Excitations in a four-leg antiferromagnetic Heisenberg spin tube

Inelastic neutron scattering is used to investigate magnetic excitations in the quasi-one-dimensional quantum spin-liquid system Cu(2)Cl(4).D(8)C(4)SO(2). Contrary to previously conjectured models that relied on bond-alternating nearest-neighbor interactions in the spin chains, the dominant interactions are actually next-nearest-neighbor in-chain antiferromagnetic couplings. The appropriate Heisenberg Hamiltonian is equivalent to that of a S=1/2 4-leg spin-tube with almost perfect one dimensionality and no bond alternation. A partial geometric frustration of rung interactions induces a small incommensurability of short-range spin correlations.     

Bond alternation defects in long polyene molecules

Recent theoretical work suggests that there is an alternation in carbon-carbon bond lengths throughout a sufficiently long polyene chain. Such alternation implies the possibility of defects in which even-numbered bonds along the chain change over from short to long. In this paper, the electronic properties of such defects are examined by Hückel molecular orbital theory, with the parameters used by Longuet-Higgins and Salem [2]. It is shown that there is a localized non-bonding molecular orbital associated with each defect (if they are well separated). In the neutral molecule, there will be an odd electron in each such molecular orbital, which should be detectable by electron spin resonance. An approximate estimate of the energy required to form a defect suggests that there should be about one per seventy carbon atoms.     

InC_n~+(n=1-10)团簇的密度泛函理论研究

利用密度泛函理论的B3LYP方法,在LANL2DZ水平上对碳基混合团簇InC_n~+(n=1—10)进行了系统研究,得到了这个团簇体系的最稳定几何构型以及基态结构的电子态、最低振动频率、总能量、自旋污染期望值、偶极距、转动常数等.计算结果显示:团簇的最稳定结构是In原子位于碳链末端的直线型构型;n为偶数的基态是三重态,除InC~+外,n为奇数的基态是单态.通过对增量结合能和能量二阶差分的计算和分析可以得出,随着团簇尺寸的增加,团簇的稳定性表现出强烈的奇强偶弱振荡规律.电离能的计算结果进一步证实了这种振荡规律的正确性.对系列团簇基态的磁性研究表明,团簇的磁矩随团簇尺寸的增加呈现出明显的奇弱偶强振荡规律.极化率的计算结果显示,极化率张量的平均值及各向异性不变量都随着团簇尺寸的增大而增大.     

Band structure of deformed armchair nanoribbon with bond alternation

Electronic energy band structure of deformed armchair graphene nanoribbons with bond alternation is studied by the tight-binding approximation. In the presence of bond alternation, all armchair graphene nanoribbons become semiconducting with small band gap opened at center of the Brillouin zone. Under tensional strain, armchair graphene nanoribbons can become metallic at the critical values of deformation and we can control the band gap of nanoribbon by its strain.     

NMR spin locking of proton magnetization under a frequency-switched Lee-Goldburg pulse sequence

The spin dynamics of NMR spin locking of proton magnetization under a frequency-switched Lee-Goldburg (FSLG) pulse sequence is investigated for a better understanding of the line-narrowing mechanism in PISEMA experiments. For the sample of oriented 15N(1,3,5,7)-labeled gramicidin A in hydrated DMPC bilayers, it is found that the spin-lattice relaxation time T(1rho)(H) in the tilted rotating frame is about five times shorter when the 1H magnetization is spin locked at the magic angle by the FSLG sequence compared to the simple Lee-Goldburg sequence. It is believed that the rapid phase alternation of the effective fields during the FSLG cycles results in averaging of the spin lock field so that the spin lock becomes less efficient. A FSLG supercycle has been suggested here to slow the phase alternation. It has been demonstrated experimentally that a modified PISEMA pulse sequence with such supercycles gives rise to about 30% line narrowing in the dipolar dimension in the PISEMA spectra compared to a standard PISEMA pulse sequence.     

Magnetic excitations in a bond-centered stripe phase: spin waves far from the semiclassical limit

Using a spin-only model, we compute spin excitation spectra in a bond-centered stripe state with long-range magnetic order. We employ a bond operator formalism, which naturally captures both dimerization and broken spin symmetry in a unified framework. At low energies, the spin excitations resemble spin waves, but at higher energies they are very similar to spin-one excitations of isolated spin ladders. Our theory does well describe neutron scattering data [Nature (London) 429, 534 (2004)]] on La2-xBaxCuO4, pointing towards bond order in this material.     

直接键连原子间的NMR偶合常数的自然杂化轨道研究V.~(13)C-~(13)C和~(13)C-~(31)P核自旋偶合常数~1J_(CC)和~1J_(CP)

在前文工作的基础上，结合ＭＮＤＯ／ＥＨＭＯ分子轨道方法和自然杂化轨道方法，具体计算了ＣＣ键和ＣＰ键的核自旋偶合常数．计算结果表明，１ＪＣＣ和１ＪＣＰ主要由成键原子的轨道杂化作用和键极性这两种结构因素所决定．为从简单价键理论角度解释和计算１ＪＣＣ和１ＪＣＰ值提供了简便直观的方法．     

Third-order optical polarizability of conjugated organic molecules

We present a theoretical investigation of the π-electron contribution to linear and non-linear polarizabilities of conjugated chains. Using simple models the respective influence of chain length and bond alternation on optical non-linearities are analyzed. The results are applied to symmetrical cyanines and polyenes.     

Coumarin-Pyrazole Hybrid with Red Shifted ESIPT Emission and AIE Characteristics - a Comprehensive Study

The newly synthesized three coumarin pyrazole hybrid excited state intramolecular proton transfer (ESIPT) dyes show efficient charge transfer from the pyrazole ring and the coumarin towards the electron withdrawing dicyanovinylene group as revealed from the frontier molecular orbitals. Aggregation induced emission enhancement (AIEE) studies with 2-((3-(4-hydroxy-2-oxo-2H-chromen-3-yl)-1-phenyl-1H-pyrazol-4-yl)methylene) malononitrile showed 9 fold increase in the emission enhancement in 90% DMF-H₂O mixture. Lippert-Mataga theory explained the solvatochromic behavior of the dyes in various solvents. The charge transfer characteristics and non-linear optical (NLO) properties have been supported and correlated with bond length alternation, bond order alternation and vibrational spectrum. As values of bond order alternation (BOA) tend to be more towards negative and as the value of α increases β decreases while the values of γ depends on the values of α and β. The observed values of γ are positive which revealed that β contributes significantly. The dyes exhibit linear and NLO properties superior to urea. (E)-2-(3-(2-(3-(4-Hydroxy-2-oxo-2H-chromen-3-yl)-1-phenyl-1H-pyrazol-4-yl)vinyl)-5,5-dimethylcyclohex-2-en-1-ylidene) malononitrile shows enhanced linear and non-linear properties among the three dyes.     

Spin transport in bimetallic pentalene complexes

Spin transport in bimetallic pentalene complexes (CpM(pentalene)M′Cp;M,M′=Fe,Co,Ni) between two gold electrodes was investigated, using a Green’s function formalism under density functional theory. Variation of the metal atom species in the complexes gives a considerable change in their spin properties, with hetero-bimetallic complexes containing an odd number of electrons exhibiting spin filter behaviour. In contrast, alternation in the contact condition, whether Cp-anchoring or adducting by sulphur-gold bonds, had almost no effect on spin filter behaviour, but did lead to variation in electrical conduction. We examined suitable bimetallic pentalene complexes in order to enhance their spin filter efficiency.     

Quantum-tunneling-induced Kondo effect in single molecular magnets

We consider transport through a single-molecule magnet strongly coupled to metallic electrodes. We demonstrate that, for a half-integer spin of the molecule, electron and spin tunneling cooperate to produce both quantum tunneling of the magnetic moment and a Kondo effect in the linear conductance. The Kondo temperature depends sensitively on the ratio of the transverse and easy-axis anisotropies in a nonmonotonic way. The magnetic symmetry of the transverse anisotropy imposes a selection rule on the total spin for the occurrence of the Kondo effect which deviates from the usual even-odd alternation.     

Spin twists, domain walls, and the cluster spin-glass phase of weakly doped cuprates

We examine the role of spin twists in the formation of domain walls, often called stripes, by focusing on the spin textures found in the cluster spin glass phases of and . To this end, we derive improved analytic expressions for the spin distortions produced by a frustrating bond, both near the core region of the bond and in the far field, and then derive an improved expression for interaction energies between such bonds. We critique our analytical theory by comparison to numerical solutions of this problem and find excellent agreement. By looking at collections of small numbers of such bonds localized in some region of a lattice, we demonstrate the stability of small “clusters” of spins, each cluster having its own orientation of its antiferromagnetic order parameter. Then, we display a domain wall corresponding to spin twists between clusters of locally ordered spins showing how spin twists can serve as a mechanism for stripe formation. Since the charges are localized in this model, we emphasize that these domain walls are produced in a situation for which no kinetic energy is present in the problem. Received 10 January 2000     

The mechanisms of spin delocalization through sigma bonds delocalization to gamma protons [1]

A study of the mechanisms of spin delocalization to γ hydrogens was conducted by investigating the change in spin density distribution upon geometrical deformation. The mechanisms are expressed in valence bond pictures. The spin densities were calculated using the SCF-MO-INDO method for the propyl radical. An empirical expression for γ-hydrogen hfsc's is proposed.     

Spin polarization of the injected carriers in C-doped BN nanotubes

We investigated BN nanotubes with two carbon substitutions for one boron atom and one nitrogen atom based on density functional theory (DFT) with local spin density approximation (LSDA). When the two carbon dopants are separated without C-C bond formation, we found that the injected carriers are spin polarized, although there is no net spin in the neutral systems. Here we call the material as ‘spin polarizer’ which can polarize the carriers passing through.     

Time correlations in quantum spin chains and the X-ray absorption problem

We study the isotropic spin one-half XY-chain with one arbitrarily modified coupling. The transverse auto-correlation functions of the spin at the modified bond are considered atT=0 and their asymptotic time dependence is determined. The results are applied to other one- and two-dimensional problems.     

Electron correlation in C(4N+2) carbon rings: aromatic versus dimerized structures

The electronic structure of C(4N+2) carbon rings exhibits competing many-body effects of Huckel aromaticity, second-order Jahn-Teller (SOJT), and Peierls instability at large sizes. This leads to possible ground state structures with aromatic, bond angle, or bond length alternated geometry. Highly accurate quantum Monte Carlo results indicate the existence of a crossover between C10 and C14 from bond angle to bond length alternation. The aromatic isomer is always a transition state. The gap opening mechanism is the SOJT effect, which coalesces with the Peierls regime as N-->infinity.     

Valence bond entanglement entropy

We introduce for SU(2) quantum spin systems the valence bond entanglement entropy as a counting of valence bond spin singlets shared by two subsystems. For a large class of antiferromagnetic systems, it can be calculated in all dimensions with quantum Monte Carlo simulations in the valence bond basis. We show numerically that this quantity displays all features of the von Neumann entanglement entropy for several one-dimensional systems. For two-dimensional Heisenberg models, we find a strict area law for a valence bond solid state and multiplicative logarithmic corrections for the Néel phase.     

Emergence of superconductivity, valence bond order, and Mott insulators in Pd[(dmit)2] based organic salts

The EtMe(3)P and EtMe(3)Sb triangular organic salts are distinguished from other Pd[(dmit)(2)] based salts, as they display valence bond and no long-range order, respectively. Under pressure, a superconducting phase is revealed in EtMe(3)P near the boundary of valence bond order. We use slave-rotor theory with an enlarged unit cell to study competition between uniform and broken translational symmetry states, offering a theoretical framework capturing the superconducting, valence bond order, spin liquid, and metallic phases on an isotropic triangular lattice. Our finite temperature phase diagram manifests a remarkable resemblance to the phase diagram of the EtMe(3)P salt, where the reentrant transition of insulator-metal-insulator type can be explained by an entropy difference between the metal and U(1) spin liquid. We predict different temperature dependence of the specific heat between the spin liquid and metal.     

Spin gap in the zigzag spin-1/2 chain cuprate Sr(0.9)Ca(0.1)CuO(2)

We report a comparative study of (63)Cu nuclear magnetic resonance spin lattice relaxation rates T(1)(-1) on undoped SrCuO(2) and Ca-doped Sr(0.9)Ca(0.1)CuO(2) spin chain compounds. A temperature independent T(1)(-1) is observed for SrCuO(2) as expected for an S=1/2 Heisenberg chain. Surprisingly, we observe an exponential decrease of T(1)(-1) for T<90 K in the Ca-doped sample evidencing the opening of a spin gap. The data analysis within the J(1)-J(2) Heisenberg model employing density-matrix renormalization group calculations suggests an impurity driven small alternation of the J(2)-exchange coupling as a possible cause of the spin gap.     

自旋多重度对钛三聚体原子结构的影响

基于密度泛函理论，获得了钛三聚体在不同自旋多重度下的原子结构，并计算了对应的振动频率.发现自旋多重度对钛三聚体的原子结构有重要的影响，特别是对键长和振动频率.考虑自旋多重度后，钛三聚体的基态结构为自旋七重态. 关键词： 钛三聚体 自旋多重度 原子结构