519 research outputs found
Amplifier for scanning tunneling microscopy at MHz frequencies
Conventional scanning tunneling microscopy (STM) is limited to a bandwidth of
circa 1kHz around DC. Here, we develop, build and test a novel amplifier
circuit capable of measuring the tunneling current in the MHz regime while
simultaneously performing conventional STM measurements. This is achieved with
an amplifier circuit including a LC tank with a quality factor exceeding 600
and a home-built, low-noise high electron mobility transistor (HEMT). The
amplifier circuit functions while simultaneously scanning with atomic
resolution in the tunneling regime, i.e. at junction resistances in the range
of giga-ohms, and down towards point contact spectroscopy. To enable high
signal-to-noise and meet all technical requirements for the inclusion in a
commercial low temperature, ultra-high vacuum STM, we use superconducting
cross-wound inductors and choose materials and circuit elements with low heat
load. We demonstrate the high performance of the amplifier by spatially mapping
the Poissonian noise of tunneling electrons on an atomically clean Au(111)
surface. We also show differential conductance spectroscopy measurements at
3MHz, demonstrating superior performance over conventional spectroscopy
techniques. Further, our technology could be used to perform impedance matched
spin resonance and distinguish Majorana modes from more conventional edge
states
Diverse Epidemiology of Leptospira Serovars Notified in New Zealand, 1999-2017
(c) The Author/sPublishe
Life events and hemodynamic stress reactivity in the middle-aged and elderly
Recent versions of the reactivity hypothesis, which consider it to be the product of stress exposure and exaggerated haemodynamic reactions to stress that confers cardiovascular disease risk, assume that reactivity is independent of the experience of stressful life events. This assumption was tested in two substantial cohorts, one middle-aged and one elderly. Participants had to indicate from a list of major stressful life events up to six they had experienced in the previous two years. They were also asked to rate how disruptive and stressful they were, at the time of occurrence and now. Blood pressure and pulse rate were measured at rest and in response to acute mental stress. Those who rated the events as highly disruptive at the time of exposure and currently exhibited blunted systolic blood pressure reactions to acute stress. The present results suggest that acute stress reactivity may not be independent of stressful life events experience
IgG-Immune Complexes Promote B Cell Memory by Inducing BAFF
Memory B cell responses are vital for protection against infections, but must also be regulated to prevent autoimmunity. Cognate T cell help, somatic hypermutation, and affinity maturation within germinal centers (GCs) are required for high affinity memory B cell formation; however, the signals that commit GC B cells to the memory pool remain unclear. In this study, we identify a role for IgG immune complexes (ICs), FcγRs, and BAFF during the formation of memory B cells in mice. We found that early secretion of IgG in response to immunization with a T-dependent antigen leads to IC-FcγR interactions that induce DCs to secrete BAFF which acts at or upstream of Bcl-6 in activated B cells. Loss of CD16, hematopoietic cell-derived BAFF, or blocking IC:FcγR regions in vivo diminished the expression of Bcl-6, the frequency of GC and memory B cells, and secondary antibody responses. BAFF also contributed to the maintenance and/or expansion of the Tfh population, although it was dispensable for their formation. Thus, early antibody responses contribute to the optimal formation of B cell memory through IgG-ICs and BAFF. Our work defines a new role for FcγRs in GC and memory B cell responses
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