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

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

Ultrafast-pulse diagnostic using third-order frequency-resolved optical gating in organic films

We report on the diagnostic of ultrafast pulses by frequency-resolved optical gating (FROG) based on strong third-harmonic generation (THG) in amorphous organic thin films. The high THG conversion efficiency of these films allows for the characterization of sub-nanojoule short pulses emitting at telecommunication wavelengths using a low cost portable fiber spectrometer

Third-order optical autocorrelator for time-domain operation at telecommunication wavelengths

We report on amorphous organic thin films that exhibit efficient third-harmonic generation at telecommunication wavelengths. At 1550 nm, micrometer-thick samples generate up to 17 µW of green light with input power of 250 mW delivered by an optical parametric oscillator. This high conversion efficiency is achieved without phase matching or cascading of quadratic nonlinear effects. With these films, we demonstrate a low-cost, sensitive third-order autocorrelator that can be used in the time-frequency domain

Cracks Cleave Crystals

The problem of finding what direction cracks should move is not completely solved. A commonly accepted way to predict crack directions is by computing the density of elastic potential energy stored well away from the crack tip, and finding a direction of crack motion to maximize the consumption of this energy. I provide here a specific case where this rule fails. The example is of a crack in a crystal. It fractures along a crystal plane, rather than in the direction normally predicted to release the most energy. Thus, a correct equation of motion for brittle cracks must take into account both energy flows that are described in conventional continuum theories and details of the environment near the tip that are not.Comment: 6 page

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

Elastic forces that do no work and the dynamics of fast cracks

Elastic singularities such as crack tips, when in motion through a medium that is itself vibrating, are subject to forces orthogonal to the direction of motion and thus impossible to determine by energy considerations alone. This fact is used to propose a universal scenario, in which three dimensionality is essential, for the dynamic instability of fast cracks in thin brittle materials.Comment: 8 pages Latex, 1 Postscript figur

Necessary and sufficient condition for longitudinal magnetoresistance

Since the Lorentz force is perpendicular to the magnetic field, it should not affect the motion of a charge along the field. This argument seems to imply absence of longitudinal magnetoresistance (LMR) which is, however, observed in many materials and reproduced by standard semiclassical transport theory applied to particular metals. We derive a necessary and sufficient condition on the shape of the Fermi surface for non-zero LMR. Although an anisotropic spectrum is a pre-requisite for LMR, not all types of anisotropy can give rise to the effect: a spectrum should not be separable in any sense. More precisely, the combination $k_{\rho}v_{\phi}/v_{\rho}$, where $k_\rho$ is the radial component of the momentum in a cylindrical system with the z-axis along the magnetic field and $v_{\rho} (v_{\phi}$) is the radial (tangential) component of the velocity, should depend on the momentum along the field. For some lattice types, this condition is satisfied already at the level of nearest-neighbor hopping; for others, the required non-separabality occurs only if next-to-nearest-neighbor hopping is taken into account.Comment: 7 pages, 2 figure