Nature of spin excitations in two-dimensional mott insulators: undoped cuprates and other materials

We investigate the excitation spectrum of a two-dimensional resonating valence bond (RVB) state. Treating the pi-flux phase with antiferromagnetic correlations as a variational ground state, we recover the long wavelength magnon as an "RVB exciton." However, this excitation does not exhaust the entire spectral weight and the high-energy spectrum is dominated by fermionic excitations. The latter can be observed directly by inelastic neutron scattering, and we predict their characteristic energy scales along different high symmetry directions in the magnetic Brillouin zone. We also interpret experimental results on two magnon Raman scattering and midinfrared absorption within this scenario.     

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.     

Five-fold way to new high <Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> superconductors

Discovery of high T _c superconductivity in La_2?x Ba _x CuO₄ by Bednorz and Muller in 1986 was a breakthrough in the 75-year long search for new superconductors. Since then new high T _c superconductors, not involving copper, have also been discovered. Superconductivity in cuprates also inspired resonating valence bond (RVB) mechanism of superconductivity. In turn, RVB theory provided a new hope for finding new superconductors through a novel electronic mechanism. This article first reviews an electron correlation-based RVB mechanism and our own application of these ideas to some new noncuprate superconducting families. In the process we abstract, using available phenomenology and RVB theory, that there are five directions to search for new high T _c superconductors. We call them five-fold way. As the paths are reasonably exclusive and well-defined, they provide more guided opportunities, than before, for discovering new superconductors. The five-fold ways are (i) copper route, (ii) pressure route, (iii) diamond route, (iv) graphene route and (v) double RVB route. Copper route is the doped spin-½ Mott insulator route. In this route one synthesizes new spin-½ Mott insulators and dopes them chemically. In pressure route, doping is not external, but internal, a (chemical or external) pressure-induced self-doping suggested by organic ET-salts. In the diamond route we are inspired by superconductivity in boron-doped diamond and our theory. Here one creates impurity band Mott insulators in a band insulator template that enables superconductivity. Graphene route follows from our recent suggestion of superconductivity in doped graphene, a two-dimensional broadband metal with moderate electron correlations, compared to cuprates. Double RVB route follows from our recent theory of doped spin-1 Mott insulator for superconductivity in iron pnictide family.     

RVB theory of the Hubbard model on the triangular lattice

We propose a Green's function technique, to investigate finite-temperature properties of the Hubbard model on the triangular lattice. The lattices are covered by dimers. The method is exact in two limits:U=0 or decoupled dimers. We apply this approximate method to calculate the ground state energy, the specific heat and the single-particle spectral weight for the 1/2-filled case. The largest lattice considered has 16×16 sites. The approximate ground state energy as a function of the on-site interactionU oscillates around the exact energyin the 1/2-filled case. We find two peaks in the specific heat. ForU5t the single-particle spectral weight splits into upper and lower Hubbard bandasymmetrically. Thus in the 1/2-filled case the chemical potential is placed in the upper band leading to a metallic state. The approximate technique yields a finite zero-point entropy for mediumU. All the investigations signal a RVB state in the range of mediumU as formerly proposed by Callaway.     

Holon kinetic energy in the RVB state

We have used a low energy variational RVB wave function to evaluate the kinetic energy of holons in the dilute limit. We report results on a 5×5 cluster; from the first 8 moments of the density of states, we estimate a lower band edge for the holon, which is fairly insensitive to details of the RVB wave function. This band edge is higher than the corresponding one for a hole in the Néel state, by about 0.08t, in the limitU/t.     

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.     

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     

Systematic improvement of variational Monte Carlo using Lanczos iterations

We present a new numerical technique which combines the variational Monte Carlo and the Lanczos methods without suffering from the fermion sign problem. Lanczos iterations allow systematic improvement of trial wavefunctions while Monte Carlo sampling permits treatment of large lattices. As in the usual Lanczos method we find it useful to symmetrize the starting wavefunction in order to accelerate convergence. We apply our method to the 2D AFM Heisenberg model in the fermionic electron representation, which allows us to compare with results from the equivalent bosonic spin representation. Using d-wave RVB states as starting wavefunctions shows that after only one iteration between 70 and 80% of the difference between the variational energy and the ground state energy (as determined by GFMC) is recovered, and a similar improvement is observed in the second iteration. Leaving the spin-singlet sector by introducing antiferromagnetic correlations reduces the symmetry and the relative improvement in energy drops below 50% for one iteration. Our method allows us also to see trends in observables. Relative to the d-wave RVB states we find an enhancement in the spinspin correlations, consistent with the expectation that the true ground state has long-range order.     

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.     

Phase coherence of RVB wave function

We establish the phase coherence of a variational resonating valence bond (RVB) wave function in the case of nearest neighbor singlets on a square lattice in the fermion representation. The result applies both in the half filled band case and in the doped case.     

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.     

muSR study of the quantum dynamics in the frustrated S=3/2 kagomé bilayers

We present muSR experiments in the S=3/2 kagomé bilayer compound Ba(2)Sn(2)ZnGa(10-7p)Cr(7p)O22 [BSZCGO(p)] and compare it to the isostructural SrCr(9p)Ga(12-9p)O19 [SCGO(p)], including for the latter new results for p > or =0.89. Quantum-dynamical low energy magnetic excitations are evidenced in this novel compound. We study the evolution of the muon relaxation rate with p, T, and field. A phenomenological model for the muon relaxation based on sporadic dynamics due to spin excitations in a singlet sea proposed by Uemura et al. is extended to all fields and T range. Its connection to the RVB picture is discussed, and we argue that such coherent states might mediate the interactions between "impurities" which induce the spin glass freezing.     

Resonating valence bond wave functions for strongly frustrated spin systems

The resonating-valence-bond (RVB) theory for two-dimensional quantum antiferromagnets is shown to be the correct paradigm for large enough "quantum frustration." This scenario, proposed a long time ago but never confirmed by microscopic calculations, is strongly supported by a new type of variational wave function, which is extremely close to the exact ground state of the J(1)-J(2) Heisenberg model for 0.4 less than approximately J(2)/J(1) less than approximately 0.5. This wave function is proposed to represent the generic spin-half RVB ground state in spin liquids.     

The effect of zero-point spin wave motions on the hole propagation in the strongly correlated Hubbard model

The kinetic energy of holes doped in a two dimensional antiferromagnetic state is dicussed, taking into account the effect of zero-point spin wave motions. The lower band edge of the hole density of states is estimated from its lower order moments in the strong electron correlation limit. A relatively large narrowing of the density of states is obtained, nearly the same as that of the RVB case. Consequences of the result are discussed in connection with the relative stability between the antiferromagnetic state and the RVB state.     

Deriving the Hawking Temperature of (Massive) Global Monopole Spacetime via a Topological Formula

In this work, we study the Hawking temperature of the global monopole spacetime (non-spherical symmetrical black hole) based on the topological method proposed by Robson, Villari, and Biancalana (RVB). By connecting the Hawking temperature with the topological properties of black holes, the Hawking temperature of the global monopole spacetime can be obtained by the RVB method. We also discuss the Hawking temperature in massive gravity, and find that the effect of the mass term cannot be ignored in the calculation of the Hawking temperature; the corrected Hawking temperature in massive gravity can be derived by adding an integral constant, which can be determined by the standard definition.     

Marked influence of the nature of the chemical bond on CP-violating signature in molecular ions HBr(+) and HI(+)

Heavy polar molecules offer a great sensitivity to the electron electric dipole moment (EDM). To guide emerging searches for EDMs with molecular ions, we estimate the EDM-induced energy corrections for hydrogen halide ions HBr(+) and HI(+) in their respective ground X (2)Pi(3/2) states. We find that the energy corrections due to EDM for the two ions differ by an unexpectedly large factor of 15. We demonstrate that a major part of this enhancement is due to a dissimilarity in the nature of the chemical bond for the two ions: the bond that is nearly of ionic character in HBr(+) exhibits predominantly a covalent nature in HI(+). We conclude that because of this enhancement the HI(+) ion may be a potentially competitive candidate for the EDM search.     

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.     

a-Si3N4纳米粒子的激光法制备及能级结构研究

给出激光化学汽相沉积法制备a-Si₃N₄纳米粒子的原理和经验公式,在特定工艺参数下获得平均粒径为6.5nm的优质a-Si₃N₄纳米粒子,用紫外光谱研究其能级结构,发现与小粒子有关的峰状光谱结构和能带分裂现象,描述了a-Si₃N₄纳米粒子的物理结构图象,确认硅错键≡Si—Si≡,硅悬挂键≡Si₃⁰在富硅a-Si₃ 关键词：