More Like this

1. Dipole Effects on Electron Transfer are Enormous

   https://doi.org/10.1002/anie.201802637  

   Krzeszewski, Maciej; Espinoza, Eli M.; Červinka, Ctirad; Derr, James B.; Clark, John A.; Borchardt, Dan; Beran, Gregory J.; Gryko, Daniel T.; Vullev, Valentine I. (June 2018, Angewandte Chemie International Edition)

   Molecular dipoles present important, but underutilized, methods for guiding electron transfer (ET) processes. While dipoles generate fields of Gigavolts per meter in their vicinity, reported differences between rates of ET along versus against dipoles are often small or undetectable. Herein we show unprecedentedly large dipole effects on ET. Depending on their orientation, dipoles either ensure picosecond ET, or turn ET completely off. Furthermore, favorable dipole orientation makes ET possible even in lipophilic medium, which appears counterintuitive for non‐charged donor–acceptor systems. Our analysis reveals that dipoles can substantially alter the ET driving force for low solvent polarity, which accounts for these unique trends. This discovery opens doors for guiding forward ET processes while suppressing undesired backward electron transduction, which is one of the holy grails of photophysics and energy science. 

   more » « less
2. From Feast to Famine: A Systematic Study of Accretion onto Oblique Pulsars with 3D GRMHD Simulations

   https://doi.org/10.3847/2041-8213/ad16eb  

   Murguia-Berthier, Ariadna; Parfrey, Kyle; Tchekhovskoy, Alexander; Jacquemin-Ide, Jonatan (January 2024, The Astrophysical Journal Letters)

   Abstract Disk-fed accretion onto neutron stars can power a wide range of astrophysical sources ranging from X-ray binaries, to accretion-powered millisecond pulsars, ultraluminous X-ray sources, and gamma-ray bursts. A crucial parameter controlling the gas–magnetosphere interaction is the strength of the stellar dipole. In addition, coherent X-ray pulsations in many neutron star systems indicate that the star's dipole moment is oblique relative to its rotation axis. Therefore, it is critical to systematically explore the 2D parameter space of the star's magnetic field strength and obliquity, which is what this work does, for the first time, in the framework of 3D general-relativistic magnetohydrodynamics. If the accretion disk carries its own vertical magnetic field, this introduces an additional factor: the relative polarity of the disk and stellar magnetic fields. We find that depending on the strength of the stellar dipole and the star–disk relative polarity, the neutron star's jet power can either increase or decrease with increasing obliquity. For weak dipole strength (equivalently, high accretion rate), the parallel polarity results in a positive correlation between jet power and obliquity, whereas the antiparallel orientation displays the opposite trend. For stronger dipoles, the relative-polarity effect disappears, and jet power always decreases with increasing obliquity. The influence of the relative polarity gradually disappears as obliquity increases. Highly oblique pulsars tend to have an increased magnetospheric radius, a lower mass accretion rate, and enter the propeller regime at lower magnetic moments than aligned stars. 

   more » « less
3. Real Space and Time Imaging of Collective Headgroup Dipole Motions in Zwitterionic Lipid Bilayers

   https://doi.org/10.3390/membranes13040442  

   Bolmatov, Dima; Collier, C Patrick; Zav’yalov, Dmitry; Egami, Takeshi; Katsaras, John (April 2023, Membranes)

   Lipid bilayers are supramolecular structures responsible for a range of processes, such as transmembrane transport of ions and solutes, and sorting and replication of genetic materials, to name just a few. Some of these processes are transient and currently, cannot be visualized in real space and time. Here, we developed an approach using 1D, 2D, and 3D Van Hove correlation functions to image collective headgroup dipole motions in zwitterionic phospholipid bilayers. We show that both 2D and 3D spatiotemporal images of headgroup dipoles are consistent with commonly understood dynamic features of fluids. However, analysis of the 1D Van Hove function reveals lateral transient and re-emergent collective dynamics of the headgroup dipoles—occurring at picosecond time scales—that transmit and dissipate heat at longer times, due to relaxation processes. At the same time, the headgroup dipoles also generate membrane surface undulations due a collective tilting of the headgroup dipoles. A continuous intensity band of headgroup dipole spatiotemporal correlations—at nanometer length and nanosecond time scales—indicates that dipoles undergo stretching and squeezing elastic deformations. Importantly, the above mentioned intrinsic headgroup dipole motions can be externally stimulated at GHz-frequency scale, enhancing their flexoelectric and piezoelectric capabilities (i.e., increased conversion efficiency of mechanical energy into electric energy). In conclusion, we discuss how lipid membranes can provide molecular-level insights about biological learning and memory, and as platforms for the development of the next generation of neuromorphic computers. 

   more » « less
4. Mechanism of silicon-nanowire-diode orientation in DC electric fields

   https://doi.org/10.1063/5.0165100  

   Hoang, Minh-Thang; Deshmukh, Nishant; Mohabir, Amar T; Feldman, Leonard C; Filler, Michael A; Shan, Jerry W (October 2023, Applied Physics Letters)

   Doped semiconductor nanowires are emerging as next-generation electronic colloidal materials, and the efficient manipulation of such nanostructures is crucial for technological applications. In fluid suspension, pn nanowires (pn NWs), unlike homogeneous nanowires, have a permanent dipole, and thus, experience a torque under an external DC field that orients the nanowire with its n-type end in the direction of the field. Here, we quantitatively measure the permanent dipoles of various Si nanowire pn diodes and investigate their origin. By comparing the dipoles of pn NWs of different lengths and radii, we show that the permanent dipole originates from non-uniform surface-charge distributions, rather than the internal charges at the p–n junction as was previously proposed. This understanding of the mechanism for pn NWs orientation has relevance to the manipulation, assembly, characterization, and separation of nanowire electronics by electric fields. 

   more » « less
5. Normal-mode oscillations for the circular and dipolar states of a filled hexagonal magnetic dipole cluster

   https://doi.org/10.1063/5.0076147  

   Haugen, Peter T.; Smith, Andrew D.; Edwards, Boyd F. (June 2022, Chaos: An Interdisciplinary Journal of Nonlinear Science)

   We analyze the rotational dynamics of six magnetic dipoles of identical strength at the vertices of a regular hexagon with a variable-strength dipole in the center. The seven dipoles spin freely about fixed axes that are perpendicular to the plane of the hexagon, with their dipole moments directed parallel to the plane. Equilibrium dipole orientations are calculated as a function of the relative strength of the central dipole. Small-amplitude perturbations about these equilibrium states are calculated in the absence of friction and are compared with analytical results in the limit of zero and infinite central dipole strength. Normal modes and frequencies are presented. Bifurcations are seen at two critical values of the central dipole strength, with bistability between these values. 

   more » « less