Xy State Research Articles

An optical and electrical study of tunnelling mechanisms through indirect gap single barrier GaAs/AlAs/GaAs heterostructures

We present an electroluminescence (EL) and transport study of the tunnelling mechanisms in GaAs/AlAs/GaAs single barrier p-i-n tunnelling structures. We show that both Г-X and XГ tunnelling processes arise from either elastic transfer involving the longitudinal X z barrier states, or momentum conserving phonon assisted transfer involving transverse X xy states. The tunnelling process is shown to be very strongly sequential.

High pressure study of Γ-X mixing in InAs/GaAs quantum dots

We have measured low-temperature photoluminescence spectra of InAs quantum dots embedded in a GaAs crystalline matrix under hydrostatic pressures up to 7 GPa. Below 4.2 GPa the spectra are dominated by the Γ-like electron-heavy hole (HH) exciton transition in the InAs dots. Above 4.2 GPa the spectra show two X-related luminescence bands which are attributed to the indirect type-I transition between X xy and HH states of the dots and the type-II transition from X states in GaAs to InAs HH states, respectively. In the Γ-X crossover regime we find evidence for a pronounced mixing interaction between InAs Γ-like and GaAs X-like states. The corresponding interaction potential is estimated to be 9 meV.

Ambiguities in the Effective Stark Hamiltonian

We show that a Stark Hamiltonian can be transformed to an effective one which is block-diagonal with respect to polyads of interacting vibrational states. The transformation does not modify the block diagonal parameters of the effective dipole moment operator, whereas parameters of the polarizability tensor have additional contributions. These contributions are however small. The effective Stark Hamiltonian describing the quadratic Stark effect has been written for 0100 and 0001 interacting vibrational states of tetrahedral XY 4 molecules. We found a strong correlation between the effective dipole moment parameters and the parameters of the effective polarizability tensor in this Hamiltonian. To remove this correlation, the effective Stark Hamiltonian has been transformed to reduced form. Numerical tests of these predictions have been performed.

Covalent binding parameters and the ground state wavefunctions of vanadyl ion complexes

The Steven’s method of molecular orbitals for octahedral complexes containing transition metal ions has been used for estimating the binding parameters and interpreting theg factors of VO 2 + ion in single crystals. The expressions forg factors have been given in terms ofK ‖ andK ⊥ taking into account the tetragonal crystalline field and covalent binding effects. Computations show thatK ⊥ should be less than 0.066 in order to fit the experimentalg values. The ground state wave function (GSWF) of VO 2 + ion doped in different single crystals has been estimated using crystal field theory. The GSWF is found to be ind xy state with slight admixture of the excited states\(d_{x^2 - y^2 } \),d xz andd yz. The hyperfine interaction parameterP and Fermi contact termX have also been estimated.

Dominant rovibrational interactions in the local-mode limit

It is shown that typical structures computed for the fundamental stretching states of XY 3 and XY 4 molecules can be directly accounted for if local-axis symmetry is introduced. The method is extended to XY 6 tops and to other higher excited states.

On the role of background scattering in the resonance interaction of slow electrons with molecules

The problem of interaction of slow electrons e- with molecules XY has been formulated directly for the collision T operator. A basis of continuous spectrum states has been used in which the K matrix, related to the collision operator, is additive with respect to the direct and the resonance interaction mechanism. This makes it possible to perform an analytical study on the contribution of these mechanisms to the processes of scattering and dissociative attachment (DA) of electrons as well as to define the validity of the theoretical approaches available. It has been shown that the elastic background scattering decreases the DA rate in the low-energy case and can be taken into account by a redefinition of the effective non-local optical potential which is introduced in the configuration interaction method to describe nuclear motion in intermediate states of XY-. The inelastic background scattering distorts the form of the resonance peaks and increases the DA rate due to the appearance of new reaction channels taking place when the molecule undergoes vibrational excitation in the collision.

Reduced Hamiltonian for 0100 and 0001 interacting states of tetrahedral XY 4 molecules: Calculated r 2J 2 and r 2J 3-type parameters for ν2 and ν4 bands of methane

Ambiguity of the effective Hamiltonian for the interacting E and F 2 states of tetrahedral XY 4 molecules was studied. Unitary transformations to yield a reduced form of the Hamiltonian were suggested, the latter including only empirically determinable parameters. The r 2J 2 and r 2J 3 -type spectroscopic parameters for the 0001 and 0100 dyad of CH 4 molecule were calculated by means of contact transformations. It was shown that sets of spectroscopic parameters derived by several authors for the interacting ν 2 and ν 4 bands of CH 4 and SiH 4 may be related via certain unitary transformations.

Some effects of spin-orbit interaction on rotational levels and rotational line intensities in vibrationally unexcited 2A, 2E, and 2F electronic states of open-shell XY 4 molecules

Rotational energy levels in vibronic ground states of 2 A, 2 E, and 2 F electronic states of open-shell XY 4 molecules, as well as rotational line intensities for allowed transitions between such states, are discussed, including the effects of spin-orbit interaction and tetrahedral splittings. Jahn-Teller effects are assumed to be small, and are only taken into account implicitly, through their contributions to various parameters in the effective Hamiltonian. Qualitative information is obtained by considering several limiting-case coupling schemes among the electron spin angular momentum S, the electron orbital angular momentum L, and the pure rotational angular momentum R. These limiting cases are similar in spirit to Hund's coupling cases in diatomic molecules, but differ sufficiently from the latter to make detailed correspondences unhelpful. Quantitative information on rotational energy levels and line intensities is obtained numerically by diagonalizing a Hamiltonian matrix set up in a basis set characterized by uncoupled moleculefixed projections of S, L, and the total angular momentum J, and symmetrized so that all basis set functions belong to a definite species in the subgroup D 2 d of the true point group T d . Hamiltonian matrix elements are determined by ladder operator techniques. Three sample calculated spectra, corresponding to p( 2F 2)-s( 2A 1) , d( 2E)-p( 2F 2) , and d( 2F 2)-p( 2F 2) are presented. As one might expect, when the spin-orbit constant A is set equal to zero, then both qualitative and quantitative aspects of the rotational-electronic problem in open-shell XY 4 molecules can be mapped easily onto discussions of the rotation-vibration problem from the CH 4 literature.

XY-Ising crossover transition in the one-dimensional antiferromagnet TMMC: Effect of a magnetic field

Quasi-elastic neutron scattering experiments have been performed in the one-dimensional antiferromagnet (CD 3) 4NMnCl 3 (TMMC) at low temperature, 1 < T < 10 K, in the presence of a magnetic field, 0 < H < 40 kOe. Our results show that in zero field TMMC is in an XY state for T ≲ 5 K. In a field applied along a direction perpendicular to the chain axis, TMMC undergoes an XY-Ising crossover transition.

Effect of a magnetic field on the correlation length in a one-dimensional antiferromagnet

New results of quasi-elastic neutron scattering on the one-dimensional antiferromagnet TMMC establish that below 5 K TMMC is actually in an XY state. The effect of a magnetic field is described in terms of crossover transitions associated with a reduction of the spin dimensionality. TMMC undergoes an XY-Ising transition when H is applied perpendicular to the chain axis.

The effect of the crystal field on ESR parameters of V 4+ ions

The ground state wavefunction of V 4+ ion in some diamagnetic lattices has been determined with the help of the different spin hamiltonian parameters obtained from ESR studies. The d xy state with slight admixture of the excited states d x 2− y 2 , d xz and d yz has been found to represent the ground state wavefunction. The hyperfine interaction parameter and the Fermi contact term for each of the lattices containing V 4+ ions has also been estimated. The hyperfine interaction parameter has been further used to explain the nature of chemical bonding in complexes.