We have successfully grown high-quality single crystals of SrFe 2 As 2 and A 0.6 K 0.4 Fe 2 As 2 ͑A = Sr, Ba͒ using flux method. The resistivity, specific heat, and Hall coefficient have been measured. For parent compound SrFe 2 As 2 , an anisotropic resistivity with c / ab as large as 130 is obtained at low temperatures. A sharp drop in both in-plane and out-plane resistivities due to the spin-density-wave ͑SDW͒ instability is observed below 200 K. The angular dependence of in-plane magnetoresistance shows twofold symmetry with field rotating within ab plane below SDW transition temperature. This is consistent with a stripe-type spin ordering in SDW state. In K-doped A 0.6 K 0.4 Fe 2 As 2 ͑A = Sr, Ba͒, the SDW instability is suppressed and the superconductivity appears with T c above 35 K. The rather low anisotropy in upper critical field between H ʈ ab and H ʈ c indicates that interplane coupling plays an important role in hole-doped Fe-based superconductors.The recent discovery of superconductivity with transition temperature T c ϳ 26 K in LaFeAsO 1−x F x has generated tremendous interest in scientific community. 1 Shortly after this discovery, the T c was raised to 41-55 K by replacing La by rare-earth Ce, Sm, Pr, Nd, etc., making those systems with T c exceeding 50 K. 2-5 The undoped quaternary compounds crystallize in a tetragonal ZrCuSiAs-type structure, which consists of alternate stacking of edge-sharing Fe 2 As 2 tetrahedral layers and La 2 O 2 tetrahedral layers along c axis. Very recently, superconductivity with T c of up to 38 K was discovered in AFe 2 As 2 ͑A = Ba, Sr, Ca͒ upon K or Na doping. 6-10 AFe 2 As 2 compounds crystallize in a tetragonal ThCr 2 Si 2 -type structure with identical Fe 2 As 2 tetrahedral layers as in LaFeAsO, but separated by single elemental A layers. These compounds contain no oxygen in A layers. The simpler structure of AFe 2 As 2 system makes it more suitable for research of intrinsic physical properties of Fe-based compounds.Except for a relatively high transition temperature, the system displays many interesting properties. The existence of a spin-density-wave ͑SDW͒ instability in parent LaFeAsO ͑Ref. 11͒ was indicated by specific heat, optical measurements, and first-principles calculations, and subsequently confirmed by neutron-scattering, 12 NMR, 13 sR, 14 and Mössbauer 15 spectroscopic measurements. The superconductivity only appears when SDW instability was suppressed by doping carriers or applying pressure. The competition between superconductivity and SDW instability was identified in other rare-earth substituted systems. 2,16,17 Besides the SDW instability, structural distortions from tetragonal to monoclinic were also observed for both ReFeAsO ͑Re = rare earth͒ and AFe 2 As 2 ͑A = Ba, Sr, Ca͒. 18-23 The structural transition temperatures were found to occur at slightly higher than SDW transition temperature in LaFeAsO, 12 but the two transitions occur simultaneously in AFe 2 As 2 ͑A = Ba, Sr, Ca͒. 18,19,24 The band-structure calculation and neutronscattering exp...

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Superradiance and subradiance concerning enhanced and inhibited collective radiation of an ensemble of atoms have been a central topic in quantum optics. However, precise generation and control of these states remain challenging. Here we deterministically generate up to 10-qubit superradiant and 8-qubit subradiant states, each containing a single excitation, in a superconducting quantum circuit with multiple qubits interconnected by a cavity resonator. The √ N -scaling enhancement of the coupling strength between the superradiant states and the cavity is validated. By applying appropriate phase gate on each qubit, we are able to switch the single collective excitation between superradiant and subradiant states. While the subradiant states containing a single excitation are forbidden from emitting photons, we demonstrate that they can still absorb photons from the resonator. However, for even number of qubits, a singlet state with half of the qubits being excited can neither emit nor absorb photons, which is verified with 4 qubits. This study is a step forward in coherent control of collective radiation and has promising applications in quantum information processing.

Fractal dendritic morphologies will be formed when metal materials are synthesized in nonequilibrium conditions. In the past, scientists mainly studied the effect of noise and anisotropy on morphology evolution. However, in this Article, fractal dendritic silver is synthesized through a simple wet-chemical method, spontaneous galvanic displacement between Ag ions and Zn plate, and through Monte Carlo simulation we found that walking of ions within the diffusion layer can affect the morphology of the whole shape, and anisotropy only affects the shape in local area. As the longitudinal velocity of silver ions walking within the diffusion layer increases, the ions are more easily distributed on the hollow part of fractal silver, and thus the fractal silver will transform from loose fractal (LF) to dense branch morphology (DBM), and shape densities and fractal dimensions of fractal morphologies also increase.