Magneto-optical study of the superconducting gap of MgB(2) single crystals

We present magneto-optical reflectivity results in the basal plane of the hexagonal MgB(2). The data were collected on a mosaic of MgB(2) single crystals with T(c)=38 K from the ultraviolet down to the far infrared as a function of temperature and magnetic field oriented along the c axis. In the far infrared, there is a clear signature of the superconducting gap with a gap ratio 2 Delta/k(B)T(c) approximately 1.2, well below the weak-coupling value. The gap is suppressed in an external magnetic field, which is a function of temperature. We extract the upper critical field H(c2) along the c axis. The temperature dependence of H(c2) is compatible with the Helfand-Werthamer behavior.     

High-resolution photoemission study of MgB2

We have performed high-resolution photoemission spectroscopy on MgB2 and observed opening of a superconducting gap with a narrow coherent peak. We found that the superconducting gap is s like with the gap value ( Delta) of 4.5+/-0.3 meV at 15 K. The temperature dependence (15-40 K) of the gap value follows well the BCS form, suggesting that 2Delta/k(B)T(c) at T = 0 is about 3. No pseudogap behavior is observed in the normal state. The present results strongly suggest that MgB2 is categorized into a phonon-mediated BCS superconductor in the weak-coupling regime.     

Tunneling spectroscopy in small grains of superconducting MgB(2).

We report on tunneling spectroscopy experiments in small grains of the new binary intermetallic superconductor MgB(2). Experiments have been performed at 2.5 K using a low temperature scanning tunneling microscope. Good fit to the BCS model is obtained, with a gap value of 2 meV. In the framework of this model, this value should correspond to a surface critical temperature of 13.2 K. No evidence of gap anisotropy has been found.     

常压和高压合成MgB2的低温比热及两个超导能隙研究

测量了常压和高压合成MgB2超导样品6K—80K的比热,并在数据拟合中考虑了爱因斯坦振动模的贡献,得到其电子比热数据.解释了超声测量得到的德拜温度和低温比热拟合结果之间的巨大差异.并且在12K附近观察到了异常的电子比热,此异常来源于MgB2的第二个能隙.此外,对高压和常压合成对样品的影响做了初步分析. 关键词： 低温比热 MgB2 能隙     

Scanning tunneling spectroscopy in MgB2

We present scanning tunneling microscopy measurements of the surface of superconducting MgB2 with a critical temperature of 39 K. In zero magnetic field the conductance spectra can be analyzed in terms of the standard BCS theory with a smearing parameter gamma. The value of the superconducting gap is 5 meV at 4.2 K, with no experimentally significant variation across the surface of the sample. The temperature dependence of the gap follows the BCS form, fully consistent with phonon-mediated superconductivity in this novel superconductor. The application of a magnetic field induces strong pair breaking as seen in the conductance spectra in fields up to 6 T.     

Specific heat of single crystal MgB2: a two-band superconductor with two different anisotropies

Heat-capacity measurements of a 39 microg MgB2 single crystal in fields up to 14 T and below 3 K allow the determination of the low-temperature linear term of the specific heat, its field dependence, and its anisotropy. Our results are compatible with two-band superconductivity, the band carrying the smaller gap being isotropic, that carrying the larger gap having an anisotropy of approximately 5. Three different upper critical fields are thus needed to describe the superconducting state of MgB2.     

Superconducting gap in MgB(2): electronic Raman scattering measurements of single crystals

Polarization-resolved Raman scattering measurements were performed on MgB(2) single crystals to determine the magnitude, symmetry, and temperature dependence of the superconducting gap. A single sharp peak due to Cooper pair breaking appears in the electronic continuum below T(c), reaching a maximum Raman shift of 105 +/- 1 cm(-1) [2 Delta(0)/k(B)T(c) = 3.96 +/- 0.09] and showing up to 5 cm(-1) anisotropy between polarized and depolarized spectra. The temperature dependence of 2 Delta follows that predicted from BCS theory, while the anisotropy decreases with decreasing temperature. It is concluded that the Raman results are consistent with a slightly anisotropic s-wave gap in a conventional BCS superconductor.     

原位法MgB_2-B_4C复相超导体合成及相含量调控研究

以B4C和Mg为原料合成的MgB2-B4C复相超导体具有高的临界电流密度(Jc)和高的超导转变温度(Tc),是一种有潜力的实用MgB2超导材料,其成相机理对复相MgB2超导体的相含量调控和磁通钉扎研究具有重要意义。结合经典烧结理论,研究了B4C-Mg真空固相烧结制备MgB2-B4C复相超导体的超导相形成和晶粒生长过程,给出了B4C-Mg的金斯特林格扩散模型和MgB2晶粒生长过程。通过选择B4C原料粒径,MgB2-B4C复相超导体超导相体积相含量在18%-88%范围可控。相含量88%的MgB2-B4C复相超导体临界转变温度达33.5K,转变宽度1.5K。10 K环境6T外场下电流密度可以达到1×104A/cm2,表明MgB2-B4C复相超导体具有良好的磁通钉扎行为。     

Anisotropy of the upper critical field in MgB2: the two-gap Ginzburg-Landau theory

The upper critical field in MgB2 is investigated in the framework of the two-gap Ginzburg-Landau theory. A variational solution of linearized Ginzburg-Landau equations agrees well with the Landau level expansion and demonstrates that spatial distributions of the gap functions are different in the two bands and change with temperature. The temperature variation of the ratio of two gaps is responsible for the upward temperature dependence of in-plane Hc2 as well as for the deviation of its out-of-plane behavior from the standard angular dependence. The hexagonal in-plane modulations of Hc2 can change sign with decreasing temperature.     

Time-resolved photoexcitation of the superconducting two-gap state in MgB2 thin films

Femtosecond pump-probe studies show that carrier dynamics in MgB2 films is governed by the sub-ps electron-phonon (e-ph) relaxation present at all temperatures, the few-ps e-ph process well pronounced below 70 K, and the sub-ns superconducting relaxation below T(c). The amplitude of the superconducting component versus temperature follows the superposition of the isotropic dirty gap and the three-dimensional pi gap dependences, closing at two different T(c) values. The time constant of the few-ps relaxation exhibits a double divergence at temperatures corresponding to the T(c)'s of the two gaps.     

用多个速率升温热重法测定MgB_2的氧化活化能

用热分析仪研究了 Mg B2 在高温空气气氛下的热稳定性和氧化情况 ,结果表明 Mg B2 在 4 0 0℃开始氧化 ,70 0℃后氧化非常强烈 ,用多个速率升温热重法测定 Mg B2 的氧化活化能为 80 k J/ mol左右。     

Definitive experimental evidence for two-band superconductivity in MgB2

The superconducting-gap of MgB2 has been studied by high-resolution angle-resolved photoemission spectroscopy. The results show that superconducting gaps with values of 5.5 and 2.2 meV open on the sigma band and the pi band, respectively, but both the gaps close at the bulk transition temperature, providing a definitive experimental evidence for the two-band superconductivity with strong interband pairing interaction in MgB2. The experiments validate the role of k-dependent electron-phonon coupling as the origin of multiple-gap superconductivity as well as the high transition temperature of MgB2.     

原位法MgB2-B4C复相超导体的成相与特性

以B4C粉末与Mg粉为原料,采用真空同相反应法成功制备了复相MgB2超导块材,研究了不同制备工艺下MgB2的成相,测量分析了材料的X射线衍射谱、扫描电镜图像、低温电输运特性等.结果显示,合成的复相超导材料含MgB2超导相、B4C绝缘相和少量MgB2C2杂相,MgB2超导相含量与B4C粒径、合成温度、反应时间等有关,并在...     

MgB_2超导体输运特性的研究

文中对 Mg B2 超导体的制备过程 ,对其同位素效应、电阻率、I- V曲线、热电势、电导涨落效应、能隙、霍尔效应、热重 -差热分析实验等输运特性进行了较全面的综述 ,并讨论了在 Mg B2 超导体基础上的元素替代效应。     

Prediction of high T(c) superconductivity in hole-doped LiBC

The layered lithium borocarbide LiBC, isovalent with and structurally similar to the superconductor MgB2, is an insulator due to the modulation within the hexagonal layers (BC vs B2). We show that hole doping of LiBC results in Fermi surfaces of B-C p sigma character that couple very strongly to B-C bond stretching modes, precisely the features that lead to superconductivity at T(c) approximately equal to 40 K in MgB2. Comparison of Li(0.5)BC with MgB2 indicates the former to be a prime candidate for electron-phonon coupled superconductivity at substantially higher temperature than in MgB2.     

Evidence for strong-coupling s-wave superconductivity in MgB2: (11)B NMR Study

We have investigated a gap structure in a newly discovered superconductor, MgB2, through measurement of the (11)B nuclear spin-lattice relaxation rate, (11)(1/T(1)). (11)(1/T(1)) is proportional to the temperature (T) in the normal state, and decreases exponentially in the superconducting (SC) state, revealing a tiny coherence peak just below T(c). The T dependence of 1/T(1) in the SC state can be accounted for by an s-wave SC model with a large gap size of 2Delta/k(B)T(c) approximately 5 which suggests it is in a strong-coupling regime.     

Search for E(2g) phonon modes in MgB2 single crystals by point-contact spectroscopy

The electron-phonon interaction in magnesium diboride MgB2 single crystals is investigated by point-contact (PC) spectroscopy. For the first time the electron coupling with E(2g) phonon modes is resolved in the PC spectra. The correlation between intensity of the extremely broad E(2g) modes in the PC spectra and value of the superconducting gap is established. Our observations favor current theoretical models for electron-phonon mediated superconductivity in MgB2, and they better match the harmonic phonon model.     

MgB2超导薄膜正常态电阻特性研究

研究了MgB2超导薄膜正常态电阻的影响因素,探讨了与之相应的超导电性机制。MgB2超导薄膜样品采用两步异位退火的Mg扩散方法制备,通过电阻~温度曲线测量、扫描电子显微镜形貌观测和光学金相显微镜观察等方法研究了所制备薄膜的基本特性。实验结果表明,先驱硼薄膜的纯度严重影响着所生成的MgB2超导薄膜的转变温度,退火温度越高,影响越大;退火温度高的样品,正常态电阻大;退火时间长的样品正常态电阻大。     

Fermi surface topology and the upper critical field in two-band superconductors: application to MgB2

Recent measurements of the anisotropy of the upper critical field B(c2) on MgB2 single crystals have shown a puzzling strong temperature dependence. Here, we present a calculation of the upper critical field based on a detailed modeling of band structure calculations that takes into account both the unusual Fermi surface topology and the two gap nature of the superconducting order parameter. Our results show that the strong temperature dependence of the B(c2) anisotropy can be understood as an interplay of the dominating gap on the sigma band, which possesses a small c-axis component of the Fermi velocity, with the induced superconductivity on the pi-band possessing a large c-axis component of the Fermi velocity. We provide analytic formulas for the anisotropy ratio at T=0 and T=T(c) and quantitatively predict the distortion of the vortex lattice based on our calculations.     

Boron isotope effect in superconducting MgB2

We report the preparation method of and boron isotope effect for MgB2, a new binary intermetallic superconductor with a remarkably high superconducting transition temperature T(c)(10B) = 40.2 K. Measurements of both temperature dependent magnetization and specific heat reveal a 1.0 K shift in T(c) between Mg11B2 and Mg10B2. Whereas such a high transition temperature might imply exotic coupling mechanisms, the boron isotope effect in MgB2 is consistent with the material being a phonon-mediated BCS superconductor.