bluuebunny/crossref_metadata_2025 huggingface dataset:
Polariton amplification in semiconductor microcavities
Authors: Saba M. , Kundermann S. , Ciuti C. , Guillet T. , Staehli J. L. , Deveaud B.
Published: July 2003

Microcavity exciton polaritons, the fundamental optical excitations of semiconductor microcavities with quantum wells inside, have been proposed as promising candidates for observing stimulated scattering, condensation and other phenomena related to the bosonic nature of excitons. Having a light mass, quantum degeneracy of polaritons can be reached at low densities and high temperatures. But the radiative time of polaritons is very short (in the picosecond range) and usually prevents an efficient thermalization and cooling of the excited cloud of polaritons. A 'coherently‐driven condensate', not corresponding to a thermal equilibrium, but featuring multiple occupation of single‐particle states, can however be created by an external laser source resonantly exciting polaritons. Under this condition, stimulated parametric scattering of polaritons can provide huge optical gain on a weak probe pulse shined on the sample. In this work we demonstrate that this phenomenon can survive at temperatures close to room temperature and could be achieved in the next future even above this limit. Clever sample designs favour the thermal robustness of polariton parametric amplification, but from the experimental data it turns out that the parameter that ultimately limits the highest temperature for polariton parametric scattering is the exciton binding energy.
Semiconductor excitons in photonic crystals
Authors: Eichmann R. , Pasenow B. , Meier T. , Stroucken T. , Thomas P. , Koch S. W.
Published: July 2003

Optical properties of a semiconductor quantum well in the vicinity of a two‐dimensional photonic crystal are investigated. Due to the periodic spatial modulation of the dielectric environment the effective Coulomb interaction potential exhibits the periodicity of the photonic crystal. As a consequence, the excitonic binding energy varies periodically by up to 50% depending on the spatial position of the exciton relative to the structured dielectric. The self image charge effects result in a position dependent shift of the single‐particle bandgap such that the spectral position of the absorption spectrum also develops the periodicity of the photonic surrounding.
Microscopic theory of the semiconductor terahertz response
Authors: Kira Mackillo , Hoyer Walter , Koch Stephan W.
Published: July 2003

A microscopic theory for the terahertz response of excited semiconductor systems is presented. It is verified that the theory yields the correct effective‐mass Drude limit. For a fully interacting incoherent system, the terahertz response is shown to be determined by the electron–hole correlations. The theory is evaluated for the example of excitonic populations in a quantum‐well structure.
Exciton dynamics studied via internal THz transitions
Authors: Kaindl R. A. , Hägele D. , Carnahan M. A. , Lövenich R. , Chemla D. S.
Published: July 2003

We employ a novel, ultrafast terahertz probe to investigate the dynamical interplay of optically‐induced excitons and unbound electron–hole pairs in GaAs quantum wells. Resonant creation of heavy‐hole excitons induces a new low‐energy oscillator linked to transitions between the internal exciton degrees of freedom. The time‐resolved terahertz optical conductivity is found to be a probe well suited for studies of fundamental processes such as formation, relaxation and ionization of excitons.
How fast do charged particles get dressed?
Authors: Leitenstorfer A. , Huber R. , Tauser F. , Brodschelm A.
Published: July 2003

On a femtosecond timescale, we observe how Coulomb screening and collective scattering build up in an extreme nonequilibrium electron–hole plasma photoinjected in GaAs. To this end, we generate a plasma within 10 fs and probe the subsequent polarization response of the system with uncertainty‐limited temporal resolution via ultrabroadband THz spectroscopy. We show that the intrinsic material becomes conductive instantaneously upon carrier injection, whereas collective effects such as Coulomb screening and plasmon scattering exhibit a delayed onset. Thus, the ultrafast formation of dressed quasiparticles is directly monitored for the first time. The timescale for these phenomena is of the order of the inverse plasma frequency. Our findings support recent quantum kinetic simulations.
Microscopic modelling of semiconductor‐based quantum devices: a predictive simulation strategy
Authors: Iotti R. C. , Rossi F.
Published: July 2003

We review and discuss the first fully three‐dimensional study of non‐equilibrium carrier dynamics governing semiconductor‐based intersubband optoelectronic devices like, e.g., quantum‐cascade lasers. First, a multisubband Monte Carlo simulation scheme in a kinetic Boltzmann‐like approach is presented. Then, its generalisation into a density‐matrix quantum‐transport formalism is discussed. This allows us to directly access microscopic key‐features of the electron relaxation dynamics (without resorting to phenomenological parameters) as well as to investigate the nature, coherent vs incoherent, of charge injection/transport processes. Besides providing a quantitative investigation into the operation of these devices, our kinetic analysis can indeed serve as a predictive tool for the evaluation of new designs and strategies. Applications are presented concerning quantum‐cascade devices, both state‐of‐the‐art mid‐infrared lasers as well as novel Thz emitters.
Infrared absorption by excitons in Cu2O
Authors: Jörger M. , Tsitsishvili E. , Fleck T. , Klingshirn C.
Published: July 2003

In a differential absorption experiment the induced infrared transitions from the excitonic 1s to the 2p levels in Cu2O have been investigated. Intermediate densities of 1s excitons were created by cw‐laser excitation while the interexcitonic 1s to 2p transitions were probed simultaneously, using Fourier spectroscopy. Our data give evidence for a surprisingly large splitting of the 2p level (≈3.7 meV) the origin of which is a matter of speculation. An analysis of lineshape and width of the transitions results in a ratio of the effective masses m1s/m2p which deviates from the literature value.
Theory of the lineshape of quantum well intersubband transitions: optical dephasing and light propagation effects
Authors: Waldmüller Inès , Woerner Michael , Förstner Jens , Knorr Andreas
Published: July 2003

We outline a theoretical description of the absorption linewidth of quantum well intersubband transitions by solving Maxwell's equations for a non‐local susceptibility including many particle effects. We show that the intersubband absorption results from a complex interplay between mean‐field effects, dephasing contributions and light propagation effects, all being very sensitive to subband dispersion.
Level repulsion of exciton states in disordered semiconductor nanostructures
Authors: Zimmermann R. , Runge E. , Savona V.
Published: July 2003

Exciton states in semiconductor nanostructures with disorder exhibit level repulsion, a concept known from random matrix theory. Its manifestations in time‐resolved Rayleigh spectra and in the autocorrelation of optical nearfield spectra are explained, and available experimental results are critically discussed. It is proposed to investigate the intrasubband absorption in quantum wells with emphasis on the aspects of level repulsion.
Level‐statistics in the resonant Rayleigh scattering dynamics of monolayer‐split excitons
Authors: Kocherscheidt G. , Langbein W. , Savona V.
Published: July 2003

The dynamics of resonant Rayleigh scattering (RRS) from excitons localized in a disorder potential with large monolayer islands is investigated theoretically and experimentally. Experimentally, the coherent RRS dynamics from GaAs single quantum wells (SQWs) is deduced by means of passively stabilized spectral interferometry. The new technique is discussed in detail. The results retrieved by spectral interferometry are complemented by data obtained by time‐resolved and spectral speckle analysis. A strong dependence of the monolayer thickness of the SQW on the RRS spectra and the RRS dynamics is found. Changes in the well width corresponding to a fraction of one monolayer lead to drastic changes in the spectral and time‐resolved RRS intensity. A comparison between spectra and dynamics contradicts the assumption of uncorrelated exciton levels. The RRS intensity is derived theoretically by solving the Schrödinger equation for the center‐of‐mass motion of the 1s‐exciton in a disorder potential taking into account both interfaces with monolayer islands, the short range correlated disorder by segregation, and a correlation between the interfaces. The simulations reproduce qualitatively all observed features, determining in this way the characteristic disorder parameters like island sizes and interface roughness.
Density‐matrix description of frequency‐resolved secondary emission from quantum wells
Authors: Mannarini Gianandrea , Zimmermann Roland , Kocherscheidt Gerrit , Langbein Wolfgang
Published: July 2003

The frequency‐resolved secondary emission from excitons in a single 8 nm‐wide quantum well is investigated using speckle analysis. We model these experiments starting with a Hamiltonian in the basis of disorder eigenstates of excitons, interacting both with light and acoustical phonons. The distinction between incoherent and coherent secondary emission is intimately related with the decomposition of a two‐time exciton density‐matrix into a diagonal, incoherent part and a product of polarizations. The latter gives rise to speckling (intensity variations over observation angle), and is called resonant Rayleigh scattering. The results of our simulation agree pretty well with the experimental data and allow a determination of the coherent and incoherent exciton distributions.
Autocorrelation spectroscopy on single GaInAs/GaAs quantum wells
Authors: Neuberth Ulrich , von Freymann Georg , Wegener Martin , Nau Siegfried , Stolz Wolfgang
Published: July 2003

We investigate an atomically rough single GaInAs/GaAs quantum well by means of autocorrelation spectroscopy. The data are based on thousands of individual low‐temperature nano‐photoluminescence spectra and exhibit a peak around 1.8 meV energy difference. On the same sample, we measure the topography of one of the buried heterointerfaces which is possible by using highly‐selective etching and atomic force microscopy.
Ultrafast dephasing in strongly correlated systems
Authors: Perakis I. E.
Published: July 2003

Using projection techniques we derive the equation of motion for the third‐order nonlinear optical polarization of a strongly correlated system. We do not assume a Hartree–Fock ground state, but include the ground state correlations via time‐independent interaction parameters and energies. Our theory can be used to describe the ultrafast nonlinear optical response of a strongly correlated cold electron gas, such as the two‐dimensional electron gas in the quantum Hall effect regime. For this system, we show that the collective electronic excitations lead to a time‐dependent coupling of the Landau level magnetoexcitons and a strong non‐Markovian exciton dephasing. We discuss the signatures in the transient four‐wave‐mixing spectrum.
Phase diagram of the quantum degenerate electron–hole system in diamond
Authors: Nagai Masaya , Shimano Ryo , Horiuch Kenji , Kuwata‐Gonokami Makoto
Published: July 2003

We measured the emission spectra under strong excitation above the band gap using femtosecond pulses and observed the exciton Mott transition at high temperature and the liquid–gas transition below 160 K from temperature and excitation density dependences of emission spectra. From these results we determined the phase diagram of electron–hole system in diamond.
Nonlinear optical dynamics of excitons and trions
Authors: Portella‐Oberli M. T. , Ciulin V. , Kutrowski M. , Wojtowicz T. , Deveaud B.
Published: July 2003

We investigate the nonlinear optical dynamics of excitons and trions in CdTe modulation‐doped quantum wells by time and spectrally resolved pump–probe experiments. We find that the nonlinearities induced by excitons are different from those due to the presence of the trions. From a comparison between the exciton and trion behaviours, we show that, at low temperature, trions are formed from excitons in about 10 picoseconds and, as the temperature is increased, a thermal equilibrium is reached between exciton and trion populations within 5 picoseconds.
Coherent control of carrier population and spin in (111)‐GaAs
Authors: Stevens Martin J. , Bhat R. D. R. , Sipe J. E. , van Driel H. M. , Smirl Arthur L.
Published: July 2003

We report independent coherent control of carrier population and spin in (111)‐oriented GaAs arising through quantum interference of the transition amplitudes associated with one‐ and two‐photon absorption of ∼100 fs phase‐locked optical pulses. We demonstrate this coherent control using various combinations of pulse polarizations and crystal orientations. In addition, we present a phenomenological framework for the spin and population control, and present predictions based on a macroscopic symmetry analysis that agree with our experimental observations.
Excitonic polarons in semiconductor quantum dots
Authors: Ferreira R. , Verzelen O. , Bastard G.
Published: July 2003

We review different aspects concerning the effects of the interaction between confined carriers and longitudinal optical phonons on the optical response of semiconductor quantum dots.
Analytical results for nonlinear optical signals from quantum dots coupled to phonons
Authors: Axt V. M. , Krummheuer B. , Vagov A. , Kuhn T.
Published: July 2003

The impact of pure dephasing processes on the optically induced nonlinear dynamics of quantum dots is analyzed. Concentrating on small dots pure dephasing can be studied analytically in the limit of ultrafast excitation. General conclusions from the analysis of the closed form formulas include the prediction that four‐wave‐mixing (FWM) spectra of single dots change from asymmetric to symmetric lineshapes with increasing pulse delay. Furthermore, in contrast to phenomenological dephasing models the delay dependence of the time integrated FWM amplitude |PFWM| generated by a strongly inhomogeneously broadened ensemble deviates from the real time dependence of |Plin|2, where Plin is the linear polarization emitted by a single dot. Numerical results are presented for FWM signals from dots coupled to bulk acoustic phonons. Surprisingly, the initial decay time of the signal depends non‐monotonously on the temperature. In addition, it is shown that for certain material parameters the coupling to the smooth continuum of acoustic phonons may result in pronounced oscillations.
Influence of coupling between localized and continuum states in InGaN quantum‐dot systems
Authors: Schneider H. C. , Chow W. W. , Koch S. W.
Published: July 2003

This paper investigates the influence of the coupling between localized and continuum states on the optical absorption in wide‐bandgap nitride‐based quantum‐dot structures under high‐excitation conditions. Calculating the optical susceptibility in the presence of strong many‐body Coulomb interactions and the quantum‐confined Stark effect, we show that the interplay of these effects results in absorption/gain features that depend on the quantum‐dot dimensions in a nontrivial way.
Dephasing of biexcitons in InGaAs quantum dots
Authors: Borri P. , Langbein W. , Schneider S. , Woggon U. , Sellin R. L. , Ouyang D. , Bimberg D.
Published: July 2003

We present measurements of the dephasing time of biexcitonic transitions in self‐organized InGaAs quantum dots in the temperature range from 10 K to 70 K. We use an heterodyne four‐wave mixing technique that allows direct measurement of the dephasing time in an inhomogeneously broadened quantum‐dot ensemble. By applying an electrical injection, we progressively increase the number of carriers in the quantum dots and investigate the passage from a four‐wave mixing response dominated by the one‐exciton transition to a response dominated by biexcitonic and multiexcitonic transitions. The temperature dependence of the homogeneous broadening of the biexciton‐to‐exciton transition is extrapolated to zero injection current and its physical origin in terms of radiative recombination and phonon interactions is discussed.
Nonequilibrium theory of photoluminescence excitation spectroscopy in semiconductors
Authors: Hannewald Karsten , Glutsch Stephan , Bechstedt Friedhelm
Published: July 2003

We present a microscopic theory of photoluminescence excitation spectroscopy (PLE) in semiconductors that is not restricted to thermalized carrier distributions. The phenomenological assumption of equivalence between PLE and absorption spectra is confirmed only for thermal equilibrium. For nonequilibrium, i.e, small time intervals between laser‐pulse excitation and luminescence detection, our quantum‐kinetic simulations reveal significant differences between PLE and absorption signals which can be attributed to incomplete relaxation and bottleneck effects, and should be observable in experiment.
Anomalously fast decay of the LH–HH exciton Raman coherence
Authors: Borca C. N. , VanEngen Spivey Amelia G. , Cundiff Steven T.
Published: July 2003

We perform 3‐pulse transient four‐wave‐mixing on a GaAs/AlGaAs multiple quantum well. This allows us to measure both the decay of the Raman coherence between the heavy‐hole and light‐hole excitons and the decay of the respective optical coherences. We observe that the decay rate of the Raman coherence is greater than the sum of decay rates of the optical coherences. This cannot happen within a Markov approximation for the scattering processes, as verified by simulations. The simulations show that such behavior can occur only in the presence of anti‐correlated scattering processes in a non‐Markovian regime.
Microscopic theory for the nonlinear optical response of the Si(111)‐(2×1) surface exciton
Authors: Reichelt M. , Meier T. , Rohlfing M. , Koch S. W.
Published: July 2003

A novel microscopic approach for computing nonlinear optical properties of semiconductor surfaces is outlined. The linear and nonlinear response of the Si(111)‐(2×1) surface is studied using a combination of ab‐initio band‐structure theory and equation of motion technique. The usefulness of the method is shown by reproducing known excitonic effects in the linear differential reflectivity spectrum. To additionally demonstrate the applicability of the approach for the nonlinear regime, the nonlinear optical response in Stark effect configuration is examined.
Non‐classical excitonic transport in quantum wells
Authors: Zhao Hui , Dal Don B. , Moehl S. , Kalt H.
Published: July 2003

We report investigations on excitonic transport in ZnSe quantum wells using far‐field nano‐photoluminescence enhanced by a solid immersion lens. The 250‐nm spatial resolution and 5‐ps temporal resolution allow the access to non‐classical transport regimes. From zero‐phonon‐line spectroscopy we deduce the spatial distribution of the PL intensity under different cw excitation condition and its temporal evolution after a pulsed excitation, respectively. The periodic quenching of the transport length as function of excitation excess energy and the nonlinear expansion of the PL spot observed in time‐resolved experiments reveal the dominance of hot‐exciton effects. The experimental data are well modelled by a Monte Carlo simulation. Spatially resolved phonon‐sideband spectroscopy is used to observe directly the coherent transport and the energy relaxation of hot excitons during transport. The coherence length is measured to be 300–400 nm at low temperatures and the excitons are found to remain hot during transport on a length scale of several micrometers.
Spin‐sensitive bleaching and spin relaxation in QWs
Authors: Schneider Petra , Ganichev S. D. , Kainz J. , Rössler U. , Wegscheider W. , Weiss D. , Prettl W. , Bel'kov V. V. , Golub L. E. , Schuh D.
Published: July 2003

Spin‐sensitive saturation of absorption of infrared radiation has been investigated in p‐type GaAs QWs. It is shown that the absorption saturation of circularly polarized radiation is mostly controlled by the spin relaxation time of the holes. The saturation behavior has been investigated for different QW widths and in dependence on the temperature with the result that the saturation intensity substantially decreases with narrowing of QWs. Spin relaxation times were experimentally obtained by making use of calculated (linear) absorption coefficients for inter‐subband transitions.
Signatures of biexcitons and triexcitons in coherent non‐degenerate semiconductor optics
Authors: Meier T. , Sieh C. , Finger E. , Stolz W. , Rühle W. W. , Thomas P. , Koch S. W. , Wieck A. D.
Published: July 2003

Signatures of exciton and two‐exciton states in semiconductor systems are investigated experimentally and theoretically. A set of quantum well samples with different well widths is studied using coherent excitation spectroscopy. The experimental results are analyzed using a microscopic model demonstrating the importance of biexciton correlations and disorder effects. Additional numerical investigations of absorption changes in small nanorings are employed to analyze optical transitions from two‐exciton to three‐exciton resonances.
Exciton fine structure in interfacial quantum dots
Authors: Phillips R. T. , Steffan A. G. , Newton S. R. , Reinecke T. L. , Kotlyar R.
Published: July 2003

We have studied, by ‘micro‐magneto‐photoluminescence’, the eigenstates of excitons localised in the interfacial potential of narrow GaAs/Al0.31Ga0.69As quantum wells. These behave in many respects as quantum dot excitons, and give narrow emission lines that permit direct resolution of Zeeman splittings. Some dots show small zero‐field splitting (<100 μeV), varying down to below the limit of resolution of our technique (≈25 μeV). For these dots the excitonic g‐factor depends on well‐width, and also shows variation with the applied field B. The variation with B is related to the band mixing which is more prominent in these weakly‐confined dots than in, for example, typical Stranski–Krastanow dots. The magnitude of the term linear in B can be calculated in a simplified model of the confined state, which gives a method of estimating the lateral extent of the confining potential. For particular transitions which are found at the extreme low‐energy end of the emission from wells of 2 nm and 3 nm width, a different behaviour is found. These show much larger zero‐field splitting of around 1 meV, and much smaller diamagnetic shift than that in the more usual dots with small zero‐field splitting. The measured spin fine structure of these states is compared with that calculated from a spin Hamiltonian for a heavy‐hole exciton, which gives a unique determination of the full three‐dimensional g‐factor for a single quantum dot.
Generation of strongly polarization‐correlated photon pairs by cascaded emission from individual CdSe/ZnSe quantum dots
Authors: Ulrich S. M. , Strauf S. , Michler P. , Bacher G. , Forchel A.
Published: July 2003

By combined pulsed auto‐ and cross‐correlation measurements (4 K) on individual self‐assembled CdSe quantum dots, besides triggered single photon emission promising for high repetition frequencies, in particular the generation of time‐correlated photon pairs from the sequential radiative biexciton‐exciton decay is demonstrated. For the photon pairs a strong collinear polarization degree up to 84.5% was observed which is related to an exciton fine‐structure splitting. Together with the measured exciton lifetime an estimate for the relaxation time (≥480 ps) between the fine‐structure levels is given.
K‐dependent exchange interaction of quadrupole excitons in Cu2O
Authors: Dasbach G. , Fröhlich D. , Stolz H. , Klieber R. , Suter D. , Bayer M.
Published: July 2003

The long‐ and short‐range exchange interaction for the quadrupole allowed transition to the yellow 1S orthoexciton in Cu2O is derived. While the three orthoexciton states still remain degenerate, when treating the exchange up to the order of K‐linear terms, a fine structure arises when K‐quadratic terms are included. This splitting is investigated experimentally by high resolution spectroscopy. Exchange contributions of few μeV are resolved and compared to theory. The impact of strain on the exciton fine structure is discussed and evaluated.
Direct observation of all‐optical injection of spin‐polarized currents
Authors: Hübner J. , Klude M. , Hommel D. , Bhat R. D. R. , Sipe J. E. , van Driel H. M. , Rühle W. W.
Published: July 2003

Ballistic currents of spin‐polarized carriers are generated by quantum interference of one and two photon absorption. Polarization and relative phase of the exciting femtosecond laser fields control the magnitude and direction of the currents as well as the electron spin orientation. These spin‐polarized currents are experimentally detected by a phase‐dependent spatial shift of the circularly analyzed photoluminescence in cubic ZnSe.
Exciton–phonon interaction in semiconductors with intermediate polar coupling
Authors: Schäfer W. , Gartner P. , Jahnke F.
Published: July 2003

The importance of vertex corrections in the description of the absorption spectra in semiconductors is discussed. These corrections are responsible for the excitonic correlation of the phonon‐scattered carriers and it is shown that these correlations are essential for the description of excitonic sidebands in the case of intermediate coupling. Different approximation schemes are compared in numerical examples.
Dynamics of two‐pair correlations induced by simultaneous excitation of excitonic and band‐to‐band continuum transitions
Authors: Wühr J. , Axt V. M. , Kuhn T.
Published: July 2003

The impact of all kinds of two‐pair correlations (biexcitons, exciton–exciton scattering states, two free electron–hole pairs) on four‐wave‐mixing (FWM) signals generated by ultra‐short optical pulses that simultaneously excite excitons and free electron–hole pairs is analyzed. Signatures of two‐pair correlations are identified by comparing the full theory with mean‐field (MF) results. For systems with strong exciton binding two‐pair correlations suppress changes of the line width of the excitonic emission that are predicted by the MF theory when the central excitation energy is varied above the band edge. For systems with weakly bound excitons the presence of two‐pair correlations leads to an emission in between the exciton and the band edge with a complex line shape which strongly depends on the excitation energy.
Camouflage third‐harmonic generation in the nonperturbative few‐cycle regime
Authors: Tritschler T. , Mücke O. D. , Morgner U. , Kärtner F. X. , Wegener M.
Published: July 2003

Focusing 5 fs optical pulses, with a Rabi energy comparable to the carrier photon energy, derived from a mode‐locked Ti:sapphire laser oscillator onto ZnO crystals, leads to large contributions in the emitted light, which depend on the carrier‐envelope offset frequency of the laser oscillator. Moreover, we discuss that an additional contribution at twice the carrier‐envelope offset frequency is due to camouflage third‐harmonic generation – a process, which looks like second‐harmonic generation in an inversion symmetric medium at first sight.
Electronic properties of superconducting NbSe2 nanotubes
Authors: Ivanovskaya V. V. , Enyashin A. N. , Medvedeva N. I. , Ivanovskii A. L.
Published: July 2003

Atomic models of zigzag (n,0)‐ and armchair (n,n)‐like nanotubes (n = 7–58) of the superconducting NbSe2 are constructed and their electronic properties and bond indices are investigated using the tight‐binding band theory. The calculations performed show that all the considered NTs are uniformly metallic‐like. The density of states at the Fermi level contains a considerable contribution from Nb4d states, which will be significant for the superconductivity. The stability and electronic properties of the NbSe2 NTs depend on the atomic configuration and diameter of the tubes. Nb—Se covalent bonds were found as basic in NbSe2 tubes. According to our estimations, zigzag‐like configurations of the NbSe2 nanotubes are more stable.
Papers presented at the 7th International Workshop on Nonlinear Optics and Excitation Kinetics in Semiconductors (NOEKS 7)
Authors: Wegener Martin
Published: July 2003

The 7th International Workshop on Nonlinear Optics and Excitation Kinetics in Semiconductors (NOEKS 7) was held at Uni‐versit�t Karlsruhe from 24‐28 February 2003. Topics of NOEKS 7 were: Ultrafast dynamics (coherent effects, coherent control, quantum kinetics, THz‐experiments), photonic crystals (2D and 3D photonic band gap materials), quantum dot physics (quantum dots, quantum wires), spin effects (spin dephasing, spin transport), disorder‐related effects, organic semiconductors, semiconductor quantum optics (luminescence, photon statistics), device physics (quantum cascade lasers, superlattices, interband lasers), and Bose‐Einstein condensation of excitons.

The cover picture is taken from Ref. [1] and shows the spatial pattern of the indirect exciton PL intensity in GaAs/AlGaAs coupled quantum wells. Around the laser excitation spot, it ex‐hibits an external ring structure with a periodic array of circular‐shaped structures which is due to the formation of an exciton gas with macroscopic coherence.

The full proceedings of NOEKS 7, guest‐edited by Martin Wegener, are published in physica status solidi (c) 0, No. 5 (2003). Since 1988, all previous NOEKS proceedings have also appeared in physica status solidi.
Kinetic isotope effects in the active site of B. subtilis chorismate mutase
Authors: Worthington Sharon E. , Roitberg Adrian E. , Krauss Morris
Published: July 2003

Kinetic isotope effects are determined for the enzyme‐catalyzed Claisen rearrangement of chorismate to prephenate using computational methods. The calculated kinetic isotope effects (KIEs) compare reasonably with the few available experimental values with both the theory and experiment obtaining a large KIE for the ether oxygen, indicating large polarization of the transition‐state geometry. Because there is a question of the extent that the experimental rate constants are for chemistry as the rate‐limiting step, the KIEs for all the atoms of the substrate are reported with the exception of the carboxylate groups. A substantial number of large regular and inverse isotope effects are predicted for the hydrogens on the cyclohexadienyl ring related to activation of the reactant and charge reorganization in the transition state. A large KIE is predicted for the hydrogen atom bound to the ether carbon atom because the largest valency change and charge transfer occurs at the ether bond in both the reactant and tansition state. Observation of the overall pattern of predicted KIEs would ensure that conditions are favorable for the rate‐limiting chemistry. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 287–292, 2003
DFT study on cooperativity in the interactions of hydrazoic acid clusters
Authors: Xia Qi Ying , Xiao He Ming , Ju Xue Hai , Gong Xue Dong
Published: July 2003

Density functional theory at the B3LYP level with the 6‐311G** basis set is performed to calculate the systems consisting of up to four hydrazoic acid molecules. The dimers are found to exhibit cyclic and chain structures with N … H contacts at ca. 2.1–2.7 Å. However, there are only cyclic structures with N … H contacts at ca. 2.0–2.3 Å and 2.0–2.1 Å in the trimer and tetramer, respectively. Hydrogen bond distances in the trimer and tetramer are shorter than those in the cyclic dimer as a result of the stronger interaction between molecules. The contribution of cooperative effect to the interaction energy is significant. After the correction of the basis set superposition error and zero‐point energy, the binding energies are −10.69, −29.34, and −54.26 kJ·mol−1 for the most stable dimer, trimer, and tetramer, respectively. The calculated IR shifts for NH stretching mode increase with the size of the cluster growths, reaching more than 200 cm−1 in the tetramer. For the most stable clusters, the transition from the monomer to dimer, dimer to trimer, and trimer to tetramer involve changes of −14.40, −25.68, and −31.88 kJ·mol−1 for the enthalpies at 298.15 K and 1atm, respectively. We also perform Mulliken populations analysis and find the Mulliken populations on intermolecular N … H increasing in the sequence of the dimer, trimer, and tetramer. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 279–286, 2003
Structure and stability of B6, B, and B clusters
Authors: Li Qian Shu , Jin Qiao , Luo Qiong , Tang Au Chin , Yu Jian Kang , Zhang Hong Xing
Published: July 2003

We investigated various isomers of B6, B, and B clusters with ab initio [Hartree–Fock (HF), MP2)] and density functional theory (DFT) methods. Ten B6 isomers, 6 B isomers, and 6 B isomers are determined to be local minima on their potential energy hypersurfaces by the HF, B3LYP, B3PW91, and MP2 methods. Fourteen of these structures are first reported. The most stable neutral B6 cluster is the capped pentagonal pyramid (C5v), in agreement with the results reported previously. Hexagon B (C2h) isomer and fan‐shaped B (C2v) isomer are found to be the most stable on the cationic and anionic energy hypersurfaces, respectively. Natural bond orbital analysis suggests that there are three‐centered bonds in the most stable B6 neutral and ionic clusters. The multicentered bonds are responsible for the special stability of the lowest‐energy isomer. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 269–278, 2003
Direct solution of the Schrödinger equation by a parallel genetic algorithm: Cases of an exactly solvable 2‐D interacting oscillator and the hydrogen atom
Authors: Saha Rajendra , Bhattacharyya S. P. , Taylor Christopher D. , Zhao Yong , Cundari Thomas R.
Published: July 2003

The Schrödinger equation for an exactly solvable 2‐D interacting oscillator problem is solved numerically by application of a parallel genetic algorithm on a fixed coordinate grid (n × n). The critical energy evaluation step for the wave function strings is parallelized. A fast Fourier transform is used for computing the kinetic energy while the potential energy is obtained by quadrature. Comparison with the exact result indicates viability of the method. As a second case, the hydrogen atom problem is solved by the same method. Parallel performance of the code is also assessed. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 243–250, 2003
Coupled‐perturbed Hartree–Fock theory for quasi–one‐dimensional periodic systems: Calculation of static and dynamic nonlinear optical properties of model systems
Authors: Martinez A. , Otto P. , Ladik J.
Published: July 2003

An alternative method to solve the coupled‐perturbed Hartree–Fock (CPHF) equations for infinite quasi–one‐dimensional systems is presented. The new procedure follows a proposal made by Langhoff, Epstein, and Karplus to obtain perturbed wavefunctions free from arbitrary phase factors in each order of perturbation. It is based on the intermediate orthonormalization of the perturbed wavefunctions (which is different from the usual one) and a corresponding selection of the Lagrangian multipliers. In this way it is possible to incorporate the orthonormalization conditions into the set of CPHF equations. Moreover, a new, advantageous procedure to determine the derivatives of the wavefunction with respect to the quasimomentum k is presented. We report calculations of the dipole moment, the polarizability α, and the first hyperpolarizability β for different polymers (poly‐HF, poly‐H2O, trans‐polyacetylene, polyyne, and polycarbonitrile) for different frequencies. These results are extensively compared with oligomer calculations. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 251–268, 2003
Effect of knowledge integration activities on students' perception of the earth's crust as a cyclic system
Authors: Kali Yael , Orion Nir , Eylon Bat‐Sheva
Published: July 2003

Systems thinking is regarded as a high‐order thinking skill required in scientific, technological, and everyday domains. However, little is known about systems thinking in the context of science education. In the current research, students' understanding of the rock cycle system after a learning program was characterized, and the effect of a concluding knowledge integration activity on their systems thinking was studied. Answers to an open‐ended test were interpreted using a systems thinking continuum, ranging from a completely static view of the system to an understanding of the system's cyclic nature. A meaningful improvement in students' views of the rock cycle toward the higher side of the systems thinking continuum was found after the knowledge integration activity. Students became more aware of the dynamic and cyclic nature of the rock cycle, and their ability to construct sequences of processes representing material transformation in relatively large chunks significantly improved. Success of the knowledge integration activity stresses the importance of postknowledge acquisition activities, which engage students in a dual process of differentiation of their knowledge and reintegration in a systems context. We suggest including such activities in curricula involving systems‐based contents, particularly in earth science, in which systems thinking can bring about environmental literacy. © 2003 Wiley Periodicals, Inc. J Res Sci Teach 40: 545–565, 2003
Use of learner‐centered instruction in college science and mathematics classrooms
Authors: Walczyk, Jeffrey J. , Ramsey Linda L.
Published: July 2003

Learner‐centered approaches to science and mathematics instruction assume that only when students are active participants will learning be deep, enduring, and enjoyable, and transfer to contexts beyond the classroom. Although their beneficial effects are well known, the extent to which learner‐centered practices are used in college classrooms may be low. Surveys of undergraduate science and math majors reveal general dissatisfaction with how courses in their majors are taught, and their number is half what it was 2 decades ago. In response, federally funded systemic reform initiatives have targeted increasing the use learner‐centered instruction in science and mathematics courses to improve undergraduate education generally and the training of preservice teachers specifically. Few data exist regarding how effective these initiatives have been or how frequently learned‐centered instruction occurs as assessed from faculty's perspective, which may not corroborate undergraduate perceptions. Accordingly, a survey was developed to assess the use of learner‐centered techniques and was administered to science and math professors of Louisiana over the Internet. The return rate was 28%. Analyses reveal that they are used infrequently, but when used, are applied to all aspects of teaching. Data also suggest that federal funding has been slightly effective in promoting its use. © 2003 Wiley Periodicals, Inc. J Res Sci Teach 40: 566–584, 2003
Development of an instrument for measuring cognitive conflict in secondary‐level science classes
Authors: Lee Gyoungho , Kwon Jaesool , Park Sang‐Suk , Kim Jung‐Whan , Kwon Hyeok‐Gu , Park Hac‐Kyoo
Published: July 2003

Based on conceptual change theory, cognitive conflict is known as an important factor in conceptual change even though there are still questions about its positive and negative effects on science learning. However, there is no reliable method by which to assess the cognitive conflict students experience in their learning. The purpose of this research was to develop an instrument for measuring secondary students' cognitive conflict levels as they learned science. The results of this study indicate that our instrument is a valid and reliable tool for measuring cognitive conflict levels. Factor analysis supported the model that cognitive conflict consists of four constructs: recognition of an anomalous situation, interest, anxiety, and cognitive reappraisal of the conflict situation. Implications for instruction and possibilities for future research are discussed. © 2003 Wiley Periodicals, Inc. J Res Sci Teach 40: 585–603, 2003
Gibt es Präferenzen bei der grafischen Darstellung logischer Relationen?
Authors: Runde Anne , Bromme Rainer , Stahl Elmar
Published: July 2003

Zusammenfassung: In einer Untersuchung schätzten 64 Personen in einem vollständigen Paarvergleich Strukturdiagramme mit verschiedenen Variationen im Hinblick darauf ein, wie gut sie unterschiedliche logische Relationen darstellen. Die logischen Relationen wurden innerhalb von Sätzen dargeboten, die bezüglich der Nennungsreihenfolge von mehr oder weniger umfangreichen Begriffen variierten. Eine conjointanalytische Auswertung ergab, dass die bevorzugte Darstellungsform weitgehend unabhängig von der Art der logischen Relation ist. Bezüglich der Nennungsreihenfolge der Begriffe ergab sich in Abhängigkeit der logischen Relation eine Präferenz für eine Übereinstimmung der grafischen Darstellung mit den Oberflächenmerkmalen der Sätze. Die Befunde werden insbesondere im Hinblick auf ihre didaktischen Konsequenzen diskutiert.
Familien mit hochbegabten Jugendlichen
Authors: Schilling Susanne R. , Sparfeldt Jörn R. , Rost Detlef H.
Published: July 2003

Zusammenfassung: Bislang vorliegende Ergebnisse zu angeblichen oder tatsächlichen “Besonderheiten” von Familien mit hochbegabten Jugendlichen sind widersprüchlich, was sich (weitgehend) mit methodischen Schwächen erklären lässt. In der vorliegenden Studie werden 84 Familien mit einem hochbegabten Jugendlichen mit 95 Familien mit einem durchschnittlich begabten Jugendlichen verglichen. Die Daten stammen aus dem “Marburger Hochbegabtenprojekt”. Drei Datenquellen werden unabhängig voneinander herangezogen: Jugendliche, Mütter und Väter. Erhoben werden die Familiensystemvariablen Kohäsion, Demokratischer Familienstil (Adaptabilität), Organisation, Leistungsorientierung und Kommunikation. Väter und Mütter schätzen zusätzlich die kulturelle Orientierung der Familie ein. Weder für die Jugendlichen noch für die Mütter noch für die Väter lässt sich ein statistisch und/oder praktisch bedeutsamer Zusammenhang der Begabung des Jugendlichen mit den Beurteilungen des Familiensystems belegen. Abweichende Resultate in der Literatur sind vermutlich auf methodische Mängel und/oder “labeling”-Effekte zurückzuführen. Die durchschnittliche Übereinstimmung zwischen Müttern und Vätern beträgt r = .46, zwischen Jugendlichen und Müttern r = .38 und zwischen Jugendlichen und Vätern r = .35.
Hängen Ergebnisse einer Lehrveranstaltungs-Evaluation von der Häufigkeit des Veranstaltungsbesuches ab?
Authors: Berger Uwe , Schleußner Christine
Published: July 2003

Zusammenfassung: Anhand der Daten einer Lehrveranstaltungs-Evaluation wird der Zusammenhang zwischen den Bewertungen und der Häufigkeit des Besuches von Vorlesungen untersucht. Einbezogen wurden 169 Vorlesungen und 288 Seminare, die zwischen 1999 und 2002 von insgesamt 13 497 Studierenden auf jeweils 13 Items an der Medizinischen Fakultät der Friedrich-Schiller-Universität Jena evaluiert wurden. Es zeigte sich ein signifikanter Zusammenhang mit großer Effektstärke zwischen dem Vorlesungsbesuch und der Bewertung der didaktischen Fähigkeiten der Lehrenden in der Weise, dass Studierende die Vorlesung um ca. eine halbe Note (gut vs. gut-befriedigend) besser bewerteten, wenn sie diese zu über 80 % besuchten. Dies kann als Mahnung zur Vorsicht bei beabsichtigten Vergleichen von Lehrenden, Fächern und Fakultäten allein auf der Basis von Lehrveranstaltungs-Evaluationen verstanden werden und widerspricht Resultaten einschlägiger Studien zu anderen möglichen Verzerrungsfaktoren, wie Besuchsgrund, Arbeitsaufwand, Notenstrenge, Veranstaltungsgröße, Status des/der Lehrenden usw., die die Bewertungen kaum beeinflussen. Im Einklang mit anderen Studien fand sich ein deutlicher Effekt des Vorinteresses auf die Evaluations-Ergebnisse (r = .59).
Conceptual Change: kognitiv oder situiert?
Authors: Stark Robin
Published: July 2003

Zusammenfassung: Es werden zwei kognitive Ansätze zu Conceptual Change ausführlich dargestellt: der “Rahmentheorieansatz” von Vosniadou ( Vosniadou & Brewer, 1992 ) und der “Kategorisierungsansatz” von Chi (1992) . Vosniadou beschreibt Conceptual Change als graduelle Revision mentaler Modelle, bei Chi wird Conceptual Change als Überwindung von ontologischen Kategorisierungsfehlern konzipiert. Beide “Defizit orientierten” Ansätze werden aus einer situierten Perspektive kritisiert. Hierbei werden das Verdinglichungsproblem sowie Probleme des vernachlässigten Kontexts, der Wissensdiagnostik, der Fokussierung auf initiales Lernen in wohl strukturierten Domänen und schließlich das Problem des kognitiven Bias diskutiert. Als Beispiel für einen situierten Ansatz wird ein Kontextmodell von Caravita und Halldén (1994) vorgestellt. Zudem wird auf die “radikale” situierte Position von Säljö (1999) eingegangen. Im Ausblick werden Möglichkeiten einer gegenseitigen Befruchtung kognitiver und situierter Conceptual-Change-Ansätze diskutiert.
Cognitive Load beim Lernen aus Lösungsbeispielen
Authors: Renkl Alexander , Gruber Hans , Weber Sandra , Lerche Thomas , Schweizer Karl
Published: July 2003

Zusammenfassung: Die Effektivität des Lernens aus Lösungsbeispielen im Vergleich zum Lernen durch Problemlösen wird derzeit mit der Cognitive Load-Theorie erklärt: Beim Lernen aus Lösungsbeispielen wird das Arbeitsgedächtnis weniger belastet, daher bleibt mehr Raum für Lern- und Verstehensprozesse. Um diese These direkt experimentell zu überprüfen, wurde das dual task-Paradigma eingesetzt. 80 Studierende der Geistes- und Sozialwissenschaften wurden auf die Zellen eines 2 × 2-faktoriellen Designs (Faktor 1: Lernen aus Lösungsbeispielen vs. Lernen durch Problemlösen; Faktor 2: mit vs. ohne Zweitaufgabe) verteilt. Der Lernstoff war Wahrscheinlichkeitsrechnung. Der Lernerfolg wurde mit einem Nachtest erfasst. Das Befundmuster hinsichtlich des Lernerfolgs und der Reaktionszeiten auf eine Zweitaufgabe in den vier experimentellen Gruppen stimmte mit den aus der Cognitive-Load-Theorie abgeleiteten Hypothesen überein.
Mentale Personenrepräsentationen:
Authors: Lemche Erwin
Published: July 2003

Zusammenfassung. Die vorliegende Arbeit legt eine neurophysiologisch fundierte Systematik der Komponenten des Konstruktes mentaler Repräsentationen wichtiger Anderer und des eigenen Selbst vor. Hierbei werden auf zwei Ebenen je drei Forschungsfelder übersichtsartig dargestellt. Die erste Ebene skizziert anhand von neurologischen, neurobiologischen und neuroimaging-Befunden die putativen neurobiologischen Grundlagen des Selbst und anderer Personen, der menschlichen Bindungsrepräsentation in Gesichtsneuronen, sowie ihrer mentalistischen Anteile (theory of mind). Die zweite Ebene folgt der ersten in der Beschreibung der frühkindlichen Entwicklung der Gesichtswahrnehmung und Spiegel-Selbsterkennung, der Konstitution der Bindungsrepräsentation, sowie schließlich der Entwicklung der Sprache für mentale Zustände. Als ein Ansatz zur Untersuchung von Struktur-Funktions-Bezügen wird vorgeschlagen, bisher unverbundene Teilbereiche der verschiedenen Forschungsstränge dergestalt zu verbinden, daß eine mögliche wechselseitige Abhängigkeit der Domänen korrelativ untersucht werden kann.