18 research outputs found
STM imaging of electronic waves on the surface of BiTe: topologically protected surface states and hexagonal warping effects
Scanning tunneling spectroscopy studies on high-quality BiTe crystals
exhibit perfect correspondence to ARPES data, hence enabling identification of
different regimes measured in the local density of states (LDOS). Oscillations
of LDOS near a step are analyzed. Within the main part of the surface band
oscillations are strongly damped, supporting the hypothesis of topological
protection. At higher energies, as the surface band becomes concave,
oscillations appear which disperse with a particular wave-vector that may
result from an unconventional hexagonal warping term.Comment: 4 pages, 4 figures. Revised manuscript with improved analysis and
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Analysis and Interpretation of Hard X-ray Emission from the Bullet Cluster (1E0657-56), the Most Distant Cluster of Galaxies Observed by RXTE
Evidence for non-thermal activity in clusters of galaxies is well established
from radio observations of synchrotron emission by relativistic electrons. New
windows in the Extreme Ultraviolet and Hard X-ray ranges have provided for more
powerful tools for the investigation of this phenomenon. Detection of hard
X-rays in the 20 to 100 keV range have been reported from several clusters of
galaxies, notably from Coma and others. Based on these earlier observations we
identified the relatively high redshift cluster 1E0657-56 (also known as RX
J0658-5557) as a good candidate for hard X-ray observations. This cluster, also
known as the bullet cluster, has many other interesting and unusual features,
most notably that it is undergoing a merger, clearly visible in the X-ray
images. Here we present results from a successful RXTE observations of this
cluster. We summarize past observations and their theoretical interpretation
which guided us in the selection process. We describe the new observations and
present the constraints we can set on the flux and spectrum of the hard X-rays.
Finally we discuss the constraints one can set on the characteristics of
accelerated electrons which produce the hard X-rays and the radio radiation.Comment: Accepted by the Astrophysical Journa
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Strongly Dispersive Transient Bragg Grating for High Harmonics
We create a transient Bragg grating in a high harmonic generation medium using two counterpropagating pulses. The Bragg grating disperses the harmonics in angle and can diffract a large bandwidth with temporal resolution limited only by the source size
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Time-resolved pump-probe experiments at the LCLS
The first time-resolved x-ray/optical pump-probe experiments at the SLAC Linac Coherent Light Source (LCLS) used a combination of feedback methods and post-analysis binning techniques to synchronize an ultrafast optical laser to the linac-based x-ray laser. Transient molecular nitrogen alignment revival features were resolved in time-dependent x-ray-induced fragmentation spectra. These alignment features were used to find the temporal overlap of the pump and probe pulses. The strong-field dissociation of x-ray generated quasi-bound molecular dications was used to establish the residual timing jitter. This analysis shows that the relative arrival time of the Ti:Sapphire laser and the x-ray pulses had a distribution with a standard deviation of approximately 120 fs. The largest contribution to the jitter noise spectrum was the locking of the laser oscillator to the reference RF of the accelerator, which suggests that simple technical improvements could reduce the jitter to better than 50 fs
Imaging nonequilibrium atomic vibrations with x-ray diffuse scattering
For over a century, x-ray scattering has been the most powerful tool for
determining the equilibrium structure of crystalline materials. Deviations from
perfect periodicity, for example due to thermal motion of the atoms, reduces
the intensity of the Bragg peaks as well as produces structure in the diffuse
scattering background. Analysis of the thermal diffuse scattering (TDS) had
been used to determine interatomic force constants and phonon dispersion in
relatively simple cases before inelastic neutron scattering became the
preferred technique to study lattice dynamics. With the advent of intense
synchrotron x-ray sources, there was a renewed interest in TDS for measuring
phonon dispersion. The relatively short x-ray pulses emanating from these
sources also enables the measurement of phonon dynamics in the time domain.
Prior experiments on nonequilibrium phonons were either limited by
time-resolution and/or to relatively long wavelength excitations. Here we
present the first images of nonequilibrium phonons throughout the Brillouin
zone in photoexcited III-V semiconductors, indium-phosphide and
indium-antimonide, using picosecond time-resolved diffuse scattering. In each
case, we find that the lattice remain out of equilibrium for several hundred
picoseconds up to nanoseconds after laser excitation. The non-equilibrium
population is dominated by transverse acoustic phonons which in InP are
directed along high-symmetry directions. The results have wide implications for
the detailed study of electron-phonon and phonon-phonon coupling in solids.Comment: 10 pages, 3 figure
On the Radio and Optical Luminosity Evolution of Quasars
We calculate simultaneously the radio and optical luminosity evolutions of
quasars, and the distribution in radio loudness R defined as the ratio of radio
and optical luminosities, using a flux limited data set containing 636 quasars
with radio and optical fluxes from White et al. We first note that when dealing
with multivariate data it is imperative to first determine the true
correlations among the variables, not those introduced by the observational
selection effects, before obtaining the individual distributions of the
variables. We use the methods developed by Efron and Petrosian which are
designed to obtain unbiased correlations, distributions, and evolution with
redshift from a data set truncated due to observational biases. It is found
that the population of quasars exhibits strong positive correlation between the
radio and optical luminosities. With this correlation, whether intrinsic or
observationally induced accounted for, we find that there is a strong
luminosity evolution with redshift in both wavebands, with significantly higher
radio than optical evolution. We also construct the local radio and optical
luminosity functions and the density evolution. Finally, we consider the
distribution of the radio loudness parameter R obtained from careful treatment
of the selection effects and luminosity evolutions with that obtained from the
raw data without such considerations. We find a significant difference between
the two distributions and no clear sign of bi-modality in the true distribution
for the range of R values considered. Our results indicate therefore, somewhat
surprisingly, that there is no critical switch in the efficiency of the
production of disk outflows/jets between very radio quiet and very radio loud
quasars, but rather a smooth transition. Also, this efficiency seems higher for
the high-redshift and more luminous sources in the considered sample.Comment: 15 pages, 15 figures, accepted to ApJ, updated to in press versio
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Photon Enhanced Thermionic Emission for Solar Energy Harvesting Final Report to the Global Climate and Energy Project
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The Quasiparticle Puzzle: Reconciling ARPES and FTSTS Studies of Bi2212
Angle Resolved Photoemission Spectroscopy (ARPES) probes the momentum-space electronic structure of materials, and provides invaluable information about the high-temperature superconducting cuprates. Likewise, cuprates real-space, inhomogeneous electronic structure is elucidated by Scanning Tunneling Spectroscopy (STS). Recently, STS has exploited quasiparticle interference (QPI) - wave-like electrons scattering off impurities to produce periodic interference patterns - to infer properties of the QP in momentum-space. Surprisingly, some interference peaks in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} (Bi-2212) are absent beyond the antiferromagnetic (AF) zone boundary, implying the dominance of particular scattering process. Here, we show that ARPES sees no evidence of quasiparticle (QP) extinction: QP-like peaks are measured everywhere on the Fermi surface, evolving smoothly across the AF zone boundary. This apparent contradiction stems from different natures of single-particle (ARPES) and two-particle (STS) processes underlying these probes. Using a simple model, we demonstrate extinction of QPI without implying the loss of QP beyond the AF zone boundary
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THz Pump and X-Ray Probe Development at LCLS
We report on measurements of broadband, intense, coherent transition radiation at terahertz frequencies, generated as the highly compressed electron bunches in Linear Coherent Light Source (LCLS) pass through a thin metal foil. The foil is inserted at 45{sup o} to the electron beam, 31 m downstream of the undulator. The THz emission passes downward through a diamond window to an optical table below the beamline. A fully compressed 350-pC bunch produces up to 0.5 mJ in a nearly half-cycle pulse of 50 fs FWHM with a spectrum peaking at 10 THz. We estimate a peak field at the focus of over 2.5 GV/m. A 20-fs Ti:sapphire laser oscillator has recently been installed for electro-optic measurements. We are developing plans to add an x-ray probe to this THz pump, by diffracting FEL x rays onto the table with a thin silicon crystal. The x rays would arrive with an adjustable time delay after the THz. This will provide a rapid start to user studies of materials excited by intense single-cycle pulses and will serve as a step toward a THz transport line for LCLS-II
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Demonstrating the Model Nature of the High-Temperature Superconductor HgBa2CuO4+d
The compound HgBa{sub 2}CuO{sub 4+{delta}} (Hg1201) exhibits a simple tetragonal crystal structure and the highest superconducting transition temperature (T{sub c}) among all single Cu-O layer cuprates, with T{sub c} = 97 K (onset) at optimal doping. Due to a lack of sizable single crystals, experimental work on this very attractive system has been significantly limited. Thanks to a recent breakthrough in crystal growth, such crystals have now become available. Here, we demonstrate that it is possible to identify suitable heat treatment conditions to systematically and uniformly tune the hole concentration of Hg1201 crystals over a wide range, from very underdoped (T{sub c} = 47 K, hole concentration p {approx} 0.08) to overdoped (T{sub c} = 64 K, p {approx} 0.22). We then present quantitative magnetic susceptibility and DC charge transport results that reveal the very high-quality nature of the studied crystals. Using XPS on cleaved samples, we furthermore demonstrate that it is possible to obtain large surfaces of good quality. These characterization measurements demonstrate that Hg1201 should be viewed as a model high-temperature superconductor, and they provide the foundation for extensive future experimental work