Doping dependent evolution of magnetism and superconductivity in Eu(1-x)K(x)Fe2As2 (x = 0-1) and temperature dependence of the lower critical field H(c1)

We have synthesized polycrystalline samples of Eu(1-x)K(x)Fe2As2 (x = 0-1) and carried out systematic characterization using x-ray diffraction, ac and dc magnetic susceptibility, and electrical resistivity measurements. A clear signature of the coexistence of a superconducting transition (T(c) = 5.5 K) with spin density wave (SDW) ordering is observed in our underdoped sample with x = 0.15. The SDW transition disappears completely for the x = 0.3 sample and superconductivity arises below 20 K. The superconducting transition temperature Tc increases with increase in the K content and a maximum Tc = 33 K is reached for x = 0.5, beyond which it decreases again. The doping dependent Tx phase diagram is extracted from the magnetic and electrical transport data. It is found that magnetic ordering of Eu moments coexists with the superconductivity up to x = 0.6. The isothermal magnetization data taken at 2 K for the doped samples suggest the 2+ valence state of the Eu ions. We also present the temperature dependence of the lower critical field H(c1) of the superconducting polycrystalline samples. The values of H(c1)(0) obtained for x = 0.3, 0.5, and 0.7 after taking the demagnetization factor into account are 202, 330, and 212 Oe, respectively. The London penetration depth λ(T) calculated from the lower critical field does not show exponential dependence at low temperature, as would be expected for a fully gapped clean s-wave superconductor. In contrast, it shows a T2 power law feature up to T = 0.3Tc, as observed in Ba(1-x)K(x)Fe2As2 and BaFe(2-x)Co(x)As2.     

Superconductivity in Li OHFe S single crystals with a shrunk c-axis lattice constant

By using a hydrothermal ion-exchange method, we have successfully grown superconducting crystals of Li OHFe S with Tcof about 2.8 K. Being different from the sister sample(Li1-xFex)OHFe Se, the energy dispersion spectrum analysis on Li OHFe S shows that the Fe/S ratio is very close to 1:1, suggesting an almost charge neutrality and less electron doping in the Fe S planes of the system. Comparing with the non superconducting Li OHFe S crystal, each peak of the X-ray diffraction pattern of the superconducting crystal splits into two, and the diffraction peaks locating at lower reflection angles are consistent with that of non-superconducting ones. The rest set of diffraction peaks with higher reflection angles is corresponding to the superconducting phase, suggesting that the superconducting phase may has a shrunk c-axis lattice constant. Magnetization measurements indicate that the magnetic shielding due to superconductivity can be quite high under a weak magnetic field. The resistivity measurements under various magnetic fields show that the upper critical field is quite low, which is similar to the tetragonal Fe S superconductor.     

GdBa_2Cu_(3-x)Fe_xO_(7-δ)超导体系结构相变及拉曼谱特征

采用固态反应法制备了GdBa2Cu3-xFexO7-δ(x=0.00-0.30)系列样品,利用X射线衍射、拉曼光谱以及电测量技术对体系的晶体结构、拉曼散射谱特征以及电输运特性进行了系统研究。结果表明,当Fe掺杂量在x=0.05-0.10区间时,体系的晶体结构发生了从正交相到四方相的转变。通过对拉曼光谱中典型振动模的指认及振动模随Fe掺杂量的变化规律,得到了拉曼谱随体系正交-四方结构相变的变化特征:对于具有正交相的x=0.00-0.05样品,拉曼谱具有五个与正交相结构相对应的特征峰;而对于具有四方相的x=0.10-0.30样品,随Fe掺杂量增加,振动模强度变弱,且典型振动模发生了不同程度的展宽或频移。电输运测量表明,随Fe掺杂量的增加,超导临界温度Tc降低,正常态电阻率增加且发生了金属-半导体相变。     

Effect of Zn impurity in K0.8Fe(2-δ-x)Zn(x)Se2

A series of K(0.8)Fe(2-δ-x)Zn(x)Se(2) single-crystal samples with nominal compositions 0?≤?x?≤?0.05 were grown and their physical properties were measured in order to study the effect of Zn impurity. It is found that the Zn impurity (x?≤?0.02) does not affect the superconducting transition temperature T(c) significantly. Meanwhile the hump in resistivity which corresponds to the transition from the insulating to metallic phase quickly shifts towards low temperatures. The results imply that there should be a phase separation in this system and Zn impurity causes the enhancement of the insulating phase. The negligible effect of Zn impurity on T(c) suggests an s-wave pairing in the superconducting phase. Meanwhile there is a possibility that the Zn impurity may selectively enter into the insulting phase.     

SmFeAsO0.78F0.22中Mn掺杂对超导电性的抑制效应

本文报道了Mn替代Fe对最佳掺杂的Sm-1111体系超导电性的影响.通过电输运及磁性质的测量发现Mn对Fe的替代迅速抑制了超导电性.随着Mn含量的增加,体系的电阻率ρ和磁化率M逐渐增大.对于低掺杂x＜0.25%和未掺杂样品,M接近零,对于≤1%,M从室温到超导转变几乎不变,实验指出Fe2+失去其磁性.我们提出这是由于F...     

Evidence for line nodes in the superconducting energy gap of noncentrosymmetric CePt3Si from magnetic penetration depth measurements

We report measurements of the magnetic penetration depth lambda(T) in high-quality CePt3Si samples down to 0.049 K. We observe a linear temperature dependence below T approximately equal to 0.16Tc, which is interpreted as evidence for line nodes in the energy gap of the low-temperature phase of this material. A kink in lambda(T) at about 0.53 K may be associated with the second superconducting transition recently reported. The results are discussed in terms of the symmetry of the superconducting order parameter.     

Microstructure and structural phase transitions in iron-based superconductors

Crystal structures and microstructural features, such as structural phase transitions, defect structures, and chemical and structural inhomogeneities, are known to have profound effects on the physical properties of superconducting materials. Recently, many studies on the structural properties of Fe-based high-Tc superconductors have been published. This review article will mainly focus on the typical microstructural features in samples that have been well characterized by physical measurements. （i） Certain common structural features are discussed, in particular, the crystal structural features for different superconducting families, the local structural distortions in the Fe2Pn2 （Pn = P, As, Sb） or FeeCh2 （Ch = S, Se, Te） blocks, and the structural transformations in the 122 system. （ii） In FeTe（Se） （11 family）, the superconductivity, chemical and structural inhomogeneities are investigated and discussed in correlation with superconductivity. （iii） In the Ko.sFe1.6＋xSe2 system, we focus on the typical compounds with emphasis on the Fe-vacancy order and phase separations. The microstructural features in other superconducting materials are also briefly discussed.     

Anisotropy of the upper critical field in c-axis oriented MgB2 thin films

C-axis oriented MgB_2 thin films were synthesized on single-crystal MgO (111) substrates using a chemical vapour deposition technique. The as-formed films revealed a sharp superconducting transition temperature of 38K with the transition width 0.2K. The temperature dependence of the upper critical magnetic field H_{c2}(T) in the films was determined via resistivity for magnetic field H parallel and perpendicular to the c axis of the films. Using the Werthamer－Helfand－Hohenberg formula, we obtained the anisotropy ratio of the upper critical field γ=1.2.     

Y_(1-x)Pr_xBa_2Cu_3O_(7-δ)单晶的生长与退火效应研究

采用顶部籽晶提拉法成功生长出了Y1-xPrxBa2Cu3O7-δ(x=0.07215,0.23960)单晶样品。利用X射线结构分析技术对生成单晶进行了表征,通过对样品进行了不同时间的退火处理,结合超导磁化特性测量,讨论了单晶样品超导转变温度Tc随退火温度和Pr替代含量的变化,并根据单晶样品磁化特性曲线,从低场下的布拉格玻璃态到高场下的磁通玻璃态的相变角度对尖峰效应进行了初步分析。     

Superconductivity in dense MgB2 wires

MgB2 becomes superconducting just below 40 K. Whereas porous polycrystalline samples of MgB2 can be synthesized from boron powders, in this Letter we demonstrate that dense wires of MgB2 can be prepared by exposing boron filaments to Mg vapor. The resulting wires have a diameter of 160 microm, are better than 80% dense, and manifest the full chi = -1/4pi shielding in the superconducting state. Temperature-dependent resistivity measurements indicate that MgB2 is a highly conducting metal in the normal state with rho(40 K) = 0.38 microOmega cm. By using this value, an electronic mean-free path, l approximately 600 A can be estimated, indicating that MgB2 wires are well within the clean limit. Tc, Hc2(T), and Jc data indicate that MgB2 manifests comparable or better superconducting properties in dense wire form than it manifests as a sintered pellet.     

Critical point and the nature of the pseudogap of single-layered copper-oxide Bi2Sr2-xLaxCuO6+delta superconductors

We apply strong magnetic fields of H=28.5 to 43 T to suppress superconductivity (SC) in the cuprates Bi2Sr2-xLaxCuO6+delta (x=0.65, 0.40, 0.25, 0.15, and 0), and investigate the low temperature (T) normal state by 63Cu nuclear spin-lattice relaxation rate (1/T1) measurements. We find that the pseudogap (PG) phase persists deep inside the overdoped region but terminates at x approximately 0.05, which corresponds to the hole doping concentration of approximately 0.21. Beyond this critical point, the normal state is a Fermi liquid that persists as the ground state when superconductivity is removed by the magnetic field. A comparison of the superconducting state with the H-induced normal state in the x=0.40 (Tc=32 K) sample indicates that there remains substantial part of the Fermi surface even in the fully developed PG state, which suggests that the PG and SC are coexisting matters.     

Resistivity superconducting transition in single-crystalline Cd0.95Ni0.05Sb system consisting of non-superconducting CdSb and NiSb phases

Resistivity superconducting transition has been for the first time found in single crystal of two-component 0.95(CdSb)–0.05(NiSb) system. End members of the system are not superconductors under normal conditions. Insulating behavior in temperature dependence of the electrical resistivity, which is due to hopping conductivity, precedes the transition. The resistivity superconducting transition is rather broad, since at cooling down the electrical resistivity starts to fall at 10.5 K, whereas zero resistivity is reached only at ~2.3 K. Longitudinal magnetic field gradually depresses superconductivity and shifts the superconducting transition to lower temperatures. Under magnetic field above 0.5 T, superconductivity is totally destroyed. Main features observed in the resistivity superconducting transition, including its unusually big width and insulating electrical behavior above the transition, can be related to inhomogeneity of the single crystal studied. According to XRD and SEM examinations, the single crystal consists of major CdSb phase and minor NiSb phase. The NiSb phase forms inhomogeneities in the CdSb matrix. Micro-sized needle-like NiSb crystals and nano-sized Ni_1-xSb_x clusters can be considered as typical inhomogeneities.     

Synthesis of a new alkali metal-organic solvent intercalated iron selenide superconductor with T(c)?≈?45?K

We report on a new iron selenide superconductor with a T(c) onset of 45?K and the nominal composition Li(x)(C(5)H(5)N)(y)Fe(2-z)Se(2), synthesized via intercalation of dissolved alkaline metal in anhydrous pyridine at room temperature. This superconductor exhibits a broad transition, reaching zero resistance at 10?K. Magnetization measurements reveal a superconducting shielding fraction of approximately 30%. Analogous phases intercalated with Na, K and Rb were also synthesized and characterized. The superconducting transition temperature of Li(x)(C(5)H(5)N)(y)Fe(2-z)Se(2) is clearly enhanced in comparison to those of the known superconductors FeSe(0.98) (T(c)?～?8?K) and A(x)Fe(2-y)Se(2) (T(c)?～?27-32?K) and is in close agreement with critical temperatures recently reported for Li(x)(NH(3))(y)Fe(2-z)Se(2). Post-annealing of intercalated material (Li(x)(C(5)H(5)N)(y)Fe(2-z)Se(2)) at elevated temperatures drastically enlarges the c-parameter of the unit cell (～44%) and increases the superconducting shielding fraction to nearly 100%. Our findings indicate a new synthesis route leading to possibly even higher critical temperatures for materials in this class: by intercalation of organic compounds between Fe-Se layers.     

Crystal orientation in Bi-based superconducting glass-ceramics prepared in high magnetic field

A high magnetic field of 10 T was applied to the crystallization process of Bi₂Sr₂CaCu₂O_x superconducting precursor glasses, and the effect of high magnetic field on crystal grain orientations and superconducting properties were examined from electrical resistivity measurements, X-ray diffraction analyses and scanning electron microscope observations. The glass-ceramics prepared in a high magnetic field show better superconducting properties (higher critical temperature, larger critical current density, and smaller normal-state resistivity) compared with the samples crystallized in a normal heat-treatment with no magnetic field. It was found that Bi2212 crystal grains with a plate-like shape tend to stack to the direction of the magnetic field, i.e., the orientation of the c-axis of the Bi2212 phase to the direction of the magnetic field.     

Er1-xDyxNi2B2C体系中超导和磁性的共存和相互作用

本在2K～20K温区内系统地研究了Er1-xDyxNi2B2C体系中超导转变温度Tc的反铁磁转变温度TN随Dy掺杂含量x的变化．实验发现x=0．3和x=0．8附近的样品具有复杂的磁结构．这些洋品有两个磁转变温度(TN’和TN)．对于该体系发现了两个主要的特征：1)在x=0．3附近，超导被抑制，TN’出现一个小的峰值；2)在x=0．8附近，Tc出现一个低谷，TN’出现一个大的宽峰．TN’在x=0．3和x=0．8附近的异常来源于改系统中超导和磁性的共存和相互作用。     

Ru掺杂La0.7Ca0.2Ba0.1Mn1-xRuxO3的磁性和输运性质

利用固相反应法制备了Ru掺杂La0.7Ca0.2Ba0.1Mn1-xRuxO3(x=0～0.06)的多晶样品,探讨了Ru掺杂对体系结构,输运性质以及磁电阻的影响.多晶X射线衍射证实所有样品均保持简单立方钙钛矿结构.通过零场冷却(ZFC)和加场冷却(FC)下的磁化曲线的测量发现随温度降低样品发生了顺磁(PM)到铁磁(FM)的相变,且样品的居里温度(Tc)随Ru掺杂发生了显著的变化,从x=0.00时的306.7K,下降到x=0.02时的294.3K,紧接着又上升到x=0.04时的302.4K.测得居里温度明显高于La0.7Ca0.2Ba0.1Mn1-xMoxO3体系,而且其磁性也大为增强.由零场和外加磁场H=1T测量得到样品的ρ～T曲线表明随温度降低样品同时发生了从绝缘体到金属的转变,绝缘体-金属转变温度低于相应的居里温度.适量的Ru掺杂降低了样品的电阻率,增强了低温时的磁电阻.     

Superconductivity in Sm-doped CaFe_2As_2 single crystals

In this article,the Sm-doping single crystals Ca_(1-x)Sm_xFe_2As_2(x = 0 ~0.2) were prepared by the Ca As flux method,and followed by a rapid quenching treatment after the high temperature growth.The samples were characterized by structural,resistive,and magnetic measurements.The successful Sm-substitution was revealed by the reduction of the lattice parameter c,due to the smaller ionic radius of Sm~(3+)than Ca~(2+).Superconductivity was observed in all samples with onset T_c varying from 27 K to 44 K upon Sm-doping.The coexistence of a collapsed phase transition and the superconducting transition was found for the lower Sm-doping samples.Zero resistivity and substantial superconducting volume fraction only happen in higher Sm-doping crystals with the nominal x 0.10.The doping dependences of the c-axis length and onset T_c were summarized.The high-T_c observed in these quenched crystals may be attributed to simultaneous tuning of electron carriers doping and strain effect caused by lattice reduction of Sm-substitution.     

Magnetotransport and superconductivity of α-uranium

We have measured the electrical resistivity, magnetoresistance and Hall effect on several new single-crystal samples and one polycrystalline sample of α-U. The residual resistivity ratios of these samples vary from 13 to 315. Matthiessen's law appears to hold above the onset of the charge-density wave phase transitions that begin near 43?K, but not below this temperature. Sharp features at all three charge-density wave transitions are observed and the effects of high magnetic fields on them are presented and discussed. The magnetoresistance is anisotropic, reaches 1000% at 2?K and 18?T and does not exhibit Kohler scaling. The Hall coefficient is positive, independent of magnetic field and slightly temperature dependent above about 40?K in agreement with earlier studies. Below 40?K the Hall coefficient changes sign as the temperature falls, varies with field and becomes much more strongly negative at the lowest temperatures than has been reported. Some of our results suggest that a spin-density wave may coexist with the charge-density wave states. Superconductivity is observed in two of our samples; we argue that it is intrinsic to α-U and suggest that it is consistent with a two-band model. Several parameters characterizing the transport and superconductivity of α-U are estimated.     

Correlated electron state in Ce(1-x)Yb(x)CoIn5 stabilized by cooperative valence fluctuations

X-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat measurements on Ce(1-x)Yb(x)CoIn5 (0≤x≤1) reveal that many of the characteristic features of the x=0 correlated electron state are stable for x≤0.775 and that phase separation occurs for x>0.775. The stability of the correlated electron state is apparently due to cooperative behavior of the Ce and Yb ions, involving their unstable valences. Low-temperature non-Fermi liquid behavior is observed and varies with x, even though there is no readily identifiable quantum critical point. The superconducting critical temperature T(c) decreases linearly with x towards 0 K as x→1, in contrast with other HF superconductors where T(c) scales with T(coh).