Magnetic flux dynamics in critical state of one-dimensional discrete superconductor

We give a theoretical description of avalanche-like dynamics of magnetic flux in the critical state of "hard" type-II superconductors using a model of a one-dimensional multijunction SQUID that well reproduces the main magnetic properties of these objects. We show that the system under consideration demonstrates the self-organized criticality. The avalanches of vortices manifest themselves as jumps of the total magnetic flux in the sample. The sizes of these jumps have a power-law distribution. Our results are in qualitative agreement with experiments.Comment: 7 pages, 5 figure

Hypersensitive transport in a phase model with multiplicative stimulus

In a simple system with periodic symmetric potential, the phase model under effect of strong multiplicative noise or periodic square wave, we found a giant response, in the form of directed flux, to an ultrasmall dc signal. The resulting flux demonstrates a bell-shaped dependence on multiplicative noise correlation time and occurs even in the case of large (compared to the signal) additive noise.Comment: 3 EPS figures, submitted to Phys.Lett.

Exploring intergroup conflict and community-based participatory research partnerships over time

Community-based participatory research (CBPR) is a growing practice by which academics and community partners conduct collaborative health-based research. While CBPR fosters productive partnerships, there is increasing research on interpersonal group dynamics and the ways that intersecting factors, such as gender and ethnicity, affect the social interactions within CBPR. This paper explores the tensions inherent in large interdisciplinary community-based participatory research partnerships, through the examination of a long-standing community–academic partnership focused on advancing public health. Nine qualitative interviews were conducted between November 2019 and January 2020 with steering committee members from a long-standing collaborative partnership that conducts research to inform public health action. While the collaborative process was generally positive, we also uncovered less pleasant aspects of CBPR projects that are infrequently discussed in the literature, such as role confusion and power dynamics. Leadership style was seen as the driving force shaping how other team concerns were perceived. Not having structures in place to facilitate relationship development, or clear documentation of procedures, rules and norms, led to team complications. Team members suggested that a renewed focus on organisational structure would contribute to role clarity and organisation. The results highlight the complexity of working on interdisciplinary mixed community–academic teams, specifically the ways in which interdisciplinary, collaborative research can be a complicated, meandering process, often without clear-cut answers to sometimes simple questions

A new type of CP symmetry, family replication and fermion mass hierarchies

We study a two-Higgs-doublet model with four generalised CP symmetries in the scalar sector. Electroweak symmetry breaking leads automatically to spontaneous breaking of two of them. We require that these four CP symmetries can be extended from the scalar sector to the full Lagrangian and call this requirement the principle of maximal CP invariance. The Yukawa interactions of the fermions are severely restricted by this requirement. In particular, a single fermion family cannot be coupled to the Higgs fields. For two fermion families, however, this is possible. Enforcing the absence of flavour-changing neutral currents, we find degenerate masses in both families or one family massless and one massive. In the latter case the Lagrangian is highly symmetric, with the mass hierarchy being generated by electroweak symmetry breaking. Adding a third family uncoupled to the Higgs fields and thus keeping it massless we get a model which gives a rough approximation of some features of the fermions observed in Nature. We discuss a number of predictions of the model which may be checked in future experiments at the LHC.Comment: 24 pages. Version published in EPJC. Minor changes as suggested by the refere

Anisotropy parameters of superconducting MgB2_2

Data on macroscopic superconducting anisotropy of MgB$_2$ are reviewed. The data are described within a weak coupling two-gaps anisotropic s-wave model of superconductivity. The calculated ratio of the upper critical fields $\gamma_H=H_{c2,ab}/H_{c2,c}$ increases with decreasing temperature in agreement with available data, whereas the calculated ratio of London penetration depths $\gamma_{\lambda}=\lambda_c/\lambda_{ab}$ decreases to reach $\approx 1.1$ at T=0. Possible macroscopic consequences of $\gamma_{\lambda}\ne\gamma_H$ are discussed.Comment: accepted to Physica C, special MgB2 issu

Diffusion and Current of Brownian Particles in Tilted Piecewise Linear Potentials: Amplification and Coherence

Overdamped motion of Brownian particles in tilted piecewise linear periodic potentials is considered. Explicit algebraic expressions for the diffusion coefficient, current, and coherence level of Brownian transport are derived. Their dependencies on temperature, tilting force, and the shape of the potential are analyzed. The necessary and sufficient conditions for the non-monotonic behavior of the diffusion coefficient as a function of temperature are determined. The diffusion coefficient and coherence level are found to be extremely sensitive to the asymmetry of the potential. It is established that at the values of the external force, for which the enhancement of diffusion is most rapid, the level of coherence has a wide plateau at low temperatures with the value of the Peclet factor 2. An interpretation of the amplification of diffusion in comparison with free thermal diffusion in terms of probability distribution is proposed.Comment: To appear in PR

QCD Corrections to QED Vacuum Polarization

We compute QCD corrections to QED calculations for vacuum polarization in background magnetic fields. Formally, the diagram for virtual $e\bar{e}$ loops is identical to the one for virtual $q\bar{q}$ loops. However due to confinement, or to the growth of $\alpha_s$ as $p^2$ decreases, a direct calculation of the diagram is not allowed. At large $p^2$ we consider the virtual $q\bar{q}$ diagram, in the intermediate region we discuss the role of the contribution of quark condensates \left and at the low-energy limit we consider the $\pi^0$, as well as charged pion $\pi^+\pi^-$ loops. Although these effects seem to be out of the measurement accuracy of photon-photon laboratory experiments they may be relevant for $\gamma$-ray burst propagation. In particular, for emissions from the center of the galaxy (8.5 kpc), we show that the mixing between the neutral pseudo-scalar pion $\pi_0$ and photons renders a deviation from the power-law spectrum in the $TeV$ range. As for scalar quark condensates \left and virtual $q\bar{q}$ loops are relevant only for very high radiation density $\sim 300 MeV/fm^3$ and very strong magnetic fields of order $\sim 10^{14} T$.Comment: 15 pages, 4 figures; Final versio

Quasi-long-range order in the random anisotropy Heisenberg model: functional renormalization group in 4-\epsilon dimensions

The large distance behaviors of the random field and random anisotropy O(N) models are studied with the functional renormalization group in 4-\epsilon dimensions. The random anisotropy Heisenberg (N=3) model is found to have a phase with the infinite correlation radius at low temperatures and weak disorder. The correlation function of the magnetization obeys a power law < m(x) m(y) >\sim |x-y|^{-0.62\epsilon}. The magnetic susceptibility diverges at low fields as \chi \sim H^{-1+0.15\epsilon}. In the random field O(N) model the correlation radius is found to be finite at the arbitrarily weak disorder for any N>3. The random field case is studied with a new simple method, based on a rigorous inequality. This approach allows one to avoid the integration of the functional renormalization group equations.Comment: 12 pages, RevTeX; a minor change in the list of reference

Neutral Higgs boson pair production via γγ\gamma\gamma collision in the minimal supersymmetric standard model at linear colliders

We investigate in detail the $\gamma\gamma$ fusion production mechanisms of two neutral Higgs bosons ($h^0A^0$, $H^0A^0$, $h^0H^0$ and $H^0H^0$) within the framework of the mSUGRA-inspired minimal supersymmetric standard model(MSSM) at an $e^+e^-$ linear colliders, which provide a probe of the trilinear Higgs self-couplings. We calculate the dependence of the production rates on Higgs boson masses, the ratio of the vacuum expectation values $\tan \beta$ and the CMS energy $\sqrt{s}$. We find that the cross section for the $H^0H^0$ production at LC can reach $0.2 fb$, while the cross section of $A^0H^0$ production is only $10^{-4}\sim 10^{-3} fb$ under our parameters.Comment: Accepted by Phys. Rev.

Bosonic Excitations in Random Media

We consider classical normal modes and non-interacting bosonic excitations in disordered systems. We emphasise generic aspects of such problems and parallels with disordered, non-interacting systems of fermions, and discuss in particular the relevance for bosonic excitations of symmetry classes known in the fermionic context. We also stress important differences between bosonic and fermionic problems. One of these follows from the fact that ground state stability of a system requires all bosonic excitation energy levels to be positive, while stability in systems of non-interacting fermions is ensured by the exclusion principle, whatever the single-particle energies. As a consequence, simple models of uncorrelated disorder are less useful for bosonic systems than for fermionic ones, and it is generally important to study the excitation spectrum in conjunction with the problem of constructing a disorder-dependent ground state: we show how a mapping to an operator with chiral symmetry provides a useful tool for doing this. A second difference involves the distinction for bosonic systems between excitations which are Goldstone modes and those which are not. In the case of Goldstone modes we review established results illustrating the fact that disorder decouples from excitations in the low frequency limit, above a critical dimension $d_c$, which in different circumstances takes the values $d_c=2$ and $d_c=0$. For bosonic excitations which are not Goldstone modes, we argue that an excitation density varying with frequency as $\rho(\omega) \propto \omega^4$ is a universal feature in systems with ground states that depend on the disorder realisation. We illustrate our conclusions with extensive analytical and some numerical calculations for a variety of models in one dimension