A large-N analysis of the local quantum critical point and the spin-liquid phase

We study analytically the Kondo lattice model with an additional nearest-neighbor antiferromagnetic interaction in the framework of large-N theory. We find that there is a local quantum critical point between two phases, a normal Fermi-liquid and a spin-liquid in which the spins are decoupled from the conduction electrons. The local spin susceptibility displays a power-law divergence throughout the spin liquid phase. We check the reliability of the large-N results by solving by quantum Monte Carlo simulation the N=2 spin-liquid problem with no conduction electrons and find qualitative agreement. We show that the spin-liquid phase is unstable at low temperatures, suggestive of a first-order transition to an ordered phase.Comment: 4 pages and 1 figur

An analysis of B_{d,s} mixing angles in presence of New Physics and an update of Bs -> K0* anti-K0*

We discuss a simple approach to measure the weak mixing angles phi_s and phi_d of the Bs and Bd systems in the presence of New Physics. We present a new expression that allows one to measure directly the New Physics mixing angles if New Physics contributes significantly to the mixing only. We apply the method to specific penguin-mediated B->PP, B->PV and B ->VV modes. We provide a very stringent and simple bound on the direct CP asymmetries of all these modes, the violation of which is a signal of New Physics in decay. Within the same theoretical framework, an updated prediction for the branching ratio of Bs->K0* anti-K0* is presented, which can be compared with a recent LHCb analysis.Comment: 11 pages, 3 figure

Nonlocal interactions in doped cuprates: correlated motion of Zhang-Rice polarons

In-plane, inter-carrier correlations in hole doped cuprates are investigated by ab initio multiconfiguration calculations. The dressed carriers display features that are reminiscent of both Zhang-Rice (ZR) CuO4 singlet states and Jahn-Teller polarons. The interaction between these quasiparticles is repulsive. At doping levels that are high enough, the interplay between long-range unscreened Coulomb interactions and long-range phase coherence among the O-ion half-breathing vibrations on the ZR plaquettes may lead to a strong reduction of the effective adiabatic energy barrier associated to each polaronic state. Tunneling effects cannot be neglected for a relatively flat, multi-well energy landscape. We suggest that the coherent, superconducting quantum state is the result of such coherent quantum lattice fluctuations involving the in-plane O ions. Our findings appear to support models where the superconductivity is related to a lowering of the in-plane kinetic energy

Metamagnetism and Lifshitz Transitions in Models for Heavy Fermions

We investigate metamagnetic transitions in models for heavy fermions by considering the doped Kondo lattice model in two dimensions. Results are obtained within the framework of dynamical mean field and dynamical cluster approximations. Universal magnetization curves for different temperatures and Kondo couplings develop upon scaling with the lattice coherence temperature. Furthermore, the coupling of the local moments to the magnetic field is varied to take into account the different Land\'e factors of localized and itinerant electrons. The competition between the lattice coherence scale and the Zeeman energy scale allows for two interpretations of the metamagnetism in heavy fermions: Kondo breakdown or Lifshitz transitions. By tracking the single-particle residue through the transition, we can uniquely conclude in favor of the Lifshitz transition scenario. In this scenario, a quasiparticle band drops below the Fermi energy which leads to a change in topology of the Fermi surface.Comment: 8 pages, 7 figure

Heavy-fermion and spin-liquid behavior in a Kondo lattice with magnetic frustration

We study the competition between the Kondo effect and frustrating exchange interactions in a Kondo-lattice model within a large-${\cal N}$ dynamical mean-field theory. We find a T=0 phase transition between a heavy Fermi-liquid and a spin-liquid for a critical value of the exchange $J_c = T_{K}^0$, the single-impurity Kondo temperature. Close to the critical point, the Fermi liquid coherence scale $T^\star$ is strongly reduced and the effective mass strongly enhanced. The regime $T>T^\star$ is characterized by spin-liquid magnetic correlations and non-Fermi-liquid properties. It is suggested that magnetic frustration is a general mechanism which is essential to explain the large effective mass of some metallic compounds such as LiV$_2$O$_4$.Comment: 7 pages, 1 figure. Late

Numerical simulation of hydraulic processes in anaerobic bioreactors

The aim of the work was to determine the rate and uniformity of temperature distribution in the amount of biogas plant constructed by UrFU, "Avanguard" PLC and "Gildia-M" Ltd on the stage of warming and achieving the rated work conditions corresponding to the mesophilic fermentation process. A computer model of the biogas reactor, considering the main unit features, was created. The installation heating was carried out by uniform thermal energy supply through the outer surface of the vessel full of substrate. Numerical solution was achieved by the finite volume method which has the following features. The calculation results in the form of temperature fields and the substrate particle trajectories for several time moments are given in this paper. The results analysis shows the possibility to use the model for the thermohydraulic computation of various operation modes of the bioreactor. © 2014 WIT Press.International Journal of Safety and Security Engineering;International Journal of Sustainable Development and Planning;WIT Transactions on Ecology and the Environmen

A device to characterize optical fibres

ATLAS is a general purpose experiment approved for the LHC collider at CERN. An important component of the detector is the central hadronic calorimeter; for its construction more than 600,000 Wave Length Shifting (WLS) fibres (corresponding to a total length of 1,120 Km) have been used. We have built and put into operation a dedicated instrument for the measurement of light yield and attenuation length over groups of 20 fibres at a time. The overall accuracy achieved in the measurement of light yield (attenuation length) is 1.5% (3%). We also report the results obtained using this method in the quality control of a large sample of fibres.Comment: 17 pages 20 figeres submitted to NIM journa

Electronic states and magnetic excitations in LiV2O4: Exact diagonalization study

Motivated by recent inelastic neutron scattering experiment we examine magnetic properties of LiV2O4. We consider a model which describes the half-filled localized A1g spins interacting via frustrated antiferromagnetic Heisenberg exchange and coupled by local Hund's interaction with the 1/8-filled itinerant Eg band, and study it within an exact diagonalization scheme. In the present study we limited the analysis to the case of the cluster of two isolated tetrahedrons. We obtained that both the ground state structure and low-lying excitations depend strongly on the value of the Hund's coupling which favors the triplet states. With increasing temperature the triplet states become more and more populated which results in the formation of non-zero residual magnetic moment. We present the temperature dependence of calculated magnetic moment and of the spin-spin correlation functions at different values of Hund's coupling and compare them with the experimental results.Comment: 7 pages. 6 eps figure

On the heavy-fermion behavior of the pyrochlore transition-metal oxide LiV2O4LiV_{2}O_{4}

Motivated by the heavy fermion Fermi liquid (HFFL) features observed at low-$T$ in the pyrochlore $LiV_{2}O_{4}$, we consider a material-specific model that includes aspects of the local quantum chemistry, the geometrically frustrated lattice structure, and strong correlations in a {\it single} approach. In particular, we show how geometrical frustration (GF) gives rise to a crossover scale, $T^{*}<<J$, the intersite (AF) exchange, below which the metallic system shows HFFL features. Our scenario is a specific realization of the importance of GF effects in driving HFFL behavior in $LiV_{2}O_{4}$, and provides a natural understanding of various puzzling features observed experimentally.Comment: 4 pages, 3 figure