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We present the results of a high-resolution x-ray scattering study of the thermal roughening of Ag(l 10). Using scattering from the bulk-forbidden (llO) surface peak as a probe, we obtain direct information about thermal variations in the height-height correlation function. Specifically, we find that the surface undergoes a roughening transition at TR =450 ± 25 °C. Above TR, the rough phase is characterized by logarithmically divergent height fluctuations.PACS numbers: 68.35.Rh, 61.50.Ks, 64.60.Cn At low temperatures, the equilibrium interface between a crystalline solid and its vapor is a well-defined crystalline surface. Alternatively, the thermal excitation of capillary waves causes the surface of a liquid to fluctuate with an amplitude (roughness) which diverges logarithmically with the size of the system. In between these two extremes, many of the faces of a crystal in equilibrium with its vapor disappear. Theoretical arguments suggest that the disappearance of a crystalline surface occurs at a critical point at which the surface-vapor interface transforms from smooth to rough. l On an atomic level, this transition results from the proliferation of atomic steps and islands on the crystalline surface. 2 " 4 The concept of a roughening transition has played a central role in theories of crystal growth and equlibrium crystal shapes, l and has been used empirically by experimentalists for many years. Nonetheless, the nature of the transition, and indeed its very existence, remain controversial. 5 Roughness is most simply characterized by the surface or interface height-height correlation function, C(r) = ([/*(r) -/z(O)] 2 ), where r is a two-dimensional vector on the interface and Mr) is the height of the interface at position r. When the surface of a crystal is smooth, the asymptotic limit of C(r) is a constant. Above the roughening transition temperature TR, C(r) is predicted to diverge logarithmically 2 ; that is, the surface height is no longer well defined. The scattering from a smooth surface is a series of Bragg rods with 5-function cross sections. There exist specific points along these rods at which scattering from alternate planes of atoms exactly cancels. These points (known as bulk-forbidden or antiBragg points) are sensitive to surface steps and thus probe C(r). When the surface becomes logarithmically rough, the scattering from these points evolves from a 5 function into a power-law line shape with an exponent rj. This exponent is a direct measure of the rate of the logarithmic divergence of C(r) and, hence, of the surface roughness.Recent atom-scattering experiments have suggested the roughening of vicinal (1 In) surfaces of Cu and Ni. 6,7 We present here an x-ray scattering study of the lowindex (110) surface of Ag. X-ray scattering provides significantly better resolution than atom scattering, as well as line shapes which are not affected by multiple and inelastic scattering, effects which often make atomscattering and LEED data difficult to interpret. 6 In particular, x-ray scattering measur...

The energy extraction efficiency is a figure of merit for a free-electron laser (FEL). It can be enhanced by the technique of undulator tapering, which enables the sustained growth of radiation power beyond the initial saturation point. In the development of a single-pass x-ray FEL, it is important to exploit the full potential of this technique and optimize the taper profile a w ðzÞ. Our approach to the optimization is based on the theoretical model by Kroll, Morton, and Rosenbluth, whereby the taper profile a w ðzÞ is not a predetermined function (such as linear or exponential) but is determined by the physics of a resonant particle. For further enhancement of the energy extraction efficiency, we propose a modification to the model, which involves manipulations of the resonant particle's phase. Using the numerical simulation code GENESIS, we apply our model-based optimization methods to a case of the future FEL at the MAX IV Laboratory (Lund, Sweden), as well as a case of the LCLS-II facility (Stanford, USA).

Research at modern light sources continues to improve our knowledge of the natural world, from the subtle workings of life to matter under extreme conditions. Free-electron lasers, for instance, have enabled the characterization of biomolecular structures with sub-ångström spatial resolution, and paved the way to controlling the molecular functions. On the other hand, attosecond temporal resolution is necessary to broaden our scope of the ultrafast world. Here we discuss attosecond pulse generation beyond present capabilities. Furthermore, we review three recently proposed methods of generating attosecond x-ray pulses. These novel methods exploit the coherent radiation of microbunched electrons in undulators and the tailoring of the emitted wavefronts. The computed pulse energy outperforms pre-existing technologies by three orders of magnitude. Specifically, our simulations of the proposed Soft X-Ray Laser (SXL) at MAX IV (Lund, Sweden) show that a pulse duration of 50-100 attoseconds and a pulse energy up to 5 microjoules is feasible with the novel methods. In addition, the methods feature pulse shape control, enable the incorporation of orbital angular momentum, and can be used in combination with modern compact free-electron laser setups.