Search Results (305 total)

The interface formation of Nb-doped SrTiO₃ single crystals and (Ba,Sr)TiO₃ thin films with Pt has been studied by using photoelectron spectroscopy with in situ sample preparation. For the single crystal sample, a Schottky barrier height for electrons of 0.5–0.6 eV is determined after deposition of Pt in vacuum environment. After annealing in 0.05 Pa oxygen pressure, a strong increase in the barrier height to ≥1.2 eV is observed. X-ray induced photovoltages of up to 0.7 eV are observed in this case and have to be taken into account for a proper determination of the barrier height. A subsequent annealing in vacuum reduces the barrier again. Hence, the barrier height can be reversibly switched between an oxidized state with a large barrier height and a reduced state with a low barrier height. Quantitative analysis of the barrier heights indicates that the changes are related to the changes of interfacial defect concentration. Due to the occurrence of a Ti³⁺ related signal, the defects are identified as oxygen vacancies. The same effects are observed at interfaces between Pt and (Ba,Sr)TiO₃ thin films with a smaller absolute value of the barrier height in the oxidized state of ∼1 eV. Deposition of (Ba,Sr)TiO₃ onto a metallic Pt substrate also results in a barrier height of 1.0 eV.

Bulk and surface sensitive x-ray spectroscopic techniques are applied in tandem to show that the valence band edge for In₂O₃ is found significantly closer to the bottom of the conduction band than expected on the basis of the widely quoted bulk band gap of 3.75 eV. First-principles theory shows that the upper valence bands of In₂O₃ exhibit a small dispersion and the conduction band minimum is positioned at Γ. However, direct optical transitions give a minimal dipole intensity until 0.8 eV below the valence band maximum. The results set an upper limit on the fundamental band gap of 2.9 eV.

The beam-spin asymmetries in the hard exclusive electroproduction of photons on the proton (e→p→epγ) were measured over a wide kinematic range and with high statistical accuracy. These asymmetries result from the interference of the Bethe-Heitler process and of deeply virtual Compton scattering. Over the whole kinematic range (x_B from 0.11 to 0.58, Q² from 1 to 4.8  GeV², -t from 0.09 to 1.8  GeV²), the azimuthal dependence of the asymmetries is compatible with expectations from leading-twist dominance, A≃asin⁡ϕ/(1+ccos⁡ϕ). This extensive set of data can thus be used to constrain significantly the generalized parton distributions of the nucleon in the valence quark sector.

We report a limit on the fractional temporal variation of the proton-to-electron mass ratio as 1 / (m_P/m_e)∂ / ∂t(m_P/m_e)=(-3.8±5.6)×10^-14  yr^-1, obtained by comparing the frequency of a rovibrational transition in SF₆ with the fundamental hyperfine transition in Cs. The SF₆ transition was accessed using a CO₂ laser to interrogate spatial 2-photon Ramsey fringes. The atomic transition was accessed using a primary standard controlled with a Cs fountain. This result is direct and model-free.

The beam spin asymmetry (BSA) in the exclusive reaction e→p→epπ⁰ was measured with the CEBAF 5.77 GeV polarized electron beam and Large Acceptance Spectrometer (CLAS). The x_B,Q²,t, and ϕ dependences of the π⁰ BSA are presented in the deep inelastic regime. The asymmetries are fitted with a sinϕ function and their amplitudes are extracted. Overall, they are of the order of 0.04–0.11 and roughly independent of t. This is the signature of a nonzero longitudinal-transverse interference. The implications concerning the applicability of a formalism based on generalized parton distributions, as well as the extension of a Regge formalism at high photon virtualities, are discussed.

The dynamics of a genetically labeled cell population may be used to infer the laws of cell division in mammalian tissue. Recently, we showed that in mouse tail skin, where proliferating cells are confined to a two-dimensional layer, cells proliferate and differentiate according to a simple stochastic model of cell division involving just one type of proliferating cell that may divide both symmetrically and asymmetrically. Curiously, these simple rules provide excellent predictions of the cell population dynamics without having to address the cells’ spatial distribution. Yet, if the spatial behavior of cells is addressed by allowing cells to diffuse at random, one deduces that density fluctuations destroy tissue confluence, implying some hidden degree of spatial regulation of cell division. To infer the mechanism of spatial regulation, we consider a two-dimensional model of cell fate that preserves the overall population dynamics. By identifying the resulting behavior with a three-species variation of the voter model, we predict that proliferating cells in the basal layer should cluster. Analysis of empirical correlations of cells stained for proliferation activity confirms that the expected clustering behavior is indeed seen in nature. As well as explaining how cells maintain a uniform two-dimensional density, these findings present an interesting experimental example of voter-model statistics in biology.

We examine the results of two measurements by the CLAS collaboration, one of which claimed evidence for a Θ⁺ pentaquark, while the other found no such evidence. The unique feature of these two experiments was that they were performed with the same experimental setup. Using a Bayesian analysis, we find that the results of the two experiments are in fact compatible with each other, but that the first measurement did not contain sufficient information to determine unambiguously the existence of a Θ⁺. Further, we suggest a means by which the existence of a new candidate particle can be tested in a rigorous manner.

The exclusive electroproduction process e→p→e^'nπ⁺ was measured in the range of the photon virtuality Q²=1.7-4.5 GeV², and the invariant mass range for the nπ⁺ system of W=1.15-1.7 GeV using the CEBAF Large Acceptance Spectrometer. For the first time, these kinematics are probed in exclusive π⁺ production from protons with nearly full coverage in the azimuthal and polar angles of the nπ⁺ center-of-mass system. The nπ⁺ channel has particular sensitivity to the isospin ½ excited nucleon states, and together with the pπ⁰ final state will serve to determine the transition form factors of a large number of resonances. The largest discrepancy between these results and present modes was seen in the σ_LT^' structure function. In this experiment, 31,295 cross section and 4,184 asymmetry data points were measured. Because of the large volume of data, only a reduced set of structure functions and Legendre polynomial moments can be presented that are obtained in model-independent fits to the differential cross sections.

High-resolution photoelectron spectroscopy of in situ prepared films of GeSb₂Te₄ reveals significant differences in electronic and chemical structure between the amorphous and the crystalline phase. Evidence for two different chemical environments of Ge and Sb in the amorphous structure is found. This observation can explain the pronounced property contrast between both phases and provides new insight into the formation of the amorphous state.

The photoproduction of vector mesons on various nuclei has been studied using the CLAS detector at Jefferson Laboratory. The vector mesons, ρ, ω, and ϕ, are observed via their decay to e⁺e^-, in order to reduce the effects of final-state interactions in the nucleus. Of particular interest are possible in-medium effects on the properties of the ρ meson. The ρ mass spectrum is extracted from the data on various nuclei, ²H, C, Fe, and Ti. We observe no significant mass shift and some broadening consistent with expected collisional broadening for the ρ meson.

The cross section and decay angular distributions for the coherent ϕ-meson photoproduction on the deuteron have been measured for the first time up to a squared four-momentum transfer t=(p_γ-p_ϕ)²=-2 GeV²/c², using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The cross sections are compared with predictions from a rescattering model. In a framework of vector meson dominance, the data are consistent with the total ϕ-N cross section σ_ϕN at about 10 mb. If vector meson dominance is violated, a larger σ_ϕN is possible by introducing a larger t slope for the ϕN→ϕN process than that for the γN→ϕN process. The decay angular distributions of the ϕ are consistent with helicity conservation.

We study the dynamics of vortices in ideal and weakly interacting Bose-Einstein condensates using a Ritz minimization method to solve the two-dimensional Gross-Pitaevskii equation. For different initial vortex configurations we calculate the trajectories of the vortices. We find conditions under which a vortex-antivortex pair annihilates and is created again. For the case of three vortices we show that at certain times two additional vortices may be created, which move through the condensate and annihilate each other again. For a noninteracting condensate this process is periodic, whereas for small interactions the essential features persist, but the periodicity is lost. The results are compared to exact numerical solutions of the Gross-Pitaevskii equation confirming our analytical findings.

Differential cross sections for the reaction γp→pπ⁰ have been measured with the CEBAF Large Acceptance Spectrometer (CLAS) and a tagged photon beam with energies from 0.675 to 2.875 GeV. The results reported here possess greater accuracy in the absolute normalization than previous measurements. They disagree with recent CB-ELSA measurements for the process at forward scattering angles. Agreement with the SAID and MAID fits is found below 1 GeV. The present set of cross sections has been incorporated into the SAID database, and exploratory fits have been extended to 3 GeV. Resonance couplings have been extracted and compared to previous determinations.

Photoproduction of the cascade resonances has been investigated in the reactions γp→K⁺K⁺(X) and γp→K⁺K⁺π^-(X). The mass splitting of the ground state (Ξ^-,Ξ⁰) doublet is measured to be 5.4±1.8 MeV/c², consistent with existing measurements. The differential (total) cross sections for the Ξ^- have been determined for photon beam energies from 2.75 to 3.85 (4.75) GeV and are consistent with a production mechanism of Y^*→K⁺Ξ^- through a t-channel process. The reaction γp→K⁺K⁺π^-[Ξ⁰] has also been investigated to search of excited cascade resonances. No significant signal of excited cascade states other than the Ξ^-(1530) is observed. The cross-section results of the Ξ^-(1530) have also been obtained for photon beam energies from 3.35 to 4.75 GeV.

The rules governing cell division and differentiation are central to understanding the mechanisms of development, aging, and cancer. By utilizing inducible genetic labeling, recent studies have shown that the clonal population in transgenic mouse epidermis can be tracked in vivo. Drawing on these results, we explain how clonal fate data may be used to infer the rules of cell division and differentiation underlying the maintenance of adult murine tail-skin. We show that the rates of cell division and differentiation may be evaluated by considering the long-time and short-time clone fate data, and that the data is consistent with cells dividing independently rather than synchronously. Motivated by these findings, we consider a mechanism for cancer onset based closely on the model for normal adult skin. By analyzing the expected changes to clonal fate in cancer emerging from a simple two-stage mutation, we propose that clonal fate data may provide a novel method for studying the earliest stages of the disease.

New cross sections for the reaction ep→e^'ηp are reported for total center-of-mass energy W=1.5-2.3 GeV and invariant squared momentum transfer Q²=0.13-3.3 GeV². This large kinematic range allows the extraction of new information about response functions, photocouplings, and ηN coupling strengths of baryon resonances. A sharp structure is seen at W~1.7 GeV. The shape of the differential cross section is indicative of the presence of a P-wave resonance that persists to high Q². Improved values are derived for the photocoupling amplitude for the S₁₁(1535) resonance. The new data greatly expand the Q² range covered, and an interpretation of all data with a consistent parametrization is provided.

We investigate the effects of a nearly uniform Bose-Einstein condensate (BEC) on the properties of immersed trapped impurity atoms. Using a weak-coupling expansion in the BEC-impurity interaction strength, we derive a model describing polarons, i.e., impurities dressed by a coherent state of Bogoliubov phonons, and apply it to ultracold bosonic atoms in an optical lattice. We show that, with increasing BEC temperature, the transport properties of the impurities change from coherent to diffusive. Furthermore, stable polaron clusters are formed via a phonon-mediated off-site attraction.

The reaction ²H(e,e^′p)n has been studied with full kinematic coverage for photon virtuality 1.75<Q²<5.5  GeV². Comparisons of experimental data with theory indicate that for very low values of neutron recoil momentum (p_n<100  MeV/c) the neutron is primarily a spectator and the reaction can be described by the plane-wave impulse approximation. For 100<p_n<750  MeV/c, proton-neutron rescattering dominates the cross section, while Δ production followed by the NΔ→NN transition is the primary contribution at higher momenta.

We have recorded the Doppler profile of a well-isolated rovibrational line in the ν₂ band of ¹⁴NH₃. Ammonia gas was placed in an absorption cell thermalized by a water-ice bath. By extrapolating to zero pressure, we have deduced the Doppler width which gives a first measurement of the Boltzmann constant k_B by laser spectroscopy. A relative uncertainty of 2×10^-4 has been obtained. The present determination should be significantly improved in the near future and contribute to a new definition of the kelvin.

The data analysis for the reaction ¹H(e,e^'π⁺)n, which was used to determine values for the charged pion form factor F_π for values of Q²= 0.6–1.6 GeV², has been repeated with careful inspection of all steps and special attention to systematic uncertainties. Also the method used to extract F_π from the measured longitudinal cross section was critically reconsidered. Final values for the separated longitudinal and transverse cross sections and the extracted values of F_π are presented.

The nature of the band gap of indium oxide is still a matter of debate. Based on optical measurements the presence of an indirect band gap has been suggested, which is 0.9 to 1.1 eV smaller than the direct band gap at the Γ point. This could be caused by strong mixing of O 2p and In 4d orbitals off Γ. We have performed extensive density functional theory calculations using the LDA+U and the GGA+U methods to elucidate the contribution of the In 4d states and the effect of spin-orbit coupling on the valence band structure. Although an indirect band gap is obtained, the energy difference between the overall valence band maximum and the highest occupied level at the Γ point is less than 50 meV. It is concluded that the experimental observation cannot be related to the electronic structure of the defect free bulk material.

Differential cross sections for the reaction γp→K^*0Σ⁺ are presented in the photon energy range of 1.7 to 3.0 GeV. The K^*0 was detected by its decay products, K⁺π^-, in the Continuous Electron Beam Accelerator Facility's large acceptance spectrometer (CLAS) detector at the Thomas Jefferson National Accelerator Facility. These data are the first K^*0 photoproduction cross sections ever published over a broad range of angles. Comparison with a theoretical model based on the vector and tensor K^*-quark couplings shows good agreement with the data, except at forward angles, suggesting that the role of scalar κ meson exchange should be investigated.

We report measurements of the exclusive electroproduction of K⁺Λ and K⁺Σ⁰ final states from a proton target using the Continuous Electron Beam Accelerator Facility (CEBAF) large-acceptance spectrometer (CLAS) detector at the Thomas Jefferson National Accelerator Facility. The separated structure functions σ_T,σ_L,σ_TT, and σ_LT were extracted from the Φ- and ε-dependent differential cross sections taken with electron beam energies of 2.567, 4.056, and 4.247 GeV. This analysis represents the first σ_L/σ_T separation with the CLAS detector, and the first measurement of the kaon electroproduction structure functions away from parallel kinematics. The data span a broad range of momentum transfers from 0.5≤Q²≤2.8GeV² and invariant energy from 1.6≤W≤2.4 GeV, while spanning nearly the full center-of-mass angular range of the kaon. The separated structure functions reveal clear differences between the production dynamics for the Λ and Σ⁰ hyperons. These results provide an unprecedented data sample with which to constrain current and future models for the associated production of strangeness, which will allow for a better understanding of the underlying resonant and nonresonant contributions to hyperon production.

We have examined the spin structure of the proton in the region of the nucleon resonances (1.085  GeV<W<1.910  GeV) at an average four momentum transfer of Q²=1.3  GeV². Using the Jefferson Lab polarized electron beam, a spectrometer, and a polarized solid target, we measured the asymmetries A_∥ and A_⊥ to high precision, and extracted the asymmetries A₁ and A₂, and the spin structure functions g₁ and g₂. We found a notably nonzero A_⊥, significant contributions from higher-twist effects, and only weak support for polarized quark-hadron duality.