Detection of non-Gaussian Fluctuations in a Quantum Point Contact

An experimental study of current fluctuations through a tunable transmission barrier, a quantum point contact, are reported. We measure the probability distribution function of transmitted charge with precision sufficient to extract the first three cumulants. To obtain the intrinsic quantities, corresponding to voltage-biased barrier, we employ a procedure that accounts for the response of the external circuit and the amplifier. The third cumulant, obtained with a high precision, is found to agree with the prediction for the statistics of transport in the non-Poissonian regime.Comment: 4 pages, 4 figures; published versio

Measurement of Counting Statistics of Electron Transport in a Tunnel Junction

We present measurements of the time-dependent fluctuations in electrical current in a voltage-biased tunnel junction. We were able to simultaneously extract the first three moments of the tunnel current counting statistics. Detailed comparison of the second and the third moment reveals that counting statistics is accurately described by the Poissonian distribution expected for spontaneous current fluctuations due to electron charge discreteness, realized in tunneling transport at negligible coupling to environment.Comment: bibliography expande

Colloidal particles at a nematic-isotropic interface: effects of confinement

When captured by a flat nematic-isotropic interface, colloidal particles can be dragged by it. As a result spatially periodic structures may appear, with the period depending on a particle mass, size, and interface velocity~\cite{west.jl:2002}. If liquid crystal is sandwiched between two substrates, the interface takes a wedge-like shape, accommodating the interface-substrate contact angle and minimizing the director distortions on its nematic side. Correspondingly, particles move along complex trajectories: they are first captured by the interface and then `glide' towards its vertex point. Our experiments quantify this scenario, and numerical minimization of the Landau-de Gennes free energy allow for a qualitative description of the interfacial structure and the drag force.Comment: 7 pages, 9 figure

On the Cooling of Electrons in a Silicon Inversion Layer

The cooling of two-dimensional electrons in silicon-metal-oxide semiconductor field effect transistors is studied experimentally. Cooling to the lattice is found to be more effective than expected from the bulk electron-phonon coupling in silicon. Unexpectedly, the extracted heat transfer rate to phonons at low temperatures depends cubically on electron temperature, suggesting that piezoelectric coupling (absent in bulk silicon) dominates over deformation potential. According to our findings, at 100 mK, electrons farther than 0.1 mm from the contacts are mostly cooled by phonons. Using long devices and low excitation voltage we measure electron resistivity down to 100 mK and find that some of the "metallic" curves, reported earlier, turn insulating below about 300 mK. This finding renders the definition of the claimed 2D metal-insulator transition questionable. Previous low temperature measurements in silicon devices are analyzed and thumb rules for evaluating their electron temperatures are provided.Comment: 5 pages, 4 figures. Discussion corrected and a few references adde

Fractal-like hierarchical organization of bone begins at the nanoscale

INTRODUCTION: The components of bone assemble hierarchically to provide stiffness and toughness. Deciphering the specific organization and relationship between bone’s principal components—mineral and collagen—requires answers to three main questions: whether the association of the mineral phase with collagen follows an intrafibrillar or extrafibrillar pattern, whether the morphology of the mineral building blocks is needle- or platelet-shaped, and how the mineral phase maintains continuity across an extensive network of cross-linked collagen fibrils. To address these questions, a nanoscale level of three-dimensional (3D) structural characterization is essential and has now been performed. RATIONALE: Because bone has multiple levels of 3D structural hierarchy, 2D imaging methods that do not detail the structural context of a sample are prone to interpretation bias. Site-specific focused ion beam preparation of lamellar bone with known orientation of the analyzed sample regions allowed us to obtain imaging data by 2D high-resolution transmission electron microscopy (HRTEM) and to identify individual crystal orientations. We studied higher-level bone mineral organization within the extracellular matrix by means of scanning TEM (STEM) tomography imaging and 3D reconstruction, as well as electron diffraction to determine crystal morphology and orientation patterns. Tomographic data allowed 3D visualization of the mineral phase as individual crystallites and/or aggregates that were correlated with atomic-resolution TEM images and corresponding diffraction patterns. Integration of STEM tomography with HRTEM and crystallographic data resulted in a model of 3D mineral morphology and its association with the organic matrix. RESULTS: To visualize and characterize the crystallites within the extracellular matrix, we recorded imaging data of the bone mineral in two orthogonal projections with respect to the arrays of mineralized collagen fibrils. Three motifs of mineral organization were observed: “filamentous” (longitudinal or in-plane) and “lacy” (out-of-plane) motifs, which have been reported previously, and a third “rosette” motif comprising hexagonal crystals. Tomographic reconstructions showed that these three motifs were projections of the same 3D assembly. Our data revealed that needle-shaped, curved nanocrystals merge laterally to form platelets, which further organize into stacks of roughly parallel platelets separated by gaps of approximately 2 nanometers. These stacks of platelets, single platelets, and single acicular crystals coalesce into larger polycrystalline aggregates exceeding the lateral dimensions of the collagen fibrils, and the aggregates span adjacent fibrils as continuous, cross-fibrillar mineralization. CONCLUSION: Our findings can be described by a model of mineral and collagen assembly in which the mineral organization is hierarchical at the nanoscale. First, the data reveal that mineral particles are neither exclusively needle- nor platelet-shaped, but indeed are a combination of both, because curved acicular elements merge laterally to form slightly twisted plates. This can only be detected when the organic extracellular matrix is preserved in the sample. Second, the mineral particles are neither exclusively intrafibrillar nor extrafibrillar, but rather form a continuous cross-fibrillar phase where curved and merging crystals splay beyond the typical dimensions of a single collagen fibril. Third, in the organization of the mineral phase of bone, a helical pattern can be identified. This 3D observation, integrated with previous studies of bone hierarchy and structure, illustrates that bone (as a material, as a tissue, and as an organ) follows a fractal-like organization that is self-affine. The assembly of bone components into nested, helix-like patterns helps to explain the paradoxical combination of enhanced stiffness and toughness of bone and results in an expansion of the previously known hierarchical structure of bone to at least 12 levels

The Evolution of Quasiparticle Charge in the Fractional Quantum Hall Regime

The charge of quasiparticles in a fractional quantum Hall (FQH) liquid, tunneling through a partly reflecting constriction with transmission t, was determined via shot noise measurements. In the nu=1/3 FQH state, a charge smoothly evolving from e*=e/3 for t=1 to e*=e for t<<1 was determined, agreeing with chiral Luttinger liquid theory. In the nu=2/5 FQH state the quasiparticle charge evolves smoothly from e*=e/5 at t=1 to a maximum charge less than e*=e/3 at t<<1. Thus it appears that quasiparticles with an approximate charge e/5 pass a barrier they see as almost opaque.Comment: 4 pages, Correct figure 3 and caption include

Electron shot noise beyond the second moment

The form of electron counting statistics of the tunneling current noise in a generic many-body interacting electron system is obtained. The third correlator of current fluctuations (the skewness of the charge counting distribution) has a universal relation with the current I and the quasiparticle charge q. This relation C_3 = q^2 I holds in a wide bias voltage range, both at large and small eV/kT, thereby representing an advantage compared to the Schottky formula. We consider the possibility of using the counting statistics for detecting quasiparticle charge at high temperature.Comment: 4 pages, 2 figure

Colored noise in the fractional Hall effect: duality relations and exact results

We study noise in the problem of tunneling between fractional quantum Hall edge states within a four probe geometry. We explore the implications of the strong-weak coupling duality symmetry existent in this problem for relating the various density-density auto-correlations and cross-correlations between the four terminals. We identify correlations that transform as either ``odd'' or ``anti-symmetric'', or ``even'' or ``symmetric'' quantities under duality. We show that the low frequency noise is colored, and that the deviations from white noise are exactly related to the differential conductance. We show explicitly that the relationship between the slope of the low frequency noise spectrum and the differential conductance follows from an identity that holds to {\it all} orders in perturbation theory, supporting the results implied by the duality symmetry. This generalizes the results of quantum supression of the finite frequency noise spectrum to Luttinger liquids and fractional statistics quasiparticles.Comment: 14 pages, 3 figure