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本文提出一个非晶态非过渡金属超导体的Tc经验公式,Tc=Aλ<ω>1/2/(<ω>/ω0+(1+λ)/20),式中A=(1/5)(K1/2)。计算值和实验值,以及和Garland理论值的比较表明,Tc经验公式能很好地描述非晶态超导体的Tc值。
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本文报道,通过对Y1-xCaxBa2Cu3-xMxO7-δ(M=Fe,Ni)体系样品的晶体结构、氧含量、正常态电阻率与温度的关系,以及超导转变温度等测量,并与YBa2Cu3-xMxO7-δ(M=Fe,Ni)体系进行比较,发现Y1-xCaxBa2Cu3-xFexO7-δ体系的Tc显著地高于相应x值的YBa2Cu3-xFexO7-δ体系,而Y1-xCaxBa2Cu3-xNixO7-δ体系则相反,Tc低于仅Ni替代的体系,表明Ca和Fe同时替代时两者引起的载流子浓度(nH)变化相互补偿,抑制了仅Fe替代时引起的nH和Tc急剧下降;而作Ca和Ni同时替代时主要的不是两者引起载流子浓度变化的相互补偿,Ca和Ni替代效应之间的关联较弱。作者认为,对Y1-xCaxBa2Cu3-xFexO7-δ体系属于CuO2平面外的元素替代,这时载流子浓度是决定Tc的主要因素;而对Y1-xCaxBa2Cu3-xNixO7-δ体系,由于Ni2+离子主要占据Cu(Ⅱ)位,它导致磁拆对效应,Ni2+离子的拆对效应是引起Tc下降的直接原因。
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本文用数值解方法从Eliashberg方程计算出超导临界温度Tc,并考察Tc对有效声子谱的依赖关系。在这个研究中,a2F(ω)被取为双δ函数谱,并允许其中的谱参数可以在很宽范围内改变。作者发现在λ<∧区域(即在Tc级数解的收敛圆外),Tc除了依赖λ和矩比外,还依赖Tc级数解的收敛半径倒数Λ;它们之间的关系是有规律的。在这些结果的启示下,本文在μ*=0情形,用弥合数值解的方法得到一个适用于λ<Λ区域的Tc近似公式。接着,本文作者对吉光达和吴杭生的一篇文章进行了研究,指出:该文提出的超导体分类建议及其工作的主要结论是对的。但其中对决定A型超导体临界温度主要参量问题进行的分析,只适用于这样一些A型超导体,它们的收敛半径倒数Λ或者比λ0小,或者虽比λ0大、但λ又小于λ0,其中λ0是个依赖谱形状的参量,它的定义在正文中给出。对另一些A型超导体(λ0<λ<Λ),决定Tc的主要参量不再是λ,而是δ=1/∧0.5(ω1/2/ωlog)5.5λ1.55。
关键词: 相似文献
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Abstract Here we report a theoretical calculation of the band structure and superconductivity of Se in the bcc phase. The energy band structure and the effect of pressure on the band structure is obtained by means of the Linear Muffin-Tin Orbital method within the atomic sphere approximation. The superconducting transition temperature (Tc) is calculated using McMillan's formula and we predict the value of Tc at 115.3 Gpa as 2.3 K. Further increase in presssure decreases the Tc values. The normal state electrical resistivity at 115.3 Gpa is 1.43 fl cm, with further increase in pressure the resistivity decreases, which is a typical behaviour of number of elemental metals under pressure. 相似文献
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E. Torikai K. Nagamine K. Nishiyama E. Hirose Y. Ikedo T. Hashimoto P. Birrer I. Tanaka H. Kojima S. Srinivas T.P. Das S. Maekawa K. Yamada S. Hosoya Y. Endoh 《Hyperfine Interactions》1997,105(1-4):175-179
The paramagnetic (μ-O) state formed at the oxygen site in high Tc LaSrCuO was used to probe an interaction between the localized moment of the paramagnetic electron and the conduction electrons
via zero field μ- spin relaxation. The enhanced relaxation rates consistently observed in the superconducting state of various Sr concentrations
are explained as an effect of spin‐pairing in the high Tc supercurrent.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
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The thermal conductivity of the ferromagnetic insulator K2CuCl4 · 2H2O has been measured near its Curie temperature Tc. The measurements were made as a function of temperature in constant external magnetic field and as a function of field along isotherms. The results indicate a relaxation rate for magnetic critical scattering of phonons varies as H?1/2. 相似文献
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I. Shirotani S. Yoshida Y. Wada T. Kajiwara M. Murakami S. Yoshimura 《Solid State Communications》1981,38(7):653-656
The electrical resistivity of N-n-propylpyridinium-TCNQ2 (NPPy-TCNQ2) and N-n-butylpyridinium-TCNQn (NBPy-TCNQn) has been measured as a function of temperature and pressure. Phase transitions in these salts have been studied at high pressures. The transition temperature (Tc) in NPPy-TCNQ2 at atmospheric pressure increased with increasing pressure at the rate of dTc/dP = + 12.0 degkbar?1. The value of volume change calculated from the Clapeylon-Clausius relation was + 4.4 cm3 mol?1. The electrical resistivity along the a- and c-axis increased with increasing pressure below 7 kbar. This anomalous electrical behaviour is closely related to the crystal structure of NPPy-TCNQ2. The resistivity dropped sharply at about 11 kbar. This abrupt change may be due to a new pressure induced phase transition.The Tc of the NBPy-TCNQn increased remarkably with increasing pressure up to 0.7 kbar, above which the phase transition disappeared. The phase transitions of N-n-alkyl-substituted pyridinium TCNQ salts depend strongly on the nature of cations. 相似文献
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C.R. Pasquier N. Kang B. Salameh P. Auban-Senzier D. Jérome S. Brazovskii 《Physica B: Condensed Matter》2012,407(11):1806-1809
We report an investigation at the endpoint region of the spin density wave state in (TMTSF)2PF6 where metal and superconductivity emerge. Thanks to resistivity measurements along the three main crystallographic directions, we are able to follow the texture in this phase coexistence regime. In this respect, superconductivity is used as a decoration technique of the metallic pattern. We show that metal (superconductivity) emerges first along the c? direction in a counterintuitive manner. Then metal (superconductivity) domains evolves from filaments along the c? axis towards slabs perpendicular to the a-axis which melt together in the homogeneous phase at high pressure. This evolution is compatible with the proposition of the formation of a soliton phase in the vicinity of the critical pressure of the (TMTSF)2PF6 phase diagram. 相似文献
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《Solid State Ionics》2006,177(19-25):1883-1886
We have carried out the electrical conductivity and NMR measurements and investigated the characteristic features of the electrical conductivity in a paraelastic phase of Tl2SeO4. It was found from electrical conductivity measurement that the activation energy in the paraelastic phase (above Tc(= 661 K)) is 0.98 eV. We also found that 205Tl-NMR line width drastically decreases above Tc and becomes approximately 0.5 Gauss. This result indicates that in the paraelastic phase the mobile Tl ions exist. Moreover this result is consistent with the existence of the anomalously large hopping motion of Tl ions observed in X-ray diffraction measurement. Furthermore the activation energy estimated from the motional narrowing of 205Tl-NMR absorption line is 0.92 eV and is in agreement with that obtained from the conductivity measurement. From these results, it is deduced that the mobile Tl ions play an important role in the appearance of electrical conductivity in the paraelastic phase. 相似文献
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《Current Applied Physics》2005,5(3):254-258
The normal state of magnesium diboride, the simplest metallic system showing the macroscopic quantum coherence at high temperature (high Tc superconductivity––HTcS) shows some features that are common with the cuprates, shedding light on the pairing mechanism for Tc amplification in HTcS: (1) two or more electronic components associated with multiple Fermi surface portions; (2) crossing of electronic topological transitions (ETT) where the Fermi surface changes its topology and (3) phonon softening in extended Kohn anomalies (EKA) over a large wave-vector range. 相似文献
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D. R. Mishra 《Pramana》2008,70(3):535-541
The effects of gadolinium doping at calcium site on the normal and superconducting properties of Bi-2223 system were studied.
The Gd-doped (BiPb)-2223 series of specimens, namely Batch I, II and III were sintered at three different sintering temperatures
830, 850 and 895°C respectively. The properties investigated are (1) the normal state resistivity with a view to study metal-to-insulator
transition, (2) the XRD patterns of the specimens with a view to study the relative composition of (BiPb)-2212 and (BiPb)-2223
phases and (3) the superconducting fluctuation behaviour (SFB) with a view to determine the effect of doping, if any, on the
dimensionality of the fluctuation conductivity of the system. The normal state resistivity of Gd-substituted Bi-2223 specimens
shows metallic, semiconducting and insulating behaviour. The T
c
(R = 0) values indicate that (BiPb)-2223 phase is responsible for the observed superconducting transitions in Batch I and Batch
II specimens with Gd concentrations x ≤ 0.7. This observation is further confirmed in the analysis of XRD patterns of these specimens. Gadolinium, being a magnetic
impurity, has pair breaking effect near the Fermi level and decreases T
c
(R = 0). The analysis of the superconducting fluctuation behaviour (SFB) shows a 2D dimensionality without any cross-over.
Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users. 相似文献
Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users. 相似文献
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Hartwig Schmidt 《Zeitschrift für Physik A Hadrons and Nuclei》1970,232(5):443-451
Thermal fluctuations of the order parameter in the superconducting state are investigated, particularly near the transition temperature, using the time dependent Ginzburg-Landau theory. These fluctuations give rise to a contribution of the dynamical conductivity, which strongly increases as the temperature is raised towardsT c , in contrast to the temperature dependence of the electromagnetic response due to the static order parameter. At the transition temperature this contribution joins continuously to the extra response (conductivity, susceptibility) which has been calculated and observed in the temperature region aboveT c , where it represents the onset of superconductivity in the normal state. Particularly the dynamical conductivity due to fluctuations is calculated and discussed for bulk material, thin films and thin wires belowT c . The temperature and frequency dependence should be observable in microwave experiments. 相似文献