David Roundy

A tunable carbon nanotube electromechanical oscillator

Nanoelectromechanical systems (NEMS) hold promise for a number of scientific and technological applications. In particular, NEMS oscillators have been proposed for use in ultrasensitive mass detection, radio-frequency signal processing, and as a model system for exploring quantum phenomena in macroscopic systems. Perhaps the ultimate material for these applications is a carbon nanotube. They are the stiffest material known, have low density, ultrasmall cross-sections and can be defect-free. Equally important, a nanotube can act as a transistor and thus may be able to sense its own motion. In spite of this great promise, a room-temperature, self-detecting nanotube oscillator has not been realized, although some progress has been made. Here we report the electrical actuation and detection of the guitar-string-like oscillation modes of doubly clamped nanotube oscillators. We show that the resonance frequency can be widely tuned and that the devices can be used to transduce very small forces.

Improving accuracy by subpixel smoothing in the finite-difference time domain

Finite-difference time-domain (FDTD) methods suffer from reduced accuracy when modeling discontinuous dielectric materials, due to the inhererent discretization (pixelization). We show that accuracy can be significantly improved by using a subpixel smoothing of the dielectric function, but only if the smoothing scheme is properly designed. We develop such a scheme based on a simple criterion taken from perturbation theory and compare it with other published FDTD smoothing methods. In addition to consistently achieving the smallest errors, our scheme is the only one that attains quadratic convergence with resolution for arbitrarily sloped interfaces. Finally, we discuss additional difficulties that arise for sharp dielectric corners.

The ideal strength of tungsten

Abstract Using pseudopotential density functional theory within the local-density approximation, we calculate the ideal shear strengths of W for slip on {110}, {112} and {123} planes allowing for complete structural relaxation orthogonal to the applied shear. The strengths in the weak directions on all planes are found to be very nearly equal (about 18GPa, or 11% of the shear modulus G). Moreover, the shear instability occurs at approximately the same applied shear strain (17–18%). This unusual isotropy is explained in terms of the atomic configurations of high-energy saddle points reached during shear. Analysis of these saddle points may also offer a simple explanation for the prevalence of the pencil glide of dislocations on planes containing a (111) direction in bcc metals. Finally, we calculate the ideal cleavage strengths of W on {100} and compare our calculated ideal shear and cleavage strengths with experimental nanoindentation and whisker measurements. All these results can be rather simply understood using a Frenkel–Orowan crystallographic model.

First-principles calculation of the superconducting transition in MgB2 within the anisotropic Eliashberg formalism

We present a study of the superconducting transition in MgB2 using the ab initio pseudopotential density-functional method, a fully anisotropic Eliashberg equation, and a conventional estimate for mu*. Our study shows that the anisotropic Eliashberg equation, constructed with ab initio calculated momentum-dependent electron-phonon interaction and anharmonic phonon frequencies, yields an average electron-phonon coupling constant lambda = 0.61, a transition temperature T-sub c = 39 K, and a boron isotope-effect exponent alpha-sub B = 0.32. The calculated values for T-sub c, lambda, and alpha-sub B are in excellent agreement with transport, specific-heat,and isotope-effect measurements, respectively. The individual values of the electron-phonon coupling lambda (k,k-prime) on the various pieces of the Fermi surface, however, vary from 0.1 to 2.5. The observed T-sub c is a result of both the raising effect of anisotropy in the electron-phonon couplings and the lowering effect of anharmonicity in the relevant phonon modes.

Ideal strengths of bcc metals

Abstract We present ab initio ideal strength calculations in body-centered cubic (bcc) tungsten for ‘pencil-glide’ slip on {110}, {112}, and {123} planes and for the {100} cleavage strength. We use these results to analyze the tensile and shear strengths of other bcc metals. In all bcc metals, the minimum shear strength on any plane containing 〈111〉 is ≈0.11/S〈111〉, where S〈111〉 is the elastic compliance for any shear in a 〈111〉 direction. The ideal cleavage strength on {100} for many bcc metals is ≈0.083/s11 where s11 is the single crystal elastic compliance. Comparison of the ideal shear and tensile strengths offers a measure of the inherent ductility or brittleness of a material.

Elastic effects of vacancies in strontium titanate: Short- and long-range strain fields, elastic dipole tensors, and chemical strain

We present a study of the local strain effects associated with vacancy defects in strontium titanate and report the first calculations of elastic dipole tensors and chemical strains for point defects in perovskites. The combination of local and long-range results will enable determination of x-ray scattering signatures that can be compared with experiments. We find that the oxygen vacancy possesses a special property -- a highly anisotropic elastic dipole tensor which almost vanishes upon averaging over all possible defect orientations. Moreover, through direct comparison with experimental measurements of chemical strain, we place constraints on the possible defects present in oxygen-poor strontium titanate and introduce a conjecture regarding the nature of the predominant defect in strontium-poor stoichiometries in samples grown via pulsed laser deposition. Finally, during the review process, we learned of recent experimental data, from strontium titanate films deposited via molecular-beam epitaxy, that show good agreement with our calculated value of the chemical strain associated with strontium vacancies.

Modeling a Suspended Nanotube Oscillator

We present a general study of oscillations in suspended one-dimensional elastic systems clamped at each end, exploring a wide range of slack (excess length) and downward external forces. Our results apply directly to recent experiments in nanotube and silicon nanowire oscillators. We find the behavior to simplify in three well-defined regimes which we present in a dimensionless phase diagram. The frequencies of vibration of such systems are found to be extremely sensitive to slack.

Microcavity confinement based on an anomalous zero group-velocity waveguide mode.

We propose and demonstrate a mechanism for small-modal-volume high-Q cavities based on an anomalous uniform waveguide mode that has zero group velocity at a nonzero wave vector. In a short piece of a uniform waveguide with a specially designed cross section, light is confined longitudinally by small group-velocity propagation and transversely by a reflective cladding. The quality factor Q is greatly enhanced by the small group velocity for a set of cavity lengths that are separated by approximately pi/k0, where k0 is the longitudinal wave vector for which the group velocity is zero.

Darcs: distributed version management in haskell

A common reaction from people who hear about darcs, the source control system I created, is that it sounds like a great tool, but it is a shame that it is written in Haskell. People think that because darcs is written in Haskell it will be a slow memory hog with very few contributors to the project. I will give a somewhat historical overview of my experiences with the Haskell language, libraries and tools.I will begin with a brief overview of the darcs advanced revision control system, how it works and how it differs from other version control systems. Then I will go through various problems and successes I have had in using the Haskell language and libraries in darcs, roughly in the order I encountered them. In the process I will give a bit of a tour through the darcs source code. In each case, I will tell about the problem I wanted to solve, what I tried, how it worked, and how it might have worked better (if that is possible).

The non-linear elastic behavior and ideal shear strength of Al and Cu

Abstract We present recent ab initio calculations of the ideal shear strengths of aluminum and copper using pseudopotential density functional theory within the local density approximation. Structural relaxations orthogonal to the applied shear significantly reduce the values of ideal shear strength, resulting in strengths of 8–9% of the shear modulus for both Al and Cu. However, the geometry of the relaxations in Al and Cu is very different. To some degree, this can be explained using experimentally measured third-order elastic constants.