104 research outputs found
A spatially and temporally localized sub-laser-cycle electron source
We present an experimental and numerical study of electron emission from a
sharp tungsten tip triggered by sub-8 femtosecond low power laser pulses. This
process is non-linear in the laser electric field, and the non-linearity can be
tuned via the DC voltage applied to the tip. Numerical simulations of this
system show that electron emission takes place within less than one optical
period of the exciting laser pulse, so that an 8 fsec 800 nm laser pulse is
capable of producing a single electron pulse of less than 1 fsec duration.
Furthermore, we find that the carrier-envelope phase dependence of the emission
process is smaller than 0.1% for an 8 fsec pulse but is steeply increasing with
decreasing laser pulse duration.Comment: 4 pages, 5 figure
Induced superconductivity in the two-dimensional topological insulator phase of cadmium arsenide
Hybrid structures between conventional, s-wave superconductors and
two-dimensional topological insulators (2D TIs) are a promising route to
topological superconductivity. Here, we investigate planar Josephson junctions
fabricated from hybrid structures that use thin films of cadmium arsenide
(Cd3As2) as the 2D TI material. Measurements of superconducting interference
patterns in a perpendicular magnetic field are used to extract information
about the spatial distribution of the supercurrent. We show that the
interference patterns are distinctly different in junctions with and without
mesa-isolation, respectively. In mesa-defined junctions, the bulk of the 2D TI
appears to be almost completely shunted by supercurrent flowing along the
edges, while the supercurrent is much more uniform across the junction when the
Cd3As2 film extends beyond the device. We discuss the possible origins of the
observed behaviors.Comment: Accepted for publication in APL Material
Direct visualization of coexisting channels of interaction in CeSb
Our understanding of correlated electron systems is vexed by the complexity of their interactions. Heavy fermion compounds are archetypal examples of this physics, leading to exotic properties that weave magnetism, superconductivity and strange metal behavior together. The Kondo semimetal CeSb is an unusual example where different channels of interaction not only coexist, but have coincident physical signatures, leading to decades of debate about the microscopic picture describing the interactions between the f moments and the itinerant electron sea. Using angle-resolved photoemission spectroscopy, we resonantly enhance the response of the Ce f electrons across the magnetic transitions of CeSb and find there are two distinct modes of interaction that are simultaneously active, but on different kinds of carriers. This study reveals how correlated systems can reconcile the coexistence of different modes on interaction-by separating their action in momentum space, they allow their coexistence in real space.11Ysciescopu
High-temporal-resolution electron microscopy for imaging ultrafast electron dynamics
Ultrafast Electron Microscopy (UEM) has been demonstrated to be an effective
table-top technique for imaging the temporally-evolving dynamics of matter with
subparticle spatial resolution on the time scale of atomic motion. However,
imaging the faster motion of electron dynamics in real time has remained beyond
reach. Here, we demonstrate more than an order of magnitude (16 times)
enhancement in the typical temporal resolution of UEM by generating isolated 30
fs electron pulses, accelerated at 200 keV, via the optical-gating approach,
with sufficient intensity for efficiently probing the electronic dynamics of
matter. Moreover, we investigate the feasibility of attosecond optical gating
to generate isolated subfemtosecond electron pulses, attaining the desired
temporal resolution in electron microscopy for establishing the Attomicroscopy
to allow the imaging of electron motion in the act.Comment: 19 Pages, 4 Figure
Magnetic torque anomaly in the quantum limit of Weyl semimetals
Electrons in materials with linear dispersion behave as massless Weyl- or Dirac-quasiparticles, and continue to intrigue due to their close resemblance to elusive ultra-relativistic particles as well as their potential for future electronics. Yet the experimental signatures of Weyl-fermions are often subtle and indirect, in particular if they coexist with conventional, massive quasiparticles. Here we show a pronounced anomaly in the magnetic torque of the Weyl semimetal NbAs upon entering the quantum limit state in high magnetic fields. The torque changes sign in the quantum limit, signalling a reversal of the magnetic anisotropy that can be directly attributed to the topological nature of the Weyl electrons. Our results establish that anomalous quantum limit torque measurements provide a direct experimental method to identify and distinguish Weyl and Dirac systems
Organic residues in archaeology - the highs and lows of recent research
YesThe analysis of organic residues from archaeological materials has become increasingly important to our understanding of ancient diet, trade and technology. Residues from diverse contexts have been retrieved and analysed from the remains of food, medicine and cosmetics to hafting material on stone arrowheads, pitch and tar from shipwrecks, and ancient manure from soils. Research has brought many advances in our understanding of archaeological, organic residues over the past two decades. Some have enabled very specific and detailed interpretations of materials preserved in the archaeological record. However there are still areas where we know very little, like the mechanisms at work during the formation and preservation of residues, and areas where each advance produces more questions rather than answers, as in the identification of degraded fats. This chapter will discuss some of the significant achievements in the field over the past decade and the ongoing challenges for research in this area.Full text was made available in the Repository on 15th Oct 2015, at the end of the publisher's embargo period
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