Persistent currents in carbon nanotubes based rings

Persistent currents in rings constructed from carbon nanotubes are investigated theoretically. After studying the contribution of finite temperature or quenched disorder on covalent rings, the complexity due to the bundle packing is addressed. The case of interacting nanotori and self-interacting coiled nanotubes are analyzed in details in relation with experiments.Comment: 7 sections, 9 figure

Ground-state configurations in ferromagnetic nanotori

Magnetization ground states are studied in toroidal nanomagnets. The energetics associated to the ferromagnetic, vortex and onion-like configurations are explicitly computed. The analysis reveals that the vortex appears to be the most prominent of such states, minimizing total energy in every torus with internal radius $r\gtrsim10\,{\rm nm}$ (for Permalloy). For $r\lesssim10\,{\rm nm}$ the vortex remains the most favorable pattern whenever $R/\ell_{ex}\gtrsim1.5$ ($R$ is the torus external radius and $\ell_{ex}$ is the exchange length), being substituted by the ferromagnetic state whenever $R/\ell_{ex}\lesssim1.5$.Comment: 16 pages, 9 figures, 3 apendices, Revtex forma

Carbon Nanotori Reinforced Lubricants in Plastic Deformation Processes

This research presents the effects of carbon nanotori structures (CNst) dispersed as reinforcement for metal-working and metal-forming lubricants. Synthetic (SL) and deep drawing (DD) nanolubricants were prepared following a two-step method at 0.01 wt.%, 0.05 wt.%, and 0.10 wt.% filler fractions. Slight increases in viscosity (\u3c6%) for nanolubricants were observed as filler fraction was increased through various measured temperatures. Tribological behavior of nanolubricants displayed superb improvements under antiwear and extreme pressure conditions. The load carrying capacity (poz) increased by 16% and 22% at merely 0.01 wt.% CNst reinforcement and up to 73% and 107% at 0.10 wt.% filler fraction for SL and DD nanolubricants, respectively, compared to conventional materials. Additionally, at 0.10 wt.% wear scar evaluations showed a highest benefit of 16% and 24%, for SL and DD nanolubricants, respectively. This enhancement is attributed to diverse mechanisms such as rolling/sliding and load bearing effects, tribofilm formation, and CNst tribosintering behavior (at high pressures) onto metallic surfaces due to nanostructures size and morphology and their interlayer relationship among conventional lubricants. View Full-Tex

Properties of Molecules in Weak and Strong Magnetic Fields

Magnetic fields alter the properties of molecules, affecting the electron distribution, the electron configuration and the molecular geometry. In weak magnetic fields, the changes are subtle. Electrons as charged particles placed in magnetic field start following specific pathways, giving rise to magnetically induced ring currents. They follow the contour of the molecule, as well as form vortices around certain molecular rings and chemical bonds. Strong ring currents arise near atomic nuclei due to the core electrons. Magnetically induced currents are a unique fingerprint of the molecular structure but they also serve as an indicator for electron delocalisation, aromatic properties and applicability in optoelectronics. Various organic molecules were investigated using the gauge-including magnetically-induced current density approach. It has been demonstrated that heteroatoms alter the ring-current pathways and the current strength, and thereby affect molecular aromaticity.The topology of Möbius systems has been shown to depend both on the twist of the molecular rings of a series of [40]annulenes, as well as on their spatial folding (writhe). The investigation of a series of toroidal carbon nanotubes showed helical current flow in one of the chiral molecules in the study, which is a pre-requisite for the generation of anapole moment when the molecule is placed in a magnetic field. Very strong magnetic fields beyond achievable on Earth cause major changes in the electron configuration of atoms and molecules. Orbitals with high angular momentum and high-spin configurations become lower in energy than the typical zero-field occupation. Weak magnetic fields can be studied as a perturbation to the zero-field Hamiltonian. However, as the field strength increases, the magnetic interaction becomes equally strong as the electrostatic one. The explicit treatment of the magnetic field strength involves the angular momentum operator in the \schr, thus leading to complex orbitals. Therefore, new quantum chemistry software is necessary. A benchmark study for the performance of a traditional implementation based on Gaussian-type orbitals versus a fully numerical code has been done at the \acl{hf} level. After determining the accuracy of the method, small hydrocarbon molecules have been investigated, which showed that they exist as bound molecules in high-spin configurations where only the core electrons of the carbon atom are paired.Kuten tiedämme, magneetti vetää rautaesineitä puoleensa. Magneettien avulla kiinnitetään lappuja jääkaapin oveen, suljetaan kaappien ovet ja älypuhelimen kotelo. Lääkärit tutkivat potilaita vahvan magneettikentän avulla, magnetisoitunut neula kompassissa osoittaa pohjoiseen, ja tietokoneen kovalevy lukee sille talletetut tiedot magneetin avulla. Näiden ilmiöiden salaisuus piilee elektronien ja magneettikentän välisissä vuorovaikutuksissa. Molekyylitasolla magnetismi aikaansaa elektroniliikkeen molekyylin ympäri. Elektronit kiertävät myös tiettyjä atomiryhmiä renkaanmuotoisilla poluilla. Koska jokaisella molekyylillä on omanlainen elektronijakauma, niin näitä elektronipolkuja tutkimalla saadaan tietoja molekyylin ominaisuuksista. Polut kertovat mm. molekyylin soveltuvuudesta aurinkokenno- ja akkukäyttöön. Väitöskirjassa on tutkittu erilaisia orgaanisia molekyylejä sekä toroidimaisia – eli renkaankaltaisia – hiilinanoputkia. Laitetta, joka suoraan pystyisi mittaamaan elektroniliikettä magneettikentässä ei ainakaan vielä ole olemassa, joten tutkimus on suoritettu teoreettisen mallinnuksen avulla, kvanttikemiallisia menetelmiä käyttäen. Laboratoriossa valmistetut magneetit voivat olla jopa miljoona kertaa maapallon omaa magneettikenttää voimakkaampia. Sellaista ainetta, joka kestäisi sitä valtavaa voimaa, jolla vahva magneettikenttä vaikuttaa kappaleeseen, ei ole olemassa. Maailmankaikkeudesta, tiettyjen tähtien läheisyydessä, löytyy kuitenkin jopa miljardikertaisesti vahvempia magneettikenttiä. Elämänsä loppuvaiheessa tähti voi kutistua pieneksi, erittäin tiheäksi kappaleeksi – niin sanotuksi valkoiseksi kääpiöksi. Mikäli alkuperäinen tähti on ollut riittävän iso, lopuksi jää kappale, joka on niin tiheä, että atomitkin hajoavat. Tällaista taivaankappaletta kutsutaan neutronitähdeksi. Erittäin vahva magneettikenttä aiheuttaa huomattavia muutoksia molekyylien elektronirakenteissa, mikä puolestaan johtaa uusiin ja pääosin arvaamattomiin ominaisuuksiin. Näiden ominaisuuksien tutkiminen onkin väitöskirjan toinen aihe. Perinteiset kvanttikemian ohjelmistot eivät pysty mallintamaan magneettikentän aiheuttamia muutoksia elektronirakenteessa. Väitöskirjassa tutkittiin uudentyyppisten ohjelmistojen tarkkuutta vahvassa magneettikentässä olevien molekyylien mallinnuksessa. Tutkimuksen kohteena oli pienten molekyylien elektronikonfiguraatio, geometria ja sidosten vahvuus; ominaisuuksia, joita ei aikaisemmin juurikaan ole tutkittu

An Introductory Note on the Spectrum and Energy of Molecular Graphs

Graph Theory is one branch of Mathematics that laid the foundations of the structural studies in Chemistry. The fact that every molecule or compound can be represented as a network of vertices (elements) and edges (bonds) provoked the question of the predictability of the physical and chemical properties of molecules and compounds. Spectrum, π-electron energy, Spectral Radius etc. are predictable using graph theoretical methods. This is an introductory paper about spectrum and energy of molecular graphs

Elastic Theory of Defects in Toroidal Crystals

We report a comprehensive analysis of the ground state properties of axisymmetric toroidal crystals based on the elastic theory of defects on curved substrates. The ground state is analyzed as a function of the aspect ratio of the torus, which provides a non-local measure of the underlying Gaussian curvature, and the ratio of the defect core-energy to the Young modulus. Several structural features are discussed,including a spectacular example of curvature-driven amorphization in the limit of the aspect ratio approaching one. The outcome of the elastic theory is then compared with the results of a numerical study of a system of point-like particles constrained on the surface of a torus and interacting via a short range potential.Comment: 24 pages, 24 figure