Coherent control of nanomagnet dynamics via ultrafast spin torque pulses

The magnetization orientation of a nanoscale ferromagnet can be manipulated using an electric current via the spin transfer effect. Time domain measurements of nanopillar devices at low temperatures have directly shown that magnetization dynamics and reversal occur coherently over a timescale of nanoseconds. By adjusting the shape of a spin torque waveform over a timescale comparable to the free precession period (100-400 ps), control of the magnetization dynamics in nanopillar devices should be possible. Here we report coherent control of the free layer magnetization in nanopillar devices using a pair of current pulses as narrow as 30 ps with adjustable amplitudes and delay. We show that the switching probability can be tuned over a broad range by timing the current pulses with the underlying free-precession orbits, and that the magnetization evolution remains coherent for more than 1 ns even at room temperature. Furthermore, we can selectively induce transitions along free-precession orbits and thereby manipulate the free magnetic moment motion. We expect this technique will be adopted for further elucidating the dynamics and dissipation processes in nanomagnets, and will provide an alternative for spin torque driven spintronic devices, such as resonantly pumping microwave oscillators, and ultimately, for efficient reversal of memory bits in magnetic random access memory (MRAM).Comment: 4 pages, 3 figures, submitted to Nature Physic

Globular Cluster Scale Sizes in Giant Galaxies: Orbital Anisotropy and Tidally Under-filling Clusters in M87, NGC 1399, and NGC 5128

We investigate the shallow increase in globular cluster half-light radii with projected galactocentric distance $R_{gc}$ observed in the giant galaxies M87, NGC 1399, and NGC 5128. To model the trend in each galaxy, we explore the effects of orbital anisotropy and tidally under-filling clusters. While a strong degeneracy exists between the two parameters, we use kinematic studies to help constrain the distance $R_\beta$ beyond which cluster orbits become anisotropic, as well as the distance $R_{f\alpha}$ beyond which clusters are tidally under-filling. For M87 we find $R_\beta > 27$ kpc and $20 < R_{f\alpha} 13$ kpc and $10 < R_{f\alpha} < 30$ kpc. The connection of $R_{f\alpha}$ with each galaxy's mass profile indicates the relationship between size and $R_{gc}$ may be imposed at formation, with only inner clusters being tidally affected. The best fitted models suggest the dynamical histories of brightest cluster galaxies yield similar present-day distributions of cluster properties. For NGC 5128, the central giant in a small galaxy group, we find $R_\beta > 5$ kpc and $R_{f\alpha} > 30$ kpc. While we cannot rule out a dependence on $R_{gc}$, NGC 5128 is well fitted by a tidally filling cluster population with an isotropic distribution of orbits, suggesting it may have formed via an initial fast accretion phase. Perturbations from the surrounding environment may also affect a galaxy's orbital anisotropy profile, as outer clusters in M87 and NGC 1399 have primarily radial orbits while outer NGC 5128 clusters remain isotropic.Comment: 16 pages, 7 figures, 4 tables, Accepted for publication in MNRA

Enhancing Cation Diffusion and Suppressing Anion Diffusion via Lewis-Acidic Polymer Electrolytes

Solid polymer electrolytes (SPEs) have the potential to increase both the energy density and stability of lithium-based batteries, but low Li^+ conductivity remains a barrier to technological viability. SPEs are designed to maximize Li^+ diffusivity relative to the anion while maintaining sufficient salt solubility. It is thus remarkable that poly(ethylene oxide) (PEO), the most widely used SPE, exhibits Li^+ diffusivity that is an order of magnitude smaller than that of typical counterions at moderate salt concentrations. We show that Lewis-basic polymers like PEO favor slow cation and rapid anion diffusion, while this relationship can be reversed in Lewis-acidic polymers. Using molecular dynamics, polyboranes are identified that achieve up to 10-fold increases in Li^+ diffusivities and significant decreases in anion diffusivities, relative to PEO in the dilute-ion regime. These results illustrate a general principle for increasing Li^+ diffusivity and transference number with chemistries that exhibit weaker cation and stronger anion coordination

Athletes' experiences of social support during their transition out of elite sport: An interpretive phenomenological analysis

Objectives The sources and types of social support that athletes receive during the transition out of sport have been well documented. However, less is known about how athletes perceive, mobilise, and manage supportive relationships. This study aimed therefore to gain a more comprehensive insight into the ways that social support may influence how athletes adjust to life following retirement from elite sport. Design The study was designed according to the principles of Interpretative Phenomenological Analysis. Method Eight former British elite athletes (four male and four female) from eight different Olympic sports were recruited using criterion-based purposive sampling strategies. Data collected using semi-structured interviews were analysed to explore subjective experiences of social support during transition. Results Participants' perception of feeling cared for and understood enabled support to be effective. There were variations in participants' ability to seek out and ask for support and those who found this difficult also found transition a more distressing experience. As transitions progressed, the adjustment process was closely linked to the participant's evolving sense of self. New social relationships and social roles fostered a sense of feeling supported, as well as providing opportunities to support others (e.g., other retired athletes). Providing support helped the participants to experience a sense of growth that facilitated adjustment to life after sport. Conclusions The content of support was largely dependent on context; that is, perceptions of supporters were just as important, if not more so, than specific support exchanges. Stigma around asking for help was a barrier to support seeking

Athletes’ retirement from elite sport: A qualitative study of parents and partners’ experiences

Objectives Athletes’ experiences of transition out of elite sport have been well documented. Less is known, however, about how the family members of athletes experience the process of transition. This study aimed to gain an in-depth understanding of parents and partners’ experiences and the way that they managed and interpreted their role in the process of transition. Method Individual semi-structured interviews were conducted with the parents and partners (two male and five female) of seven retired elite athletes from the UK. Data were analysed according to the principles of interpretative phenomenological analysis. Results Parents and partners experienced their own transition as they navigated uncertainty and upheaval in their own lives when the athletes retired. Parents and partners had to renegotiate their identity as they adjusted to changing roles and dynamics in their close relationships. Providing support to the former athletes was complicated by parents and partners’ own difficulties during transition and they often felt unsure about their role as a supporter. Parents and partners often experienced difficulties in their relationship with the athletes during their transition, but things improved as time went by. This was due to better communication and a willingness to share their feelings about their experiences. This helped parents and partners to gain a positive perspective on their transition and a sense that their relationship with the athletes had strengthened and grown. Conclusions Transition is often a shared experience and the findings of the present study underline the value of exploring transition at the level of the family or partnership as well as the individual

Big bang nucleosynthesis as a probe of fundamental "constants"

Big Bang nucleosynthesis (BBN) is the earliest sensitive probe of the values of many fundamental particle physics parameters. We have found the leading linear dependences of primordial abundances on all relevant parameters of the standard BBN code, including binding energies and nuclear reaction rates. This enables us to set limits on possible variations of fundamental parameters. We find that 7Li is expected to be significantly more sensitive than other species to many fundamental parameters, a result which also holds for variations of coupling strengths in grand unified (GUT) models. Our work also indicates which areas of nuclear theory need further development if the values of ``constants'' are to be more accurately probed.Comment: Refereed article to be published in Nuclear Physics in Astrophysics III Proceedings, J. Phys. G. Special Issue. Based on work in collaboration with C. Wetterich (Heidelberg). 6 page

Chemically Specific Dynamic Bond Percolation Model for Ion Transport in Polymer Electrolytes

We introduce a coarse-grained approach for characterizing the long-timescale dynamics of ion diffusion in general polymer electrolytes using input from short molecular dynamics trajectories. The approach includes aspects of the dynamic bond percolation model [ J. Chem. Phys. 1983, 79, 3133−3142] by treating ion diffusion in terms of hopping transitions on a fluctuating lattice. We extend this well-known approach by using short (i.e., 10 ns) molecular dynamics (MD) trajectories to predict the distribution of ion solvation sites that comprise the lattice and to predict the rate of hopping among the lattice sites. This yields a chemically specific dynamic bond percolation (CS-DBP) model that enables the description of long-timescale ion diffusion in polymer electrolytes at a computational cost that makes feasible the screening of candidate materials. We employ the new model to characterize lithium-ion diffusion properties in six polyethers that differ by oxygen content and backbone stiffness: poly(trimethylene oxide), poly(ethylene oxide-alt-trimethylene oxide), poly(ethylene oxide), poly(propylene oxide), poly(ethylene oxide-alt-methylene oxide), and poly(methylene oxide). Good agreement is observed between the predictions of the CS-DBP model and long-timescale atomistic MD simulations, thus providing validation of the model. Among the most striking results from this analysis is the unexpectedly good lithium-ion diffusivity of poly(trimethylene oxide-alt-ethylene oxide) by comparison to poly(ethylene oxide), which is widely used. Additionally, the model straightforwardly reveals a range of polymer features that lead to low lithium-ion diffusivity, including the competing effects of the density of solvation sites and polymer stiffness. These results illustrate the potential of the CS-DBP model to screen polymer electrolytes on the basis of ion diffusivity and to identify important design criteria