arXiv subjects

David Tomanek

Publications and source records attributed to David Tomanek.

46 records · Page 3Linked to original sources

arXiv2007

Effect of electron and hole doping on the structure of C, Si, and S nanowires

We use ab initio density functional calculations to study the effect of electron and hole doping on the equilibrium geometry and electronic structure of C, Si, and S monatomic wires. Independent of doping, all these nanowires are found to be metallic. In absence of doping, C wires are straight, whereas Si and S wires display a zigzag structure. Besides two preferred bond angles of 60 deg and 120 deg in Si wires, we find an additional metastable bond angle of 90 deg in S wires. The equilibrium geometry and electronic structure of these nanowires is shown to change drastically upon electron and hole doping.

cond-mat.mtrl-sci↗

arXiv2006

Ab initio simulations of excited carrier dynamics in carbon nanotubes

Combining time-dependent density functional calculations for electrons with molecular dynamics simulations for ions, we investigate the dynamics of excited carriers in a (3,3) carbon nanotube at different temperatures. Following an hv=6.8 eV photoexcitation, the carrier decay is initially dominated by efficient electron-electron scattering. At room temperature, the excitation gap is reduced to nearly half its initial value after ~230 fs, where coupling to phonons starts dominating the decay. We show that the onset point and damping rate in the phonon regime change with initial ion velocities, a manifestation of temperature dependent electron-phonon coupling.

cond-mat.mes-hall↗

arXiv2006

Interplay between structure and magnetism in $Mo_{12} S_9 I_9$ nanowires

We investigate the equilibrium geometry and electronic structure of Mo$_{12}$S$_{9}$I$_{9}$ nanowires using ab initio Density Functional calculations. The skeleton of these unusually stable nanowires consists of rigid, functionalized Mo octahedra, connected by flexible, bi-stable sulphur bridges. This structural flexibility translates into a capability to stretch up to approximate 20% at almost no energy cost. The nanowires change from conductors to narrow-gap magnetic semiconductors in one of their structural isomers.

cond-mat.mtrl-sci↗

arXiv2002

Giant magneto-conductance in twisted carbon nanotubes

Using the Landauer-Buttiker formalism, we calculate the effect of structural twist on electron transport in conducting carbon nanotubes. We demonstrate that even a localized region of twist scatters the propagating pi electrons and induces the opening of a (pseudo-) gap near the Fermi level. The subsequent conductance reduction may be compensated by an applied axial magnetic field, leading to a twist-induced, giant positive magneto-conductance in clean armchair nanotubes.

cond-mat.mes-hall↗

arXiv2001

Paradoxical Magnetic Cooling in a Structural Transition Model

In contrast to the experimentally widely used isentropic demagnetization process for cooling to ultra-low temperatures we examine a particular classical model system that does not cool, but rather heats up with isentropic demagnetization. This system consists of several magnetite particles in a colloidal suspension, and shows the uncommon behavior of disordering structurally while ordering magnetically in an increasing magnetic field. For a six-particle system, we report an uncommon structural transition from a ring to a chain as a function of magnetic field and temperature.

cond-mat.stat-mech↗

arXiv2000

Electronic and Structural Properties of Carbon Nano-Horns

We use parametrized linear combination of atomic orbitals calculations to determine the stability, optimum geometry and electronic properties of nanometer-sized capped graphitic cones, called ``nano-horns''. Different nano-horn morphologies are considered, which differ in the relative location of the five terminating pentagons. Simulated scanning tunneling microscopy images of the various structures at different bias voltages reflect a net electron transfer towards the pentagon vertex sites. We find that the local density of states at the tip, observable by scanning tunneling spectroscopy, can be used to discriminate between different tip structures. Our molecular dynamics simulations indicate that disintegration of nano-horns at high temperatures starts in the highest-strain region near the tip.

cond-mat.mtrl-sci↗

arXiv2000

Unusually High Thermal Conductivity of Carbon Nanotubes

Combining equilibrium and non-equilibrium molecular dynamics simulations with accurate carbon potentials, we determine the thermal conductivity $λ$ of carbon nanotubes and its dependence on temperature. Our results suggest an unusually high value $λ{\approx}6,600$~W/m$\cdot$K for an isolated (10,10) nanotube at room temperature, comparable to the thermal conductivity of a hypothetical isolated graphene monolayer or diamond. Our results suggest that these high values of $λ$ are associated with the large phonon mean free paths in these systems; substantially lower values are predicted and observed for the basal plane of bulk graphite.

cond-mat.mtrl-sci↗

arXiv1999

Orientational Melting in Carbon Nanotube Ropes

Using Monte Carlo simulations, we investigate the possibility of an orientational melting transition within a "rope" of (10,10) carbon nanotubes. When twisting nanotubes bundle up during the synthesis, orientational dislocations or twistons arise from the competition between the anisotropic inter-tube interactions, which tend to align neighboring tubes, and the torsion rigidity that tends to keep individual tubes straight. We map the energetics of a rope containing twistons onto a lattice gas model and find that the onset of a free "diffusion" of twistons, corresponding to orientational melting, occurs at T_OM > 160 K.

cond-mat.mtrl-sci↗

arXiv1996

Thermodynamics of finite magnetic two-level systems

We use Monte Carlo simulations to investigate the thermodynamical behaviour of aggregates consisting of few superparamagnetic particles in a colloidal suspension. The potential energy surface of this classical two-level system with a stable and a metastable `ring' and `chain' configuration is tunable by an external magnetic field and temperature. We determine the complex `phase diagram' of this intriguing system and analyze thermodynamically the nature of the transition between the ring and the chain `phase'.

cond-mat↗

arXiv1994

Magnetism of small transition metal clusters and the effects of isomerisation

We investigate the magnetic properties of small transition metal clusters using a simple statistical model, which requires some input data from ab initio spin-density functional calculations. In our study we consider a thermodynamically equilibrated ensemble of clusters with different structures, spin multiplicities, and ground state energies. We calculate the physical properties of this system by weighting the individual configurations according to the Boltzmann statistics. We find that presence of isomers with very similar ground state energies, yet very different magnetic properties, gives rise to a rich magnetic behaviour of the system which differs significantly from what would be expected for single configurations. We apply the present model to to determine the magnetic susceptibility of a cluster ensemble of Langevin paramagnets. Our results show that some of the anomalies in the magnetic behaviour of transition metal clusters might be understood in the framework of our model which is, of course, limited by the extremely high computational effort needed to obtain the input data.

cond-mat↗