Losses in the Energy of Low-Energy Electrons in Fluorene, Fluorenone, and Diiodofluorenone Vapors

Electron energy loss spectra (EELS) of fluorene, fluorenone, and diiodofluorenone vapors excited by monokinetic electrons of energies 15–50 eV have been obtained. The singlet and triplet absorption bands of these molecules have been calculated. Comparison of these bands with the experimental EELSs and optical absorption spectra has shown that the forbiddenness of singlet-triplet transitions is not completely removed in the process of interaction of molecules with electrons. The presence of heavy iodine atoms in the diiodofluorenone molecule enhances singlet-triplet transitions. Bands of overtones of stretching vibrations of the CH groups of the benzene rings have been detected near the peak of elastic scattering of electrons of the molecules studied.     

Critical exponents for alternation in Heisenberg antiferromagnetic chains

The ground-state energy per site, λ(δ), and singlet-triplet gap, ΔE(δ), of regular (δ=0) and alternating (δ ≤ 0.01) Heisenberg antiferromagnetic chains are obtained by extrapolating exact numerical solutions of even and odd rings and chains of N ≤ 20 spins. The critical exponents of 1.72 ± 0.02 for λ(δ) and 0.9 ± 0.1 for ΔE(δ) are compared with recent theoretical results for spin-Peierls transitions. Interactions among spinless fermions alter the instability to alternation.     

On the magneto-optical activity of singlet-triplet transitions in near-symmetric tops

Expressions are derived for the magnetic rotational strengths associated with singlet-triplet transitions in molecules which are Hund's coupling case (b) near-symmetric tops. The analysis closely parallels that developed by Hougen in his treatment of dipole intensities in these systems.     

Photoacoustic Spectroscopic Studies of Mono-Substituted Naphthalene Molecules

The present paper deals with the Photoacoustic (PA) studies of mono-substituted (hydroxy) naphthalene molecules, namely 1-naphthol and 2-naphthol in boric acid glass in the region 250–400 nm. The electronic transitions of these molecules observed experimentally, have been interpreted using the optimised geometries and CNDO/S-CI method. Assignments of observed electronic transitions are made on the basis of singlet-singlet and singlet-triplet transitions. The PA spectra of 1-naphthol and 2- Maphthol are compared with its parent molecule i.e. naphthalene molecule, in terms of charge transfer character of -OH group and polarization of molecules. It is observed that the non-radiative transitions in mono-substituted naphthalene molecules shift towards the higher wavelength region when compared with naphthalene molecule, which may be attributed to increase in the charge density in the substituted ring.     

Singlet-Triplet Transitions of a Pöschl-Teller Quantum Dot

We study the energy spectra of a two-dimensional two-electron quantum dot (QD) with Pöschl-Teller confining potential under the influence of perpendicular homogeneous magnetic field. Calculations are made by using the method of numerical diagonalization of Hamiltonian matrix within the effective-mass approximation. A ground-state behavior (spin singlet-triplet transitions) as a function of the strength of a magnetic field is found. We find that the dot radius R of a Pöschl-Teller potential is important for the ground-state transition and the feature of ground-state for a Pöschl-Teller QD and a parabolic QD is similar when R is larger. The larger the well depth, the higher the magnetic field for the singlet-triplet transition of the ground-state of two interacting electrons in a Pöschl-Teller QD.     

Application of Tietz Potential to Study Singlet-Triplet Transition of a Two-Electron Quantum Dot

In this paper, we have studied electronic properties of a two-electron quantum dot using Tietz confining potential in the presence of an external magnetic field. In this regard, we have applied diagonalization procedure of Hamilonian matrix. We have calculated singlet-triplet ground state transitions as a function of the magnetic field. The obtained results show that the dot size of the Tietz potential has an important role in the ground state transition. The singlet-triplet transition of the ground state shifts towards lower magnetic field when the quantum size increases. Our results yield much less transitions than that of previous results [R.G. Nazmitdinov, N.S. Simonovic, and M.J. Rost, Phys. Rev. B 65 (2002) 155307].     

Application of Tietz Potential to Study Singlet-Triplet Transition of a Two-Electron Quantum Dot

In this paper, we have studied electronic properties of a two-electron quantum dot using Tietz confining potential in the presence of an external magnetic field. In this regard, we have applied diagonalization procedure of Hamilonian matrix. We have calculated singlet-triplet ground state transitions as a function of the magnetic field. The obtained results show that the dot size of the Tietz potential has an important role in the ground state transition. The singlet-triplet transition of the ground state shifts towards lower magnetic field when the quantum size increases. Our results yield much less transitions than that of previous results [R.G. Nazmitdinov, N.S. Simonovic, and M.J. Rost, Phys.Rev. B 65(2002) 155307].     

The influence of paramagnetic molecules on singlet-triplet transitions

It is shown that singlet-triplet transitions may obtain a finite probability when exchange interaction occurs between the absorbing molecule and a paramagnetic molecule.

According to this mechanism no quantitative relation exists between the transition dipole strength and the paramagnetic susceptibility of the perturbing molecule.     

A tight-binding representation of electron-hole exchange interaction in semiconductors

The electron-hole exchange interaction in semiconductors is analyzed in the framework of the empirical tight-binding method. It is demonstrated that intra-atomic and interatomic contributions to the long-range exchange interaction enter in an inequivalent way. In particular, for the Γ₆×Γ₇ exciton in a spherical nanocrystal with a cubic lattice, the dipole-dipole contribution associated only with the intra-atomic (or intra-site) transitions does not lead to singlet-triplet splitting of the exciton level. The interatomic transitions, for example, anion-to-cation transitions between the nearest neighbors in binary semiconductor compounds, determine the so-called monopole-monopole contribution to the exchange splitting of the Γ₆×Γ₇ exciton, and this contribution does not vanish in a spherical nanocrystal.     

Low energy electron spectroscopy of CO in the 10.600–14.400 eV energy loss range

Excitation spectra of CO have been obtained at low electron impact energy in the 10.600–13.400 eV energy loss range for scattering angles from 10 to 120°, with a 35 meV experimental resolution. The angular behaviour of the observed peaks is used to discriminate singlet-singlet and singlet-triplet transitions. Previously calculated Rydberg states are observed, in particular the triplet analogue of the F¹Σ⁺ state. A new high energy valence triplet state is identified; the first observed vibrational level is at 11.595 eV and the vibrational spacing is 90 meV. Upper levels are strongly affected by predissociation.     

Electromagnetic contributions to the nucleon-nucleon interaction

The helicity amplitudes of the nucleon-nucleon one-photon exchange interaction are calculated. The anomalous magnetic moments of the proton and neutron are included in the calculations. The formalism is especially suitable for the small angle proton-proton interaction at intermediate energies. The electromagnetic phase shifts in the Born approximation are defined and calculated. The three cases of the proton-proton, the neutron-proton and the neutron-neutron interactions are dealt with separately. Analytic expressions are given for sums defined by divergent partial waves. We indicate the pecularities of the neutron-proton interaction: the singlet-triplet transitions and the t⁻¹ singularity in the differential cross section. These peculiarities require an improved treatment of the neutron-proton interaction. A modified formalism is suggested.     

Doped Mott insulators are insulators: hole localization in the cuprates

We demonstrate that a Mott insulator lightly doped with holes is still an insulator at low temperature even without disorder. Hole localization obtains because the chemical potential lies in a pseudogap which has a vanishing density of states at zero temperature. The energy scale for the pseudogap is set by the nearest-neighbor singlet-triplet splitting. As this energy scale vanishes if transitions, virtual or otherwise, to the upper Hubbard band are not permitted, the fundamental length scale in the pseudogap regime is the average distance between doubly occupied sites. Consequently, the pseudogap is tied to the noncommutativity of the two limits U-->infinity (U the on-site Coulomb repulsion) and L -->infinity (the system size).     

Cotunneling spectroscopy in few-electron quantum dots

Few-electron quantum dots are investigated in the regime of strong tunneling to the leads. Inelastic cotunneling is used to measure the two-electron singlet-triplet splitting above and below a magnetic field driven singlet-triplet transition. Evidence for a nonequilibrium two-electron singlet-triplet Kondo effect is presented. Cotunneling allows orbital correlations and parameters characterizing entanglement of the two-electron singlet ground state to be extracted from dc transport.     

Low energy electron spectroscopy of N2 in the 11.8–13.8 eV energy range

In the 11.8–13.8 eV energy range differential threshold and energy loss spectra of electrons scattered by N₂ molecules have been obtained at an incident energy of 14.3 eV and with a 30 meV experimental resolution. The study of the angular behaviour of the observed peaks permits us to distinguish between singlet-singlet and singlet-triplet transitions. The predicted F³Π_u and G³Π_u Rydberg states are observed. Also some levels of unknown triplet states are seen at 13.155, 13.395 and 13.635 eV.     

Collision frequency shift of hyperfine transitions in atomic hydrogen at a low temperature

The experimental results on the collision frequency shift of ESR in diluted two-and three-dimensional atomic hydrogen at ultralow temperature have been analyzed. The apparent contradiction between experiment and theory is resolved by taking into account the relation between the symmetry of the state of two atoms and their total electron spin S. For example, transitions between symmetric and antisymmetric states of a pair of atoms induced by a symmetric perturbation are forbidden. If symmetric and antisymmetric states are, respectively, pure electronic triplets (S= 1) and singlets (S = 0), this results in the cancellation of the singlet-triplet transitions. Thus, the collision frequency shift of b → c and b → a transitions vanishes in a completely electron and nuclear spin-polarized gas (hyperfine state b). The comparison is performed with experiments in ultracold alkali vapors.     

Kondo effect and spin filtering in triangular artificial atoms

We studied, strongly correlated states in triangular artificial atoms. Symmetry-driven orbital degeneracy of the single particle states can give rise to an SU(4) Kondo state with entangled orbital and spin degrees of freedom, and a characteristic phase shift δ=π/4. Upon application of a Zeeman field, a purely orbital Kondo state is formed with somewhat smaller Kondo temperature and a fully polarized current through the device. The Kondo temperatures are in the measurable range. The triangular atom also provides a tool to systematically study the singlet-triplet transitions observed in recent experiments [Phys. Rev. Lett., 88 (2002) 126803, cond-mat/0208268 (2002)].     

On the possible effect of a magnetic field on the breaking of mechanically loaded covalent chemical bonds

It was established using the EPR method that a magnetic field (B=0.6 T) does not affect the process of the appearance of free radicals that are formed as a result of the breaking of covalent chemical bonds in macromolecules of mechanically loaded fibers of polycaproamide. On this basis, as well as on the basis of some other data, doubts were cast on the possibility of using the assumption (frequently discussed in the literature) on the influence of a magnetic field on the rate of transformation of defect complexes due to singlet-triplet transitions upon the fluctuational lengthening of covalent bonds in such complexes for explaining some manifestations of the magnetoplastic effect in ionic crystals.     

Experimental signature of phonon-mediated spin relaxation in a two-electron quantum dot

We observe an experimental signature of the role of phonons in spin relaxation between triplet and singlet states in a two-electron quantum dot. Using both the external magnetic field and the electrostatic confinement potential, we change the singlet-triplet energy splitting from 1.3 meV to zero and observe that the spin relaxation time depends nonmonotonously on the energy splitting. A simple theoretical model is derived to capture the underlying physical mechanism. The present experiment confirms that spin-flip energy is dissipated in the phonon bath.     

Characterization of an intermediate case exciton in the emission of Cd-doped and pure AgBr

Abstract

Pulsed emission data and ODMR spectra of the 580 nm band provide evidence for an intermediate case exciton in pure and Cd²⁺-doped AgBr. This interpretation is based on the observed g-factor and the absence of a wavelength shift of the emission after pulsed excitation in pure AgBr. It is supported by the response of the ODMR spectra as the microwave modulation frequency is changed. The two central ODMR lines observed in AgBr (g = 1. 78) are assigned to transitions from the m_s=±1 levels to the m_s = 0 level of an intermediate case triplet exciton, which has an exchange coupling (singlet-triplet splitting) of -1. 9 ± 0. 2 cm^?1.     

Singlet-triplet splitting of geminate electron-hole pairs in conjugated polymers

The singlet-triplet splitting of geminate polaron pairs in a ladder-type conjugated polymer has been studied by the thermally stimulated luminescence technique. The energy gap separating the singlet and triplet states of the geminate pairs is measured to be in the range of 3-6 meV, depending on the polymer morphology. The results of correlated quantum-chemical calculations on a long ladder-type oligomer are fully consistent with the observed values of the geminate polaron pair singlet-triplet gap. Such low splitting values have important implications for the spin-dependent exciton formation in conjugated polymers.