394 research outputs found
Interplay of chiral and helical states in a Quantum Spin Hall Insulator lateral junction
We study the electronic transport across an electrostatically-gated lateral
junction in a HgTe quantum well, a canonical 2D topological insulator, with and
without applied magnetic field. We control carrier density inside and outside a
junction region independently and hence tune the number and nature of 1D edge
modes propagating in each of those regions. Outside the 2D gap, magnetic field
drives the system to the quantum Hall regime, and chiral states propagate at
the edge. In this regime, we observe fractional plateaus which reflect the
equilibration between 1D chiral modes across the junction. As carrier density
approaches zero in the central region and at moderate fields, we observe
oscillations in resistance that we attribute to Fabry-Perot interference in the
helical states, enabled by the broken time reversal symmetry. At higher fields,
those oscillations disappear, in agreement with the expected absence of helical
states when band inversion is lifted.Comment: 5 pages, 4 figures, supp. ma
Frictional cooling of positively charged particles
One of the focuses of research and development towards the construction of a
muon collider is muon beam preparation. Simulation of frictional cooling shows
that it can achieve the desired emittance reduction to produce high-luminosity
muon beams. We show that for positively charged particles, charge exchange
interactions necessitate significant changes to schemes previously developed
for negatively charged particles. We also demonstrate that foil-based schemes
are not viable for positive particles.Comment: 15 pages, 6 figure
Comparison of Upper Neck Loading in Young Adult and Elderly Volunteers During Low Speed Frontal Impacts
Cervical pain and injuries are a major health problem globally. Existing neck injury criteria are based on experimental studies that included sled tests performed with volunteers, post-mortem human surrogates and animals. However, none of these studies have addressed the differences between young adults and elderly volunteers to date. Thus, this work analyzed the estimated axial and shear forces, and the bending moment at the craniocervical junction of nine young volunteers (18–30 years old) and four elderly volunteers (>65 years old) in a low-speed frontal deceleration. Since the calculation of these loads required the use of the mass and moment of inertia of the volunteers' heads, this study proposed new methods to estimate the inertial properties of the head of the volunteers based on external measurements that reduced the error of previously published methods. The estimated mean peak axial force (Fz) was −164.38 ± 35.04 N in the young group and −170.62 ± 49.82 N in the elderly group. The average maximum shear force (Fx) was −224.42 ± 54.39 N and −232.41 ± 19.23 N in the young and elderly group, respectively. Last, the estimated peak bending moment (My) was 13.63 ± 1.09 Nm in the young group and 14.81 ± 1.36 Nm in the elderly group. The neck loads experienced by the elderly group were within the highest values in the present study. Nevertheless, for the group of volunteers included in this study, no substantial differences with age were observed
Energy research with neutrons (ErwiN) and installation of a fast neutron powder diffraction option at the MLZ, Germany
The need for rapid data collection and studies of small sample volumes in the range of cubic millimetres are the main driving forces for the concept of a new high-throughput monochromatic diffraction instrument at the Heinz Maier-Leibnitz Zentrum (MLZ), Germany. A large region of reciprocal space will be accessed by a detector with sufficient dynamic range and microsecond time resolution, while allowing for a variety of complementary sample environments. The medium-resolution neutron powder diffraction option for `energy research with neutrons\u27 (ErwiN) at the high-flux FRM II neutron source at the MLZ is foreseen to meet future demand. ErwiN will address studies of energy-related systems and materials with respect to their structure and uniformity by means of bulk and spatially resolved neutron powder diffraction. A set of experimental options will be implemented, enabling time-resolved studies, rapid parametric measurements as a function of external parameters and studies of small samples using an adapted radial collimator. The proposed powder diffraction option ErwiN will bridge the gap in functionality between the high-resolution powder diffractometer SPODI and the time-of-flight diffractometers POWTEX and SAPHiR at the MLZ
The OSACA Database and a Kinematic Analysis of Stars in the Solar Neighborhood
We transformed radial velocities compiled from more than 1400 published
sources, including the Geneva--Copenhagen survey of the solar neighborhood
(CORAVEL-CfA), into a uniform system based on the radial velocities of 854
standard stars in our list. This enabled us to calculate the average weighted
radial velocities for more than 25~000 HIPPARCOS stars located in the local
Galactic spiral arm (Orion arm) with a median error of +-1 km/s. We use these
radial velocities together with the stars' coordinates, parallaxes, and proper
motions to determine their Galactic coordinates and space velocities. These
quantities, along with other parameters of the stars, are available from the
continuously updated Orion Spiral Arm CAtalogue (OSACA) and the associated
database. We perform a kinematic analysis of the stars by applying an
Ogorodnikov-Milne model to the OSACA data. The kinematics of the nearest single
and multiple main-sequence stars differ substantially. We used distant
(r\approx 0.2 kpc) stars of mixed spectral composition to estimate the angular
velocity of the Galactic rotation -25.7+-1.2 km/s/kpc, and the vertex
deviation,l=13+-2 degrees, and detect a negative K effect. This negative K
effect is most conspicuous in the motion of A0-A5 giants, and is equal to
K=-13.1+-2.0 km/s/kpc.Comment: 16 pages, 8 figure
Structure and degeneracy of vortex lattice domains in pure superconducting niobium: A small-angle neutron scattering study
High-purity niobium exhibits a surprisingly rich assortment of vortex lattice (VL) structures for fields applied parallel to a fourfold symmetry axis, with all observed VL phases made up of degenerate domains that spontaneously break some crystal symmetry. Yet a single regular hexagonal VL domain is observed at all temperatures and fields parallel to a threefold symmetry axis. We report a detailed investigation of the transition between these lush and barren VL landscapes, discovering new VL structures and phase transitions at high fields. We show that the number and relative population of VL domains is intrinsically tied to the underlying crystal symmetry. We discuss how subtle anisotropies of the crystal may generate the remarkable VLs observed. © 2009 The American Physical Society
Orbital textures and charge density waves in transition metal dichalcogenides
Low-dimensional electron systems, as realized naturally in graphene or
created artificially at the interfaces of heterostructures, exhibit a variety
of fascinating quantum phenomena with great prospects for future applications.
Once electrons are confined to low dimensions, they also tend to spontaneously
break the symmetry of the underlying nuclear lattice by forming so-called
density waves; a state of matter that currently attracts enormous attention
because of its relation to various unconventional electronic properties. In
this study we reveal a remarkable and surprising feature of charge density
waves (CDWs), namely their intimate relation to orbital order. For the
prototypical material 1T-TaS2 we not only show that the CDW within the
two-dimensional TaS2-layers involves previously unidentified orbital textures
of great complexity. We also demonstrate that two metastable stackings of the
orbitally ordered layers allow to manipulate salient features of the electronic
structure. Indeed, these orbital effects enable to switch the properties of
1T-TaS2 nanostructures from metallic to semiconducting with technologically
pertinent gaps of the order of 200 meV. This new type of orbitronics is
especially relevant for the ongoing development of novel, miniaturized and
ultra-fast devices based on layered transition metal dichalcogenides
Theory of current-driven motion of Skyrmions and spirals in helical magnets
We study theoretically the dynamics of the spin textures, i.e., Skyrmion
crystal (SkX) and spiral structure (SS), in two-dimensional helical magnets
under external current. By numerically solving the Landau-Lifshitz-Gilbert
equation, it is found that (i) the critical current density of the motion is
much lower for SkX compared with SS in agreement with the recent experiment,
(ii) there is no intrinsic pinning effect for SkX and the deformation of the
internal structure of Skyrmion reduces the pinning effect dramatically, (iii)
the Bragg intensity of SkX shows strong time-dependence as can be observed by
neutron scattering experiment.Comment: 4 pages, 3 figure
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