A. Baldwin - Louisiana State University

A. Baldwin
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A. Baldwin
Louisiana State University
Baton Rouge
United States

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Nuclear Experiment (6)
Nuclear Theory (4)
Solar and Stellar Astrophysics (3)
Astrophysics of Galaxies (2)
Physics - Strongly Correlated Electrons (2)
Physics - Soft Condensed Matter (2)
Physics - Statistical Mechanics (1)
High Energy Physics - Experiment (1)
Physics - Optics (1)
Physics - Materials Science (1)
High Energy Astrophysical Phenomena (1)

Publications Authored By A. Baldwin

Proper motions (PMs) are crucial to fully understand the internal dynamics of globular clusters (GCs). To that end, the Hubble Space Telescope (HST) Proper Motion (HSTPROMO) collaboration has constructed large, high-quality PM catalogues for 22 Galactic GCs. We highlight some of our exciting recent results: the first directly-measured radial anisotropy profiles for a large sample of GCs; the first dynamical distance and mass-to-light (M/L) ratio estimates for a large sample of GCs; and the first dynamically-determined masses for hundreds of blue-straggler stars (BSSs) across a large GC sample. Read More

The charge and magnetic form factors, FC and FM, of 3He have been extracted in the kinematic range 25 fm-2 < Q2 < 61 fm-2 from elastic electron scattering by detecting 3He recoil nuclei and electrons in coincidence with the High Resolution Spectrometers of the Hall A Facility at Jefferson Lab. The measurements are indicative of a second diffraction minimum for the magnetic form factor, which was predicted in the Q2 range of this experiment, and of a continuing diffractive structure for the charge form factor. The data are in qualitative agreement with theoretical calculations based on realistic interactions and accurate methods to solve the three-body nuclear problem. Read More

The nematic twist-bend (TB) phase, exhibited by certain achiral thermotropic liquid crystalline (LC) dimers, features a nanometer-scale, heliconical rotation of the average molecular long axis (director) with equally probable left- and right-handed domains. On meso to macroscopic scales, the TB phase may be considered as a stack of equivalent slabs or "pseudo-layers", each one helical pitch in thickness. The long wavelength fluctuation modes should then be analogous to those of a smectic-A phase, and in particular the hydrodynamic mode combining "layer" compression and bending ought to be characterized by an effective layer compression elastic constant $B_{eff}$ and average director splay constant $K_1^{eff}$. Read More

We make use of the Hubble Space Telescope proper-motion catalogs derived by Bellini et al. (2014) to produce the first radial velocity-dispersion profiles sigma(R) for blue straggler stars (BSSs) in Galactic globular clusters (GCs), as well as the first dynamical estimates for the average mass of the entire BSS population. We show that BSSs typically have lower velocity dispersions than stars with mass equal to the main-sequence turnoff mass, as one would expect for a more massive population of stars. Read More

An $S=1$ antiferromagnetic polymeric chain, [Ni(HF$_2$)(3-Clpy)$_4$]BF$_4$ (py = pyridine), has previously been identified to have intrachain, nearest-neighbor antiferromagnetic interaction strength $J/k_{\mathrm{B}} = 4.86$ K and single-ion anisotropy (zero-field splitting) $D/k_{\mathrm{B}} = 4.3$ K, so the ratio $D/J = 0. Read More

Affiliations: 1Louisiana State University, 2Louisiana State University, 3Louisiana State University, 4Louisiana State University, 5SRON, Netherlands Institute for Space Research, 6IMAPP, Radboud UniversityNijmegen, 7SRON, Netherlands Institute for Space Research, 8Texas Tech University, 9University of Warwick, 10University of Warwick, 11University of Alberta, 12Jodrell Bank Centre for Astrophysics, 13Louisiana State University, 14Massachussettes Institute of Technology, 15Florida Atlantic University, 16Louisiana State University

We present optical lightcurves of variable stars consistent with the positions of X-ray sources identified with the Chandra X-ray Observatory for the Chandra Galactic Bulge Survey. Using data from the Mosaic-II instrument on the Blanco 4m Telescope at CTIO, we gathered time-resolved photometric data on timescales from $\sim2$ hr to 8 days over the $\frac{3}{4}$ of the X-ray survey containing sources from the initial GBS catalog. Among the lightcurve morphologies we identify are flickering in interacting binaries, eclipsing sources, dwarf nova outbursts, ellipsoidal variations, long period variables, spotted stars, and flare stars. Read More

The charge form factor of $^$4He has been extracted in the range 29 fm$^{-2}$ $\le Q^2 \le 77$ fm$^{-2}$ from elastic electron scattering, detecting $^4$He nuclei and electrons in coincidence with the High Resolution Spectrometers of the Hall A Facility of Jefferson Lab. The results are in qualitative agreement with realistic meson-nucleon theoretical calculations. The data have uncovered a second diffraction minimum, which was predicted in the $Q^2$ range of this experiment, and rule out conclusively long-standing predictions of dimensional scaling of high-energy amplitudes using quark counting. Read More

Affiliations: 1Liquid Crystal Institute, Kent State University, Kent, USA, 2Department of Physics, Kent State University, Kent, USA, 3Department of Physics, Kent State University, Kent, USA, 4Liquid Crystal Institute, Kent State University, Kent, USA, 5Department of Physics, Kent State University, Kent, USA

Using dynamic light scattering, we measure for the first time the temperature-dependent elastic moduli and associated orientational viscosity coefficients of the nematic phase in a self-assembled lyotropic chromonic liquid crystal. The bend K3 and splay K1 moduli are an order of magnitude higher than the twist K2 constant. The ratio K3/K1 shows an anomalous increase with temperature; we attribute this to the shortening of the aggregates as temperature increases. Read More

Background: The strength of electron capture for medium mass nuclei has a significant effect on the evolution of supernovae. There is insufficient knowledge of these strengths and very little data for important radioactive nuclei. Purpose: Determine whether it is feasible to obtain EC strength from studies of T_o+1 excitations in (p,n) reactions, and whether this might yield information for radioactive nuclei. Read More

We study the ground state phases of Bose-Fermi mixtures in one-dimensional optical lattices with quantum Monte Carlo simulations using the Canonical Worm algorithm. Depending on the filling of bosons and fermions, and the on-site intra- and inter-species interaction, different kinds of incompressible and superfluid phases appear. On the compressible side, correlations between bosons and fermions can lead to a distinctive behavior of the bosonic superfluid density and the fermionic stiffness, as well as of the equal-time Green functions, which allow one to identify regions where the two species exhibit anticorrelated flow. Read More

We use determinant Quantum Monte Carlo simulations and exact diagonalization to explore insulating behavior in the Hubbard model with a bimodal distribution of randomly positioned local site energies. From the temperature dependence of the compressibility and conductivity, we show that gapped, incompressible Mott insulating phases exist away from half filling when the variance of the local site energies is sufficiently large. The compressible regions around this Mott phase are metallic only if the density of sites with the corresponding energy exceeds the percolation threshold, but are Anderson insulators otherwise. Read More

Affiliations: 1Kent State University, 2Kent State University, 3Kent State University, 4Kent State University, 5Kent State University, 6Kent State University, 7Kent State University, 8Kent State University, 9Kent State University, 10Kent State University, 11Kent State University, 12LBNL, 13LBNL, 14LBNL, 15LBNL, 16LBNL, 17LBNL, 18Michigan State University, 19Michigan State University, 20McGill University, 21McGill University

Triple-differential cross sections for neutrons from high-multiplicity La-La collisions at 250 and 400 MeV per nucleon and Nb-Nb collisions at 400 MeV per nucleon were measured at several polar angles as a function of the azimuthal angle with respect to the reaction plane of the collision. The reaction plane was determined by a transverse-velocity method with the capability of identifying charged-particles with Z=1, Z=2, and Z > 2. The flow of neutrons was extracted from the slope at mid-rapidity of the curve of the average in-plane momentum vs the center-of-mass rapidity. Read More

We report observations from the (p,n) reaction on 12C at 135 MeV. The experiment was performed with the beam-swinger neutron time-of-flight system at the Indiana University Cyclotron Facility. Neutrons were detected in large-volume plastic scintillation detectors located in three detector stations at 0 deg, 24 deg, and 45 deg with respect to the undeflected beam line; the flight paths were 91 m, 91 m, and 74 m, respectively. Read More

We measured neutron triple-differential cross sections from multiplicity-selected Au-Au collisions at 150, 250, 400, and 650 \AMeV. The reaction plane for each collision was estimated from the summed transverse velocity vector of the charged fragments emitted in the collision. We examined the azimuthal distribution of the triple-differential cross sections as a function of the polar angle and the neutron rapidity. Read More

We measured the first azimuthal distributions of triple--differential cross sections of neutrons emitted in heavy-ion collisions, and compared their maximum azimuthal anisotropy ratios with Boltzmann--Uehling--Uhlenbeck (BUU) calculations with a momentum-dependent interaction. The BUU calculations agree with the triple- and double-differential cross sections for positive rapidity neutrons emitted at polar angles from 7 to 27 degrees; however, the maximum azimuthal anisotropy ratio for these free neutrons is insensitive to the size of the nuclear incompressibility modulus K characterizing the nuclear matter equation of state. Read More