31 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
Two-dimensional vortex behavior in highly underdoped YBa_2Cu_3O_{6+x} observed by scanning Hall probe microscopy
We report scanning Hall probe microscopy of highly underdoped superconducting
YBa_2Cu_3O_{6+x} with T_c ranging from 5 to 15 K which showed distinct flux
bundles with less than one superconducting flux quantum (Phi_0) through the
sample surface. The sub-Phi_0 features occurred more frequently for lower T_c,
were more mobile than conventional vortices, and occurred more readily when the
sample was cooled with an in-plane field component. We show that these features
are consistent with kinked stacks of pancake vortices.Comment: 11 pages, 8 figures, accepted for publication in Physical Review
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STM Studies of TbTe3: Evidence for a Fully Incommensurate Charge Density Wave
We observe unidirectional charge density wave ordering on the cleaved surface of TbTe{sub 3} with a Scanning Tunneling Microscope at {approx}6 K. The modulation wave-vector q{sub CDW} as determined by Fourier analysis is 0.71 {+-} 0.02 x2{pi}/c. (Where c is one edge of the in-plane 3D unit cell.) Images at different tip-sample voltages show the unit cell doubling effects of dimerization and the layer below. Our results agree with bulk X-ray measurements, with the addition of (1/3) x2{pi}/a ordering perpendicular to the CDW. Our analysis indicates that the CDW is incommensurate
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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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Dissipative Cryogenic Filters with Zero DC Resistance
The authors designed, implemented and tested cryogenic RF filters with zero DC resistance, based on wires with a superconducting core inside a resistive sheath. The superconducting core allows low frequency currents to pass with negligible dissipation. Signals above the cutoff frequency are dissipated in the resistive part due to their small skin depth. The filters consist of twisted wire pairs shielded with copper tape. Above approximately 1 GHz, the attenuation is exponential in {radical}{omega}, as typical for skin depth based RF filters. By using additional capacitors of 10 nF per line, an attenuation of at least 45 dB above 10 MHz can be obtained. Thus, one single filter stage kept at mixing chamber temperature in a dilution refrigerator is sufficient to attenuate room temperature black body radiation to levels corresponding to 10 mK above about 10 MHz
Examining electron-boson coupling using time-resolved spectroscopy
Nonequilibrium pump-probe time domain spectroscopies can become an important
tool to disentangle degrees of freedom whose coupling leads to broad structures
in the frequency domain. Here, using the time-resolved solution of a model
photoexcited electron-phonon system we show that the relaxational dynamics are
directly governed by the equilibrium self-energy so that the phonon frequency
sets a window for "slow" versus "fast" recovery. The overall temporal structure
of this relaxation spectroscopy allows for a reliable and quantitative
extraction of the electron-phonon coupling strength without requiring an
effective temperature model or making strong assumptions about the underlying
bare electronic band dispersion.Comment: 23 pages, 4 figures + Supplementary Material and movies, to appear in
PR
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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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Normal Modes of Black Hole Accretion Disks
This paper studies the hydrodynamical problem of normal modes of small adiabatic oscillations of relativistic barotropic thin accretion disks around black holes (and compact weakly magnetic neutron stars). Employing WKB techniques, we obtain the eigen frequencies and eigenfunctions of the modes for different values of the mass and angular momentum of the central black hole. We discuss the properties of the various types of modes and examine the role of viscosity, as it appears to render some of the modes unstable to rapid growth
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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