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Manipulating matter at the nanometre scale is important for many electronic, chemical and biological advances, but present solid-state fabrication methods do not reproducibly achieve dimensional control at the nanometre scale. Here we report a means of fashioning matter at these dimensions that uses low-energy ion beams and reveals surprising atomic transport phenomena that occur in a variety of materials and geometries. The method is implemented in a feedback-controlled sputtering system that provides fine control over ion beam exposure and sample temperature. We call the method "ion-beam sculpting", and apply it to the problem of fabricating a molecular-scale hole, or nanopore, in a thin insulating solid-state membrane. Such pores can serve to localize molecular-scale electrical junctions and switches and function as masks to create other small-scale structures. Nanopores also function as membrane channels in all living systems, where they serve as extremely sensitive electro-mechanical devices that regulate electric potential, ionic flow, and molecular transport across cellular membranes. We show that ion-beam sculpting can be used to fashion an analogous solid-state device: a robust electronic detector consisting of a single nanopore in a Si3N4 membrane, capable of registering single DNA molecules in aqueous solution.

A new monoenergetic MeV positron beam source was designed and constructed at the Lawrence Livermore National Laboratory (LLNL) positron facility. The positron source provides a low emittance beam with high energy resolution for channeling, scattering, and in-flight annihilation experiments. New beam optics for extracting slow positrons from a moderator mounted in a Pelletron electrostatic accelerator is presented. A beam line was designed and constructed to energy analyze the output of the accelerator and to provide adjustable size and angular divergence beams to a target station. A post target large angle acceptance magnet separates positrons transmitted through the target from forward gamma ray emission.

As a step towards developing fast positrons as a probe to measure electron density and spin magnetic density in crystals, an experiment has been performed to measure the multiple scattering distribution of MeV positrons penetrating through thin polycrystalline films in both random and channeling directions. Image plate detectors were used to determine the angular distribution of emergent positrons. Image plate sensitivity was measured for high-energy positrons and for the background radiation encountered in our experiment. The image plates showed good sensitivity and good linearity for MeV positrons. The multiple scattering results are compared with the theory for random direction and show disagreement due to crystal effects and quantum mechanical considerations.

Energy-resolved two-quantum annihilation in flight of positrons with energies ranging from 10 to 71.6 keV was observed. An energy-dispersive two-detector coincidence system was used to observe the sum and difference energies of the {gamma} rays from annihilating positron-electron pairs. For positrons penetrating carbon foils the c/v dependence of the annihilation cross section is confirmed. Spectra obtained from gold-coated carbon foils show evidence of in-flight annihilation with gold M-shell electrons. (c) 1999 The American Physical Society.

Energetic, positively charged particles travelling along a low-index crystal direction undergo many highly correlated, small-angle scattering events; the effect of these interactions is to guide or ‘channel’ (refs 1,2,3,4,5,6,7,8) the particles through the lattice. Channelling effectively focuses positive particles into the interstitial regions of the crystal: nuclear collisional processes such as Rutherford backscattering are suppressed, while the number of interactions with valence electrons increases. The interaction of channelled positrons with electrons produces annihilation radiation that can in principle9,10,11,12 serve as a quantitative, spatially selective probe of electronic charge and spin densities within the crystal: in the interstitial regions, two-photon annihilation is enhanced relative to single-photon annihilation, because the latter process requires a nuclear recoil to conserve momentum. Here we report observations of single- and two-photon annihilation from channelled positrons, using a monoenergetic beam flux of 105 particles per second. Comparison of these two annihilation modes demonstrates the ability of channelled positrons to selectively sample valence electrons in a crystal. Useful practical implementation of the technique will require the development of more intense positron beams with fluxes approaching 107 particles per second.

We report on the first direct energy resolved measurements of two quantum annihilation in flight (TQAF) using monoenergetic positrons below 100 keV. A monoenergetic positron beam at Washington State University with variable kinetic energy up to 70 keV was guided onto thin aluminum targets. To clearly identify in flight positron annihilation a two dimensional Doppler broadening technique was employed utilizing two HPGe detectors. The method uniquely identifies TQAF by providing the total energy of the annihilating electron-positron pair. A basic understanding of the fundamental TQAF process has been obtained using this high resolution technique. With this fundamental understanding we believe several new avenues of experimental exploration are possible.

X-ray bound states trapped on a curved surface are realized by coupling subangstrom photons at grazing angles into a bent silicon wafer with adjustable radius. At small radii of curvature the usual ray approximation breaks down and new phenomena associated with wave optical behavior of x-ray surface eigenmodes are revealed. For such samples, tunneling is shown to extinguish all but the ground state surface eigenmode, whose presence may be detected after extremely long distances of propagation along the surface.

The subtleties of the interplay between classical and quantum pictures are revealed particularly clearly in connection with the effect of channeling. The role of this interplay is described and related to a new probe for valence electron and spin magnetic densities in thin films, which is based upon two-photon annihilation in flight of channeled MeV positrons. The channeling effect is used to steer positrons into the interstices of a crystal, where the valence electron density is high. Transmission measurements have been performed to test our quantitative understanding of the spatial and momentum distribution of channeled positrons. We calculate annihilation rates in a gold crystal and show that the probe is highly sensitive to valence electrons. Channeling states in iron are shown to be sensitive to different regions of the spin magnetic density. With a polarized beam of MeV positrons and the possibility for selective state population, we show that the spatial distribution of the magnetization density can be probed.

Using the x-ray standing-wave method, we have measured directly the thermal vibration amplitudes [l angle][ital u][sup 2][r angle] of symmetric As dimers on a Si(001)-(2[times]1) surface. For sample temperatures in the range 300 K [le][ital T][le]650 K, the results are Debye-like. Above 650 K [l angle][ital u][sup 2][r angle] varies more rapidly, indicating the onset of defect-mediated processes. We also found that at room temperature the bond length of the As dimers is 2.58[plus minus]0.04 A and that, independent of temperature, they sit 1.40[plus minus]0.01 A above the top bulk-extrapolated silicon (004) plane. These results provide a critical test for theoretical structure calculations.

Nanopores fabricated in free-standing amorphous silicon thin films were observed to close under 3 keV argon ion irradiation. The closing rate, measured in situ, exhibited a memory effect: at the same instantaneous radius, pores that started larger close more slowly. An ion-stimulated viscous flow model is developed and solved in both a simple analytical approximation for the small-deformation limit and in a finite element solution for large deformations. The finite-element solution exhibits surprising changes in cross-section morphology, which may be extremely valuable for single biomolecule detection, and are untested experimentally. The finite-element solution reproduces the shape of the measured nanopore radius versus fluence behavior and the sign and magnitude of the measured memory effect. We discuss aspects of the experimental data not reproduced by the model, and successes and failures of the competing adatom diffusion model.

Properly fashioned electromagnetic fields coupled to microscopic dielectric objects can be used to create arrays of extended crystalline and noncrystalline structures. Organization can be achieved in two ways: In the first, dielectric matter is transported in direct response to the externally applied standing wave optical fields. In the second, the external optical fields induce interactions between dielectric objects that can also result in the creation of complex structures. In either case, these new ordered structures, whose existence depends on the presence of both light and polarizable matter, are referred to as optical matter.

We present two new x-ray optical effects observed on a whispering-gallery waveguide. The first is a gradual in-surface deflection of guided waves caused by the non-Euclidean intrinsic curvature of the waveguide surface. The second is the excitation of in-surface dynamical diffraction modes that reflects the strong influence of the surface photonic band structure of a crystalline waveguide. Both phenomena portend a rich array of basic and applied x-ray optical potentialities.

We have considered the possibility of creating a matter-antimatter Bose-Einstein condensate in the laboratory and concluded that a committed and focussed program could succeed in making this new “extraordinary” material available for scientific study. In this paper we describe the experimental requirements of the scheme and the mechanisms by which a positronium Bose-Einstein condensate may be both formed and observed.

We suggest the binding of neutral atoms to a current carrying wire through the interaction between the atomic magnetic dipole moment and the wire's magnetic field. The theoretical description is based upon an extension of the concept of supersymmetry to multicomponent wave functions. A solution for spin $\frac{1}{2}$ particles is obtained directly in coordinate space. Spin 1 particles are considered as well. Experimentally, the system should be immediately realizable for 25 mK sodium atoms around a wire with a diameter of $0.5\ensuremath{\mu}\mathrm{m}$ and a current of 400 mA.

We present a combined scanning tunneling microscopy and ballistic electron emission microscopy study of electron transport across an epitaxial Pb/Si(111) interface. Experiments with a self-assembled Pb nanoscale wedge reveal the phenomenon of confinement-enhanced interfacial transport: a proportional increase of the electron injection rate into the semiconductor with the frequency of electron oscillations in the Pb quantum well.

We present quantitative measurements of the spatial density profile of Bose-Einstein condensates of sodium atoms confined in a new '4D' magnetic bottle. The condensates are imaged in transmission with near resonant laser light. We demonstrate that the Thomas-Fermi surface of a condensate can be determined to better than 1%. More generally, we obtain excellent agreement with mean-field theory. We conclude that precision measurements of atomic scattering lengths and interactions between phase separated cold atoms in a harmonic trap can be measured with high precision using this method., 15 pages, 3 figures. Submitted 10/30/97, accepted for publication in Phys. Rev. A Rapid Comm

We consider a waveguide for cold neutral atoms with a magnetic moment proportional to their spin angular momentum. The waveguide consists of a thin wire carrying a constant current. For the spin-1 case, we find bound states for the two-dimensional part of the motion around the wire, and present numerical and approximate analytic results for these bound states. In an experiment, the bound states can decay due to various effects, and we calculate the time scales involved.

A high resolution angular study of positrons transmitted through a thin single crystal of Si clearly reveals a detailed fine structure due to strong quantum channeling effects. The beam transmitted in the forward direction displays many features associated with dynamical diffraction effects and long coherence lengths. Calculations are presented showing that in flight annihilation of channeled positrons can serve as a solid state probe of electron and spin densities in thin crystals.

Channeling effects have been observed for 1.1 GeV/ c protons and pions transmitted through a 1 mm thick germanium single crystal.

We report the use of MeV ion-irradiation-induced plastic deformation of amorphous materials to fabricate electrodes with nanometer-sized gaps. Plastic deformation of the amorphous metal Pd(80)Si(20) is induced by 4.64 MeV O(2+) ion irradiation, allowing the complete closing of a sub-micrometer gap. We measure the evolving gap size in situ by monitoring the field emission current-voltage (I-V) characteristics between electrodes. The I-V behavior is consistent with Fowler-Nordheim tunneling. We show that using feedback control on this signal permits gap size fabrication with atomic-scale precision. We expect this approach to nanogap fabrication will enable the practical realization of single molecule controlled devices and sensors.

We report a material-dependent critical temperature for ion beam sculpting of nanopores in amorphous materials under keV ion irradiation. At temperatures below the critical temperature, irradiated pores open at a rate that soon saturates with decreasing temperature. At temperatures above the critical temperature, the pore closing rate rises rapidly and eventually saturates with increasing temperature. The observed behavior is well described by a model based on adatom diffusion, but is difficult to reconcile with an ion-stimulated viscous flow model.

We argue that it is possible to bind a neutral atom in stable orbits around a wire charged by a time-varying sinusoidal voltage. Both classical and quantum-mechanical theories for this system are discussed, and a unified approach to the Kapitza picture of effective potentials associated with high-frequency fields is presented. It appears that cavities and waveguides for neutral-atomic-matter waves may be fashioned from these considerations

We present conclusive results on the structures and coverages of the submonolayer phases of Pb on the Si(111) 7\ifmmode\times\else\texttimes\fi{}7 surface based on scanning-tunneling-microscopy, low-energy-electron-diffraction, and Rutherford backscattering measurements. For room-temperature deposition at low coverage on the 7\ifmmode\times\else\texttimes\fi{}7 surface, we find that Pb atoms occupy sites above and between the Si adatoms (with a preference for the faulted half of the unit cell) leaving the 7\ifmmode\times\else\texttimes\fi{}7 structure intact. After annealing to 450 \ifmmode^\circ\else\textdegree\fi{}C, some Pb atoms occupy Si(111) 7\ifmmode\times\else\texttimes\fi{}7 adatom sites, while others have nucleated to form small areas of an unusual \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 phase which has a 1:1 ratio of Si:Pb.

Some exact solutions to the linear Boltzmann transport equation in a one-dimensional space are presented. These results may be applied to demonstrate the trend towards statistical equilibrium of fast channeled particles in the transverse plane. Statistical mechanics of channeled particle motion is then discussed in terms of the random string approximation and further considerations.

The momentum profiles from positron annihilation with core electrons in Al have been observed with a recently developed two-detector Doppler-broadening technique. Independent-particle-model (IPM) calculations are in good agreement with the experimental results beyond twice the Fermi momentum and suggest the absence of strong positron-electron correlation effects in this region. Comparisons of calculations with properly normalized momentum profiles indicate that high-momentum core contributions may provide information on the nature of vacancy-type defects and self-trapping effects.

The charge state dependence of channeled ion energy loss has been determined for a series of ions ranging from fluorine to chlorine along the $〈110〉$ direction in a silicon crystal. Energy losses for both bare ions and ions partially clothed with bound electrons at $\frac{E}{A}\ensuremath{\cong}3$ MeV/amu have been measured. The energy-loss rate for bare ions follows a strict ${Z}_{1}^{2}$ scaling and agrees reasonably well with quantal perturbation calculations without the need for polarization or Bloch corrections. An explanation for this result is discussed. The clothed-ion energy losses appear to demonstrate screening effects that agree qualitatively with simple estimates. The angular dependence of the observed energy-loss effects is also presented.

Classical calculations of the intensity of channeling radiation from GeV positrons moving in continuum potentials agree reasonably well with experimental data. However, generally, the peaks of the experimental spectrum appear at 5–10% lower frequencies, and they are broader. Since multiple scattering was expected to cause such changes, the classical formula for the intensity has been extended to include this effect. While the calculated broadening is of the right order, it appears that multiple scattering is not the major cause of the shift.

Interference effects have been observed when coherently related x-ray beams are Compton scattered from an atomic system. We use dynamical diffraction methods to prepare the initial x-ray state and observe the coherent Compton effect in both crosssection and profile measurements.

A double crystal x‐ray monochromator system has been developed for use on synchrotron radiation sources. The system consists of two separate Bragg reflecting crystals constrained by a mechanical linkage system enabling it to tune continuously in x‐ray energy, always maintaining an exit beam of constant offset and direction relative to the incident beam. Accurate parallelism between the two crystals is maintained by a piezoelectrically‐controlled analog feedback circuit.

The presence and properties of x-ray evanescent waves are demonstrated detecting their absorption and their excitation in the vicinity of a target surface. The role of microscopic reversibility in relating the two cases is discussed and practical applications of the phenomena to probe interfaces are suggested.

With use of bromine atoms, deposited on the surface of a silicon single crystal, the presence of x-ray standing waves above the crystal surface has been detected under conditions of strong Bragg diffraction.

The channeling effect has been observed to reduce the ionization energy loss by about a factor of 3 for 1.35-GeV/c protons and ..pi../sup +/ traversing a 0.67-mm germanium crystal. (AIP)

An x-ray standing-wave-field apparatus has been constructed that demonstrates an impurity atomic position resolution of \ensuremath{\sim}0.02 \AA{}. The method is applied to the measurement of ion-implantation-induced lattice relaxation. It is pointed out that the method is also suited to determine surface impurity locations perpendicular to crystal surfaces.

Tunneling images of single atomic steps on Si(111) have been obtained with a tunneling microscope. The 7\ifmmode\times\else\texttimes\fi{}7 reconstruction is observed to persist essentially undistorted right up to the steps. The position of each step coincides with a 7\ifmmode\times\else\texttimes\fi{}7 unit-mesh edge in both the upper and lower terraces adjacent to it. Asymmetry within the 7\ifmmode\times\else\texttimes\fi{}7 unit mesh is observed and correlated with faulting of terraces adjacent to the step. The results are described by an extension of the dimer adatom stacking-fault model of Takayanagi et al.

The azimuthal distributions of protons transmitted through thin silicon single crystals near the axis were measured using a two-dimensional position-sensitive detector. The data are composed of ringlike distributions with strong azimuthal and transverse energy dependence. The azimuthal distributions are compared with theoretical predictions based on the random string approximation using different forms of the interatomic potential. ''Blocking'' in the transverse plane is also observed. In addition, from an analysis of the radial spreading of the distribution the effects of inelastic scattering in the transverse plane are clearly seen.

A new instrument called the tunneling microscope has recently been developed that is capable of generating real-space images of surfaces showing atomic structure. These images offer a new view of matter on an atomic scale. The current capabilities and limitations and the physics involved in the technique are discussed along with specific results from a study of silicon crystal surfaces.

A critical analysis of channeled-ion energy-loss experiments is presented with the goal of commenting on the presence of the Barkas effect of Z/sub 1//sup 3/ corrections to the stopping power. Accurate charge-density values are obtained for silicon and used to evaluate Bloch and straggling effects in the data. The remaining contributions to the data show a clear Z/sub 1//sup 3/ dependence that can be explained with an electron-gas model for the Barkas effect.

Channeling effects have been observed for 1.1 GeV/ c protons and pions transmitted through a 1 mm thick germanium single crystal.

Strong directional effects have been observed when 1.15 and 1.35 GeV/ c protons, π + and π − , and deuterons penetrate through germanium crystals. Transmission, wide-angle scattering, and an attempt to do blocking distributions are obtained from experiments using position-sensitive drift chambers. Corresponding measurements of energy loss in germanium crystals are also presented. These experimental results are compared to theory for energy loss, and satisfactory agreement is obtained.

Energy loss of 2-, 6-, and 15-GeV/c positive and negative protons, kaons, and pions transmitted through thin germanium and silicon crystals are measured using the crysta\l{}s as intrinsic detectors. For particles incident in directions far from axial or planar directions the results are in accord with the relativistic Beth\'e-Bloch stopping theory. The density effect plays an important role at these energies. The spectral distribution of energy losses is in good agreement with the Landau distribution. For positive particles aligned with an axis or a plane, the energy loss is considerably reduced compared to random. The measurements agree well with theoretical calculations. Negative particles, well aligned to a crystal axis, show increased energy loss compared to random by as much as \ensuremath{\sim}20%.

Strong directional effects for nuclear-reaction probabilities have been observed when 15 GeV/ c protons and pions are incident on a 4.2 mm Ge single crystal. In the random situation, our measurements are in agreement with Glauber's theory of diffraction scattering and with published particle-production data. When protons are incident in an aligned direction, the nuclear-reaction probabilities fall off very drastically but in a way which is in agreement with standard channeling theory; for aligned negative pions where a simple channeling theory is lacking, there is some experimental indication that nuclear-reaction probabilities are enhanced compared to the corresponding random rates, an indication which is supported by detailed computer-simulation studies.

We have observed surface states indigenous to the Si(111) 7\ifmmode\times\else\texttimes\fi{}7 superlattice with the tunneling microscope. The data show the states' energies and strengths to be strongly dependent on position in the unit mesh. Implications of this for structural models of the 7\ifmmode\times\else\texttimes\fi{}7 superlattice are discussed.

Strong channeling effects have been measured for 1.1–1.35 GeV/c protons, π+, π−, and deuterons penetrating germanium crystals. Both in transmission and wide angle scattering experiments, for positive particles, yields agree qualitatively with theoretical expectations. π− results are similar to those obtained in electron-channeling studies. Energy loss for well channeled 1.35 GeV/c protons and π+ is about 1 3 of the random value.