4,283 research outputs found

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

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    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*

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    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

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    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

    Disorder effects in the quantum Heisenberg model: An Extended Dynamical mean-field theory analysis

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    We investigate a quantum Heisenberg model with both antiferromagnetic and disordered nearest-neighbor couplings. We use an extended dynamical mean-field approach, which reduces the lattice problem to a self-consistent local impurity problem that we solve by using a quantum Monte Carlo algorithm. We consider both two- and three-dimensional antiferromagnetic spin fluctuations and systematically analyze the effect of disorder. We find that in three dimensions for any small amount of disorder a spin-glass phase is realized. In two dimensions, while clean systems display the properties of a highly correlated spin-liquid (where the local spin susceptibility has a non-integer power-low frequency and/or temperature dependence), in the present case this behavior is more elusive unless disorder is very small. This is because the spin-glass transition temperature leaves only an intermediate temperature regime where the system can display the spin-liquid behavior, which turns out to be more apparent in the static than in the dynamical susceptibility.Comment: 15 pages, 7 figure

    Metamagnetism and Lifshitz Transitions in Models for Heavy Fermions

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    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

    The modulated spin liquid: a new paradigm for URu2_2Si2_2

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    We argue that near a Kondo breakdown critical point, a spin liquid with spatial modulations can form. Unlike its uniform counterpart, we find that this occurs via a second order phase transition. The amount of entropy quenched when ordering is of the same magnitude as for an antiferromagnet. Moreover, the two states are competitive, and at low temperatures are separated by a first order phase transition. The modulated spin liquid we find breaks Z4Z_4 symmetry, as recently seen in the hidden order phase of URu2_2Si2_2. Based on this, we suggest that the modulated spin liquid is a viable candidate for this unique phase of matter.Comment: 4 pages, 2 figure

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

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    We study the competition between the Kondo effect and frustrating exchange interactions in a Kondo-lattice model within a large-N{\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 Jc=TK0J_c = T_{K}^0, the single-impurity Kondo temperature. Close to the critical point, the Fermi liquid coherence scale T‚čÜT^\star is strongly reduced and the effective mass strongly enhanced. The regime T>T‚čÜT>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 LiV2_2O4_4.Comment: 7 pages, 1 figure. Late

    Numerical simulation of hydraulic processes in anaerobic bioreactors

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    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

    CP parameters of the B systems from Tevatron

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    Recent results on CP parameters of the B systems obtained by the CDF and D0 collaborations using the data samples collected at the Tevatron Collider in the period 2002 - 2007 were presented at the QCD 2008 conference (Montpellier, France). These results include measurements of the mixing phase, decay width difference, and CP violation parameters in the Bs and Bu decays.Comment: Presented at the QCD 2008 Conference (Montpellier, France); added references; corrected RCP+ paramete
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