762 research outputs found
Energy Calibration of the JLab Bremsstrahlung Tagging System
In this report, we present the energy calibration of the Hall B
bremsstrahlung tagging system at the Thomas Jefferson National Accelerator
Facility. The calibration was performed using a magnetic pair spectrometer. The
tagged photon energy spectrum was measured in coincidence with pairs
as a function of the pair spectrometer magnetic field. Taking advantage of the
internal linearity of the pair spectrometer, the energy of the tagging system
was calibrated at the level of . The absolute energy scale
was determined using the rate measurements close to the end-point of
the photon spectrum. The energy variations across the full tagging range were
found to be MeV.Comment: 15 pages, 12 figure
Resonances in Ferromagnetic Gratings Detected by Microwave Photoconductivity
We investigate the impact of microwave excited spin excitations on the DC
charge transport in a ferromagnetic (FM) grating. We observe both resonant and
nonresonant microwave photoresistance. Resonant features are identified as the
ferromagnetic resonance (FMR) and ferromagnetic antiresonance (FMAR). A
macroscopic model based on Maxwell and Landau-Lifschitz equations reveals the
macroscopic nature of the FMAR. The experimental approach and results provide
new insight in the interplay between photonic, spintronic, and charge effects
in FM microstructures.Comment: 4 pages, 4 figure
Microwave photovoltage and photoresistance effects in ferromagnetic microstrips
We investigate the dc electric response induced by ferromagnetic resonance in
ferromagnetic Permalloy (Ni80Fe20) microstrips. The resulting magnetization
precession alters the angle of the magnetization with respect to both dc and rf
current. Consequently the time averaged anisotropic magnetoresistance (AMR)
changes (photoresistance). At the same time the time-dependent AMR oscillation
rectifies a part of the rf current and induces a dc voltage (photovoltage). A
phenomenological approach to magnetoresistance is used to describe the distinct
characteristics of the photoresistance and photovoltage with a consistent
formalism, which is found in excellent agreement with experiments performed on
in-plane magnetized ferromagnetic microstrips. Application of the microwave
photovoltage effect for rf magnetic field sensing is discussed.Comment: 16 pages, 15 figure
Linking northeastern North Pacific oxygen changes to upstream surface outcrop variations
Understanding the response of the ocean to global warming, including the renewal of ocean waters from the surface (ventilation), is important for future climate predictions. Oxygen distributions in the ocean thermocline have proven an effective way to infer changes in ventilation because physical processes (ventilation and circulation) that supply oxygen are thought to be primarily responsible for changes in interior oxygen concentrations. Here, the focus is on the North Pacific thermocline, where some of the world's oceans' largest oxygen variations have been observed. These variations, described as bi-decadal cycles on top of a small declining trend, are strongest on subsurface isopycnals that outcrop into the mixed layer of the northwestern North Pacific in late winter. In this study, surface density time series are reconstructed in this area using observational data only and focusing on the time period from 1982, the first full year of the satellite sea surface temperature record, to 2020. It is found that changes in the annual maximum outcrop area of the densest isopycnals outcropping in the northwestern North Pacific are correlated with interannual oxygen variability observed at Ocean Station P (OSP) downstream at about a 10-year lag. The hypothesis is that ocean ventilation and uptake of oxygen is greatly reduced when the outcrop areas are small and that this signal travels within the North Pacific Current to OSP, with 10Â years being at the higher end of transit times reported in other studies. It is also found that sea surface salinity (SSS) dominates over sea surface temperature (SST) in driving interannual fluctuations in annual maximum surface density in the northwestern North Pacific, highlighting the role that salinity may play in altering ocean ventilation. In contrast, SSS and SST contribute about equally to the long-term declining surface density trends that are superimposed on the interannual cycles.</p
Quantized spin excitations in a ferromagnetic microstrip from microwave photovoltage measurements
Quantized spin excitations in a single ferromagnetic microstrip have been
measured using the microwave photovoltage technique. Several kinds of spin wave
modes due to different contributions of the dipole-dipole and the exchange
interactions are observed. Among them are a series of distinct dipole-exchange
spin wave modes, which allow us to determine precisely the subtle spin boundary
condition. A comprehensive picture for quantized spin excitations in a
ferromagnet with finite size is thereby established. The dispersions of the
quantized spin wave modes have two different branches separated by the
saturation magnetization.Comment: 4 pages, 3 figure
The decrease in ocean heat transport in response to global warming
The ocean is taking up additional heat but how this affects ocean circulation and heat transport is unclear. Here, using coupled model intercomparison project phase 5/6 (CMIP5/6) climate projections, we show a future decrease in poleward ocean heat transport (OHT) across all Northern Hemisphere latitudes and south of 10° S. Most notably, the CMIP5/6 multimodel mean reduction in poleward OHT for the Atlantic at 26.5° N and Indo-Pacific at 20° S is 0.093–0.304 PW and 0.097–0.194 PW, respectively, dependent on scenario and CMIP phase. These changes in OHT are driven by decline in overturning circulation dampened by upper ocean warming. In the Southern Ocean, the reduction in poleward OHT at 55° S is 0.071–0.268 PW. The projected changes are stronger in CMIP6, even when corrected for its larger climate sensitivity. This is especially noticable in the Atlantic Ocean for the weaker forcing scenarios (shared socioeconomic pathway SSP 1-2.6/representative concentration pathways RCP 2.6), where the decrease is 2.5 times larger at 26.5° N due to a stronger decline in the Atlantic meridional overturning circulation
Anomalous magnetotransport and cyclotron resonance of high mobility magnetic 2DHGs in the quantum Hall regime
Low temperature magnetotransport measurements and far infrared transmission
spectroscopy are reported in molecular beam epitaxial grown two-dimensional
hole systems confined in strained InAs quantum wells with magnetic impurities
in the channel. The interactions of the free holes spin with the magnetic
moment of 5/2 provided by manganese features intriguing localization phenomena
and anomalies in the Hall and the quantum Hall resistance. In magnetic field
dependent far infrared spectroscopy measurements well pronounced cyclotron
resonance and an additional resonance are found that indicates an anticrossing
with the cyclotron resonance
Stable AMOC off state in an eddy-permitting coupled climate model
Shifts between on and off states of the Atlantic Meridional Overturning Circulation (AMOC) have been associated with past abrupt climate change, supported by the bistability of the AMOC found in many older, coarser resolution, ocean and climate models. However, as coupled climate models evolved in complexity a stable AMOC off state no longer seemed supported. Here we show that a current-generation, eddy-permitting climate model has an AMOC off state that remains stable for the 450-year duration of the model integration. Ocean eddies modify the overall freshwater balance, allowing for stronger northward salt transport by the AMOC compared with previous, non eddy-permitting models. As a result, the salinification of the subtropical North Atlantic, due to a southward shift of the intertropical rain belt, is counteracted by the reduced salt transport of the collapsed AMOC. The reduced salinification of the subtropical North Atlantic allows for an anomalous northward freshwater transport into the subpolar North Atlantic dominated by the gyre component. Combining the anomalous northward freshwater transport with the freshening due to reduced evaporation in this region helps stabilise the AMOC off state
Comparing observed and modelled components of the Atlantic Meridional Overturning Circulation at 26° N
The Coupled Model Intercomparison Project (CMIP) allows the assessment of the representation of the Atlantic Meridional Overturning Circulation (AMOC) in climate models. While CMIP Phase 6 models display a large spread in AMOC strength, the multi-model mean strength agrees reasonably well with observed estimates from RAPID1, but this does not hold for the AMOC's various components. In CMIP Phase 6 (CMIP6), the present-day AMOC is characterized by a lack of lower North Atlantic Deep Water (lNADW) due to the small scale of Greenland–Iceland–Scotland Ridge overflow and too much mixing. This is compensated for by increased recirculation in the subtropical gyre and more Antarctic Bottom Water (AABW). Deep-water circulation is dominated by a distinct deep western boundary current (DWBC) with minor interior recirculation compared with observations. The future decline in the AMOC of 7 Sv by 2100 under a Shared Socioeconomic Pathway 5-8.5 (SSP5-8.5) emission scenario is associated with decreased northward western boundary current transport in combination with reduced southward flow of upper North Atlantic Deep Water (uNADW). In CMIP6, wind stress curl decreases with time by 14 % so that wind-driven thermocline recirculation in the subtropical gyre is reduced by 4 Sv (17 %) by 2100. The reduction in western boundary current transport of 11 Sv is more than the decrease in wind-driven gyre transport, indicating a decrease over time in the component of the Gulf Stream originating from the South Atlantic
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