On the Magnetism of the Normal State in MgB2

The experimentally observed ferromagnetism in MgB₂ in the normal state is attributed to microphase-separated inclusions of iron. This magnetic character is also observed when the iron content of the samples is reduced below 20 g/g, however in these samples the diamagnetism of MgB₂ is apparent and is measured. It is found experimentally that the diamagnetic susceptibility at room temperature of B element, MgB₂, and MgB₄ is close to the ratio 1:2:4, suggesting that the diamagnetism in these borides is confined to the boron atoms. This observation supports a picture in which the two electrons of Mg are donated to B in MgB₂.     

Point-Contact Spectroscopy in Mn-Doped MgB2 Single Crystals: Effects of Magnetic Impurities in a Two-Band Superconductor

We performed point-contact spectroscopy (PCS) measurements in Mg_1−x Mn _x B₂ single crystals, with x≤0.015 and bulk T _c down to 13.3 K. The gaps Δ _σ and Δ _π were obtained by fitting the conductance curves of the point contacts with the two-band Blonder–Tinkham–Klapwijk (BTK) model. Both Δ _σ and Δ _π decrease with the critical temperature of the junctions T _c ^A , but remain clearly distinct down to the lowest critical temperature (T _c ^A ≅9 K). Once analyzed within the Eliashberg theory, the gap trends as a function of T _c ^A can be explained by a doping-induced increase in the pair-breaking scattering within the σ band, with smaller contributions from the π-π or the σ-π channels.     

Vortex Motion and Quasiparticle Resistivity in MgB<Subscript>2</Subscript> at Microwave Frequencies

The electrical transport in superconductors at nonzero frequencies is affected by the normal and superfiuid fractions, as well as moving vortices, resulting in intricate expressions for the complex resistivity. In MgB₂, additional complications arise from the two-band nature of this material. We present an accurate study of microwave resistivity data measured in MgB₂ thin films by means of the Corbino disk broadband technique between 2 and 20 GHz. We show that a two-fluid model applies well in a relatively large region of the H–T phase diagram. Excellent agreement is found between the derived superfluid parameters (superfluid density, upper critical field) and theoretical predictions. In the same H–T region we isolate and discuss the vortex motion complex resistivity. To this end, we make use of the expressions given by the model by Coffey and Clem (CC). We show that the frequency dependence of the complex vortex resistivity recovers the CC model. However, the temperature and field dependence of the obtained parameters are at odds with the assumptions of the model. We discuss possible explanations of these oddities by considering collective pinning of vortices.     

TiC和SiC掺杂MgB2超导体的对比研究

分别制备了8％（质量分数）的TiC和5％（质量分数）的SiC掺杂的MgB2超导块材，并对比分析了这两种掺杂物对MgB2超导块材的性能影响。所有样品均在流动的Ar气保护下在不同退火温度下退火，研究发现TiC掺杂和SiC掺杂的MgB2样品的最优化工艺参数分别是900℃保温1h和720℃保温1h。随后采用XRD和SEM分别对样品进行了相成份和微观结构的分析，并采用PPMS测试了样品的磁滞回线并由Bean模型计算出了样品的临界电流密度。在4．2K，0T下TiC掺杂的五值为1．0×10^5A／cm^2，而10K，0T下SiC掺杂样品的五值为4×100A／cm^2。而且随着外加磁场的增加，SiC掺杂MgB2样品的五值下降得比TiC掺杂的MgB2样品要缓慢很多，这表明了SiC掺杂比TiC掺杂更有利于改善MgB2在高场下的超导电性能。     

Time Decay of the Nonlinear Microwave Response of Superconductors in the Critical State

We report experimental results on the second-order response of superconductors in the critical state exposed to intense pulsed microwave (mw) fields. The second-harmonic power radiated by the samples is investigated during the time interval in which the mw pulse endures, for mw pulses 1 ms long. The second-harmonic signal exhibits a time decay whose features depend on the investigated sample, the input power level, and the way in which the value of the static magnetic field has been reached. The transient is ascribed to processes of magnetic flux redistribution, which come into play when fluxons in the critical state are driven by electromagnetic (em) fields of high frequency.     

Weak Links and Phase Slip Centers in Superconducting MgB2 Wires

MgB₂ superconducting wires were produced by the Mg diffusion method. Scanning electron microscopy (SEM), optical microscopy, dispersive X-ray analysis (EDS), and XRD diffraction were used to study the physical structure and content of the wires. Magnetic properties (T _c ^m, H _c1, H _c2, J _c by the Bean model) were obtained with a SQUID magnetometer, and transport properties (T _c ^r , H _c2, resistivity and residual resistivity ratio) were measured using a standard four-lead configuration. The V-I characteristics of the wires close to the critical temperature showed a staircase response, which was attributed to the presence of weak links, creating phase slip centers. The origin of those weak links is discussed in relation to their formation and structure.     

Fabrication of Bulk Targets of MgB2 with Stoichiometric and Nonstoichiometric Contents

Powder of magnesium diboride was obtained by solid phase reaction of mixture of magnesium （〉 99% pure） and amorphous boron （〉 99% pure） powders at 650-900 ℃ temperatures in inert atmosphere. During synthesis process main attention was paid. to removing oxide layer of surfaces of powder particles by organic solvents in Glovebox, where concentration of oxygen and water steam is less than 5 ppm. Homogenization and activation of powders were conducted in a planetary nano-mill by WC balls in an inert area. Pressing of the obtained powders was conducted in an argon atmosphere. MgB2 nonstoichiometric powders contained excess boron and magnesium, Magnesium hydride was used as source of excess boron, which is fragile compound and easy to grind in nano-mill. It decomposites with metallic magnesium and hydrogen up to 280 ℃ temperature. Commercial magnesium diboride powder （Aldrich, 〉 99%） was used for fabrication of MgB2 bulk targets. Powders systems of MgB2-Mg, MgB2-MgH2, MgB2-B homogenized by nano-mill in Glove box was used for fabrication of composites with nonstoichiometric contents. The targets were cylinders with diameters of 27-52 mm and height of 5-15 mm. Consolidation of pressed powdery composites was conducted in argon atmosphere. Synthesis of MgB2 from mixture of magnesium and amorphous boron powders and simultaneous consolidation were conducted by hot pressing （HP） method. Phase content of the obtained targets were established by XRD method after dry polishing. Superconducting characteristic of the obtained samples were measured by vibrational magnetometer. The superconducting transition with an onset at 39 K was observed in a good agreement with the results of the other groups obtained on samples prepared by conventional techniques. The phase exists near the nominal composition without a significant homogeneity range.     

Unified Description of Nonlinear Effects in High Temperature Superconductor Microwave Devices

We experimentally demonstrate the equivalence of different manifestations of nonlinear response in high temperature superconductor (HTS) microwave devices. Using a combination of analytical and numerical analysis, we show that the results of intermodulation distortion measurements, harmonic generation measurements, and power-dependent resonator measurements of different coplanar waveguide structures patterned onto the same HTS thin-film sample all yield approximately the same values for the nonlinear penetration depth. The extraction of an underlying nonlinear material parameter that is independent of the specific device geometry and experimental configuration will allow our results to be quantitatively compared with other nonlinear measurements, and will therefore help in determining the dominant source(s) of nonlinear response in HTS microwave devices.     

Modulated microwave absorption in PbBi2Sr2Ca2Cu3Oy powders and possible flux melting

Temperature-swept magnetically modulated microwave absorption is reported in powdered PbBi₂Sr₂Ca₂Cu₃O _y with a range of particle sizes and in fields up to 700 mT. In low fields a signal of positive sign is observed originating from weak link regions. In higher fields two other signal appear. One is very weak and almost independent of field and is attributed to the bulk intrinsic transition atT _c112 K. The second broader and stronger negative peak moves rapidly to low temperatures with increasing field and is attributed to melting of the flux lattice.     

提高MgB2超导体临界电流密度的研究进展

临界电流是超导体载流能力大小的重要表征,论述了制备工艺、元素掺杂和物理场对MgB2超导体临界电流密度的影响.使用非晶态高纯硼粉制备致密的MgB2试样有利于样品临界电流密度的提高,添加SiC或者Ti也能起到提高临界电流密度的作用,质子照射使MgB2试样在高磁场条件下的临界电流密度增加,强磁场下烧结则有利于提高MgB2样品临界的传输电流密度.     

Transient Electromagnetic and Heat Transfer Characteristics of the MgB2 Superconductor in Liquid Helium

A transient current density analysis is carried out for the type II superconductor (SC) material MgB₂ in liquid Helium. Variable magnetic fields are used as the unknown in solving the curl-curl relations of the electromagnetic fields applying the finite element method (FEM) for 2D axis symmetric cylindrical wires. Assuming an exponential current rise, the magnetic flux, current density and temperature distribution in the SC and He are calculated. This study gives the limiting current values to avoid the normal state, the temperature distribution profile with time as well as the changes in the magnitude of the current and time constant. Oscillations and non-uniform temperature distributions are observed in He and in the SC respectively if one varies the parameters which are related to the different heat transfer mechanisms. A slight instability in the current distribution is also observed which might transfer to a thermal instability, i.e. could signal He boiling.     

Ba1-xMgxAl2Si2O8晶体结构与微波介电性能的研究

采用固相烧结法, 按化学计量比Ba_1-xMg_xAl₂Si₂O₈(x=0, 0.05, 0.1, 0.15, 0.3, 0.5)制备样品, 考察不同MgO含量对BaO-Al₂O₃-SiO₂系介电材料晶体结构及微波介电性能的影响。结果表明, MgO可以降低烧结温度, 促进六方相转变为单斜相, 当添加量x≥0.15时, 相转变可以达到100%。当x≤0.15时, 适量的MgO可以有效地促进单斜钡长石晶粒的长大。在0.05≤x≤0.1范围内, 随着MgO含量的增加, 单斜钡长石衍射峰增强, 晶粒尺寸增大, 密度、介电常数与τ_f均随MgO含量的增加而增大。在x=0.1, 烧结温度为1400℃时, 可获得综合性能相对较好的单斜钡长石, 其介电性能ε_r=6.44, Q×f=16461 GHz, τ_f= -30.6×10^-6 K^-1。     

基于预应力法的转子不平衡响应研究

利用预应力及谐响应分析的方法研究了转子的不平衡响应，得出在某频率范围内，转子在指定点处位移对频率的响应曲线以及在指定频率处，整个转子上的应力、应变响应的影响等。     

Probing the Vorticity in a Superconductor Rapidly Quenched from the Normal State

Topological defects such as vortices subsist for some time in a rapidly frozen superconducting film (W. H. Zurek, Phys. Rep. 276, 177, 1996). We propose to use as a probe of the vortex density the finite delay T _d, which, in narrow strips, exists between a current step and the voltage response. Technically, this amounts to driving a bridge into the localized hot-spot regime by means of a pump pulse (laser or electrical). Cooling of such films as epitaxial YBCO-on-MgO, or niobium-on-sapphire, requires only a few nanoseconds at low T. A time t later, a probe pulse is applied to measure T _d. The dependence t T _d is interpreted as a witness of the fossil vorticity, long after quenching into the zero-resistance regime.     

Microwave Power Dependence of Surface Resistance of High-Temperature Superconductor YBa2Cu3O y Films by Dielectric Resonator Method

The microwave power dependence of the surface resistance of YBa₂Cu₃O_y films of two different surface morphologies was measured using the dielectric resonator method. The dielectric resonator consisted of a high-quality sapphire rod sandwiched between two superconductor films. Measurements showed that the microwave power dependence of the surface resistance strongly depended on the surface morphology of the YBa₂Cu₃O_y film though the surface resistance of those films at low microwave power was comparable. In conclusion, the surface morphology is one of the reasons of the power dependence which is crucial for high power applications.     

Effects of Small Fields on the Magnetic Response of Superconducting Thin Films of MgB2

Under the influence of a magnetic field applied perpendicularly to its plane, a superconducting thin film develops dendritic patterns of penetrated regions that coexist with Meissner areas. For a thin film of MgB₂ submitted to an alternate field of moderate amplitude, the AC susceptibility measured while cooling the sample exhibits a quite unusual behavior, reentering and fluctuating with temperature. The effect is more pronounced at frequencies around 1 kHz. Two plausible explanations for the effect are discussed.     

Preparation of Nonlinear Optical Aerogels and Xerogels in Parabolic Flights

Second harmonic generation of ferroelectrical nano- and microparticles embedded in a nanostructured silica aerogel and xerogel matrix was observed. A homogenous distribution of the ferroelectrical powder was achieved during microgravity on parabolic flight. The paper describes the synthesis procedures of this new class of random non-linear optical materials. The experimental results on frequency doubling are explained with a simple model of absorption and transmission.     

Highly Sensitive,Fast Response Perovskite Photodetectors Demonstrated in Weak Light Detection Circuit and Visible Light Communication System

Organic–inorganic hybrid perovskite (OIHP) photodetectors have presented unprecedented device performance mainly owing to outstanding material properties. However, the solution‐processed OIHP polycrystalline thin films with defective surface and grain boundaries always impair the key parameter of photodetectors. Herein, a nonfullerene passivation layer exhibits more efficient passivation for OIHP materials to dramatically reduce the trap density of state, yielding a dark current as low as 2.6 × 10^?8 A cm^?2 under ?0.1 V. In addition, the strong absorption in near‐infrared (NIR) region of nonfullerene/C₆₀ heterojunction broadens the detectable range to over 900 nm by effective charge transport, ultimately leading to a specific detectivity of 1.45 × 10¹² and 7.37 × 10¹¹ cm Hz^1/2 W^?1 at 650 and 820 nm, respectively. Encouragingly, the response speed of 27 ns is obtained at 0.6 mm² of device area by removing constrain from the resistance–capacitance constant. Moreover, the prominent practical application of the photodetector is demonstrated in a weak light detection circuit and a visible light communication system. It is believed that the OIHP photodetectors with high sensitivity, NIR photoresponse, and ultrafast speed would pave the way to commercial applications.