16,989 research outputs found
Direct 3D Tomographic Reconstruction and Phase-Retrieval of Far-Field Coherent Diffraction Patterns
We present an alternative numerical reconstruction algorithm for direct
tomographic reconstruction of a sample refractive indices from the measured
intensities of its far-field coherent diffraction patterns. We formulate the
well-known phase-retrieval problem in ptychography in a tomographic framework
which allows for simultaneous reconstruction of the illumination function and
the sample refractive indices in three dimensions. Our iterative reconstruction
algorithm is based on the Levenberg-Marquardt algorithm. We demonstrate the
performance of our proposed method with simulation studies
Discovery of Broad Molecular lines and of Shocked Molecular Hydrogen from the Supernova Remnant G357.7+0.3: HHSMT, APEX, Spitzer and SOFIA Observations
We report a discovery of shocked gas from the supernova remnant (SNR)
G357.7+0.3. Our millimeter and submillimeter observations reveal broad
molecular lines of CO(2-1), CO(3-2), CO(4-3), 13CO (2-1) and 13CO (3-2), HCO^+
and HCN using HHSMT, Arizona 12-Meter Telescope, APEX and MOPRA Telescope. The
widths of the broad lines are 15-30 kms, and the detection of such broad lines
is unambiguous, dynamic evidence showing that the SNR G357.7+0.3 is interacting
with molecular clouds. The broad lines appear in extended regions (>4.5'x5').
We also present detection of shocked H2 emission in mid-infrared but lacking
ionic lines using the Spitzer IRS observations to map a few arcmin area. The H2
excitation diagram shows a best-fit with a two-temperature LTE model with the
temperatures of ~200 and 660 K. We observed [C II] at 158um and high-J
CO(11-10) with the GREAT on SOFIA. The GREAT spectrum of [C II], a 3 sigma
detection, shows a broad line profile with a width of 15.7 km/s that is similar
to those of broad CO molecular lines. The line width of [C~II] implies that
ionic lines can come from a low-velocity C-shock. Comparison of H2 emission
with shock models shows that a combination of two C-shock models is favored
over a combination of C- and J-shocks or a single shock. We estimate the CO
density, column density, and temperature using a RADEX model. The best-fit
model with n(H2) = 1.7x10^{4} cm^{-3}, N(CO) = 5.6x10^{16} cm^{-2}, and T = 75
K can reproduce the observed millimeter CO brightnesses.Comment: 19 pages, 22 figure
Interpretation of scanning tunneling quasiparticle interference and impurity states in cuprates
We apply a recently developed method combining first principles based Wannier
functions with solutions to the Bogoliubov-de Gennes equations to the problem
of interpreting STM data in cuprate superconductors. We show that the observed
images of Zn on the surface of BiSrCaCuO can only be understood
by accounting for the tails of the Cu Wannier functions, which include
significant weight on apical O sites in neighboring unit cells. This
calculation thus puts earlier crude "filter" theories on a microscopic
foundation and solves a long standing puzzle. We then study quasiparticle
interference phenomena induced by out-of-plane weak potential scatterers, and
show how patterns long observed in cuprates can be understood in terms of the
interference of Wannier functions above the surface. Our results show excellent
agreement with experiment and enable a better understanding of novel phenomena
in the cuprates via STM imaging.Comment: 5 pages, 5 figures, published version (Supplemental Material: 5
pages, 11 figures) for associated video file, see
http://itp.uni-frankfurt.de/~kreisel/QPI_BSCCO_BdG_p_W.mp
Developing the MTO Formalism
We review the simple linear muffin-tin orbital method in the atomic-spheres
approximation and a tight-binding representation (TB-LMTO-ASA method), and show
how it can be generalized to an accurate and robust Nth order muffin-tin
orbital (NMTO) method without increasing the size of the basis set and without
complicating the formalism. On the contrary, downfolding is now more efficient
and the formalism is simpler and closer to that of screened multiple-scattering
theory. The NMTO method allows one to solve the single-electron Schroedinger
equation for a MT-potential -in which the MT-wells may overlap- using basis
sets which are arbitrarily minimal. The substantial increase in accuracy over
the LMTO-ASA method is achieved by substitution of the energy-dependent partial
waves by so-called kinked partial waves, which have tails attached to them, and
by using these kinked partial waves at N+1 arbitrary energies to construct the
set of NMTOs. For N=1 and the two energies chosen infinitesimally close, the
NMTOs are simply the 3rd-generation LMTOs. Increasing N, widens the energy
window, inside which accurate results are obtained, and increases the range of
the orbitals, but it does not increase the size of the basis set and therefore
does not change the number of bands obtained. The price for reducing the size
of the basis set through downfolding, is a reduction in the number of bands
accounted for and -unless N is increased- a narrowing of the energy window
inside which these bands are accurate. A method for obtaining orthonormal NMTO
sets is given and several applications are presented.Comment: 85 pages, Latex2e, Springer style, to be published in: Lecture notes
in Physics, edited by H. Dreysse, (Springer Verlag
Third-Generation TB-LMTO
We describe the screened Korringa-Kohn-Rostoker (KKR) method and the
third-generation linear muffin-tin orbital (LMTO) method for solving the
single-particle Schroedinger equation for a MT potential. The simple and
popular formalism which previously resulted from the atomic-spheres
approximation (ASA) now holds in general, that is, it includes downfolding and
the combined correction. Downfolding to few-orbital, possibly short-ranged,
low-energy, and possibly orthonormal Hamiltonians now works exceedingly well,
as is demonstrated for a high-temperature superconductor. First-principles sp3
and sp3d5 TB Hamiltonians for the valence and lowest conduction bands of
silicon are derived. Finally, we prove that the new method treats overlap of
the potential wells correctly to leading order and we demonstrate how this can
be exploited to get rid of the empty spheres in the diamond structure.Comment: latex2e, 32 printed pages, Postscript figs, to be published in:
Tight-Binding Approach to Computational Materials Science, MRS Symposia
Proceedings No. 491 (MRS, Pittsburgh, 1998
Survival of a Diffusing Particle in a Transverse Shear Flow: A First-Passage Problem with Continuously Varying Persistence Exponent
We consider a particle diffusing in the y-direction, dy/dt=\eta(t), subject
to a transverse shear flow in the x-direction, dx/dt=f(y), where x \ge 0 and
x=0 is an absorbing boundary. We treat the class of models defined by f(y) =
\pm v_{\pm}(\pm y)^\alpha where the upper (lower) sign refers to y>0 (y<0). We
show that the particle survives with probability Q(t) \sim t^{-\theta} with
\theta = 1/4, independent of \alpha, if v_{+}=v_{-}. If v_{+} \ne v_{-},
however, we show that \theta depends on both \alpha and the ratio v_{+}/v_{-},
and we determine this dependence.Comment: 4 page
Extinction of impurity resonances in large-gap regions of inhomogeneous d-wave superconductors
Impurity resonances observed by scanning tunneling spectroscopy in the
superconducting state have been used to deduce properties of the underlying
pure state. Here we study a longstanding puzzle associated with these
measurements, the apparent extinction of these resonances for Ni and Zn
impurities in large-gap regions of the inhomogeneous BSCCO superconductor. We
calculate the effect of order parameter and hopping suppression near the
impurity site, and find that these two effects are sufficient to explain the
missing resonances in the case of Ni. There are several possible scenarios for
the extinction of the Zn resonances, which we discuss in turn; in addition, we
propose measurements which could distinguish among them.Comment: 10 pages, 8 figure
Pattern formation and selection in quasi-static fracture
Fracture in quasi-statically driven systems is studied by means of a discrete
spring-block model. Developed from close comparison with desiccation
experiments, it describes crack formation induced by friction on a substrate.
The model produces cellular, hierarchical patterns of cracks, characterized by
a mean fragment size linear in the layer thickness, in agreement with
experiments. The selection of a stationary fragment size is explained by
exploiting the correlations prior to cracking. A scaling behavior associated
with the thickness and substrate coupling, derived and confirmed by
simulations, suggests why patterns have similar morphology despite their
disparity in scales.Comment: 4 pages, RevTeX, two-column, 5 PS figures include
Three-loop HTL gluon thermodynamics at intermediate coupling
We calculate the thermodynamic functions of pure-glue QCD to three-loop order
using the hard-thermal-loop perturbation theory (HTLpt) reorganization of
finite temperature quantum field theory. We show that at three-loop order
hard-thermal-loop perturbation theory is compatible with lattice results for
the pressure, energy density, and entropy down to temperatures .
Our results suggest that HTLpt provides a systematic framework that can used to
calculate static and dynamic quantities for temperatures relevant at LHC.Comment: 24 pages, 13 figs. 2nd version: improved discussion and fixing typos.
Published in JHE
The Structure of Barium in the hcp Phase Under High Pressure
Recent experimental results on two hcp phases of barium under high pressure
show interesting variation of the lattice parameters. They are here interpreted
in terms of electronic structure calculation by using the LMTO method and
generalized pseudopotential theory (GPT) with a NFE-TBB approach. In phase II
the dramatic drop in c/a is an instability analogous to that in the group II
metals but with the transfer of s to d electrons playing a crucial role in Ba.
Meanwhile in phase V, the instability decrease a lot due to the core repulsion
at very high pressure. PACS numbers: 62.50+p, 61.66Bi, 71.15.Ap, 71.15Hx,
71.15LaComment: 29 pages, 8 figure
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