We studied the adsorption of single atoms on a semiconducting and metallic single-wall carbon nanotube from first principles for a large number of foreign atoms. The stable adsorption sites, binding energy, and the resulting electronic properties are analyzed. The character of the bonding and associated physical properties exhibit dramatic variations depending on the type of the adsorbed atom. While the atoms of good conducting metals, such as Cu and Au, form very weak bonding, atoms such as Ti, Sc, Nb, and Ta are adsorbed with relatively high binding energy. Most of the adsorbed transition-metal atoms excluding Ni, Pd, and Pt have a magnetic ground state with a significant magnetic moment. Our results suggest that carbon nanotubes can be functionalized in different ways by their coverage with different atoms, showing interesting applications such as one-dimensional nanomagnets or nanoconductors and conducting connects, etc. DOI: 10.1103/PhysRevB.67.201401 PACS number͑s͒: 73.22.Ϫf, 68.43.Bc, 73.20.Hb, 68.43.Fg Single-wall carbon nanotubes ͑SWNT's͒ can serve as templates to produce reproducible, very thin metallic wires with controllable sizes.1 These metallic nanowires can be used as conducting connects and hence are important in nanodevices based on molecular electronics. Recently, Zhang et al.2 have shown that a continuous Ti coating of varying thickness and a quasicontinuous coating of Ni and Pd can be obtained by using electron-beam evaporation techniques. Metal atoms such as Au, Al, Fe, Pb were able to form only isolated discrete particles or clusters instead of a continuous coating of SWNT's. Low-resistance contacts to metallic and semiconducting SWNT's were achieved by Ti and Ni ohmic contacts.3 Most recently, ab initio density-functional calculations 4 have indicated that stable rings and tubes of Al atoms can form around a semiconducting SWNT. It is argued that either persistent currents through these conducting nanorings, or conversely very high magnetic fields can be induced at their center. 4 It is expected that novel molecular nanomagnets and electromagnetic devices can be generated from these metallic nanostructures formed by adatom adsorption on SWNT's. As an example, one can contemplate to generate a nanodevice by the modulating adsorption of adatoms on a bare ͑8,0͒ SWNT, which is a semiconductor 5 with an energy gap of ϳ0.64 eV. This band gap can increase to 2 eV by the adsorption of a hydrogen atom.6 Then, a quantum well ͑or dot͒ can form between two barriers at the hydrogen covered sections of the ͑8,0͒ tube. This structure is connected to the metallic reservoirs through metal coated ends of SWNT's. This way a resonant tunneling device with metal reservoirs and connects at both ends can be fabricated on a single SWNT.Clearly, the study of adsorption of atoms on nanotube surfaces is essential to achieve low-resistance ohmic contacts to nanotubes, to produce nanowires with controllable size, and to fabricate functional nanodevices. In particular, it is important to know the following: ͑i͒ Ho...

The current-voltage (I-V) characteristics of armchair graphene nanoribbons under a local uniaxial tension are investigated by using first principles quantum transport calculations. It is shown that for a given value of bias-voltage, the resulting current depends strongly on the applied tension. The observed trends are explained by means of changes in the band gaps of the nanoribbons due to the applied uniaxial tension. In the course of plastic deformation, the irreversible structural changes and derivation of carbon monatomic chains from graphene pieces can be monitored by two-probe transport measurements.

The structural and electronic properties of aluminum-covered single-wall carbon nanotubes ͑SWNT's͒ are studied from first principles for a large number of coverages. Aluminum-aluminum interaction, that is stronger than aluminum-tube interaction, prevents uniform metal coverage, and hence gives rise to the clustering. However, a stable aluminum ring and aluminum nanotube with well defined patterns can also form around the semiconducting SWNT's and lead to metallization. The persistent current in the Al nanoring is discussed to show that a high magnetic field can be induced at the center of SWNT.

We calculate the localization length in a two-channel tight-binding model for correlated disordered site potential. Both intra-and interchannel correlations are taken into account. The localization length is obtained in quadratic approximation by expanding the two-channel conductance over weak disorder. The result is applied to a simple two-stranded model of DNA molecule and it is shown that a strong pair coupling between the basic nucleotides in the strands is not sufficient to delocalize electronic states.

Parts of DNA sequences known as exons and introns play very different roles in coding and storage of genetic information. Here we show that their conducting properties are also very different. Taking into account long-range correlations among four basic nucleotides that form double-stranded DNA sequence, we calculate electron localization length for exon and intron regions. Analyzing different DNA molecules, we obtain that the exons have narrow bands of extended states, unlike the introns where all the states are well localized. The band of extended states is due to a specific form of the binary correlation function of the sequence of basic DNA nucleotides.