3,134 research outputs found

    Renormalization group flow for fermions into antiferromagnetically ordered phases: Method and mean-field models

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    We present a functional renormalization group flow for many-fermion lattice models into phases with broken spin-rotational symmetry. The flow is expressed purely in terms of fermionic vertex functions. The symmetry breaking is seeded by a small initial anomalous self-energy which grows at the transition scale and which prevents a runaway-flow at nonzero scales. Focusing on the case of commensurate antiferromagnetism we discuss how the interaction vertex can be parametrized efficiently. For reduced models with long-range bare interactions we show the results of standard mean-field theory are reproduced by the fRG and how anisotropies in the spin sector change the flows. We then describe a more efficient decomposition of the interaction vertex that should allow for the treatment of more general models.Comment: revised version; 12 pages, 9 figure

    Universality in antiferromagnetic strange metals

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    We propose a theory of metals at the spin-density wave quantum critical point in spatial dimension d=2d=2. We provide a first estimate of the full set of critical exponents (dynamical exponent z=2.13z=2.13, correlation length ν=1.02\nu =1.02, spin susceptibility γ=0.96\gamma = 0.96, electronic non-Fermi liquid ητf=0.53\eta^f_\tau = 0.53, spin-wave Landau damping ητb=1.06\eta^b_\tau = 1.06), which determine the universal power-laws in thermodynamics and response functions in the quantum-critical regime relevant for experiments in heavy-fermion systems and iron pnictides. We present approximate numerical and analytical solutions of Polchinski-Wetterich type flow equations with soft frequency regulators for an effective action of electrons coupled to spin-wave bosons. Performing the renormalization group in frequency -instead of momentum- space allows to track changes of the Fermi surface shape and to capture Landau damping during the flow. The technique is easily generalizable from models retaining only patches of the Fermi surface to full, compact Fermi surfaces.Comment: 46 pages, 13 figures, typos fixed; as accepted to Physical Review

    Plasmonics: Localization and guiding of electromagnetic energy in metal/dielectric structures

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    We review the basic physics of surface-plasmon excitations occurring at metal/dielectric interfaces with special emphasis on the possibility of using such excitations for the localization of electromagnetic energy in one, two, and three dimensions, in a context of applications in sensing and waveguiding for functional photonic devices. Localized plasmon resonances occurring in metallic nanoparticles are discussed both for single particles and particle ensembles, focusing on the generation of confined light fields enabling enhancement of Raman-scattering and nonlinear processes. We then survey the basic properties of interface plasmons propagating along flat boundaries of thin metallic films, with applications for waveguiding along patterned films, stripes, and nanowires. Interactions between plasmonic structures and optically active media are also discussed

    Dynamical response of ultracold interacting fermion-boson mixtures

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    We analyze the dynamical response of a ultracold binary gas mixture in presence of strong boson-fermion couplings. Mapping the problem onto that of the optical response of a metal/semiconductor electronic degrees of freedom to electromagnetic perturbation we calculate the corresponding dynamic linear response susceptibility in the non-perturbative regimes of strong boson-fermion coupling using diagrammatic resummation technique as well as quantum Monte Carlo simulations. We evaluate the Bragg spectral function as well as the optical conductivity and find a pseudogap, which forms in certain parameter regimes.Comment: 32 pages, 13 figure

    The Establishment History Panel (BHP) 1975-2008 : Handbook Version 1.0.2

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    "The Establishment History Panel 1975-2008 V1 is a 50% sample of all establishments throughout Germany which have at least one employee liable to social security as of 30th June of a given year or, since 1999, also establishments which have at least one marginal part-time employee. This data report describes the variables of the weakly anonymous version of the 1975-2008 V1 BHP dataset, which is available to researchers via on-site use at the Research Data Centre (FDZ) or via remote data access. The BHP data are predominantly original data, with the sole exception that the original establishment numbers, which are direct identifiers for the different establishments, have been replaced by artificial establishment numbers. In order to protect the anonymity of the establishments still further, some variables are classified as particularly sensitive and are only disclosed on submission of a special application (see Chapter 1.2). The Datenreport on the BHP 1975-2008 V1 is structured as follows. In addition to the introduction, Chapter 1 contains an outline of the data as well as information regarding the so-called volume structure and about data access. Chapter 1 also contains a brief overview of the differences between this version and its predecessor BHP 1975-2006 V2.0.1 in order to make it possible for users of the older version to switch to the new one quickly. A description of the data source can be found in Chapter 2. The data preparation and data quality are discussed in Chapters 3 and 4. Chapter 5 contains detailed descriptions of the variables." (Author's abstract, IAB-Doku) ((en)) Additional Information Frequencies and labels Here you can find the German version.IAB-Betriebs-Historik-Panel, Datenaufbereitung, Datenorganisation, Datensatzbeschreibung

    Functional renormalization group for commensurate antiferromagnets: Beyond the mean-field picture

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    We present a functional renormalization group (fRG) formalism for interacting fermions on lattices that captures the flow into states with commensurate spin-density wave order. During the flow, the growth of the order parameter is fed back into the flow of the interactions and all modes can be integrated out. This extends previous fRG flows in the symmetric phase that run into a divergence at a nonzero RG scale, i.e., that have to be stopped at the ordering scale. We use the corresponding Ward identity to check the accuracy of the results. We apply our new method to a model with two Fermi pockets that have perfect particle-hole nesting. The results obtained from the fRG are compared with those in random phase approximation.Comment: revised version; 24 pages, 12 figure

    Multiorbital effects in the functional renormalization group: A weak-coupling study of the Emery model

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    We perform an instability analysis of the Emery three-band model at hole doping and weak coupling within a channel-decomposed functional renormalization group flow proposed in Phys. Rev. B 79, 195125 (2009). In our approach, momentum dependences are taken into account with improved precision compared to previous studies of related models. Around a generic parameter set, we find a strong competition of antiferromagnetic and d-wave Cooper instabilities with a smooth behavior under a variation of doping and additional hopping parameters. For increasingly incommensurate ordering tendencies in the magnetic channel, the d-wave pairing gap is deformed at its maxima. Comparing our results for the Emery model to those obtained for the two-dimensional one-band Hubbard model with effective parameters, we find that, despite considerable qualitative agreement, multi-orbital effects have a significant impact on a quantitative level.Comment: revised version: 22 pages, 11 figure

    Interplay between Point-Group Symmetries and the Choice of the Bloch Basis in Multiband Models

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    We analyze the point-group symmetries of generic multiband tight-binding models with respect to the transformation properties of the effective interactions. While the vertex functions in the orbital language may transform non-trivially under point-group operations, their point-group behavior in the band language can be simplified by choosing a suitable Bloch basis.We first give two analytically accessible examples. Then we show that, for a large class of models, a natural Bloch basis exists, in which the vertex functions in the band language transform trivially under all point-group operations. As a consequence, the point-group symmetries can be used to reduce the computational effort in perturbative many-particle approaches such as the functional renormalization group.Comment: revised version: 38 pages, 4 figure
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