Single shot phase contrast imaging using laser-produced Betatron x-ray beams

Development of x-ray phase contrast imaging applications with a laboratory scale source have been limited by the long exposure time needed to obtain one image. We demonstrate, using the Betatron x-ray radiation produced when electrons are accelerated and wiggled in the laser-wakefield cavity, that a high quality phase contrast image of a complex object (here, a bee), located in air, can be obtained with a single laser shot. The Betatron x-ray source used in this proof of principle experiment has a source diameter of 1.7 microns and produces a synchrotron spectrum with critical energy E_c=12.3 +- 2.5 keV and 10^9 photons per shot in the whole spectrum.Comment: 3 pages, 3 figure

The spontaneous emergence of ordered phases in crumpled sheets

X-ray tomography is performed to acquire 3D images of crumpled aluminum foils. We develop an algorithm to trace out the labyrinthian paths in the three perpendicular cross sections of the data matrices. The tangent-tangent correlation function along each path is found to decay exponentially with an effective persistence length that shortens as the crumpled ball becomes more compact. In the mean time, we observed ordered domains near the crust, similar to the lamellae phase mixed by the amorphous portion in lyotropic liquid crystals. The size and density of these domains grow with further compaction, and their orientation favors either perpendicular or parallel to the radial direction. Ordering is also identified near the core with an arbitrary orientation, exemplary of the spontaneous symmetry breaking

Exploring patterns of recurrent melanoma in Northeast Scotland to inform the introduction a digital self-examination intervention

Peer reviewedPublisher PD

Spectral function of the 1D Hubbard model in the U→+∞U\to +\infty limit

We show that the one-particle spectral functions of the one-dimensional Hubbard model diverge at the Fermi energy like $|\omega-\varepsilon_F|^{-3/8}$ in the $U\to +\infty$ limit. The Luttinger liquid behaviour $|\omega-\varepsilon_F|^\alpha$, where $\alpha \to 1/8$ as $U\to +\infty$, should be limited to $|\omega-\varepsilon_F| \sim t^2/U$ (for $U$ large but finite), which shrinks to a single point, $\omega=\varepsilon_F$,in that limit. The consequences for the observation of the Luttinger liquid behaviour in photoemission and inverse photoemission experiments are discussed.Comment: 4 pages, RevTeX, 2 figures on reques

Surface characterization and surface electronic structure of organic quasi-one-dimensional charge transfer salts

We have thoroughly characterized the surfaces of the organic charge-transfer salts TTF-TCNQ and (TMTSF)2PF6 which are generally acknowledged as prototypical examples of one-dimensional conductors. In particular x-ray induced photoemission spectroscopy turns out to be a valuable non-destructive diagnostic tool. We show that the observation of generic one-dimensional signatures in photoemission spectra of the valence band close to the Fermi level can be strongly affected by surface effects. Especially, great care must be exercised taking evidence for an unusual one-dimensional many-body state exclusively from the observation of a pseudogap.Comment: 11 pages, 12 figures, v2: minor changes in text and figure labellin

Critical Properties in Photoemmision Spectra for One Dimensional Orbitally Degenerate Mott Insulator

Critical properties in photoemission spectra for the one-dimensional Mott insulator with orbital degeneracy are studied by exploiting the integrable {\it t-J} model, which is a supersymmetric generalization of the SU($n$) degenerate spin model. We discuss the critical properties for the holon dispersion as well as the spinon dispersions, by applying the conformal field theory analysis to the exact finite-size energy spectrum. We study the effect of orbital-splitting on the spectra by evaluating the momentum-dependent critical exponents.Comment: 8 pages, REVTeX, 2 figures(available upon request), accepted for publication in JPSJ 68 (1999) No.

Critical Properties of Spectral Functions for the 1D Anisotropic t-J Models with an Energy Gap

We exactly calculate the momentum-dependent critical exponents for spectral functions in the one-dimensional anisotropic t-J models with a gap either in the spin or charge excitation spectrum. Our approach is based on the Bethe ansatz technique combined with finite-size scaling techniques in conformal field theory. It is found that the spectral functions show a power-law singularity, which occurs at frequencies determined by the dispersion of a massive spin (or charge) excitation.We discuss how the nontrivial contribution of a massive excitation controls the singular behavior in optical response functions.Comment: 4 pages, REVTeX, 2 figures(available upon request), accepted for publication in JPSJ 66 (1997) No.

Properties of a Luttinger Liquid with Boundaries at Finite Temperature and Size

We use bosonization methods to calculate the exact finite-temperature single-electron Green's function of a spinful Luttinger liquid confined by open boundaries. The corresponding local spectral density is constructed and analyzed in detail. The interplay between boundary, finite-size and thermal effects are shown to dramatically influence the low-energy properties of the system. In particular, the well-known zero-temperature critical behavior in the bulk always crosses over to a boundary dominated regime in the vicinity of the Fermi level. Thermal fluctuations cause an enhanced depletion of spectral weight for small energies E, with the spectral density scaling as E^2 for E much less than the temperature. Consequences for photoemission experiments are discussed.Comment: 18 pages in revtex format including 5 embedded figures (using epsf). The latest complete postscript file is available from http://fy.chalmers.se/~eggert/papers/longlutt.ps or by request from [email protected]. To appear in Phys. Rev. B (Dec. 1997

Spectral functions of the 1D Hubbard model in the U -> \infty limit: How to use the factorized wave-function

We give the details of the calculation of the spectral functions of the 1D Hubbard model using the spin-charge factorized wave-function for several versions of the U -> +\infty limit. The spectral functions are expressed as a convolution of charge and spin dynamical correlation functions. A procedure to evaluate these correlation functions very accurately for large systems is developed, and analytical results are presented for the low energy region. These results are fully consistent with the conformal field theory. We also propose a direct method of extracting the exponents from the matrix elements in more general cases.Comment: 15 pages,7 eps figures, RevTeX, needs epsf and multico

Microscopic theory of the pseudogap and Peierls transition in quasi-one-dimensional materials

The problem of deriving from microscopic theory a Ginzburg-Landau free energy functional to describe the Peierls or charge-density-wave transition in quasi-one-dimensional materials is considered. Particular attention is given to how the thermal lattice motion affects the electronic states. Near the transition temperature the thermal lattice motion produces a pseudogap in the density of states at the Fermi level. Perturbation theory diverges and the traditional quasi-particle or Fermi liquid picture breaks down. The pseudogap causes a significant modification of the coefficients in the Ginzburg-Landau functional from their values in the rigid lattice approximation, which neglects the effect of the thermal lattice motion. To appear in Physical Review B.Comment: 21 pages, RevTeX, 5 figures in uuencoded compressed tar fil