Physics Research Works

Permanent URI for this collectionhttp://hdl.handle.net/1903/1597

Browse

Now showing 1 - 20 of 226

A facet is not an island: step-step interactions and the fluctuations of the boundary of a crystal facet
(Elsevier, 2005) Pimpinelli, Alberto; Degawa, M.; Einstein, Theodore L.; Williams, Ellen D.
Accounting for variability in a teacher’s epistemology: Resources and framing
(2018-05-31) Lau, Matty; Elby, Andrew
Adenylyl cyclase mRNA localizes to the posterior of polarized DICTYOSTELIUM cells during chemotaxis
(Springer Nature, 2017-05-25) Das, Satarupa; Parker, Joshua M.; Guven, Can; Wang, Weiye; Kriebel, Paul W.; Losert, Wolfgang; Larson, Daniel R.; Parent, Carole A.
In Dictyostelium discoideum, vesicular transport of the adenylyl cyclase A (ACA) to the posterior of polarized cells is essential to relay exogenous 3′,5′-cyclic adenosine monophosphate (cAMP) signals during chemotaxis and for the collective migration of cells in head-to-tail arrangements called streams. Using fluorescence in situ hybridization (FISH), we discovered that the ACA mRNA is asymmetrically distributed at the posterior of polarized cells. Using both standard estimators and Monte Carlo simulation methods, we found that the ACA mRNA enrichment depends on the position of the cell within a stream, with the posterior localization of ACA mRNA being strongest for cells at the end of a stream. By monitoring the recovery of ACA-YFP after cycloheximide (CHX) treatment, we observed that ACA mRNA and newly synthesized ACA-YFP first emerge as fluorescent punctae that later accumulate to the posterior of cells. We also found that the ACA mRNA localization requires 3′ ACA cis-acting elements. Together, our findings suggest that the asymmetric distribution of ACA mRNA allows the local translation and accumulation of ACA protein at the posterior of cells. These data represent a novel functional role for localized translation in the relay of chemotactic signal during chemotaxis.
Adsorbates in a Box: Titration of Substrate Electronic States
(American Physical Society, 2010) Cheng, Zhihai; Wyrick, Jonathan; Luo, Miaomiao; Sun, Dezheng; Kim, Daeho; Zhu, Yeming; Lu, Wenhao; Kim, Kwangmoo; Einstein, Theodore L.; Bartels, Ludwig
Nanoscale confinement of adsorbed CO molecules in an anthraquinone network on Cu(111) with a pore size of ?4??nm arranges the CO molecules in a shell structure that coincides with the distribution of substrate confined electronic states. Molecules occupy the states approximately in the sequence of rising electron energy. Despite the sixfold symmetry of the pore boundary itself, the adsorbate distribution adopts the threefold symmetry of the network-substrate system, highlighting the importance of the substrate even for such quasi-free-electron systems.
An Extended Detailed Investigation of First and Second Order Supersymmetries for Off-Shell N = 2 and N = 4 Supermultiplets
(MDPI, 2015-06-16) Gates, Sylvester James Jr.; Parker, James; Rodgers, Vincent G. J.; Rodriguez, Leo; Stiffler, Kory
This paper investigates the d = 4, 𝒩=4 Abelian, global Super-Yang Mills system (SUSY-YM). It is shown how the 𝒩=2 Fayet Hypermultiplet (FH) and 𝒩=2 vector multiplet (VM) are embedded within. The central charges and internal symmetries provide a plethora of information as to further symmetries of the Lagrangian. Several of these symmetries are calculated to second order. It is hoped that investigations such as these may yield avenues to help solve the auxiliary field closure problem for d = 4, 𝒩=4, SUSY-YM and the d = 4, 𝒩=2 Fayet-Hypermultiplet, without using an infinite number of auxiliary fields.
Analysis of terrace-width distributions using the generalized Wigner surmise: Calibration using Monte Carlo and transfer-matrix calculations
(American Physical Society, 2004) Gebremariam, Hailu; Cohen, Saul D.; Richards, Howard L.; Einstein, Theodore L.
Measurement of terrace-width distributions (TWD’s) of vicinal surfaces is used routinely to find the dimensionless strength Ã of the elastic repulsion between steps. For sufficiently strong repulsions, the TWD can be described by a Gaussian about the mean step spacing, but controversy has arisen on the correct prefactor in the relation of the TWD variance to Ã. Instead of the various Gaussian approximations, we have advocated for several years that the TWD be fit with the generalized Wigner distribution, essentially a gamma distribution in the normalized squared TWs. The basis for this idea stems from a mapping of the step model to the Sutherland model of fermions in one dimension. While several applications to experiment have been successful, definitive comparison of the various approximations requires high-quality numerical data. We report transfer matrix and extensive Monte Carlo simulations of terrace-step-kink models to support our contentions. Our work includes investigation of finite-size effects and of the breakdown of the continuum picture for values of Ã larger than in typical experiments.
Analytic Formulas for the Orientation Dependence of Step Stiffness and Line Tension: Key Ingredients for Numerical Modeling
(SIAM, 2006) Stasevich, T. J.; Einstein, Theodore L.
We present explicit analytic, twice-differentiable expressions for the temperature-dependent anisotropic step line tension and step stiffness for the two principal surfaces of face-centered-cubic crystals, the square (001) and the hexagonal (111). These expressions improve on simple expressions that are valid only for low temperatures and away from singular orientations. They are well suited for implementation into numerical methods such as finite-element simulation of step evolution.
Analyzing Capture Zone Distributions (CZD) in Growth: Theory and Applications
(Elsevier, 2014) Einstein, Theodore L.; Pimpinelli, Alberto; Gonzalez, Diego Luis
Angular momentum branching ratios for electron-induced ionization: Atomic and model calculations
(American Physical Society, 1987) Mehl, M. J.; Einstein, Theodore L.
We present calculations of the matrix elements for electron-induced ionization of core electrons of atoms. We use both self-consistent atomic potentials for accuracy and model potentials to gain physical insight. We pay particular attention to the angular momentum distribution of the two final-state electrons, especially when one of them lies near what would be the Fermi energy in a solid (i.e., as in an absorption fine-structure experiment). For nodeless core wave functions, in the dominant channel both final-state electrons have angular momentum one greater than that of the initial core state. For sufficiently deeply bound states, this first approximate selection rule holds until the incident electron energy exceeds the ionization threshold by at least 500 eV, i.e., over the experimentally relevant range. It is also possible to determine the angular momentum distribution of the final-state electron. The EXAFS-like electron tends to have angular momentum one greater than that of the initial core state, even in some cases where the first approximate selection rule does not hold. (EXAFS is extended x-ray-absorption fine structure.) The strongest trend is that the dipole component in a partial-wave expansion of the Coulomb interaction dominates the matrix element. In these studies, careful treatment of not just the core state but also the unbound states is crucial; we show that the conventional orthogonalized plane-wave approximation is inadequate, giving incorrect ordering of the channels. For model potentials with an adjustable screening length, low-lying bound resonances are found to play an important role.
Anisotropic surface-state-mediated RKKY interaction between adatoms
(American Physical Society, 2012) Patrone, Paul N.; Einstein, Theodore L.
Motivated by recent numerical studies of Ag on Pt(111), we derive an expression for the RKKY interaction mediated by surface states, considering the effect of anisotropy in the Fermi edge. Our analysis is based on a stationary phase approximation. The main contribution to the interaction comes from electrons whose Fermi velocity vF is parallel to the vector R connecting the interacting adatoms; we show that, in general, the corresponding Fermi wave vector kF is not parallel to R. The interaction is oscillatory; the amplitude and wavelength of oscillations have angular dependence arising from the anisotropy of the surface-state band structure. The wavelength, in particular, is determined by the projection of this kF (corresponding to vF) onto the direction of R. Our analysis is easily generalized to other systems. For Ag on Pt(111), our results indicate that the RKKY interaction between pairs of adatoms should be nearly isotropic and so cannot account for the anisotropy found in the studies motivating our work. However, for metals with surface-state dispersions similar to Be(10¯10), we show that the RKKY interaction should have considerable anisotropy.
Anomalous Loss Reduction Below Two-Level System Saturation in Aluminum Superconducting Resonators
(Wiley, 2023-11-28) Tai, Tamin; Cai, Jingnan; Anlage, Steven M.
Superconducting resonators are widely used in many applications such as qubit readout for quantum computing, and kinetic inductance detectors. These resonators are susceptible to numerous loss and noise mechanisms, especially the dissipation due to two-level systems (TLS) which become the dominant source of loss in the few-photon and low temperature regime. In this study, capacitively-coupled aluminum half-wavelength coplanar waveguide resonators are investigated. Surprisingly, the loss of the resonators is observed to decrease with a lowering temperature at low excitation powers and temperatures below the TLS saturation. This behavior is attributed to the reduction of the TLS resonant response bandwidth with decreasing temperature and power to below the detuning between the TLS and the resonant photon frequency in a discrete ensemble of TLS. When response bandwidths of TLS are smaller than their detunings from the resonance, the resonant response and thus the loss is reduced. At higher excitation powers, the loss follows a logarithmic power dependence, consistent with predictions from the generalized tunneling model (GTM). A model combining the discrete TLS ensemble with the GTM is proposed and matches the temperature and power dependence of the measured internal loss of the resonator with reasonable parameters.
Arguing about argument and evidence: Disagreements and ambiguities in science education research and practice
(2022) Tang, Xiaowei; Levin, Daniel; Chumbley, Alexander; Elby, Andrew
Science education researchers agree about the importance of evidence in science practices such as argumentation. Yet, disagreements and ambiguities about what counts as “evidence” in science classrooms pervade the literature. We argue that these ambiguities and disagreements can be viewed as falling along three fault lines: (i) the source of evidence, specifically, whether it must be first-hand; (ii) whether “evidence” must always be empirical; and (iii) the extent to which evidence is inferred, and what degree of inference transforms “evidence” into something else. In this paper, after showing how these three fault lines manifest in the literature, we argue that these three dimensions of disagreements and ambiguities are not confined to research and research-based curricula; they are also salient in teachers’ classroom practice, as illustrated by a dramatic, multi-day debate between a mentor teacher and her teacher intern. After establishing the salience of the three fault lines in both research and practice, we explore whether NGSS can provide a resolution to the teachers’ debate and to the disagreements/ambiguities in the literature. Our analysis reveals that NGSS reproduces rather than resolves those three fault lines—but in doing so, it invites a resolution of a different type. Instead of providing a single, precise, context-independent definition of “evidence,” NGSS implicitly reflects a defensible view that what counts as “evidence” depends on the epistemic aims of the practices in which the students are engaged. This implied context-dependency of what counts as good evidence use, we argue, could be made explicit in an addendum document clarifying aspects of NGSS. Doing so would provide valuable guidance to teachers, teacher educators, and researchers.
Bending-rigidity-driven transition and crumpling-point scaling of lattice vesicles
(American Physical Society, 1996) Orlandini, E.; Stella, A. L.; Einstein, Theodore L.; Tesi, M. C.; Beichl, I.; Sullivan, F.
Beyond the Wigner distribution: Schrodinger equations and terrace width distributions
(American Physical Society, 2005) Richards, Howard L.; Einstein, Theodore L.
The so-called generalized Wigner distribution has earlier been shown to be an excellent approximation for the terrace width distribution (TWD) of vicinal surfaces characterized by step-step interactions that are perpendicular to the average step direction and fall off as the inverse square of the step spacing. In this paper, we show that the generalized Wigner distribution can be derived from a plausible, phenomenological model in which two steps interact with each other directly and with other steps through a position-dependent pressure. We also discuss generalizations to more general step-step interactions and show that the predictions are in good agreement with TWDs derived from numerical transfer-matrix calculations and Monte Carlo simulations. This phenomenological approach allows the step-step interaction to be extracted from experimental TWDs.
Beyond triplet: Unconventional superconductivity in a spin-3/2 topological semimetal
(American Association for the Advancement of Science, 2018-04-06) Kim, Hyunsoo; Wang, Kefeng; Nakajima, Yasuyuki; Hu, Rongwei; Ziemak, Steven; Syers, Paul; Wang, Limin; Hodovanets, Halyna; Denlinger, Jonathan D.; Brydon, Philip M. R.; Agterberg, Daniel F.; Tanatar, Makariy A.; Prozorov, Ruslan; Paglione, Johnpierre
In all known fermionic superfluids, Cooper pairs are composed of spin-1/2 quasi-particles that pair to form either spin-singlet or spin-triplet bound states. The “spin” of a Bloch electron, however, is fixed by the symmetries of the crystal and the atomic orbitals from which it is derived and, in some cases, can behave as if it were a spin-3/2 particle. The superconducting state of such a system allows pairing beyond spin-triplet, with higher spin quasi-particles combining to form quintet or septet pairs. We report evidence of unconventional superconductivity emerging from a spin-3/2 quasi-particle electronic structure in the half-Heusler semimetal YPtBi, a low-carrier density noncentrosymmetric cubic material with a high symmetry that preserves the p-like j = 3/2 manifold in the Bi-based Γ8 band in the presence of strong spin-orbit coupling. With a striking linear temperature dependence of the London penetration depth, the existence of line nodes in the superconducting order parameter Δ is directly explained by a mixed-parity Cooper pairing model with high total angular momentum, consistent with a high-spin fermionic superfluid state. We propose a k ⋅ p model of the j = 3/2 fermions to explain how a dominant J = 3 septet pairing state is the simplest solution that naturally produces nodes in the mixed even-odd parity gap. Together with the underlying topologically nontrivial band structure, the unconventional pairing in this system represents a truly novel form of superfluidity that has strong potential for leading the development of a new series of topological superconductors.
Brownian motion and shape fluctuations of single-layer adatom and vacancy clusters on surfaces: Theory and simulations
(American Physical Society, 1996) Khare, S. V.; Einstein, Theodore L.
In recent observations of Brownian motion of islands of adsorbed atoms and of vacancies with mean radius R, the cluster diffusion constant Dc varies as R1 and R2. From an analytical continuum description of the cluster's steplike boundary, we find a single Langevin equation for the motion of the cluster boundary, rather than three special cases.
Brownian motion of steps on Si(111)
(American Physical Society, 1993) Bartelt, N. C.; Goldberg, J. L.; Einstein, Theodore L.; Williams, Ellen D.; Heyraud, J. C.; Metois, J. J.
Step motion on surfaces can now be measured quantitatively. We present a formalism for analyzing equilibrium step fluctuations and apply it to real-time reflection electron microscope observations of step motion on Si(111). The time correlation functions of the step positions and of their Fourier components are compared with predictions from Langevin equations for two extreme mechanisms for step motion: edge diffusion and terrace exchange.
Capture-Zone Scaling in Island Nucleation: Universal Fluctuation Behavior
(American Physical Society, 2007) Pimpinelli, Alberto; Einstein, Theodore L.
In island nucleation and growth, the distribution of capture zones (in essence proximity cells) can be described by a simple expression generalizing the Wigner surmise (power-law rise, Gaussian decay) from random matrix theory that accounts for spacing distributions in a host of fluctuation phenomena. Its single adjustable parameter, the power-law exponent, can be simply related to the critical nucleus of growth models and the substrate dimensionality. We compare with extensive published kinetic Monte Carlo data and limited experimental data. A phenomenological theory elucidates the result.
Changes in density of states caused by chemisorption
(American Physical Society, 1975) Einstein, Theodore L.
The process of chemisorption is studied via the change in density of states when an adatom with a single level Ea bonds by a hopping parameter V to the (100) surface of an s-band simple cubic crystal. As V increases, the bond changes from a perturbative regime to the formation of a surface complex. Investigations of local and layer-summed show this dimerlike structure to be well localized near the bond. Steric effects (binding-site symmetry) are easily introduced. A damping parameter can be added to simulate decay effects of surface probes. Application of the model to photoemission angular-averaged and angular-resolved difference spectra for light gases on tungsten gives understanding of the bonding peak near the bottom of the band, of the depletion of states near the Fermi energy, and of the slight shifting of the energy-momentum-conserving peak.
Charged pion form factor between Q2=0.60 and 2.45 GeV2. II. Determination of, and results for, the pion form factor
(2008-10) Huber, G.M.; Blok, H.P.; Horn, T.; Beise, E.J.; Gaskell, D.; Mack, D.J.; Tadevosyan, V.; Volmer, J.; Abbott, D.; Aniol, K.; Anklin, H.; Armstrong, C.; Arrington, J.; Assamagan, K.; Avery, S.; Baker, O.K.; Barrett, B.; Bochna, C.; Boeglin, W.; Brash, E.J.; Breuer, H.; Chang, C.C.; Chant, N.; Christy, M.E.; Dunne, J.; Eden, T.; Ent, R.; Fenker, H.; Gibson, E.F.; Gilman, R.; Gustafsson, K.; Hinton, W.; Holt, J.; Jackson, H.; Jin, S.; Jones, M.K.; Keppel, C.E.; Kim, P.H.; Kim, W.; King, P.M.; Klein, A.; Koltenuk, D.; Kovaltchouk, V.; Liang, M.; Liu, J.; Lolos, G.J.; Lung, A.; Margaziotis, D.J.; Markowitz, P.; Matsumura, A.; McKee, D.; Meekins, D.; Mitchell, J.; Miyoshi, T.; Mkrtchyan, H.; Mueller, B.; Niculescu, G.; Niculescu, I.; Okayasu, Y.; Pentchev, L.; Perdrisat, C.; Pitz, D.; Potterveld, D.; Punjabi, V.; Qin, L.M.; Reimer, P.E.; Reinhold, J.; Roche, J.; Roos, P.G.; Sarty, A.; Shin, I.K.; Smith, G.R.; Stepanyan, S.; Tang, L.G.; Tvaskis, V.; van der Meer, R.L.J.; Vansyoc, K.; VanWestrum, D.; Vidakovic, S.; Vulcan, W.; Warren, G.; Wood, S.A.; Xu, C.; Yan, C.; Zhao, W.-X.; Zheng, X.; Zihlmann, B.; The Jefferson Lab Fπ Collaboration
The charged pion form factor, Fπ(Q2), is an important quantity that can be used to advance our knowledge of hadronic structure. However, the extraction of Fπ from data requires a model of the 1H(e,e'π+)n reaction and thus is inherently model dependent. Therefore, a detailed description of the extraction of the charged pion form factor from electroproduction data obtained recently at Jefferson Lab is presented, with particular focus given to the dominant uncertainties in this procedure. Results for Fπ are presented for Q2=0.60-2.45 GeV2. Above Q2=1.5 GeV2, the Fπ values are systematically below the monopole parametrization that describes the low Q2 data used to determine the pion charge radius. The pion form factor can be calculated in a wide variety of theoretical approaches, and the experimental results are compared to a number of calculations. This comparison is helpful in understanding the role of soft versus hard contributions to hadronic structure in the intermediate Q2 regime.