969 research outputs found
Effective Governance of Global Financial Markets:An Evolutionary Plan for Reform
Runaway electrons, which are generated in a plasma where the induced electric field exceeds a certain critical value, can reach very high energies in the MeV range. For such energetic electrons, radiative losses will contribute significantly to the momentum space dynamics. Under certain conditions, due to radiative momentum losses, a non-monotonic feature - a âbump' - can form in the runaway electron tail, creating a potential for bump-on-tail-type instabilities to arise. Here, we study the conditions for the existence of the bump. We derive an analytical threshold condition for bump appearance and give an approximate expression for the minimum energy at which the bump can appear. Numerical calculations are performed to support the analytical derivation
Finite bias Cooper pair splitting
In a device with a superconductor coupled to two parallel quantum dots (QDs)
the electrical tunability of the QD levels can be used to exploit non-classical
current correlations due to the splitting of Cooper pairs. We experimentally
investigate the effect of a finite potential difference across one quantum dot
on the conductance through the other completely grounded QD in a Cooper pair
splitter fabricated on an InAs nanowire. We demonstrate that the electrical
transport through the device can be tuned by electrical means to be dominated
either by Cooper pair splitting (CPS), or by elastic co-tunneling (EC). The
basic experimental findings can be understood by considering the energy
dependent density of states in a QD. The reported experiments add
bias-dependent spectroscopy to the investigative tools necessary to develop
CPS-based sources of entangled electrons in solid-state devices.Comment: 4 pages, 4 figure
Optical bandgap engineering in nonlinear silicon nitride waveguides
Silicon nitride is awell-established material for photonic devices and
integrated circuits. It displays a broad transparency window spanning from the
visible to the mid-IR and waveguides can be manufactured with low losses. An
absence of nonlinear multi-photon absorption in the erbium lightwave
communications band has enabled various nonlinear optic applications in the
past decade. Silicon nitride is a dielectric material whose optical and
mechanical properties strongly depend on the deposition conditions. In
particular, the optical bandgap can be modified with the gas flow ratio during
low-pressure chemical vapor deposition (LPCVD). Here we show that this
parameter can be controlled in a highly reproducible manner, providing an
approach to synthesize the nonlinear Kerr coefficient of the material. This
holistic empirical study provides relevant guidelines to optimize the
properties of LPCVD silicon nitride waveguides for nonlinear optics
applications that rely on the Kerr effect
Absorption-reduced waveguide structure for efficient terahertz generation
An absorption-reduced planar waveguide structure is proposed for increasing the efficiency of
terahertz (THz) pulse generation by optical rectification of femtosecond laser pulses with tiltedpulse-
front in highly nonlinear materials with large absorption coefficient. The structure functions
as waveguide both for the optical pump and the generated THz radiation. Most of the THz power
propagates inside the cladding with low THz absorption, thereby reducing losses and leading to the
enhancement of the THz generation efficiency by up to more than one order of magnitude, as
compared with a bulk medium. Such a source can be suitable for highly efficient THz pulse generation
pumped by low-energy (nJ-lJ) pulses at high (MHz) repetition rates delivered by compact
fiber lasers
Wet etch methods for InAs nanowire patterning and self-aligned electrical contacts
Advanced synthesis of semiconductor nanowires (NWs) enables their application
in diverse fields, notably in chemical and electrical sensing, photovoltaics,
or quantum electronic devices. In particular, Indium Arsenide (InAs) NWs are an
ideal platform for quantum devices, e.g. they may host topological Majorana
states. While the synthesis has been continously perfected, only few techniques
were developed to tailor individual NWs after growth. Here we present three wet
chemical etch methods for the post-growth morphological engineering of InAs NWs
on the sub-100 nm scale. The first two methods allow the formation of
self-aligned electrical contacts to etched NWs, while the third method results
in conical shaped NW profiles ideal for creating smooth electrical potential
gradients and shallow barriers. Low temperature experiments show that NWs with
etched segments have stable transport characteristics and can serve as building
blocks of quantum electronic devices. As an example we report the formation of
a single electrically stable quantum dot between two etched NW segments.Comment: 9 pages, 5 figure
Electronic States of Graphene Grain Boundaries
We introduce a model for amorphous grain boundaries in graphene, and find
that stable structures can exist along the boundary that are responsible for
local density of states enhancements both at zero and finite (~0.5 eV)
energies. Such zero energy peaks in particular were identified in STS
measurements [J. \v{C}ervenka, M. I. Katsnelson, and C. F. J. Flipse, Nature
Physics 5, 840 (2009)], but are not present in the simplest pentagon-heptagon
dislocation array model [O. V. Yazyev and S. G. Louie, Physical Review B 81,
195420 (2010)]. We consider the low energy continuum theory of arrays of
dislocations in graphene and show that it predicts localized zero energy
states. Since the continuum theory is based on an idealized lattice scale
physics it is a priori not literally applicable. However, we identify stable
dislocation cores, different from the pentagon-heptagon pairs, that do carry
zero energy states. These might be responsible for the enhanced magnetism seen
experimentally at graphite grain boundaries.Comment: 10 pages, 4 figures, submitted to Physical Review
- âŠ