We consider the extended Hubbard model and introduce a corresponding Heisenberg-like problem written in terms of spin operators. The derived formalism is reminiscent of Anderson's idea of the effective exchange interaction and takes into account nonlocal correlation effects. The results for the exchange interaction and spin susceptibility in the magnetic phase are expressed in terms of single-particle quantities. This fact not only can be used for realistic calculations of multiband systems but also allows us to reconsider a general description of many-body effects in the most interesting physical regimes, where the physical properties of the system are dominated by collective (bosonic) fluctuations. In the strongly spin-polarized limit, when the local magnetic moment is well defined, the exchange interaction reduces to a standard expression of the density functional theory that has been successfully used in practical calculations of magnetic properties of real materials.

We present a strong-coupling approach to the theory of high-temperature superconductivity based on the observation of a quantum critical point in the plaquette within the t,t Hubbard model. The crossing of ground-state energies in the N = 2-4 sectors occurs for parameters close to the optimal doping. The theory predicts the maximum of the d x 2 −y 2 -wave order parameter at the border between localized and itinerant electron behaviors and gives a natural explanation for the pseudogap formation via the soft-fermion mode related to local singlet states of the plaquette in the environment. Our approach follows the general line of resonating valence-bond theory stressing a crucial role of singlets in the physics of high-T c superconductors but focuses on the formation of local singlets, similar to phenomena observed in frustrated one-dimensional quantum spin models.

We experimentally demonstrate that hybrid plasmon-photon modes exist in a silver-coated glass bottle resonator. The bottle resonator is realized in a glass fiber with a smoothly varying diameter, which is subsequently coated with a rhodamine 800-dye doped acryl-glass layer and a 30 nm thick silver layer. We show by means of photoluminescence experiments supported by electromagnetic simulations that the rhodamine 800 photoluminescence excites hybrid plasmon-photon modes in such a bottle resonator, which provide a plasmon-type field enhancement at the outer silver surface and exhibit quality factors as high as 1000.

We present the TRIQS/SOM analytic continuation package, an efficient implementation of the Stochastic Optimization Method proposed by A. Mishchenko et al [Phys. Rev. B 62, 6317 (2000)]. TRIQS/SOM strives to provide a high quality open source (distributed under the GNU General Public License version 3) alternative to the more widely adopted Maximum Entropy continuation programs. It supports a variety of analytic continuation problems encountered in the field of computational condensed matter physics. Those problems can be formulated in terms of response functions of imaginary time, Matsubara frequencies or in the Legendre polynomial basis representation. The application is based on the TRIQS C++/Python framework, which allows for easy interoperability with TRIQSbased quantum impurity solvers, electronic band structure codes and visualization tools. Similar to other TRIQS packages, it comes with a convenient Python interface. PROGRAM SUMMARYProgram Title: TRIQS/SOM Project homepage: Nature of problem: Quantum Monte Carlo methods (QMC) are powerful numerical techniques widely used to study quantum many-body effects and electronic structure of correlated materials. Obtaining physically relevant spectral functions from noisy QMC results in the imaginary * Corresponding author. Current address: time/Matsubara frequency domains requires solution of an ill-posed analytic continuation problem as a post-processing step. Solution method: We present an efficient C++/Python open-source implementation of the stochastic optimization method for analytic continuation.

Research on pass band with negative phase velocity in tubular acoustic metamaterial J. Appl. Phys. 112, 053523 (2012) Control of reflectance and transmittance in scattering and curvilinear hyperbolic metamaterials Appl. Phys. Lett. 101, 091105 (2012) A fast Fourier transform implementation of the Kramers-Kronig relations: Application to anomalous and left handed propagation AIP Advances 2, 032144 (2012) Pure nonlinear optical activity in metamaterials