arXiv subjects

N. F. Johnson

Publications and source records attributed to N. F. Johnson.

63 records · Page 4Linked to original sources

arXiv1998

Doublet structures in quantum well absorption spectra due to Fano-related interference

In this theoretical investigation we predict an unusual interaction between a discrete state and a continuum of states, which is closely related to the case of Fano-interference. It occurs in a GaAs/AlxGa1-xAs quantum well between the lowest light-hole exciton and the continuum of the second heavy-hole exciton. Unlike the typical case for Fano-resonance, the discrete state here is outside the continuum; we use uniaxial stress to tune its position with respect to the onset of the continuum. State-of-the art calculations of absorption spectra show that as the discrete state approaches the continuum, a doublet structure forms which reveals anticrossing behaviour. The minimum separation energy of the anticrossing depends characteristically on the well width and is unusually large for narrow wells. This offers striking evidence for the strong underlying valence-band mixing. Moreover, it proves that previous explanations of similar doublets in experimental data, employing simple two-state models, are incomplete.

cond-mat.mes-hall↗

arXiv1998

Volatility and Agent Adaptability in a Self-Organizing Market

We present results for the so-called `bar-attendance' model of market behavior: $p$ adaptive agents, each possessing $n$ prediction rules chosen randomly from a pool, attempt to attend a bar whose cut-off is $s$. The global attendance time-series has a mean near, but not equal to, $s$. The variance, or `volatility', can show a minimum with increasing adaptability of the individual agents.

cond-mat↗

arXiv1997

Acoustic attenuation in a type-II superconductor at high magnetic fields

We have calculated the longitudinal acoustic attenuation in a type-II superconductor in high magnetic fields within a mean-field BCS theory. We predict two new features in the corresponding attenuation signal as compared to that of the Meissner state. Our analytical calculations predict the existence of oscillations in the attenuation as the external magnetic field is varied- this effect is associated with the Landau level structure of the electron states and is analogous to the well-known de Haas van Alphen oscillations in the mixed state. The attenuation directly probes the quasiparticle energies; the presence of gapless points in the quasiparticle spectrum, which is characteristic of type-II superconductors at high magnetic fields, shows up in the frequency and temperature dependence of the attenuation in the limit of low frequency and low temperature respectively. At low temperature there is no analogue to the discontinuity in the attenuation observed in the Meissner state when $\hbarω=Δ$, where $Δ$ is the quasiparticle gap and $=ω$ is the frequency. This result opens up the possibility of experimentally determining the existence and nature of the gapless points in the quasiparticle spectrum of a type-II superconductor in high magnetic fields.

cond-mat.supr-con↗

arXiv1997

Optics with Quantum Hall Skyrmions

A novel type of charged excitation, known as a Skyrmion, has recently been discovered in quantum Hall systems with filling factor near ν= 1. A Skyrmion -- which can be thought of as a topological twist in the spin density of the electron gas -- has the same charge as an electron, but a much larger spin. In this review we present a detailed theoretical investigation of the optical properties of Skyrmions. Our results provide means for the optical detection of Skyrmions using photoluminescence (PL) spectroscopy. We first consider the optical properties of Skyrmions in disordered systems. A calculation of the luminescence energy reveals a special optical signature which allows us to distinguish between Skyrmions and ordinary electrons. Two experiments to measure the optical signature are proposed. We then turn to the optical properties of Skyrmions in pure systems. We show that, just like an ordinary electron, a Skyrmion may bind with a hole to form a Skyrmionic exciton. The Skyrmionic exciton can have a lower energy than the ordinary magnetoexciton. The optical signature of Skyrmions is found to be a robust feature of the PL spectrum in both disordered and pure systems.

cond-mat.mes-hall↗

arXiv1997

Photoluminescence signatures of negatively charged magnetoexcitons

The negatively charged quasi-two-dimensional exciton ($X^-$) is studied numerically in the presence of a uniform B-field and various in-plane confinements. The corresponding photoluminescence (PL) spectra, including shakeup processes, are calculated. These calculations provide a quantitative explanation of recent experimental PL spectra. The photoluminescence signatures of charged excitons in quantum dots are also examined. Electron-hole pair distriution functions are calculated which show the structure of the charged exciton states (both ground and excited states). Increasing excitation reflects a competition between two effects: partial ionization of either one or two electrons.

cond-mat↗

arXiv1997

Skyrmionic excitons

We investigate the properties of a Skyrmionic exciton consisting of a negatively charged Skyrmion bound to a mobile valence hole. A variational wave function is constructed which has the generalized total momentum P as a good quantum number. It is shown that the Skyrmionic exciton can have a larger binding energy than an ordinary magnetoexciton and should therefore dominate the photoluminescence spectrum in high-mobility quantum wells and heterojunctions where the electron-hole separation exceeds a critical value. The dispersion relation for the Skyrmionic exciton is discussed.

cond-mat.mes-hall↗

arXiv1997

A solvable model for excitonic complexes in one dimension

It is known experimentally that stable few-body clusters containing negatively-charged electrons (e) and positively-charged holes (h) can exist in low-dimensional semiconductor nanostructures. In addition to the familiar exciton (e+h), three-body 'charged excitons' (2e+h and 2h+e) have also been observed. Much less is known about the properties of such charged excitons since three-body problems are generally very difficult to solve, even numerically. Here we introduce a simple model, which can be considered as an extended Calogero model, to calculate analytically the energy spectra for both a charged exciton and a neutral exciton in a one-dimensional nanostructure, such as a finite-length quantum wire. Apart from its physical motivation, the model is of mathematical interest in that it can be related to the Heun (or Heine) equation and, as shown explicitly, highly accurate, closed form solutions can be obtained.

cond-mat↗

arXiv1997

Ginzburg-Landau-Gor'kov Theory of Magnetic oscillations in a type-II 2-dimensional Superconductor

We investigate de Haas-van Alphen (dHvA) oscillations in the mixed state of a type-II two-dimensional superconductor within a self-consistent Gor'kov perturbation scheme. Assuming that the order parameter forms a vortex lattice we can calculate the expansion coefficients exactly to any order. We have tested the results of the perturbation theory to fourth and eight order against an exact numerical solution of the corresponding Bogoliubov-de Gennes equations. The perturbation theory is found to describe the onset of superconductivity well close to the transition point $H_{c2}$. Contrary to earlier calculations by other authors we do not find that the perturbative scheme predicts any maximum of the dHvA-oscillations below $H_{c2}$. Instead we obtain a substantial damping of the magnetic oscillations in the mixed state as compared to the normal state. We have examined the effect of an oscillatory chemical potential due to particle conservation and the effect of a finite Zeeman splitting. Furthermore we have investigated the recently debated issue of a possibility of a sign change of the fundamental harmonic of the magnetic oscillations. Our theory is compared with experiment and we have found good agreement.

supr-con↗

arXiv1997

Photoluminescence signature of skyrmions at ν= 1

The photoluminescence spectrum of quantized Hall states near filling factor ν= 1 is investigated theoretically. For ν>= 1 the spectrum consists of a right-circularly polarized (RCP) line and a left-circularly polarized (LCP) line, whose mean energy: (1) does not depend on the electron g factor for spin-1/2 quasielectrons, (2) does depend on g for charged spin-texture excitations (skyrmions). For ν< 1 the spectrum consists of a LCP line shifted down in energy from the LCP line at ν>= 1. The g-factor dependence of the red shift of the LCP line determines the nature of the negatively charged excitations.

cond-mat.mes-hall↗