Singlet and triplet bipolarons on the triangular lattice

We study the Coulomb-Fröhlich model on a triangular lattice, looking in particular at states with angular momentum. We examine a simplified model of crab bipolarons with angular momentum by projecting onto the low energy subspace of the Coulomb-Fröhlich model with large phonon frequency. Such a projection is consistent with large long-range electron-phonon coupling and large repulsive Hubbard U. Significant differences are found between the band structure of singlet and triplet states: The triplet state (which has a flat band) is found to be significantly heavier than the singlet state (which has mass similar to the polaron). We test whether the heavier triplet states persist to lower electron-phonon coupling using continuous time quantum Monte Carlo (QMC) simulation. The triplet state is both heavier and larger, demonstrating that the heavier mass is due to quantum interference effects on the motion. We also find that retardation effects reduce the differences between singlet and triplet states, since they reintroduce second order terms in the hopping into the inverse effective mass.     

Excitation of lowest electronic states of thymine by slow electrons

Excitation of lowest electronic states of the thymine molecules in the gas phase is studied by elec- tron energy loss spectroscopy. In addition to dipole-allowed transitions to singlet states, transitions to the lowest triplet states were observed. The low-energy features of the spectrum at 3.66 and 4.61 eV are identified with the excitation of the first triplet states 1₃ A′ (π → π*) and 1₃ A″ (n → π*). The higher-lying features at 4.96, 5.75, 6.17, and 7.35 eV are assigned mainly to the excitation of the π → π* transitions to the singlet states of the molecule. The excitation dynamics of the lowest states is studied. It is found that the first triplet state 1³ A′(π → π*) is most efficiently excited at a residual energy close to zero, while the singlet 2₁ A′(π → π*) state is excited with almost identical efficiency at different residual energies.     

EPR identification of the triplet ground state and photoinduced population inversion for a Si-C divacancy in silicon carbide

It is shown that intrinsic defects responsible for the semi-insulating properties of SiC represent Si-C divacancies in a neutral state (V_Si-V_C)⁰, which have the triplet ground state. The energy level scheme and the mechanism of creating the photoinduced population inversion of the triplet sublevels of the divacancy ground state are determined. It is concluded that there is a singlet excited state through which spin polarization is accomplished, and this fact opens the possibility of detecting magnetic resonance on single divacancies.     

On Electron Spin Polarization Created in the Excited Triplet State of Accessory Chlorophyll via Photoinduced Charge-Recombination of the Photosystem II Reaction Center

We present a theoretical approach to investigate the electron spin polarization (ESP) of the excited triplet state that has been detected using the time-resolved electron paramagnetic resonance (TREPR) method in the photosystem II center of the plants. We show, using the stochastic Liouville equation, that the ESP pattern created in the accessory chlorophyll (Chl_accD1) which reside near the P_D1 chlorophyll of the active branch is explained by one-step, concerted double electron transfer model, initiating from the singlet–triplet conversion of the light-induced charge-separated state composed of P_D1 radical cation and pheophytin radical anion. We also considered the sequential ESP transfer model via the triplet charge-recombination (CR) and the triplet–triplet energy transfer processes. It has been clearly shown that the ESP created in the ³Chl_accD1* is dependent on the rate constant (k _TT) of the triplet–triplet energy transfer from the intermediate triplet state created by the CR. Also we show that the relative orientation of the principal axes of the spin dipolar interaction in the intermediate triplet state (³P_D1*, as an example) may play a role in the ESP pattern, when the k _TT is smaller than the angular frequency of the Zeeman energy. We have theoretically shown that the TREPR measurement of the ESP is very powerful to investigate the primary chemical process and to characterize the intermediate as a signature of the stepwise ESP transfer.     

Excitation of lowest electronic states of the uracil molecule by slow electrons

The excitation of lowest electronic states of the uracil molecule in the gas phase has been studied by electron energy loss spectroscopy. Along with excitation of lowest singlet states, excitation of two lowest triplet states at 3.75 and 4.76 eV (±0.05 eV) and vibrational excitation of the molecule in two resonant ranges (1?C2 and 3?C4 eV) have been observed for the first time. The peak of the excitation band related to the lowest singlet state (5.50 eV) is found to be blueshifted by 0.4 eV in comparison with the optical absorption spectroscopy data. The threshold excitation spectra have been measured for the first time, with detection of electrons inelastically scattered by an angle of 180°. These spectra exhibit clear separation of the 5.50-eV-wide band into two bands, which are due to the excitation of the triplet 1³ A?? and singlet 1¹ A?? states.     

Diffusion of one- and two-electron vacancy centers in polar crystals

The change in the diffusion barrier for anion vacancies in polar crystals is calculated in the case where one electron (F centers) or two electrons (F′ centers) are captured. The lowest singlet and triplet states are considered for the two-electron system. It is shown that the capture of two electrons on the lowest metastable triplet state (which is possible under nonequilibrium conditions, for example, in the case of either exchange scattering of band electrons or thermal refilling of energy levels) leads to a decrease in the diffusion barrier for the system. However, for the two-electron center in the lowest singlet state, the potential barrier to migration, as a rule, is higher than that for a vacancy without electrons. Calculations are carried out for Gaussian functions with the inclusion of the effects of interelectron correlations and electron-phonon interactions.     

Magnetic field effect on the pairing state competition in quasi-one-dimensional organic superconductors (TMTSF)2X

We study the effect of the magnetic field on the pairing state competition in organic conductors (TMTSF)₂X by applying random phase approximation to a quasi-one-dimensional extended Hubbard model. We show that the singlet pairing, triplet pairing and the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) superconducting states may compete when charge fluctuations coexist with spin fluctuations. This rises a possibility of a consecutive transition from singlet pairing to FFLO state and further to S_z = 1 triplet pairing upon increasing the magnetic field. We also show that the singlet and S_z = 0 triplet components of the gap function in the FFLO state have “d-wave” and “f-wave” forms, respectively, which are strongly mixed.     

Intersystem crossing mechanisms and single molecule fluorescence: Terrylene in anthracene crystals

Single molecule spectroscopy requires molecules with low triplet yields and/or short triplet lifetimes. The intersystem crossing (ISC) rate may be dramatically enhanced by the host matrix. Comparing the fluorescence intensity of single terrylene molecules in para-terphenyl, naphthalene, and anthracene crystals, we found a reduction of the saturation intensity by three orders of magnitude in the latter case. The fluorescence autocorrelation function indicates that the bottleneck state is the terrylene triplet. We propose a ping-pong mechanism between host and guest. This intermolecular ISC mechanism, which can open whenever the host triplet lies lower than the guest singlet, was overlooked in previous single molecule investigations.     

Mechanism for quenching of the luminescence of 9,10-anthraquinone vapor by oxygen

The fluorescence of 9,10-anthraquinone, 1-aminoanthraquinone, 1,4-diaminoanthraquinone, and 1,5-diaminoanthraquinone is not quenched by oxygen because the singlet-triplet energy difference in these compounds is less than the energy needed for excitation of the triplet state of oxygen to the singlet state. Luminescence of 9,10-anthraquinone is quenched because it is mainly phosphorescence, for which the singlet-triplet difference is sufficient for quenching by a mechanism involving singlet oxygen formation. The weak fluorescence of 9,10-anthraquinone is not quenched. The resistance of the fluorescence of 9,10-anthraquinone vapor to quenching by oxygen and the quenching of its phosphorescence explain the different effects of oxygen on the luminescence of α-substituted and β-substituted anthraquinones known from the literature, and indicate that their singlet excited state cannot convert triplet oxygen to singlet oxygen. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 1, pp. 79–4, January–February, 2008.     

超冷钠原子弹性散射特性的精确计算

基于精确的原子之间相互作用势,系统研究了钠原子在超冷温度下的弹性散射特性,精确计算了钠原子间碰撞时的s波散射长度、有效力程、p波散射长度以及束缚态数目等散射参数.超冷温度下单重态和三重态原子间的弹性散射截面主要为s波贡献,随着碰撞能量的增加散射截面有丰富的形状共振出现,计算发现单重态和三重态散射截面分别存在显著的f波和i波形状共振.应用简并内态近似方法获得了超精细态相互作用时的s波散射长度,所得结果与精确值比较符合.     

Intramolecular energy transfer by singlet and triplet excitons in macromolecules

Absorption, luminescence and photo-oxidation of polyvinylcarbazole (PVCa) and polyvinylbenzocarbazole (PVBCa) in weak solutions have been studied. It is found that fluorescence and phosphorescence spectra, observed at 293, 77 and 4.2 K, depend on the molecular mass M of the polymer and on the experimental conditions. Due to efficient intramolecular energy transfer the fluorescence spectrum includes the emission of excimers, emission of end groups and that of internal structural defects of a macrochain. The relative role of each emission markedly depends on M. Phosphorescence is largely determined by the annihilation effects between triplet excitations, these effects being pronounced in long macrochains. The data show the excitonic nature of the singlet and triplet excitation energy transfer in the polymers studied. The migration of one-dimensional singlet and triplet excitons strongly influences the luminescent properties of PVCa and PVBCa macromolecules, as well as their photochemical reactions. The range of singlet excitons in PVCa and PVBCa macromolecules is determined both at room temperature and at low temperatures.     

Surface-induced magnetism of the solids with impurities and vacancies

Using the quantum-mechanical approach combined with the image charge method we calculated the lowest energy levels of the impurities and neutral vacancies with two electrons or holes located in the vicinity of flat surface of different solids. Unexpectedly we obtained that the magnetic triplet state is the ground state of the impurities and neutral vacancies in the vicinity of surface, while the nonmagnetic singlet is the ground state in the bulk, for e.g. He atom, Li⁺, Be⁺⁺ ions, etc. The energy difference between the lowest triplet and singlet states strongly depends on the electron (hole) effective mass μ, dielectric permittivity of the solid ε₂ and the distance from the surface z₀. For z₀=0 and defect charge ∣Z∣=2 the energy difference is more than several hundreds of Kelvins at μ=(0.5−1)m_e and ε₂=2-10, more than several tens of Kelvins at μ=(0.1−0.2)m_e and ε₂=5-10, and not more than several Kelvins at μ<0.1m_e and ε₂>15 (m_e is the mass of a free electron). Pair interaction of the identical surface defects (two doubly charged impurities or vacancies with two electrons or holes) reveals the ferromagnetic spin state with the maximal exchange energy at the definite distance between the defects (∼5-25 nm). We estimated the critical concentration of surface defects and transition temperature of ferromagnetic long-range order appearance in the framework of percolation and mean field theories, and RKKY approach for semiconductors like ZnO. We obtained that the nonmagnetic singlet state is the lowest one for a molecule with two electrons formed by a pair of identical surface impurities (like surface hydrogen), while its next state with deep enough negative energy minimum is the magnetic triplet. The metastable magnetic triplet state appeared for such molecule at the surface indicates the possibility of metastable ortho-states of the hydrogen-like molecules, while they are absent in the bulk of material. The two series of spectral lines are expected due to the coexistence of ortho- and para-states of the molecules at the surface. We hope that obtained results could provide an alternative mechanism of the room temperature ferromagnetism observed in TiO₂, HfO₂, and In₂O₃ thin films with contribution of the oxygen vacancies. We expect that both anion and cation vacancies near the flat surface act as magnetic defects because of their triplet ground state and Hund's rule. The theoretical forecasts are waiting for experimental justification allowing for the number of the defects in the vicinity of surface is much larger than in the bulk of as-grown samples.     

Impurity induced resonance states at the superconducting interface LaAlO3/SrTiO3

Motivated by the recent discovery of superconductivity on the heterointerface LaAlO₃/SrTiO₃, we theoretically investigate the impurity-induced resonance states with coexisting spin singlet s- and triplet p-wave pairing symmetries by considering the influence of Rashba-type spin-orbit interaction (RSOI). Due to the nodal structure of the mixed gap function, we find single nonmagnetic impurity-induced resonance peaks appearing in the local density of state. We also analyze the evolutions of density of states and local density of states with the weight of triplet pairing component determined by the strength of RSOI, which will be widely observed in thin films of superconductors with surface or interface-induced RSOI, or various noncentrosymmetric superconductors in terms of point contact tunneling and scanning tunneling microscopy, and thus shed light on the admixture of the spin singlet and RSOI-induced triplet superconducting states.     

C20分子异构体的电子态

使用量子化学从头计算方法优化了C₂₀分子三种异构体（环形、碗形和笼形）的单重和三重电子态几何结构。从优化的几何结构出发，由二阶微扰（MP2）方法得到的单点能给出了一个不符合Hund法则的结果，也就是三种异构体的能量都是单重电子态的最低。使用UHF波函数给出了三种异构体的总电荷密度和静电势，结果显示在笼形和碗形结构的中心区域存在负电荷。最后，本文分析了环形结构的成键特征。     

Energy transfer by singlet and triplet excitons in carbazole-containing polymers

Quantum yields of pyrene fluorescence and bis[2-(2′-benzothienyl)-pyridinato-N,C^3′](acetylacetonate)iridium [Btp₂Ir(acac)] phosphorescence upon excitation via a matrix of poly-N-epoxypropylcarbazole (PEPC) or poly-N-epoxypropyl-3,6-dibromocarbazole (DBrPEPC), respectively, were found to be lower than those for the compounds directly excited in a polystyrene (PS) matrix. It was established that the energy in PEPC was transferred to an acceptor by both singlet excitons (by migration and long-range dipole–dipole interaction) and triplet excitons (through migration and short-range exchange electron interaction); however, only by triplet excitons in DBrPEPC, which did not show any fluorescence. The energy-transfer efficiency in PEPC by singlet excitons was higher than by triplet excitons. The observed effects were explained by the fact that energy transfer to the acceptor competed with such processes as localization of the excitons in the “tail” energy states, dissociation of singlet excitons into geminal electron–hole pairs (EHP), and capture of triplet excitons by polymer oxidation products.     

NDMAP: the best material to study the Haldane's prediction

As predicted by Haldane, spin, S=1 one-dimensional (1D) Heisenberg antiferromagnet (HAF) has an energy gap between the singlet ground state and first excited triplet. On application of magnetic field, the triplet state Zeeman splits and the energy of one of the triplet state becomes zero at a critical field, H_c. Above H_c the system recovers magnetism. Then, we expect that a quasi-1D HAF will show a magnetic long-range ordering (LRO) at low temperatures due to the inter-chain coupling. This field-induced LRO has not been observed before due to complication of the crystal structure in the materials studied so far and/or technical difficulty.From a heat capacity measurement on a single crystal of an S=1 quasi-Q1D HAF, Ni(C₅H₁₄N₂)₂N₃(PF₆), we found an anomaly at a temperature in finite fields indicating a field-induced phase transition. A magnetic LRO is confirmed by a neutron diffraction measurement on the same sample. The temperature versus magnetic field phase diagram of this compound is constructed and discussed.     

A theoretical analysis of the excited states in 2-benzoylthiophene

The lower excited states of 2-benzoylthiophene have been studied using ab initio quantum chemical methods based on multiconfigurational wave functions. Six singlet and six triplet excited states have been characterized. The geometry has been optimized for the two lowest triplet states, which are responsible for the photoreactivity of the chromophore in the photosensitizing drug tiaprofenic acid. The T¹(π → π?) and T₂(n → π?) states have been found to be close in energy with the π → π? state slightly lower. The excited states have been characterized using density difference and spin density plots. The different photochemical behaviour of the two triplet states can be rationalized from the theoretical data.     

空间波函数的对称性对氦原子双光子双电离过程中电子关联的影响

通过数值求解含时薛定谔方程,研究了氦原子具有对称空间波函数的1s2s 1S态和具有反对称空间波函数的1s2s 3S态分别作为初态的双光子双电离过程. 结果表明,对于初态为单重态1s2s 1S的双光子双电离过程,两个电离电子的能量分布随激光脉冲持续时间的增加呈现由单峰到双峰的变化,这里的单峰和双峰分别意味着两个电离电子主要携带相等和不等的能量;然而对于初态为三重态1s2s 3S的双光子双电离过程,两个电离电子的能量分布随激光脉冲持续时间的增加总是保持双峰结构. 这些结果表明当原子的初态处于反对称空间波函数时,两电子的空间密度分布具有较少的重叠,从而导致电子在超短激光脉冲中电离时电子关联能无法平均分配.     

Oxidation at the Si/SiO2 interface: influence of the spin degree of freedom

We show, using first-principles spin-polarized total-energy calculations, that depending on the spin configuration of the system, the reaction of an O2 molecule with a Si-Si bond in a suboxidized region might result either in a peroxy linkage defect (for a singlet spin state) or in a perfect Si-O-Si bond plus an interstitial O atom (for a triplet spin state). Even though the singlet has a lower energy than the triplet configuration, we find a rather small probability for triplet to singlet conversion. Therefore, as the O2 in an SiO2 interstitial site has a triplet configuration, this reaction spin dependence may have a strong influence on the high quality of the Si/SiO2 interface.