405 research outputs found
Conduction Channels of One-Atom Zinc Contacts
We have determined the transmission coefficients of atomic-sized Zn contacts
using a new type of breakjunction which contains a whisker as a central bridge.
We find that in the last conductance plateau the transport is unexpectedly
dominated by a well-transmitting single conduction channel. We explain the
experimental findings with the help of a tight-binding model which shows that
in an one-atom Zn contact the current proceeds through the 4s and 4p orbitals
of the central atom.Comment: revtex4, 5 pages, 5 figure
Universal three-body recombination and Efimov resonances in an ultracold Li-Cs mixture
We study Efimov resonances via three-body loss in an ultracold two-component
gas of fermionic Li and bosonic Cs atoms close to a Feshbach
resonance at 843~G, extending results reported previously [Pires \textit{et
al.}, Phys. Rev. Lett. 112, 250404 (2014)] to temperatures around 120~nK. The
experimental scheme for reaching lower temperatures is based upon compensating
the gravity-induced spatial separation of the mass-imbalanced gases with
bichromatic optical dipole traps. We observe the first and second excited
Li-Cs-Cs Efimov resonance in the magnetic field dependence of the three-body
event rate constant, in good agreement with the universal zero-range theory at
finite temperature [Petrov and Werner, Phys. Rev. A 92, 022704 (2015)].
Deviations are found for the Efimov ground state, and the inelasticity
parameter is found to be significantly larger than those for
single-species systems
Conformal scattering for a nonlinear wave equation on a curved background
The purpose of this paper is to establish a geometric scattering result for a
conformally invariant nonlinear wave equation on an asymptotically simple
spacetime. The scattering operator is obtained via trace operators at null
infinities. The proof is achieved in three steps. A priori linear estimates are
obtained via an adaptation of the Morawetz vector field in the Schwarzschild
spacetime and a method used by H\"ormander for the Goursat problem. A
well-posedness result for the characteristic Cauchy problem on a light cone at
infinity is then obtained. This requires a control of the nonlinearity uniform
in time which comes from an estimates of the Sobolev constant and a decay
assumption on the nonlinearity of the equation. Finally, the trace operators on
conformal infinities are built and used to define the conformal scattering
operator
Electron-vibration interaction in transport through atomic gold wires
We calculate the effect of electron-vibration coupling on conduction through
atomic gold wires, which was measured in the experiments of Agra\"it et al.
[Phys. Rev. Lett. 88, 216803 (2002)]. The vibrational modes, the coupling
constants, and the inelastic transport are all calculated using a tight-binding
parametrization and the non-equilibrium Green function formalism. The
electron-vibration coupling gives rise to small drops in the conductance at
voltages corresponding to energies of some of the vibrational modes. We study
systematically how the position and height of these steps vary as a linear wire
is stretched and more atoms are added to it, and find a good agreement with the
experiments. We also consider two different types of geometries, which are
found to yield qualitatively similar results. In contrast to previous
calculations, we find that typically there are several close-lying drops due to
different longitudinal modes. In the experiments, only a single drop is usually
visible, but its width is too large to be accounted for by temperature.
Therefore, to explain the experimental results, we find it necessary to
introduce a finite broadening to the vibrational modes, which makes the
separate drops merge into a single, wide one. In addition, we predict how the
signatures of vibrational modes in the conductance curves differ between linear
and zigzag-type wires.Comment: 19 pages, 12 figure
Recovering the mass and the charge of a Reissner-Nordstr\"om black hole by an inverse scattering experiment
In this paper, we study inverse scattering of massless Dirac fields that
propagate in the exterior region of a Reissner-Nordstr\"om black hole. Using a
stationary approach we determine precisely the leading terms of the high-energy
asymptotic expansion of the scattering matrix that, in turn, permit us to
recover uniquely the mass of the black hole and its charge up to a sign
Radiotherapy for Colorectal Cancer: Current Standards and Future Perspectives
Background: Multimodal treatment approaches are indispensable for patients with advanced-stage colorectal cancer. Radiotherapy has been established as essential part of perioperative concepts and was introduced as an option to face challenges such as local relapse or oligometastases.
Methods: A literature review was performed to summarize evidence and current standards of radiotherapeutic concepts in the treatment of colorectal cancer.
Results: For stage II/III rectal cancer, neoadjuvant radiotherapy is superior to adjuvant treatment. Two preoperative regimens have been established and are commonly used with different objectives: short-course radiotherapy (SC-RT) and long-course chemoradiotherapy (LC-CRT). Both reduce the risk of local relapse. Additionally, LC-CRT aims at downsizing the tumor to potentially reduce radicalness of surgery. There is increasing evidence that not all stage II/III rectal cancer patients need neoadjuvant irradiation but also that in some cases surgery might be omitted. Stereotactic body radiotherapy (SBRT) of the liver shows high rates of local control in oligometastatic patients. Intraoperative and particle radiotherapy extend the spectrum of treatment options for locally recurrent patients.
Conclusion: Radiotherapeutic concepts are crucial for the primary management of locally advanced colorectal cancer and can essentially contribute to treatment approaches in locally recurrent, oligometastatic or palliative patients
Local energy decay of massive Dirac fields in the 5D Myers-Perry metric
We consider massive Dirac fields evolving in the exterior region of a
5-dimensional Myers-Perry black hole and study their propagation properties.
Our main result states that the local energy of such fields decays in a weak
sense at late times. We obtain this result in two steps: first, using the
separability of the Dirac equation, we prove the absence of a pure point
spectrum for the corresponding Dirac operator; second, using a new form of the
equation adapted to the local rotations of the black hole, we show by a Mourre
theory argument that the spectrum is absolutely continuous. This leads directly
to our main result.Comment: 40 page
Theoretical analysis of the conductance histograms and structural properties of Ag, Pt and Ni nanocontacts
Conductance histograms are a valuable tool to study the intrinsic conduction
properties of metallic atomic-sized contacts. These histograms show a peak
structure, which is characteristic of the type of metal under investigation.
Despite the enormous progress in the understanding of the electronic transport
in metallic nanowires, the origin of this peak structure is still a basic open
problem. In the present work we tackle this issue, extending our theoretical
analysis of Au conductance histograms [Dreher et al., PRB 72, 075435 (2005)] to
different types of metals, namely, Ag, Pt and ferromagnetic Ni. We combine
classical molecular dynamics simulations of the breaking of nanocontacts with
conductance calculations based on a tight-binding model. This combination gives
us access to crucial information such as contact geometries, strain forces,
minimum cross-sections, the conductance, transmissions of the individual
conduction channels and, in the case of Ni, the spin polarization of the
current. We shall also briefly discuss investigations of Al atomic-sized
contacts. From our analysis we conclude that the differences in the histograms
of these metals are due to (i) the very different electronic structures, which
means different atomic orbitals contributing to the transport, and (ii) the
different mechanical properties, which in a case like Pt lead to the formation
of special structures, namely monoatomic chains. Of particular interest are
results for Ni that indicate the absence of any conductance quantization, and
show how the current polarization evolves (including large fluctuations) from
negative values in thick contacts to even positive values in the tunneling
regime after rupture of the contact. Finally, we also present a detailed
analysis of the breaking forces of these metallic contacts.Comment: 22 pages, 19 figure
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