Janette L. Dunn

An analysis of the T ⊗ (e + t2) Jahn-Teller system with strong coupling

The T(X)(e+t2) Jahn-Teller system is studied in the strong-coupling approximation on a cluster model by transformation methods. An energy-minimisation procedure is used to show that the system will be localised in potential energy minima having either tetragonal, trigonal or orthorhombic symmetry depending upon the strengths of the e and t2 coupling constants. Detailed results are given for the orthorhombic case to underline the physical processes involved in the Jahn-Teller effect and provide a detailed mathematical framework for the calculation of other properties of the system (e.g. first- and second-order reduction factors, random-strain effects).

Theoretical calculations of shallow donor impurity states in GaAs/GaAlAs multi-quantum wells with magnetic fields applied parallel to the layers

A method is presented for the calculation of the energy eigenvalues and eigenstates of the electron associated with a shallow donor impurity in a GaAs/GaAlAs multi-quantum well structure in the presence of a magnetic field parallel to the layers. Compared to the case of an impurity in a magnetic field directed perpendicular to the layers, the problem is complicated by the lack of cylindrical symmetry. This lack of symmetry is reflected in the form chosen here for the basis states that describe the donor electron. By formulating the Hamiltonian and operating on these states, the energies and orthonormal ground and excited state wavefunctions of the donor electron are determined by matrix diagonalization procedures. Calculations of the transition probability between the ground and excited states yield the theoretically most probable transition energy for a given magnetic field. These calculated values are compared to available experimental data and deviations between the theory and experiments at large fields are discussed.

A SNP in the ACT gene associated with astrocytosis and rapid cognitive decline in AD

There is biochemical and animal model evidence supporting a pathological role of the ACT gene in AD. However, direct genetic evidence remains controversial and has been mostly limited to individual single nucleotide polymorphism (SNP) analysis. To resolve this apparent conflict we have used a high-density ACT SNP map, constructed haplotypes and explored correlations with phenotype. SNPs were identified by sequencing and used to construct haplotypes in 668 AD patients and 419 controls and a case-control association study was performed. Five SNPs, comprising five common haplotypes, represented 93% of ACT gene variation. Although no single SNP or haplotype was associated with AD status, a SNP in intron 2 was associated with later onset and more rapid cognitive decline (P=0.04). This SNP was both individually associated with severe astrocytosis (P=0.004) in AD patients and when combined with the signal sequence SNP (P=0.002). This suggests that astrocytosis may have a protective function for a limited period in some patients. These SNP associations either support a direct role for the ACT gene, in AD pathology or alternatively reflect linkage with polymorphisms in other genes nearby.

A complete nearest-neighbor force field model for C60.

A force field model is developed for C(60) that features 13 force constants representing all interactions between nearest-neighboring atoms. The model is compared with, and tested against, other force field models in the literature. Force constants for C(60) are then deduced by fitting the model to the 14 known optically accessible vibrational frequencies of the molecule. Finally, the model is fitted to two existing theoretical calculations of the complete vibrational spectrum of C(60). Fair agreement is obtained with the theoretical calculations, implying that interactions with atoms other than nearest neighbors are small.

Strongly coupled orbital triplet Jahn-Teller systems

Further details of a new method for studying strongly coupled orbital triplet Jahn-Teller systems (JT) are presented. The procedure, which involves unitary transformation and energy minimisation, allows coupling to both e- and t2-type cluster vibrations. Detailed results are presented for systems in which trigonal wells in the potential energy surface are energy minima. An estimate is made of the range of e- and t2-type couplings for which the method is valid. Results are also presented for systems with tetragonal energy minima. These are compared with well known results of T(X) e JT problem for the case when the t2-type coupling can be neglected.

Theoretical calculations of the transition energies of a hydrogenic impurity in GaAs/GaAlAs multi-quantum wells in magnetic fields applied at any angle

Calculations are made of the energies of the dominant transition (1s to 2p+1-like) of a hydrogenic donor impurity in a multi-quantum-well (MQW) system subject to a magnetic field applied at an angle theta to the direction of growth of the MQW layers. The model used involves constructing suitable basis states, and then performing a matrix diagonalization procedure. The results are compared to the available experimental data, including the extreme cases of fields perpendicular ( theta =0 degrees ) and parallel ( theta =90 degrees ) to the layers. The results are also found to compare well with the results of existing variational calculations.

Thermally detected optical absorption and photoluminescence in a GaAs/GaAlAs multiquantum well sample

A multiquantum well (MQW) GaAs/GaAlAs system is studied by a non-conventional technique of thermally detected optical absorption (TD-OA) in which the sample temperature variation is measured by a germanium thermometer at 0.5 K. In such a sample, for which the holder and quantum well materials are the same, the QW transitions give rise to temperature minima. The latter are interpreted in terms of intensity variations of the energy reflected at the interfaces of the wells close to the excitonic recombinations. The TD-OA spectra corresponding to the n=1 heavy-hole and light-hole excitonic transitions in the wells are examined, together with the spectra obtained by conventional photoluminescence. Doublet lines involving QW excited levels, a component of which is due to a transition only allowed by valence band mixing, are also detected as in photoluminescence excitation.

Excited-state spectroscopy of confined shallow donor impurities in a multi-quantum well

The far infrared (FIR) photoconductive response of silicon-doped GaAs/AlGaAs multi-quantum wells (MQWS) at 4.2 K reveals evidence of several transitions from the ground state to higher excited states of the confined impurity (e.g. 1s-3d+1, 1s-3p+1, 1s-3d+2, 1s-4d+2 etc.). Assignments are made by comparison with the bulk case and with available theory. Linewidths of the 1s-2p+ transition indicate that in the most favourable case minimum redistribution of the silicon dopant occurs which permits observation of higher state transitions.

A physical explanation of the ground state crossover in the H⊗h Jahn–Teller effect

By comparison with other systems, vibronic coupling between an h-type vibration and an H-type electronic level in a H⊗h Jahn–Teller system would be expected to result in a vibronic H-type ground state. However, it is already known that an A state can cross over the H state at a given coupling strength and become the ground state if the ground adiabatic potential energy surface contains minima of D3d symmetry. This is an unusual property of the H⊗h Jahn–Teller system. In this article, the physics behind the crossover is analyzed in terms of competition between tunneling paths between D3d wells of C1 and C2 symmetries. The H⊗h Jahn–Teller system is relevant to some fullerenes and other icosahedral complexes.

Effect of crossed electric and strong magnetic fields on impurity states in a quantum well

An analytical approach to the problem of an impurity electron (or hole) in a quantum well (QW) subject to crossed electric and strong magnetic external fields is developed. The magnetic field is directed perpendicular to the heteroplanes. The dependencies of the impurity energy levels upon the width of the well, upon the position of the impurity within the QW and upon the magnitudes of the external fields are obtained. If the electric field increases in magnitude, if the width of the well decreases and if the impurity shifts from the middle point of the QW, the energy of the impurity electron increases in each case. It is shown also how the results may be extended to the effect of crossed fields to the case of a confined exciton.