超导材料NbS2上临界磁场的理论分析

根据两带Ginzburg-Landau 理论计算了层状超导材料NbS₂的上临界磁场, 以及上临界磁场各向异性参数随温度的变化情况, 并将NbS₂与MgB₂, NbSe₂上临界磁场的各向异性进行比较. 所得计算结果与已有实验数据符合得很好, 充分说明了NbS₂的超导电性具有两能隙特征. NbS₂上临界磁场各向异性参数在5.0 K附近逐渐变小, 这与MgB₂和NbSe₂有相似之处. 但NbS₂的上临界磁场各向异性参数大约为7.3, 明显大于MgB₂和NbSe₂. 计算结果还表明, NbS₂较大能隙所对应能带的有效质量比约为54, 另一能带的有效质量基本为各向同性.     

Fluctuation conductivity in superconducting MgB2

According to the crystal structure of MgB₂ and band structure calculations, quasi-two-dimensional (2D) boron planes are responsible for the superconductivity. We report on critical-field and resistance measurements of 5.6-μm-thick MgB₂ films grown on a sapphire single-crystal substrate. Resistivity measurements yield a temperature dependence of the fluctuation conductivity above the critical temperature, which agrees with the Aslamazov-Larkin and Maki-Thompson theory of fluctuations in layered superconductors, indicating a quasi-two-dimensional nucleation of superconductivity in MgB₂.     

Temperature dependence of transmittance and effective surface resistance of MgB2 film

Using the Kubo formula approach and Green's function method, the temperature dependence of the transmittance and effective surface resistance of high quality MgB₂ film are calculated in the framework of the two-band model. For large interval of temperatures below T_C our results are in agreement with experimental data. We show a single-gap model based on BCS theory, which is insufficient to understand such quantities, but a two-band model with different symmetries can describes the experimental data rather well. In the two-gap model we consider that the transmittance and effective surface resistance are a weighted sum of the contributions from σ and π bonds and hybridization between them is negligible.     

Anisotropy of the upper critical field in MgB<Subscript>2</Subscript>: The two-band Ginzburg-Landau theory

The temperature dependence of the anisotropy parameter of the upper critical field \(\gamma _{H_{c2} } (T) = H_{c2}^\parallel (T)/H_{c2}^ \bot (T)\) is calculated using the two-band Ginzburg-Landau theory for layered superconductors. It is shown that the anisotropy parameter γ(T) increases with decreasing temperature. The results of the calculations are in agreement with experimental data for single crystals of MgB₂ and with other calculations.     

WEAK QUANTUM FLUX CREEP AND STRONG PINNING IN THE NEW SUPERCONDUCTOR MgB2

The newly discovered superconductor MgB₂ has a transition temperature T_c of about 40 K, which is touching the upper line of that predicted by the phonon-mediated Bardeen--Cooper--Schrieffer (BCS) theory. It is interesting to investigate the flux creep in MgB₂ and compare it with other superconductors. We have measured the magnetization relaxation of MgB₂ sintered at high temperature and high pressure. It is found that the quantum tunnelling and the thermally activated flux creep are very weak, implying a strong pinning in this material.     

Influence of shock-wave pressure up to 65 GPa on the crystal structure and superconducting properties of MgB2

The influence of shock-wave pressure treatment up to 65 GPa on the crystal structure and the superconducting transition temperature of a polycrystalline MgB₂ sample has been investigated. X-ray diffraction measurements have revealed that the shock-wave pressure does not result in any irreversible structural phase transitions in the MgB₂, except for microdistortions formed in the crystal structure of the shock-wave pressure-treated MgB₂ sample. This conclusion is in agreement with the results of superconducting transition temperature measurements of a MgB₂ sample performed before and after its shock-wave pressure treatment.     

Critical temperature of two-band superconductors in intermediate e-ph coupling: The case of MgB2

Critical temperature of two-band superconductors is studied using Eliashberg theory in approach of intermediate e-ph coupling. The results are shown to be in qualitative agreement with experimental data for the superconducting magnesium diboride, MgB₂.     

Excess current in point contacts on two-band superconductor MgB2 in magnetic field

A series of I(V) characteristics and bias-dependent differential resistance dV/dI(V) curves of point contacts made between a single crystal of two-band superconductor MgB₂ and Cu were measured in magnetic fields up to 9 T. The magnetic field dependences of the excess current in the I(V) curves were obtained and analyzed using Koshelev and Golubov's [Phys. Rev. Lett. 90, 177002 (2003)] theoretical results for the mixed state of a dirty two-band superconductor. Introducing a simple model for the excess current in the point contact in the mixed state, our data can be qualitatively described using the theoretical magnetic field dependence of the superconducting order parameter of the σ and π-bands and the averaged electronic density of states in MgB₂.     

利用电泳法在金属基底上制备MgB2超导厚膜

利用电泳技术在高熔点金属基底Ta，Mo和W上制备MgB₂超导厚膜.厚膜中的MgB2晶粒结合紧密，粒度小于1μm，呈随机取向生长.电阻测量表明沉积在Ta，Mo，W上的MgB₂厚膜的超导起始转变温度分别为36.5K，34.8K，33.4K，对应的转变宽度为0.3K，1.5K和2.0K.三种基底上制备的MgB₂厚膜的临界电流密度在不同温度下随外磁场的变化情况 基本相同，MgB₂/Mo厚膜的临界电流密 关键词： 2超导厚膜')" href="#">MgB₂超导厚膜 电泳 金属基底     

Effective action approach to the Leggett's mode in two-band superconductors

We investigate a collective excitation (Leggett's mode) corresponding to small fluctuations of the relative phase of two condensates in two-band superconductor using the effective “phase only” action. We consider the possibility of observing Leggett's mode in MgB₂ superconductor and conclude that for the known at present values of the two-band model parameters for MgB₂ Leggett's mode arises above the two-particle threshold. Received 10 May 2002 Published online 19 November 2002     

Interrelation of the critical magnetic field H c2 and the residual resistivity for the two-band superconductor MgB2

An expression that describes the upper critical magnetic field H _c2 and generalizes the Gor’kov relation has been derived for the two-band two-gap superconductor MgB₂. The expression relates the upper critical magnetic field H _c2 to the residual resistivity and the parameters of the band structure and holds in the range from the clean limit to the dirty limit. The ratios of the relaxation times τ_π/τ_σ and the mean free paths of π- and σ-band electrons for MgB₂ samples with a low defect level and Mg(B_{1 ? x} C _x )₂ samples with a partial substitution of carbon for boron are determined from experimental data.     

Extended Ginzburg-Landau formalism for two-band superconductors

Recent observation of unusual vortex patterns in MgB(2) single crystals raised speculations about possible "type-1.5" superconductivity in two-band materials, mixing the properties of both type-I and type-II superconductors. However, the strict application of the standard two-band Ginzburg-Landau (GL) theory results in simply proportional order parameters of the two bands-and does not support the "type-1.5" behavior. Here we derive the extended GL formalism (accounting all terms of the next order over the small τ=1-T/T(c) parameter) for a two-band clean s-wave superconductor and show that the two condensates generally have different spatial scales, with the difference disappearing only in the limit T→T(c). The extended version of the two-band GL formalism improves the validity of GL theory below T(c) and suggests revisiting the earlier calculations based on the standard model.     

Bulk MgB2 superconductor with high critical current density synthesized by self-propagating high-temperature synthesis method

Pure MgBMgB2 超导体 临界密度 自我传播 预热温度 超导电性SHS method, bulk MgB₂ superconductor, superconductivityProject supported by the Natural Science Foundation of Gansu province of China (Grant No ZS032-B25-019).2005-03-187/2/2005 12:00:00 AMPure MgB2 bulk samples are successfully synthesized by self-propagatlng hlgh-temperature synthesis （SHS） method. The experiments show that the best preheating temperature is 250℃, the highest Jc values of the prepared MgB2 reach 1.5×10^6A/cm^2 （10K, 0.5T） and 1.7×10^6A/cm^2 （20K, 0T）, and the MgB2 particle sizes range from 2 to 5μm. The advantages of this method are that it is simple, economical and suitable for the manufacture of bulk MgB2 materials on industrial scale.     

MgB2混合态热导率的反常增强

测量了MgB₂多晶样品的混合态热导率，磁场强度为0—7 T，温度范围为5—45 K .实验结果显示MgB₂热导率在低场下迅速上升，高场下趋于饱和，这与MgB₂的 二能隙电子结构有关.对实验结果的分析指出，低温强场下MgB₂多晶样品热导率的显著增强无法完全 用电子热导来解释，并对此进行了讨论. 关键词： 2')" href="#">MgB₂ 热导率 混合态     

STRUCTURE ANALYSIS OF NEW SUPERCONDUCTOR MgB2

The superconductivity of a sintered pellet of the newly discovered MgB₂ superconductor has been studied with magnetic measurements and its crystal structure was analysed using the Rietveld method of powder X-ray patterns. It has hexagonal symmetry (S.G. P6/mmm) with unit cell lattice parameters a=0.308136(14)nm and c=0.351782(17)nm.     

Two regimes in the magnetic field response of superconducting MgB<Subscript>2</Subscript>

Using Scanning Tunneling Microscope at low temperature we explore the superconducting phase diagram in the π-band of the two-band superconductor MgB₂. In this band the peculiar shape of the local tunneling spectra and their dynamics in the magnetic field reveal the complex character of the quasiparticle density of states (DOS). The gap in the DOS is first rapidly filled with states in raising the magnetic field up to 0.5 T and then slowly approaches the normal state value: the gap is observed up to 2 T. Such a change in the DOS dynamics suggests the existence of two terms in the DOS of the π-band: a first one, reflecting an intrinsic superconductivity in the band and a second one, originating from an inter-band coupling to the σ-band. Our findings allow a deeper understanding of the unique phase diagram of MgB₂.     

高温超导体MgB2的电子结构研究

用第一性原理能带理论计算了高温超导体MgB₂的电子结构.计算得出的电子能带说明MgB₂是一种宽能带化合物,价带主要由Mg和B原子s和p的杂化形成.费米能级处的态密度N(E_F)是0.72(states/eV).根据这些结果,初步推断出MgB₂的超导电性的微观机制不可能是电子声子耦合的BCS模型,而是有待于探索的新机制 关键词： 高温超导体 电子结构     

Angular dependence of magnetoresistivity in c-oriented MgB thin film

The anisotropy of MgB₂ is still under debate: its value, strongly dependent on the kind of sample and on the measuring method, ranges between 1.2 and 13. In this work we present our results on MgB₂ c-oriented superconducting thin film. To evaluate the anisotropy, we followed two different approaches. Firstly, magnetoresistivity was measured as a function of temperature at selected magnetic fields applied both parallel and perpendicular to the c-axis; secondly, we measured magnetoresistivity at selected temperatures and magnetic fields, varying the angle θ between the magnetic field and the c-axis. The anisotropy estimated from the ratio between the upper critical fields parallel and perpendicular to the c-axis and the one obtained in the framework of the scaling approach within the anisotropic Ginzburg-Landau theory are different but show a similar trend in the temperature dependence. Some differences in the upper critical field and in its anisotropy of our film with respect to single crystals are emphasized: some of these aspects can be accounted for by an analysis of upper critical fields within a two-band model in presence of disorder and/or crystallographic strain. Received 12 July 2002 / Received in final form 17 September 2002 Published online 29 November 2002     

纳米C和SiC掺杂对MgB2带材超导性能的影响

采用X射线衍射仪，扫描电镜，超导量子干涉仪等仪器对纳米C和SiC掺杂的MgB₂带材进行了表征，并采用标准四引线法对样品的临界电流进行了测试. 实验表明，C和SiC掺杂在提高MgB₂带材高场下的临界电流密度方面具有显著效果. 在温度为4.2 K、磁场大于9 T条件下，C和SiC掺杂样品的临界电流密度与未掺杂样品相比均提高一个数量级以上. 掺杂样品高磁场下良好的临界电流性能主要归因于C对B的替代所产生的晶格畸变、位错等缺陷和局部成分变化而导致的有效晶内钉扎作用. 实验结果表明，SiC掺杂的MgB2带材之所以具有非常好的高场电流特性，和C掺杂的样品一样， C对B的替代起到十分关键的作用. 关键词： 2带材')" href="#">MgB₂带材 C掺杂 SiC掺杂 临界电流性能     

Magnetic field dependence of Sommerfeld coefficient and penetration depth in NdFeAsO1−XFX single crystal

Anisotropic properties of Sommerfeld coefficient and penetration depth for single crystal NdFeAsO_1?xF_x has been studied by using modified phenomenological Ginzburg–Landau (GL) theory. In the above two-band superconducting system, the calculated value of Sommerfeld coefficient shows very close proximity with the experimental result as reported by Welp. Further, anisotropic ratio of penetration depth also calculated and reported for this system. The results of anisotropic properties of the above superconducting system implied that modified GL-theory in the form presented here can be applicable to the above superconducting system.