2. Nanostructures

Questions addressed:  Is it possible to induce arbitrary structural changes in nanostructures using shaped femtosecond laser pulses? Can topological defects in graphite and carbon nanotubes be eliminated optically? What is the difference between thermally induced damage and selective manipulation via an extreme nonequilibrium state? Can coherent phonons be excited in nanostructures? What are the mechanisms?

First, we demonstrated the possibility of a selective nonequilibrium cap opening of carbon nanotubes as a response to femtosecond laser excitation. By performing molecular dynamics simulations based on a microscopic electronic model we show that the laser-induced ultrafast structural changes differ dramatically from the thermally induced dimer emission. Ultrafast bond weakening and simultaneous excitation of two coherent phonon modes of different frequencies, localized in the spherical caps and cylindrical nanotube body, are responsible for the selective cap opening. 
 
We also studied the structural transitions suffered by nanodiamond upon femtosecond laser excitations. Nanodiamonds convert themselves into graphitic structures. 
 
All self-assembled nanostructures, like carbon nanotubes, exhibit structural imperfections that affect their electronic and mechanical properties and constitute a serious problem for the development of novel electronic nanodevices. Very common defects in nanotubes are pentagon-heptagon pairs, in which the replacement of four hexagons by two pentagons and two heptagons disrupts the perfect hexagonal lattice. In this work, we demonstrate that these defects can be eliminated efficiently with the help of femtosecond laser pulses. By performing nonadiabatic molecular dynamics simulations, we show that in the laser-induced electronic nonequilibrium the pentagon-heptagon pair is transformed back into four hexagons without producing any irreversible damage to the rest of the nanotube.
 
Methods: Molecular dynamics simulations on time-dependent potential energy surfaces combined with integration of equations of motion for the electronic occupations. Complementary ab-initio calculations 

Publications: [26], [25], [24], [23], [19], [17], [13], [8]  (see list of publications)

Question addressed: Explanation of the puzzling size dependence of the magnetiztion of Mn clusters? Noncollinear magnetism?
 
We propose a theoretical explanation for the puzzling size dependence of the magnetic properties of Mn_n clusters. Our approach combines noncollinear ab initio calculations and effective spin Hamiltonians. We show that a remarkable interplay between different magnetic couplings, which is already present in the dimer, leads to a complex size-dependent magnetic behavior, dominated by noncollinear magnetism, frustration, and small cluster magnetic moments. Our results are in reasonably good agreement with experiment. 

Methods: DFT-calculations including noncollinear spins (LDA and GGA approximations). Model Hamil- tonians.

Publications: [14] (see list of publications)