Chromatin-remodeling enzymes can overcome strong histone-DNA interactions within the nucleosome to regulate access of DNA-binding factors to the genetic code. By unzipping individual DNA duplexes, each containing a uniquely positioned nucleosome flanked by long segments of DNA, we directly probed histone-DNA interactions. The resulting disruption-force signatures were characteristic of the types and locations of interactions and allowed measurement of the positions of nucleosomes with 2.6-base-pair (bp) precision.

The kinetics and mechanisms of transcription are now being investigated by a repertoire of single-molecule techniques, including optical and magnetic tweezers, high-sensitivity fluorescence techniques, and atomic force microscopy.

Vitrification of aqueous cryoprotectant mixtures is essential in cryopreservation of proteins and other biological samples. Systematic measurements of critical cryoprotective agent (CPA) concentrations required for vitrification during plunge-cooling from T = 295 K to T = 77 K in liquid nitrogen are reported.

We have fabricated longitudinal nanoconstrictions in the charge-density wave conductor (CDW) NbSe3 using a focused ion beam and using a mechanically controlled break-junction technique. Conductance peaks are observed below the TP1=145K and TP2=59K CDW transitions, which correspond closely with previous values of the full CDW gaps 2Δ1 and 2Δ2 obtained from photoemission. These results can be explained by assuming CDW-CDW tunneling in the presence of an energy gap corrugation 2 comparable to Δ2, which eliminates expected peaks at ±|Δ1+Δ2|.

The formation of dislocations is central to our understanding of yield, work hardening, fracture, and fatigue1 of crystalline materials. While dislocations have been studied extensively in conventional materials, recent results have shown that colloidal crystals offer a potential model system for visualizing their structure and dynamics directly in real space 2. Although thermal fluctuations are thought to play a critical role in the nucleation of these defects, it is difficult to observe them directly.

We have probed the effects of transverse variations in pinning strength on charge-density-wave (CDW) structure in NbSe3 by x-ray micro-beam diffraction. In ribbonlike crystals having a large longitudinal step in thickness, the CDW first depins on the thick side of the step, causing rotations of the CDW wave vector. By measuring these rotations as a function of position and electric field, the corresponding shear strains are determined, allowing the CDW's shear modulus to be estimated.

We study the effects of the electronic coupling to bosonic modes on scanning tunneling microscopy (STM) into a d -wave superconductor. We propose to investigate these effects by means of a different technique: a Fourier transformed inelastic electron tunneling spectroscopy (FT-IETS). Specifically, in this technique, the Fourier spectrum of the energy derivative local density of states is addressed, which is proportional to the (d2 I d V2) (q,eV) characteristics measured in FT-IETS STM.

We present measurements of the forces on, and displacements of, an optically trapped bead along the propagation direction of the trapping laser beam (the axial direction). In a typical experimental configuration, the bead is trapped in an aqueous solution using an oil-immersion, high-numerical-aperture objective. This refractive index mismatch complicates axial calibrations due to both a shift of the trap center along the axial direction and spherical aberrations.

We discuss current- and noise-spectroscopy of small molecules weakly coupled to electrodes using master equations. The Coulomb interaction and size quantization effect on the molecule restrict the transport to the tunneling of single electrons. We consider situations where orbital-, spin- or vibrational excitations localized on the molecule play a role or even dominate the transport. For each case, we analyze mechanisms which lead to negative differential conductance (NDC) and even total suppression of the current.

The humble current-voltage (I-V) measurement has proven to be an extremely powerful probe of the physics of condensed matter systems from bulk semiconductors and superconductors to quantum dots and nanotubes. However, doing these deceptively simple measurements "right" so as to unambiguously extract particular bits of physics is hard. This is especially true of charge and spin density wave (CDW and SDW) conductors and related collective transport systems, in part because of the tremendous richness of their physics.