Fabrication and Characterization of Topological Insulator Bi2_2Se3_3 Nanocrystals

In the recently discovered class of materials known as topological insulators, the presence of strong spin-orbit coupling causes certain topological invariants in the bulk to differ from their values in vacuum. The sudden change of invariants at the interface results in metallic, time reversal invariant surface states whose properties are useful for applications in spintronics and quantum computation. However, a key challenge is to fabricate these materials on the nanoscale appropriate for devices and probing the surface. To this end we have produced 2 nm thick nanocrystals of the topological insulator Bi$_2$Se$_3$ via mechanical exfoliation. For crystals thinner than 10 nm we observe the emergence of an additional mode in the Raman spectrum. The emergent mode intensity together with the other results presented here provide a recipe for production and thickness characterization of Bi$_2$Se$_3$ nanocrystals.Comment: 4 pages, 3 figures (accepted for publication in Applied Physics Letters

Assessing Parental Self-Efficacy for Obesity Prevention Related Behaviors

Background: Reliable, valid and theoretically consistent measures that assess a parent’s self-efficacy for helping a child with obesity prevention behaviors are lacking. Objectives: To develop measures of parental self-efficacy for four behaviors: 1) helping their child get at least 60 minutes of moderate intensity physical activity every day, 2) helping one’s child consume five servings of fruits and vegetables each day, 3) limiting sugary drinks to once a week, and 4) limiting consumption of fruit juice to 6 ounces every day. Methods: Sequential methods of scale development were used. An item pool was generated based on theory and qualitative interviews, and reviewed by content experts. Scales were administered to parents or legal guardians of children 4–10 years old. The item pool was reduced using principal component analysis. Confirmatory factor analysis tested the resulting models in a separate sample. Subjects: 304 parents, majority were women (88%), low-income (61%) and single parents (61%). Ethnic distribution was 40% Black and 37% white. Results: All scales had excellent fit indices: Comparative fit index \u3e .98 and chi-squares (Pediatrics 120 Suppl 4:S229-253, 2007) = .85 – 7.82. Alphas and one-week test-retest ICC’s were ≥ .80. Significant correlations between self-efficacy scale scores and their corresponding behaviors ranged from .13-.29 (all p \u3c .03). Conclusions: We developed four, four-item self-efficacy scales with excellent psychometric properties and construct validity using diverse samples of parents

Ion-lithium collision dynamics studied with an in-ring MOTReMi

We present a novel experimental tool allowing for kinematically complete studies of break-up processes of laser-cooled atoms. This apparatus, the 'MOTReMi', is a combination of a magneto-optical trap (MOT) and a Reaction Microscope (ReMi). Operated in an ion-storage ring, the new setup enables to study the dynamics in swift ion-atom collisions on an unprecedented level of precision and detail. In first experiments on collisions with 1.5 MeV/amu O$^{8+}$-Li the pure ionization of the valence electron as well as ionization-excitation of the lithium target has been investigated

Manipulating Atomic Fragmentation Processes by Controlling the Projectile Coherence

We have measured the scattering angle dependence of cross sections for ionization in p+H2 collisions for a fixed projectile energy loss. Depending on the projectile coherence, interference due to indistinguishable diffraction of the projectile from the two atomic centers was either present or absent in the data. This shows that, due to the fundamentals of quantum mechanics, the preparation of the beam must be included in theoretical calculations. The results have far-reaching implications on formal atomic scattering theory because this critical aspect has been overlooked for several decades

Comparison of Experimental and Theoretical Fully Differential Cross Sections for Single Ionization of the 2s and 2p States of Li By O⁸⁺ Ions

This paper presents a full three-dimensional (3D) comparison between experiment and theory for 24 MeV O8+ single ionization of the 2s ground state of lithium and the 2p excited state. Two theoretical approximations are examined: the three-body continuum distorted-wave (3DW) and three-body continuum distorted-wave-eikonal initial state (3DW-EIS). Normally, there is a significant difference between these two approaches and the 3DW-EIS is in much better agreement with experiment. In this case, there is very little difference between the two approaches and both are in very good agreement with experiment. For the excited 2p state, the 3D cross sections would exhibit a mirror symmetry about the scattering plane if all three magnetic sublevels were excited in equal proportions. For the present experiment, the 2p+1 (m=+1) sublevel is dominantly excited (quantization axis is the incident beam direction) and for this case there is a magnetic dichroism which is observed both experimentally and theoretically

Manipulating Atomic Fragmentation Processes by Controlling the Projectile Coherence

We have measured the scattering angle dependence of cross sections for ionization in p+H2 collisions for a fixed projectile energy loss. Depending on the projectile coherence, interference due to indistinguishable diffraction of the projectile from the two atomic centers was either present or absent in the data. This shows that, due to the fundamentals of quantum mechanics, the preparation of the beam must be included in theoretical calculations. The results have far-reaching implications on formal atomic scattering theory because this critical aspect has been overlooked for several decades

Effect of Projectile Coherence on Atomic Fragmentation Processes

We demonstrate that the projectile coherence can have a major impact on atomic fragmentation processes. This has been overlooked for decades in formal scattering theory and may explain puzzling discrepancies between theoretical and experimental fully differential cross sections for single ionization

Real-time dynamics of the formation of hydrated electrons upon irradiation of water clusters with extreme ultraviolet light

Free electrons in a polar liquid can form a bound state via interaction with the molecular environment. This so-called hydrated electron state in water is of fundamental importance e.g.~in cellular biology or radiation chemistry. Hydrated electrons are highly reactive radicals that can either directly interact with DNA or enzymes, or form highly excited hydrogen (H∗) after being captured by protons. Here, we investigate the formation of the hydrated electron in real-time employing XUV femtosecond pulses from a free electron laser, in this way observing the initial steps of the hydration process. Using time-resolved photoelectron spectroscopy we find formation timescales in the low picosecond range and resolve the prominent dynamics of forming excited hydrogen states

Two-Stage Rotational Disordering of a Molecular Crystal Surface: C60

We propose a two-stage mechanism for the rotational surface disordering phase transition of a molecular crystal, as realized in C$_{60}$ fullerite. Our study, based on Monte Carlo simulations, uncovers the existence of a new intermediate regime, between a low temperature ordered $(2 \times 2)$ state, and a high temperature $(1 \times 1)$ disordered phase. In the intermediate regime there is partial disorder, strongest for a subset of particularly frustrated surface molecules. These concepts and calculations provide a coherent understanding of experimental observations, with possible extension to other molecular crystal surfaces.Comment: 4 pages, 2 figure

Interference Effects Due to Projectile Target Nucleus Scattering in Single Ionization of H₂ by 75-keV Proton Impact

Doubly differential cross sections (DDCSs) for single ionization of molecular hydrogen by 75-keV proton impact have been measured and calculated as a function of the projectile scattering angle and energy loss. Interference structures are observed in the scattering angular dependence of the DDCSs, which disappear, however, at electron speeds near the projectile speed. The comparison to our calculations shows that the projectile-target nucleus interaction plays a central role. Furthermore, our data suggest that for a given scattering angle, ionization favors well-defined molecular orientations