9,154 research outputs found

    Vortex Plasma in a Superconducting Film with Magnetic Dots

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    We consider a superconducting film, placed upon a magnetic dot array. Magnetic moments of the dots are normal to the film and randomly oriented. We determine how the concentration of the vortices in the film depends on the magnetic moment of a dot at low temperatures. The concentration of the vortices, bound to the dots, is proportional to the density of the dots and depends on the magnetization of a dot in a step-like way. The concentration of the unbound vortices oscillates about a value, proportional to the magnetic moment of the dots. The period of the oscillations is equal to the width of a step in the concentration of the bound vortices.Comment: RevTeX, 4 page

    Rectification in Luttinger liquids

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    We investigate the rectification of an ac bias in Luttinger liquids in the presence of an asymmetric potential (the ratchet effect). We show that strong repulsive electron interaction enhances the ratchet current in comparison with Fermi liquid systems, and the I-V curve is strongly asymmetric in the low-voltage regime even for a weak asymmetric potential. At higher voltages the ratchet current exhibits an oscillatory voltage dependence.Comment: 5 pages, Revte

    Functional renormalization group approach to correlated fermion systems

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    Numerous correlated electron systems exhibit a strongly scale-dependent behavior. Upon lowering the energy scale, collective phenomena, bound states, and new effective degrees of freedom emerge. Typical examples include (i) competing magnetic, charge, and pairing instabilities in two-dimensional electron systems, (ii) the interplay of electronic excitations and order parameter fluctuations near thermal and quantum phase transitions in metals, (iii) correlation effects such as Luttinger liquid behavior and the Kondo effect showing up in linear and non-equilibrium transport through quantum wires and quantum dots. The functional renormalization group is a flexible and unbiased tool for dealing with such scale-dependent behavior. Its starting point is an exact functional flow equation, which yields the gradual evolution from a microscopic model action to the final effective action as a function of a continuously decreasing energy scale. Expanding in powers of the fields one obtains an exact hierarchy of flow equations for vertex functions. Truncations of this hierarchy have led to powerful new approximation schemes. This review is a comprehensive introduction to the functional renormalization group method for interacting Fermi systems. We present a self-contained derivation of the exact flow equations and describe frequently used truncation schemes. Reviewing selected applications we then show how approximations based on the functional renormalization group can be fruitfully used to improve our understanding of correlated fermion systems.Comment: Review article, final version, 59 pages, 28 figure

    Fermionization, Convergent Perturbation Theory, and Correlations in the Yang-Mills Quantum Field Theory in Four Dimensions

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    We show that the Yang-Mills quantum field theory with momentum and spacetime cutoffs in four Euclidean dimensions is equivalent, term by term in an appropriately resummed perturbation theory, to a Fermionic theory with nonlocal interaction terms. When a further momentum cutoff is imposed, this Fermionic theory has a convergent perturbation expansion. To zeroth order in this perturbation expansion, the correlation function E(x,y)E(x,y) of generic components of pairs of connections is given by an explicit, finite-dimensional integral formula, which we conjecture will behave as E(x,y)∼∣x−y∣−2−2dG,E(x,y) \sim |x - y|^{-2 - 2 d_G}, \noindent for ∣x−y∣>>0,|x-y|>>0, where dGd_G is a positive integer depending on the gauge group G.G. In the case where G=SU(n),G=SU(n), we conjecture that dG=dimSU(n)−dimS(U(n−1)×U(1)),d_G = {\rm dim}SU(n) - {\rm dim}S(U(n-1) \times U(1)), \noindent so that the rate of decay of correlations increases as n→∞.n \to \infty.Comment: Minor corrections of notation, style and arithmetic errors; correction of minor gap in the proof of Proposition 1.4 (the statement of the Proposition was correct); further remark and references adde

    Explaining the effects of symptom attribution by carers on help-seeking for individuals living with dementia

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    This study investigated the effects of carer attributions on help-seeking behaviour for people with dementia using interviews with 84 carers recruited through general practice. Memory loss was the most commonly reported first symptom but psychological and behavioural symptoms were also common at onset. In over a third of individuals help-seeking was delayed for a mean of 25 months (range 6-69, SD 19.3). Help-seeking between those who attributed symptoms to dementia, or to unknown causes, and those who attributed symptoms to personality, ageing, life events or other illnesses was statistically significant ( p < 0.001). No statistically significant associations between help-seeking and patient or carer characteristics were found. There is a need to raise public awareness about the range of symptoms suggestive of dementia. Assumptions that age and other conditions may be the likely cause of an individual's cognitive decline needs to be challenged by practitioners. Attribution of symptoms to characteristics other than dementia delays help-seeking

    Quasi-long range order in glass states of impure liquid crystals, magnets, and superconductors

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    In this review we consider glass states of several disordered systems: vortices in impure superconductors, amorphous magnets, and nematic liquid crystals in random porous media. All these systems can be described by the random-field or random-anisotropy O(N) model. Even arbitrarily weak disorder destroys long range order in the O(N) model. We demonstrate that at weak disorder and low temperatures quasi-long range order emerges. In quasi-long-range-ordered phases the correlation length is infinite and correlation functions obey power dependencies on the distance. In pure systems quasi-long range order is possible only in the lower critical dimension and only in the case of Abelian symmetry. In the presence of disorder this type of ordering turns out to be more common. It exists in a range of dimensions and is not prohibited by non-Abelian symmetries.Comment: 32 page

    Riemann–Hilbert problems, Toeplitz operators and Q-classes

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    We generalize the notion of Q-classes C(Q1,Q2) , which was introduced in the context of Wiener–Hopf factorization, by considering very general 2 × 2 matrix functions Q1, Q2. This allows us to use a mainly algebraic approach to obtain several equivalent representations for each class, to study the intersections of Q-classes and to explore their close connection with certain non-linear scalar equations. The results are applied to various factorization problems and to the study of Toeplitz operators with symbol in a Q-class. We conclude with a group theoretic interpretation of some of the main results.Fundação para a Ciência e a Tecnologia (FCT/Portugal), through Project PTDC/MAT/121837/2010 and Project Est- C/MAT/UI0013/2011. The first author was also supported by the Center for Mathematical Analysis, Geometry, and Dynamical Systems and the second author was also supported by the Centre of Mathematics of the University of Minho through the FEDER Funds Programa Operacional Factores de Competitividade COMPET

    On Lagrangian formulations for arbitrary bosonic HS fields on Minkowski backgrounds

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    We review the details of unconstrained Lagrangian formulations for Bose particles propagated on an arbitrary dimensional flat space-time and described by the unitary irreducible integer higher-spin representations of the Poincare group subject to Young tableaux Y(s1,...,sk)Y(s_1,...,s_k) with kk rows. The procedure is based on the construction of Verma modules and finding auxiliary oscillator realizations for the symplectic sp(2k)sp(2k) algebra which encodes the second-class operator constraints subsystem in the HS symmetry algebra. Application of an universal BRST approach reproduces gauge-invariant Lagrangians with reducible gauge symmetries describing the free dynamics of both massless and massive bosonic fields of any spin with appropriate number of auxiliary fields.Comment: 8 pages, no figures, extended Contribution to the Proceedings of the International Workshop "Supersymmetry and Quantum Symmetries" (SQS'2011, July 18- July 23, 2011, Dubna, Russia), v.2: 9 pages, 2 references with comments in Introduction adde
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