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Ludwig, A.

Publications and source records attributed to Ludwig, A..

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Unusual Near-Horizon Cosmic-Ray-like Events Observed by ANITA-IV

ANITA’s fourth long-duration balloon flight in 2016 detected 29 cosmic-ray (CR)-like events on a background of 0.37 - 0.17 + 0.27 anthropogenic events. CRs are mainly seen in reflection off the Antarctic ice sheets, creating a phase-inverted waveform polarity. However, four of the below-horizon CR-like events show anomalous noninverted polarity, a p = 5.3 × 10 - 4 chance if due to background. Furthermore, all anomalous events are from locations near the horizon; ANITA-IV observed no steeply upcoming anomalous events similar to the two such events seen in prior flights.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Calculation Accuracy and Efficiency of a Transient Model for Submerged Entry Nozzle Clogging

A transient numerical model was proposed and validated by the current authors for nozzle clogging (Barati et al. in Powder Technol 329:181-98, 2018). The model can reproduce the experiment in pilot scale satisfactorily. In the present article, the main objective is to validate the model for application in industry process continuous casting of steel, referring to the model accuracy and calculation efficiency. The results have shown that for the complex geometry of submerged entry nozzle (SEN), where it is difficult to create hexahedron mesh in the entire domain, a mixed mesh type is recommended, i.e., the wedge mesh for regions adjacent to SEN walls and the tetrahedron mesh for inner regions. Another challenge to the calculation of real SEN clogging is the huge number of particles involved in the industry process. An artificial factor, the N-factor, has to be introduced to reduce the calculation cost. A dimensionless number (α) is defined to limit the N-factor and ensure the modeling accuracy. Simulation of a test case has indicated that by an appropriate N-factor (1000, corresponding to α = 6 × 10{sup −5}), the calculation time would be reduced significantly to a reasonable time.

36 MATERIALS SCIENCE↗
DOE OSTI2018

HiCal 2: An instrument designed for calibration of the ANITA experiment and for Antarctic surface reflectivity measurements

The NASA supported High-Altitude Calibration (HiCal)-2 instrument flew as a companion balloon to the ANITA-4 experiment in December 2016. Based on a high-voltage (HV) discharge pulser producing radio-frequency (RF) calibration pulses, HiCal-2 comprised two payloads, which flew for a combined 18 days, covering 1.5 revolutions of the Antarctic continent. ANITA-4 captured over 10,000 pulses from HiCal-2, both direct and reflected from the surface, at distances varying from 100–700 km, providing a large dataset for surface reflectivity measurements. In view of this, we present details on the design, construction and performance of HiCal-2.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

Observation of an Unusual Upward-Going Cosmic-Ray-like Event in the Third Flight of ANITA

We report here on an upward traveling, radio-detected cosmic-ray-like impulsive event with characteristics closely matching an extensive air shower. This event, observed in the third flight of the Antarctic Impulsive Transient Antenna (ANITA), a NASA-sponsored long-duration balloon payload, is consistent with a similar event reported in a previous flight. These events could be produced by the atmospheric decay of an upward-propagating τ lepton produced by a ν τ interaction, although their relatively steep arrival angles create tension with the standard model neutrino cross section. Each of the two events have a posteriori background estimates of ≲ 10 - 2 events. If these are generated by τ -lepton decay, then either the charged-current ν τ cross section is suppressed at EeV energies, or the events arise at moments when the peak flux of a transient neutrino source was much larger than the typical expected cosmogenic background neutrinos.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Antarctic surface reflectivity calculations and measurements from the ANITA-4 and HiCal-2 experiments

We report that the balloon-borne HiCal radio-frequency (RF) transmitter, in concert with the ANITA radio-frequency receiver array, is designed to measure the Antarctic surface reflectivity in the RF wavelength regime. The amplitude of surface-reflected transmissions from HiCal, registered as triggered events by ANITA, can be compared with the direct transmissions preceding them by O(10) microseconds, to infer the surface power reflection coefficient R. The first HiCal mission (HiCal-1, Jan. 2015) yielded a sample of 100 such pairs, resulting in estimates of R at highly glancing angles (i.e., zenith angles approaching 90°), with measured reflectivity for those events which exceeded extant calculations [P. W. Gorham et al., Journal of Astronomical Instrumentation, 1740002 (2017)]. The HiCal-2 experiment, flying from December 2016–January 2017, provided an improvement by nearly 2 orders of magnitude in our event statistics, allowing a considerably more precise mapping of the reflectivity over a wider range of incidence angles. We find general agreement between the HiCal-2 reflectivity results and those obtained with the earlier HiCal-1 mission, as well as estimates from Solar reflections in the radio-frequency regime [D. Z. Besson et al., Radio Sci. 50, 1 (2015)]. In parallel, our calculations of expected reflectivity have matured; herein, we use a plane-wave expansion to estimate the reflectivity R from both a flat, smooth surface (and, in so doing, recover the Fresnel reflectivity equations) and also a curved surface. Lastly, multiplying our flat-smooth reflectivity by improved Earth curvature and surface roughness corrections now provides significantly better agreement between theory and the HiCal-2 measurements.

47 OTHER INSTRUMENTATION↗
DOE OSTI2018

Universal Ratio of Coulomb Interaction to Geometric Quantization in (In, Ga)As/GaAs Quantum Dots

We study the photoluminescence of self-assembled (In,Ga)As/GaAs quantum dot ensembles with varying confinement potential height. The low energy shift of the s-shell emission with increasing excitation power gives a measure of the Coulomb interaction in these structures as it results from carrier–carrier interactions between the optically injected exciton complexes. When dividing this shift by the dot level splitting, determined by the geometric confinement, we obtain a universal function of the number of involved excitons that is independent of the confinement potential height. This shows an identical scaling of Coulomb interaction and geometric quantization with varying confinement.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Photoelectron generation and capture in the resonance fluorescence of a quantum dot

Time-resolved resonance fluorescence on a single self-assembled quantum dot (QD) is used to analyze the generation and capture of photoinduced free charge carriers. We directly observe the capture of electrons into the QD as an intensity reduction of the exciton transition. The exciton transition is quenched until the captured electron tunnels out of the dot again in the order of milliseconds. Our results demonstrate that even under resonant excitation, excited free electrons are generated and can negatively influence the optical properties of a QD.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Coherent electron-spin-resonance manipulation of three individual spins in a triple quantum dot

Quantum dot arrays provide a promising platform for quantum information processing. For universal quantum simulation and computation, one central issue is to demonstrate the exhaustive controllability of quantum states. Here, we report the addressable manipulation of three single electron spins in a triple quantum dot using a technique combining electron-spin-resonance and a micro-magnet. The micro-magnet makes the local Zeeman field difference between neighboring spins much larger than the nuclear field fluctuation, which ensures the addressable driving of electron-spin-resonance by shifting the resonance condition for each spin. We observe distinct coherent Rabi oscillations for three spins in a semiconductor triple quantum dot with up to 25 MHz spin rotation frequencies. This individual manipulation over three spins enables us to arbitrarily change the magnetic spin quantum number of the three spin system, and thus to operate a triple-dot device as a three-qubit system in combination with the existing technique of exchange operations among three spins.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Measurement of the transmission phase of an electron in a quantum two-path interferometer

A quantum two-path interferometer allows for direct measurement of the transmission phase shift of an electron, providing useful information on coherent scattering problems. In mesoscopic systems, however, the two-path interference is easily smeared by contributions from other paths, and this makes it difficult to observe the true transmission phase shift. To eliminate this problem, multi-terminal Aharonov-Bohm (AB) interferometers have been used to derive the phase shift by assuming that the relative phase shift of the electrons between the two paths is simply obtained when a smooth shift of the AB oscillations is observed. Nevertheless, the phase shifts using such a criterion have sometimes been inconsistent with theory. On the other hand, we have used an AB ring contacted to tunnel-coupled wires and acquired the phase shift consistent with theory when the two output currents through the coupled wires oscillate with well-defined anti-phase. Here, we investigate thoroughly these two criteria used to ensure a reliable phase measurement, the anti-phase relation of the two output currents, and the smooth phase shift in the AB oscillation. We confirm that the well-defined anti-phase relation ensures a correct phase measurement with a quantum two-path interference. In contrast, we find that even in a situation where the anti-phase relation is less well-defined, the smooth phase shift in the AB oscillation can still occur but does not give the correct transmission phase due to contributions from multiple paths. This indicates that the phase relation of the two output currents in our interferometer gives a good criterion for the measurement of the true transmission phase, while the smooth phase shift in the AB oscillation itself does not.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

The effect of charged quantum dots on the mobility of a two-dimensional electron gas: How important is the Coulomb scattering?

We have investigated the influence of a layer of charged self-assembled quantum dots (QDs) on the mobility of a nearby two-dimensional electron gas (2DEG). Time-resolved transconductance spectroscopy was used to separate the two contributions of the change in mobility, which are: (i) The electrons in the QDs act as Coulomb scatterers for the electrons in the 2DEG. (ii) The screening ability and, hence, the mobility of the 2DEG decreases when the charge carrier density is reduced by the charged QDs, i.e., the mobility itself depends on the charge carrier concentration. Surprisingly, we find a negligible influence of the Coulomb scattering on the mobility for a 2DEG, separated by a 30 nm tunneling barrier to the layer of QDs. This means that the mobility change is completely caused by depletion, i.e., reduction of the charge carrier density in the 2DEG, which indirectly influences the mobility.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Temperature dependent low-field measurements of the magnetocaloric ΔT with sub-mK resolution in small volume and thin film samples

We present temperature dependent ΔT measurements of the magnetocaloric effect in a thin film sample of Gd, employing magnetomodulation and detection of thermal radiation. A bulk sample of the metamagnetic material LaFe{sub 11.05}Co{sub 0.91}Si{sub 1.04} shows a strong broadening of the ΔT peak for increasing field amplitudes between 4 and 45 mT. Bulk Gd in comparison shows only a weak broadening. All investigated samples exhibit a clear quadratic dependence of ΔT on the external field H{sub ext} at the ΔT peak maximum, contrary to earlier predictions. An analytic expression is derived that interpolates between the H{sub ext}{sup 2}-behavior at low and the well-known H{sub ext}{sup 2/3}-behavior at high fields.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Hybrid architecture for shallow accumulation mode AlGaAs/GaAs heterostructures with epitaxial gates

Accumulation mode devices with epitaxially grown gates have excellent electrical stability due to the absence of dopant impurities and surface states. We overcome typical fabrication issues associated with epitaxially gated structures (e.g., gate leakage and high contact resistance) by using separate gates to control the electron densities in the Ohmic and Hall bar regions. This hybrid gate architecture opens up a way to make ultrastable nanoscale devices where the separation between the surface gates and the 2D electron gas is small. In this work, we demonstrate that the hybrid devices made from the same wafer have reproducible electrical characteristics, with identical mobility and density traces over a large range of 2D densities. In addition, thermal cycling does not influence the measured electrical characteristics. As a demonstration of concept, we have fabricated a hybrid single-electron transistor on a shallow (50 nm) AlGaAs/GaAs heterostructure that shows clear Coulomb blockade oscillations in the low temperature conductance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1968

Computer program aids dual reflector antenna system design

Computer program aids in the design of maximum efficiency dual reflector antenna systems. It designs a shaped cassegrainian antenna which has nearly 100 percent efficiency, and accepts input parameters specifying an existing conventional antenna and produces as output the modifications necessary to conform to a shaped design.

Firnett, P.↗
NASA NTRS1967

Computer programs for antenna feed system design and analysis

Fourteen computer programs have been developed for antenna feed system design and analysis. The programs cover a large spectrum of feed design problems, from primary feed pattern synthesis to the farfield pattern of the main reflector, including analyses of structural distortions.

Ludwig, A.↗
NASA NTRS1967

Plotting program

Computer program for plotting antenna radiation amplitude and phase patterns

ANTENNA↗