Growth and characterization of superconducting Ca1-xNaxFe2As2 single crystals by NaAs-flux method

High-quality superconducting Ca$_{1-x}$Na$_x$Fe$_2$As$_2$ single crystals have been successfully grown by the NaAs-flux method, with sodium doping level $x = 0.4$-0.64. The typical sizes of these crystals are more than 10 mm in $ab$-plane and $\sim 0.1$ mm along $c$-axis in thickness. X-ray diffraction, resistance and magnetization measurements are carried out to characterize the quality of these crystals. While no signature of magnetic phase transitions is detected in the normal state, bulk superconductivity is found for these samples, with a sharp transition at $T_{\rm c}$ ranging from 19.8 K ($x = 0.4$) to 34.8 K ($x = 0.64$). The doping dependences of the $c$-axis parameter and $T_{\rm c}$ are consistent with previous reports, suggesting a possible connection between the lattice parameters and superconductivity.     

Superconductivity at 38 K in the iron arsenide (Ba1-xKx)Fe2As2

The ternary iron arsenide BaFe2As2 becomes superconducting by hole doping, which was achieved by partial substitution of the barium site with potassium. We have discovered bulk superconductivity at T{c}=38 K in (Ba1-xKx)Fe2As2 with x approximately 0.4. The parent compound BaFe2As2 crystallizes in the tetragonal ThCr2Si2-type structure, which consists of (FeAs);{delta-} iron arsenide layers separated by Ba2+ ions. BaFe2As2 is a poor metal and exhibits a spin density wave anomaly at 140 K. By substituting Ba2+ for K+ ions we have introduced holes in the (FeAs);{-} layers, which suppress the anomaly and induce superconductivity. The T{c} of 38 K in (Ba0.6K0.4)Fe2As2 is the highest in hole doped iron arsenide superconductors so far. Therefore, we were able to expand this class of superconductors by oxygen-free compounds with the ThCr2Si2-type structure.     

Incommensurate-commensurate magnetic phase transition in double tungstate Li2Co(WO4)2

Magnetic susceptibility, specific heat, and neutron powder diffraction measurements have been performed on polycrystalline Li$_{2}$Co(WO$_{4}$)$_{2}$ samples. Under zero magnetic field, two successive magnetic transitions at $T_{\rm N1}\sim 9.4$ K and $T_{\rm N2}\sim 7.4$ K are observed. The magnetic ordering temperatures gradually decrease as the magnetic field increases. Neutron diffraction reveals that Li$_{2}$Co(WO$_{4}$)$_{2}$ enters an incommensurate magnetic state with a temperature dependent $\bm k$ between $T_{\rm N1}$ and $T_{\rm N2}$. The magnetic propagation vector locks-in to a commensurate value $\bm k = (1/2, 1/4, 1/4)$ below $T_{\rm N2}$. The antiferromagnetic structure is refined at 1.7 K with Co$^{2+}$ magnetic moment 2.8(1) $\mu_{\rm B}$, consistent with our first-principles calculations.     

Enhanced optical nonlinear absorption of graded Au--TiO2 composite films

This paper reports that single-layer and graded Au-TiO2 granular composite films with Au atom content 15%- 66% were prepared by using reactive co-sputtering technique. The third-order optical nonlinearity of single-layer and graded composite films was investigated by using s- and p-polarized Z-scans in femtosecond time scale. The nonlinear absorption coefficient βeff of single-layer Au-TiO2 films is measured to be -2.3×10^3-0.76×10^3 cm/GW with Au atom content 15%-66%. The βeff value of the 10-layer Au-TiO2 graded film is enhanced to be -2.1×10^4cm/GW calculated from p-polarized Z-scans, which is about ten times the maximum βeff of single-layer films. Broadened response in the wavelength region 730-860 nm of the enhanced optical nonlinearity of graded Au-TiO2 composite films was also investigated.     

Synthesis and characterization of Ca2Sn1-xCexO4 with blue luminescence originating from Ce^4＋ charge transfer transition

Ce^4＋-doped Ca2SnO4 with a one-dimensional structure, which emits bright blue light, is prepared by using a solid-state reaction method. The x-ray diffraction results show that the Ce^4＋ ions doped in Ca2SnO4 occupy the Sn^4＋ sites. The excitation and emission spectra of Ca2Sn1-xCexO4 appear to have broad bands with peaks at - 268nm and -442nm, respectively. A long excited-state lifetime （-83μs） for the emission from Ca2Sn1-xCexO4 suggests that the luminescence originates from a ligand-to-metal Ce^4＋ charge transfer （CT）. The luminescent properties of Ca2Snl_xCexO4 have been compared with those of Sr2CeO4, which is the only material reported so far to show Ce^4＋ CT luminescence. More interestingly, it is observed that the emission intensity of Ca2Sn1-xCexO4 with a small doping concentration （x - 0.03） is comparable to that of Sr2CeO4 in which the concentration of active centre is 100%.     

Superconductivity in CuIr2-xAlxTe4 telluride chalcogenides

The relationship between charge-density-wave (CDW) and superconductivity (SC), two vital physical phases in condensed matter physics, has always been the focus of scientists' research over the past decades. Motivated by this research hotspot, we systematically studied the physical properties of the layered telluride chalcogenide superconductors CuIr$_{2-x}$Al$_{x}$Te$_{4}$ ($0 \leqslant x \leqslant 0.2$). Through the resistance and magnetization measurements, we found that the CDW order was destroyed by a small amount of Al doping. Meanwhile, the superconducting transition temperature ($T_{\rm c}$) kept changing with the change of doping amount and rose towards the maximum value of 2.75 K when $x=0.075$. The value of normalized specific heat jump ($\Delta C/\gamma T_{\rm c}$) for the highest $T_{\rm c}$ sample CuIr$_{1.925}$Al$_{0.075}$Te$_{4}$ was 1.53, which was larger than the BCS value of 1.43 and showed the bulk superconducting nature. In order to clearly show the relationship between SC and CDW states, we propose a phase diagram of $T_{\rm c}$ vs. doping content.     

Spontaneous magnetostriction of Y2Fe16Al compound

The structure and magnetic properties of Y₂Fe₁₆Al compound have been investigated by means of x-ray diffraction and magnetization measurements. The Y₂Fe Fe₁₆Al compound has a hexagonal Th$_{2}$Ni$_{17}$-type structure. Negative thermal expansion was found in Y₂Fe₁₆Al compound in the temperature range from 332 to 438K by x-ray dilatometry. The coefficient of the average thermal expansion is \alpha =-3.4\times 10^-5K^-1. The spontaneous magnetostrictive deformations from 293 to 427K have been calculated based on the differences between the experimental values of the lattice parameters and the corresponding values extrapolated from the paramagnetic range. The result shows that the spontaneous volume magnetostrictive deformation \textit{$\omega $}$_{\rm S}$ decreases from 5.4$\times $10^-3to near zero with temperature increasing from 293 to 427K, the spontaneous linear magnetostrictive deformation \textit{$\lambda $}$_{\rm c}$ along the $c$ axis is much larger than the spontaneous linear magnetostrictive deformation \textit{$\lambda $}$_{\rm a}$ in basal-plane in the same temperature range except near 427K.     

Elastic collisions of sulfur and hydrogen in their ground states at low temperatures and spectroscopic parameters of SH（X^2∏） radical

This paper constructs the interaction potential of the SH（X^2∏） radical by using the coupled-cluster singlesdoubles-approximate-triples theory combining the correlation-consistent quintuple basis set augmented with the diffuse functions, aug-cc-pV5Z, in the valence range. Employing the potential, it accurately determines the spectroscopic parameters. The present De, Re, ωe, ωeχe, ae and Be values are of 3.7767eV, 0.13424nm, 2699.846 cm^-1, 47.7055 cm^-1, 0.2639cm^-1 and 9.4414 cm^-1, respectively, which are in excellent agreement with those obtained from the measure- ments. A total of 19 vibrational states has been found when J = 0 by solving the radial SchrSdinger equation of nuclear motion. The complete vibrational levels, classical turning points, initial rotation and centrifugal distortion constants when J = 0 are reported for the first time, which are in good accord with the experimental results. The total and various partial-wave cross sections are computed for the elastic collisions of sulfur and hydrogen in their ground states at low temperatures when two atoms approach each other along the SH（X^2∏） potential energy curve. Over the impact energy range from 1.0×10^-11 to 1.0×10^-4 a.u., eight shape resonances have been found in the total elastic cross sections. For each shape resonance, the resonant energy is accurately calculated. Careful investigations have pointed out that these resonances result from the 1 = 0, 1, 2, 3, 4, 6, 7, 8 partial-wave contributions.     

Effects of anisotropy and layering in the high-temperature superconductors

The key building blocks of the high-temperature copper-oxide superconductors are the CuO₂ layers, on which superconductivity tends to be localized and along which the normal-state electrical conductivity is highest. Separating these layers (or bilayers, trilayers, etc.) are locally nonsuperconducting layers, which can be modeled as proximity junctions or tunnel junctions. In this paper I summarize some consequences of a theoretical model for strongly anisotropic high-temperature superconductors in which the interlayer regions are treated as Josephson junctions. In such a model, the vortex lines threading through the structure are best visualized as stacks of two-dimensional pancake vortices connected by Josephson strings. The two-dimensional pancake vortices are centered on the layers and have Abrikosov cores, while the Josephson strings are confined to the junctions and have Josephson cores. Outside the cores, the field and current distributions can be calculated from the anisotropic Ginzburg-Landau (or London) theory. Various features of the flux-pinning anisotropy can be explained using these approaches.     

Studies of K-shell x-ray energy shifts induced by MeV/u heavy ions

This paper reports that the K x-ray spectra of the thin target 47Ag, 48Cd, 49In and 50Sn were measured by an HPGe semi-conductor detector in collisions with 84.5 MeV 6C4+ ions. Our experiment revealed the Kα x-ray energy shifts were not obvious and the Kβ1 x-ray energy shifts were about 90～110 eV. The simple model of Burch et al has been previously used to calculate the K x-ray energy shifts due to an additional vacancy in 2p orbit. The present work extends the model of Burch to calculate the x-ray energy shifts of multiple ionized atoms induced by heavy ions with kinetic energy of MeV/u. In addition to our experimental results, many other experimental results are compared with the calculated values by using the model.     

Elastic scattering of two H（^2Sg） and N（^4Su） atoms at low temperatures and accurate spectroscopic parameters of NH （X^3∑^-） radical

This paper reports that the interaction potential for the X3Z- state of NH radical is constructed at the CCSD（T）/ cc-PV6Z level of theory. Using this potential, this paper calculates the spectroscopic parameters （De, Re, ωe, ωeχe, αe and Be） and their values are of 3.578eV, 0.10368nm, 3286.833cm^-1, 78.433cm^-1, 0.6469cm^-1 and 16.6735cm^-1 respectively, which are in excellent agreement with the experiments. Then the total of 14 vibrational states has been found when J=0 by solving the radial Schrodinger equation of nuclear motion. For each vibrational state, the vibrational manifolds are reported for the first time. And last, the total cross sections, s-wave, p-wave and d-wave cross sections are computed for the elastic collisions between two ground-state atoms （hydrogen and nitrogen） at low temperatures. It finds that the total elastic cross sections are dominated by s-wave scattering when the collision energy is below 10^-6a.u. The pronounced shape resonance is found at energy of 6.1 × 10^-6a.u. Calculations have shown that the shape resonance comes from the p-wave contributions.     

Ferroelectricity in diatomic Ising chain as investigated by elastic Ising model

An elastic Ising model for a one-dimensional diatomic spin chain is proposed to explain the ferroelectricity induced by the collinear magnetic order with a low-excited energy state. A statistical theory based on this model is developed to calculate the electrical and magnetic properties of Ca₃CoMnO₆, a typical quasi-one-dimensional diatomic spin chain system. The calculated ferroelectric polarization and dielectric susceptibility show a good agreement with recently reported data on Ca₃Co_2-xMn_xO₆ (x ≈0.96) (Phys. Rev. Lett. 100 047601 (2008)), although the predicted magnetic susceptibility does not coincide well with experiment. We also address the rationality and deficiency of this model by including a first-order correction which improves the consistency between the model and experiment.     

Structural evolution and molecular dissociation of H2S under high pressures

Solid H$_{2}$S as the precursor for H$_{3}$S with incredible superconducting properties under high pressure, has recently attracted extensive attention. Here in this work, we propose two new phases of H$_{2}$S with $P$4$_{2}/n$ and $I$4$_{1}/a$ lattice symmetries in a pressure range of 0 GPa-30 GPa through first-principles structural searches, which complement the phase transition sequence. Further an $ab initio$ molecular dynamics simulation confirms that the molecular phase $P2/c$ of H$_{2}$S is gradually dissociated with the pressure increasing and reconstructs into a new $P$2$_{1}/m$ structure at 160 GPa, exhibiting the superconductivity with $T_{\rm c}$ of 82.5 K. Our results may provide a guidance for the theoretical study of low-temperature superconducting phase of H$_{2}$S.     

High-pressure study of topological semimetals XCd2Sb2 (X = Eu and Yb)

Topological materials have aroused great interest in recent years, especially when magnetism is involved. Pressure can effectively tune the topological states and possibly induce superconductivity. Here we report the high-pressure study of topological semimetals $X$Cd$_{2}$Sb$_{2}$ ($X = {\rm Eu} $ and Yb), which have the same crystal structure. In antiferromagnetic (AFM) Weyl semimetal EuCd$_{2}$Sb$_{2}$, the Néel temperature (${T}_{\rm N}$) increases from 7.4 K at ambient pressure to 50.9 K at 14.9 GPa. When pressure is above 14.9 GPa, the AFM peak of resistance disappears, indicating a non-magnetic state. In paramagnetic Dirac semimetal candidate YbCd$_{2}$Sb$_{2}$, pressure-induced superconductivity appears at 1.94 GPa, then ${ T}_{\rm c}$ reaches to a maximum of 1.67 K at 5.22 GPa and drops to zero at about 30 GPa, displaying a dome-shaped temperature-pressure phase diagram. High-pressure x-ray diffraction measurement demonstrates that a crystalline-to-amorphous phase transition occurs at about 16 GPa in YbCd$_{2}$Sb$_{2}$, revealing the robustness of pressure-induced superconductivity against structural instability. Similar structural phase transition may also occur in EuCd$_{2}$Sb$_{2}$, causing the disappearance of magnetism. Our results show that $X$Cd$_{2}$Sb$_{2}$ ($X = {\rm Eu}$ and Yb) is a novel platform for exploring the interplay among magnetism, topology, and superconductivity.     

Studies on high-moment soft magnetic FeCo/Co thin films

The dependences of soft magnetic properties and microstructures of the sputtered FeCo (=FeFeCo薄膜 溅射条件 软磁性 高饱和磁化强度FeCo film, sputtering conditions, high saturation magnetization, soft magnetic properties2005-10-263/7/2006 12:00:00 AMThe dependences of soft magnetic properties and microstructures of the sputtered FeCo （=Fe65Co35） films on Co underlayer thickness tCo, FeCo thickness tFeCo, substrate temperature Ts and taxget-substrate spacing dT-s are studied. FeCo single layer generally shows a high coercivity with no obvious magnetic anisotropy. Excellent soft magnetic properties with saturation magnetization μ0Ms of 2.35 T and hard axis coercivity Hch of 0.25 kA/m in FeCo films can be achieved by introducing a Co underlayer. It is shown that sandwiching a Co underlayer causes a change in orientation and reduction in grain size from 70 nm to about 10 nm in the FeCo layer. The magnetic softness can be explained by the Hoffmann＇s ripple theory due to the effect of grain size. The magnetic anisotropy can be controlled by changing dT-S, and a maximum of 14.3 kA/m for anisotropic field Hk is obtained with dT-S=18.0 cm.     

Elastic collisions between Si and D atoms at low temperatures and accurate analytic potential energy function and molecular constants of the SiD（χ^2П） radical

Interaction potential of the SiD(X2Π) radical is constructed by using the CCSD(T) theory in combination with the largest correlation-consistent quintuple basis set augmented with the diffuse functions in the valence range. Using the interaction potential, the spectroscopic parameters are accurately determined. The present D0, De, Re, ωe, αe and Be values are of 3.0956 eV, 3.1863 eV, 0.15223 nm, 1472.894 cm-1, 0.07799 cm-1 and 3.8717 cm-1, respectively, which are in excellent agreement with the measurements. A total of 26 vibrational states is predicted when J=0 by solving the radial Schro¨dinger equation of nuclear motion. The complete vibrational levels, classical turning points, initial rotation and centrifugal distortion constants when J=0 are reported for the first time, which are in good accord with the available experiments. The total and various partial-wave cross sections are calculated for the elastic collisions between Si and D atoms in their ground states at 1.0×10-11–1.0×10-3 a.u. when the two atoms approach each other along the SiD(X2Π) potential energy curve. Four shape resonances are found in the total elastic cross sections, and their resonant energies are of 1.73×10-5, 4.0×10-5, 6.45×10-5 and 5.5×10-4 a.u., respectively. Each shape resonance in the total elastic cross sections is carefully investigated. The results show that the shape of the total elastic cross sections is mainly dominated by the s partial wave at very low temperatures. Because of the weakness of the shape resonances coming from the higher partial waves, most of them are passed into oblivion by the strong s partial-wave elastic cross sections.     

Dy2 AlFe13 Mn3 化合物的本征磁致伸缩

通过X-射线衍射及磁测量手段研究了Dy2AlFe13Mn3化合物的结构及磁性质。研究结果表明Dy2AlFe13Mn3化合物具有六角相的Th2Ni17型结构。通过X-射线热膨胀测定法发现Dy2AlFe13Mn3化合物在245到344K的温度范围内存在负热膨胀现象，其平均热膨胀系数为α＝-1.1×10-4K-1K-1。在105到360K的温度范围内，通过比较磁性状态下的晶胞参数和由高温顺磁状态外延得到的低温顺磁状态下的晶胞参数间的差别计算了Dy2AlFe13Mn3化合物的本征磁致伸缩。结果表明Dy2AlFe13Mn3化合物的本征体磁致伸缩ωS在105到245K的温度范围内随着温度的升高而增大，由105K时的7.0×10-3 增加到245K时的9.1×10-3。随着温度的进一步升高，ωS反而减小。沿c轴方向的本征线磁致伸缩λc随着温度的升高而减小。基面内的本征线磁致伸缩λa在105到270K的温度范围内随着温度的升高而增大，从105K时的0.8×10-3增大到270K时的3.4×10-3，然后随着温度的进一步升高而减小。     

Growth of Mn5Ge3 ultrathin film on Ge（111）

The growth of Mn5Ge3 ultrathin films with different thicknesses, prepared by solid phase epitaxy, is studied. The results of scanning tunnelling microscopy and low energy electron diffraction studies show that the film can be formed and it is terminated with a （√3 × √3） R30° surface reconstruction when the thickness of Mn exceeds 3 monolayers. The magnetic properties show that the Curie temperature is about 300 K and the T^2-dependent behaviour is observed to remain up to 220 K.