Seeded x-ray free-electron laser generating radiation with laser statistical properties

The invention of optical lasers led to a revolution in the field of optics and even to the creation of completely new fields of research such as quantum optics. The reason was their unique statistical and coherence properties. The newly emerging, short-wavelength free-electron lasers (FELs) are sources of very bright coherent extreme-ultraviolet (XUV) and x-ray radiation with pulse durations on the order of femtoseconds, and are presently considered to be laser sources at these energies. Most existing FELs are highly spatially coherent but in spite of their name, they behave statistically as chaotic sources. Here, we demonstrate experimentally, by combining Hanbury Brown and Twiss (HBT) interferometry with spectral measurements that the seeded XUV FERMI FEL-2 source does indeed behave statistically as a laser. The first steps have been taken towards exploiting the first-order coherence of FELs, and the present work opens the way to quantum optics experiments that strongly rely on high-order statistical properties of the radiation.Comment: 24 pages, 10 figures, 37 reference

Ferromagnetism in a hard-core boson model

The problem of ferromagnetism -- associated with a ground state with maximal total spin -- is discussed in the framework of a hard-core model, which forbids the occupancy at each site with more than one particle. It is shown that the emergence of ferromagnetism on finite square lattices crucially depends on the statistics of the particles. Fermions (electrons) lead to the well-known instabilities for finite hole densities, whereas for bosons (with spin) ferromagnetism appears to be stable for all hole densities.Comment: 8 pages, 7 figures, RevTex

Variationnal study of ferromagnetism in the t1-t2 Hubbard chain

A one-dimensional Hubbard model with nearest and (negative) next-nearest neighbour hopping is studied variationally. This allows to exclude saturated ferromagnetism for $U < U_c$. The variational boundary $U_c (n)$ has a minimum at a ``critical density'' $n_c$ and diverges for $n \rightarrow 1$.Comment: 5 pages, LateX and 1 postscript figure. To appear in Physica

A real-time comparison between direct control, sequential pattern recognition control and simultaneous pattern recognition control using a Fitts’ law style assessment procedure

Background: Pattern recognition (PR) based strategies for the control of myoelectric upper limb prostheses are generally evaluated through offline classification accuracy, which is an admittedly useful metric, but insufficient to discuss functional performance in real time. Existing functional tests are extensive to set up and most fail to provide a challenging, objective framework to assess the strategy performance in real time. Methods: Nine able-bodied and two amputee subjects gave informed consent and participated in the local Institutional Review Board approved study. We designed a two-dimensional target acquisition task, based on the principles of Fitts' law for human motor control. Subjects were prompted to steer a cursor from the screen center of into a series of subsequently appearing targets of different difficulties. Three cursor control systems were tested, corresponding to three electromyography-based prosthetic control strategies: 1) amplitude-based direct control (the clinical standard of care), 2) sequential PR control, and 3) simultaneous PR control, allowing for a concurrent activation of two degrees of freedom (DOF). We computed throughput (bits/second), path efficiency (%), reaction time (second), and overshoot (%)) and used general linear models to assess significant differences between the strategies for each metric. Results: We validated the proposed methodology by achieving very high coefficients of determination for Fitts' law. Both PR strategies significantly outperformed direct control in two-DOF targets and were more intuitive to operate. In one-DOF targets, the simultaneous approach was the least precise. The direct control was efficient in one-DOF targets but cumbersome to operate in two-DOF targets through a switch-depended sequential cursor control. Conclusions: We designed a test, capable of comprehensively describing prosthetic control strategies in real time. When implemented on control subjects, the test was able to capture statistically significant differences (p < 0.05) in control strategies when considering throughputs, path efficiencies and reaction times. Of particular note, we found statistically significant (p < 0.01) improvements in throughputs and path efficiencies with simultaneous PR when compared to direct control or sequential PR. Amputees could readily achieve the task; however a limited number of subjects was tested and a statistical analysis was not performed with that population

Hanbury Brown and Twiss interferometry at a free-electron laser

We present measurements of second- and higher-order intensity correlation functions (so-called Hanbury Brown and Twiss experiment) performed at the free-electron laser (FEL) FLASH in the non-linear regime of its operation. We demonstrate the high transverse coherence properties of the FEL beam with a degree of transverse coherence of about 80% and degeneracy parameter of the order 10^9 that makes it similar to laser sources. Intensity correlation measurements in spatial and frequency domain gave an estimate of the FEL average pulse duration of 50 fs. Our measurements of the higher-order correlation functions indicate that FEL radiation obeys Gaussian statistics, which is characteristic to chaotic sources.Comment: 19 pages, 6 figures, 1 table, 40 reference

Statistical properties of a free-electron laser revealed by the Hanbury Brown and Twiss interferometry

We present a comprehensive experimental analysis of statistical properties of the self-amplified spontaneous emission (SASE) free-electron laser (FEL) FLASH at DESY in Hamburg by means of Hanbury Brown and Twiss (HBT) interferometry. The experiments were performed at the FEL wavelengths of 5.5 nm, 13.4 nm, and 20.8 nm. We determined the 2-nd order intensity correlation function for all wavelengths and different operation conditions of FLASH. In all experiments a high degree of spatial coherence (above 50%) was obtained. Our analysis performed in spatial and spectral domains provided us with the independent measurements of an average pulse duration of the FEL that were below 60 fs. To explain complicated behaviour of the 2-nd order intensity correlation function we developed advanced theoretical model that includes the presence of multiple beams and external positional jitter of the FEL pulses. By this analysis we determined that in most experiments several beams were present in radiating field and in one of the experiments external positional jitter was about 25% of the beam size. We envision that methods developed in our study will be used widely for analysis and diagnostics of the FEL radiation.Comment: 29 pages, 14 figures, 3 table

Towards time resolved core level photoelectron spectroscopy with femtosecond x-ray free-electron lasers

We have performed core level photoelectron spectroscopy on a W(110) single crystal with femtosecond XUV pulses from the free-electron laser at Hamburg (FLASH). We demonstrate experimentally and through theoretical modelling that for a suitable range of photon fluences per pulse, time-resolved photoemission experiments on solid surfaces are possible. Using FLASH pulses in combination with a synchronized optical laser, we have performed femtosecond time-resolved core-level photoelectron spectroscopy and observed sideband formation on the W 4f lines indicating a cross correlation between femtosecond optical and XUV pulses. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft

Exact single spin flip for the Hubbard model in d=∞d=\infty

It is shown that the dynamics of a single $\downarrow$-electron interacting with a band of $\uparrow$-electrons can be calculated exactly in the limit of infinite dimension. The corresponding Green function is determined as a continued fraction. It is used to investigate the stability of saturated ferromagnetism and the nature of the ground state for two generic non-bipartite infinite dimensional lattices. Non Fermi liquid behavior is found. For certain dopings the $\downarrow$-electron is bound to the $\uparrow$-holes.Comment: 4 pages, 3 figures included with psfig, Revtex; Phys. Rev. Lett. in press; some amendments made to clarify the calculation of the self-energy, the extrapolation of the continued fraction, and the statements on Fermi-liquid theor

Quantum Monte Carlo simulations of infinitely strongly correlated fermions

Numerical simulations of the two-dimensional t-J model in the limit $J/t \ll 1$ are performed for rather large systems (up to $N = 12 \times 12$) using a world-line loop-algorithm. It is shown that in the one-hole case with J=0, where no minus signs appear, very low temperatures ($\beta t \sim 3000$) are necessary in order to reach Nagaoka's state. J/t \ltsim 0.05 leads to the formation of partially polarized systems, whereas J/t \gtsim 0.05 corresponds to minimal spin. The two-hole case shows enhanced total spin up to the lowest attainable temperatures ($\beta t = 150$).Comment: 6 pages, 5 figure