945 research outputs found
Van der Waals epitaxy of Bi2Se3 on Si(111) vicinal surface: An approach to prepare high-quality thin films of topological insulator
Epitaxial growth of topological insulator Bi2Se3 thin films on nominally flat
and vicinal Si(111) substrates is studied. In order to achieve planner growth
front and better quality epifilms, a two-step growth method is adopted for the
van der Waal epitaxy of Bi2Se3 to proceed. By employing vicinal Si(111)
substrate surfaces, the in-pane growth rate anisotropy of Bi2Se3 is explored to
achieve single crystalline Bi2Se3 epifilms, in which threading defects and
twins are effectively suppressed. Optimization of the growth parameters has
resulted in vicinal Bi2Se3 films showing a carrier mobility of ~ 2000 cm2V-1s-1
and the background doping of ~ 3 x 1018 cm-3 of the as-grown layers. Such
samples not only show relatively high magnetoresistance but also a linear
dependence on magnetic field.Comment: 18 pages, 4 figure
Image-Domain Material Decomposition for Dual-energy CT using Unsupervised Learning with Data-fidelity Loss
Background: Dual-energy CT (DECT) and material decomposition play vital roles
in quantitative medical imaging. However, the decomposition process may suffer
from significant noise amplification, leading to severely degraded image
signal-to-noise ratios (SNRs). While existing iterative algorithms perform
noise suppression using different image priors, these heuristic image priors
cannot accurately represent the features of the target image manifold. Although
deep learning-based decomposition methods have been reported, these methods are
in the supervised-learning framework requiring paired data for training, which
is not readily available in clinical settings.
Purpose: This work aims to develop an unsupervised-learning framework with
data-measurement consistency for image-domain material decomposition in DECT
Broadband 1-3 piezoelectric composite transducer design using Sierpinski Gasket fractal geometry
Wider operational bandwidth is an important requirement of an ultrasound transducer across many applications. In nature, it can be observed that several hearing organs possess a broad operating bandwidth by having a varying length scales structure. Moreover, conventional 1-3 piezoelectric composite transducers have been widely recognized for their wider bandwidth over their piezoelectric ceramic counterparts. In this paper, a novel 1-3 piezoelectric composite design using a fractal geometry, known as the Sierpinski Gasket (SG), is proposed in order to explore the potential of further extending the operational bandwidth and sensitivity of the transducer. Two equivalent 1-3 piezocomposite designs are compared to this end, one with a conventional periodic parallelepiped shaped pillar structure and one with the SG fractal geometry, both theoretically, using a finite element (FE) analysis package, and experimentally. The transmit voltage response and open circuit voltage response are used to illustrate bandwidth improvement from the fractal composite design. Following the simulation results, a 580 kHz single element transducer, utilizing the proposed SG fractal microstructure, is fabricated using a pillar placement methodology. The performance of the prototyped device is characterized and compared with a conventional 1-3 composite design, as well as with a commercial ultrasound transducer. In the one-way transmission mode, a bandwidth improvement of 27.2 % and sensitivity enhancement of 3.8 dB can be found with the SG fractal design compared to an equivalent conventional composite design and up 105.1 % bandwidth improvement when compared to the commercial transducer. In the one-way reception mode, the bandwidth improvement for the SG fractal design is 2.5 % and 32.9 % when compared to the conventional and commercial transducers, respectively
Two-dimensional Transport Induced Linear Magneto-Resistance in Topological Insulator BiSe Nanoribbons
We report the study of a novel linear magneto-resistance (MR) under
perpendicular magnetic fields in Bi2Se3 nanoribbons. Through angular dependence
magneto-transport experiments, we show that this linear MR is purely due to
two-dimensional (2D) transport, in agreement with the recently discovered
linear MR from 2D topological surface state in bulk Bi2Te3, and the linear MR
of other gapless semiconductors and graphene. We further show that the linear
MR of Bi2Se3 nanoribbons persists to room temperature, underscoring the
potential of exploiting topological insulator nanomaterials for room
temperature magneto-electronic applications.Comment: ACS Nano, in pres
- …