Using a catalog of seismicity for Southern California, we measure how the number of triggered earthquakes increases with the earthquake magnitude. The trade-off between this relation and the distribution of earthquake magnitudes controls the relative role of small compared to large earthquakes. We show that seismicity triggering is driven by the smallest earthquakes, which trigger fewer events than larger earthquakes, but which are much more numerous. We propose that the nontrivial scaling of the number of triggered earthquakes emerges from the fractal spatial distribution of seismicity.

Large increases in the upper critical field $B_{C2}(0)$ are reported in bulk superconductors that demonstrate another novel property for nanocrystalline materials. Disordered nanocrystalline ${\mathrm{P}\mathrm{b}\mathrm{M}\mathrm{o}}_6{\mathrm{S}}_8$ superconductors were fabricated by mechanical milling and hot isostatic pressing. Conventional ${\mathrm{P}\mathrm{b}\mathrm{M}\mathrm{o}}_6{\mathrm{S}}_8$ has $B_{C2}(0)\sim 50\mathrm{T}$. The nanocrystalline materials have higher resistivity (${\rho }_N$) and $B_{C2}(0)\sim 100\mathrm{T}$. The disorder produced in these nanocrystalline materials is significantly different from that produced by doping because it increases ${\rho }_N$ and, hence, $B_{C2}(0)$ without significantly reducing the electronic density of states or superconducting transition temperature ($T_C$). Furthermore, the disorder reduces the electron mean-free path to $\sim 1\mathrm{n}\mathrm{m}$ which is more than an order of magnitude smaller than the grain size and necessary to achieve the unprecedented increase in $B_{C2}(0)$.

It is known empirically that multivalent ions generate attractions between like-charged polyelectrolytes, with different valence requirements for different systems. How multivalent must an ion be before it can condense a given polyelectrolyte? Using charge-tunable $M13$ virus rods and a family of artificial homologous “dumbbell” divalent ions of different sizes, we have constructed a multivalent ion-polyelectrolyte phase diagram, and find an experimentally motivated general criterion for like-charged attraction based on the ion valence, ion size, and the Gouy-Chapman length.

Based on the idea that the hardness of covalent crystal is intrinsic and equivalent to the sum of the resistance to the indenter of each bond per unit area, a semiempirical method for the evaluation of hardness of multicomponent crystals is presented. Applied to $\beta \mathrm{\text{−}}{\mathrm{B}\mathrm{C}}_2\mathrm{N}$ crystal, the predicted value of hardness is in good agreement with the experimental value. It is found that bond density or electronic density, bond length, and degree of covalent bonding are three determinative factors for the hardness of a polar covalent crystal. Our method offers the advantage of applicability to a broad class of materials and initializes a link between macroscopic property and electronic structure from first principles calculation.

The vibrational modes in three-dimensional ordered arrays of unembedded ${\mathrm{S}\mathrm{i}\mathrm{O}}_2$ nanospheres have been studied by Brillouin light scattering. Multiple distinct Brillouin peaks are observed whose frequencies are found to be inversely proportional to the diameter ($\approx 200–340\mathrm{n}\mathrm{m}$) of the nanospheres, in agreement with Lamb’s theory. This is the first Brillouin observation of acoustic mode quantization in a nanoparticle arising from spatial confinement. The distinct spectral peaks measured afford an unambiguous assignment of seven surface and inner acoustic modes. Interestingly, the relative intensities and polarization dependence of the Brillouin spectrum do not agree with the predictions made for Raman scattering.

Two-dimensional hydrodynamic simulations of stellar core collapse are presented which for the first time were performed by solving the Boltzmann equation for the neutrino transport including a state-of-the-art description of neutrino interactions. Stellar rotation is also taken into account. Although convection develops below the neutrinosphere and in the neutrino-heated region behind the supernova shock, the models do not explode. This suggests missing physics, possibly with respect to the nuclear equation of state and weak interactions in the subnuclear regime. However, it might also indicate a fundamental problem with the neutrino-driven explosion mechanism.

In this work we propose a ratchet effect which provides a general means of performing clocked logic operations on discrete particles, such as single electrons or vortices. The states are propagated through the device by the use of an applied ac drive. We numerically demonstrate that a complete logic architecture is realizable using this ratchet. We consider specific nanostructured superconducting geometries using superconducting materials under an applied magnetic field, with the positions of the individual vortices in samples acting as the logic states. These devices can be used as the building blocks for an alternative microelectronic architecture.

The tangential motion at the contact of two solid objects is studied. It consists of a sliding and a spinning degree of freedom (no rolling). We show that the friction force and torque are inherently coupled. As a simple test system, a sliding and spinning disk on a horizontal flat surface is considered. We calculate, and also measure, how the disk slows down and find that it always stops its sliding and spinning motion at the same moment. We discuss the impact of this coupling between friction force and torque on the physics of granular materials.

We show how a carefully chosen combination of strong linearly and circularly polarized laser fields can bind two same-sign charges, not only suppressing their Coulomb repulsion in all three spatial dimensions, but also creating an effective attraction. As an example, we show how a molecule ${\mathrm{H}\mathrm{D}}^{2+}$ stripped of both electrons can be kept bound by the laser fields.

Quantum interference of one- and two-photon excitation of unbiased semiconductors yields ballistic currents of carriers. The magnitudes and directions of the currents and the spin orientations of the carriers are controlled by the polarization and relative phase of the exciting femtosecond laser fields. We provide direct experimental evidence for the spin polarization of the optically injected spin currents by detecting a phase-dependent spatial shift of the circularly polarized photoluminescence in cubic ZnSe.

We provide the first evidence that stochastic resonance within the human brain can enhance behavioral responses to weak sensory inputs. We asked subjects to adjust handgrip force to a slowly changing, subthreshold gray level signal presented to their right eye. Behavioral responses were optimized by presenting randomly changing gray levels separately to the left eye. The results indicate that observed behavioral stochastic resonance was mediated by neural activity within the human brain where the information from both eyes converges.

Simultaneous observations of the solar atmosphere from its surface to the corona obtained with the Solar and Heliospheric Observatory (SOHO) and Transition Region and Coronal Explorer (TRACE) show a ubiquitous sequence of events that start from cancellation of photospheric magnetic fields, pass through shock formation, and result in transition region supersonic jets and microflares. These results support a novel view of the energy buildup in the solar atmosphere associated with a cascade of shock waves produced by interacting network magnetic elements in the photosphere and provide insight into the origin of the solar transition region. The findings account for the general mechanisms of energy production, transfer, and release throughout the Sun’s and stellar atmospheres.

The perpendicular critical fields of a superconducting film have been strongly enhanced by using a nanoengineered lattice of magnetic dots (dipoles) on top of the film. Magnetic-field-induced superconductivity is observed in these hybrid superconductor/ferromagnet systems due to the compensation of the applied field between the dots by the stray field of the dipole array. By switching between different magnetic states of the nanoengineered field compensator, the critical parameters of the superconductor can be effectively controlled.

We report on craters formed by balls dropped into dry, noncohesive, granular media. By explicit variation of ball density ${\rho }_b$, diameter $D_b$, and drop height $H$, the crater diameter is confirmed to scale as the $1/4$ power of the energy of the ball at impact: $D_c\sim ({\rho }_b{D}_b^{3}H{)}^{1/4}$. Against expectation, a different scaling law is discovered for the crater depth: $d\sim ({\rho }_b^{3/2}{D}_b^{2}H{)}^{1/3}$. The scaling with properties of the medium is also established. The crater depth has significance for granular mechanics in that it relates to the stopping force on the ball.

We observe experimentally that particle systems that would otherwise mix can be made to segregate and vice versa simply by adding moisture. Using a newly developed theoretical approach, we generate phase diagrams that exhibit both mixed and segregated phases and show how the location of phase boundaries may be manipulated via modifying the mechanical and surface properties of the particles. These results have implications for industrial mixing/separation processes as well as novel particle production methods (e.g., engineered agglomerates with precisely prescribed compositions).

A near contact atomic force microscope operated at low-temperature is used for vertical manipulation of selected single atoms from the $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}(7\times 7)$ surface. The strong repulsive short-range chemical force interaction between the closest atoms of both tip apex and surface during a soft nanoindentation leads to the removal of a selected silicon atom from its equilibrium position at the surface without additional perturbation of the $(7\times 7)$ unit cell. Deposition of a single atom on a created vacancy at the surface is achieved as well. These manipulation processes are purely mechanical, since neither bias voltage nor voltage pulse is applied between probe and sample. Differences in the mechanical response of the two nonequivalent adatoms of the $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}(7\times 7)$ with the load applied is also detected.

The atomic level structure of the element fermium was investigated for the first time using a sample of $2.7\times {10}^{10}$ atoms of the isotope ${}^{255}\mathrm{F}\mathrm{m}$ with a half-life of 20.1 h. The atoms were evaporated from a filament and stored in the argon buffer gas of an optical cell. Atomic levels were sought by the method of resonance ionization spectroscopy using an excimer-dye-laser combination. Two atomic levels were found at wave numbers $(25099.8\pm 0.2)$ and $(25111.8\pm 0.2){\mathrm{c}\mathrm{m}}^{-1}$. Partial transition rates to the $5f^{12}7s^2$ ${}^{3}H_6^e$ ground state have been determined from their saturation characteristics. Multiconfiguration Dirac-Fock calculations suggest that the leading orders of these levels could be the $5f^{12}7s7p$ ${}^{5}I_6^o$ and $5f^{12}7s7p$ ${}^{5}G_5^o$ terms.

We present a theoretical foundation for the beaming of light displayed by a single subwavelength aperture in an appropriately corrugated metal film [H. J. Lezec et al., Science 297, 820 (2002)]. Good agreement is found between calculations and experimental data. We show that beaming is due to the formation of electromagnetic surface resonances and that the beam direction, width, and wavelength at which it occurs can be selected by tuning geometrical parameters of the structure.

We propose a scheme to observe strongly correlated fermionic phenomena with bosonic atoms in optical lattices. For different values of the sign and strength of the scattering lengths, it is possible to reach either a “superconducting” regime, where the system exhibits atomic pairing, or a Luttinger liquid behavior. We identify the range of parameters where these phenomena appear, illustrate our predictions with numerical calculations, and show how to detect the presence of pairing.

We show that the band structure of a carbon nanotube (NT) can be dramatically altered by mechanical strain. We employ an atomic force microscope tip to simultaneously vary the NT strain and to electrostatically gate the tube. We show that strain can open a band gap in a metallic NT and modify the band gap in a semiconducting NT. Theoretical work predicts that band gap changes can range between $\pm 100\mathrm{m}\mathrm{e}\mathrm{V}$ per $1\%$ stretch, depending on NT chirality, and our measurements are consistent with this predicted range.