arXiv subjects

S. Lee

Publications and source records attributed to S. Lee.

At least 235 records · Page 13Linked to original sources

arXiv2008

Velocity distribution measurements in atomic beams generated using laser induced back-ablation

We present measurements of the velocity distribution of calcium atoms in an atomic beam generated using a dual-stage laser back-ablation apparatus. Distributions are measured using a velocity selective Doppler time-of-flight technique. They are Boltzmann-like with rms velocities corresponding to temperatures above the melting point for calcium. Contrary to a recent report in the literature, this method does not generate a sub-thermal atomic beam.

physics.atom-ph↗

arXiv2008

Design and Electronics Commissioning of the Physics Prototype of a Si-W Electromagnetic Calorimeter for the International Linear Collider

The CALICE collaboration is studying the design of high performance electromagnetic and hadronic calorimeters for future International Linear Collider detectors. For the electromagnetic calorimeter, the current baseline choice is a high granularity sampling calorimeter with tungsten as absorber and silicon detectors as sensitive material. A ``physics prototype'' has been constructed, consisting of thirty sensitive layers. Each layer has an active area of 18x18 cm2 and a pad size of 1x1 cm2. The absorber thickness totals 24 radiation lengths. It has been exposed in 2006 and 2007 to electron and hadron beams at the DESY and CERN beam test facilities, using a wide range of beam energies and incidence angles. In this paper, the prototype and the data acquisition chain are described and a summary of the data taken in the 2006 beam tests is presented. The methods used to subtract the pedestals and calibrate the detector are detailed. The signal-over-noise ratio has been measured at 7.63 +/- 0.01. Some electronics features have been observed; these lead to coherent noise and crosstalk between pads, and also crosstalk between sensitive and passive areas. The performance achieved in terms of uniformity and stability is presented.

physics.ins-det↗

arXiv2008

Two-step model versus one-step model of the inter-polarization conversion and statistics of CdSe/ZnSe quantum dot elongations

The magneto-optical inter-polarization conversions by a layer of quantum dots have been investigated. Various types of polarization response of the sample were observed as a function of external magnetic field and of the orientation of the sample. The full set of experimental dependences is analyzed in terms of a one-step and a two-step model of spin evolution. The angular distribution of the quantum dots over the directions of elongation in the plane of the sample is taken into account in terms of the two models, and the model predictions are compared with experimental observations.

cond-mat.other↗

arXiv2008

Nanogaps with very large aspect ratios for electrical measurements

For nanoscale electrical characterization and device fabrication it is often desirable to fabricate planar metal electrodes separated by large aspect ratio gaps with interelectrode distances well below 100 nm. We demonstrate a self-aligned process to accomplish this goal using a thin Cr film as a sacrificial etch layer. The resulting gaps can be as small as 10 nm and have aspect ratios exceeding 1000, with excellent interelectrode isolation. Such Ti/Au electrodes are demonstrated on Si substrates and are used to examine a voltage-driven transition in magnetite nanostructures. This shows the utility of this fabrication approach even with relatively reactive substrates.

cond-mat.mtrl-sci↗

arXiv2007

A global 86 GHz VLBI survey

We present results from a large global VLBI(Very Long Baseline Interferometry) survey of compact radio sources at 86 GHz which started in October 2001. The main goal of the survey is to increase the total number of objects accessible for future 3mm-VLBI imaging by factors of 3-5. The survey data reach the baseline sensitivity of 0.1 Jy, and image sensitivity of better than 10 mJy/beam. To date, a total of 127 compact radio sources have been observed. The observations have yielded images for 109 sources, and only 6 sources have not been detected. Flux densities and sizes of core and jet components of all detected sources have been measured using Gaussian model fitting. From these measurements, brightness temperatures have been estimated, taking into account resolution limits of the data. Here, we compare the brightness temperatures of the cores and secondary jet components with similar estimates obtained from surveys at longer wavelengths (e.g. 15 GHz). This approach can be used to study questions related to mechanisms of initial jet acceleration (accelerating or decelerating sub-pc jets?) and jet composition (electron-positron or electron-proton plasma?).

astro-ph↗

arXiv2007

Time-dependent universal conductance fluctuations in mesoscopic Au wires: implications

In cold, mesoscopic conductors, two-level fluctuators lead to time-dependent universal conductance fluctuations (TDUCF) manifested as $1/f$ noise. In Au nanowires, we measure the magnetic field dependence of TDUCF, weak localization (WL), and magnetic field-driven (MF) UCF before and after treatments that alter magnetic scattering and passivate surface fluctuators. Inconsistencies between $L_ϕ^{\rm WL}$ and $L_ϕ^{\rm TDUCF}$ strongly suggest either that the theory of these mesoscopic phenomena in weakly disordered, highly pure Au is incomplete, or that the assumption that the TDUCF frequency dependence remains $1/f$ to very high frequencies is incorrect. In the latter case, TDUCF in excess of $1/f$ expectations may have implications for decoherence in solid-state qubits.

cond-mat.mes-hall↗

arXiv2007

Two-phonon contributions to inelastic x-ray scattering spectra of MgB2

Two-phonon contributions to meV-resolved inelastic x-ray scattering spectra of MgB2 at 300K are identified, in good agreement, in both intensity and energy, with a harmonic calculation using the force constant matrix from ab-inito LDA calculations. This contribution impacts the determination of the linewidth of the E2g phonon mode that is so important for the high Tc of this material. To the best of our knowledge, this is the first observation of peaks in measurements of phonon dispersion (q>0) due to 2-phonon scattering in a non-rare-gas solid.

cond-mat.supr-con↗

arXiv2006

Mesoscopic conductance effects in InMnAs structures

Quantum corrections to the electrical conduction of magnetic semiconductors are comparatively unexplored. We report measurements of time-dependent universal conductance fluctuations (TDUCF) and magnetic field dependent universal conductance fluctuations (MFUCF) in micron-scale structures fabricated from two different In$_{1-x}$Mn$_{x}$As thin films. TDUCF and MFUCF increasing in magnitude with decreasing temperature are observed. At 4 K and below, TDUCF are suppressed at finite magnetic fields independent of field orientation.

cond-mat.mes-hall↗

arXiv2006

Strong magnetic scattering from TiO$_{x}$ adhesion layers

Electronic phase coherence in normal metals is incredibly sensitive to magnetic scattering. As a result, the weak localization magnetoresistance and time-dependent universal conductance fluctuations are powerful probes of magnetic impurities. We report measurements of these effects in Au and Ag nanowires with a 1.5 nm thick Ti adhesion layer underneath the deposited metal. The results indicate an anomalously large magnetic impurity concentration due to the Ti layer. Results suggest that this magnetic scattering and its evolution are related to the oxidation state of the Ti.

cond-mat.mes-hall↗

arXiv2006

On the Search For Transits of the Planets Orbiting Gl 876

We report the results of a globally coordinated photometric campaign to search for transits by the P ~ 30 d and P ~ 60 d outer planets of the 3-planet system orbiting the nearby M-dwarf Gl 876. These two planets experience strong mutual perturbations, which necessitate use of a dynamical (four-body) model to compute transit ephemerides for the system. Our photometric data have been collected from published archival sources, as well as from our photometric campaigns that were targeted to specific transit predictions. Our analysis indicates that transits by planet "c" (P ~ 30 d) do not currently occur, in concordance with the best-fit i = 50 degree co-planar configuration obtained by dynamical fits to the most recent radial velocity data for the system. Transits by planet "b" (P ~ 60 d) are not entirely ruled out by our observations, but our data indicate that it is very unlikely that they occur. Our experience with the Gl 876 system suggests that a distributed ground-based network of small telescopes can be used to search for transits of very low mass M-stars by terrestrial-sized planets.

astro-ph↗

arXiv2005

Phase diagram of the three band half-filled Cu-O two-leg ladder

We determine the phase diagram of the half-filled two-leg ladder both at weak and strong coupling, taking into account the Cu d_{x^2-y^2} and the O p_x and p_y orbitals. At weak coupling, renormalization group flows are interpreted with the use of bosonization. Two different models with and without outer oxygen orbitals are examined. For physical parameters, and in the absence of the outer oxygen orbitals, the D-Mott phase arises; a dimerized phase appears when the outer oxygen atoms are included. We show that the circulating current phase that preserves translational symmetry does not appear at weak coupling. In the opposite strong-coupling atomic limit the model is purely electrostatic and the ground states may be found by simple energy minimization. The phase diagram so obtained is compared to the weak-coupling one.

cond-mat.str-el↗

arXiv2004

Time-dependent universal conductance fluctuations and coherence in AuPd and Ag

Quantum transport phenomena allow experimental assessment of the phase coherence information in metals. We report quantitative comparisons of coherence lengths inferred from weak localization magnetoresistance measurements and time-dependent universal conductance fluctuation data. We describe these two measurements and their analysis. Strong agreement is observed in both quasi-2D and quasi-1D AuPd samples, a metal known to have high spin-orbit scattering. However, quantitative {\it disagreement} is seen in quasi-1D Ag wires below 10 K, a material with intermediate spin-orbit scattering. We consider explanations of this discrepancy, with particular emphasis on the theoretical expressions used to analyze the field dependence of the conductance fluctuations. We also discuss the mechanism of the suppression of conductance fluctuations at high drive levels, and dephasing mechanisms at work in these systems.

cond-mat.mes-hall↗

arXiv2004

$p$-$sd$ shell gap reduction in neutron-rich systems and cross-shell excitations in $^{20}$O

Excited states in $^{20}$O were populated in the reaction $^{10}$Be($^{14}$C,$α$) at Florida State University. Charged particles were detected with a particle telescope consisting of 4 annularly segmented Si surface barrier detectors and $γ$ radiation was detected with the FSU $γ$ detector array. Five new states were observed below 6 MeV from the $α$-$γ$ and $α$-$γ$-$γ$ coincidence data. Shell model calculations suggest that most of the newly observed states are core-excited 1p-1h excitations across the $N = Z = 8$ shell gap. Comparisons between experimental data and calculations for the neutron-rich O and F isotopes imply a steady reduction of the $p$-$sd$ shell gap as neutrons are added.

nucl-ex↗

arXiv2004

Pressure dependence of the upper critical field of MgB2 and of YNi2B2C

We present measurements of H$_{c2}(T)$ under pressure in MgB$_2$ and in YNi$_2$B$_2$C. The changes in the shape of H$_{c2}(T)$ are interpreted within current models and show the evolution of the main Fermi surface velocities $v_F$ and electron-phonon coupling parameters $λ$ with pressure. In MgB$_2$ the electron-phonon coupling strength of the nearly two dimensional $σ$ band, responsible for the high critical temperature, is more affected by pressure than the $π$ band coupling, and the hole doping of the $σ$ band decreases. In YNi$_2$B$_2$C, the peculiar positive curvature of H$_{c2}(T)$ is weakened by pressure.

cond-mat.supr-con↗

arXiv2004

Excess current in point contacts on two-band superconductor MgB$_2$ in magnetic field

Series of $I(V)$ characteristic and bias-dependent differential resistance $dV/dI(V)$ for point-contacts on the single crystals of two-band superconductor MgB$_2$ were measured in magnetic field up to 9 T. We have obtained magnetic field dependences of the excess current on the $I(V)$ curves and analyzed them using Koshelev and Golubov (Phys. Rev. Lett. {\bf 90}, 177002 (2003)) theoretical results for the mixed state of a dirty two-band superconductor. Introducing a simple model for the excess current in the point contact in the mixed state our data can be qualitatively described utilizing theoretical magnetic filed behavior of the $σ$ and $π$-band superconducting order parameters and the corresponding averaged electronic density of states in MgB$_2$.

cond-mat.supr-con↗

arXiv2004

Carbon-substitution dependent multiple superconducting gap of MgB$_{2}$: a "sub-meV" resolution photoemission study

``Sub-meV'' resolution photoemission spectroscopy was used to study carbon-substitution dependence on the multiple superconducting gap of Mg(B$_{1 - x}$C$_{x})_{2}$. Two features corresponding to $σ$ and $π$ gaps are clearly observed in the raw spectra up to carbon concentration x = 7.5 {%}. The observed x dependence of the two gaps shows a qualitatively different behavior: a marked change of the $σ$ gap proportional to the $T_{c}$ variation and a negligible one of the $π$ gap. This as well as the temperature dependence can be explained with the two-band mean-field theory. Implications from the present study are discussed.

cond-mat.supr-con↗

arXiv2004

Quantum coherence in a ferromagnetic metal: time-dependent conductance fluctuations

Quantum coherence of electrons in ferromagnetic metals is difficult to assess experimentally. We report the first measurements of time-dependent universal conductance fluctuations in ferromagnetic metal (Ni$_{0.8}$Fe$_{0.2}$) nanostructures as a function of temperature and magnetic field strength and orientation. We find that the cooperon contribution to this quantum correction is suppressed, and that domain wall motion can be a source of coherence-enhanced conductance fluctuations. The fluctuations are more strongly temperature dependent than those in normal metals, hinting that an unusual dephasing mechanism may be at work.

cond-mat.mes-hall↗

arXiv2004

Effect of Plasma Irradiation on $Cd I_2$ films

The effect of plasma irradiation is studied systematically on a 4H polytype (002) oriented ${\rm CdI_2}$ stoichiometric film having compressive residual stress. Plasma irradiation was found to change the orientation to (110) of the film at certain moderate irradiation distances. A linear decrease in grain size and residual stress was observed with decreasing irradiation distance (or increasing ion energy) consistent with both structural and morphological observations. The direct optical energy gap ${\rm E_g}$ was found to increase linearly at the rate ${\rm 15μeV/atm}$ with the compressive stress. The combined data of present compressive stress and from earlier reported tensile stress show a consistent trend of ${\rm E_g}$ change with stress. The iodine-iodine distance in the unit cell could be responsible for the observed change in ${\rm E_g}$ with stress.

cond-mat.mtrl-sci↗