Recalculation of 4kF correlations in one-dimensional systems

The 4kF response function is recalculated for the one-dimensional electron system in the presence of the umklapp matrix element g3. It is shown that the 1/ω2-4γc singularity is replaced by a finite value below the energy gap set by g3

Uspostavljanje strujnih staza i pogoršanje svojstava elektronskih sklopova zasnovanih na amorfnim organskim slojevima

The recent advent of new flat-panel organic displays follows a long struggle for the extended device lifetime. Many modifications proposed along this way have been based on trial and error experimentation. In this paper, we show that some recent major improvements may have been implicitly related to fighting against the strong current filamentation, intrinsic for charge injection and conduction in organic amorphous thin films. We first recapitulate some major causes of current filamentation in thin amorphous organic layers. Then we consider the charge transport in devices with a high-mobility injection layer, with a smoothened organic heterojunction surface, that operate at lower electric fields, and the devices with doped transport layers. We show that these conditions, known to decrease the device degradation rates, may separately lead to the current homogenization in organic films.Nedavni uzlet u proizvodnji organskih zaslona uslijedio je nakon duge borbe za produljenje vijeka trajanja tih uređaja. Mnoga su poboljšanja pri tom nastala kao rezultat eksperimentalnih pokušaja i promašaja. U ovom članku pokazujemo da su najvažnije nedavne preinake vrlo vjerojatno mikroskopski povezane s prostornim ujednačavanjem električne struje. Pri tom smo najprije ponovili glavne poznate razloge za pojavu prostornog uspostavljanja strujnih staza, pojave svojstvene za injekciju i vođenje u tankim organskim amorfnim slojevima. Potom smo numeričkim simulacijama redom razmotrili utjecaje: tankog injekcijskog sloja velike elektronske pokretljivosti; zaglađene naspram hrapave granice organskih slojeva; slabog nasuprot jakog operativnog električnog polja, te utjecaj punjenja na transport u organskim transportnim slojevima. Pokazujemo da sve te preinake u strukturi i načinu rada, znane da dovode do produljenja vijeka trajanja uređaja, također uzrokuju prostorno ujednačavanje struje u tankim slojevima

Current filamentation and degradation in electronic devices based on amorphous organic layers

The recent advent of new flat-panel organic displays follows a long struggle for the extended device lifetime. Many modifications proposed along this way have been based on trial and error experimentation. In this paper, we show that some recent major improvements may have been implicitly related to fighting against the strong current filamentation, intrinsic for charge injection and conduction in organic amorphous thin films. We first recapitulate some major causes of current filamentation in thin amorphous organic layers. Then we consider the charge transport in devices with a high-mobility injection layer, with a smoothened organic heterojunction surface, that operate at lower electric fields, and the devices with doped transport layers. We show that these conditions, known to decrease the device degradation rates, may separately lead to the current homogenization in organic films

Dynamic structure factor of a one-dimensional Peierls system

The Newtonian dynamics of a one-dimensional system with a complex order parameter and an anharmonic potential energy of the Landau type is examined in a wide temperature range using numerical Monte Carlo–molecular dynamics simulations. Results are discussed with an emphasis on the incommensurate Peierls systems. Dispersive-mode behavior found previously in the quartic region near TMF can be followed to lower temperatures where the frequency of the phonon mode decreases and the damping increases. Below 0.4TMF the dynamic structure factor is characterized by the overdamped mode down to 0.3TMF, when the separation between the phase and the amplitude mode becomes effective. The relation between the different regimes in S(k,ω) and the pseudogap in the electronic spectrum is also briefly discussed

Uspostavljanje strujnih staza i pogoršanje svojstava elektronskih sklopova zasnovanih na amorfnim organskim slojevima

The recent advent of new flat-panel organic displays follows a long struggle for the extended device lifetime. Many modifications proposed along this way have been based on trial and error experimentation. In this paper, we show that some recent major improvements may have been implicitly related to fighting against the strong current filamentation, intrinsic for charge injection and conduction in organic amorphous thin films. We first recapitulate some major causes of current filamentation in thin amorphous organic layers. Then we consider the charge transport in devices with a high-mobility injection layer, with a smoothened organic heterojunction surface, that operate at lower electric fields, and the devices with doped transport layers. We show that these conditions, known to decrease the device degradation rates, may separately lead to the current homogenization in organic films.Nedavni uzlet u proizvodnji organskih zaslona uslijedio je nakon duge borbe za produljenje vijeka trajanja tih uređaja. Mnoga su poboljšanja pri tom nastala kao rezultat eksperimentalnih pokušaja i promašaja. U ovom članku pokazujemo da su najvažnije nedavne preinake vrlo vjerojatno mikroskopski povezane s prostornim ujednačavanjem električne struje. Pri tom smo najprije ponovili glavne poznate razloge za pojavu prostornog uspostavljanja strujnih staza, pojave svojstvene za injekciju i vođenje u tankim organskim amorfnim slojevima. Potom smo numeričkim simulacijama redom razmotrili utjecaje: tankog injekcijskog sloja velike elektronske pokretljivosti; zaglađene naspram hrapave granice organskih slojeva; slabog nasuprot jakog operativnog električnog polja, te utjecaj punjenja na transport u organskim transportnim slojevima. Pokazujemo da sve te preinake u strukturi i načinu rada, znane da dovode do produljenja vijeka trajanja uređaja, također uzrokuju prostorno ujednačavanje struje u tankim slojevima

Particle-Energy Distribution and Effective Temperature for the Hopping Transport in One-dimensional Disordered System

Recent numerical simulations of the electronic hopping conduction in disordered organics have indicated that the effective temperature may be suitable to describe the ensemble of electrons in the electric field. However, the reasons for the emergence of the effective temperature have not been clarified, and only phenomenological expressions for the effective temperature have been proposed based on numerical simulations. We address these questions here for an analytically tractable one-dimensional model with uncorrelated Gaussian disorder. The exact relation for the distributions of particles in energy is obtained, the analytical expression for the effective temperature is derived, and the limits of the applicability of the concept of the effective temperature are drawn

Injection and strong current channeling in organic disordered media

We consider charge injection from a metal into amorphous organic molecular media with correlated disorder. It is shown that correlations, known to be essential for understanding the field dependence of the carrier mobility, also strongly influence the injection current distribution. In particular, we find that the injection hot spots are intrinsic for metal/organic interfaces, even for perfectly flat surfaces. The current density variations reach several orders of magnitude for realistic material parameters. The injection hot spots further induce current channels in the bulk of the material that extend a hundred nanometers beyond the injection surface. For electronic devices based on thin amorphous organic films, as are the organic light-emitting diodes, this current channeling is expected to have a serious impact on device characteristic and performance

Recombination at heterojunctions in disordered organic media: Modeling and numerical simulations

Multilayer organic electroluminescent devices derive their advantages over their single-layer counterparts from the processes occurring at heterojunctions in organic media. These processes significantly differ from those in the bulk of the material. This paper presents three-dimensional modeling, numerical simulations, and a discussion of transport and recombination in a system with a heterojunction. We consider partial cross sections for the creation of excitons and exciplexes, and probabilities for recombination in the respective channels. We examine the influence of the energy barrier, electric field, site-energy disorder, and structural disorder at an organic-organic interface on transport and recombination. In particular, we investigate optimal parameter domains for recombination in the exciton channel. The interface roughness, unlike the site-energy disorder, is found to strongly affect the partial cross sections

Beyond the effective temperature: The electron ensemble at high electric fields in disordered organics

Hopping between localized polaronic states, with a Gaussian distribution in energy, is regarded as the main mechanism of electric conduction in disordered organics. Several authors have recently suggested that the hopping electrons, subjected to an electric field, can be described as a homogeneous ‘overheated’ gas, with its “effective temperature” sufficient for a parametrization of the ensemble and the current. It is not clear how such a picture could be reconciled with the observed strongly oriented filamentization of the flow. We show, through extensive numeric simulations, that the picture is misleading as it can overestimate the electron mobility by orders of magnitude. The reason lies in deviations of the average site occupancies from the effective Boltzmann distribution. The ensemble can be described by a distribution function with two parameters—the effective temperature and the variance of the occupancy deviations. The two are connected by a simple universal relation. The spatial structure of the occupancy deviations is found to be connected to the current filaments and its neglect is recognized as the cause for the failure of mobility calculations based on the overheated gas concept. Thus we identify those aspects, lying beyond the overheated gas picture, that are of a fundamental importance to a proper understanding of the transport in disordered organics

Electric-field-gradient analysis of high-Tc superconductors

The qualitative and quantitative features of the electronic charge distribution in cuprate superconductors are reexamined on the basis of a comprehensive set of nuclear quadrupole resonance and NMR measurements. A systematic analysis of measured electric-field gradients is performed within the tight-binding approach, commonly used for electronic models in cuprates. Both in-plane and out-of-plane sites and orbitals are considered. Special attention is given to the generic pd model involving only the σ orbitals in the CuO2 planes. This model is checked against the experimental data. The physical origin of several material-dependent features is clarified. It is shown that in La2-xSrxCuO4 the 3d3z2-r2 orbital of the in-plane copper and the 2pz apex oxygen orbital are occupied by a substantially smaller fraction of holes than the in-plane σ orbitals, at variance with proposals lending importance to copper-apex oxygen hybridization. At the in-plane oxygen site a sizable 2pz admixture is found at the Fermi level of YBa2Cu3O7. The in-plane charge distribution of Tl2Ba2CuO4 is found to be qualitatively similar to that of the high-temperature tetragonal phase of La2-xSrxCuO4. It appears that the additional holes are shared among copper and oxygen sites in similar proportions. This is shown to be compatible with the large Ud Emery model provided that the difference between the p and d atomic energies is comparable to the first neighbor overlap energy