systems. Furthermore, by taking account of the smearing of superconducting gap structure resulting from the recombination of quasi-particles and by modifying the BCS relative jump rate as S(E, E', Γ) = Re{1-Δ2/[(E-iΓ)(E'-iΓ)]}, the calculated results of internal friction and ultrasonic attenuation below Tc are in good agreement with the experimental data. The superconducting gap Δ and the damping rate Γ for both BiSrCaCuO and TlBaCaCuO have also been obtained, they are in accordance with those got by tunneling spectrum and NMR methods, etc. 相似文献
3.
Explicit expressions and results for Tc(n) of the layer compounds of Bi and T1 are obtained within the generalized soliton pairing framework, in which Tc, is raised by the cooperation l i of the two couplings, intralayer and interlayer. The calculated values are in good agreement I with the observed Tc's. The maximum possible Tc(∞) is predicted at≈ 200 K. 相似文献
4.
ZHANG Li-yuan 《理论物理通讯》1989,12(1):27-32
Using the free carrier-negative U center interacting model, we have investigated the superconducting transitionr,temperature Tc, the specific heat jump at Tc, the coherence length, the penetration depth, the thermodynamic critical field and the isotope effect for the high-Tc oxide superconductor. The theoretical values of these physical quantities are consistent with the experimental values. Further, the residual electronic specific heat in the superconducting state is explained as the result of the inherent instability in this superconductive mechanism. 相似文献
5.
We describe the molecular ferromagnet decamethylchromocenium tetracyanoethanide [CrCp2*] [TCNE] by a quasi-one-dimensional alternating-spin model with ferromagnetic exchange interaction. The spin-wave theory is used to discuss the magnetic excitation of the molecular ferromagnet. We obtain the magnetizations corresponding to different external magnetic fields without interchain exchange interaction which is consistent with the experimental results. The formula of the Curie temperature Tc is obtained by using the thermodynamic Green's function method. It is found to be related to the spin of donors, to the intrachain exchange interaction and to the spatial anisotropic parameter. The result obtained by this Tc formula is consistent with the experimental result Tc = 3.6 K for [CrCp2*][TCNE]. 相似文献
6.
The variational-cumulant expansion (VCE)methodis used to calculate the quantum XY model, for which the trial action S0 does not commute with the action S, with arbitrary magnitude of spin on various one-, twe and three-dimensional lattices. The free energy of the model is expanded to the third order and the analytical formulae of critical temperature Tc are given for each order. To testify this method in a wider range, the Heisenberg model on different lattices is also calculated, which has more comparable results. Weighted fitting method of statistical mathematics is adopted to predict Tc of infinite order. 相似文献
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The determination of Tc in dilute alloys is examined. As an example, magnetization vs field data in a Pd0.95Gd0.05 alloy are presented. For this alloy, it was not possible to determine a value of Tc using Arrott-Belov-Kouvel (ABK) plots; the conditions for the applicability of ABK plots were not satisfied. Results of very-low-field ac and dc magnetization vs T, which depend mainly on the demagnetization factor, yield a more precise measure of Tc. We conclude that the latter procedures are more useful than ABK plots for determination of Tc in dilute alloys. 相似文献
9.
PREPARATION,SUPERCONDUCTIVITY AND STRUCTURE OF THE LaBaCaCu3O7-x SUPERCONDUCTORS WITH Tc HIGHER THAN 80 K
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In this paper, the preparation process of the tetragonal LaBaCaCu3O7-x (LBCCO) superconductor with Tc=84 K is repored. The superconductivity and structure of the LBCCO superconducting phase are investigated by means of X-ray diffraction (XRD), transmission electron microscope(TEM) and superconductivity measurements, The results indicate that the superconducting phase with Tc=82K in the LBCCO system has a tetragonal structure with a=0.3966nm, c=l.1838nm and space group P4/mmm. About 50% of the Ca and La atoms are located in 1d sites (i.e. Y site in the YBCO structure). It is confirmed that there is in fact no orthorhombic to tetragonal transition in this material. The relationship between the oxygen content and Tc is also presented. 相似文献
10.
通过对FeSe进行化学插层可以将其超导转变温度(Tc)从约8 K提高到40 K以上,实现高温超导电性.最近,我们对两种插层FeSe高温超导材料(Li0.84Fe0.16)OHFe0.98Se和Li0.36(NH3)yFe2Se2开展了高压调控研究,发现压力会首先抑制高温超导相(称为SC-I相),然后在临界压力Pc以上诱导出第二个高温超导相(称为SC-Ⅱ相),呈现出双拱形T-P超导相图.这两个体系的Pc分别约为5和2 GPa,两个体系SC-Ⅱ相的最高Tc分别可以达到约52和55 K,比相应SC-I相的初始Tc提高了10 K.对(Li0.84Fe0.16)OHFe0.98Se的正常态电输运性质分析表明,SC-I和SC-Ⅱ相的正常态分别具有费米液体和非费米液体行为,意味着这两个超导相可能存在显著差异.此外,还发现这两个体系的SC-Ⅱ相的Tc与霍尔系数倒数1/RH(∝载流子浓度ne)具有很好的线性依赖关系.对(Li0.84Fe0.16)OHFe0.98Se的高压X射线衍射测量排除了其在10 GPa以内发生结构相变的可能,因此Pc以上SC-Ⅱ相的出现和载流子浓度的增加很可能起源于压力导致的费米面重构. 相似文献
11.
In a previous paper we established a set of nonlocal Ginzburg-Landan equations near Tc. In this paper this set of equations is generalized to the cases well below Tc. the generalization is characterized by two new kernel functions which can be reduced to a similar form aa that obtained previously. The form factor resulted from first principle calculation is in good agreement with neutron scattering data. 相似文献
12.
By using hopping parameter expansion and cumulant expansion, as an example, we calculate analytically the effect of fermion determinant to average plaquette energy < Ep > up to k6 order. The corrections of the fermion determinant to < Ep > are controllable small in our approximation. 相似文献
13.
Fn the present paper, the linear Eliashberg equation is converted into a new integral equation and from it, the following TC formula is derived analytically in the weak coupling limit:Tc=28/πωlog·(πlog/πc)μ*/λ*-μ* exp{-1+λ/λ-μ*}, where In γ=C=0.5772 is the Eater constant. 相似文献
14.
ZHOU BangRong 《理论物理通讯》2000,33(3):451-456
The discrete symmetry breaking and possible restoration at finite temperature T are analyzed in 20 Gross-Neveu model by the real-time thermal field theory in the fermion bubble approximation. The dynamical fermion mass m is proven to, be scale-independent and this fact indicates the equivalence between the fermion bubble diagram approximation and the meanfield approximation used in the auxiliary scalar field approach. Reproducing of the nonzero critical temperature Tc = 0.567m(O), m(0) is the dynamical fermion mass at T = 0, shows the equivalence between the real-time and the imaginary-time thermal. field theories in this problem. However, in the real-time formalism, more results including absence of scalar bound state, the equation of criticality curve of chemical potential-temperature and the ln(Tc/T) behavior of m2 at T ≤ Tc can be easily obtained. The last one indicates the second-order phase transition feature of the symmetry restoration. 相似文献
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Measurements of upper critical field as a function of temperature and Fe-concentration are reported for the amorphous superconductors ((Mo.6Ru.4).80G.20)1−cFec, where G is a glass-former. The linear shapes of the Hc2 vs. T curves near Tc agree well with predictions made by Matsura, Ichinose, and Nagaoka, for TKondo > Tc. 相似文献
17.
The standard variational-cumulant expansion (VC) combined with a scheme for the enumeration of connected diagrams is employed to study the simple Ising model with the second-order phase transition. This model is evaluated to the 5th order approximations. A comparison with the 2-dimensional exact results shows that thermodynamic quantities converge rather slowly to the exact values in the intermediate region. A strict analytical calculation of the critical temperature with the 5th order approxinlation both for Ji and cumulant expansion suggests that VC chn achieve rather high standard accuracy if the variational parameters Ji could be determined up to higher degrees of corrections rather than the first one. Anomalies are found for U, C and M at extremely low temperature. A detailed study of M implies that thermodynamic formulae should be carefully chosen in VC approach so as to avoid these anomalies. 相似文献
18.
在铁基超导体中,FeSe具有最简单的晶体结构和化学组成,而且其超导转变温度具有较大的调控空间,因此适合作为超导机理研究和应用的载体.高质量样品的研制是物性研究和器件应用的前提,本文系统地研究了利用激光脉冲沉积技术制备FeSe薄膜的工艺条件,在多种衬底上成功地制备出高质量的β-FeSe薄膜,并首次实现了超导临界转变温度从小于2 K到14 K的连续调控,这为FeSe超导机理研究提供了样品支持.为探究FeSe薄膜超导电性变化的起因,从β-FeSe超导电性与晶格常数c正相关出发,基于简单的费米面填充假设,第一性原理计算可以很好地解释晶格常数c的变化规律,但该假设并不能完全符合角分辨光电子能谱实验给出的电子结构演变过程.因此β-FeSe薄膜的超导电性、晶格结构和电子结构三者之间的关系还有待澄清,该问题的解决将为FeSe超导机理研究提供重要的线索,而上述系列高质量的β-FeSe薄膜样品恰好能为该问题的研究提供理想的载体.本文根据实验和已有的相关研究结果,详细介绍了FeSe薄膜的脉冲激光沉积制备及其优化,以期为后续的薄膜研究应用提供参考. 相似文献
19.
ZHOU BangRong 《理论物理通讯》1999,32(3):425-434
The gap equation at finite temperature in the top-quark condensate scheme of electroweak symmetry breaking is proven to have the identical form in both the imaginary and real time formalisms of thermal field theory. By means of the gap equation, combined with the basic relation to define the vacuum expectation value v of the effective Higgs field, we analyze the dependence on temperature T and chemical potential p of the dynamical top-quark mass as the order parameter characteristic of symmetry breaking, and obtain the p-T criticality curve for symmetry restoration. We find out that the critical temperature Tc = 2υ for μ = 0 and the critical chemical potential μc = 2 π υ / √3 for T = 0. When μ = 0, the top-quark mass near Tc has the leading (Tc2 - T2)1/2 behavior with an extra factor dependent on temperature T and the momentum cutoff Λ. However, it is generally argued that the symmetry restoration at T ≥ Tc is still a second-order phase transition. 相似文献
20.
在还原气氛保护下利用高温固相法合成了化学组分为(M1,M2)10(PO4)6X2(M1=Ca,Sr,Ba;M2=Eu,Mn;X=F,Cl,Br)的可被紫光激发的蓝光、绿光和红光荧光粉,制备了紫光LED芯片+蓝光荧光粉+YAG荧光粉的二基色白光LED;紫光LED芯片+蓝光荧光粉+红光荧光粉的二基色白光LED,以及紫光LED芯片+蓝光荧光粉+绿光荧光粉+红光荧光粉的三基色白光LED。测试了所有制备的白光LED在不同的直流电驱动下的色度坐标、相关色温和显色指数。 相似文献