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 $e^+e^-$ 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 $\pm 0.1$. The absolute energy scale was determined using the $e^+e^-$ rate measurements close to the end-point of the photon spectrum. The energy variations across the full tagging range were found to be $<3$ 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

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

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 10 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

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

Ocean and atmosphere influence on the 2015 European heatwave

During the summer of 2015, central Europe experienced a major heatwave that was preceded by anomalously cold sea surface temperatures (SSTs) in the northern North Atlantic. Recent observation-based studies found a correlation between North Atlantic SST in spring and European summer temperatures, suggesting potential for predictability. Here we show, by using a high-resolution climate model, that ocean temperature anomalies, in combination with matching atmospheric and sea-ice initial conditions were key to the development of the 2015 European heatwave. In a series of 30-member ensemble simulations we test different combinations of ocean temperature and salinity initial states versus non-initialised climatology, mediated in both ensembles by different atmospheric/sea-ice initial conditions, using a non-standard initialisation method without data-assimilation. With the best combination of the initial ocean, and matching atmosphere/sea-ice initial conditions, the ensemble mean temperature response over central Europe in this set-up equals 60% of the observed anomaly, with 6 out of 30 ensemble-members showing similar, or even larger surface air temperature anomalies than observed