8,478 research outputs found
Highly charged ions with E1, M1, and E2 transitions within laser range
Level crossings in the ground state of ions occur when the nuclear charge Z
and ion charge Z_ion are varied along an isoelectronic sequence until the two
outermost shells are nearly degenerate. We examine all available level
crossings in the periodic table for both near neutral ions and highly charged
ions (HCIs). Normal E1 transitions in HCIs are in X-ray range, however level
crossings allow for optical electromagnetic transitions that could form the
reference transition for high accuracy atomic clocks. Optical E1 (due to
configuration mixing), M1 and E2 transitions are available in HCIs near level
crossings. We present scaling laws for energies and amplitudes that allow us to
make simple estimates of systematic effects of relevance to atomic clocks. HCI
clocks could have some advantages over existing optical clocks because certain
systematic effects are reduced, for example they can have much smaller thermal
shifts. Other effects such as fine-structure and hyperfine splitting are much
larger in HCIs, which can allow for richer spectra. HCIs are excellent
candidates for probing variations in the fine-structure constant, alpha, in
atomic systems as there are transitions with the highest sensitivity to
alpha-variation
Optical transitions in highly-charged californium ions with high sensitivity to variation of the fine-structure constant
We study electronic transitions in highly-charged Cf ions that are within the
frequency range of optical lasers and have very high sensitivity to potential
variations in the fine-structure constant, alpha. The transitions are in the
optical despite the large ionisation energies because they lie on the
level-crossing of the 5f and 6p valence orbitals in the thallium isoelectronic
sequence. Cf16+ is a particularly rich ion, having several narrow lines with
properties that minimize certain systematic effects. Cf16+ has very large
nuclear charge and large ionisation energy, resulting in the largest
alpha-sensitivity seen in atomic systems. The lines include positive and
negative shifters
Dissipationless Anomalous Hall Current in the Ferromagnetic Spinel CuCrSeBr
In a ferromagnet, an applied electric field invariably produces an
anomalous Hall current that flows perpendicular to the plane
defined by and (the magnetization). For decades, the question
whether is dissipationless (independent of the scattering rate),
has been keenly debated without experimental resolution. In the ferromagnetic
spinel CuCrSeBr, the resistivity (at low temperature)
may be increased 1000 fold by varying (Br), without degrading the .
We show that (normalized per carrier, at 5 K) remains unchanged
throughout. In addition to resolving the controversy experimentally, our
finding has strong bearing on the generation and study of spin-Hall currents in
bulk samples.Comment: 7 pages, 6 figure
The financial stress index: identification of systemic risk conditions
This paper develops a financial stress index for the United States, the Cleveland Financial Stress Index (CFSI), which provides a continuous signal of financial stress and broad coverage of the areas that could indicate it. The index is based on daily public-market data collected from four sectors of the fi nancial markets—the credit, foreign exchange, equity, and interbank markets. A dynamic weighting method is employed to capture changes in the relative importance of these four sectors as they occur. In addition, the design of the index allows the origin of the stress to be identified. We compare the CFSI to alternative indexes, using a detailed benchmarking methodology, and show how the CFSI can be applied to systemic stress monitoring and early warning system design. To that end, we investigate alternative stress-signaling thresholds and frequency regimes and then establish optimal frequencies for filtering out market noise and idiosyncratic episodes. Finally, we quantify a powerful CFSI-based rating system that assigns a probability of systemic stress to ranges of CFSI outcomes.Systemic risk ; Risk assessment
Theory of Interfacial Plasmon-Phonon Scattering in Supported Graphene
One of the factors limiting electron mobility in supported graphene is remote
phonon scattering. We formulate the theory of the coupling between graphene
plasmon and substrate surface polar phonon (SPP) modes, and find that it leads
to the formation of interfacial plasmon-phonon (IPP) modes, from which the
phenomena of dynamic anti-screening and screening of remote phonons emerge. The
remote phonon-limited mobilities for SiO, HfO, h-BN and
AlO substrates are computed using our theory. We find that h-BN
yields the highest peak mobility, but in the practically useful high-density
range the mobility in HfO-supported graphene is high, despite the fact
that HfO is a high- dielectric with low-frequency modes. Our
theory predicts that the strong temperature dependence of the total mobility
effectively vanishes at very high carrier concentrations. The effects of
polycrystallinity on IPP scattering are also discussed.Comment: 33 pages, 7 figure
- …