949 research outputs found
A Large Effective Phonon Magnetic Moment in a Dirac Semimetal
We investigated the magnetoterahertz response of the Dirac semimetal
CdAs and observed a particularly low frequency optical phonon, as well
as a very prominent and field sensitive cyclotron resonance. As the cyclotron
frequency is tuned with field to pass through the phonon, the phonon become
circularly polarized as shown by a notable splitting in their response to
right- and left-hand polarized light. This splitting can be expressed as an
effective phonon magnetic moment that is approximately 2.7 times the Bohr
magneton, which is almost four orders of magnitude larger than ab initio
calculations predict for phonon magnetic moments in nonmagnetic insulators.
This exceedingly large value is due to the coupling of the phonons to the
cyclotron motion and is controlled directly by the electron-phonon coupling
constant. This field tunable circular-polarization selective coupling provides
new functionality for nonlinear optics to create light-induced topological
phases in Dirac semimetals.Comment: 15 pages for main text and SI; To appear in Nano Letters (2020
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
Zeeman field-induced two-dimensional Weyl semimetal phase in cadmium arsenide
We report a topological phase transition in quantum-confined cadmium arsenide
(Cd3As2) thin films under an in-plane Zeeman field when the Fermi level is
tuned into the topological gap via an electric field. Symmetry considerations
in this case predict the appearance of a two-dimensional Weyl semimetal (2D
WSM), with a pair of Weyl nodes of opposite chirality at charge neutrality that
are protected by space-time inversion (C2T) symmetry. We show that the 2D WSM
phase displays unique transport signatures, including saturated resistivities
on the order of h/e^2 that persist over a range of in-plane magnetic fields.
Moreover, applying a small out-of-plane magnetic field, while keeping the
in-plane field within the stability range of the 2D WSM phase, gives rise to a
well-developed odd integer quantum Hall effect, characteristic of degenerate,
massive Weyl fermions. A minimal four-band k.p model of Cd3As2, which
incorporates first-principles effective g factors, qualitatively explains our
findings.Comment: Accepted for publication in Physical Review Letter
Two-dimensional topological insulator state in cadmium arsenide thin films
Two-dimensional topological insulators (2D TIs) are a highly desired quantum
phase but few materials have demonstrated clear signatures of a 2D TI state. It
has been predicted that 2D TIs can be created from thin films of
three-dimensional TIs by reducing the film thickness until the surface states
hybridize. Here, we employ this technique to report the first observation of a
2D TI state in epitaxial thin films of cadmium arsenide, a prototype Dirac
semimetal in bulk form and a 3D TI in thin films. Using magnetotransport
measurements with electrostatic gating, we observe a Landau level spectrum and
quantum Hall effect that are in excellent agreement with those of an ideal 2D
TI. Specifically, we observe a crossing of the zeroth Landau levels at a
critical magnetic field. We show that the film thickness can be used to tune
the critical magnetic field. Moreover, a larger change in film thickness causes
a transition from a 2D TI to a 2D trivial insulator, just as predicted by
theory. The high degree of tunability available in epitaxial cadmium arsenide
heterostructures can thus be used to fine-tune the 2D TI, which is essential
for future topological devices.Comment: Accepted for publication in Physical Review Letters. Fixed issues
with the display of Fig. 3 in the pdf in this versio
Dynamics of Competitive Evolution on a Smooth Landscape
We study competitive DNA sequence evolution directed by {\it in vitro}
protein binding. The steady-state dynamics of this process is well described by
a shape-preserving pulse which decelerates and eventually reaches equilibrium.
We explain this dynamical behavior within a continuum mean-field framework.
Analytical results obtained on the motion of the pulse agree with simulations.
Furthermore, finite population correction to the mean-field results are found
to be insignificant.Comment: 4 pages, 2 figures, revised, to appear in Phys. Rev. Let
Thermal receptivity of free convective flow from a heated vertical surface: linear waves
Numerical techniques are used to study the receptivity to small-amplitude thermal disturbances of the boundary layer flow of air which is induced by a heated vertical flat plate. The fully elliptic nonlinear, time-dependent Navier–Stokes and energy equations are first solved to determine the steady state boundary-layer flow, while a linearised version of the same code is used to determine the stability characteristics. In particular we investigate (i) the ultimate fate of a localised thermal disturbance placed in the region near the leading edge and (ii) the effect of small-scale surface temperature oscillations as means of understanding the stability characteristics of the boundary layer. We show that there is a favoured frequency of excitation for the time-periodic disturbance which maximises the local response in terms of the local rate of heat transfer. However the magnitude of the favoured frequency depends on precisely how far from the leading edge the local response is measured. We also find that the instability is advective in nature and that the response of the boundary layer consists of a starting transient which eventually leaves the computational domain, leaving behind the large-time time-periodic asymptotic state. Our detailed numerical results are compared with those obtained using parallel flow theory
The value of source data verification in a cancer clinical trial
Background
Source data verification (SDV) is a resource intensive method of quality assurance frequently used in clinical trials. There is no empirical evidence to suggest that SDV would impact on comparative treatment effect results from a clinical trial.
Methods
Data discrepancies and comparative treatment effects obtained following 100% SDV were compared to those based on data without SDV. Overall survival (OS) and Progression-free survival (PFS) were compared using Kaplan-Meier curves, log-rank tests and Cox models. Tumour response classifications and comparative treatment Odds Ratios (ORs) for the outcome objective response rate, and number of Serious Adverse Events (SAEs) were compared. OS estimates based on SDV data were compared against estimates obtained from centrally monitored data.
Findings
Data discrepancies were identified between different monitoring procedures for the majority of variables examined, with some variation in discrepancy rates. There were no systematic patterns to discrepancies and their impact was negligible on OS, the primary outcome of the trial (HR (95% CI): 1.18(0.99 to 1.41), p = 0.064 with 100% SDV; 1.18(0.99 to 1.42), p = 0.068 without SDV; 1.18(0.99 to 1.40), p = 0.073 with central monitoring). Results were similar for PFS. More extreme discrepancies were found for the subjective outcome overall objective response (OR (95% CI): 1.67(1.04 to 2.68), p = 0.03 with 100% SDV; 2.45(1.49 to 4.04), p = 0.0003 without any SDV) which was mostly due to differing CT scans.
Interpretation
Quality assurance methods used in clinical trials should be informed by empirical evidence. In this empirical comparison, SDV was expensive and identified random errors that made little impact on results and clinical conclusions of the trial. Central monitoring using an external data source was a more efficient approach for the primary outcome of OS. For the subjective outcome objective response, an independent blinded review committee and tracking system to monitor missing scan data could be more efficient than SDV
Feasibility of free breathing Lung MRI for Radiotherapy using non-Cartesian k-space acquisition schemes
Objective:
To test a free-breathing MRI protocol for anatomical and functional assessment during lung cancer radiotherapy by assessing two non-Cartesian acquisition schemes based on T1 weighted 3D gradient recall echo sequence: (i) stack-of stars (StarVIBE) and (ii) spiral (SpiralVIBE) trajectories. Methods:
MR images on five healthy volunteers were acquired on a wide bore 3T scanner (MAGNETOM Skyra, Siemens Healthcare, Erlangen, Germany). Anatomical image quality was assessed on: (1) free breathing (StarVIBE), (2) the standard clinical sequence (volumetric interpolated breath-hold examination, VIBE) acquired in a 20 second (s) compliant breath-hold and (3) 20 s non-compliant breath-hold. For functional assessment, StarVIBE and the current standard breath-hold time-resolved angiography with stochastic trajectories (TWIST) sequence were run as multiphase acquisitions to replicate dynamic contrast enhancement (DCE) in one healthy volunteer. The potential application of the SpiralVIBE sequence for lung parenchymal imaging was assessed on one healthy volunteer. Ten patients with lung cancer were subsequently imaged with the StarVIBE and SpiralVIBE sequences for anatomical and structural assessment. For functional assessment, free-breathing StarVIBE DCE protocol was compared with breath-hold TWIST sequences on four prior lung cancer patients with similar tumour locations. Image quality was evaluated independently and blinded to sequence information by an experienced thoracic radiologist. Results:
For anatomical assessment, the compliant breath-hold VIBE sequence was better than free-breathing StarVIBE. However, in the presence of a non-compliant breath-hold, StarVIBE was superior. For functional assessment, StarVIBE outperformed the standard sequence and was shown to provide robust DCE data in the presence of motion. The ultrashort echo of the SpiralVIBE sequence enabled visualisation of lung parenchyma. Conclusion:
The two non-Cartesian acquisition sequences, StarVIBE and SpiralVIBE, provide a free-breathing imaging protocol of the lung with sufficient image quality to permit anatomical, structural and functional assessment during radiotherapy. Advances in knowledge:
Novel application of non-Cartesian MRI sequences for lung cancer imaging for radiotherapy. Illustration of SpiralVIBE UTE sequence as a promising sequence for lung structural imaging during lung radiotherapy
Genetically engineered minipigs model the major clinical features of human neurofibromatosis type 1.
Neurofibromatosis Type 1 (NF1) is a genetic disease caused by mutations in Neurofibromin 1 (NF1). NF1 patients present with a variety of clinical manifestations and are predisposed to cancer development. Many NF1 animal models have been developed, yet none display the spectrum of disease seen in patients and the translational impact of these models has been limited. We describe a minipig model that exhibits clinical hallmarks of NF1, including café au lait macules, neurofibromas, and optic pathway glioma. Spontaneous loss of heterozygosity is observed in this model, a phenomenon also described in NF1 patients. Oral administration of a mitogen-activated protein kinase/extracellular signal-regulated kinase inhibitor suppresses Ras signaling. To our knowledge, this model provides an unprecedented opportunity to study the complex biology and natural history of NF1 and could prove indispensable for development of imaging methods, biomarkers, and evaluation of safety and efficacy of NF1-targeted therapies
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