2,697 research outputs found
Tearing of free-standing graphene
We examine the fracture mechanics of tearing graphene. We present a molecular dynamics simulation of the propagation of cracks in clamped, free-standing graphene as a function of the out-of-plane force. The geometry is motivated by experimental configurations that expose graphene sheets to out-of-plane forces, such as back-gate voltage. We establish the geometry and basic energetics of failure and obtain approximate analytical expressions for critical crack lengths and forces. We also propose a method to obtain graphene's toughness. We observe that the cracks' path and the edge structure produced are dependent on the initial crack length. This work may help avoid the tearing of graphene sheets and aid the production of samples with specific edge structures.CAPESNational Science Foundation DMR 1002428Physic
Field induced phase transitions in the helimagnet Ba2CuGe2O7
We present a theoretical study of the two-dimensional spiral antiferromagnet
Ba2CuGe2O7 in the presence of an external magnetic field. We employ a suitable
nonlinear sigma model to calculate the T=0 phase diagram and the associated
low-energy spin dynamics for arbitrary canted fields, in general agreement with
experiment. In particular, when the field is applied parallel to the c axis, a
previously anticipated Dzyaloshinskii-type incommensurate-to-commensurate phase
transition is actually mediated by an intermediate phase, in agreement with our
earlier theoretical prediction confirmed by the recent observation of the
so-called double-k structure. The sudden pi/2 rotations of the magnetic
structures observed in experiment are accounted for by a weakly broken U(1)
symmetry of our model. Finally, our analysis suggests a nonzero
weak-ferromagnetic component in the underlying Dzyaloshinskii-Moriya
anisotropy, which is important for quantitative agreement with experiment.Comment: 17 pages, 14 figures. Corrected typos in the abstrac
Development of materials and process technology for dual alloy disks
Techniques for the preparation of dual alloy disks were developed and evaluated. Four material combinations were evaluated in the form of HIP consolidated and heat treated cylindrical and plate shapes in terms of elevated temperature tensile, stress rupture and low cycle fatigue properties. The process evaluation indicated that the pe-HIP AF-115 rim/loose powder Rene 95 hub combination offered the best overall range of mechanical properties for dual disk applications. The feasibility of this dual alloy concept for the production of more complex components was demonstrated by the scale up fabrication of a prototype CFM-56 disk made from this AF-115/Rene 95 combination. The hub alloy ultimate tensile strength was approximately 92 percent of the program goal of 1520 MPa (220 ksi) at 480 C (900 F) and the rim alloy stress rupture goal of 300 hours at 675 C (1250 F)/925 MPa (134 ksi) was exceeded by 200 hours. The low cycle fatigue properties were equivalent to those exhibited by HIP and heat treated alloys. There was an absence of rupture notch sensitivity in both alloys. The joint tensile properties were approximately 85 percent of the weaker of the two materials (Rene 95) and the stress rupture properties were equivalent to those of the weaker of the two materials (Rene 95)
Dynamics of Epidemics
This article examines how diseases on random networks spread in time. The
disease is described by a probability distribution function for the number of
infected and recovered individuals, and the probability distribution is
described by a generating function. The time development of the disease is
obtained by iterating the generating function. In cases where the disease can
expand to an epidemic, the probability distribution function is the sum of two
parts; one which is static at long times, and another whose mean grows
exponentially. The time development of the mean number of infected individuals
is obtained analytically. When epidemics occur, the probability distributions
are very broad, and the uncertainty in the number of infected individuals at
any given time is typically larger than the mean number of infected
individuals.Comment: 4 pages and 3 figure
Dynamical stability of the crack front line
Dynamical stability of the crack front line that propagates between two
plates is studied numerically using the simple two-dimensional mass-spring
model. It is demonstrated that the straight front line is unstable for low
speed while it becomes stable for high speed. For the uniform model, the
roughness exponent in the slower speed region is fairly constant around 0.4 and
there seems to be a rough-smooth transition at a certain speed. For the
inhomogeneous case with quenched randomness, the transition is gradual.Comment: 14 pages, 7 figure
Cracks in rubber under tension exceed the shear wave speed
The shear wave speed is an upper limit for the speed of cracks loaded in
tension in linear elastic solids. We have discovered that in a non-linear
material, cracks in tension (Mode I) exceed this sound speed, and travel in an
intersonic range between shear and longitudinal wave speeds. The experiments
are conducted in highly stretched sheets of rubber; intersonic cracks can be
produced simply by popping a balloon.Comment: 4 pages, 5 eps figure
Steady-State Cracks in Viscoelastic Lattice Models II
We present the analytic solution of the Mode III steady-state crack in a
square lattice with piecewise linear springs and Kelvin viscosity. We show how
the results simplify in the limit of large width. We relate our results to a
model where the continuum limit is taken only along the crack direction. We
present results for small velocity, and for large viscosity, and discuss the
structure of the critical bifurcation for small velocity. We compute the size
of the process zone wherein standard continuum elasticity theory breaks down.Comment: 17 pages, 3 figure
Antiferromagnetic coupling of the single-molecule magnet Mn12 to a ferromagnetic substrate
We investigate magnetic coupling between a monolayer of prototype
single-molecule magnets Mn12 and a ferromagnetic Ni(111) substrate through S,
using density-functional theory (DFT) and a DFT+U method. Our DFT and DFT+U
calculations show that the Mn12 molecules favor antiferromagnetic coupling to
the Ni substrate, and that they possess magnetic moments deviated from the
magnetic moments of isolated Mn12 molecules. We find that the magnetic easy
axis of the Mn12 on Ni (whole system) is dictated by that of the Ni substrate.
The antiferromagnetic coupling is, dominantly, caused by superexchange
interactions between the magnetic moments of the Mn and the Ni substrate via
the S, C, and O anions. Our findings can be observed from x-ray magnetic
circular dichroism or scanning tunneling microscopy
- …