Intersubband spin-density excitations in quantum wells with Rashba spin splitting

In inversion-asymmetric semiconductors, spin-orbit coupling induces a k-dependent spin splitting of valence and conduction bands, which is a well-known cause for spin decoherence in bulk and heterostructures. Manipulating nonequilibrium spin coherence in device applications thus requires understanding how valence and conduction band spin splitting affects carrier spin dynamics. This paper studies the relevance of this decoherence mechanism for collective intersubband spin-density excitations (SDEs) in quantum wells. A density-functional formalism for the linear spin-density matrix response is presented that describes SDEs in the conduction band of quantum wells with subbands that may be non-parabolic and spin-split due to bulk or structural inversion asymmetry (Rashba effect). As an example, we consider a 40 nm GaAs/AlGaAs quantum well, including Rashba spin splitting of the conduction subbands. We find a coupling and wavevector-dependent splitting of the longitudinal and transverse SDEs. However, decoherence of the SDEs is not determined by subband spin splitting, due to collective effects arising from dynamical exchange and correlation.Comment: 10 pages, 4 figure

Physical activity from childhood to adulthood and cognitive performance in midlife

Introduction: Physical activity (PA) has been suggested to protect against old-age cognitive deficits. However, the independent role of childhood/youth PA for adulthood cognitive performance is unknown. This study investigated the association between PA from childhood to adulthood and midlife cognitive performance.Methods: This study is a part of the Cardiovascular Risk in Young Finns Study. Since 1980, a population-basedcohort of 3,596 children (age 3-18 years) have been followed-up in 3-9-year intervals. PA has been queried in all study phases. Cumulative PA was determined in childhood (age 6-12 years), adolescence (age 12-18 years), young adulthood (age 18-24 years) and adulthood (age 24-37 years). Cognitive performance was assessed using computerized neuropsychological test, CANTAB®, (N=2,026, age 34-39 years) in 2011.Results: High PA in childhood (β 0.119, 95% confidence interval (CI) 0.055–0.182) and adolescence (β 0.125, 95% CI 0.063–0.188) were associated with better reaction time in midlife independent of PA in other age frames. Additionally, an independent association of high PA in young adulthood with better visual processing and sustained attention in midlife was observed among men (β 0.101, 95% CI 0.001–0.200). There were no associations for other cognitive domains.Conclusion: Cumulative exposure to PA from childhood to adulthood was found to be associated with better midlife reaction time. Furthermore, cumulative PA exposure in young adulthood and adulthood was associated with better visual processing and sustained attention in men. All associations were independent of participants PA level in other measured age frames. Therefore, a physically active lifestyle should be adopted already in childhood, adolescence and young adulthood and continued into midlife to ensure the plausible benefits of PA on midlife cognitive performance.</p