Baruch Horovitz

Spectroscopic studies of photoexcitations in regioregular and regiorandom polythiophene films

Using a variety of optical probe techniques we studied the steady state and transient dynamics of charged and neutral photoexcitations in thin films of poly-3-alkyl thiophene with regioregular order, which forms self-assembled lamellae structures with increased interchain interaction, as well as regiorandom order that keeps a chain-like morphology. In regiorandom polythiophene films we found that intrachain excitons with correlated photoinduced absorption and stimulated emission bands are the primary photoexcitations; they give rise to a moderately strong photoluminescence band, and long-lived triplet excitons and intrachain charged polarons. In regioregular polythiophene films, on the contrary we found that the primary photoexcitations are excitons with much larger interchain component; this results in lack of stimulated emission, vanishing intersystem crossing, and a very weak photoluminescence band. The long-lived photoexcitations in regioregular polythiophene films are interchain excitons and delocalized polarons (DP) within the lamellae, with very small relaxation energy. The characteristic properties of the DP species are thoroughly investigated as a function of the alkyl side group of the polymer backbone, film deposition conditions and solvents used, as well as at high hydrostatic pressure. The quantum interference between the low energy absorption band of the DP species and a series of photoinduced infrared active vibrations, which give rise to antiresonances that are superimposed on the electronic absorption band is studied and explained by a Fano-type interference mechanism, using the amplitude mode model.

Gauge field induced by ripples in graphene

We study the effects of quenched height fluctuations (ripples) in graphene on the density of states (DOS). We show that at strong ripple disorder, a divergence in the DOS can lead to an ordered ground state. We also discuss the formation of dislocations in corrugated systems, buckling effects in suspended samples, and the changes in the Landau levels due to the interplay between a real magnetic field and the gauge potential induced by ripples.

Raman scattering and infrared phonons in polyacetylene

Abstract The Raman active normal modes of polyacetylene are described in terms of amplitude modes associated with the Peierls gap. A narrow distribution in the effective electron-phonon coupling constant accounts for the spectrum and its dependence on laser frequency. The resonant Raman condition determines a functional dependence of the gap on the coupling constant which is consistent with the Peierls relation. All infrared phonons induced by either doping or photogeneration are determined by the Raman data and a pinning parameter.

Thermal metal in network models of a disordered two-dimensional superconductor

We study the symmetry class for localization which arises from models of noninteracting quasiparticles in disordered superconductors that have neither time-reversal nor spin-rotation invariance. Two-dimensional systems in this category, which is known as class D, can display phases with three different types of quasiparticle dynamics: metallic, localized, or with a quantized (thermal) Hall conductance. Correspondingly, they can show a variety of delocalization transitions. We illustrate this behavior by investigating numerically the phase diagrams of network models with the appropriate symmetry and show the appearance of the metallic phase.

QUANTUM HALL PLATEAU TRANSITIONS IN DISORDERED SUPERCONDUCTORS

We study a delocalization transition for non-interacting quasiparticles moving in two dimensions, which belongs to a new symmetry class. This symmetry class can be realised in a dirty, gapless superconductor in which time reversal symmetry for orbital motion is broken, but spin rotation symmetry is intact. We find a direct transition between two insulating phases with quantized Hall conductances of zero and two for the conserved quasiparticles. The energy of quasiparticles acts as a relevant, symmetry-breaking field at the critical point, which splits the direct transition into two conventional plateau transitions.

Freezing transitions and the density of states of two-dimensional random Dirac Hamiltonians

Using an exact mapping to disordered Coulomb gases, we introduce a method to study two-dimensional Dirac fermions with quenched disorder in two dimensions that allows us to treat nonperturbative freezing phenomena. For purely random gauge disorder it is known that the exact zero-energy eigenstate exhibits a freezinglike transition at a threshold value of disorder σ = σ t h = 2. Here we compute the dynamical exponent z that characterizes the critical behavior of the density of states around zero energy, and find that it also exhibits a phase transition. Specifically, we find that ρ(E = 0 + i∈) ∼ ∈ 2 / z - 1 [and ρ(E)∼E 2 / z - 1 ] with z = 1 + σ for σ 2. For a finite system size L<∈ - t / z /: we find large sample to sample fluctuations with a typical ρ,(0)∼L z - 2 . Adding a scalar random potential of small variance δ, as in the corresponding quantum Hall system, yields a finite noncritical ρ(0)∼δ α whose scaling exponent α exhibits two transitions, one at σ t h /4 and the other at σ t h . These transitions are shown to he related to the one of a directed polymer on a Cayley tree with random signs (or complex) Boltzmann weights. Some observations are made for the strong disorder regime relevant to describe transport in the quantum Hall system.

Raman scattering from charge-density waves and application to polyacetylene

Oscillations in an electronic gap formed by a charge-density wave are described in terms of Raman-active amplitude modes. In a multi-phonon system the authors find the Raman shifts and their intensity ratios in terms of a parameter lambda which contains all effects of electron-phonon, electron-electron and electron-disorder interactions; the single assumption in the theory is the adiabatic limit. The theory is compared with data on polyacetylene where the relation of lambda with the electronic gap yields information on the type of disorder. For an electron-phonon system an ab initio calculation of the Raman cross section is given and compared with more conventional calculations.

Resonant Raman Scattering in the Polyacetylene System

Abstract The amplitude mode approach for describing the resonant raman scattering in dimerized chains is reviewed and applied to the polyacetylene system. trans-(CH)x is disordered with a variable electron-phonon coupling constant. cis-(CH) x is ordered with an extrinsic gap of ∼ 5% of the full gap. A distribution of extrinsic gaps is shown to exist In trans-(CH) x chains in partially isomerized polyacetylene.

Superconductors with broken time-reversal symmetry: Spontaneous magnetization and quantum Hall effects

Broken time-reversal symmetry ~BTRS! in d1id8 as well as in d1is superconductors is studied and is shown to yield current carrying surface states. We evaluate the temperature and thickness dependence of the resulting spontaneous magnetization and show a marked difference between weak and strong BTRS. We also derive the Hall conductance which vanishes at zero wave vector q and finite frequency v; however, at finite q,v it has an unusual structure. The chirality of the surface states leads to quantum Hall effects for spin and heat transport in d1id8 superconductors.

Landau-level mixing and spin degeneracy in the quantum Hall effect

We study the dynamics of electrons in a magnetic field using a network model with two channels per link with random mixing in a random intrachannel potential; the channels represent either two Landau levels or two spin states. We consider channel mixing as a function of the energy separation of the two extended states and show that its effect changes from repulsion to attraction as the energy separation increases. For two Landau levels this leads to level floating at low magnetic fields, while for Zeeman-split spin states we predict level attraction at high magnetic fields, accounting for electron spin resonance data. We also study random mixing of two degenerate channels, while the intrachannel potential is periodic (nonrandom). We find a single extended state with a localization exponent \ensuremath{\nu}\ensuremath{\approx}1.1 for real scattering at nodes; the general case also has a single extended state, though the localized nature of nearby states sets in at unusually large scales.