ABSTRACT
The charge mobility of molecular semiconductors is limited by the large fluctuation of intermolecular transfer integrals, often referred to as off-diagonal dynamic disorder, which causes transient localization of the carriers' eigenstates. Using a recently developed theoretical framework, we show here that the electronic structure of the molecular crystals determines its sensitivity to intermolecular fluctuations. We build a map of the transient localization lengths of high-mobility molecular semiconductors to identify what patterns of nearest-neighbour transfer integrals in the two-dimensional (2D) high-mobility plane protect the semiconductor from the effect of dynamic disorder and yield larger mobility. Such a map helps rationalizing the transport properties of the whole family of molecular semiconductors and is also used to demonstrate why common textbook approaches fail in describing this important class of materials. These results can be used to rapidly screen many compounds and design new ones with optimal transport characteristics.
ABSTRACT
For Dirac electrons the Klein paradox implies that the confinement is difficult to achieve with an electrostatic potential although it can be of great importance for graphene-based devices. Here, ab initio and tight-binding approaches are combined and show that the wave function of Dirac electrons can be localized in rotated graphene bilayers due to the Moire pattern. This localization of wave function is maximum in the limit of the small rotation angle between the two layers.
Subject(s)
Graphite/chemistry , Models, Chemical , Nanostructures/chemistry , Nanostructures/ultrastructure , Computer Simulation , Electrons , Particle Size , RotationABSTRACT
An extreme sensitivity of the magnetic properties to the atomic structure has been observed in Al(Si)-Mn and Al-Pd-Mn quasicrystals, approximants, and liquids with similar Mn concentrations (of the order of 8%- 20%). Here, we show that the effect of the local environment of the Mn atoms is not sufficient to explain this complex behavior. A new model is presented, which, taking into account Mn-Mn interactions mediated by the conduction electrons over large distances ( 5 A and more), allows one to understand why only a small fraction of the Mn atoms carry a localized magnetic moment in quasicrystals and why a large proportion is magnetic in liquids.
ABSTRACT
We derive a generalized Drude formula for the optical conductivity of quasicrystals, assuming a diffusion law in a perfect quasicrystal L(t) approximately t(beta), where L(t) measures the spreading of a wave packet in a time t. We show that the so-called Drude peak in the optical conductivity, characteristic of metals, is strongly modified. If beta<1/2 the Drude peak is even replaced by a dip and the dc conductivity increases when disorder increases. An interpretation of the experimental results on icosahedral and decagonal phases is proposed.
ABSTRACT
An experiment performed on barium atoms has permitted us to observe an infrared emission whose spectral and temporal analysis has led us to identify it with collision-induced superradiance. Tentative theoretical interpretation is given in the dressed-atom formalism using the monochromatic approximation.
ABSTRACT
We describe an experimental study of a new regime of superradiance characterized by relaxation oscillations, which is obtained in the case of long-duration pumping pulses. A qualitative interpretation of our results is given using the mean-field approximation in conjunction with broadband excitation. A more quantitative interpretation is also given, in which propagation effects are explicitely taken into account and the excitation is purely monochromatic.
