Summary form only given. Fundamental processes in atoms, molecules, as well as condensed matter are triggered or mediated by the motion of electrons inside or between atoms. Electronic dynamics on atomic length scales tends to unfold within tens to thousands of attoseconds (1 attosecond [as] = 10-18 s). Recent breakthroughs in laser science are now opening the door to watching and controlling these hitherto inaccessible microscopic dynamics. The key to accessing the attosecond time domain is the control of the electric field of (visible) light, which varies its strength and direction within less than a femtosecond (1 femtosecond = 1000 attoseconds). Atoms exposed to a few oscillations cycles of intense laser light are able to emit a single extreme ultraviolet (XUV) burst lasting less than one femtosecond. Full control of the evolution of the electromagnetic field in laser pulses comprising a few wave cycles have recently allowed the reproducible generation and measurement of isolated sub-femtosecond XUV pulses, demonstrating the control of microscopic processes (electron motion and photon emission) on an attosecond time scale. These tools have enabled us to visualize the oscillating electric field of visible light with an attosecond "oscilloscope", to control single-electron and probe multi-electron dynamics in atoms, molecules and solids. Recent experiments hold promise for the development of an attosecond X-ray source, which may pave the way towards 4D electron imaging with sub-atomic resolution in space and time.

Attosecond Physics

The response of the worldwide scientific community to the discovery in 2008 of superconductivity at T c = 26 K in the Fe-based compound LaFeAsO1−x F x has been very enthusiastic. In short order, ot...

The puzzle of high temperature superconductivity in layered iron pnictides and chalcogenides

Kamihara and coworkers' report of superconductivity at ${T}_{c}=26\text{ }\text{ }\mathrm{K}$ in fluorine-doped LaFeAsO inspired a worldwide effort to understand the nature of the superconductivity in this new class of compounds. These iron pnictide and chalcogenide (FePn/Ch) superconductors have Fe electrons at the Fermi surface, plus an unusual Fermiology that can change rapidly with doping, which lead to normal and superconducting state properties very different from those in standard electron-phonon coupled ``conventional'' superconductors. Clearly, superconductivity and magnetism or magnetic fluctuations are intimately related in the FePn/Ch, and even coexist in some. Open questions, including the superconducting nodal structure in a number of compounds, abound and are often dependent on improved sample quality for their solution. With ${T}_{c}$ values up to 56 K, the six distinct Fe-containing superconducting structures exhibit complex but often comparable behaviors. The search for correlations and explanations in this fascinating field of research would benefit from an organization of the large, seemingly disparate data set. This review provides an overview, using numerous references, with a focus on the materials and their superconductivity.

Superconductivity in iron compounds

Superconductivity has been observed in iron-arsenic (or iron pnictide) compounds with critical transition temperatures as high as 56 K, inviting comparison with the high-Tc copper oxides. The copper oxides have quasi-two-dimensional electronic properties, which led to speculation that extreme anisotropy (direction dependence) is a requirement for superconductivity at temperatures above 40 K. Yuan et al. report that this is not the case: the superconducting properties of (Ba,K)Fe2As2 are in fact quite isotropic. This behaviour is strikingly different from all previously known layered superconductors. It is shown that the superconducting properties of (Ba,K)Fe2As2 are quite isotropic. Such behaviour is strikingly different from all previously known layered superconductors, and indicates that reduced dimensionality in these compounds is not a prerequisite for 'high-temperature' superconductivity. Superconductivity was recently observed1,2,3,4,5,6,7 in iron-arsenic-based compounds with a superconducting transition temperature (Tc) as high as 56 K, naturally raising comparisons with the high-Tc copper oxides. The copper oxides have layered crystal structures with quasi-two-dimensional electronic properties, which led to speculation that reduced dimensionality (that is, extreme anisotropy) is a necessary prerequisite for superconductivity at temperatures above 40 K (refs 8, 9). Early work on the iron-arsenic compounds seemed to support this view7,10. Here we report measurements of the electrical resistivity in single crystals of (Ba,K)Fe2As2 in a magnetic field up to 60 T. We find that the superconducting properties are in fact quite isotropic, being rather independent of the direction of the applied magnetic fields at low temperature. Such behaviour is strikingly different from all previously known layered superconductors9,11, and indicates that reduced dimensionality in these compounds is not a prerequisite for ‘high-temperature’ superconductivity. We suggest that this situation arises because of the underlying electronic structure of the iron-arsenic compounds, which appears to be much more three dimensional than that of the copper oxides. Extrapolations of low-field single-crystal data incorrectly suggest a high anisotropy and a greatly exaggerated zero-temperature upper critical field.

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Nearly isotropic superconductivity in (Ba,K)Fe 2 As 2

Models of few optical cycle solitons beyond the slowly varying envelope approximation

We theoretically investigate the high-order harmonic generation in a helium atom with a two-color optical field synthesized by an intense 6 fs pulse at 800 nm and a relatively weak 21.3 fs pulse at 400 nm. When the frequency-doubled pulse is properly time shifted with respect to the fundamental pulse, an ultrabroad extreme ultraviolet supercontinuum spectrum with a 148 eV spectral width can be generated which directly creates an isolated 65 as pulse even without phase compensation. We explain this extraordinary phenomenon by analyzing maximum electron kinetic energies at different return times.

Generation of an extreme ultraviolet supercontinuum in a two-color laser field.

We have successfully grown high-quality single crystals of ${\text{SrFe}}_{2}{\text{As}}_{2}$ and ${A}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_{2}{\text{As}}_{2}$($A=\text{Sr}$, Ba) using flux method. The resistivity, specific heat, and Hall coefficient have been measured. For parent compound ${\text{SrFe}}_{2}{\text{As}}_{2}$, an anisotropic resistivity with ${\ensuremath{\rho}}_{c}/{\ensuremath{\rho}}_{ab}$ 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 (SDW) 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 ${A}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_{2}{\text{As}}_{2}$($A=\text{Sr}$, Ba), the SDW instability is suppressed and the superconductivity appears with ${T}_{c}$ above 35 K. The rather low anisotropy in upper critical field between $H\ensuremath{\parallel}ab$ and $H\ensuremath{\parallel}c$ indicates that interplane coupling plays an important role in hole-doped Fe-based superconductors.

Transport and anisotropy in single-crystalline SrFe 2 As 2 and A 0.6 K 0.4 Fe 2 As 2 ( A = Sr , Ba) superconductors

Feynman's path integral approach is to sum over all possible spatiotemporal paths to reproduce the quantum wave function and the corresponding time evolution, which has enormous potential to reveal quantum processes in the classical view. However, the complete characterization of the quantum wave function with infinite paths is a formidable challenge, which greatly limits the application potential, especially in the strong-field physics and attosecond science. Instead of brute-force tracking every path one by one, here we propose a deep-learning-performed strong-field Feynman's formulation with a preclassification scheme that can predict directly the final results only with data of initial conditions, so as to attack unsurmountable tasks by existing strong-field methods and explore new physics. Our results build a bridge between deep learning and strong-field physics through Feynman's path integral, which would boost applications of deep learning to study the ultrafast time-dependent dynamics in strong-field physics and attosecond science and shed new light on the quantum-classical correspondence.

Deep Learning for Feynman's Path Integral in Strong-Field Time-Dependent Dynamics.

We theoretically investigate carrier-envelope phase dependence of few-cycle ultrashort laser pulse propagation in a polar molecule medium. Our results show that a soliton pulse can be generated during the two-photon resonant propagation of few-cycle pulse in the polar molecule medium. Moreover, the main features of the soliton pulse, such as pulse duration and intensity, depend crucially on the carrier-envelope phase of the incident pulse, which could be utilized to determine the carrier-envelope phase of a few-cycle ultrashort laser pulse from a mode-locked oscillator.

Carrier-envelope phase dependence of few-cycle ultrashort laser pulse propagation in a polar molecule medium

We present a method to generate an extremely short unipolar half-cycle pulse based on resonant propagation of a few-cycle pulse through asymmetrical media with periodic subwavelength structure. Moreover, single- and few-cycle gap solitons with different frequencies are found to split from one incident few-cycle ultrashort pulse. These solitons with various frequencies provide evidence for the generation of different parametric waves during the strong light-matter coupling in asymmetrical media under the extreme nonlinear optics condition. Because of the pulse self-shaping process during the course of resonant propagation, the generated low-frequency sideband and another broadband continuum sideband ranging from the visible to the near-infrared regime couple together, which results in the generation of the subfemtosecond unipolar half-cycle pulse. A time-frequency analysis is preformed which corroborates the mechanism. The generated unipolar half-cycle pulse might be utilized to control and probe the ultrafast electronic dynamics.

Xiaohong Song

Papers

Generation of an extreme ultraviolet supercontinuum in a two-color laser field.

Transport and anisotropy in single-crystalline SrFe 2 As 2 and A 0.6 K 0.4 Fe 2 As 2 ( A = Sr , Ba) superconductors

Deep Learning for Feynman's Path Integral in Strong-Field Time-Dependent Dynamics.

Carrier-envelope phase dependence of few-cycle ultrashort laser pulse propagation in a polar molecule medium

Unipolar half-cycle pulse generation in asymmetrical media with a periodic subwavelength structure