Relationship between transport anisotropy and nematicity in FeSe

We thank the Max Planck Society for financial support. C. W. H., A. P. M., and C. T. acknowledge support by the DFG (DE) through the Collaborative Research Centre SFB 1143 (Projects C09 and A04). C. T. acknowledges support by the DFG (DE) through the Cluster of Excellence on Complexity and Topology in Quantum Matter ct.qmat (EXC 2147). Work in Japan was supported by Grants-in-Aid for Scientific Research (KAKENHI) (No. JP19H00649 and No. JP18H05227), and Grant-in-Aid for Scientific Research on innovative areas “Quantum Liquid Crystals” (No. JP19H05824 and No. JP20H05162) from Japan Society for the Promotion of Science (JSPS).The mechanism behind the nematicity of FeSe is not known. Through elastoresitivity measurements it has been shown to be an electronic instability. However, thus far measurements have extended only to small strains, where the response is linear. Here, we apply large elastic strains to FeSe and perform two types of measurement. (1) Using applied strain to control twinning, the nematic resistive anisotropy at temperatures below the nematic transition temperature Ts is determined. (2) Resistive anisotropy is measured as nematicity is induced through applied strain at fixed temperature above Ts. In both cases, as nematicity strengthens, the resistive anisotropy peaks at about 7%, then decreases. Below ≈40  K, the nematic resistive anisotropy changes sign. We discuss possible implications of this behavior for theories of nematicity. In addition, we report the following. (1) Under experimentally accessible conditions with bulk crystals, stress, rather than strain, is the conjugate field to the nematicity of FeSe. (2) At low temperatures the twin boundary resistance is ∼10% of the sample resistance, and must be properly subtracted to extract intrinsic resistivities. (3) Biaxial in-plane compression increases both in-plane resistivity and the superconducting critical temperature Tc, consistent with a strong role of the yz orbital in the electronic correlations.Publisher PDFPeer reviewe

Does femtosecond time-resolved second-harmonic generation probe electron temperatures at surfaces?

Femtosecond pump-probe second-harmonic generation (SHG) and transient linear reflectivity measurements were carried out on polycrystalline Cu, Ag and Au in air to analyze whether the electron temperature affects Fresnel factors or nonlinear susceptibilities, or both. Sensitivity to electron temperatures was attained by using photon energies near the interband transition threshold. We find that the nonlinear susceptibility carries the electron temperature dependence in case of Ag and Au, while for Cu the dependence is in the Fresnel factors. This contrasting behavior emphasizes that SHG is not a priori sensitive to electron dynamics at surfaces or interfaces, notwithstanding its cause.Comment: 11 pages, 4 figure

Predictors of loneliness during the Covid-19 pandemic in people with dementia and their carers in England: findings from the DETERMIND-C19 study

Objectives To identify factors that predict the risk of loneliness for people with dementia and carers during a pandemic. Methods People with dementia and their carers completed assessments before (July 2019–March 2020; 206 dyads) and after (July–October 2020) the first Covid-19 ‘lockdown’ in England. At follow-up, the analytic sample comprised 67 people with dementia and 108 carers. We built a longitudinal path model with loneliness as an observed outcome. Carer type and social contacts at both measurements were considered. Other social resources (quality of relationship, formal day activities), wellbeing (anxiety, psychological wellbeing) and cognitive impairment were measured with initial level and change using latent growth curves. We adjusted for socio-demographic factors and health at baseline. Results In carers, higher levels of loneliness were directly associated with non-spouse coresident carer type, level and increase of anxiety in carer, more formal day activities, and higher cognitive impairment in the person with dementia. In people with dementia, non-spouse coresident carer type, and higher initial levels of social resources, wellbeing, and cognitive impairment predicted the changes in these factors; this produced indirect effects on social contacts and loneliness. Conclusion Loneliness in the Covid-19 pandemic appears to be shaped by different mechanisms for people with dementia and their carers. The results suggest that carers of those with dementia may prioritize providing care that protects the person with dementia from loneliness at the cost of experiencing loneliness themselves. Directions for the promotion of adaptive social care during the Covid-19 pandemic and beyond are discussed

Effects of sea level rise on economy of the United States

We report the first ex post study of the economic impact of sea level rise. We apply two econometric approaches to estimate the past effects of sea level rise on the economy of the USA, viz. Barro type growth regressions adjusted for spatial patterns and a matching estimator. Unit of analysis is 3063 counties of the USA. We fit growth regressions for 13 time periods and we estimated numerous varieties and robustness tests for both growth regressions and matching estimator. Although there is some evidence that sea level rise has a positive effect on economic growth, in most specifications the estimated effects are insignificant. We therefore conclude that there is no stable, significant effect of sea level rise on economic growth. This finding contradicts previous ex ante studies

Spin asymmetry for the 16O(gamma,pi- p) reaction in the Delta(1232) region within an effective Lagrangian approach

The spin asymmetry of the photon in the exclusive A(gamma,pi N)A-1 reaction is computed employing a recently developed fully relativistic model based on elementary pion production amplitudes that include a consistent treatment of the spin-3/2 nucleon resonances. We compare the results of this model to the only available data on Oxygen [Phys. Rev. C 61 (2000) 054609] and find that, contrary to other models, the predicted spin asymmetry compares well to the available experimental data in the Delta(1232) region. Our results indicate that no major medium modifications in the Delta(1232) properties are needed in order to describe the measured spin asymmetries.Comment: 16 pages, 7 figures. To be published in Physics Letters

Simulation of dimensionality effects in thermal transport

The discovery of nanostructures and the development of growth and fabrication techniques of one- and two-dimensional materials provide the possibility to probe experimentally heat transport in low-dimensional systems. Nevertheless measuring the thermal conductivity of these systems is extremely challenging and subject to large uncertainties, thus hindering the chance for a direct comparison between experiments and statistical physics models. Atomistic simulations of realistic nanostructures provide the ideal bridge between abstract models and experiments. After briefly introducing the state of the art of heat transport measurement in nanostructures, and numerical techniques to simulate realistic systems at atomistic level, we review the contribution of lattice dynamics and molecular dynamics simulation to understanding nanoscale thermal transport in systems with reduced dimensionality. We focus on the effect of dimensionality in determining the phononic properties of carbon and semiconducting nanostructures, specifically considering the cases of carbon nanotubes, graphene and of silicon nanowires and ultra-thin membranes, underlying analogies and differences with abstract lattice models.Comment: 30 pages, 21 figures. Review paper, to appear in the Springer Lecture Notes in Physics volume "Thermal transport in low dimensions: from statistical physics to nanoscale heat transfer" (S. Lepri ed.

Observation of exclusive DVCS in polarized electron beam asymmetry measurements

We report the first results of the beam spin asymmetry measured in the reaction e + p -> e + p + gamma at a beam energy of 4.25 GeV. A large asymmetry with a sin(phi) modulation is observed, as predicted for the interference term of Deeply Virtual Compton Scattering and the Bethe-Heitler process. The amplitude of this modulation is alpha = 0.202 +/- 0.028. In leading-order and leading-twist pQCD, the alpha is directly proportional to the imaginary part of the DVCS amplitude.Comment: 6 pages, 5 figure