Coupling of structure to magnetic and superconducting orders in quasi-one-dimensional K2Cr3As3\text{K}_2\text{Cr}_3\text{As}_3

Quasi-one-dimensional $A_2\text{Cr}_3\text{As}_3$ (with $A = \text{K, Cs, Rb}$) is an intriguing new family of superconductors which exhibit many similar features to the cuprate and iron-based unconventional superconductor families. Yet in contrast to these systems, no charge or magnetic ordering has been observed which could provide the electronic correlations presumed necessary for an unconventional superconducting pairing mechanism - an absence which defies predictions of first principles models. We report the results of neutron scattering experiments on polycrystalline $\text{K}_2\text{Cr}_3\text{As}_3$ $(T_c \sim 7\text{K})$ which probed the low temperature dynamics near $T_c$ . Neutron diffraction data evidence a strong response of the nuclear lattice to the onset of superconductivity while inelastic scattering reveals a highly dispersive column of intensity at the commensurate wavevector $q = (00\frac{1}{2})$ which loses intensity beneath $T_c$ - indicative of short-range magnetic fluctuations. Using linear spin-wave theory we model the observed scattering and suggest a possible structure to the short-range magnetic order. These observations suggest that $\text{K}_2\text{Cr}_3\text{As}_3$ is in close proximity to a magnetic instability and that the incipient magnetic order both couples strongly to the lattice and competes with superconductivity - in direct analogy with the iron-based superconductors

Ultra-efficient Cooling in Ferromagnet-Superconductor Microrefrigerators

A promising scheme for electron microrefrigeration based on ferromagnet-superconductor contacts is presented. In this setup, cooling power densities up to 600 nW/$\mu$m$^2$ can be achieved leading to electronic temperature reductions largely exceeding those obtained with existing superconductor-normal metal tunnel contacts. Half-metallic CrO$_2$/Al bilayers are indicated as ideal candidates for the implementation of the device.Comment: 9 pages, 3 figures, submitted to Applied Physics Letter

Superconductor-semiconductor magnetic microswitch

A hybrid superconductor--two-dimensional electron gas microdevice is presented. Its working principle is based on the suppression of Andreev reflection at the superconductor-semiconductor interface caused by a magnetic barrier generated by a ferromagnetic strip placed on top of the structure. Device switching is predicted with fields up to some mT and working frequencies of several GHz, making it promising for applications ranging from microswitches and storage cells to magnetic field discriminators.Comment: 4 pages, 3 figures, minor changes to tex

Andreev interference in adiabatic pumping

Within the scattering approach, we develop a model for adiabatic quantum pumping in hybrid normal/superconductor systems where several superconducting leads are present. This is exploited to study Andreev-interference effects on adiabatically pumped charge in a 3-arm beam splitter attached to one normal and two superconducting leads with different phases of the order parameters. We derive expressions for the pumped charge through the normal lead for different parameters for the scattering region, and elucidate the effects due to Andreev interference. In contrast to what happens for voltage-driven transport, Andreev interference does not yield in general a pumped current which is a symmetric function of the superconducting-phase difference.Comment: 4 pages, 1 figur

Factors associated with avoidable hospitalisation of children younger than 2 years old: the 2006 Brazilian National Demographic Health Survey

INTRODUCTION: Ambulatory Care Sensitive Conditions (ACSC) are conditions for which hospitalisation is thought to be avoidable with the use of effective preventive care and early disease management. The objective of this study was to estimate the rate of avoidable hospitalisations in children younger than 24 months of age participating in a Brazilian national representative survey and to identify the risk factors for such hospitalisations. METHODS: We analysed data from a cross-sectional study of 1901 children from the 2006 Brazilian National Demographic Health Survey of Women and Children (NDHS). The children’s socioeconomic, biological and maternal characteristics, nutritional status, and access to healthcare were tested; variables with p < 0.20 were selected to fit a Poisson regression. RESULTS: The prevalence of avoidable hospitalisation was 11.8 % (95 % Confidence Interval [CI], 9.0, 15.2); the prevalence was higher in the Southeast (40.1 %) and Northwest (21.7 %) macro-regions. The multivariate model identified five risk factors for avoidable hospitalisation: male gender (Prevalence Ratio [PR] = 1.48, p = 0.004), low socioeconomic level (PR = 1.51, p = 0.005), children from mothers younger than 20 years of age (PR = 1.41, p = 0.031), not breastfed within the first hour of life (PR = 1.29, p = 0.034), and neonatal hospitalisation (PR = 1.66, p = 0.043). CONCLUSIONS: To decrease the costs associated with avoidable hospitalisations, health managers and professionals should focus their efforts on providing effective primary healthcare to families of low socioeconomic levels, particularly prenatal and paediatric care, as well as encouraging breastfeeding and supporting young mothers. Strategies to improve children’s health by controlling such hospitalisations in Brazil should consider all residence areas and geopolitical macro-regions

Entanglement detection for electrons via witness operators

We discuss an implementation of the entanglement witness, a method to detect entanglement with few local measurements, in systems where entangled electrons are generated both in the spin and orbital degrees of freedom. We address the efficiency of this method in various setups, including two different particle-hole entanglement structures, and we demonstrate that it can also be used to infer information on the possible dephasing afflicting the devices.Comment: 12 pages, 5 figures; published versio

Does Giant Magnetoresistance Survive in Presence of Superconducting Contact?

The giant magnetoresistance (GMR) of ferromagnetic bilayers with a superconducting contact (F1/F2/S) is calculated in ballistic and diffusive regimes. As in spin-valve, it is assumed that the magnetization in the two ferromagnetic layers F1 and F2 can be changed from parallel to antiparallel. It is shown that the GMR defined as the change of conductance between the two magnetic configurations is an oscillatory function of the thickness of F2 layer and tends to an asymptotic positive value at large thickness. This is due to the formation of quantum well states in F2 induced by Andreev reflection at the F2/S interface and reflection at F1/F2 interface in antiparallel configuration. In the diffusive regime, if only spin-dependent scattering rates in the magnetic layers are considered (no difference in Fermi wave-vectors between spin up and down electrons) then the GMR is supressed due to the mixing of spin up and down electron-hole channels by Andreev reflection.Comment: 7 pages, 4 figures, submitted to Phys.Rev.Let

Crossed Andreev reflection-induced magnetoresistance

We show that very large negative magnetoresistance can be obtained in magnetic trilayers in a current-in-plane geometry owing to the existence of crossed Andreev reflection. This spin-valve consists of a thin superconducting film sandwiched between two ferromagnetic layers whose magnetization is allowed to be either parallelly or antiparallelly aligned. For a suitable choice of structure parameters and nearly fully spin-polarized ferromagnets the magnetoresistance can exceed -80%. Our results are relevant for the design and implementation of spintronic devices exploiting ferromagnet-superconductor structures.Comment: 5 pages, 4 figures, final published versio