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.     

自旋极化有机电致发光器件中单线态与三线态激子的形成及调控

由于有机半导体（OSC）材料自旋弛豫时间长、自旋扩散长度大,OSC自旋器件逐渐成为研究热点.对于有机电致发光器件（OLED）,通过自旋极化电极调控单线态和三线态激子比率是提高其效率的有效方法.本文从漂移扩散方程和载流子浓度连续性方程出发,结合朗之万定律建立了一个自旋注入、输运、复合的理论模型.计算了OSC中的极化电子、空穴浓度,得出了单线态和三线态激子的比率.分析了电场强度、自旋相关界面电导、电极和OSC电导率匹配和电极极化率等因素的影响.计算结果表明：两电极注入反向极化的载流子并提高载流子自旋极化率,有     

Berry phase and shot noise for spin-polarized and entangled electrons

Shot noise for entangled and spin-polarized states in a four-probe geometric setup has been studied by adding two rotating magnetic fields in an incoming channel. Our results show that the noise power oscillates as the magnetic fields vary. The singlet, entangled triplet and polarized states can be distinguished by adjusting the magnetic fields. The Berry phase can be derived by measuring the shot noise power.     

Time-resolved luminescence and singlet oxygen formation under illumination of hypericin in acetone

We present time- and spectral-resolved phosphorescence study of hypericin as well as evolution of singlet oxygen formation and elimination under the illumination of hypericin in acetone solution. The obtained time-resolved hypericin phosphorescence can be satisfactorily fitted by using a two-exponential decay curve. The value of shorter component is about 0.29 μs and is independent of the hypericin concentration in the studied range (2–200 μM). The rise time of singlet oxygen production matches this value perfectly. It confirms that singlet oxygen formation represents the significant channel of hypericin triplet state deactivation under aerobic conditions at room temperature. The total phosphorescence intensity of hypericin is linearly proportional to the hypericin concentration within the studied concentration range, but the singlet oxygen phosphorescence intensity exhibits saturation behavior. This observation is a result of at least two effects: quenching of singlet oxygen by hypericin, as well as quenching of singlet oxygen by singlet oxygen.     

Solving the differential Yakubovsky equations for p 3He scattering by the cluster-reduction method

The cluster-reduction method has been used to solve numerically the differential equations for the s-wave Yakubovsky components in the nppp system. Proton scattering on a ³He nucleus has been considered for energies below the three-body threshold in the singlet and triplet spin states of the system. Nucleon-nucleon interaction has been simulated by the MT I–III potential, and the Coulomb interaction between the protons has been taken into account. Effective equations that describe the relative motion of clusters have been derived. The low-energy behavior of phase shifts has been analyzed. The values of ¹ A=8.2 fm and ³ A=7.7 fm have been obtained for, respectively, the singlet and triplet scattering lengths. The calculated phase shifts agree well with experimental data.     

Doubly-charged scalar bosons from the doublet

We consider the extended Higgs models, in which one of the isospin doublet scalar fields carries the hypercharge Y=3/2. Such a doublet field Φ3/2 is composed of a doubly charged scalar boson as well as a singly charged one. We first discuss a simple model with Φ3/2 (Model I), and study its collider phenomenology at the LHC. We then consider a new model for radiatively generating neutrino masses with a dark matter candidate (Model II), in which Φ3/2 and an extra Y=1/2 doublet as well as vector-like singlet fermions carry the odd quantum number for an unbroken discrete Z₂ symmetry. We also discuss the neutrino mass model (Model III), in which the exact Z₂ parity in Model II is softly broken. It is found that the doubly charged scalar bosons in these models show different phenomenological aspects from those which appear in models with a Y=2 isospin singlet field or a Y=1 triplet one. They could be clearly distinguished at the LHC.     

Quantum critical behavior of the Kondo necklace model with aperiodic exchange modulation

The low energy behavior of the Kondo necklace model with an aperiodic exchange modulation is studied using a representation for the localized and conduction electron spins, in terms of local Kondo singlet and triplet operators at zero and finite temperature for arbitrary d dimensions. A decoupling scheme on the double time Green's functions is used to find the dispersion relation for the excitations of the system. We determined the dependence between the chemical aperiodic exchange modulation and the spin gap in 1d, 2d and 3d, at zero temperature and in the paramagnetic side of the phase diagram. On the other hand, at low but finite temperatures, the line of Néel transitions in the antiferromagnetic phase is calculated in function of the aperiodic exchange modulation.     

磁场对异质界面上D-中心的影响

势阱中的类氢杂质的能级问题一直为学术界所长期关注。讨论了异质界面上中性施主D⁰和负施主离子D^-的能量随垂直于界面的磁场的变化情况,同时将磁场和势阱结合起来考虑其对类氢杂质的影响。研究发现随磁场的增大,其对D⁰基态能的影响越来越大,对其束缚能的影响逐渐变小,而对D^-中心,磁场的作用使得D^-由非束缚态转变为束缚态。计算中分别选取了两种不同的波函数,分析了这两种波函数的适用范围,利用变分的方法得到此结构中D⁰中心的基态能量和束缚能与D^-中心角动量L=-1自旋三重态的本征能量和束缚能随磁场的变化关系,找到了此三重态由非束缚态转变到束缚态对应磁场的阈值。     

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.     

Triplet-to-singlet exciton formation in poly(p-phenylene-vinylene) light-emitting diodes

The triplet to singlet exciton formation ratio in a MEH-PPV light-emitting diode is measured by comparing the triplet-induced absorptions with optical and electric excitations at the same singlet exciton density. The ratio is a strong universal decreasing function of the averaged electric field. Using 4 ns for singlet to triplet intersystem crossing time, the ratio is significantly larger than the spin-independent value 3 at intermediate field but is reduced to about 2 for higher field.     

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.     

Spinfluctuations versus phonons: The case of Heavy-Fermion superconductivity

Superconductivity of Heavy-Fermions, with an emphasis on hexagonal UPt₃, is investigated microscopically on the basis of the LNCA-approximation for strongly correlated electrons in a Kondo lattice and of Eliashberg theory for anisotropic singlet and triplet superconducting order. The effective interaction kernel incorporates exchange of spin fluctuations carried by strongly renormalized particle-hole excitations as well as exchange of phonons generated by the breathing mechanism. Particular care is taken to include realistic anisotropies in band structure and couplings and to expand interaction kernel and order parameter in appropriate Fermi surface harmonics. Separate interactions and corresponding gap equations are used for pseudo-spin-singlet and -triplet channel, with a restriction to the case of weak spin-orbit coupling. Our main results include a detailed study of phonon- and spin-fluctuation mechanisms as a possible source for superconductivity and, in combination, their mutual influence and competition. Furthermore, the questions regarding singlet or triplet order, conventional or unconventional symmetry, order parameter zeros and phase transitions between different superconducting states are addressed. We propose a possible scenario of Heavy-Fermion superconductivity.     

Type-IV superconductivity: Cooper pairs with broken inversion and time-reversal symmetries in conventional superconductors

The vortex phase in a singlet superconductor in the absence of impurities is shown to be absolutely unstable with respect to the appearance of a triplet component that breaks both the inversion and time-reversal symmetries of Cooper pairs. The symmetry breaking paramagnetic effects are demonstrated to be of the order of unity if the orbital upper critical field, H_c2(0), is of the order of the Clogston paramagnetic limiting field, H_p. We suggest a generic phase diagram of such a type-IV superconductor that is a singlet one at H = 0 and characterized by a mixed singlet-triplet order parameter with broken time-reversal symmetry in the vortex phase. The possibility to observe type-IV superconductivity in clean organic, high-T_c, MgB₂, and other superconductors is discussed.     

The hydrogen molecule in an arbitrarily oriented magnetic field

Summary The modifications induced by a magnetic field of arbitrary direction and intermediate strength (i.e not larger than 2.35·10⁵ tesla, the ?atomic tesla?) on the lowest singlet and triplet energy states of the hydrogen molecule are studied. Using a linear combination of products of field-modified atomic orbitals, it is found that increasing the field strength the depth of the singlet energy well increases and the equilibrium internuclear distance decreases, yielding more rigid and localized nuclear vibrations. For sufficiently strong fields perpendicular to the internuclear axis, the triplet state exhibits a bonding behaviour. An explanation of the above results is given in terms of the field-modified electronic-charge distributions in the internuclear region. Based on the thesis submitted by S. Basile to the University of Palermo for graduation in Physics.     

The scaling of the current voltage characteristics of the YBa2Cu3O7:Y2BaCuO5 composites

We have investigated features in the current voltage (I—V) characteristics of YBa₂Cu₃O₇:Y₂BaCuO₅ composite sinters which indicate the occurrence of a second-order phase transition. For each sinter composition similar families of characteristics are found for a wide range of magnetic fields. The relation between V and I is seen to be of the form V = f(I, ζ(B, T)) where a single parameter ξ contains all of the field and temperature dependence. The locus of points in the B, T plane for which ζ(B, T) is a constant have been examined, and a low field and a high field régime have been identified. Samples of different composition show families of I—V characteristics which scale onto one another in a simple way, hence we are led to suppose the V = f(I, ζ(B, T, p)) where p is the dilution fraction. Data taken at temperatures below the phase transition agrees well with a simple scaling picture of dissipation close to a second order phase transition, with exponents which are independent of magnetic field and composition. Above the transition, however, the behaviour is more complex.     

Avian magnetoreception model realized by coupling a magnetite-based mechanism with a radical-pair-based mechanism

Many animal species have been proven to use the geomagnetic field for their navigation, but the biophysical mechanism of magnetoreception has remained enigmatic. In this paper, we present a special biophysical model that consists of magnetite-based and radical-pair-based mechanisms for avian magnetoreception. The amplitude of the resultant magnetic field around the magnetic particles corresponds to the geomagnetic field direction and affects the yield of singlet/triplet state products in the radical-pair reactions. Therefore, in the proposed model, the singlet/triplet state product yields are related to the geomagnetic field information for orientational detection. The resultant magnetic fields corresponding to two materials with different magnetic properties are analyzed under different geomagnetic field directions. The results show that ferromagnetic particles in organisms can provide more significant changes in singlet state products than superparam- agnetic particles, and the period of variation for the singlet state products with an included angle in the geomagnetic field is approximately 180 when the magnetic particles are ferromagnetic materials, consistent with the experimental results obtained from the avian magnetic compass. Further, the calculated results of the singlet state products in a reception plane show that the proposed model can explain the avian magnetoreception mechanism with an inclination compass.     

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.     

EPR studies of photoinduced electron transfer in triad model compounds of photosynthesis

Time-resolved EPR spectra are reported for porphyrin-quinone-quinone and porphyrin-porphyrin-quinone triads obtained after photoexcitation in the nematic and soft glass phase of liquid crystals. Spin-polarized EPR spectra were observed for the triplet states of the porphyrin created by spin-selective intersystem crossing (ISC) from the excited singlet state and those of the charge-separated radical pair states (RP) generated by electron transfer (ET) processes. The EPR polarization patterns of the RP are discussed in terms of the favored decay channel of the photoexcited singlet state of the porphyrin donor. The decay pathway may either be singlet ET to the quinone(s) followed by singlet/triplet mixing to yield RPs with triplet character or triplet ET after ISC from the porphyrin singlet to the triplet state, or a superposition of both pathways. It is demonstrated that the nature of the linking bridge between donor and acceptor, i.e., aliphatic cyclohexylene or aromatic phenylene, significantly influences the ET mechanism and thus the polarization patterns of the RP spectra. Using liquid crystals, information about the orientation of the guest molecules in the liquid crystal matrix with respect to the long axes of the liquid crystal molecules can be obtained. In the porphyrin-porphyrin-quinone triads the energy and ET processes strongly depend on the type of metallation of the porphyrins, specifically, whether the distal, the vicinal or both porphyrins bear a zinc atom.     

单线态分裂的超快光谱学研究

有机分子中的单线态分裂过程能将单个光激发的单线态激子转化成两个三线态激子. 借助此载流子倍增效应, 太阳能电池可以更有效地利用太阳光谱中的高能光子, 进而突破单结太阳能电池效率的理论极限. 因此, 单线态分裂备受关注. 本文回顾学术界对单线态分裂物理图像的认识以及争议, 结合课题组近年来的一些结果, 重点总结此领域中运用瞬态光谱学方法取得的实验进展, 讨论有关多激子中间暗态机理的不同观点, 并介绍单线态分裂材料的发展以及器件应用.     

Competition of spin fluctuations and phonons in superconductivity of ZrZn(2)

It has long been suspected that spin fluctuations in ZrZn2 may lead to a triplet superconductivity. We point out another possibility, an inhomogeneous singlet (Fulde-Ferrell) state. We calculated the electronic structure, as well as the zone center phonons and their coupling with electrons. We find that the exchange splitting is nonuniform and the Fermi surface exhibits substantial nesting. Both factors favor a Fulde-Ferrell state at parts of the Fermi surface. We find a substantial coupling of Zr rattling modes with electrons, which can provide the necessary pairing in the s-channel.