An official website of the United States government
Abstract This report is on the observation and analysis of nonlinear magnetoelectric effects (NLME) for in-plane currents perpendicularly to the hexagonal axis in single crystals and liquid phase epitaxy grown thin films of barium hexaferrite. Measurements involved tuning of ferromagnetic resonance (FMR) at 56–58 GHz in the multidomain and single domain states in the ferrite by applying a current. Data on the shift in the resonance frequency with input electric power was utilized to estimate the variations in the magnetic parameter that showed a linear dependence on the input electric power. The NLME tensor coefficients were determined form the estimated changes in the magnetization and uniaxial anisotropy field. The estimated NLME coefficients for in-plane currents are shown to be much higher than for currents flowing along the hexagonal axis. Although the frequency shift of FMR was higher for the single domain resonance, the multi-domain configuration is preferable for device applications since it eliminates the need for a large bias magnetic field. Thus, multidomain resonance with current in the basal plane is favorable for use in electrically tunable miniature, ferrite microwave signal processing devices requiring low operating power.
more »
« less
Direct observation of shift and ballistic photovoltaic currents
The quantum phenomenon of shift photovoltaic current was predicted decades ago, but this effect was never observed directly because shift and ballistic currents coexist. The atomic-scale relaxation time of shift, along with the absence of a photo-Hall behavior, has made decisive measurement of shift elusive. Here, we report a facile, direct-current, steady-state method for unambiguous determination of shift by means of the simultaneous measurements of linear and circular bulk photovoltaic currents under magnetic field, in a sillenite piezoelectric crystal. Comparison with theoretical predictions permits estimation of the signature length scale for shift. Remarkably, shift and ballistic photovoltaic currents under monochromatic illumination simultaneously flow in opposite directions. Disentangling the shift and ballistic contributions opens the way for quantitative, fundamental insight into and practical understanding of these radically different photovoltaic current mechanisms and their relationship.
more »
« less
- Award ID(s):
- 1705440
- PAR ID:
- 10110188
- Date Published:
- Journal Name:
- Science Advances
- Volume:
- 5
- Issue:
- 1
- ISSN:
- 2375-2548
- Page Range / eLocation ID:
- eaau5588
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Deep-reaching warming along the boundary of the Antarctic Circumpolar Current and the subtropical gyre is a consistent feature of multidecadal observational estimates and projections of future climate. In the Indian basin, the maximum ocean heat content change is collocated with the powerful Agulhas Return Current (ARC) in the west and Subantarctic Front (SAF) in the east, forming a southeastward band we denote as the ARC–SAF. We find that this jet-confined warming is linked to a poleward shift of these strong currents via the thermal wind relation. Using a suite of idealized ocean-only and partially coupled climate model experiments, we show that strong global buoyancy flux anomalies consistently drive a poleward shift of the ARC–SAF circulation and the associated heat content change maximum. To better understand how buoyancy addition modifies this circulation in the absence of wind stress change, we next apply buoyancy perturbations only to certain regions. Buoyancy addition across the Indian and Pacific Oceans (including the ARC–SAF) gives rise to a strong baroclinic circulation response and modest poleward shift. In contrast, buoyancy addition in the North Atlantic drives a vertically coherent poleward shift of the ARC–SAF, which we suggest is associated with an ocean heat content perturbation communicated to the Southern Ocean via planetary waves and advected eastward along the ARC–SAF. Whereas poleward-shifting circulation and banded warming under climate change have been previously attributed to poleward-shifting winds in the Southern Ocean, we show that buoyancy addition can drive this circulation change in the Indian sector independent of changing wind stress. Significance StatementThis research aims to identify which changes at the atmosphere–ocean interface cause ocean warming localized within strong Southern Ocean currents under climate change. Whereas previous regional studies have emphasized the role of changes in Southern Hemisphere winds, we show that these currents are also sensitive to additional heat and freshwater input into the ocean—even in the faraway North Atlantic. Adding heat and freshwater shifts the currents southward, which is dynamically tied to the “band” of ocean warming seen in both long-term observations and climate change projections. We demonstrate that the warming climate will modify ocean circulation in unexpected ways; the consequences for the ocean’s ability to continue removing anthropogenic heat and carbon from the atmosphere remain poorly understood.more » « less
-
Abstract The existence of Birkeland magnetic field‐aligned current (FAC) system was proposed more than a century ago, and it has been of immense interest for investigating the nature of solar wind‐magnetosphere‐ionosphere coupling ever since. In this paper, we present the first application of deep learning architecture for modeling the Birkeland currents using data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). The model uses a 1‐hr time history of several different parameters such as interplanetary magnetic field (IMF), solar wind, and geomagnetic and solar indices as inputs to determine the global distribution of Birkeland currents in the Northern Hemisphere. We present a comparison between our model and bin‐averaged statistical patterns under steady IMF conditions and also when the IMF is variable. Our deep learning model shows good agreement with the bin‐averaged patterns, capturing several prominent large‐scale features such as the Regions 1 and 2 FACs, the NBZ current system, and the cusp currents along with their seasonal variations. However, when IMF and solar wind conditions are not stable, our model provides a more accurate view of the time‐dependent evolution of Birkeland currents. The reconfiguration of the FACs following an abrupt change in IMF orientation can be traced in its details. The magnitude of FACs is found to evolve with e‐folding times that vary with season and MLT. When IMF Bz turns southward after a prolonged northward orientation, NBZ currents decay exponentially with an e‐folding time of∼25 min, whereas Region 1 currents grow with an e‐folding time of 6–20 min depending on the MLT.more » « less
-
Abstract Since the discovery of the large‐scale field‐aligned currents it is widely acknowledged that gaps exist between the Region 1 (R1) and Region 2 (R2) currents in which the current values are relatively small as compared to neighboring regions. Assuming that the field‐aligned currents are generated by plasma pressure gradients, we analyzed data collected by the THEMIS satellites between 2007 and 2011 to identify regions with very low plasma pressure gradients (pressure plateaus), which could be responsible for the appearance of these gaps. It was found that the pressure profiles with low radial gradients are typically located between 8 and 10 Radii around the Earth. Projections of pressure plateau regions onto ionospheric altitudes, for both individual events and on a statistical basis, coincide with the locations of gaps between Iijima and Potemra field‐aligned currents. The role played by identified pressure plateaus in shaping the pattern of large‐scale field‐aligned currents is discussed.more » « less
-
null (Ed.)A microsecond time-scale photonic lift-off (PLO) process was used to fabricate mechanically flexible photovoltaic devices (PVs) with a total thickness of less than 20 μm. PLO is a rapid, scalable photothermal technique for processing extremely thin, mechanically flexible electronic and optoelectronic devices. PLO is also compatible with large-area devices, roll-to-roll processing, and substrates with low temperature compatibility. As a proof of concept, PVs were fabricated using CuInSe2 nanocrystal ink deposited at room temperature under ambient conditions on thin, plastic substrates heated to 100 °C. It was necessary to prevent cracking of the brittle top contact layer of indium tin oxide (ITO) during lift-off, either by using a layer of silver nanowires (AgNW) as the top contact or by infusing the ITO layer with AgNW. This approach could generally be used to improve the mechanical versatility of current collectors in a variety of ultrathin electronic and optoelectronic devices requiring a transparent conductive contact layer.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1126/sciadv.aau5588
