Structure and correlation effects in semiconducting SrTiO₃

We have investigated the effects of structure change and electron correlation on SrTiO₃ single crystals using angle-resolved photoemission spectroscopy. We show that the cubic to tetragonal phase transition at 105 K is manifested by a charge transfer from in-plane (dyz and dzx) bands to out-of-plane (dxy) band, which is opposite to the theoretical predictions. Along this second-order phase transition, we find a smooth evolution of the quasiparticle strength and effective masses. The in-plane band exhibits a peak-dip-hump lineshape, indicating a high degree of correlation on a relatively large (170 meV) energy scale, which is attributed to the polaron formation

Polarization control at the microscopic and electronic structure observatory

The new Microscopic and Electronic Structure Observatory (MAESTRO) at the Advanced Light Source (ALS) in Berkeley provides X-rays of variable polarization, produced by an elliptically polarized undulator (EPU), for angle resolved photoemission (ARPES) and photoemission electron microscopy (PEEM) experiments. The interpretation of photoemission data, in particular of dichroism effects in ARPES, requires the precise knowledge of the exact polarization state. Numerical simulations show that the first harmonics of the EPU at MAESTRO provides soft X-rays of almost 100% on axis polarization. However, the higher harmonics as well as the downstream optical elements of the beamline, have a considerable impact on the polarization of the light delivered to the experimental end-station. Employing a simple reflective polarimeter, the polarization is characterized for variable EPU and beamline settings and the overall degree of polarization in the MAESTRO end-stations is estimated to be on the order of 83%

Visualizing Atomic-Scale Negative Differential Resistance in Bilayer Graphene

We investigate the atomic-scale tunneling characteristics of bilayer graphene on silicon carbide using the scanning tunneling microscopy. The high-resolution tunneling spectroscopy reveals an unexpected negative differential resistance (NDR) at the Dirac energy, which spatially varies within the single unit cell of bilayer graphene. The origin of NDR is explained by two near-gap van Hove singularities emerging in the electronic spectrum of bilayer graphene under a transverse electric field, which are strongly localized on two sublattices in different layers. Furthermore, defects near the tunneling contact are found to strongly impact on NDR through the electron interference. Our result provides an atomic-level understanding of quantum tunneling in bilayer graphene, and constitutes a useful step towards graphene-based tunneling devices. DOI: 10.1103/PhysRevLett.110.036804X11109sciescopu

String attractors : Verification and optimization

String attractors [STOC 2018] are combinatorial objects recently introduced to unify all known dictionary compression techniques in a single theory. A set γ ⊆ [1.n] is a k-attractor for a string S ∈ Σn if and only if every distinct substring of S of length at most k has an occurrence crossing at least one of the positions in γ. Finding the smallest k-attractor is NP-hard for k ≥ 3, but polylogarithmic approximations can be found using reductions from dictionary compressors. It is easy to reduce the k-attractor problem to a set-cover instance where the string's positions are interpreted as sets of substrings. The main result of this paper is a much more powerful reduction based on the truncated suffix tree. Our new characterization of the problem leads to more efficient algorithms for string attractors: we show how to check the validity and minimality of a k-attractor in near-optimal time and how to quickly compute exact solutions. For example, we prove that a minimum 3-attractor can be found in O(n) time when |Σ| ∈ O(3+ϵ√log n) for some constant ϵ > 0, despite the problem being NP-hard for large Σ. © Dominik Kempa, Alberto Policriti, Nicola Prezza, and Eva Rotenberg.Peer reviewe

Strictly One-Dimensional Electron System in Au Chains on Ge(001) Revealed By Photoelectron K-Space Mapping

Atomic nanowires formed by Au on Ge(001) are scrutinized for the band topology of the conduction electron system by k-resolved photoemission. Two metallic electron pockets are observed. Their Fermi surface sheets form straight lines without undulations perpendicular to the chains within experimental uncertainty. The electrons hence emerge as strictly confined to one dimension. Moreover, the system is stable against a Peierls distortion down to 10 K, lending itself for studies of the spectral function. Indications for unusually low spectral weight at the chemical potential are discussed.Comment: 4 pages, 4 figures - revised version with added Fig. 2e) and additional reference

Reduced variance analysis of molecular dynamics simulations by linear combination of estimators

International audienceBuilding upon recent developments of force-based estimators with a reduced variance for the computation of densities, radial distribution functions or local transport properties from molecular simulations, we show that the variance can be further reduced by considering optimal linear combinations of such estimators. This control variates approach, well known in Statistics and already used in other branches of computational Physics, has been comparatively much less exploited in molecular simulations. We illustrate this idea on the radial distribution function and the one-dimensional density of a bulk and confined Lennard-Jones fluid, where the optimal combination of estimators is determined for each distance or position, respectively. In addition to reducing the variance everywhere at virtually no additional cost, this approach cures an artefact of the initial force-based estimators, namely small but non-zero values of the quantities in regions where they should vanish. Beyond the examples considered here, the present work highlights more generally the underexplored potential of control variates to estimate observables from molecular simulations

Cluster sum rules for three-body systems with angular-momentum dependent interactions

We derive general expressions for non-energy weighted and energy-weighted cluster sum rules for systems of three charged particles. The interferences between pairs of particles are found to play a substantial role. The energy-weighted sum rule is usually determined by the kinetic energy operator, but we demonstrate that it has similar additional contributions from the angular momentum and parity dependence of two- and three-body potentials frequently used in three-body calculations. The importance of the different contributions is illustrated with the dipole excitations in $^6$He. The results are compared with the available experimental data.Comment: 11 pages, 3 figures, 2 table

Apoio social e rastreamento de câncer uterino e de mama entre trabalhadoras de enfermagem

This cross-sectional epidemiological study was carried out at three public hospitals in Rio de Janeiro, RJ, Brazil. It aimed at analyzing the association between social support and cervical and breast cancer early detection practices among nurses. Data were collected through a multidimensional questionnaire focusing on socio-demographic and occupational variables, the performance of the breast self-exam, the Papanicolaou smear test, and social support (Medical Outcomes Study scale). Statistical associations were evaluated through the Chi-square test (p d" 0.05). Logistic regression tests were used for multivariate analysis. Higher levels of social support consistently increased the chances of individuals reporting adequate practices concerning breast self-examination and having Pap smear tests performed, regardless of socio-demographic or occupational variables. These results corroborate the hypotheses that social support has a positive effect on the regular practice of self-care.Estudio epidemiológico, seccional, realizado en tres hospitales públicos de Río de Janeiro, en Brasil, con el objetivo de analizar la asociación entre el apoyo social y las prácticas de detección precoz de cáncer uterino y de mama entre trabajadoras de enfermería. Los datos fueron recolectados a través de un cuestionario multidimensional referente a variables socio demográficas y ocupacionales, a la realización del Papanicolaou, al auto examen de las mamas y al apoyo social (escala Medical Outcomes Studiy). Las asociaciones estadísticas fueron evaluadas a través de la prueba chi-cuadrado con niveles de significancia de 5%. Se utilizó la regresión logística para el análisis multivariado. De manera consistente, mayores niveles de apoyo social aumentaron las chances de la realización de prácticas adecuadas, del examen Papanicolaou y del auto examen de las mamas, después de ajuste por variables socios demográficos y ocupacionales. Esos resultados corroboran las hipótesis sobre la contribución positiva del apoyo social para la práctica regular de cuidados con la salud.Estudo epidemiológico, seccional, realizado em três hospitais públicos do Rio de Janeiro, Brasil, com o objetivo de analisar a associação entre o apoio social e práticas de detecção precoce de câncer uterino e de mama entre trabalhadoras de enfermagem. Os dados foram coletados através de questionário multidimensional referente a variáveis sociodemográficas, ocupacionais e à realização do Papanicolaou, autoexame das mamas e apoio social (escala Medical Outcomes Study). Associações estatísticas foram avaliadas através do teste qui-quadrado com níveis de significancia de 5%. Utilizou-se a regressão logística para análise multivariada. De maneira consistente, maiores níveis de apoio social aumentaram as chances de realização de práticas adequadas do exame Papanicolaou e de autoexame das mamas, após ajuste por variáveis sociodemográficas e ocupacionais. Esses resultados corroboram as hipóteses sobre a contribuição positiva do apoio social para a prática regular de cuidados com a saúde

Semi-relativistic description of quasielastic neutrino reactions and superscaling in a continuum shell model

The so-called semi-relativistic expansion of the weak charged current in powers of the initial nucleon momentum is performed to describe charge-changing, quasielastic neutrino reactions $(\nu_\mu,\mu^-)$ at intermediate energies. The quality of the expansion is tested by comparing with the relativistic Fermi gas model using several choices of kinematics of interest for ongoing neutrino oscillation experiments. The new current is then implemented in a continuum shell model together with relativistic kinematics to investigate the scaling properties of $(e,e')$ and $(\nu_\mu,\mu^-)$ cross sections.Comment: 33 pages, 10 figures, to appear in PR