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We report the discovery of a new and unexpected kind of metal in two dimensions (2D), which exists in the presence of scattering by local magnetic moments. The experiment was carried out on a 2D electron system in silicon, where the local magnetic moments have been induced by disorder and their number was varied using substrate bias ( V sub ). In the new metal, the conductivity decreases as σ ( n s , T )= σ ( n s , T =0)+ A ( n s ) T 2 ( n s – carrier density) to a non-zero value as temperature T →0. In three dimensions, this T 2 dependence is well known, and results from Kondo scattering by local magnetic moments. In 2D, however, the existence of a metal with d σ /d T >0 is very surprising. As the number of local moments is reduced, the range of temperatures [ T T m ( V sub )] where they dominate transport becomes smaller. For T > T m , we observe the usual 2D metallic behavior with d σ /d T

We review some experimental and theoretical results on the metal-to-insulator transition (MIT) observed at zero magnetic field (B=0) in several two-dimensional electron systems (2DES). Scaling of the conductance and magnetic field dependence of the conductance provide convincing evidence that the MIT is driven by Coulomb interactions among the carriers and is dramatically sensitive to spin polarization of the carriers., Comment: 8 pages, LaTeX, figure label changed

We present our results on the manipulation of individual viruses using an advanced interface for atomic force microscopes (AFMs). We show that the viruses can be dissected, rotated, and translated with great facility. We interpret the behavior of tobacco mosaic virus with a mechanical model that makes explicit the competition between sample-substrate lateral friction and the flexural rigidity of the manipulated object. The manipulation behavior of tobacco mosaic virus on graphite is shown to be consistent with values of lateral friction observed on similar interfaces and the flexural rigidity expected for macromolecular assemblies. The ability to manipulate individual samples broadens the scope of possible studies by providing a means for positioning samples at specific binding sites or predefined measuring devices. The mechanical model provides a framework for interpreting quantitative measurements of virus binding and mechanical properties and for understanding the constraints on the successful, nondestructive AFM manipulation of delicate samples.

We have studied the response of the tunnel current and the feedback circuit during surface modification by an STM tip. Our studies show that several classes of events occur when “writing” by applying voltage pulses. We have studied W and Au tips on Au surfaces and PtRh tips on Pt surfaces. Strong correlation is found between the tunnel current response and the kind of surface change that occurs. One dramatic discovery is the formation of a wire between the tip and surface, which is characterized by an electrical shorting of the tip to the surface until the feedback circuit of the tip retracts enough to break the wire. Using a sophisticated graphics display system and allowing pulse placement by pointing at the desired location, we are able to distinguish easily between changes in surface morphology and changes in tip shape and length.

In narrow high-mobility conductors the predominant source of scattering is reflection of carriers off the confining potential. We demonstrate that by changing the geometry of the intersection of the Hall probes with the conductor, the Hall resistance can be quenched, negative or enhanced. More complex junction geometries can lead to one of these phenomena for one field polarity and to another for the other field polarity. At liquid helium temperatures these results can be explained by following trajectories. In the milli-Kelvin range fluctuations are superimposed. At high fields strong resonant depressions of the Hall resistance are found which may be associated with bound states in the region of the cross.

We have fabricated gated, asymmetric rings which, in principle, enable interference between electron waves to be varied with a gate voltage. Although close to the minimum dimensions currently achievable, the results are far from clear-cut, and imply that there are many problems to be overcome before useful devices based on quantum interference can be demonstrated, even supposing that the geometries proposed will actually work. We describe the methods used to analyse our data to show any consistent interference effects caused by the gate voltage variation. Narrower, higher-mobility devices are required in order to produce more easily-resolved gate voltage-controlled interference effects.

We investigated the temperature dependence of resistivity of a high mobility Si-MOSFET in the insulating regime near a 2D metal–insulator transition. Coulomb hopping in a wide range of temperature and carrier density was found. Quantitative analysis of the results suggests that electron–electron interaction is screened by the metal-gate as the localization length increases. The hopping is highly correlated, i.e. the observed hopping energy is one order of magnitude smaller than the expected value from a single-particle hopping picture.

The Aharonov-Bohm effect in normal metal rings together with the associated conductance fluctuations in wires are clear signatures that experiments are now being performed in a quantum regime where conventional transport concepts do not apply. Ensemble averaging and taking the thermodynamic limit should not be used if comparisons to experiments are to be made. It is shown that by varying both the size of the sample as well as the temperature, we can study how ensemble averaging and energy averaging transforms a coherent quantum system into a classical one.

Measurements of the nuclear spin echo decays are reported in solid H2 after two-pulse and three-pulse sequences. The characteristic times T2 and τE describe the spin-spin interaction and the stimulated echo decay respectively. A sharp increase in τE is observed upon cooling into the long-range orientationally ordered phase, and a continuous one during freezing into a quadrupolar glass. However, no significant change is observed for T2 during either process.

We report measurements of the NMR lineshape, the second and fourth moments ( M 2 and M 4 ), and of the longitudinal relaxation tine in H 2 single crystals. Although a sharp phase transition cannot be excluded, our results for ortho concentrations between 0.15 and 0.4 suggest rather a continuous freezing of rotational motion to a “glass state” as T decreases. In this regime; the NMR spectrum is found to be very nearly isotropic.

Measurements of the magnetoresistance of very small Au rings have revealed Aharonov-Bohm oscillations with a fundamental period h/e . These oscillations persist to very large magnetic field. The temperature dependence of the oscillations is consistent with predictions that apply in the absence of inelastic scattering.

Solid H2 samples were investigated with a phase-coherent pulsed NMR technique. Upon application of a three-pulse sequence, up to four stimulated echoes were observed. These echoes were identified as quasi-quadrupolar ones in their nature, and brought about by the intramolecular dipolar interactions. The decay of the primary stimulated echo was observed, indicating the presence of cross-relaxation between the spin states. A density matrix analysis using the quasi-quadrupolar model was successful in accounting for the results.