d-Wave resonating valence bond states of fermionic atoms in optical lattices

We study controlled generation and measurement of superfluid d-wave resonating valence bond (RVB) states of fermionic atoms in 2D optical lattices. Starting from loading spatial and spin patterns of atoms in optical superlattices as pure quantum states from a Fermi gas, we adiabatically transform this state to an RVB state by a change of the lattice parameters. Results of exact time-dependent numerical studies for ladders systems are presented, suggesting generation of RVB states on a time scale smaller than typical experimental decoherence times.     

Fate of the resonating valence bond in graphene

We apply a variational wave function capable of describing qualitatively and quantitatively the so-called "resonating valence bond" (RVB) in realistic materials, by improving standard ab initio calculations by means of quantum Monte Carlo methods. In this framework we clearly identify the Kekulé and Dewar contributions to the chemical bond of the benzene molecule and establish the corresponding RVB energy of these structures (?0.01 eV/atom). We apply this method to unveil the nature of the chemical bond in undoped graphene, providing an estimate of the RVB energy gain, and show that this picture remains only within a small "resonance length" of a few atomic units.     

Phase transition to the resonating valence bond state

A Ginzburg-Landau expansion for the free energy functional of the resonating valence bond state is performed for the mean field approximation (MFA) and for a functional integral approach (FIA) which includes correlations. Phase diagrams obtained in both approximations are presented. The FIA differs form the MFA in three main aspects: (i) Above the mean field transition temperature an instability exists towards the formation of degenerate singlet pair states, indicating the onset of the RVB state. (ii) The extendeds-wave phase is favoured over the extendedd-wave phase. (iii) Phase fluctuations are included, destroying off-diagonal order in the absence of holes.     

The resonating valence bond state and high temperature superconductivity

A fluctuation theory is developed for the RVB state of high temperature superconductors in the Anderson model of high temperature superconductivity. The energy spectrum of fermion pairs in the RVB state is calculated by summing up a selected class of planar diagrams over all orders of perturbation theory. It is explicitly shown that the fermion pairs of the RVB state are pre-existing Cooper pairs which by undergoing a Bose condensation can become super-conducting Copper pairs. A part of the phase diagram is constructed showing conditions under which the RVB state becomes unstable with respect to both superconducting and magnetically ordered states.     

Mesoscopic resonating valence bond system on a triple dot

We theoretically introduce a mesoscopic pendulum from a triple dot. The pendulum is fastened through a singly occupied dot (spin qubit). Two other strongly capacitively coupled islands form a double-dot charge qubit with one electron in excess oscillating between the two low-energy charge states (1,0) and (0,1). The triple dot is placed between two superconducting leads. Under realistic conditions, the main proximity effect stems from the injection of resonating singlet (valence) bonds on the triple dot. This gives rise to a Josephson current that is charge- and spin-dependent and, as a consequence, exhibits a distinct resonance as a function of the superconducting phase difference.     

Gossamer superconductor,Mott insulator,and resonating valence bond state in correlated electron systems

Gutzwiller variational method is applied to an effective two-dimensional Hubbard model to examine the recently proposed gossamer superconductor by Laughlin (LANL cond-mat/0209269). The ground state at half filled electron density is a gossamer superconductor for smaller intrasite Coulomb repulsion U and a Mott insulator for larger U. The gossamer superconducting state is similar to the resonating valence bond superconducting state, except that the chemical potential is approximately pinned at the mid of the two Hubbard bands away from the half filled.     

2D multipartite valence bond states in quantum anti-ferromagnets

A quantum anti-ferromagnetic spin-1 model is characterised on a 2D lattice with the following requirements: (i) The Hamiltonian is made out of nearest neighbour interactions. (ii) It is homogeneous, translational and rotational invariant. (iii) The ground state is a real singlet state of SU(2) (non-chiral). (iv) It has a local spin-1 representation. Along the way to characterise the system, connections with classical statistical mechanics and integrable models are explored. Finally, the relevance of the model in the physics of low dimensional anti-ferromagnetic Mott-Hubbard insulators is discussed.     

Electronic model for CoO2 layer based systems: chiral resonating valence bond metal and superconductivity

Takada et al. have reported superconductivity in layered Na(x)CoO(2)yH(2)O (T(c) approximately equal to 5 K). We model a reference neutral CoO2 layer as an orbitally nondegenerate spin-1/2 antiferromagnetic Mott insulator on a triangular lattice and Na(x)CoO(2)yH(2)O as electron doped Mott insulators described by a t-J model. It is suggested that at optimal doping chiral spin fluctuations enhanced by the dopant dynamics lead to a gapful d-wave superconducting state. A chiral resonating valence bond (RVB) metal, a parity and time (PT) reversal violating state with condensed RVB gauge fields, with a possible weak ferromagnetism, and low temperature p-wave superconductivity are also suggested at higher dopings.     

Optical implementation and entanglement distribution in Gaussian valence bond states

We study Gaussian valence bond states of continuous variable systems obtained as the outputs of projection operations from an ancillary space of M infinitely entangled bonds connecting neighboring sites applied at each ofN sites of a harmonic chain. The entanglement distribution in Gaussian valence bond states can be controlled by varying the input amount of entanglement engineered in a (2M+ 1)-mode Gaussian state known as the building block, which is isomorphic to the projector applied at a given site. We show how this mechanism can be interpreted in terms of multiple entanglement swapping from the chain of ancillary bonds, through the building blocks. We provide optical schemes to produce bisymmetric three-mode Gaussian building blocks (which correspond to a single bond, M = 1), and study the entanglement structure in the output Gaussian valence bond states. Finally, the usefulness of such states for quantum communication protocols with continuous variables, like telecloning and teleportation networks, is discussed. The text was submitted by the authors in English.     

Mott insulator to high Tc superconductor via pressure: resonating valence bond theory and prediction of new systems

Mott insulator superconductor transition, via pressure and no external doping, is studied in orbitally nondegenerate spin-1 / 2 systems. It is presented as another resonating valence bond route to high T(c) superconductivity. We propose a "strong coupling" hypothesis that views long range Coulomb force driven first order Mott transition as a self-doping process that also preserves superexchange on the metal side. We present a two-species t-J model where conserved N0 doubly occupied (e(-)) sites and N0 empty sites (e(+)) hop in the background of N-2N(0) singly occupied (neutral) sites in a lattice of N sites. An equivalence to the regular t-J model is made. Some old and new systems are predicted to be candidates for pressure-induced high T(c) superconductivity.     

共振价键波函数在高压液氢量子蒙卡模拟中的适用性研究

在共振价键理论基础上, 选取高压液氢电子主要占据轨道的线性组合作为基组, 构建由Jastrow项和反对称孪生函数乘积项 (AGP) 组成的波函数. 考虑电子关联作用的共振价键 (RVB) 波函数得出的能量值低于LDA能量值; 当满足r_s<1.75或T >15000 K时引入backflow项以改善波函数结点面, 改善后的能量值下降约1 mHa/atom, 能量方差值变小. 将构建的RVB波函数与电子-离子耦合的蒙特卡罗法 (CEIMC) 相结合, 计算结果与实验及其他ab-initio结果相符合, 获得的液氘单次冲击Hugoniot曲线基本通过所有加载类型实验误差棒, 液氘在50.3 GPa处具有最大压缩率4.48, 在100–120 GPa内未发现压缩率有急剧增大的现象. 构建的RVB 波函数能够适用于较宽密度与温度范围内(1.0< r_s<2.2, 2800 K< T<60000 K)液氢的模拟, 与CEIMC法相结合可提高液氢冲击特性的模拟精度. 关键词： 共振价键理论 波函数 量子蒙卡法 液氢     

Resonating valence bond states in doped square-lattice antiferromagnets: finite cluster calculations

We discuss some general features of the resonating valence bond (RVB) ansatz for the ground state of quantums=1/2 Heisenberg antiferromagnets (AFMs). For finite clusters of up to 16 spins on the square lattice we compare the exact ground state with a short-range correlated RVB trial wave function. Since in the pure square-lattice AFM the short-range correlated RVB state differs significantly from the real ground state we discuss different mechanisms that favour the RVB state. In particular, we study the influence of anisotropy, disorder and frustration, which could be relevant for slightly doped high-T _c superconducting materials. Furthermore, we discuss the influence of holes on the realization of a RVB state. We found that exchange disorder and, in particular, frustration and holes can favour a short-range correlated magnetic state, which is well described by a RVB state.     

Positive energy bound states in resonating group calculations

Spurious resonances occuring in numerical resonating group calculations off the oscillator limit are explained as being due to positive energy bound states. A generalization of Swan's version of the Levinson theorem is given which takes into account these states.     

Electron-nucleon resonance model of valence bond

The formation of valence bonds is traditionally considered as the displacement of electronic orbitals in space between interacting atoms. At the same time, a number of experimental facts, in particular, the existence of four-dimensional structural symmetry between the filling of electron shells and periodic properties of elements, require a change in the standard valence concept. A model in which valence bonds are caused by the resonant interaction between the shell electrons of one atom and the protons of another atom seems to be more adequate. The text was submitted by the authors in English.     

Mean energies of hybridized valence states

A method of determining the relative signs of J _AB and J _BX in nuclear spin systems of the type AB₂X _q is described and applied to the AB₂X₃ proton spin system of 2-butyn-1-ol. Variation of the relative chemical shift v _A—v _B (through changes in temperature or in the composition of the solvent) causes certain pairs of ‘labile’ transitions in the X spectrum to coalesce on one side or other of the X spectrum. The information about the relative signs of the coupling constants is obtained by noting the two different critical values of v _A-v _B at which these coalescences occur. Since the method remains applicable even if the third coupling constant J _AX is vanishingly small, it is particularly useful in the determination of the signs of long-range spin coupling constants where the usual double resonance methods often break down. It is concluded that the long-range coupling J(H-C-C≡C-C-H) of 2-butyn-1-ol has a positive sign.     

Analytic description of valence band nonparabolicity in silicon

Setting out from our solutions of the Kane equation for hole energies in the ε?δ₀ and δ₀?ε?ε_g regions and using the method of continuous fractions we obtain simple dispersion relations which accurately describe the valence bands in the non-parabolic region εRδ₀. We also examine the change in the constant energy surfaces of heavy, light, and spin-split holes as the energy increases.     

The electronic states of silicon monofluoride: Low-lying valence states

Hartree-Fock and configuration interaction calculations are reported for the lowest valence state of the SiF molecule. Deduced spectroscopic constants are in good agreement with the experimental values. The dipole moment calculated for SiF is consistent with a charge distribution corresponding to a Si⁺F^? configuration.