Physical Sciences and Mathematics Commons^™

Self-Compensation In Semiconductors: The Zn Vacancy In Ga-Doped Zno, David C. Look, K. D. Leedy, L. Vines, B. G. Svensson, A. Zubiaga, F. Tuomisto, Daniel R. Doutt, L. J. Brillson

Physics Faculty Publications

Self-compensation, the tendency of a crystal to lower its energy by forming point defects to counter the effects of a dopant, is here quantitatively proven. Based on a new theoretical formalism and several different experimental techniques, we demonstrate that the addition of 1.4 × 10²¹-cm⁻³ Ga donors in ZnO causes the lattice to form 1.7 × 10²⁰-cm⁻³ Zn-vacancy acceptors. The calculated V_Zn formation energy of 0.2 eV is consistent with predictions from density functional theory. Our formalism is of general validity and can be used to investigate self-compensation in any degenerate semiconductor material.

Magnetic Properties Of Monolayer Co Islands On Ir(111) Probed By Spin-Resolved Scanning Tunneling Microscopy, Jessica E. Bickel, Focko Meier, Jens Brede, André Kubetzka, Kirsten Von Bergmann, Roland Wiesendanger

Physics Faculty Publications

As the characteristic length scale of devices continues to decrease, it is essential to understand the fundamental magnetic properties of reduced dimension structures. This paper examines the electronic and magnetic properties of two-dimensional nanoscale Co islands on an Ir(111) surface using spin-polarized scanning tunneling microscopy. The pseudomorphic Co islands investigated are ferromagnetic and single domain, with the magnetic easy axis normal to the sample surface. Remarkably, the coercivity of these islands is greater than 4 T and magnetic saturation of the islands requires an applied field of at least 5 T.

Variable Renewable Energy In Modeling Climate Change Mitigation Scenarios, Falko Ueckerdt, Robert J. Brecha, Gunnar Luderer, Patrick Sullivan, Eva Schmid, Nico Bauer, Diana Böttger

Physics Faculty Publications

This paper addresses the issue of how to account for short‐term temporal variability of renewable energy sources and power demand in long‐term climate change mitigation scenarios in energy‐economic models. An approach that captures in a stylized way the major challenges to the integration of variable renewable energy sources into power systems has been developed. As a first application this approach has been introduced to REMIND‐D, a hybrid energy‐economy model of Germany. An approximation of the residual load duration curve is implemented. The approximating function endogenously changes depending on the penetration and mix of variable renewable power. The approach can thus …

Thermal Annealing Effect On Spin Coherence In Zno Single Crystals, Z. Yang, David C. Look, H. M. Zhou, W. V. Kawakami, R. K. Kawakami, P. K. L. Yu, J. L. Liu

Physics Faculty Publications

The spin coherence time (T₂^*) in ZnO single crystals at 8.5 K decreases significantly from ∼11.2 ns to ∼2.3 ns after annealing at 500 °C, as indicated by time-resolved Kerr-rotation pump-probe magneto-optical spectroscopy. The annealing-induced spin coherence degradation in ZnO arises neither from crystallinity degradation during the annealing process, as confirmed by x-ray rocking curves; nor from reflection variations of the probe laser beam induced by surface roughness changes during the annealing process, as confirmed by atomic force microscopy. Temperature-dependent Hall-effect studies indicate that decreased mobility and increased shallow-donor concentration in the annealing-induced surface conducting layer …

Arcade 2 Observations Of Galactic Radio Emission, A. Kogut, D. J. Fixsen, S. M. Levin, M. Limon, P. M. Lubin, P. Mirel, M. Seiffert, Jack Singal, T. Villela, E. Wollack, C. A. Wuensche

Physics Faculty Publications

We use absolutely calibrated data from the ARCADE 2 flight in 2006 July to model Galactic emission at frequencies 3, 8, and 10 GHz. The spatial structure in the data is consistent with a superposition of free–free and synchrotron emission. Emission with spatial morphology traced by the Haslam 408 MHz survey has spectral index βsynch = −2.5±0.1, with free–free emission contributing 0.10±0.01 of the total Galactic plane emission in the lowest ARCADE 2 band at 3.15 GHz. We estimate the total Galactic emission toward the polar caps using either a simple plane-parallel model with csc |b| dependence or a model …

Arcade 2 Measurement Of The Absolute Sky Brightness At 3-90 Ghz, D. J. Fixsen, A. Kogut, S. Levin, M. Limon, P. Lubin, P. Mirel, M. Seiffert, Jack Singal, E. Wollack, T. Villela, C. A. Wuensche

Physics Faculty Publications

The ARCADE 2 instrument has measured the absolute temperature of the sky at frequencies 3, 8, 10, 30, and 90 GHz, using an open-aperture cryogenic instrument observing at balloon altitudes with no emissive windows between the beam-forming optics and the sky. An external blackbody calibrator provides an in situ reference. Systematic errors were greatly reduced by using differential radiometers and cooling all critical components to physical temperatures approximating the cosmic microwave background (CMB) temperature. A linear model is used to compare the output of each radiometer to a set of thermometers on the instrument. Small corrections are made for the …

Interpretation Of The Arcade 2 Absolute Sky Brightness Measurement, M. Seiffert, D. J. Fixsen, A. Kogut, S. M. Levin, M. Limon, P. M. Lubin, P. Mirel, Jack Singal, T. Villela, E. Wollack, C. A. Wuensche

Physics Faculty Publications

We use absolutely calibrated data between 3 and 90 GHz from the 2006 balloon flight of the ARCADE 2 instrument, along with previous measurements at other frequencies, to constrain models of extragalactic emission. Such emission is a combination of the cosmic microwave background (CMB) monopole, Galactic foreground emission, the integrated contribution of radio emission from external galaxies, any spectral distortions present in the CMB, and any other extragalactic source. After removal of estimates of foreground emission from our own Galaxy, and an estimated contribution of external galaxies, we present fits to a combination of the flat-spectrum CMB and potential spectral …

Logistic Curves, Extraction Costs And The Effective Size Of Oil Resources, Robert J. Brecha

Physics Faculty Publications

The size of potential fossil fuel resources is an issue of perennial interest and controversy. Fundamentally, there appears to be a conflict in interpretation of available data for both past and future extraction histories. As fossil-fuel prices rose dramatically over the past several years, the question of resources once again became acute. In this paper we concentrate on conventional and non-conventional oil resources and make four main points, with the overarching theme that one can determine an effective oil resource that represents significantly less availability for consumption than usually posited by tallying resources in place.

First, looking at oil production …

Dynamics Of Fingering Convection I: Small-Scale Fluxes And Large-Scale Instabilities, Adrienne L. Traxler, Stephan Stellmach, Pascale Garaud, T. Radko, N. Brummell

Physics Faculty Publications

Double-diffusive instabilities are often invoked to explain enhanced transport in stably stratified fluids. The most-studied natural manifestation of this process, fingering convection, commonly occurs in the ocean's thermocline and typically increases diapycnal mixing by 2 orders of magnitude over molecular diffusion. Fingering convection is also often associated with structures on much larger scales, such as thermohaline intrusions, gravity waves and thermohaline staircases. In this paper, we present an exhaustive study of the phenomenon from small to large scales. We perform the first three-dimensional simulations of the process at realistic values of the heat and salt diffusivities and provide accurate estimates …

Dynamics Of Fingering Convection Ii: The Formation Of Thermohaline Staircases, Stephan Stellmach, Adrienne L. Traxler, Pascale Garaud, N. Brummell, T. Radko

Physics Faculty Publications

Regions of the ocean's thermocline unstable to salt fingering are often observed to host thermohaline staircases, stacks of deep well-mixed convective layers separated by thin stably stratified interfaces. Decades after their discovery, however, their origin remains controversial. In this paper we use three-dimensional direct numerical simulations to shed light on the problem. We study the evolution of an analogous double-diffusive system, starting from an initial statistically homogeneous fingering state, and find that it spontaneously transforms into a layered state. By analysing our results in the light of the mean-field theory developed in Part 1 (Traxler et al., J. Fluid …

Mie Scattering In The Time Domain. Part Ii. The Role Of Diffraction, James A. Lock, Philip Laven

Physics Faculty Publications

The p = 0 term of the Mie-Debye scattering amplitude contains the effects of external reflection and diffraction. We computed the reflected intensity in the time domain as a function of the scattering angle and delay time for a short electromagnetic pulse incident on a spherical particle and compared it to the predicted behavior in the forward-focusing region, the specular reflection region, and the glory region. We examined the physical consequences of three different approaches to the exact diffraction amplitude, and determined the signature of diffraction in the time domain. The external reflection surface wave amplitude gradually replaces the diffraction …

Mie Scattering In The Time Domain. Part I. The Role Of Surface Waves, James A. Lock, Philip Laven

Physics Faculty Publications

We computed the Debye series p = 1 and p = 2 terms of the Mie scattered intensity as a function of scattering angle and delay time for a linearly polarized plane wave pulse incident on a spherical dielectric particle and physically interpreted the resulting numerical data. Radiation shed by electromagnetic surface waves plays a prominent role in the scattered intensity. We determined the surface wave phase and damping rate and studied the structure of the p = 1, 2 surface wave glory in the time domain. (C) 2011 Optical Society of America

Experimental Observation Of Ion Beams In The Madison Helicon Experiment, Matt Wiebold, Yung-Ta Sung, John E. Scharer

Physics Faculty Publications

Argon ion beams up to E_b=165 eV at P_rf=500 W are observed in the Madison Helicon eXperiment (MadHeX) helicon source with a magnetic nozzle. A two-grid retarding potential analyzer (RPA) is used to measure the ion energy distribution, and emissive and rf-filtered Langmuir probes measure the plasma potential, electron density, and temperature. The supersonic ion beam (M=v_i/c_s up to 5) forms over tens of Debye lengths and extends spatially for a few ion-neutral charge-exchange mean free paths. The parametric variation of the ion beam energy is explored, including flow rate, rf power, and …

Surrogate Measurement Of The 238Pu(N,F) Cross Section, J. J. Ressler, J. T. Burke, J. Escher, C. Angell, M. S. Basunia, C. W. Beausang, L. A. Bernstein, D. L. Bleuel, R. J. Casperson, B. L. Goldblum, J. Gostic, R. Hatarik, R. Henderson, R. O. Hughes, J. Munson, L. Phair, T. J. Ross, N. D. Scielzo, E. Swanberg, I. Thompson, M. Wiedeking

Physics Faculty Publications

The neutron-induced fission cross section of ²³⁸Pu was determined using the surrogate ratio method. The (n,f) cross section over an equivalent neutron energy range 5–20 MeV was deduced from inelastic α-induced fission reactions on ²³⁹Pu, with ²³⁵U(α,α′f) and ²³⁶U(α,α′f) used as references. These reference reactions reflect ²³⁴U(n,f) and ²³⁵U(n,f) yields, respectively. The deduced ²³⁸Pu(n,f) cross section agrees well with standard data libraries up to …

Propagation Dynamics Of Controlled Cross-Talk Via Interplay Between Chi((1)) And Chi((3)) Processes, Paul S. Hsu, George R. Welch, James R. Gord, Anil K. Patnaik

Physics Faculty Publications

We investigate theoretically and experimentally the propagation dynamics of a nonlinear cross-talk effect between two probe channels in a double-ladder system and show that an interplay between χ⁽¹⁾ and χ⁽³⁾ processes leads to the control of cross-talk. We derive analytical solutions to describe the propagation dynamics of the probe fields with the cross-talk effect built in. From the analytical results we identify and examine the regimes of interest where contributions of either χ⁽¹⁾ or χ⁽³⁾ or both are significant. The control of cross-talk is demonstrated experimentally, and good quantitative agreement is found between the analytical solutions …

Prioritizing Investment In Residential Energy Efficiency And Renewable Energy: A Case Study For The U.S. Midwest, Robert J. Brecha, Austin Mitchell, Kevin P. Hallinan, J. Kelly Kissock

Physics Faculty Publications

Residential building energy use is an important contributor to greenhouse gas emissions and in the United States represents about 20% of total energy consumption. A number of previous macro-scale studies of residential energy consumption and energy-efficiency improvements are mainly concerned with national or international aggregate potential savings. In this paper we look into the details of how a collection of specific homes in one region might reduce energy consumption and carbon emissions, with particular attention given to some practical limits to what can be achieved by upgrading the existing residential building stock. Using a simple model of residential, single-family home …

Ga-Doped Zno Grown By Pulsed Laser Deposition In H-2: The Roles Of Ga And H, David C. Look, T. C. Droubay, J. S. Mccloy, Z. H. Zhu, S. A. Chambers

Physics Faculty Publications

Highly conductive thin films of ZnO doped with Ga were grown by pulsed laser deposition with 10 mTorr of H₂ in the growth chamber. Compared with a more conventional method of producing conductive films of ZnO, i.e., growth in O₂ followed by annealing in forming gas (5% H₂ in Ar), the H₂ method requires no postgrowth anneal and also produces higher carrier concentrations and lower resistivities with better depth uniformity. As an example, a 65-nm-thick sample had a room-temperature mobility of 32 cm²/V s, a concentration of 6.8×10²⁰ cm⁻³, and a …

Impact Of Dispersion On Amplitude And Frequency Noise In A Yb-Fiber Laser Comb, Lora Nugent-Glandorf, Todd A. Johnson, Yohei Kobayashi, Scott A. Diddams

Physics Faculty Publications

We describe a Yb-fiber-based laser comb, with a focus on the relationship between the net-cavity dispersion and frequency noise on the comb. While tuning the net-cavity dispersion from anomalous to normal, we measure the relative intensity noise, offset frequency (f_CEO) linewidth, and the resulting frequency noise spectrum on the f_CEO. We find that the laser operating at zero net-cavity dispersion has many advantages, including an approximately 100× reduction in free-running f_CEO linewidth and frequency noise power spectral density when compared to the normal-dispersion regime. At the zero-dispersion point, we demonstrate a phase-locked f_{CEO …}

Optimization Of The Nanolens Consisting Of Coupled Metal Nanoparticles: An Analytical Approach, Greg Sun, Jacob B. Khurgin

Physics Faculty Publications

Using a simple and intuitive analytical approach, we perform optimization of a nanolens composed of coupled metal nanoparticles capable of subwavelength focusing of light inside the narrow gap separating the particles. Specifically, we optimize the structure of two nanospheres of different sizes to achieve maximum field enhancement at an off-center position in the gap. We demonstrate that the nanolens of two or more spheres acts simultaneously as an efficient antenna with large dipole and an efficient cavity with small effective volume.

Turbulent Mixing And Layer Formation In Double-Diffusive Convection: 3d Numerical Simulations And Theory, Erica Rosenblum, Pascale Garaud, Adrienne L. Traxler, Stephan Stellmach

Physics Faculty Publications

Double-diffusive convection, often referred to as semi-convection in astrophysics, occurs in thermally and compositionally stratified systems which are stable according to the Ledoux criterion but unstable according to the Schwarzschild criterion. This process has been given relatively little attention so far, and its properties remain poorly constrained. In this paper, we present and analyze a set of three-dimensional simulations of this phenomenon in a Cartesian domain under the Boussinesq approximation. We find that in some cases the double-diffusive convection saturates into a state of homogeneous turbulence, but with turbulent fluxes several orders of magnitude smaller than those expected from direct …

Efficient Decomposition Of Cosmic Microwave Background Polarization Maps Into Pure E, Pure B, And Ambiguous Components., Emory F. Bunn

Physics Faculty Publications

Separation of the B component of a cosmic microwave background (CMB) polarization map from the much larger E component is an essential step in CMB polarimetry. For a map with incomplete sky coverage, this separation is necessarily hampered by the presence of ambiguous modes which could be either E or B modes. I present an efficient pixel-space algorithm for removing the ambiguous modes and separating the map into pure E and B components. The method, which works for arbitrary geometries, does not involve generating a complete basis of such modes and scales the cube of the number of pixels on …

The Efficacy Of Galaxy Shape Parameters In Photometric Redshift Estimation: A Neural Network Approach, Jack Singal, M. Shmakova, B. Gerke, R. L. Griffith, J. Lotz

Physics Faculty Publications

We present a determination of the effects of including galaxy morphological parameters in photometric redshift estimation with an artificial neural network method. Neural networks, which recognize patterns in the information content of data in an unbiased way, can be a useful estimator of the additional information contained in extra parameters, such as those describing morphology, if the input data are treated on an equal footing. We use imaging and five band photometric magnitudes from the All-wavelength Extended Groth Strip International Survey. It is shown that certain principal components of the morphology information are correlated with galaxy type. However, we find …

Radial Motion In A Central Potential For Singular Mass Densities, Ulrich Zürcher, Miron Kaufman

Physics Faculty Publications

We study the radial motion of an object in the gravitational field produced by an isotropic mass density that is singular at the origin. This problem applies to elliptical galaxies and can be used to illustrate motion in a central field appropriate for an intermediate-level mechanics course.

Torque Around The Center Of Mass: Dynamic Stability During Quadrupedal Arboreal Locomotion In The Siberian Chipmunk (Tamias Sibiricus), Andrew R. Lammers, Ulrich Zurcher

Physics Faculty Publications

When animals travel on tree branches, avoiding falls is of paramount importance. Animals swiftly running on a narrow branch must rely on movement to create stability rather than on static methods. We examined how Siberian chipmunks (Tamias sibiricus) remain stable while running on a narrow tree branch trackway. We examined the pitch, yaw, and rolling torques around the center of mass, and hypothesized that within a stride, any angular impulse (torque during step time) acting on the center of mass would be canceled out by an equal and opposite angular impulse. Three chipmunks were videotaped while running on a 2 …

The Arcade 2 Instrument, Jack Singal, D. J. Fixsen, A. Kogut, S. Levin, M. Limon, P. Lubin, P. Mirel, M. Seiffert, T. Villela, E. Wollack, C. A. Wuensche

Physics Faculty Publications

The second generation Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE 2) instrument is a balloon-borne experiment to measure the radiometric temperature of the cosmic microwave background and Galactic and extragalactic emission at six frequencies from 3 to 90 GHz. ARCADE 2 utilizes a double-nulled design where emission from the sky is compared to that from an external cryogenic full-aperture blackbody calibrator by cryogenic switching radiometers containing internal blackbody reference loads. In order to further minimize sources of systematic error, ARCADE 2 features a cold fully open aperture with all radiometrically active components maintained at near 2.7 K without …

Theory Of Optical Emission Enhancement By Coupled Metal Nanoparticles: An Analytical Approach, Greg Sun, Jacob B. Khurgin

Physics Faculty Publications

We present an analytical “coupled mode” model explaining enhancement of emission by an emitter placed within complexes of metal nanoparticles and apply it for an important case of an emitter placed inside the gap of two coupled Au nanospheres. This approach has dual advantages of exposing the underling physics of the enhancement and revealing a straightforward path toward optimization.

Coherent Photoproduction Of Π+ From 3he, R. Nasseripour, Gerard P. Gilfoyle, Et. Al.

Physics Faculty Publications

We have measured the differential cross section for the γ³He→ π⁺t reaction. This reaction was studied using the Continuous Electron Beam Accelerator Facility (CEBAF) Large Acceptance Spectrometer (CLAS) at Jefferson Lab. Real photons produced with the Hall-B bremsstrahlung tagging system in the energy range from 0.50 to 1.55 GeV were incident on a cryogenic liquid ³He target. The differential cross sections for the γ³He→ π⁺t reaction were measured as a function of photon-beam energy and pion-scattering angle. Theoretical predictions to date cannot explain the large cross sections except at backward angles, …

Episodic Tremors And Slip In Cascadia In The Framework Of The Frenkel-Kontorova Model, Naum I. Gershenzon, Gust Bambakidis, Ernest Hauser, Abhijit Ghosh, Kenneth C. Creager

Physics Faculty Publications

The seismic moment for regular earthquakes is proportional to the cube of rupture time. A second class of phenomena, collectively called slow earthquakes, has very different scaling. We propose a model, inspired from the phenomenology of dislocation dynamics in crystals, that is consistent with the scaling relations observed in the Cascadia episodic tremor and slip (ETS) events. Two fundamental features of ETS are periodicity and migration. In the northern Cascadia subduction zone, ETS events appear every 14.5 months or so. During these events, tremors migrate along-strike with a velocity of 10 km/day and simultaneously zip back and forth in the …

Electric Field Tunable Magnetic Properties Of Lead-Free Na0.5bi0.5tio3/Cofe2o4 Multiferroic Composites, S Narendra Babu, Seong Gi Min, Leszek Malkinski

Physics Faculty Publications

Lead-free multiferroic particulate composites of Na_0.5Bi_0.5TiO₃ (NBT) and CoFe₂O₄ (CFO) have been synthesized by solid-state sintering method. A systematic study of structural, magnetic and magnetoelectric (ME) properties is undertaken. Structural and morphology studies carried out by x-ray diffraction and field emission scanning electron microscopy indicate formation of single phase for parent phases and presence of both phases in the composites. Magnetic properties are investigated using vibrating sample magnetometer and ferromagnetic resonance (FMR) measurements at room temperature. Strong ME coupling is demonstrated in NBT-CFO 70-30 mol% composite by an electrostatically tunable FMR field …

Calorimetric Evidence Of Strong-Coupling Multiband Superconductivity In Fe(Te0.57se0.43) Single Crystal, J. Hu, T. J. Liu, B. Qian, A. Rotaru, L. Spinu, Z. Q. Mao

Physics Faculty Publications

We have investigated the specific heat of optimally doped iron chalcogenide superconductor Fe(Te_0.57Se_0.43) with a high-quality single crystal sample. The electronic specific heat C_e of this sample has been successfully separated from the phonon contribution using the specific heat of a nonsuperconducting sample (Fe_0.90Cu_0.10)(Te_0.57Se_0.43) as a reference. The normal-state Sommerfeld coefficient γ_n of the superconducting sample is found to be ~26.6 mJ/mol K², indicating intermediate electronic correlation. The temperature dependence of C_e in the superconducting state can be best fitted using a double-gap …