When using Hartree-Fock (HF) trial wave functions in quantum Monte Carlo calculations, one faces, in case of HF instabilities, the HF symmetry dilemma in choosing between the symmetry-adapted solution of higher HF energy and symmetry-broken solutions of lower HF energies. In this work, we have examined the HF symmetry dilemma in hydrogen rings which present singlet instabilities for sufficiently large rings. We have found that the symmetry-adapted HF wave function gives a lower energy both in variational Monte Carlo and in fixed-node diffusion Monte Carlo.

X-ray free-electron lasers have enabled new approaches to the structural determination of protein crystals that are too small or radiation-sensitive for conventional analysis. For sufficiently short pulses, diffraction is collected before significant changes occur to the sample, and it has been predicted that pulses as short as 10 fs may be required to acquire atomic-resolution structural information.

A maple seed falls in a characteristic helical motion. A crude analogy with autorotation of a wind turbine suggests that the torque due to the aerodynamic force would initiate the gyration of the seed. We were therefore surprised that a seed with a torn wing gyrates in a similar manner as a full-winged seed. In fact, a seed with only a sliver of leading edge can still gyrate. Thus the gyrating motion appears not to fully depend on the aerodynamic force. If, on the other hand, the aerodynamic force is completely absent, a seed would fall from rest like a rock in a vacuum.

DNA unzipping is a powerful tool to study protein-DNA interactions at the single-molecule level. In this chapter, we provide a detailed and practical guide to performing this technique with an optical trap, using nucleosome studies as an example. We detail protocols for preparing an unzipping template, constructing and calibrating the instrument, and acquiring, processing, and analyzing unzipping data. We also summarize major results from utilization of this technique for the studies of nucleosome structure, dynamics, positioning, and remodeling. © 2012 Elsevier Inc.

Motivated by recent experiments on the finite temperature Mott transition in VO 2 films, we propose a classical coarse-grained dielectric breakdown model where each degree of freedom represents a nanograin which transitions from insulator to metal with increasing temperature and voltage at random thresholds due to quenched disorder. We describe the properties of the resulting nonequilibrium metal-insulator transition and explain the universal characteristics of the resistance jump distribution.

Proteins are known to undergo a dynamical transition at around 200 K but the underlying mechanism, physical origin, and relationship to water are controversial. Here we report an observation of a protein dynamical transition as low as 110 K. This unexpected protein dynamical transition precisely correlated with the cryogenic phase transition of water from a high-density amorphous to a low-density amorphous state. The results suggest that the cryogenic protein dynamical transition might be directly related to the two liquid forms of water proposed at cryogenic temperatures.

We have fabricated large (≤400 μm diameter) high tensile stress SiN membrane mechanical resonators and measured the resonant frequency and quality factors (Q) of different modes of oscillation using optical interferometric detection. We observe that the dissipation (Q -1) is limited by clamping loss for pure radial modes, but higher order azimuthal modes are limited by a mechanism which appears to be intrinsic to the material. The observed dissipation is strongly dependent on size of the membrane and mode type.

We develop the analysis of x-ray intensity correlations from dilute ensembles of identical particles in a number of ways. Firstly, we show that the three-dimensional (3D) particle structure can be determined if the particles can be aligned with respect to a single axis having a known angle with respect to the incident beam. Secondly, we clarify the phase problem in this setting and introduce a data reduction scheme that assesses the integrity of the data even before particle reconstruction is attempted.

We calculate the vortex structures of an elongated two-component Bose-Einstein condensate. We study how these structures depend on the intracomponent and intercomponent interaction strengths. We present analytical and numerical results respectively at weak and strong interactions; finding lattices with different interlocking geometries: triangular, square, rectangular, and double core. © 2011 American Physical Society.

An alternative measure of X-ray absorption spectroscopy (XAS) called inverse partial fluorescence yield (IPFY) has recently been developed that is both bulk sensitive and free of saturation effects. Here we show that the angle dependence of IPFY can provide a measure directly proportional to the total X-ray absorption coefficient, μ(E). In contrast, fluorescence yield (FY) and electron yield (EY) spectra are offset and/or distorted from μ(E) by an unknown and difficult to measure amount.