804 research outputs found
Energy-based mechanical model for mixed mode failure of laminated composites
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76674/1/AIAA-12639-806.pd
Mode I failure of laminated polymeric composites
A mechanical model for the prediction of mode I delamination failure of a laminated double cantilever beam (DCB) type specimen is presented. The volume of material ahead of the crack tip that experiences high stresses due to the presence of the crack tip was replaced by a nonlinear elastic spring foundation. The volume undergoing global deformation due to the external loading was replaced by a beam. The spring foundation was characterized by a covalent interatomic force law as a constitutive law and a non-uniform strain distribution throughout the spring length. Experimental data of fracture toughness for PEEK adhesive joints were used to partially characterize the spring foundation. Experimental results from mode I fracture tests performed to verify the current model are presented. The current model matched the experimental results closely for PEEK and BP907 adhesive joints for a wide range of adhesive layer thickness. It also reproduced load vs displacement curves of E7T1/G40 and E719/IM7 composite specimens very closely. The work presented contributes a new fracture model for prediction of delamination of laminated composite structures.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31339/1/0000249.pd
Isotropic three-dimensional gap in the iron-arsenide superconductor LiFeAs from directional heat transport measurements
The thermal conductivity k of the iron-arsenide superconductor LiFeAs (Tc ~
18K) was measured in single crystals at temperatures down to T~50mK and in
magnetic fields up to H=17T, very close to the upper critical field Hc2~18T.
For both directions of the heat current, parallel and perpendicular to the
tetragonal c-axis, a negligible residual linear term k/T is found as T ->0,
revealing that there are no zero-energy quasiparticles in the superconducting
state. The increase in k with magnetic field is the same for both current
directions and it follows closely the dependence expected for an isotropic
superconducting gap. There is no evidence of multi-band character, whereby the
gap would be different on different Fermi-surface sheets. These findings show
that the superconducting gap in LiFeAs is isotropic in 3D, without nodes or
deep minima anywhere on the Fermi surface. Comparison with other iron-pnictide
superconductors suggests that a nodeless isotropic gap is a common feature at
optimal doping (maximal Tc).Comment: 4 pages, 3 figure
Anisotropy of the coherence length from critical currents in the stoichiometric superconductor LiFeAs
Miniature Hall-probe arrays were used to measure the critical current
densities for the three main directions of vortex motion in the stoichiometric
LiFeAs superconductor. These correspond to vortices oriented along the c-axis
moving parallel to the ab-plane, and to vortices in the ab-plane moving
perpendicular to, and within the plane, respectively. The measurements were
carried out in the low-field regime of strong vortex pinning, in which the
critical current anisotropy is solely determined by the coherence length
anisotropy parameter, {\epsilon}_{\xi}. This allows extraction of
{\epsilon}_{\xi} at magnetic fields far below the upper critical field B_c2. We
find that increasing magnetic field decreases the anisotropy of the coherence
length
Domain Wall Enabled Hysteresis-Free Steep Slope Switching in MoS Transistors
The device concept of operating ferroelectric field effect transistors (FETs)
in the negative capacitance (NC) regime offers a promising route for achieving
energy-efficient logic applications that can outperform the conventional CMOS
technology, while the viable mechanisms for stabilizing the NC mode remain a
central topic of debate. In this work, we report hysteresis-free steep slope
switching in few-layer and bilayer MoS transistors back-gated by single
layer polycrystalline PbZrTiO films. The devices exhibit
current on/off ratios up to 810 within an ultra-low gate voltage
window of V = 0.5 V and subthreshold swing as low as 9.7 mV/decade at
room temperature, transcending the 60 mV/decade Boltzmann limit. Unlike
previous studies, the quasi-static NC mode is realized in a ferroelectric
without involving an additional dielectric layer. Theoretical modeling reveals
the dominant role of the metastable polar states within ferroelectric domain
walls in enabling the NC mode in the MoS transistors. Our findings shed
light into a new mechanism for NC operation, providing a simple yet effective
material strategy for developing high speed, low-power 2D nanoelectronics.Comment: 15 pages, 5 figure
Small anisotropy of the lower critical field and -wave two-gap feature in single crystal LiFeAs
The in- and out-of-plane lower critical fields and magnetic penetration
depths for LiFeAs were examined. The anisotropy ratio is
smaller than the expected theoretical value, and increased slightly with
increasing temperature from 0.6 to . This small degree of anisotropy
was numerically confirmed by considering electron correlation effect. The
temperature dependence of the penetration depths followed a power
law() below 0.3, with 3.5 for both and
. Based on theoretical studies of iron-based superconductors, these
results suggest that the superconductivity of LiFeAs can be represented by an
extended -wave due to weak impurity scattering effect. And the
magnitudes of the two gaps were also evaluted by fitting the superfluid density
for both the in- and out-of-plane to the two-gap model. The estimated values
for the two gaps are consistent with the results of angle resolved
photoemission spectroscopy and specific heat experiments.Comment: 10 pages, 5 figure
Experimental and theoretical studies on the structure of N-doped carbon nanotubes: Possibility of intercalated molecular N2
The concentration distribution and electronic structure of N atoms doped in multiwalled banboo-like carbon nanotubes (CNTs) are examined by photon energy-dependent x-ray photoelectron spectroscopy and x-ray absorption near edge structure. The inner part of the nanotube wall has a higher N concentration and contains molecular N-2 presumably intercalated between the graphite layers. These results are supported by the self-consistent charge-density-functional-based tight-binding calculation of double-walled CNTs, showing that the intercalation of N-2 is energetically possible and the graphite-like N structure conformer becomes more stable when the inner wall is more heavily doped. (C) 2004 American Institute of Physicsclose656
- …