The pairing of electronic states in alternant hydrocarbons

The pairing of Hückel molecular orbitals in an alternant hydrocarbon holds in a more general sense for wave functions which make complete allowance for the correlation of π electrons within the atomic orbital scheme. The new pairing property holds exactly in the scope of Pariser, Parr, and Pople's approximations, and leads to an exact correspondence between every detail of the excited states, the electronic spectra and electron resonance spectra of positive and negative hydrocarbon ions. Neutral molecules have a half-filled electron shell in which electrons and holes are on an equal footing. This causes the electron distribution to be uniform in every electronic state, and leads to two kinds of excited state—‘even’ and ‘odd’—as Pariser first suggested. Transitions between states of the same parity are forbidden. In neutral radicals the spin density vanishes in all the bonds, and in both radicals and molecules the bond orders vanish between atoms of the same set (starred or unstarred).     

Spin triplet pairing in high temperature superconductors

Various superconductivity mechanisms show that the electron pair is in spin singlet (S=0) belowTc. In contrast with these, our EPR measurements show that the electron pair is in spin triplet (S=1) rather than the spin singlet. Since an intense zero field absorption was observed in the high temperature superconductors of YBa₂Cu₃O_7–y , and YBa₂Cu₃O_7–y F₁, which violated various spin-pairing singlet superconductivity mechanisms.     

Low energy electronic excitations in the layered cuprates studied by copper L3 resonant inelastic x-ray scattering

We have measured the resonant inelastic x-ray scattering (RIXS) spectra at the Cu L3 edge in a variety of cuprates. Exploiting a considerably improved energy resolution (0.8 eV) we recorded significant dependencies on the sample composition and orientation, on the scattering geometry, and on the incident photon polarization. The RIXS final states correspond to two families of electronic excitations, having local (dd excitations) and nonlocal (charge-transfer) character. The dd energy splitting can be estimated with a simple crystal field model. The RIXS at the L3 edge demonstrates here a great potential, thanks to the resonance strength and to the large 2p spin-orbit splitting.     

Low energy nuclear states in124Te

The energy level scheme of¹²⁴Te has been established on the basis of singlesγ-spectra, andγ-γ coincidence measurements carried out using a dual parameter data collection system. Ge(Li) detectors were used to study the gamma spectra produced in theβ ^? decay of¹²⁴Sb. Eleven new transitions and two new energy levels are proposed. Relative intensities, branching ratios,α(K) and logft values were calculated and multipolarities, spins and parities deduced. Comparisons are made with predictions of the Interacting Boson Model: in a first order perturbation approximation to theSU (5) limit, using the complete IBA-1 Hamiltonian, and in theO(6) limit.     

Phonon mediated interaction for triplet pairing in Pd

The pairing interaction due to phonon exchange is calculated for the triplet p-state of Pd, using the atomic-site representation for the heavy d electrons. It is found that this interaction is repulsive, λ^Ph₁ < 0, and that its magnitude is small compared with the singlet s-state interaction of BCS, λ^Ph₀. The net interaction due to both phonons and spin-fluctuations is attractive but it is so small that the observation of a triplet p-state in Pd appears unlikely at the lowest accessible temperatures.     

Radiationless intermolecular energy transfer with participation of acceptor excited triplet states

Radiationless energy transfer between like and unlike molecules has been experimentally studied under conditions where acceptor molecules have been excited to the triplet state Homogeneous singlet-triplet-triplet migration has been discovered in highlyconcentrated chlorophyll “a” and pheophytin “a” solutions in castor oil at 183 K by measuring the variation of pigment relative quantum yields of fluorescence and triplet state formation as a function of exciting pulse intensity. Heterogeneous single-triplet-triplet energy transfer has been observed in solid solutions of different complex organic molecules (perylene + phenanthrene, Na-fluorescein+chlorophyll “a”, pyrene+Mg-phthalocyanine) as the fluorescent donor state quenching in the presence of acceptor triplet-excited molecules. Primary emphasis is placed on a direct observation of the effect of energy transfer on the excited-state lifetime of the donor. The benzophenone phosphorescence quenching (shortening of phosphorescence lifetime) in the presence of Mg-mesoporphyrin triplet molecules has been found to be caused by the heterogeneous triplet-triplet-triplet energy transfer. Good agreement of the theoretical and experimental results permits us to conclude that all types of observed transfer processes are described by the Förster-Galanin theory for dipole-dipole radiationless energy transfer with no additional assumptions.     

Intermolecular energy transfer in mixed laser dyes: photophysical properties of triplet states

Triplet-singlet energy transfer in laser dyes have been studied in EPA at 77K using N₂ laser as an excitation source. Phosphorescence of the donor (D) and the delayed fluorescence of the acceptor (A) and their lifetimes have been measured for coumarin 102 (D)-rhodamine B(A) and 9(10H)-acridone (D)-rhodamine 6G(A) dye systems as a function of acceptor concentration. These data yield energy transfer rate constants of ∼10³ dm³ mol⁻¹ s⁻¹ for the donor acceptor combinations, consistent with the Forster mechanism. The phosphorescence quantum efficiency and other spectral parameters are also reported.     

Odd triplet pairing in clean superconductor/ferromagnet heterostructures

We study triplet pairing correlations in clean ferromagnet (F)/superconductor (S) nanojunctions, via fully self-consistent solution of the Bogoliubov-de Gennes equations. We consider FSF trilayers, with S being an s-wave superconductor, and an arbitrary angle alpha between the magnetizations of the two F layers. We find that contrary to some previous expectations, triplet correlations, odd in time, are induced in both the S and F layers in the clean limit. We investigate their behavior as a function of time, position, and alpha. The triplet amplitudes are largest at times on the order of the inverse Debye frequency, and at that time scale they are long-ranged in both S and F. The zero temperature condensation energy is found to be lowest when the magnetizations are antiparallel.     

Triplet-triplet transfer of electronic energy upon nonequilibrium vibrational excitation of triplet molecules

The specific features of the triplet-triplet (T-T) transfer of electronic excitation energy in a gas phase upon nonequilibrium vibrational excitation of the triplet molecules of a donor were studied for an anthraquinone-diacetyl donor-acceptor pair using the time-resolved slow fluoresence of anthraquinone and sensitized phosphorescence of diacetyl. It is shown that in the gas phase, which allows regular control of the number of collisions, competition between the processes of T-T transfer and intermolecular vibrational relaxation is observed for nanosecond time resolution. The T-T transfer rate for the molecular system investigated exceeded the rate of intermolecular vibrational relaxation k_V in the triplet state T₁ of the donor. The effectiveness of the T-T transfer of energy by vibrationally excited molecules turned out to be higher than the effectiveness of transfer by thermalized ones, but even the highest of them was much less than unity. An increase in the equilibrium temperature of vapors led to a decrease in the effectiveness of transfer for both vibrationally excited and thermalized triplet molecules, thus indicating the importance of the collisional complex in the intermolecular process studied. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 4, pp. 474–479, July–August, 2000.     

Low energy density of states in several disordered systems

Disordered systems of arbitrary dimension consisting of randomly distributed scatterers are studied in the low energy limit (Roger Waxler, Ph.D. thesis, Columbia University). We evaluate the density of electronic energy levels for long range, attractive, and electric dipole scatterers.     

Symmetries in the triplet superconducting states

Depending upon the point-group symmetry of the system several phases occur within the superconducting state. The gap parameter for four different point group symmetries is calculated.     

Low-lying quasiparticle states and hidden collective charge instabilities in parent cobaltate superconductors

We report a state-of-the-art photoemission (angle-resolved photoemission spectroscopy) study of high quality single crystals of NaxCoO2 the series focusing on the fine details of the low-energy states. The Fermi velocity is found to be small (<0.5 eV A) and only weakly anisotropic over the Fermi surface at all dopings, setting the size of the pair wave function to be on the order of 10-20 nm. In the low-doping regime, the exchange interlayer splitting vanishes and two-dimensional collective instabilities such as 120 degrees -type fluctuations become kinematically allowed. Our results suggest that the unusually small Fermi velocity and the unique symmetry of kinematic instabilities distinguish cobaltates from most other oxide superconductors.     

High spin states and pairing vibrations

The Generator-Coordinate-Method is applied to describe yrast-bands and excited vibrational bands of pairing-vibrational orβ-vibrational type in deformed rare earth nuclei. The model wave functions are supplied by the Pairing+Quadrupole-Model with angular momentum — and particle number projection. Results are shown for the nucleus¹⁷⁰Yb. The question if backbending may be produced by the intersection of the ground and the neutron pairing vibrational band is discussed.     

RKKY interaction in layered superconductors with anisotropic pairing

The RKKY interaction between localized moments in layered d-wave superconductors is considered at T ? T _c . It is shown that the interaction is anisotropic and consists of two terms: conventional oscillating one and the positive term, which is proportional to the gap function and decreases in the 2D case inversely proportional to the distance. Both of these terms reveal crossover to the more rapid decay law at large distances, exponential one away from the nodal directions of order parameter and power-like law close to them. If the background in the normal state is ferromagnetic, d-wave superconductivity results in helicoid ordering with a star of helicoid axes fixed by nodes. The period of this skew structure has unconventional ξ^1/2 dependence on the correlation length.     

Key pairing interaction in layered doped ionic insulators

A controversial issue on whether the electron-phonon interaction (EPI) is crucial for high-temperature superconductivity or it is weak and inessential has remained one of the most challenging problems of contemporary condensed matter physics. We employ a continuum random phase approximation for the dielectric response function allowing for a self-consistent semianalytical evaluation of the EPI strength, electron-electron attractions, and the carrier mass renormalization in layered high-temperature superconductors. We show that the Fr?hlich EPI with high-frequency optical phonons in doped ionic lattices is the key pairing interaction, which is beyond the BCS-Migdal-Eliashberg approximation in underdoped superconductors, and it remains a significant player in overdoped compounds.     

DNA分子能带结构与电子态研究

DNA分子链内的巡游电子数与其结构和位形密切相关，可变的电子数会导致这类软物质费米面处能带结构的变化.在紧束缚近似下，计入电子 晶格的相互作用，计算了DNA分子不同巡游电子数下的能带结构及态密度，对碱基对不同排列情况下DNA分子可能的电属性进行了讨论. 关键词： DNA 态密度 电晶相互作用     

Radiative triplet luminescence and metastable states in a-P

Previous ODMR studies showed a PL emission near 1eV which was associated with a triplet excited state. We discuss further the possible models for the ODMR signals, and show by measurement of the ODMR waveform and of the PL decay, that the experimental results are consistent with an assignment of this band to a spin-forbidden triplet exciton emission, with a lifetime ～ 4 msec. We note, however, that with chopped excitation, luminescence transients are observed which are characteristic of long-lived (τ ～ 1s) optically-created non-radiative centres.