One-step replica symmetry breaking solution for a highly asymmetric two-sublattice fermionic Ising spin glass model in a transverse field

The one-step replica symmetry breaking (RSB) is used to study a two-sublattice fermionic infinite-range Ising spin glass (SG) model in a transverse field $\Gamma$. The problem is formulated in a Grassmann path integral formalism within the static approximation. In this model, a parallel magnetic field $H$ breaks the symmetry of the sublattices. It destroys the antiferromagnetic (AF) order, but it can favor the nonergodic mixed phase (SG+AF) characterizing an asymmetric RSB region. In this region, intra-sublattice disordered interactions $V$ increase the difference between the RSB solutions of each sublattice. The freezing temperature shows a higher increase with $H$ when $V$ enhances. A discontinue phase transition from the replica symmetry (RS) solution to the RSB solution can appear with the presence of an intra-sublattice ferromagnetic average coupling. The $\Gamma$ field introduces a quantum spin flip mechanism that suppresses the magnetic orders leading them to quantum critical points. Results suggest that the quantum effects are not able to restore the RS solution. However, in the asymmetric RSB region, $\Gamma$ can produce a stable RS solution at any finite temperature for a particular sublattice while the other sublattice still presents RSB solution for the special case in which only the intra-sublattice spins couple with disordered interactions.Comment: 11 pages, 8 figures, accepted for publication in Phys. Rev.

Iteratively reweighted compressive sensing based algorithm for spectrum cartography in cognitive radio networks

© 2014 IEEE. Spectrum cartography is the process of constructing a map showing Radio Frequency signal strength over a finite geographical area. In our previous work we formulated spectrum cartography as a compressive sensing problem and we illustrated how cartography can be used in the context of discovering spectrum holes in space that can be exploited locally in cognitive radio networks. This paper investigates the performance of compressive sensing based approach to cartography in a fading environment where realtime channel estimation is not feasible. To accommodate for lack of channel information we take an iterative approach. We extend the well-known iteratively reweighted ℓ1 minimisation approach by exploiting spatial correlation between two points in space. We evaluate the performance in an urban environment where Rayleigh fading is prominent. Our numerical results show a significant improvement in the probability of accurately making a spectrum sensing decision, in comparison to the well-known weighted approach and the traditional compressive sensing based method

Tricritical behaviour of Ising spin glasses with charge fluctuations

We show that tricritical points displaying unusal behaviour exist in phase diagrams of fermionic Ising spin glasses as the chemical potential or the filling assumes characteristic values. Exact results for infinite range interaction and a one loop renormalization group analysis of thermal tricritical fluctuations for finite range models are presented. Surprising similarities with zero temperature transitions and a new $T=0$ tricritical point of metallic quantum spin glasses are derived.Comment: 4 pages, 1 Postscript figure, minor change

Nonanalytic quantum oscillator image of complete replica symmetry breaking

We describe the effect of replica symmetry breaking in the field distribution function P(h) of the T=0 SK-model as the difference between a split Gaussian and the first excited state $\psi_1$ of a weakly anharmonic oscillator with nonanalytic shift by means of the analogy $P(h)|\psi_1(x)|$. New numerical calculations of the leading 100 orders of replica symmetry breaking (RSB) were performed in order to obtain P(h), employing the exact mapping between density of states $\rho(E)$ of the fermionic SK-model and P(h) of the standard model, as derived by Perez-Castillo and Sherrington. Fast convergence towards a fixed point function $\rho(E)$ for infinite steps of RSB is observed. A surprisingly small number of harmonic oscillator wave-functions suffices to represent this fixed point function. This allows to determine an anharmonic potential V(x) with nonanalytic shift, whose first excited state represents $\rho(E)$ and hence P(h). The harmonic potential with unconventional shift $V_2(x)\sim (|x|-x_0)^2=(x-x_0\,sign(x))^2$ yields already a very good approximation, since anharmonic couplings of $V(x)-V_2(x)\sim |x|^{m}, m>2,$ decay rapidly with increasing m. We compare the pseudogap-forming effect of replica symmetry breaking, hosted by the fermionic SK-model, with the analogous effect in the Coulomb glass as designed by Davies-Lee-Rice and described by M\"uller-Pankov.Comment: 11 pages, 3 figures, submitted to Phil. Mag., special edition in honour of David Sherrington's 70th birthda

A measure of personal information in mobile data

© 2020 IEEE. This paper describes fundamental aspects of a framework for privacy-preserving data sharing in a mobile context. The principal technical challenge is measuring the level of personal information (PI) in datasets that are shared for the delivery or enhancement of mobile enabled services. Another challenge is determining the threshold delineating a 'reasonable likelihood' of an individual being identifiable from the data. The risk of reidentification defines personally identifiable information (PII). The measure of PI must go beyond simply analysing personal attributes captured in data and consider preference revealed through use of services, temporal and spatial aspects of data, as well as context for use of services. Keywords-data sharing, privacy, mobile services

Magnetic Gaps related to Spin Glass Order in Fermionic Systems

We provide evidence for spin glass related magnetic gaps in the fermionic density of states below the freezing temperature. Model calculations are presented and proposed to be relevant for explaining resistivity measurements which observe a crossover from variable-range- to activated behavior. The magnetic field dependence of a hardgap and the low temperature decay of the density of states are given. In models with fermion transport a new metal-insulator transition is predicted to occur due to the spin-glass gap, anteceding the spin glass to quantum paramagnet transition at smaller spin density. Important fluctuation effects due to finite range frustrated interactions are estimated and discussed.Comment: 4 pages, 1 Postscript figure, revised version accepted for publication in Physical Review Letter

Akzeptanz von Tiergesundheitsplänen bei Landwirten – Ergebnisse einer Befragung bei 60 Betrieben

In organic farming the ambitious claims in enhancing and keeping animal health are often not realised. The implementation of animal health plans should clear this deficit effectively. Experiences with British and Danish herd health plans showed that the acceptance of plans is an essential part for its successful transfer into practice. But anyhow, this aspect has not been regarded sufficiently. To avoid similar mistakes like done in former institution tests a social study is integrated into German projects that deal with the implementation of animal health plans in poultry, dairy, and pig hus-bandry. To get more information about the acceptance, the study requires farmers’ attitudes to herd health plans, the motivation to animal health and financial and work capabilities as well

A cooperative multihop radio resource allocation in next generation networks

With the objectives of improving 3G networks coverage by using relaying nodes, a game theoretic approach to radio resource allocation for the downlink capacity is introduced. One of the basic issues not examined in the literature is the importance of fair resource sharing among mobile nodes located along a multihop link. We describe a novel technique for providing a resource allocation mechanism in a multihop relaying network. The resource allocation problem is formulated as a cooperative game using Nash Bargaining Solution (NBS), which allows mobile nodes to fairly share a downlink bandwidth among themselves. Sharing of the downlink capacity between multiple nodes using a noncooperative approach is inefficient when the radio resource is scarce. If upstream nodes manipulate their location at the head of the multihop link to exploit the downlink capacity, downstream nodes will suffer disproportionately. The undesirable properties can be avoided by means of a cooperative agreement in which all nodes share the radio resources equally, where downstream nodes are allowed to pay compensation to prevent upstream nodes from exploiting the downlink capacity and encourage them to cooperate. © 2005 IEEE

Superconductors with Magnetic Impurities: Instantons and Sub-gap States

When subject to a weak magnetic impurity potential, the order parameter and quasi-particle energy gap of a bulk singlet superconductor are suppressed. According to the conventional mean-field theory of Abrikosov and Gor'kov, the integrity of the energy gap is maintained up to a critical concentration of magnetic impurities. In this paper, a field theoretic approach is developed to critically analyze the validity of the mean field theory. Using the supersymmetry technique we find a spatially homogeneous saddle-point that reproduces the Abrikosov-Gor'kov theory, and identify instanton contributions to the density of states that render the quasi-particle energy gap soft at any non-zero magnetic impurity concentration. The sub-gap states are associated with supersymmetry broken field configurations of the action. An analysis of fluctuations around these configurations shows how the underlying supersymmetry of the action is restored by zero modes. An estimate of the density of states is given for all dimensionalities. To illustrate the universality of the present scheme we apply the same method to study `gap fluctuations' in a normal quantum dot coupled to a superconducting terminal. Using the same instanton approach, we recover the universal result recently proposed by Vavilov et al. Finally, we emphasize the universality of the present scheme for the description of gap fluctuations in d-dimensional superconducting/normal structures.Comment: 18 pages, 9 eps figure

Disorder-driven superconductor-normal metal phase transition in quasi-one-dimensional organic conductors

Effects of non-magnetic disorder on the critical temperature T_c and on diamagnetism of quasi-one-dimensional superconductors are reported. The energy of Josephson-coupling between wires is considered to be random, which is typical for dirty organic superconductors. We show that this randomness destroys phase coherence between wires and that T_c vanishes discontinuously at a critical disorder-strength. The parallel and transverse components of the penetration-depth are evaluated. They diverge at different critical temperatures T_c^{(1)} and T_c, which correspond to pair-breaking and phase-coherence breaking respectively. The interplay between disorder and quantum phase fluctuations is shown to result in quantum critical behavior at T=0, which manifests itself as a superconducting-normal metal phase transition of first-order at a critical disorder strength.Comment: 12 pages, 3 figure