Material structures of reduced dimensions exhibit electrical and mechanical properties different from those in the bulk. Measurements of room-temperature electronic transport in pulled metallic nanowires are presented, demonstrating that the conductance characteristics depend on the length, lateral dimensions, state and degree of disorder, and elongation mechanism of the wire. Conductance during the elongation of short wires (length l approximately 50 angstroms) exhibits periodic quantization steps with characteristic dips, correlating with the order-disorder states of layers of atoms in the wire predicted by molecular dynamics simulations. The resistance R of wires as long as l approximately 400 angstroms exhibits localization characteristics with In R(l) approximately l(2).

Physical Review B 57, 4872 [1998]) Energetics and quantized conductance in jellium-modeled nanowires are investigated using the local-density-functionalbased shell correction method, extending our previous study of uniform-in-shape wires [C. Yannouleas and U. Landman, J. Phys. Chem. B 101, 5780 (1997)] to wires containing a variable-shaped constricted region. The energetics of the wire (sodium) as a function of the length of the volumeconserving, adiabatically shaped constriction, or equivalently its minimum width, leads to formation of self-selecting magic wire configurations, i.e., a discrete configurational sequence of enhanced stability, originating from quantization of the electronic spectrum, namely, formation of transverse subbands due to the reduced lateral dimensions of the wire. These subbands are the analogs of shells in finite-size, zero-dimensional fermionic systems, such as metal clusters, atomic nuclei, and 3 He clusters, where magic numbers are known to occur. These variations in the energy result in oscillations in the force required to elongate the wire and are directly correlated with the stepwise variations of the conductance of the nanowire in units of 2e 2 /h. The oscillatory patterns in the energetics and forces, and the correlated stepwise variation in the conductance are shown, numerically and through a semiclassical analysis, to be dominated by the quantized spectrum of the transverse states at the narrowmost part of the constriction in the wire.

The transport properties of three-dimensional quantum microconstrictions in field-free conditions and under the influence of magnetic fields of arbitrary strengths and directions are studied via a generalized Büttiker model ͓Phys. Rev. B 41, 7906 ͑1990͔͒. It is shown that conductance quantization is influenced by the geometry of the microconstriction ͑that is, its length and the shape of its transverse cross section͒. In a weak longitudinal magnetic field, when r c ӷd, where r c is the cyclotron radius and d the effective transverse size of the narrowing of the microconstriction, the conductance exhibits Aharonov-Bohm-type behavior. This behavior transforms in the strong-field limit, r c Ӷd, into Shubnikov-de Haas oscillations with a superimposed Aharonov-Bohm fine structure. The dependence of the Aharonov-Bohm-type features on the length of the microconstriction and on temperature are demonstrated. Transverse magnetic fields lead to depopulation of the magnetoelectric subbands, resulting in a steplike decrease of the conductance upon increasing the strength of the applied magnetic field.

The thermodynamic and spectral properties of a two-dimensional electron gas with an antidot in a strong magnetic field, r, ro, where r, is the cyclotron radius and ro is the antidot efFective radius, are studied via a solvable model with the antidot confinement potential U-1/r . The edge states localized at the antidot boundary result in an Aharonov-Bohm-type oscillatory dependence of the magnetization as a function of the magnetic field flux through the antidot. These oscillations are superimposed on the de Haasvan Alphen oscillations. In the strong-field limit, A'co, -e+, where co, is the cyclotron frequency and e+ is the Fermi energy, the amplitude of the Aharonov-Bohm-type oscillations of the magnetization due to the contribution of the lowest edge state is -p&k+r, (p& is the Bohr magneton and k+ is the Fermi wave vector). When the magnetic field is decreased, higher edge states can contribute to the magnetization, leading to the appearance of a beating pattern in the Aharonov-Bohm oscillations. The role of temperature in suppressing the oscillatory contribution due to higher edge states is analyzed. Rapid oscillations of the magnetization as a function of the Aharonov-Bohm flux, occurring on a scale of a small fraction of the flux quantum hc/e, are demonstrated.The appearance of a manifold of nonequidistant frequencies in the magneto-optical-absorption spectrum, due to transitions between electronic edge states localized near the antidot boundary, is predicted.

and plasmons in coaxial carbon nanotubes and multishell fullerenes are modeled in analogy with coupled collective excitations in finite, layered, two-dimensional-electron-gas, planar semiconductor superlattices. The curvature of the surface of these complex carbon clusters plays an important role in shaping the dimensionality ͑one dimensional, two dimensional, or three dimensional͒ of the plasmons. Direct crossover from a one-dimensional to a three-dimensional regime is found under readily fulfilled conditions for carbon nanotubes in the case of small finite longitudinal momentum transfer បq, while for qϭ0 bulk graphitic plasmons fail to develop. For large q, a two-dimensional behavior is found. The case of multishell fullerenes resembles in all instances the qϭ0 behavior of carbon nanotubes. Such behavior correlates with the observed systematic redshift of the strong interstellar absorption band as compared to the plasmon of bulk oriented graphite ͑i.e., the 5.7 eV position of the former compared to the 6.2 eV energy of the latter͒. Furthermore, in the case of plasmons in carbon nanotubes, a special surface mode can develop for large q, due to the difference in the values of the dielectric constants between the graphitic structures and the surrounding medium.

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